How to Prevent and Clean Fouled RO Membranes for Optimal Performance

Fouling of the RO membrane is more or less a normal phenomenon of most RO systems since the pretreatment of the feed water before the RO membrane is designed to reduce fouling substances as much as possible and technically could not remove all of them. Fortunately, with the correct cleaning frequency, most foulants can be removed from the membrane. The cleaning frequency could be minimized as long as the pretreatment is well maintained without upset conditions such as an uncontrolled change in feed water composition and uncontrolled biological contamination. Sometimes mistakes in the system operation such as too high recovery and failure of chemical dosing systems could end up with fouling the membrane.

Cleaning the fouled membranes can be accomplished by suitable cleaning agents at alkaline (up to pH 12) and acidic (pH2) conditions because RO membranes are stable at the pH conditions and an elevated temperature (45℃).

Many foulants, particularly clay-type soils, can compact with time as the foulant layer increases in depth.

As the foulant compacts, it will become more difficult to remove them during cleaning. Thus the time of cleaning must not cleaning must not be delayed too long.

Timing for Cleaning

Elements should be cleaned immediately when one of the following symptoms is detected :

☞ Loss of to 15% in normalized permeate flow rate.

☞ Decrease of 0.5% in salt rejection.

The differential pressure (feed pressure – concentrate pressure) ΔP increases by 15% from the reference conditions (initial performance established during the first 24 to 48 hours of operation).

Clean in place Procedure

To clean a system, follow these six basic steps:

1. Prepare and mix the appropriate cleaning solution, and check pH and temperature of the solution.

• Displace the solution in the vessels either by flushing with permeate water or by pumping the cleaning solution at low pressure and low flow. To prevent dilution of the cleaning solution, the process water can be dumped to drain until the cleaning solution has filled the vessels.
• Recycle the solution through the elements and back to the tank.
• Soak the elements for 1 hour.
• Recycle at the flow rates listed in Table 1 for an hour. The turbulence will displace the foulants from the membrane. Do not exceed 0.7 bar pressure drop per element; if the pressure drop is too great, reduce the flow.
• Flush the system with clean permeated water or pre-filtered raw water.

Choosing right cleaning chemicals is important since harsh and frequent cleaning will shorten the membrane life, and sometimes a wrong choice cleaning chemicals can worsen the fouling situation. The cleaning will be more effective if it is tailored to the specific fouling problem. Therefore, the type of foulants should be determined prior RO cleaning, therefore are helpful ways to determine the type of foulants as shown below ;

Analyze the plant performance data ;

Analyze the feed water to fine potential fouling substances ;

Check the results of previous cleanings which may indicate specific fouling substances ; Analyze the foulants collected with a membrane filter used for SDI measurement ; Analyze the deposits in the cartridge filter ;

Inspect the inner surface of the feed line tubing and the feed end scroll of the RO element. If it is reddish – brown,

Fouling by iron is possible. Biological fouling or an organic material deposits it is often slimy or gelatinous.

. The acid cleaners are to re-dissolve inorganic deposits including iron, while the alkaline cleaners are to remove organic fouling biological matter.

Sulfuric acid should not be used for cleaning because of the risk of calcium sulfate scaling.

For the preparation of the cleaning solutions, RO permeate is referred, but pre-filtered raw water may be used.

The raw water could have some buffering ability, so more acid or hydroxide may be needed reach the desired pH level,

Which is about 2(Two) for acid cleaning and about 12(Twenty) for alkaline cleaning at 30℃, respectively. At 35℃, the pH limits is in the range of 2(Two) to 11(Eleven).

Foulant	Cleaning Chemical	Comments
Inorganic Salts (CaCO3, CaSO4, BaSO4)	0.2% Hydrochloric Acid (HCl). 0.5% Phosphoric Acid (H3PO4) 2.0% Citric Acid	Best O.K O.K
Metal Oxides (Iron)	0.5% Phosphoric Acid (H3PO4) 1.0% Sodium Hydrosulfate (Na2S2O4)	Good Good

Inorganic Colloids (Silt)	0.1% Sodium Hydroxide (NaOH), 30℃ 0.025 Sodium Dodecylsulfate / 0.1% NaOH, 30℃	Good Good
Bio films	0.1% Sodium hydroxide, 30℃ 1.0% Sodium EthyleneDiamineTetra Acetic Acid (Na4EDTA) and 0.1% NaOH, 30℃	Best Best     When     bio film contains Inorganic scaling
Organics	0.025%      Sodium     Dodecylsulfate      /    0.1% NaOH, 30℃ 0.1% Sodium Triphosphate / 1% Na4 EDTA	Good Good
Silica	0.1% Sodium Hydroxide, 30℃ 1.0% Sodium EthyleneDiamineTetra Acetic Acid (Na4EDTA) and 0.1% NaOH, 30℃	O.K O.K

Operation Mode:

	Idle	Pre service Flush	Service to Product	Service to Drain	Flush	CIP	Remark
RO        high pressure pump	Stop	Stop	Run	Run	Stop	Stop	2 operation 1 stand-by
RO cleaning pump	Stop	Stop	Stop	Stop	Stop	Run	1 operation 1 stand-by
Service inlet valve	Closed	Opened	Opened	Opened	Opened	Closed
Product outlet	Closed	Closed	Opened	Closed	Closed	Closed
Product
conc.	Opened	Opened	Closed	Opened	Opened	Closed
outlet
Flush inlet
/ RO cleaning solution	Closed	Closed	Closed	Closed	Opened	Opened
inlet
Flush
outlet / RO cleaning solution	Closed	Closed	Closed	Closed	Opened	Opened
outlet