Current status and trend of reverse osmosis concentrated water disposal

The reverse osmosis concentrated water is the discharge water of the reverse osmosis device, the impurities in the raw water are concentrated in it, and direct discharge will have an adverse effect on the environment.

Compared with processes such as ion exchange, electrodialysis, and distillation, reverse osmosis and nanofiltration processes have the advantages of energy saving, less chemicals, high quality of water, and easy expansion of equipment in terms of water desalination and purification. Since the 1980s, it has been widely used in the purification treatment of industrial and domestic water in European and American countries. At present, reverse osmosis and nanofiltration processes have become a standard water treatment process. my country began to use reverse osmosis technology on a large scale in the late 1990s, mainly focusing on the desalination of power plant boiler make-up water, bottled drinking water, and food and medical water. Recently, some large-scale industrial wastewater reuse devices will also be reverse osmosis. As the core process. According to rough estimates, the total installed capacity of reverse osmosis in my country is about 500,000-800,000 m3/h, and the newly added processing capacity in 2003 was 50,000-60,000 m3/h, with an annual growth rate of 20-30% in the past three years. Although the reverse osmosis process produces chemical wastewater only when the device is cleaned, no new pollutants are produced during normal operation. However, it must be noted that while high-quality desalinated water, impurities in the influent water are concentrated. If the reverse osmosis concentrated water is not properly disposed of and is directly discharged into the natural water body, it will inevitably have an adverse effect on the water environment. At present, there is basically no special treatment for reverse osmosis concentrated water in China, and most of them use on-site discharge.

Wastes from reverse osmosis and nanofiltration systems include by-products such as pretreatment sludge, cleaning fluid, protection fluid, and concentrated water. Except for concentrated water, other by-products are consistent with traditional water treatment facilities, and there are ready-made disposal methods for reference. Due to the high salt content of concentrated water, the traditional water supply and drainage treatment cannot effectively solve this problem. The quality of concentrated water is affected by the quality of raw water, treatment chemicals and recovery rate. Generally, the simple estimation method when determining the concentration of concentrated water is calculated by 100% desalination rate.

From the calculation formula of the concentration factor CF, it can be calculated that the TDS of the raw water is 3,000 mg/l and the TDS of the concentrated water is 12,000 mg/l when the recovery rate is 75%. If the recovery rate reaches 90%, TDS will reach 30,000 mg/l. A membrane system with a processing capacity of 10 Mgal/d (millions of gallons/day), the production of concentrated water at a recovery rate of 75% is 3.3Mgal/d, and the production of concentrated water at a recovery rate of 90% is 1.1Mgal/d. The balance between the quality and quantity of concentrated water has a great influence on the disposal of concentrated water, and the choice of recovery rate is also affected by the possible disposal methods. Sometimes the concentrated water is mixed with other water or waste water and then discharged, which is a better choice in terms of feasibility and economy. For example, mixed discharge of concentrated water with treated urban drainage, industrial wastewater or power plant cooling water.

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