Chemical cleaning of RO system or NF system

When shall we process chemical cleaning

When physical cleaning can no longer can restore the membrane performance of RO system or NF system, the chemical cleaning is required at this time.

1) 15 % drop in permeate water flow under standardized conditions

2) The pressure difference is increased to 1.5 times the initial value

3) The quality of the permeate water drops by 10-15%.

Frequency of chemical cleaning

1) When the membrane element is slightly polluted, the membrane element should be cleaned in time. Severe pollution will affect the cleaning effect because the chemical agent is not easy to penetrate deeply into the pollution layer, and the pollutants are not easy to be washed out of the membrane and other factors. If the performance of the membrane element is reduced to 30-50% of the normal value, it is difficult to clean and restore the original performance of the RO system or NF sytsem.

2) The cleaning cycle of the membrane is determined according to the actual pollution situation on site. The normal cleaning cycle is every 3-12 months. If cleaning is performed more than once in a month, it is necessary to improve the pretreatment; if the cleaning cycle is once every 1-3 months, focus on adjusting and optimizing the operating parameters of the existing system. Even if the system has not been polluted for a long time, in order to better ensure the normal operation of the system, it is generally considered to carry out chemical cleaning every 6 months.

3) When inorganic flocculants are used in the pretreatment process, there are often incompletely reacted inorganic salts that do not form flocs that can be filtered out. The user should ensure that no excess flocculant enters the membrane system. Excess flocculant can be determined by the SDI test device.

Introduction of conventional cleaning solution

1) Low pH wash with 2.0% (W) citric acid (C6H8O7). It is very effective for removing inorganic salt scale (such as calcium carbonate scale, calcium sulfate, barium sulfate, strontium sulfate scale, etc.), metal oxides or hydroxides (iron, manganese, copper, nickel, zinc, etc.), and inorganic colloids. Commercially available citric acid is a powdered solid.

2) A high pH (pH 10) wash solution of 2.0 (W)% STPP (sodium tripolyphosphate Na5P3O10) and 0.8% (W) Na-EDTA. It is designed to remove calcium sulfate scale and light to moderate natural organic contaminants. STPP has the function of inorganic chelating agent and detergent. Na-EDTA is a chelating organic chelating cleaning agent that can effectively remove divalent and trivalent cations and metal ions. Commercially available STPP and Na-EDTA are both powder solids.

3) 2.0 (W)% STPP (sodium tripolyphosphate Na5P3O10) and 0.25% (W) Na-DDBS [sodium dodecylbenzenesulfonate, C6H5(CH2)12-SO3Na] mixed pH 10 The solution. This cleaning solution is used to remove heavy natural organic matter (NOM) contamination. STPP has the function of inorganic chelating agent and detergent, while Na-DDBS is an anionic detergent.

4) 0.5% (W) hydrochloric acid low pH cleaning solution (pH is 2.5), mainly used to remove inorganic scale (such as calcium carbonate scale, calcium sulfate, barium sulfate, strontium sulfate scale, etc.), metal oxide/hydroxide substances (iron, manganese, copper, nickel, aluminum, etc.), and inorganic colloids. This cleaning solution is stronger than solution 1. Hydrochloric acid is a strong acid solution, and relevant precautions must be observed when preparing.

Chemical cleaning equipment

Cleaning equipment generally includes cleaning tank filters, circulating pumps, pressure gauges, thermometers, pressure gauges, valves, sampling points and pipelines. The volume of the cleaning water tank must meet the requirements of the connecting hoses, pipelines and the replacement water volume in the RO system pressure vessel.

Precautions and preparations before chemical cleaning

1)  Before using the drug, carefully read the drug safety form (MSDS) obtained from the drug company and the description of the drug

2) When operating, wear safety glasses, protective gloves and work clothes

3) Calibrate the pH meter before operation

4) Estimate the volume of cleaning solution ready be used

5) Ensure that all cleaning chemicals are completely dissolved and mixed before the cleaning solution enters the system

6) The temperature and pH range of the cleaning solution meet the specified values

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