Reverse Osmosis Cleaning

After a period of normal operation, reverse osmosis membrane elements can become fouled by suspended solids or sparingly soluble salts that may be present in the feed water. The most common types of fouling include calcium carbonate precipitation, calcium sulfate precipitation, metal oxide precipitation (iron, manganese, copper, nickel, aluminum, etc.), silica deposits, mixtures of inorganic or organic deposits, NOM (natural organic matter), synthetic organic matter (such as scale inhibitors/dispersants, cationic polyelectrolytes), and microbial contamination (algae, mold, fungi).

The nature and rate of fouling depend on various factors, such as feed water quality and system recovery rate. Fouling is usually gradual; if not controlled early, it can damage the membrane elements in a relatively short time. Cleaning the membrane elements is recommended when fouling is confirmed, either before a long-term shutdown or as part of routine maintenance.

Chemical cleaning or physical flushing is required when a reverse osmosis system (or unit) exhibits the following symptoms:

Permeate flow rate decreases by 10-15% compared to normal feedwater pressure;

Feedwater pressure, after temperature correction, increases by 10-15% to maintain normal permeate flow rate;

Permeate water quality decreases by 10-15%, while salt permeability increases by 10-15%;

Feedwater pressure increases by 10-15%;

Pressure differential between different sections of the system increases significantly (this parameter may not be monitored by instruments).

The above cleaning principles still apply if key parameters remain unchanged, even when operating data is not standardized. Maintaining stable operating parameters primarily refers to permeate flow rate, permeate back pressure, recovery rate, temperature, and TDS.

The cleaning cycle for fouled reverse osmosis membranes depends on the actual site conditions. A normal cleaning cycle is every 3-12 months. If cleaning is required more than once a month, further adjustments and improvements to the reverse osmosis pretreatment system are necessary, such as additional investment or redesign of the reverse osmosis system. If cleaning is performed every 3 months, modifications can be made to the existing equipment.

When the membrane element is only slightly fouled, cleaning the membrane element is crucial. Severe fouling hinders the penetration of chemicals into the fouled layer, affecting the cleaning effect. If the membrane element's performance drops to 30-50% of its normal value, it is impossible to fully restore the membrane element to its initial performance at the time of manufacture.

In reverse osmosis system design, reverse osmosis product water can be used to flush contaminants in the system to reduce the cleaning frequency. Soaking the membrane element with product water helps dissolve and remove fouling, reducing the frequency of chemical cleaning.

The type of contaminant to clean and the cleaning method depend on the on-site fouling situation. For complex situations where several types of fouling exist simultaneously, the cleaning method is to use low-pH and high-pH cleaning solutions alternately.

After cleaning, the water flow increased, indicating that the cleaning was effective to some extent. However, if it quickly became clogged again, it indicates a problem with your feed water. There are three possible reasons:

1. The feed water SDI is too high, and the system recovery rate is relatively high, leading to a significant tendency for scaling in the system.

2. Insufficient dosage of reverse osmosis antiscalant, or incorrect selection of the type.

3. Microbial contamination of the system. Bacterial proliferation leads to membrane fouling and clogging.