Publications

Improving Water Quality Can Yield a Better Crop

by Nancy Walery
San Diego County Farm Bureau 

Whether you grow flowers, nursery products, fruits, vegetables, or produce anything in which water is central to a great agricultural crop, you don’t want to settle for water quality that works against you and your production goals. But if you’re irrigating your crops with unadulterated H20 from the tap, chances are good you’re doing yourself and your ag products a disservice that’s resulting in reduced yields at harvest time.

Applied Membranes, Inc., a Vista-based designer, manufacturer and distributor of commercial, industrial and residential reverse osmosis (RO) filtration systems and components, is the grower’s solution to water quality problems. A SDCFB Business Supporting Member since 2009 and in business since 1983, AMI has supplied standard and custom water filtration and purification systems and services to a wide range of customers, from residential households to Fortune 500 companies offering a broad range of applications including ultrapure, potable, dialysis, pharmaceutical, water recycling, seawater desalination, and more. Company owner Gil Dhawan has seen the proof that RO systems provide growers—even dairy farmers—with impressive improvements in yields.

“Anything the grower grows requires good quality water,” said Dhawan, who has worked in the field of membrane technology since 1972 and holds a Doctorate in Chemical Engineering from the University of Waterloo in Ontario, Canada. “Many times, the grower’s water quality—whether from a well or from municipal sources, is not as good as it should be and could be a factor in not only the crop volume, but also product size, uniformity and overall quality.”

What reverse osmosis systems can do for agriculture is remove—or at least considerably reduce—the sodium chloride that is so prevalent in most water samples Dhawan analyzes. In addition, it can even out the variances in water quality that occur from municipal water that can come from mixed sources during different times of the year.

“Salt is a significant factor in agricultural situations because the high chloride levels reduce crop yields,” said Dhawan, who has authored dozens of technical articles on the subject of RO technology. “Our goal is to reduce that to minimal levels. But we don’t have to remove all of it; each crop has a different threshold limit for these compounds, so as long as we can bring the level below that threshold, there will be an improvement in the crop. After we conduct a detailed analysis of a customer’s water composition, we design their system around that data so that the system provides optimum water quality.”

Dhawan said that one of his first reverse osmosis systems sold in the agriculture sector went to a local orchid grower, and it completely changed the quality and consistency of his products. Even dairy farmers will see a marked improvement in milk production from cows who drink RO water, Dhawan said, citing a study by a New Mexico dairy farmer who is also a veterinarian from UC Davis. In that study, two different water bins were provided—one with regular tap water, the other—located farther away—with RO water. It was found that more animals would walk the longer distance to drink the RO water, and they drank more, which increased their milk production.

AMI membrane products are used in more than 100 countries around the worlds and are made in USA in facilities that have earned the ISO 9001:2015 certification, an international quality standard.

Another advantage to working with AMI is the technical expertise and robust parts inventory availability that comes with your system—not just when you buy it, but during the entire lifespan of your AMI equipment. Should you ever need a reanalysis of the effectiveness of your system due to changes in your water quality or other suspected problems, AMI stands ready with the technical expertise to make the proper adjustments quickly and accurately. A strong inventory control system assures that AMI never runs out of your specific filter replacement or other components, and the volume of parts movement ensures competitive pricing. The company, which carries a wide range of filters and housings manufactured under the AMI label, is also a large stocking distributor for many of the well-known brand-name filters.

“Most companies providing similar products and services are not as technically oriented as AMI to provide the critical analysis and customized recommendations the customer is looking for,” Dhawan said. “Over time, RO systems will likely need some kind of technical assistance. So, with our more technical business model, when our customers call looking for service and support, that’s when we will shine.”

Applied Membranes, Inc. is located at 2450 Business Park Dr., Vista, CA 92081 and can be reached at (760) 727-3711, toll free at (800) 321-9321. Also be sure to visit AMI’s comprehensive website at www.appliedmembranes.com.

San Diego County Farm Bureau has a strong roster of Business Supporting members dedicated to serving the local agriculture community. For a complete list of Business Supporting members and the services they provide, visit the San Diego County Farm Bureau website.

By Gil K. Dhawan | 1997

Introduction

Ultrafiltration and low pressure reverse osmosis are finding increasing use as a final treatment step in the production of ultrapure, particle free and organic free water. Membrane systems are especially attractive for the point-of-use as required in electronic, pharmaceutical, and potable water applications.
Several advancements have been made over the past three years. These include better membranes, better analytical equipment, and better understanding of the effect of impurities on the use of water. This paper reviews some of these developments and gives examples of some of the applications for ultrafiltration and low pressure RO.

Point-of-Use Water for the Electronic Industry

In the electronic industry, water standards are periodically revised to reflect changing needs, changing raw water quality, and the availability of more sophisticated analytical techniques.
The starting point for producing ultrapure water is the Feedwater which is generally obtained from city water supply. The standard for this water is set by the Federal Environmental Protection Agency. Table 1 gives some of the more common contaminants and their maximum concentrations allowed in drinking water.
There has been a great deal of discussion on the quality of water required for final rinsing of wafers. Several water quality specifications have been proposed and projected for the point-of-use water (Table 2.)
The technology to produce the quality of water demanded by the electronic industry has improved over the last five years. The major areas of concern in the ultrapure water are the growth of microorganisms and the presence of colloidal materials. Colloidal contamination can be caused by impurities present in the raw water such as humic acid or colloidal silica, bacterial growth in the system, and bacterial byproducts such as pyrogens. Other sources of contamination are leaching of materials from piping, valves, gauges, pumps, and controls used in the water purification system.
To obtain and maintain the highest quality of water it is essential to use materials for piping and other components that do not leach out in the ultrapure water. Fluoropolymer is replacing PVC as construction material for ultrapure water systems.
At present, the final treatment step for the point-of-use water in the electronic industry is filtration through a 0.2 micron microporous filter to remove microorganisms and colloidal materials that may still be present in the water at that point. Substantial experience now indicates that 0.2 micron filtration is not adequate to maintain the high quality of water.

A recent study (Ref. 2) shows that ultrafiltration provides much superior quality of water at point-of-use than the conventional 0.2 micron filters. Microporous membrane systems are in the 0.2 to 10 micron particle removal range and operate with inlet pressures from 5 to 100 psi. Ultrafiltration, on the other hand, removes particles from 0.001 to 0.05 micron.

One such system designed for point-of-use in micro-electronics industry is a 5 gallon per minute system supplied by Millipore. This system uses spiral wound ultrafiltration membranes with a 0.006 micron rating and a final 0.2 micron membrane. Figure 1 shows the Millipore system with the entire piping constructed of fluoropolymer materials. The system has a sanitization injection post at the inlet to the system. The reject valve for the system is factory set at 0.1 gallon per minute so that a very high recovery (98%) is obtained. Periodically the valve is opened to allow full flow and a flushing action across the membrane.
These systems have been tested (Ref. 2) using colloid retention. These tests were carried out both in the laboratory and in the field. In addition, the wafers were tested to determine the performance of the point-of-use system.

The first comparison of point-of-use microfilters and ultrafilter was done by using a modified Silt Density Index. The D.I. water flows from the point-of-use filter under test into an SDI filter. The rate of plugging of the SDI filter is an indication of the colloidal removal efficiency of the point-of-use filter. A higher rate of plugging would mean a relatively poor efficiency in removing colloids and vice versa.

Figure 2 summarizes the results of this testing. The graph shows that 0.1 and 0.2 micron filters allowed the passage of about the same amount of particles and colloids. A much better performance was shown by the ultrafilter with a 100,000 molecular weight (0.006 micron).

The results of the field test are given in Figure 3. The plot shows plugging of a 0.2 micron filter with DI water filtered through a 0.2 micron filter versus plugging of water treated by ultrafiltration. Clearly, ultrafiltration has produced a much higher quality water.

And, finally, the point-of-use ultrafiltration was also evaluated by measuring wafer contamination. Table 3 gives the results of these tests in four different locations. The table shows a significant reduction of particles on wafers when ultrafiltration was used, as compared with the 0.2 micron filter. Locations 1 and 4 also show the effects of not upgrading the piping downstream from ultrafiltration.

More work is being done by different researches in understanding and improving the quality of water at the point-of-use, but it is clear that point-of-use ultrafiltration provides much better quality rinse water than was possible before with 0.2 micron filters (Table 4).

Conclusions

The applications of ultrafiltration and low pressure reverse osmosis for point-of-use water treatment looks very promising. The development of high flux membranes and “softener” type RO membranes is sure to expedite the use of these membranes at the point-of-use, to meet the very high water quality requirements in a number of industries. We can expect to see an increasing use of these processes in the production of ultrapure water.

References

1. Motomura, H., Microcontamination, March 1984
2. Accomazzo, M. and Gaudet, P.W., Point-of-Use Ultrafiltration of Deionized Water and Effects on Microelectronics Devices Quality, Millipore Corporation.
3. California State Department of Health Services, Sanitary Engineering Branch
4. United States Pharmacopeia, XX Edition, Mack Publishing Company, 1980.
5. Pharmaceutical Technology, October 1983, Part IIb of the report of PMA’s Deionization Water Committee.

Figure 1  Ultrafiltration System  Designed for Point-of-Use in the Micro-electronics Industry	Figure 2   Comparison of Point-of-Use Microfilters and Ultrafilter Using a Modified Silt Density Index	Figure 3 Comparison of UF and 0.2 Micron Filter
UF system using spiral wound ultrafiltration membranes with a 0.006 micron rating and a final 0.2 micron membrane.	Plugging of 0.2 Micron Membrane (293 mm disc) Downstream of Test Filters at Constant Flow Rate (2.0 GPM)	Plugging of 0.2 Micron Disc Filter in DI Water Loop Before and After Ultrafiltration

Table 1 - Primary Drinking Water Standards (Ref. 3)

Table 2 - Specifications for Ultrapure Water for the Semiconductor Industry

a.  Measured with a resistivity meter.

b.  Measured by direct microscopic count.  Particles retained on a poly-carbonate filter are stained and counted with an optical microscope or, when particles are less than 0.1 micron, witha  scanning electron microscope.

c. Measured by ultraviolet oxidation-resistivity detection (Barnstead's Photochem).  Wet oxidation-infrared detection is used for determining total organic carbon levels higher than 0.2 ppm.

d. Measured by the culture method (ASTM F60-68).

e. Measured by the colorimetric-molybdate reactive silica method (ASTM 0689-80).

f. Measured by Winkler's titration.

Table 3 - Point-Of-Use Ultrafiltration Evaluation Using Wafer Contamination

Particle Counts on Wafers

*Plumbing after UF not upgraded

Table 4 - Point-of-Use Ultrafiltration Performance

Case Study:   Zero Discharge Industrial Wastewater Treatment at R.D. Nixon Power Plant

by Gil K. Dhawan | 1983

Abstract

A zero discharge wastewater system designed by the Fluid Systems Division of UOP Inc. at San Diego, California and CH₂M Hill of Bellevue, Washington has been operating successfully at the R.D. Nixon Power Plant since October, 1980.   Operating savings are estimated at 50 percent over traditional alternatives.  Process design for the facility was arrived at after examining several options.  Unit operations for the system include clarification, filtration, reverse osmosis, and vapor recompression evaporation.   The various options of the performance of the facility are reviewed.

Introduction

The R.D. Nixon Power Plant is located in Fountain Colorado.  The plant site selection was based on a number of factors including its proximity to customers.  This location put the plant 15 miles from the municipal sewer line and adjacent to a local freshwater stream.  Any liquid discharged into this stream must comply with stringent effluent discharge requirements.  However, discharge of plant effluent into the municipal sewer line would require a long pipeline at prohibitive costs.  Therefore, the only viable option was to treat and recycle the plant effluent. The wastewater treatment system was designed by CH₂M Hill, and the reverse osmosis system was designed and supplied by Fluid Systems.  The overall cost of the wastewater system was approximately half the estimated cost of installing and operating a pipeline to the municipal sewage plant.

Supply Water

Supply water for the power plant is obtained from wells located near the plant.  Total requirement for the power plant averages around 1700 gallons per minute (GPM).  Almost 90% of this usage is for make-up water for the cooling tower.

The design analysis of the well water is given in Table 1.   From the analysis it is clear that the concentration cycles in the cooling tower would lead to the precipitation of calcium phosphate, calcium sulfate, or silica. 

Well water hardness can be as high as 600 mg/l of calcium carbonate.  The supply water is, therefore, softened by ion exchange resins (zeolite).  The total hardness is reduced to less than 15 mg/l (Table 1).  The softened water is used for the cooling tower and other routine uses in the plant.

Make-Up of Wastewater

Power plant effluent comes from a number of sources.  These are as follows:

• Cooling tower blowdown
• Softener regenerant wastewater
• Ash sluicewater blowdown
• Demineralizer regeneration waste
• Floor drains
• Chemical cleaning wastes

In fact, all plant wastewater other than sewage is treated by the wastewater treatment system.  

Nearly 60% of the total wastewater comes from cooling tower blowdown.  The concentration cycles are limited by the solubility of the calcium phosphate still remaining in the softened water, as shown in a typical analysis of the cooling water blowdown (Table 1).

The regenerant wastewater from the softener is the next largest source of wastewater, contributing nearly 18% of the total.   As expected, the softener regenerant wastewater has very high concentrations of calcium.

The third largest source of wastewater is the blowdown of the ash sluicewater (15%).  A slipstream of cooling tower blowdown is used as make-up water to remove the bottom ash from the ash hoppers in the coal-fired power plant.  The mixture of ash and water is allowed to settle in the ash ponds, and the supernatant from the last pond is continuously pumped back to the sluicewater tank.  A portion of the liquid from the sluicewater is blown down continuously from the upstream of the sluicewater tank.

The demineralizer regeneration waste contributes roughly 5% of the wastewater.  The demineralizer is used to produce high quality water for the boiler. 

The majority of the water in the floor drains is water that was used for flushing various pump seals.  The floor drains contribute about 2 percent of the total wastewater from the power plant. 

A representative analysis of some of the major wastewater streams and the combined plant wastewater is given in Table 1.  The combined wastewater analysis does not include the softener regenerant stream. 

The definition of waste streams from the power plant was the first step toward a process design for the wastewater treatment.   The following section examines the process design options for various water and wastewater needs.

Table 1
Design Composition of Major Water, Wastewater and Treated Wastewater Streams

Wastewater Treatment – Process Design

CH₂M Hill developed the process design for the Nixon Power Plant.  Design decisions focused on four main considerations:

1. Water treatment of well water for the cooling tower
2. Treatment and recycle of combined plant wastewater
3. Water treatment for boiler feedwater
4. Concentration and disposal of the final wastewater from (3).

Several options were considered for each step.  A brief summary of the rationale for each process selection is given below.

1. Cooling Tower Water

Analysis showed that the cooling tower water quality requirement could be met by the removal of those scale-forming ions that limit the cycles of concentration in the cooling tower.  The limiting ions in the composition of the well water supply are Ca^-- and PO⁼₄.  Processes considered for the removal of these ions included lime-soda softening and ion exchange (zeolite) softening.  Ion exchange softening was selected because the capital and chemical costs were lower.

2. Treatment and Recycle of Combined Plant Wastewater

The processes considered here were as follows:

• Sidestream lime-soda softening (SS L/S)
• Reverse osmosis (RO)
• Vapor recompression evaporation (VRE)

Sidestream lime-soda softening would remove calcium, magnesium, silica, and phosphate ions from the cooling water.   For the water quality in the cooling tower, SS L/S was more expensive than reverse osmosis due to the requirement for a two-stage system to ensure hardness removal, higher chemical costs, and the higher cost of cooling tower construction materials required by a more saline environment.  The quality of cooling tower blowdown required with SS/LS was similar to the reverse osmosis concentrate; therefore, final evaporation costs were roughly equal.

Capital and operating costs for reverse osmosis are an order of magnitude lower than for VRE.  On the other hand, there is a practical limitation to the maximum concentration of salts in water with reverse osmosis.  Therefore, a combination of reverse osmosis and VRE was the most economical solution for the wastewater treatment.  The projected quality of the permeate from reverse osmosis was considered suitable for cooling tower make-up water, and the condensate from the VRE could be used as boiler feedwater. 

3. Boiler Feedwater

Boiler feedwater criteria is as follows:

Studies indicated that the entire need for boiler feedwater could be met by polishing the condensate return from the VRE by mixed bed ion exchange.  However, a complete demineralizer train including carbon, cation, degassing, anion, and mixed bed units would be necessary as a back-up.  Since permeate from the reverse osmosis units is of better quality than the softened supply water, the reverse osmosis permeate could be diverted to the demineralizer train if VRE condensate supply were cut off.

Also, the excess condensate return could be sent to the cooling tower.  

4. Final Waste Concentration

To further reduce the volume of the concentrate from the vapor recompression evaporator, evaporation ponds were considered.  Since land was available, evaporation ponds offered substantial savings to processing alternatives such as spray drying.

5. Other Waste Streams

For other low volume and/or low frequency waste streams, the following decisions were made:

Economic Comparison of Process Design Alternatives

An economic comparison of the different process design alternatives done by CH2M Hill is given in Figure 1.  Based on these costs and other considerations discussed above, the process scheme for case 1A was selected.    Evaporation ponds were chose for further concentration of the concentrate from the evaporator, and the solids collected in evaporation ponds were to be disposed of in landfills.  

Figure 1
Zero Discharge Process Alternatives

• IX  = Ion Exchange Softening 
• L  = Cold Lime Softening
• L/S  = Lime Soda Softening
• SSL/S  = Side Stream Lime Soda Treatment
• RO  = Reverse Osmosis
• VRE = Vapor Recompression Evaporation

Wastewater Treatment System

The wastewater from all sources, except the cooling tower, flows into an equalization basin.  From there it is pumped through a mixing tank.  Through valve control the wastewater to be treated can be obtained from the equalization basin, the cooling tower, or both (see Figure 2 and Figure 3).  Chlorine, alum, and polymer are added to the wastewater and allowed to mix in the mixing tank.  From the mixing tank the wastewater is pumped to the three parallel clarification and filtration units.  Each unit contains a flocculation chamber, a clarification section employing tube settlers, and a mixed media filter.  The filters and tubes are automatically backwashed.

The filtration system is automated and a cationic polymer and ferric chloride are added continuously in proportion to the volume of wastewater.   The filters are automatically backwashed if the filtered water has a turbidity of higher than 0.9 JTU.

The clarified and filtered water flows into a clear well.   The clear well serves a s a supply water tank for the reverse osmosis process as well as for backwash and surface wash for the media filter.  The backwash wastewater flows into a sump which also collects water from the floor drains.  The wastewater from the sump is pumped to the bottom ash ponds.  Bottom ash is sluiced out of the wet-bottom ash hopper to the ash ponds once or twice a shift with large quantities of water.   The resulting slurry settles in three 3-acre ponds connected in series, and the supernatant from the last pond is continuously pumped back to the sluicewater tank. 

The water from the clear well is pumped through a cartridge filter to the suction of the high pressure pump for the reverse osmosis system.  Scale inhibitor and sulfuric acid are added to the water prior to the cartridge filter.

The reverse osmosis system is divided into three parallel units containing Fluid Systems’ Model 8150 membrane elements.  The operating conditions for the reverse osmosis system are listed in Table 2.  The permeate from the reverse osmosis system is collected in a tank from where it can be transferred to either the cooling tower directly or to the boiler, if required.  The concentrate from the reverse osmosis system is collected in another tank from where it is transferred to the evaporation feed tank.  The evaporation feed tank also receives regeneration wastewater from the softener.  Two reverse osmosis units can meet the normal plant flow requirements, and the third is on standby.  Occasionally, the three units are run to meet peak demand.

After pH adjustment and scale inhibitor addition, the liquid from the evaporation feed tank is pumped to the evaporator for a final volume reduction.   The condensate from the evaporator is collected in a tank and pumped to the boiler feedwater supply.

The waste liquid from the evaporator is sent to a decant basin where the salts are allowed to settle.  The overflow liquid from the decant basin goes to evaporation ponds.  Periodically, the salts from the decant basins are removed by solid waste handling equipment.

Figure 2
Water Flow Balance Wastewater Treatment System

Figure 3
Ray D. Nixon Generating Sation Effluent Treatment Filtration & Desalting Systems  (click to enlarge)

Operation of Wastewater Treatment System

The R.D. Nixon Power Plant has an excellent program to operate and maintain the wastewater treatment system.  The entire operation can be monitored from a central control room, and an IBM computer prints out a weekly maintenance schedule.  Water samples are taken at selected points for a more detailed analysis.  The operation and maintenance of the wastewater system requires one operator and three maintenance personnel on weekdays and just one operator on evening shifts and weekends.

Performance of the Wastewater Treatment System

The wastewater treatment system has been in operation since October, 1980 with very few problems.  The system proved to be well designed and has performed as expected.  The major problems included high calcium concentrations in the feedwater to the reverse osmosis systems.  This was corrected by diverting the softener regenerant water from the equalization basin to the evaporator.  Another problem arose because of improper coagulation in the pretreatment section.  This was alleviated by changing the coagulant from aluminum sulfate to ferric chloride.

The reverse osmosis system has performed extremely well, and no major problems have been encountered.  The original membranes are still in use producing the design capacity and projected quality of product water.

The analysis of feedwater, permeate, and concentrate is given in Table 3 at start-up of the reverse osmosis system in October, 1980, and in Figure 4 for September, 1982.  These analyses indicate that the reverse osmosis system is still removing over 95% of the total dissolved solids in the feedwater.   Cleaning of membranes with recommended cleaning solutions has been necessary once every two months.

A high concentration of silica encountered in the feedwater has not been a problem.  Under the reverse osmosis operating conditions (Table 2), the maximum silica allowable concentration in the feed is 70 mg/l.  At higher silica concentrations, the recovery of the reverse osmosis is adjusted downwards.

No unscheduled shutdowns of the reverse osmosis system have occurred.  Scheduled shutdowns of the system include yearly power plant shutdown for a month and shutdown when there is a low level in the equalization basin. 

Table 2
Reverse Osmosis System Operating Conditions

Table 3
Reverse Osmosis Performance Data | Test Results on October 6, 1980  (Analysis in mg/l)

Figure 4
Wastewater Treatment - Typical Water Analysis Through Treatment Steps

Conclusions

Power plant wastewater can be treated and recycled by using pretreatment, reverse osmosis, and vapor recompression evaporation.

For situations where water is in a short supply and water costs are high, or when plant effluent must meet stringent standards, the zero discharge system described here provides the most economical and viable solution.

By Gil K. Dhawan | July 1978

Abstract

Emulsified oily water presents a difficult treatment problem in a large number of metal fabricating and machining industries.  The conventional methods of disposal include chemical treatment, incineration and haulage by the waste disposal companies.  This paper describes a new approach for the treatment of the oily waste water.

A typical system would include pre-treatment (such as settling or screening), ultrafiltration and some post-treatment.  If the oil can be recycled, the post treatment is generally settling to increase the concentration of the oil.  If the oil cannot be recycled, the post-treatment is usually incineration. The concentration of the recovered oil can be as high as 90% oil by volume.

The ultrafiltration system dewaters the oily water solution up to 90% as it passes over the membrane system developed by Electrohome.  Each module contains tubes that act as the pressure vessels supporting the cylindrical membranes.  With each pass through the tubes water is removed and the oily water gets more concentrated.

The present systems are being used on a variety of oily water situations in a number of metal industries.  This paper discusses the data collected from these operating systems.  Examples of some of the plants in operation are given.  In most cases the savings in the haulage costs and reduction in the incineration costs can pay for the system in less than two years.  Where recovery is possible the payback is even more attractive.  Other advantages include the low energy consumption, minimal use of chemicals, and the simplicity of the systems.

Introduction

Ultrafiltration is filtration on a molecular level.  Its basis is a membrane of controlled pore size which discriminates between large and small molecules.  The molecules retained by the membrane may be dissolved in solution or they may be visible aggregates.  The principle of ultrafiltration is shown schematically in Figure1 and Figure 2.

The liquid to be treated is applied against a membrane surface under controlled conditions of pressure and flow.  The membrane allows water and smaller molecular components in water to go through the membrane.  This stream is called the permeate.   The larger molecular components do not go through the membrane and get concentrated in the water.  This stream is called the concentrate.

The performance of an ultrafiltration process is defined by two parameters.  These are: 

• Permeate Rate:   This is the rate at which the permeate is produced and is usually measured as gallons per day per square foot of the membrane surface.
  
  
• Volume Reduction:  This is defined as:    [Volume of Permeate Removed/Original Volume of the liquid] x 100.

These factors will be discussed in more detail in the following sections.

Figure 1:   Process of Ultrafiltration (UF)

Figure 2:  Schematic of Ultrafiltration UF System

Factors Affecting the Performance of Ultrafiltration

There are several factors that can affect the performance of the ultrafiltration system.  These are listed below:

•  Flow Across the Membrane Surface
  The permeate rate is directly proportional to the velocity of the liquid across the membrane.  Increasing the velocity of the liquid also reduces the fouling of the membrane by suspended solids.  Generally an optimum velocity of the liquid is arrived at by a compromise between the pump horse power and the increase in permeate rate.
  
  
• Operating Pressure
  The permeate rate is also directly proportional to the applied pressure across the membrane.  Excessing pressure can, however, lead to an irreversible compaction of the membrane.  The compaction will cause a permanent drop in the permeate rate.   
  
  Generally the maximum recommended pressure for ultrafiltration membranes is about 100 pounds per square inch.
  
  
• Operating Temperature
  The permeate rate is directly proportional to the liquid temperature.  Normally the operating temperature is the highest temperature the membrane and the system can withstand.  Ultrafiltration membranes are now available in materials that can be steam cleaned

Application of Ultrafiltration to Emulsified Oily Waste

Emulsified oily wastes are produced in metal working plants.  These include machine shops, automotive plants, rolling mills, and others.  Emulsified oily waste is also generated in teh textile industry.  Typical characteristics of this waste are as follows:

• Contaminants
  The emulsified waste invariably contains other contaminants.  These include unemulsified hydraulic and machine oils, metal fines, dirt, lint, etc.
  
  
• Oil Content
  The oil content of hte emulsified oil waste is generally between 1 and 10% oil by volume.
  
  
• pH
  Most emulsified wastes are alkaline.  The pH is usually between 6 and 12.
  
  
• Temperature
  The temperature of the waste varies between 60° and 140°F.  

The emulsified oily waste produces a disposal problem and is not acceptable to the municipal treatment systems.  The conventional solutions to this problem are:

• Chemical Method
  This process uses mixing tanks where an acid and emulsion breaker are added.  This is followed by settling of the liquid to remove the free oil which can be incinerated.  The oil free liquid is then neutralized before it is discharged to the sewer.  
  
  This method is not economical for small sizes due to the relatively high cost for tanks, chemical injection and control systems.  In addition, it requires large space and high operating costs.
  
  
• Incineration
  The other method commonly used by most plants is to transport the waste to the nearest disposal company that will incinerate the waste.  The disposal cost by this method varies between 25 cents and 45 cents per gallon.  This is a rather expensive way to treat the emulsified oil problem.
  
  

• Ultrafiltration
  Ultrafiltration can be used to dewater the emulsified oil waste.  Up to 95% of teh water can be removed by ultrafiltration.  This water can either be reused for making fresh emulsions or sewered.  The concentrate, which is as little as 5% of the original waste, can either be reused or incinerated.  The disposal costs by incernation are now only 5% of the original disposal costs.   
  
  In addition. the ultrafiltration process is simple, continuous, and uses  relatively very small amounts of chemicals.  When the oil can be reused, the ultrafiltration process provides a closed loop operation

Operation of Ultrafiltration Systems for Oil Waste

The effect of certain operating parameterws on the performance of ultrafiltration systems was outlined in an earlier section of this paper.  Generally, for this application the following operations are used:

The  high flow across the membrane surface reduces the rate of fouling of the membrane.  Fouling may be defined as the deposition of suspended solids on the membrane.  The rate of fouling is dependent on the concentration of free  oil and contaminants, type of oil, and type and concentration of other additives in the oil (such as emulsifiers, rust inhibitors, etc.).

Membranes are therefore periodically washed using a detergent solution.  The frequency of cleaning is determined in each case by monitoring the performance of the system.  Usually a weekly cleaning is sufficcient to maintain a satisfactory level of performance.

Performance of Ultrafiltration Systems

Performance data for ultrafiltration systems have been obtained for several emulsified oil situations.  These include cutting oils from machining operations, wash waters from automotive plants, and rolling mill coolants.  The information was obtained from laboratory tests, pilot plant and operating systems. Although there are variations that can be made for most emulsified oil treatments.  These are listed below:

• Ultrafiltration can be used ot remove between 90-95% of hte water in emulsion (for typical test results see Table 1).
  
  
• The permeate rate is independent of the oil concentration up to a certain oil concentration  This concentration is different for each emulsion, and generally indicates the point at which the emulsion breaks down (See Figure 3 and Figure 4).
  
  During the concentration of emulsified oily water a certain amount of the emulsifiers are removed in the permeate.  This causes the emulsion to break down and release some free oil.  The free oil forms an oily layer on the membrane.  It is this oily layer on the membrane that produces a sharp drop in the permeate rate (see Figure 3 and Figure 4).
  
  
• In some cases the permeate can be reused.  Where even higher quality of permeate is required, it can be further treated by reverse osmosis or carbon absorption system.
  
  
• The final concentrate of the emulsified oil can be either incinerated or recycled.  Normally reuse of the concentrate is possible when there is only one type of oil-emulsifier package in the combined effluent.  One example of this recovered oil is given in Table 2 where 200 gallons of emulsified oil is recycled every day, resulting in savings of about $500/day.
  
  
• The permeate rate decreases with time but can be recovered to its original value by a regular washing of the membrane (Figure 5).  The wash solution removes any oily deposit on the membrane surface, and generally takes about two hours of re-circulation in the system.  The frequency of the wash cycle will vary with each application.

Table 1 - Typical Test Data for an Emulsified Oil Application

*Note:  Percent Volume Reduction equals  [Volume of Water Removed/Original Volume of Liquid] x 100

Table 2 - Quality of Concentrate vs. Original Oil

 Figure 3 - Membrane Flux vs. Total Solids

32°; 30 GPM; 50 psig; 4-5% Oil Initial Feed

Figure 4 - Performance of Ultrafiltration Systems Using Oily Water

Test 5; 50 psig; 30 GPM; pH 6-7

 Figure 5 - Performance of UF System at Budd Automotive, Kitchener

Economics of the Ultrafiltration System

The following example will illustrate the attractive payback of using an ultrafiltration system:

Design Conditions

Capital Costs

The capital cost for an ultrafiltration system to treat 2,000 gpd of emulsified oil waste water is $40,680.

Operating Costs

Payback Calculations

A survey of the automotive plants in Ontario indicated that the disposal of oily wastewater costs an average of about $0.25 per gallon.  A summary of payback calculations for the ultrafiltration system is given below:

Conclusions

Ultrafiltration is an efficient and economically attractive solution to the emulsified oil waste problem.  The process is simple and requires no chemicals.  Where the oil can be reused the ultrafiltration makes a closed loop system possible.  The payback for the system is very attractive, even when there is no recovery of the oil.

By Dr. Gil Dhawan, Applied Membranes, Inc.

Last few years have seen a rapid growth in sales of residential reverse osmosis overseas. Membrane requirements for these systems are different than those sold in the United States. Tap pressures available in many locations are as low as 10psi. This article discusses how equipment manufacturers are adjusting their system design and the selection of membranes to meet the requirements.

Minimum Pressure vs. Feed TDS

For reverse osmosis systems to produce product water, a minimum pressure to overcome the natural osmotic pressure of the water must be applied to the membrane.  This pressure depends on the types of ions present and their concentration in the water.  The osmotic pressure does not depend on the type of membrane.  Roughly, every 100 ppm of total dissolved solids (TDS) contributes about 1 psi of osmotic pressure.

For instance, if the TDS of feed water is 2,000 ppm then the natural osmotic pressure for this water is about 20 psi.  In this case, a pressure of at least 20 psi must be applied before any permeate will come through the membrane.  Generally, the applied pressure is at least twice the osmotic presure for a viable reverse osmosis system.

If applied pressure is not sufficient to overcome the natural osmotic pressure, a pump must be used to raise the applied pressure.  At least three different pump systems are available now to satisfy the requirements for residential systems.  The selection of the optimum pump system takes into account such factors as noise level, voltage, capacity and cost.

Higher Capacity Membranes

The permeate capacity of a membrane depends on several factors including the water temperature and net available pressure.

Net available pressure  = Applied Pressure - Osmotic Pressure

If a pressurized tank is used, the tank pressure must also be subtracted from the applied pressure to get the net available pressure.

The lower the net pressure, the lower the permeate rate will be.  In order to compensate for the lower net available pressure, membranes with higher rated capacity are used.  

Membrane manufacturers have packaged more membrane are in the same physical size of 2"×12" elements that are currently used in the residential systems.  The same membrane housings used for standard residential elements can be employed for these larger capacity elements.  CTA elements with capacities up to 22 gallons/day and thin film elements up to 36 gallons per day (tested at 60 psi, 77 degrees Fahrenheit and with 500 ppm TDS water) are available in 2" diameter and 12" length.

As applied pressures get lower and approach the osmotic pressure, the effect on the permeate flow and permeate quality gets more pronounced.

Table 1 gives an example of how the permeate quality and quantity for a membrane may change as pressure is reduced.  This analysis is based on a 500 ppm TDS water, 60 degrees Fahrenheit temperature and applied pressures as shown.  The data in the tables below is given to demonstrate the change in performance at very low pressures.  Actual values may vary with water analysis, membrane flux, and other factors.   In general, higher flux membranes show a relatively lower drop in performance and lower pressures.

Table 1* - Projected Performance at Very Low Pressures

• Reference Conditions:  60 Degrees Fahrenheit, 500 ppm TDS
• Recovery:  < 10%
• Membrane:  FT-30 (DOW FilmTec)

* Based on Feed

** Data supplied by Jack Loos, Dow Chemical, San Diego, CA 

In a different study, experimental values were obtained for permeate flow and quality performance for a CTA and a thin-film membrane at very low pressure. These are shown in Figure 2.

Figure 2 - Performance at Very Low Pressures Lab Study

(Courtesy Steve Laird, Fastek Corporation, Liverpool, NY)

By G.K. Dhawan | Presented at Membrane Technology Planning Conference Boston Massachusetts – October 29, 1985

Abstract

Reverse Osmosis and ultrafiltration are being used in a variety of applications. Potentially there are hundreds of other applications where these technologies can be used. This paper discusses some of the techniques that can be used to investigate new market areas and to increase the chances of success in commercialization of each application.

Background

The potential of membrane technology has been known since early 1960’s.  (See Table 1) It was not until 10 years later that first commercially available membranes were used in applications other than desalting.

 Table 1 - Membrane Technology Potential Applications

Even then only a few of these efforts resulted in success. Notwithstanding these early setbacks the industry had an optimistic look on the future of membrane technology in wide array of membrane applications. For instance, Table 2 shows results of a 1970 study of waste water recovery applications:

 Table 2 - View of Potential RO & UF Markets

Actual sales in 1985 for the markets listed in Table 2 were a tiny fraction of the projected figures.  There are several reasons for this.  First, each application development requires extensive testing, understanding of the industry, understanding of the competition and market development.  Second, in many cases, in order to make the membrane technology commercially viable, process modifications, membrane modifications or new membrane development, special operating conditions or a combination of these may be necessary.  All these factors point to long development time and high cost of application development.  Membrane application demonstration tests done in a laboratory are deceptively simple.  We feel that simplicity of such test and lack of understanding of how membranes and processes behave, has led to an unrealistic expectation of the size of membrane markets.  On the other hand, once we understand these factors we can work to maximize the chance of success.  Each application development, therefore, is a process of carrying out a planned set of tasks based on sound engineering practice and lessons learned in the past in the membrane (RO/UF) industry.  The rest of the paper will discuss these tasks and experiences.

Is Membrane Technology The Answer?

This is one of the first questions that need to be answered. We need to know the following information for the application:

• Technical Feasibility
• Economic Feasibility
• Market Potential
• Customer Acceptance

We will discuss these in more detail in the following sections.

Technical Feasibility

Before an expensive and time consuming development program is launched, we need to analyze the technical capabilities and limitations of membrane technology. In particular, we need to know the following:

• Minimum performance targets for the membrane system. For instance, minimum membrane rejection capability or minimum volume reduction may be specified. A quick check at this point may indicate whether or not any membrane or any membrane system is capable of meeting these requirements.
• Major areas of concern should be defined. For example, these may include amount particular component lost in the permeate, effect of using membrane technology on the overall process, how to dispose of the concentrate in case of waste stream, etc. The cost or feasibility impact of these factors should be estimated.

Economic Feasibility

This involves not only the usual criteria of overall cost and economic benefits but also factors like product quality or yield improvement, cost of concentrate disposal (water and waste water), impact on overall process, water reuse, etc.  Combining membrane process with a conventional process may be more economical than either process.

Pilot Plant Design

For all new applications, it is necessary to carry out a planned pilot plant test on site. In order for this testing to be meaningful, the design of the pilot plant must consider the following:

• Size of the membrane module
• Operating conditions
• Volume of liquid sample tested
• Test duration
• Data collection

Membrane module used in the pilot plant should be of commercial size. Operating conditions should be selected to meet the membrane specifications. Modifications of the operating conditions to optimize the membrane performance. The test should be carried out with sufficient time to give meaningful information on membrane fouling. In determining this time one would need to consider the type of feed, the variation in the quality of this feed with time, past experience and data on a similar feed and a few other factors. These considerations would also affect the type of data and the frequency at which it should be collected.

Factors Limiting Membrane Applications

There are several factors that may limit the application of membrane technology for a particular situation. These include:

• Recovery
• Membrane Life
• Application Development Time
• Risks and Guarantees
• Application Industry

Recovery is defined as the fraction of feed water that is converted into permeate. It is obtained by dividing the permeate flow by feed flow. Higher recoveries are desired for waste water applications to reduce the total waste water into the smallest volume for final disposal.  Higher recoveries are also desired in water-short areas.  Recovery for a given application depends on the total ionic content (osmotic pressure), concentration of sparingly soluble salts, quality of permeate, etc.  If desired recovery cannot be achieved, the membrane application may not be feasible.  Membrane life is related to the operating cost of the systems.  Proper design of membrane element, correct operation and maintenance of the system.  Fouling of membranes limits the application of membrane technology in many applications.

Developing a new application in membrane technology takes a considerable time and expense.  Although initial test and application data can be completed relatively quickly, long term effects on membrane performance under variety of situations possible in that industry take a considerable effort.  This can lead to frustration and possibly a premature end to the applications development program. 

In many waste water and process applications, membrane system supplier is reluctant to offer performance or membrane life guarantees.  On the other hand, the user of the membrane system is not willing to take a risk with new technology.  Equipment manufacturer needs more field experience whereas the end user needs proof and long performance record.

Finally, an understanding of each application industry, its needs and its attitudes must be well understood.  System design may need to be modified to suit special requirements of each industry.

From Pilot Testing to System Design

System design is yet another step in the process of developing new applications that can make a big difference in the overall success.  Improper system design, in the past, has led to shorter membrane life, more frequent cleaning, higher energy costs and reduced membrane efficiency.  And, of course, a poorly designed system can lead to a loss of confidence in membrane technology by both the customer and the industry.

Market Potential and Customer Acceptance

An important consideration for developing a new application is its potential market size.  Membrane technology has suffered from extremely optimistic market forecast put forward by many different organizations. Such forecasts have led to bitter disappointments and misgivings about the real potential of membrane technology. Impact of factors such as time to develop the technology, cost of development, customer acceptance of membrane technology and the strength of competing technology, must be carefully weighed on predicting market potential for each application.

I will illustrate this by two applications that I have developed. First application is in the treatment of emulsified oily water. Ultrafiltration is used to concentrate oil in waste water and permeate is discharged to sewer or recycle. In 1974 we carried out an extensive laboratory and pilot testing program. Our work demonstrated that savings in waste water haulage costs would pay for the ultrafiltration system in less than one year in many cases. The metal finishing industry was very skeptical of ultrafiltration technology and was reluctant to purchase this equipment. Finally, we had to overcome this problem by renting or leasing the ultrafiltration units until enough track records had been established in the industry. This process took more than five years.

In a different application, the objective was to concentrate maple sap from about 1% sugar to about 10% sugar concentration. Here reverse osmosis with its low energy consumption can be used to remove 90% of the water. The reverse osmosis concentrate can then be further concentrated by a conventional evaporator. In 1978, I investigated this application and discovered that a few reverse osmosis units had been sold in this market. These were standard reverse osmosis units originally designed for water treatment. We studied the customer and industry needs, designed reverse osmosis systems to match these and undertook a testing and demonstration program. Maple sap industry responded to this and over 200 reverse osmosis units were sold over the next two years.

Successful Applications

Some of the successful applications of reverse osmosis and ultrafiltration are listed below.

• Treatment of water for potable, ultrapure, boiler feed, rinse and other industrial uses.
• Waste water treatment applications such as electro-deposition paint, oily water, electroplating rinse water, cooling water, municipal waste water, cheese whey, dyes, solvents, colloids, etc.
• Food processing applications such as milk concentration, juice clarifications, low alcohol beer, etc.

Conclusions

• Developing new markets for reverse osmosis and ultrafiltration requires careful development of application and design data in the laboratory and field.
• Market forecasts must be moderated with factors such as technology limitations, time and effort required to develop an application and others.
• Lack of thorough understanding of application industry and insufficient application tests data have resulted in failures or long delays in the introduction of membrane technology.
• Understanding and experience gained to date can help towards more successes in commercialization of membrane technology.

By G.K. Dhawan, 1985

Introduction to Membrane Fouling

All membranes lose their performance with time.  One of the major causes for the loss of performance is due to substances that deposit on the membrane surface.  Although the term fouling is used for deposit of any materials on the membrane, the coating of the membrane surface can be due to the following:

• Fouling
• Scaling

Fouling

Fouling of membranes is due to the suspended or emulsified materials that may be present in the feed water to the reverse osmosis system. Examples of such materials are:  silica, oil, clay, iron, sulfur and humic acids.  These substances can be present in a very fine or colloidal form.  Even the typical 5 micron cartridge filters used upstream from a reverse osmosis system may not completely remove these foulants.

Membrane Fouling and Scaling

The concentration of all materials in the feed water - dissolved and suspended - is highest near the membrane surface.  As permeate is removed through the membrane, all impurities are left behind near the membrane surface.  The layer of water next to the membrane surface (boundary layer) gets more and more concentrated in the dissolved and suspended materials.  These concentrations reach a certain steady level depending on feed velocity, element recovery and membrane permeate flux (gallons per square foot of permeate produced per day).

It is important to follow membrane manufacturers' recommendations on minimum feed flow, maximum element recovery and maximum element flux.  These recommendations are based on the element size and quality of feed water being treated.  The concentrations of the dissolved and suspended solids in the boundary layer control the performance of the membrane.  Higher concentrations mean higher osmotic pressure, higher tendency of suspended solids to coagulate and coat the membrane surface, and higher likelihood of scaling to take place.  Maintaining proper operating conditions for the membrane is the key preventative step to minimize membrane fouling.

Antiscalant Injection

For non-residential systems, another option to avoid calcium carbonate and calcium sulfate scaling is by the use of antiscalants.  These are injected directly into the feed water upstream from the cartridge filter.  Dosage of antiscalant depends on the feed water analysis but usually is between 2 to 5 ppm.  In simplified terms, the antiscalants delay the scale formation process.  This delay is sufficient to avoid precipitation of calcium carbonate and calcium sulfate on the membrane surface.  As this delay is for a finite period, scaling can take place in systems on shut down.  For this reason, it is a good practice to flush the membranes with permeate or feed water at shut down.  By this flush, the concentrated solution in the membrane is displaced by the permeate or feed water.

Dispersant Injection

For suspended or colloidal materials, a dispersant can be injected in the feed water.  The usual dosage for a dispersant is 10 ppm.  Dispersants keep fine suspended solids from coagulating and coming down on the membrane surface.  Proper use of dispersants can minimize fouling due to problem particulates that are difficult to prefilter.

Acid Injection

Adjusting the pH of the feed water is another way to control calcium carbonate scaling.  The net effect of lowering the feed pH with acid injection is to convert bicarbonate alkalinity to carbon dioxide and thereby prevent the formation of calcium carbonate.  For reasons of handling and safety, acid injection is not used for residential or small commercial systems.

Reduce Recovery

Membrane recovery is defined as the ratio of permeate flow to feed flow for that membrane.  Recovery can be reduced by increasing the feed flow.  Another way to reduce recovery is to decrease the operating pressure.  Lower operating pressure produces a lower amount of permeate.  If the feed flow can be maintained near the original value, then a lower recovery is obtained.

The effect of lower recovery is to reduce the overall concentration of all substances in the reverse osmosis system.  More favorable boundary layer conditions are also achieved by reducing the system recovery.

Membrane Cleaning

Even with all the preventative care given to a reverse osmosis system, some fouling of the membranes will take place.  Cleaning of the membranes can improve membrane performance.  Membranes can be cleaned using Cleaning Solutions approved by the membrane manufacturer.  It is not economical to clean membranes used in the residential reverse osmosis systems. 

Conclusions

Membrane fouling and scaling can be minimized by proper design and operating conditions.  Important variables that control the membrane fouling must be considered in designing an operating the reverse osmosis system.

Related Literature:

		Membrane Cleaning Guidelines When to Clean Membranes, Common Foulants and Their Associated Symptoms, Cleaning Sequence, and Cleaning Instructions
		Membrane Cleaning and Support Chemicals Acid Cleaner, Alkaline Cleaner, Antiscalants, Dispersants, and Preservative

By: Gil K. Dhawan Ph.D., P.E., Applied Membranes, Inc.

Before discussing membrane properties and performance, it is appropriate to define and discuss reverse osmosis briefly.

Osmosis can be defined as the spontaneous passage of a liquid from a dilute to a more concentrated solution across an ideal semipermeable membrane which allows the passage of the solvent (water) but not the dissolved solids (solutes). (See Fig. 1.) The transfer of the water from one side of the membrane to the other continues until the head or pressure (P) is large enough to prevent any net transfer of the solvent (water) to the more concentrated solution. At equilibrium, the quantity of water passing in either direction is equal, and the pressure (P) is then defined as the osmotic pressure of the solution having that particular concentration of dissolved solids.

If a piston is placed on the more-concentrated solution side of a semipermeable membrane (see Fig. 2) and a pressure, P, is applied to the solution, the following conditions can be realized: (1) P is less than the osmotic pressure of the solution and the solvent still flows spontaneously toward the more concentrated solution; (2) P equals the osmotic pressure of the solution and solvent flows at the same rate in both directions, i.e., no net change in water levels; (3) P is greater than the osmotic pressure of the solution and solvent flows from the more concentrated solution to the "pure" solvent side of the membrane. Condition (3) shown in Fig. II-2, represents the phenomenon of reverse osmosis.

The osmotic pressure of a solution increases with the concentration of a solution. A rule of thumb, which is based on sodium chloride, is that the osmotic pressure increases by approximately 0.01 psi for each milligram/liter. This approximation works well for most natural waters. However, high-molecular-weight organics produce a much lower osmotic pressure. For example, sucrose gives approximately 0.001 psi for each milligram/liter.

Several methods are available for measuring the osmotic pressure. It can be calculated from the depression of the vapor pressure of a solution, by depression of the freezing point, and by the equivalent of the ideal gas law equation. Some calculated values for common components are listed in Table 1. Several devices are commercially available for direct measurement of the osmotic pressure. These measure the pressure necessary to stop the flow of water through a membrane.

The procedure that we use to measure the osmotic pressure of a solution is to measure the water flux through a module under operating conditions at several pressures. If a plot of water flux versus pressure is extrapolated to zero water flux, the intercept is the osmotic pressure. This gives the effective osmotic pressure, including any concentration polarization. Care must be taken to either maintain constant recovery or correct for the variation in concentration.

Attempting to measure the osmotic pressure of a solution directly by operating at a pressure just sufficient to obtain zero flow is impractical because the membranes are not perfect semipermeable membranes. This technique would measure the difference in osmotic pressure between the feed and product water. At low pressures the salt rejection is relatively poor, so that a false osmotic pressure somewhat lower than the real value would be determined.

Typical Osmotic Pressure at 25 deg C (77 deg F)

Note: Based on the above data for commonly present ionic species, a useful rule of thumb for estimating osmotic pressure of a natural water supply requiring demineralization is 10 psi per 1,000 mg/l (ppm).

By G.K. Dhawan, President of Applied Membranes, Inc.

Ultrafiltration is a separation process using membranes with pore sizes in the range of 0.1 to 0.001 micron. Typically, ultrafiltration will remove high molecular-weight substances, colloidal materials, and organic and inorganic polymeric molecules. Low molecular-weight organics and ions such as sodium, calcium, magnesium chloride, and sulfate are not removed. Because only high-molecular weight species are removed, the osmotic pressure differential across the membrane surface is negligible. Low applied pressures are therefore sufficient to achieve high flux rates from an ultrafiltration membrane. Flux of a membrane is defined as the amount of permeate produced per unit area of membrane surface per unit time. Generally flux is expressed as gallons per square foot per day (GFD) or as cubic meters per square meters per day.

Ultrafiltration membranes can have extremely high fluxes but in most practical applications the flux varies between 50 and 200 GFD at an operating pressure of about 50 psig in contrast, reverse osmosis membranes only produce between 10 to 30 GFD at 200 to 400 psig.

Ultrafilter vs. Conventional Filter

Ultrafiltration, like reverse osmosis, is a cross-flow separation process. Here liquid stream to be treated (feed) flows tangentially along the membrane surface, thereby producing two streams. The stream of liquid that comes through the membrane is called permeate. The type and amount of species left in the permeate will depend on the characteristics of the membrane, the operating conditions, and the quality of feed. The other liquid stream is called concentrate and gets progressively concentrated in those species removed by the membrane. In cross-flow separation, therefore, the membrane itself does not act as a collector of ions, molecules, or colloids but merely as a barrier to these species.

Conventional filters such as media filters or cartridge filters, on the other hand, only remove suspended solids by trapping these in the pores of the filter-media. These filters therefore act as depositories of suspended solids and have to be cleaned or replaced frequently. Conventional filters are used upstream from the membrane system to remove relatively large suspended solids and to let the membrane do the job of removing fine particles and dissolved solids. In ultrafiltration, for many applications, no prefilters are used and ultrafiltration modules concentrate all of the suspended and emulsified materials.

Concentration Polarization

When a membrane is used for a separation, the concentration of any species being removed is higher near the membrane surface than it is in the bulk of the stream. This condition is known as concentration polarization and exists in all ultrafiltration and reverse osmosis separations. The result of concentration polarization is the formation of a boundary layer of substantially high concentration of substances being removed by the membrane. The thickness of the layer and its concentration depend on the mass of transfer conditions that exist in the membrane system. Membrane flux and feed flow velocity are both important in controlling the thickness and the concentration in the boundary layer. The boundary layer impedes the flow of water through the membrane and the high concentration of species in the boundary layer produces a permeate of inferior quality in ultrafiltration applications relatively high fluid velocities are maintained along the membrane surface to reduce the concentration polarization effect.

Recovery

Recovery of an ultrafiltration system is defined as the percentage of the feed water that is converted into the permeate, or:

Ultrafiltration Membranes

Ultrafiltration Membrane modules come in plate-and-frame, spiral-wound, and tubular configurations. All configurations have been used successfully in different process applications. Each configuration is especially suited for some specific applications and there are many applications where more than one configuration is appropriate. For high purity water, spiral-wound and capillary configurations are generally used. The configuration selected depends on the type and concentration of colloidal material or emulsion. For more concentrated solutions, more open configurations like plate-and-frame and tubular are used. In all configurations the optimum system design must take into consideration the flow velocity, pressure drop, power consumption, membrane fouling and module cost.

Membrane Materials

A variety of materials have been used for commercial ultrafiltration membranes, but polysulfone and cellulose acetate are the most common. Recently thin-film composite ultrafiltration membranes have been marketed. For high purity water applications the membrane module materials must be compatible with chemicals such as hydrogen peroxide used in sanitizing the membranes on a periodic basis.

Molecular-Weight Cutoff

Pore sizes for ultrafiltration membranes range between 0.001 and 0.1 micron. However, it is more customary to categorize membranes by molecular-weight cutoff. For instance, a membrane that removes dissolved solids with molecular weights of 10,000 and higher has a molecular weight cutoff of 10,000. Obviously, different membranes even with the same molecular-weight cutoff, will have different pore size distribution. In other words, different membranes may remove species of different molecular weights to different degrees. Nevertheless, molecular-weight cutoff serves as a useful guide when selecting a membrane for a particular application.

Factors Affecting the Performance of Ultrafiltration 

There are several factors that can affect the performance of an ultrafiltration system. A brief discussion of these is given here. FLOW ACROSS THE MEMBRANE SURFACE The permeate rate increases with the flow velocity of the liquid across the membrane surface. Flow velocity if especially critical for liquids containing emulsions or suspensions. Higher flow also means higher energy consumption and larger pumps. Increasing the flow velocity also reduces the fouling of the membrane surface. Generally, an optimum flow velocity is arrived at by a compromise between the pump horsepower and increase in permeate rate.

Operating Pressure

Permeate rate is directly proportional to the applied pressure across the membrane surface. However, due to increased fouling and compaction, the operating pressures rarely exceed 100 psig and are generally around 50 psig. In some of the capillary-type ultrafiltration membrane modules the operating pressures are even lower due to the physical strength limitation imposed by the membrane module.

Operating Temperature

Permeate rates increase with increasing temperature. However, temperature generally is not a controlled variable. It is important to know the effect of temperature on membrane flux in order to distinguish between a drop in permeate due to a drop in temperature and the effect of other parameters.

Performance of Ultrafiltration Systems

In high purity water systems, ultrafiltration is slowly replacing the traditional 0.2-micron cartridge filters. In Japan, practically all of the semiconductor industry follows this practice. An ultrafiltration membrane with a molecular-weight cutoff of 10,000 has a nominal pore size of 0.003 micron. When an ultrafiltration membrane is used instead of a 0.2-micron cartridge filter, particle removal efficiency is greatly improved. In addition, ultrafiltration membranes are not susceptible to the problem of bacteria growing through them, as is the case with 0.2-micron filters. In a recent study (1), the performance of an ultrafilter was compared with that of a 0.2-micron cartridge filter. Some of these results are given in Table A. The Ultrafilter used in the study had a molecular-weight cutoff of 100,000- (pore size 0.006 micron). As the requirements for the quality of high purity water become more stringent, we can expect to see an increasing use of ultrafiltration as a final filter.

Table A - Effectiveness of Ultrafiltration Particle Counts on Waters

*Plumbing after UF not upgraded

Operation and Maintenance of UF Systems

Ultrafiltration system operation and maintenance is similar to that of reverse osmosis systems. Daily records of feed and permeate flow, feed pressure and temperature, and pressure drop across the system should be kept. Membranes should be cleaned when the system permeate rate drops by 10% or more. Feed flow is critical to the operation of ultrafiltration systems. A drop in feed flow may be due to a problem in the prefilter (if any), with the flow control valve, or with the pump itself. When the system is shut down for more than two days, a bacteriocide should be circulated through the membranes. At restart, permeate should be diverted to drain until all the bacteriocide is removed.

Conclusions

Ultrafiltration will find an increasing application in the production of high purity water. The basic principles outlined here should help in the understanding and use of this technology.

Reference:

¹ Gaudet, P.W. "Point-of-use Ultrafiltration of Deionized Water and Effects of Microelectronics Device Quality, American Society for Testing and Materials", 1984.

Glossary of Terms

• Feed - Liquid to be treated by the ultrafiltration system. 
• Permeate - Liquid stream that passes through the membrane. 
• Concentrate - Remaining Portion of the liquid stream after the permeate has been 
• Recovery - Expressed as percentage, this defines the permeate rate as a fraction of the feed rate. Recovery provides an immediate measure of the maximum concentrations in the system and it affects permeate quality, pump size, power consumption and membrane fouling. 
• Flux - Permeate flow per unit area of membrane per unit time (gallons/ft²/day) 
• Rejection - Percent removal of a particular species by the membrane. Expressed as 1-CP CF where CP is the concentration in the permeate, and CF is the concentration in the feed. 
• Flow Velocity - Rate at which the liquid goes along the membrane surface, expresse d in length per unit time (ft/sec).

By: G.K. Dhawan Ph.D., P.E., Applied Membranes, Inc.

Introduction

Reverse Osmosis technology is evolved into a widely used process for the purification of water. Well designed and properly operated systems give a trouble-free performance over long periods of time. Membranes in these systems have a long useful life. On the other hand mistakes made during the design or operation of reverse osmosis systems can lead to ongoing problems and reduced membrane life.

This article reviews some of the common mistakes made during the design and operation of the reverse osmosis systems.

Membrane Performance

There is one simple but extremely important fact in keeping the membranes at their peak performance:  "Keep the membrane surface clean".

All impurities in water are removed at the membrane surface. The dynamics of this separation step must ensure that concentrated materials are not accumulating at the membrane surface. If concentrations are allowed to build up near the membrane, precipitation of low solubility substances will follow resulting in a decline in membrane performance. 

Water Analysis

Understanding the water analysis and the potential problems caused by the sparingly soluble substances are crucial for the success of a reverse osmosis system. Many reverse osmosis systems have been designed and sold with no or incomplete water analysis. Some of these mistakes are difficult to fix in teh field and may even require discarding the existing system and starting all over again.

Recovery

Recovery is defined as the ration of the permeate flow to feed flow.

% Recovery = (Permeate Flow ÷ Feed Flow) × 100

In residential systems the recovery is expressed in terms of ratio of brine flow to permeate flow. For example, the brine: permeate flow ration may be 5:1. This can be converted into recovery as follows:

Feed flow = Permeate Flow + Brine Flow
% Recovery = (Permeate Flow × 100) ÷ Feed Flow
or = 100 ÷ 6 = 16.7%

It is recommended that for most tap waters the recovery for each membrane be maintained between 10 to 15%. Operating membranes at higher than recommended recovery will result in fouling of the membrane surface.

Membrane Flux

All membranes have one common limitation. They can only produce a maximum flow of a certain maximum permeate flow for a given water. This limit is controlled by the quality of feed water and not by the make of the membrane. For example, a maximum permeate flow for most tap water applications is 25 gallons per square foot per day. When membranes are run at fluxes higher than this value, fouling takes place.

Feed Flow

A minimum feed flow must be maintained throughout the membrane. Feed velocity helps to reduce build up of concentrated materials at the membrane surface. When several membranes are being used, the arrangement of these membranes is crucial in maintaining proper flow velocities. This arrangement must be checked against other related factors such as higher pumping costs, recycle flow, etc.

System Shut-Down

Fouling tendency of feed water when flowing through membranes is quite different than that of stagnant water at shut down. Certain suspended solids may settle on membrane surface during stagnant periods. On the other hand silica is found to crystallize during shut down. A proper flush cycle can eliminate these problems.

Residential Systems

Residential reverse osmosis systems need to take into consideration all of the parts described above. In addition, there are some other factors that require special attention in residential systems. Most of these concerns are due to an improper selection of some key components in the manufacturing of these systems.

• Flow Restrictors: Poor quality flow restrictors may cause systems to run at higher recoveries resulting in shorter membrane life. 
• Prefilters: Sediment and carbon filters used in the pretreatment of the residential systems must not shed fibers or release carbon fines. 
• Check Valves: A faulty check valve can cause a back pressure on the permeate side of the membrane element resulting in a physical damage to the membrane.

Summary

Mistakes in the design and operation of reverse osmosis systems can be avoided by following the recommendations outlined in this paper. There are no short cuts in providing systems that give trouble free performance with a long useful membrane life.

Diagrams and References

By: Gil K. Dhawan Ph.D., P.E., Applied Membranes, Inc.

Seawater desalination to convert seawater into potable water is being used in many parts of the world. Reverse Osmosis process using thin-film composite membranes has evolved over the last 20 years and has brought down the cost of desalination. Major improvements in the membranes, energy recovery, pumps and pressure vessels have brought down the cost of desalinated water significantly.

The key technology in the desalination process is Reverse Osmosis. In this process sea water is forced against semi-permeable membranes under pressure in a continuous flow condition. The high salt content of sea water requires that the operating pressure for Reverse Osmosis must be between 60-70 bar. As the water permeates through the membrane most of the dissolved impurities removed and 99.5% of the total salt is removed. The impurities are left behind in the flowing water and the concentrated stream from the membranes is discharged to the ocean. The design of the complete system must optimize the flows, the area of the membranes and other conditions to keep the system operation at the highest efficiency possible.

Applied Membranes has installed a number of Reverse Osmosis Sytems for seawater desalination. A typical system consists of filtration, UV, chemical injection followed by reverse osmosis Membranes. The Table below gives typical performance of a sea water system:

Water Quality of Seawater RO

• Recovery: 45% Operating 
• Pressure: 900 PSI 

A successful desalination system requires proper understanding and design to overcome the high salt content and large number of micro-organisms present in the seawater. The high degree of turbidity and corrosiveness of the seawater also requires measures to overcome these. Experience in seawater desalination and recent improvements in energy recovery and low energy membranes has brought the cost of seawater desalination down as shown below:

Operating Costs – Desalination

Not Included in the Cost:

• Intake Water
• Concentration Discharge
• Building Cost
• Amortization of Equipment
• Labor to Monitor & Maintain

The future of sea water desalination looks very good. The problem of a lack of potable water and increase in drought in many parts of the world in coastal areas can be solved by sea water desalination. Hundreds of seawater systems are producing drinking water or process water for municipalities, resorts, hotels, off-shore drills, ships, yachts and military use. The size of these systems varies from 100 gallons per day to millions of gallons per day.

Conclusions

• RO Desalination of seawater will find more applications because of the lower cost of desalination.
• Proper design, operation and maintenance are essential to reduce these costs further.
• Careful consideration must be given to both the intake of sea water to the system and the discharge of the concentrate from the RO membranes.

Written By: William H. Stroman, Laboratory Projects Analyst, San Diego Gas & Electric, 1995
Presented at:  Applied Membranes, Inc. Seminar:  Design, Operation & Maintenance of Reverse Osmosis Systems by Dr. Gil Dhawan.

Experience to Date in Troubleshooting Problems Encountered with the Encina Power Plant Reverse Osmosis System

 Abstract

A shift in the early 1980's from base load to cycling operations of the generating units increased the demand for boiler water makeup.  Due to the loss in production with unit evaporators during low load operation or those evaporators being out of service when the generating units were off line for economics, the decision was made to install a reverse osmosis system (RO) in front of an existing plant demineralizer (DI).  Due to budget constraints in the early 1980's the decision was made to have a contract with a vendor to build and sell RO product to supply the service run gallonage to 1.8 million gallons from 100,000 gallons and allowed for the removal of the existing evaporators on the other 4 units.  After 4 years of a five year contract, the option to purchase the vendor operated system was exercised in Jun 1988.  This paper discusses the experience to date, in troubleshooting the RO system, membrane scaling problems, and modifications to improve operation of SDG&E's Encina Power Plant reverse osmosis system since taking over operation responsibilities from the vendor.

Background

SDG&E has two RO system contracts that were built, operated, serviced, and maintained by the vendor to supply RO product feed supply to existing power plant demineralizers.  Each contract had options at 4 years and 5 to purchase the RO system and end the service contract.  Both RO systems were competitively bid with the main criteria requiring operation at 75% recovery and a minimum of 92% rejection of the feedwater constituents to specified gallonage e.g. Encina Power Plant 160 gpm and South Bay Power Plant 60 gpm.  The Encina RO system consists of primary filtering, cartridge filtering, acid injection for pH control, high pressure pumps, vessels and cellulose acetate (CA) membranes aligned into a 4:3:2 array, degasifier and degasifier product pumps, reference figure 1.   In the event the contract specifications couldn't be met, the vendor was required to bring in portable demineralizers.

At the end of four years, an evaluation determined it was economically beneficial for SDG&E to purchase the Encina Power Plant RO system and re-bid the service/maintenance contract.  After purchasing the RO system, the intent was to bid our servicing and maintenance to a vendor.  The original vendor lost the service/maintenance to a competitor.  Two days prior to changing service vendors the high pressure pump failed which resulted in first pass membrane damage by debris from the failed pump.  It took approximately 30 days to get the system back into acceptable production.  Due to residual oil being burned in the boilers, additional water usage for fuel atomization was required.  This further increased the RO/DI makeup requirements and with the RO system only able to produce ~90 gpm, portable demineralizers were brought in to help meet the water demand.

The new vendor was reluctant to accept the responsibility for service and maintenance of the RO system until the high pressure pump was repaired and the system was brought back within contract specifications.  The RO system declined occurred, resulting in the feed pressure exceeding 500 psi caused leaks in the fiber glass wrap on two of the pressure vessels.

During the restoration period, the new vendor expressed concern with the system design, condition of the system equipment, and the high membrane fouling rate (NOTE:  This information was made available during the bidding process).  As a result, SDG&E replaced the RO membranes, overhauled the primary filters, replaced two leaking pressure vessels, installed a larger cartridge filter and added a chlorine injection system.  A new short term contract was renegotiated and SDG&E ultimately took over maintenance and service of the RO system during 1989.

Membrane scaling from aluminum in the city water feed has been a chronic problem since the RO system originally went into service in 1984.  Once the system was restored from the pump failure, efforts were then directed towards coping with the aluminum scaling problem.  The solution for solving the problem was to lower the feed pH from the normal 5.7 to 4.8-5.0 range.  At the 4.8-5.0 pH the solubility of the aluminum increases back to within the range of what it is in the feedwater.

The pump failure also pointed out many areas of concern with the RO system, with the main one being very little operating or system flexibility in the event of a problem with the RO system.  For example the original system required two high pressure pumps to be in service in order to achieve design flow of 160 gpm.  After taking over the operating and maintenance responsibilities in 1989, design modifications were formulated and in 1991 the original single system 4:3:2 array was separated into a two pass configuration, reference figure 2.  The two pass configuration can be operated in parallel with a net 224 gpm or in series with a net of 120 gpm RO product.  The series mode allows for the product of Train A to be the feed to Train B to produce less than 10 ppm degasified product for feed supply to the demineralizer.

Scope

Due to the poor performance of the RO unit and the problems with the service vendors, portable demineralizers were required to meet the plant makeup requirements.  Because this was expensive ($50,000), plant engineering and laboratory personnel evaluated the RO system.  The objective was to identify RO system component problems and what would be required to improve system operation, reliability and the taking over of service/maintenance by plant personnel.

Reverse Osmosis Unit Problem Solving

Discussions with several service companies, membrane manufacturers, contractors and other RO system users, yielded many different options for improving the system.  To assist in the investigation, Process Scientific, Inc. (PSI) was contracted to evaluate the system and respond to a punch list of specific items compiled by SDG&E.  After sorting through the various inputs received from PSI, vendors, manufacturers, and other RO system users, a corrective action plan was formulated.

The first decision was to determine who would operate and maintain the RO system, a vendor or plant personnel.  The decision was to renegotiate the service contract and reduce the term of the contract to 15 months, then take over service and maintenance of the RO unit.  The plan developed was to use the 15 month period to implement system improvements to make the system less maintenance intensive, build a chemical cleaning skid, put a normalization program in place, and train plant operations and maintenance personnel.

The second step was to evaluate RO performance.  The investigation determined aluminum scaling coupled with the marginally designed RO system required significant manpower to maintain the unit within desired specifications, e.g. 160 gpm product flow @ 92% minimum rejection of the feedwater constituents.

Evaluation of the RO Components:

City Water Feed

Alum pretreatment chemical used by the Southern California Metropolitan Water District is the main contributor of the RO membrane scaling of the first pass elements.  The aluminum scaling results in the loss of product flow and increases the RO feedwater pressure as required to maintain the desired product flow.  If membranes are not chemically cleaned in a timely manner, the higher membrane pressure can lead to irreversible compaction of the membranes.  Discussions regarding pretreatment with service companies, membrane manufacturers, contractors, and other RO system users yielded many different modes of corrective actions of which some are discussed in the following component evaluations.

Sand Filters

The sand filters (primary filtering) were not providing good pretreatment of the city water based on the silt density index (SDI) tests performed on the filter effluent.  Before the filters were overhauled the SDI’s average 5.5.  After the filters were overhauled the SDI’s varied between 4.5 to 5.6 (varies due to changes in seasonal water quality).  Typically, an SDI of 5 indicates a high fouling rate, an SDI of 5 indicates moderate fouling, and an SDI of 3 indicates a fouling range that requires membrane cleaning once every 3 to 6 months.

Analyses of the micron filers used during the SDI testing found low organics (1%) and high aluminum (58%) and iron (42%) fouling.  Samples of the feed water inlet and effluent of each filter was done to determine if internal corrosion was taking place.  The analyses found that iron was in the feed water and not from the filters. 

One of the recommendations made was to inject acid into the inlet of the sand filters to precipitate the aluminum out in the filter media.  The precipitated aluminum is later removed during the backwash cycle.  The sand filters were not removed during the filter backwash cycle.  The sand filters were not designed for low pH (5.7) feedwater.   The low pH environment would cause corrosion within the filters and iron fouling problems in the membrane from the corrosion byproducts.

To upgrade the filters to be more corrosion resistant would cost ~$75,000.  An additional expense would also be incurred to modify the inlet piping to allow for the 10-30 second contact time after removal in the sand filters.  Since this option was cost prohibitive, it was given a low priority on the corrective action punch list.

The filter manufacturer representative inspected the filters and recommended re-bedding with #30 sand in lieu of a blend of anthracite and sand during the overhaul.  The sand media will filter down to smaller particle size than the anthracite/sand blend and proper bed expansion of the blend could not be achieved with the existing vessel height.

Cartridge Filters

The cartridge filter had a flow rating that would allow for a feedwater flow of 230 gpm.  The 5 micron filters were fouling and reaching the change out pressure (delta 10 psig) within 1-7 days.  This rate of fouling was also experienced by other RO users in this area.  The cartridge filter was replaced with a larger one to increase the filter surface area.  NOTE:   After replacing the cartridge filter and lowering the feedwater pH to 4.8-5.0, the change out frequency now often exceeds 30 days.

Chlorine Injection

As part of the immediate corrective action plan, a chlorine injection system was installed to control the free chlorine residual between 0.05 to 0.1 ppm in the product.  This helped to control biofouling in the membranes.

Methods for Controlling Aluminum Fouling

Several vendors recommended that the aluminum be dropped out of solution via pH adjustment of the feedwater prior to the sand filters to allow for its removal by the filtration process. As previously noted, the sand filters were not compatible with the lowered pH feedwater.  Since it was not economically feasible to replace the original filters and modify the feedwater piping to produce the desired contact time, this option was placed low on the corrective action punch list.

Another recommendation made was to soften the city water and change out the RO cellulose acetate membranes to thin film membranes.  This was not considered based on economics (e.g. capital $176,000), annual softener operating costs for the softener regenerations ($9,000), and the additional waste generated from the softener regenerations.  NOTE:  If this was a new plant design,  pre-treating with water softeners can be a good option if the design utilizes the RO brine waste for other uses (e.g. cooling tower makeup, pump cooling) to reduce overall water requirements to minimize plant wastes.

The last option considered and implemented was to lower the feed water pH from the original control 5.7 pH to 4.8-5.0 pH downstream of the sand filters and before the cartridge filter.   This matched the solubility of the aluminum with that of the incoming city water and prevented it from dropping out of solution on to the membranes.

The lowered pH control (4.8-5.0) also improves the membrane hydrolysis of the CA membranes which can help to increase the membrane life span.  The 4.8-5.0 pH control does increase the monthly acid cost by approximately $35 but the benefits gained in cartridge filter replacement alone far exceed the increased cost of additional acid.  Since the change was implemented, the interval between membrane cleanings has increased from 5 weeks to approximately 3-5 months.  In addition, the cartridge filter elements change out frequency changed from once every 2-7 days to every 4 weeks, saving ~$500/month in consumables and labor.

The additional sulfuric acid added for lowering the feedwater pH from 5.7 to 4.8-5.0 also increases the amount of sulfate ions available towards the precipitation of barium, calcium, magnesium, and strontium sulfates.  The RO unit performance projections by Fluid Systems indicate that only barium exceeds saturation in the RO brine @ 75% recovery.   After the array was separated into a two pass system and the aluminum scaling problem was eliminated, we began to note problems with barium scaling in the second pass and is discussed in more detail later in the paper.

RO Membranes

The CA membranes were replaced in April 1989.  The membrane replacement was done after completing  items listed on the corrective action plan e.g. overhaul of the primary filters, installation of the chlorine injection, larger cartridge filter, lower feed pH (4.8-5.0), new pressure regulator and interstage pressure gages. 

Pressure Gages

Pressure gages (total 2) were added at the effluent of the first vessel array (first inner stage pressure).  In conjunction with the existing system feed and concentration pressure gages, the new gages allow for determination of which part of the system has the differential increase.   Using the changes in differential pressure readings of the separate passes, the cleaning effort can then be directed to the specific pass that actually requires cleaning (e.g. clean only the first, second pass or third pass as required to maintain the system within desired operating specifications).

Performance Monitoring

System performance monitoring is done using the NORMPRO normalization software package provided by Fluid Systems.  The purpose of the normalized data is to compare daily operating data to a standard set of operating conditions.

The normalization program is based on ASTM D-4516, Standard Practice for Standardizing Reverse Osmosis Performance Data.  The membranes are now cleaned based on 10% maximum increase between current concentrate/feed differential pressure and baseline concentrate/feed differential pressure.  This helps prevent excessive membrane fouling/scaling and irreversible membrane damage from occurring.  (NOTE:  Depending on the cause of scaling or fouling and how successful the cleaning is in restoring the system, a maximum of 15% can be used as the criterion for determining when to clean).

October 1991:  The Original System was Changed into a Two-Pass Configuration

When the high pressure pump failed in 1988, it showed how important it is to have an RO system that is flexible and reliable when problems occur.  Having one RO system with two high pressure pumps with each having less than 100% design capacity does not help provide overall system reliability and efficiency.

During October 1991 the existing 4:3:2 array RO system was separated into a two stage RO system with Train A having a 5:2 array and Train B a 4:2 array that can operate in series or parallel depending on system water requirements, reference figure 2.  In parallel operations the combined output is 224 gpm with a degasified product quality less than 50 ppm, and in series, the combined effluent is 120 gpm with the degasified product quality less than 10 ppm.

The water quality from the series operation also increases the demineralizer gallonage throughout by an additional 2-3 times.  NOTE:  If the RO product storage tank has a floating roof or nitrogen cap to reduce the re-absorption rate of carbon dioxide, the DI anion exchanger gallonage throughout can be increased by as much as 20-40%.

The downstream unit (Train B) has not experienced any problems with fouling or scaling nor required a membrane cleaning since it started up in the two pass configuration (~3 years).  The downstream unit should also have significantly longer membrane life span than the lead unit.

The array change also incorporated a post shut down city water flush (acidified and chlorinated) to purge the concentrated RO brine water from the membranes.  The flush helps to prevent precipitation and organic growth during idle periods.  NOTE:  If possible, RO product should be used for the post shut down rinse to ensure thorough removal or dilution of the concentrated constituents that may be at or near saturation from the membrane brine feed channels.  In addition, while the RO is in standby, there is potential for the RO product used in flushing to reach ionic balance with various minerals that may be present on the membrane surfaces that can be removed when the RO is returned to service.

Ongoing Efforts to Prevent Barium Scale Formation

Based on the NORMPRO normalization program, it was determined that the reverse osmosis (RO) Train A required membrane cleaning, reference figure 3.  The Train A second pass was cleaned April 23 and the first pass was cleaned on April 24, 1992.   This was the first cleaning since the RO was modified into a two train system in October 1991.  The two step cleaning process using Flocon 103 A differential pressure across the second pass.

After the cleaning, the feed pressure was decreased and the differential pressure increased.  This implied that the membrane surface was cleaned and that the brine feed channel was still plugged.  On May 7, 1992 the last two elements of the second pass vessels were replaced (total of 4 elements).  The element replacement reduced the pressure drop across the second pass back to when it was first separated into the two pass configuration.

The elements removed from the second pass had barium deposition present in the inlet of each element with the worst being the last element in series in each vessel.  Two of the four elements removed from the second pass vessels were taken to Fluid Systems to be cut open for inspection.  The #3 element of four had bands of deposits (later determined to be barium) along eth bottom section areas in the membrane, reference figure 4 and figure 5.  The #4 element of four in series in the pressure vessel had bands of barium that were 4-6 times wider than #3 element and had more general deposits in the top sections of the membrane element, reference figure 6.

Based on the bands of barium noted being at the bottom or six o’clock position it appears that the barium in its supersaturated state is precipitating out of solution when changes in velocity occurs (i.e. unit shut down).  After the initial precipitation occurs, it then becomes the seed for other particles to grow on and induces the formation of coagulated colloidal particles that increase the membrane fouling rate.   The membrane scaling also increases the pressure drop across the membranes.  After the initial precipitation occurs, a seed is formed for other particles to grow on that induces the formation of coagulated colloidal particles which increases the pressure drop across the membrane.  As the fouling increases, the pressure drop across the elements also increases to the point what membrane cleaning is required to prevent damage to the membrane elements.

An evaluation was done on the post shutdown flush used to eliminate potential barium, calcium, magnesium, and strontium saturation when the RO system goes into standby.  (NOTE:  The flush water is maintained to the same ranges as the in service specifications, e.g. pH 4.8-5.0, chlorine 0.05-0.1 ppm).  The evaluation found that even at city water pressure (regulated to ~60 psi) RO product was being produced @ 50% recovery and the flush water was not adequately removing the brine concentrate out of the membrane elements.  The recommendation was made and implemented to bypass the brine control valve during the post shutdown flush.  This modification reduced the product flow and increased the rinse flow rate without increasing the constituent concentration in the rinse water.

It appears that during the RO membrane cleaning, cross contamination of the first pass is occurring by the cleaning solution during the second pass cleaning.  The contamination occurs when the cleaning solution enters the common brine header between the first and second passes.  With the inlet feedwater valve closed to the first pass, the cleaning solution (estimated to be ~3 gpm) enters via the interstage header into the first pass and through the membranes.  The 3 gpm flow through the membranes then concentrates the cleaning chemical, debris and dissolved constituents from the second pass clean onto the feedwater side of the first pass membranes.  Any foulants that are in the solution (i.e. barium, calcium, strontium) from the second pass cleaning, enters the first pass resulting in some scale deposition onto the membranes.

Based on past cleaning experience, barium and strontium are normally not present in the first pass cleaning as was the case during the cleaning, reference figure 7, cleaning analyses.  Due to the concentration of various elements taking place in the RO system, (i.e. product removal concentrating the feedwater into brine) the potential for barium, calcium, magnesium, and strontium to become saturated would normally take place in the second pass downstream elements and not in the first pass.

Prior to the conversion to the two pass RO system, the RO headers were disconnected and a cleaning header was installed.  This allowed for 100% isolation between the pass being cleaned and the remaining RO pass not being cleaned.

When the RO system was upgraded, the industry method of cleaning uses the existing brine, feedwater and product headers during the cleaning.  The rationale for the cleaning method, was that the pass not being cleaned would have very little or no flow due to feedwater valve being closed.  However, most  cleaning procedures do not monitor the post clean rinse after each cleaning step as thoroughly as our procedure, which uses DI water for rinsing and continuous conductivity measurements until it reaches 10 mmhos to note cross contamination.

In August 1993, isolation valves were installed on the Train A RO system to ensure 100% isolation of the first or second pass during membrane cleaning.  In addition to eliminating cross contamination of the pass not being cleaned, the modification also reduced the time for the cleaning hook up, dismantling and improved ease of the overall cleaning.  Faster post clean rinse down is also achieved, which contributes in waste reduction, and the decrease of the cleaning outage and costs.  Note:  There is concern in the RO industry with the potential risk of irreversible membrane damage if the isolation cleaning header valve is closed while the RO is in service.  With proper operating procedures which includes a return to service check off protocol and locked in service valves this can be eliminated.

Antiscalant Chemistry

The antiscalant chemistry used in pretreating the RO feedwater has been primarily adapted from chemistries utilized in cooling towers and boiler water programs. While there are documented success with the RO antiscalant pretreatment there are also poor success stories.  There is no all-purpose antiscalant chemistry that is suitable for pretreating all of the variations of constituents that are present in RO and cooling tower feedwater.

Cooling tower chemistry deals primarily with water recirculating through a cooling tower and heat exchangers which become concentrated via tower evaporation.  The limiting factors are typically the materials of construction, the temperature of the surfaces being cooled, constituents in the water used for tower makeup and the chemistry program objectives (e.g. prevention and control of scaling, corrosion and biological activity).

Boiler chemistry deals with bulk boiler water concentrations that are dependent on the operating pressure which can cycle the feed water constituents by 20-100 times, and in areas of high heat flux the concentration of impurities can be very high.  As a result, emphasis is placed on quality boiler makeup for removal of undesired constituents and tailored boiler chemistry programs to prevent corrosion and scale formation to maintain boiler cleanliness.

While there have been significant efforts made by industry to tailor RO pretreatment chemistry over the past few years.  We have had limited success in evaluations done with six different antiscalant and dispersant chemistries which has led us to try switching out the acid used for adjusting the feedwater pH.

The present pretreatment chemical being used is a blend of phosphonates and dispersants which helps to control barium, calcium, magnesium, silica and strontium sulfate scaling.  However, the effectiveness of the dispersants is significantly reduced when the feedwater pH is lowered as presently done to control aluminum scaling.  The decision then must be made which scalant does one want to deal with and its subsequent consequences?

Normally, the approach would be to increase the dosage rate of the pretreatment chemical to compensate for the loss in dispersant effectiveness.  However, increasing the dosage rate also increases the amount of phosphonates which can lead to calcium, magnesium, and silica interactions that could result in membrane scaling.

Antiscalant Injection

During the July 6, 1994 RO Cleaning, laboratory personnel noted high turbidity in the first pass cleaning solution (normally turbidity and debris are noted only in the second pass cleaning e.g. barium deposition).  Analyses of the cleaning skid and strontium, reference deposit analyses in figure 8.  A potential problem with injection being in front of the cartridge filters.  The dispersant could disperse the trapped debris from the cartridge filter into the first pass elements which can result in plugging of the feed channel spacers.  To eliminate this problem, the injection line was relocated to inject downstream of the cartridge filter.

Brine Reject Fouling Monitor

In an effort to appropriately address what is occurring with the brine chemistry, a membrane fouling monitor utilizing a 2.5 inch x 2.5 inch scavenger element was built.  The objective of the fouling monitor is to evaluate various dosage rates of the antiscalant being used, based on using the RO brine reject as the feed to the fouling monitor, reference figure 9.

Due to the scaling being accumulative over several months and that the NORMPRO membrane normalization program was noting the same trend, this effort was later discontinued.  However, analysis from the membrane biopsy confirmed high barium, strontium scale constituents were present.  These results also correlated with what was found in the biopsies of the last stage elements removed from the second pass, reference scale analysis in figure 10.

After change over to the hydrochloric acid pretreatment (discussed in next section), the fouling monitor will be used as a long term monitoring device (e.g. the approach is similar to the use of corrosion coupons to measure the corrosion rates in the cooling tower chemistry program).  Whenever the RO Train A, second pass is cleaned, the scavenger element will be removed and cut open to analyze the constituents present in the membrane (NOTE:  With the scavenger element cost @ $25-50, it is much cheaper to cut open than a last stage element @ ~$1,250).   The analyses will be used to trend the constituents in the back end elements of the second pass.  If the barium scaling can be mitigated, the trending of scale constituents would be useful in determining other long term limitations or refinements of the pretreatment chemistry program (e.g. antiscalant, dispersant, hydrochloric acid, RO system recovery rate, pH control point).

Acid Pretreatment Change to Hydrochloric From Sulfuric Acid

Due to the difficulty of removing barium and to lesser extent strontium sulfate scalants during the second pass membrane cleanings, additional measures were required to restore the RO performance back within specifications.  To date there have been two options taken to restore the RO system back to original specifications:

1) Chemical clean the second pass, then remove of the back two elements (reference elements # 3 and #4 in figure 4) from the second pass vessels (total of 4 elements).  The membranes that are removed are sent out to have them individually cleaned by a vendor to remove barium deposition.  NOTE:  the vendor utilizes a single element vessel to clean the membrane element with various cleaners element using forward and reverse cleaning flows in order to restore the membrane performance back within specifications.

2) Rotate the Train A second pass last stage elements with the Train B second pass last elements once every two months.  The Train B unit with its feedwater being the RO product from the Train A unit will flush out the barium in approximately 7 days.  The cost per two month rotation is $346 (requires ~16 man hours plus replacement o-rings/grease).  The rotation approach could lower the annual membrane cleaning requirement to approximately 3 per year and eliminate the need for option #1, of sending out the membranes for further individual cleaning.  NOTE:  Due to the increased handling which could induce problems in Train A or B and quickly erode any potential savings, the preferred option is #1.

Unless a new chemistry specifically for barium scaling is developed, the remaining options are to lower the RO system recovery or to replace the sulfuric acid pretreatment with hydrochloric acid.  Lowering the RO system recovery significantly increases the overall cost to produce RO product it would not be considered unless it was absolutely necessary.

Using hydrochloric acid reduces the amount of sulfates available in the brine concentrate, which helps to lower the potential for barium sulfate formation.   Based on the computer projections, with hydrochloric acid feed the barium sulfate saturation at the membrane surface lowers to 38 versus 54 times saturation with sulfuric acid.  With the scaling problem at the last stage element, the changeover to hydrochloric acid and its reduction of 30% in barium sulfate formation may reduce the scaling rate or potentially eliminate the scale formation problem.

Due to the differences in price between the 36% sulfuric acid and 31% hydrochloric acid an increase the pretreatment operating costs @ 50% operating capacity is $1,782 and at 100% capacity is $3,653.  NOTE:  36% sulfuric acid is used in lieu of the cheaper 93% sulfuric acid for the safety of plant personnel.  A vendor visits the plant as required to top off the 500 gallon storage tank without requiring the need for plant personnel.  Better pH control is also achieved with the diluted 36% versus the concentrated 93% sulfuric acid.  Good pH control is also expected with the 31% hydrochloric acid.

If the changeover to hydrochloric acid is successful in reducing or eliminating the barium scale formation, the membrane cleaning frequency could potentially be reduced to less than 2 times a year.  In addition, the required membrane rotation/restoration could be eliminated for an annual savings of $3,483 (based on 3.5 cleanings a year).   The combination of these two benefits could potentially reduce the annual overall membrane cleaning costs by ~$5,800.  Based on the present 50% operating capacity factor the total savings in membrane cleanings versus the increase in acid cost of $1,782 for pretreatment should result in an annual savings of ~$4,000.  The estimated cost for changeover to hydrochloric acid is $955 and is expected to take place by November 1994.

Summary

When we first started investigating why the RO unit was performing poorly, it quickly became apparent that the primary problem was membrane scaling in the first pass.   After extensive analyses it was determined that the major scalant was aluminum.  There were three options available for reducing the aluminum scalant, (1) upgrade the prefiltering system to trap out aluminum, (2) install water softeners and pretreat by softening the feedwater, (3) lower the feed pH after the primary filters to keep the aluminum in solution and discharge it in the RO brine reject.  We opted for the 3^rd option due to the ease of implementation and economics.   We were successful in changing the interval between membrane cleanings from 5 weeks to approximately 3-5 months.

With the aluminum scaling under control the next problem to develop was barium scale problems in the second pass elements.  To date a number of measures have been employed to reduce the impact of barium scalant.  The easiest have been evaluations of various antiscalants and antiscalant/dispersant formulations.  Overall the best (although limited) success has been with the combination of antiscalant/dispersant.  However, coupled with the reduction in sulfate with the changeover to hydrochloric acid for adjusting the feedwater pH, the reduction in barium sulfate saturation may be enough to exit the second pass last stage elements to either eliminate or increase the interval between membrane cleanings associated with the barium scaling.

Even though the vendor is typically the expert in the field, end users need to be actively involved in the original bid specifications.  Our experience has taught us that the minimum considerations and requirements for RO system operation are as follows:

• The minimum acceptable criteria (e.g. 92-98% removal of the feedwater constituents, desired gallonage rate) should be designed around the worst case water constituents and lowest feed temperature.
  
  
•  To have better system flexibility and reliability, the system should consist of a minimum of two “50-70%” of a desired product flow RO units.  If problems with one of the RO units e.g. pump failure, down for membrane cleaning, the plant still has the ability to produce RO product for makeup.
  
  
• Each RO system should have, 1) post shut down brine flush that bypasses the brine control valve while flushing is taking place (RO product water provides the best result and if city water is used the pH and chlorine should be maintained within normal service specifications), 2) acid, antiscalant and chlorine chemical injection, 3) inter-stage pressure indication, 4) adequately designed primary and secondary cartridge filters.
  
  
• The inter-stage header between the array stages should have isolation valves for use during chemical cleanings.  This helps to prevent cross contamination of the cleaning solutions from compromising the pass that isn’t being cleaned.
  
  
• Flow rate during membrane cleaning needs to supply sufficient velocity across the brine channel mesh spacer between the membrane sheets to prevent the potential for scaling and plugging of the brine feed channel during the cleaning.   If the velocity is too low, precipitation can occur which becomes the seed for further scale formation of coagulated colloidal particles, increasing the pressure drop across the membranes and the subsequent requirement for more cleanings.
  
  
• At the time of purchase a used RO system, the unit should be re-membraned with new membranes as originally specified (as a minimum) in the contract.  The seller should also provide membrane element serial numbers with the date of manufacture, performance guarantees and a drawing showing the exact location of each membrane element.
  
  
• If the RO operator is a vendor, they should provide the end user with data on the performance (e.g. pressure, conductivity, system flow rates, pH) of the RO system and provide a weekly report.  The reports should use an agreed upon software normalization program to determine membrane performance (e.g. Fluid Systems NORMPRO).  The normalization program should be based on ASTM-D4516, Standard Practice for Standardizing Reverse Osmosis Performance Data, Annual Books of ASTM Standards, Water and Environmental Technology, Section 11.  These reports will provide documentation on how the RO system is operating and be useful towards making any decisions on purchasing or continuing to have vendor operate the RO system.  This same criteria applies to the end user if they are the operator of the OR system.  The percent rejection criteria e.g. 92%, should be based on the feed water constituents before any pretreatment chemical additions.  This prevents the end user from being penalized by overfeeding of treatment chemicals.
  
  
• Organic fouling has been noted during membrane inspection that is related to the system being exposed to sunlight.  A roof was erected to the RO system during 1993.  Subsequent second pass membrane rotations have noted a significant drop in the amount of algae present on the elements since the roof was erected.   The roof over the RO pad helped to prevent ultraviolet ray deterioration of the PVC piping and protect process monitoring instrumentation from the elements.
  
  
• A potential problem with the dispersant pretreatment chemistry is with the chemical injection being in front of the cartridge filters.  The dispersant could disperse the trapped debris from the cartridge filter into the first pass elements which can result in plugging of the feed channel spacers.  To eliminate this problem the injection line should be located to inject downstream of the cartridge filter.
  
  
• To reduce the adsorption rate of carbon dioxide back into the RO product (which increases the loading on the anion exchange resins), RO product should be fed directly to the demineralizer, or if routed to a product storage tank, employ a floating roof or nitrogen cap system.
  
  
• With the significant change in the silica percentage due to its removal by the RO, an evaluation between strong base Type I and Type II resins should be done.  In most cases the option to use Type II anion resin will significantly increase the overall DI service run gallonage.  The same is also true with RO pretreatment that the DI cation regeneration could replace the two step with a single step acid regeneration.
  
  
• The method used for deposit analyses encountered problems for accurately determining the true amount of barium being present and a new method was utilized.  The new procedure used is the flux method, which requires drying, ashing, and solubilizing of solid wastes using lithium metaborate (Libo2), fusion is used for the determination of inorganic constituents by inductively coupled plasma – atomic emission spectrometric (ICP) or by atomic absorption (AA).

This method, ASTM method “Standard Practice for Dissolution of Solid Waste by Lithium Metaborate Fusion”, is intended for the solubilization of nonvolatile inorganic constituents, particularly barium, in solid waste.  The fusion method is also appropriate for analyses of the silicate matrix.  This procedure was used in the laboratory when classical dissolution methods, SW-846, failed to obtain the estimated sample concentrations of barium.

Acknowledgement

I would like to acknowledge the tremendous effort of the Encina Power Plant maintenance, operation and engineering staff and specifically two plant laboratory personnel, Tom McCluskey and Pedro Lopez.

Applied Membranes Inc. looks to expand unique membrane offerings in U.S. dairy sector as part of strategic growth

By Alyssa Mitchell, Cheese Market News
August 21, 2020

VISTA, Calif. — Applied Membranes Inc. — a Vista, California-based global manufacturer and distributor of commercial and residential reverse osmosis (RO) membranes, systems and components — is looking to expand its reach in the U.S. dairy sector.

Applied Membranes CEO Gil Dhawan founded the company in 1983 when he identified a need in the market for a supplier that could manufacture unique membranes and systems for companies of various sizes. At that time, Dhawan was working for major membrane companies and collaborating with Dr. Srinivasa Sourirajan, the co-inventor of the very first RO membrane. Dhawan did extensive pilot plant work in many applications and sold complete systems with applications including cheese whey fractionation in lactose and protein, juice concentration, maple sap concentration and gelatin concentration.

When Dhawan decided to create Applied Membranes Inc., RO technology was relatively new, and he saw a need for a company that could not only sell products but also train people on how to create these systems. He first conducted a series of educational seminars, and many of those attendees still are customers today. Customers also expressed an interest in building their own systems for their operations.

“The development of Applied Membranes Inc. really can be expressed as customer-driven,” Dhawan says. “We didn’t have a defined market at first. Whatever customers wanted at the time, I would develop the system, and we would provide the solutions, and this added to our expertise and different applications. There were not many companies like us in the business at the time.”

The company has been manufacturing RO systems, RO membranes and water filtration components under the Applied Membranes label for more than 35 years.
Since its inception, Applied Membranes has taken the approach of collaborating with customers to develop solutions, Dhawan adds.

“Working with the real process and treatment needs, we have supplied pilot plants and test support. We are the only company in the world that rolls its own microfiltration (MF),ultrafiltration (UF), nanofiltration (NF) and RO membranes using the best membranes available. We custom design to meet the special needs for the specific application. We use different fabrics and feed channels to minimize membrane fouling and increase the useful life of the membrane elements.”
In addition to servicing its own customers, Applied Membranes also works with larger companies to develop these applications for those companies’ customers.
“Larger companies don’t have the resources to put toward developing some of these niche applications. That’s where we come in,” Dhawan says. “We can develop systems under other companies’ names so they can supply to their customers.”

He adds Applied Membranes also supplies heat sanitizable membranes for dialysis and back washable spiral wound elements.
“Our field experience in ultrahigh purity, beverage and food applications with RO,NF,UF and MF systems gives us unique insight into tailoring the spiral wound elements to perform at the optimum level,” Dhawan says.

While Applied Membranes has been supplying dairy membrane elements for more than five years, these elements have been exported to customers in Asia, Dhawan notes.
“We set up a sales office in Shanghai to service these customers, and they are very pleased with our quality and performance,” he says. “We wanted to do the same in the United States but needed a salesperson experienced in the U.S. market.”

With this in mind, Applied Membranes earlier this year added Rich Pankau to its team to help develop the U.S. market.
Dhawan notes this aligns with the company’s strategic growth plan as it recently moved into a new 156,000-square-foot building, much larger than its previous space, which will allow the company to focus on expanding specialty membranes.

Water purification and membrane water treatment are the basis of all membrane applications, he adds, noting, however, that each application has its own specific needs and interactions with each membrane.
“To be successful, we must understand all aspects of the application and industry requirements almost as well as the experts in that field,” Dhawan says. “We will combine our vast experience in other applications with the unique requirements of the dairy industry to create the best solution.

“Like in other markets we have developed, we are planning to go everywhere in the country where we establish a need for another supplier,” he adds. “We are going to be competitive and responsive to customer needs.” CMN