Comprehensive Analysis of Reverse Osmosis Membrane Fouling and Solutions

Reverse osmosis membrane fouling

1. Damage to the reverse osmosis membrane performance, resulting in membrane fouling

（ 1) Polyester reinforced non-woven fabric, approximately 120μm thick;

（ 2) Polysulfone porous intermediate support layer, approximately 40μm thick;

（ 3) Polyamide ultra-thin separation layer, approximately 0.2μm thick.

Based on its performance structure, the possible reasons for damage to the reverse osmosis membrane performance are as follows:

（ 1) Non-standard maintenance of new reverse osmosis membranes;

（ 2) Storage time exceeding 1 year under standard maintenance;

（ 3) Non-standard maintenance of reverse osmosis membranes during shutdown;

（ 4) Ambient temperature below 5℃;

（ 5) System operating under high pressure;

（ 6) Improper shutdown operation.

2. Frequent changes in water quality causing membrane fouling

Changes in raw water quality compared to the design water quality increase the pretreatment load. Due to the increase in inorganic substances, organic substances, microorganisms, particulate matter, and colloids in the influent, the probability of membrane fouling increases.

3. Untimely cleaning and incorrect cleaning methods causing membrane fouling

During use, in addition to the normal attenuation of membrane performance, untimely cleaning and incorrect cleaning methods are also important factors leading to serious membrane fouling.

4. Failure to add chemicals correctly

In the use of composite polyamide membranes, because polyamide membranes have poor chlorine resistance, the failure to correctly add chlorine and other disinfectants during use, coupled with insufficient user attention to microbial prevention, easily leads to microbial contamination.

5. Membrane surface wear

Membrane elements are blocked by foreign objects or the membrane surface is worn (such as sand particles). In this case, the detection method should be used to detect the elements in the system 'Find the damaged components, modify the pretreatment, and replace the membrane components'

Clean the membrane

Even though the slurry undergoes various pretreatment measures, after long-term use, deposits and scaling may still occur on the membrane surface, causing membrane pore blockage, reduced water production, and therefore regular cleaning of the polluted membrane is necessary. However, the reverse osmosis membrane system cannot wait until the pollution is very serious before cleaning, as this will increase the cleaning difficulty, increase the cleaning steps, and prolong the cleaning time. The cleaning time must be grasped correctly to remove the fouling in time.

（ 1) Cleaning principles:

Understand the local water quality characteristics, perform chemical analysis of pollutants, and select the optimal cleaning agent and cleaning method based on the results analysis to provide a basis for finding the optimal method under specific water supply conditions;

（ 2) Cleaning conditions:

A. Product water volume is 5%-10% lower than normal.

B. To ensure product water volume, the water supply pressure after temperature correction is increased by 10%-15%.

C. The conductivity (salt content increase) of the permeate water increases by 5%-10%.

D. Multi-stage RO system, the pressure drop in different stages increases significantly.

（ 3) Cleaning methods

First, perform system backwashing;

Then perform negative pressure cleaning;

Mechanical cleaning is necessary;

Then perform chemical cleaning;

Ultrasonic cleaning is possible if conditions permit;

Online electric field cleaning is a good method, but it is expensive;

Because chemical cleaning has a better effect, other methods are somewhat difficult to implement, and although the agents provided by various suppliers have different names and usage methods, their principles are roughly the same.

（ 4) The cleaning steps are as follows:

Cleaning a single-stage system:

A. Prepare the cleaning solution;

B. Low flow input of cleaning solution;

C. Circulation;

D. Soaking;

E. High flow water pump circulation;

F. Rinsing;

G. Restart the system.

Cleaning for specific pollutants includes: cleaning sulfate scale, cleaning carbonate scale, cleaning iron and manganese pollution, cleaning organic pollution, etc.