Chemical cleaning of reverse osmosis system for reclaimed water treatment in thermal power plant

As the production water of thermal power plant, reclaimed water increases the risk of reverse osmosis membrane fouling or being polluted. The abnormal conditions are mainly manifested in the changes of feed water pressure, differential pressure, water flow and desalination rate. It should be judged by combining the pretreatment mode and raw water quality to determine the type of fouling or pollutants. Acid and EDTA salt are mainly used to clean calcium carbonate scale and calcium sulfate scale, non oxidizing bactericide is used for microbial and bacterial pollution, NaHSO3 solution is mainly used for iron pollution, and NaOH solution with low concentration is used for inorganic colloid pollution. The temperature and pH value of the cleaning agent should be strictly controlled during the cleaning process.

Reverse osmosis technology is a membrane separation technology which takes pressure difference as driving force to separate purified water from saltwater. It has the advantages of high desalination efficiency, low operating cost, simple operation and small floor area, and is widely used in boiler make-up water treatment of thermal power plants. At present, because of the shortage of water resources in China, the new thermal power plant no longer allows groundwater as production water, and the simple treatment of urban sewage becomes the only source of water for production. The change of water source makes the reverse osmosis membrane appear scaling, performance decline and pollution in production, which will cause the water quality of boiler make-up water poor, and affect the water vapor quality of the system. When the above problems occur, the reasons should be analyzed in time, and the reverse osmosis membrane should be cleaned in a targeted manner to avoid long-term irreversible damage and affect the water production performance.

Selection of chemical cleaning agents for reverse osmosis membrane

When it is confirmed that the reverse osmosis membrane module is contaminated, chemical cleaning must be carried out. Different chemical cleaning agents should be used for different pollutants. The fouling or scaling of reverse osmosis membrane is affected by many factors, such as the type of pollutants and the material of membrane itself. Acid and EDTA salt are mainly used to clean calcium carbonate scale and calcium sulfate scale; Non oxidizing bactericides were used for microbial and bacterial contamination; The source of iron pollution is corroded iron in pipeline or filter, and the main cleaning agent is sodium bisulfite; Inorganic colloid pollution is mainly caused by improper pretreatment of reclaimed water, resulting in inorganic colloid entering the reverse osmosis system, and low concentration NaOH solution is mainly used for cleaning.

Matters needing attention in chemical cleaning process of reverse osmosis membrane

Preparation before cleaning

According to the abnormal phenomenon of reverse osmosis membrane, it is determined that chemical cleaning is needed for reverse osmosis. Sample and analyze the pollutants or scale on the reverse osmosis membrane to determine the chemical composition of the pollutants or scale, and then determine the chemical formula to be used according to table 1. Before chemical cleaning, check whether the electric heating device can work normally to ensure that the temperature of the cleaning solution is not lower than 15 ℃.

Configuration of cleaning solution

The configuration of chemical cleaning agent is the basis of chemical cleaning process, and the concentration of chemical cleaning agent directly affects the effect of chemical cleaning. Pour the reagent into the chemical cleaning box according to the calculated dosage. When water is injected to the full level line of chemical tank, start the circulating pump of cleaning solution and stir for about 5 minutes to make it fully mixed. Detect the pH value and adjust the pH value to the required range.

Soak

Immersion is a key process in the process of reverse osmosis chemical cleaning. It can not only make the chemical solution react with the pollutants for enough time, but also make the pollutants fall off from the surface of the membrane and dissolve in the chemical solution to achieve the purpose of chemical cleaning. In the process of chemical cleaning, we must ensure enough soaking time.

Cycle cleaning process

Circulating cleaning is the main process of chemical cleaning in reverse osmosis system. In this process, the chemical solution has physical and dynamic contact with the pollutants inside the membrane, and further reactions such as penetration, friction and shear occur, so as to achieve the purpose of chemical cleaning. The pH value of cleaning solution is an important measurement parameter in the cleaning process. The cleaning status and cleaning stage of the system can be judged by the change of pH value. Precision test paper method or portable pH meter can be used for detection on site.

Conclusion

Due to the change of the influent water quality of reverse osmosis, the problems such as the increase of pressure difference and the decrease of desalination rate often occur during the operation of reverse osmosis device, and the workload of operation and maintenance is increased. In case of abnormal conditions, it is necessary to judge the type of pollution and determine the chemical cleaning agent in combination with various water supply indexes. Meanwhile, the temperature and pH value of the cleaning agent should be strictly controlled in the chemical cleaning process to ensure sufficient soaking time so that the cleaning agent and pollutants can react completely. In daily work, it should be carried out in strict accordance with the national standards to ensure the quality of reverse osmosis water supply, which can effectively reduce the pollution of reverse osmosis membrane and prolong the membrane cleaning cycle.

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