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Membrane Maintenance and Cleaning Information
Membrane Cleaning Chemicals Product support

 

When to clean Membranes

In normal operation, the membrane in reverse osmosis elements can become fouled by suspended solids, microorganisms, and mineral scale. These deposits build up during operation and cause loss in water output or salt rejection, or both.

Elements should be cleaned whenever the normalized permeate water output rate drops by 10% from its initial flow rate (the flow rate established during the first 24 to 48 hours of operation), when salt passage in the product water increases over 5-10%, or when normalized pressure drop across the membrane increases by 10-15%.

It should be noted that the water output rate will drop if feedwater temperature decreases (see Temperature Correction). This is normal and does not indicate membrane fouling.

Comparison of Fouled and Clean Membrane
When to Clean Membranes


Common Membrane Foulants and Their Associated Symptoms

Foulant Symptoms Solution (AMI Part #)
Biological Growth Element may have strong odor, possible mold growth on scroll end. Element will likely exhibit low permeate flow, but salt rejection will usually be as good if not better than original test. TF: AM-22, AM-23
CTA: AM-33
Carbonate Scale Usually on tap water or brackish water elements only. The element may be noticeably heavier than normal. Element will exhibit low permeate flow and poor salt rejection. TF: AM-11
CTA: AM-44
Iron Fouling Rust coloring seen on end of scroll. Possibly some large rust flakes from iron plumbing. Element will exhibit low permeate flow and poor salt rejection. Rust colored reject water may be seen on start of baseline test. TF: AM-11
CTA: AM-44
Silica Scale (Precipitated) Same characteristics as Carbonate Scale TF: AM-99
Silt or Carbon Fines Brown or black material on scroll end. Low flow, good early stages. High flow and very poor rejection in later stages due to the abrasive effects of the material on the membrane. AM-55

Membrane Cleaning Sequence

Whether the system needs acid or alkaline cleaning will depend on the type of foulant suspected. If CaCO3 is the known scalant, acid cleaning alone may be sufficient. Otherwise both kinds of cleaning are needed and it is recommended to start with the alkaline cleaning then follow with the acid cleaning after the system has been flushed.
  1. Alkaline Cleaning (if required)
  2. Flush
  3. Acid Cleaning
  4. Flush
  5. Sanitizing
  6. Flush

Note:  Acid cleaning may be performed alone, but alkaline cleanings should always be followed by an acid cleaning after the system has been flushed.


Membrane Cleaning Procedure (How to Clean RO Membranes)

Use RO permeate water if possible, preferred. Whether the system needs acid (AM-11) or alkaline (AM-22) cleaning will depend on the type of foulant suspected. If both kinds of cleaning are desired, we recommend starting with the alkaline cleaning, then cleaning with the acid. Note: If CaCO3 is the known scalant, acid cleaning alone may be sufficient.

Membrane Cleaning System

Connect cleaning tank and pump system to the membrane system. It may be necessary to clean one tube at a time (see flow requirements below). Pump pressure must not exceed 60 psi. Permeate and concentrate lines must return to the cleaning tank. Include a 10 micron filter in the feed line to the membrane.

Membrane Cleaning Procedure

Add the cleaner slowly (for the proper amounts, check the detailed procedure for that cleaner) to cleaning tank water, and mix well.

CAUTION: Mix with care and wear protective clothing.

CAUTION: Do not allow the cleaning solution temperature to exceed 120 degrees F. Do not allow the flows to exceed 4 gpm for 2½" elements, 12 gpm for 4" elements, or 40 gpm for 8" elements. Recirculate solution.

CAUTION: Flush thoroughly before cleaning with other cleaners. Cleaning chemicals may react with one another or with foulants to produce additional fouling on the membrane.


Cleaning Flow Rates for Spiral Wound Membranes

Membrane Size Volume (Gallons) Medium Flow (GPM) High Flow (GPM)
4" × 40" 2.3 4 10
4" Magnum 3.5 4 10
6" × 40" 4.0 12 20
8" × 40" 6.0 25 35
8" Magnum 8.5 25 35

Estimate of Cleaning Solution Volume

V = El × Vol × 5

EL = Number of Membrane Elements
Vol = Volume of Membrane Element Size from Flow Table Above

Estimate of Total Flow Required

HTF = NV × HF
MTF = NV × MF

HTF = High Total Flow
MTF = Medium Total Flow
NV = Number of Pressure Vessels in Parallel
HF = High Flow from Flow Table Above
MF = Medium Flow from Flow Table Above

 


Membrane Cleaner Selection Guide Table

Foulant Cleaner, for Membrane Type
Thin Film RO (TF or TFC) Cellulose Triacetate (CA or CTA)
Mineral Scale
(Calcium Carbonate, Calcium Sulfate, Barium Sulfate, Strontium Sulfate, Calcium Flouride)
AM-11 AM-44
Metal Oxides (Iron and Mangenese) AM-124 AM-124
Inorganic Colloids (Silt) AM-55 AM-33
Biofoulants AM-55 AM-55
Organics AM-22 or AM-23 AM-33
Silicates AM-99 AM-99
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