Application of high-efficiency reverse osmosis membrane elements in sewage treatment

RO membrane separation technology

RO membrane separation technology uses the static pressure difference across the membrane as the driving force. Small molecules, such as water molecules, overcome the osmotic pressure to pass through the RO membrane, thereby separating them from impurities. The operating pressure is generally between 1.5 and 10.5 MPa, and the system can retain small molecules of 1 to 10A.

In water treatment, RO, as a key equipment, can remove more than 97% of soluble inorganic matter, 99% of organic matter with a relative molecular weight of 300 or above, more than 99% of various particles including bacteria, and 95% of SiO2 in water.

The largest domestic application area for reverse osmosis membranes remains large-scale boiler feed water, followed by various industrial pure water and drinking water markets. Membrane applications have also reached a significant scale in the electronics, semiconductor, pharmaceutical, medical, food, beverage, alcohol, chemical, environmental protection, metallurgy, and textile industries. Future potential applications include wastewater treatment for power plant cooling water, large-scale seawater desalination, brackish water desalination, and large-scale municipal and industrial wastewater treatment.

Application of RO membrane in sewage treatment

1  Application in high-salt wastewater treatment

(1) The influent treatment of high-mineralization wastewater, represented by mine water, is characterized by high salinity, especially groundwater outflow, with an average salt content of more than 1000 mg/L. The organic components of SS containing a large amount of Ca2+Mg2+K+, Cl-, SO42-, HCO3 and HCO3- are less than 1.5 mg/L. For mining areas with severe water shortages, the use of reverse osmosis technology for deep treatment of production and domestic water has been widely promoted.

Using mineral water as a pretreatment agent, flocculation, sedimentation, and rapid filtration were added to remove most of the SSS in the water, resulting in a reverse osmosis inlet turbidity of less than 1 NTU. After reverse osmosis treatment, the effluent turbidity removal rate approached 100%, with a desalination rate of 96%.

  (2) Metallurgical wastewater treatment

The steel industry, a resource-intensive, water-intensive and highly polluting industry, accounts for 14% of China's total water consumption. Steel industry wastewater is widely used in the metallurgical industry. The wastewater has a complex composition and fluctuates widely in various parameters, particularly high levels of Ca2+, Mg2+, Fe2+, Mn2+, SO42-, F, and SiO2. Failure to pre-remove the high-metal extractants present on the reverse osmosis membrane can lead to serious inorganic pollution.

For wastewater from a steel plant undergoing secondary biochemical treatment, aeration is first performed in an aeration tank to oxidize Fe2+ to Fe3+. Simultaneously, NaClO is added to enhance the Fe2+ oxidation capacity and bactericidal efficacy. Lime milk is added to adjust the pH, followed by flocculation with PAM and PAC. Sedimentation, rapid filtration, and activated carbon adsorption are then used to remove organic matter, residual chlorine, and heavy metal ions. Following UF treatment, the effluent is treated with a reducing agent. Scale inhibitors and acid enter the RO system.

2  Application in the treatment of refractory organic wastewater

Printing and dyeing wastewater and petrochemical wastewater treatment, printing and dyeing wastewater, in addition to a large amount of dyes, slurries, inorganic salts, acids and alkalis, has the characteristics of up to 4000 times the color, large water volume, high content of organic pollutants, diverse water quality, poor biodegradability, etc. The deep treatment of landfill leachate mainly comes from the sedimentation of landfills, and the pollutants mainly come from microbial decomposition of garbage and precipitation leaching. The water quality is complex, and the COD is much higher than that of urban sewage. In addition, the leachate also contains a variety of metal ions, such as Fe2+, Cd2+, Cr3+, Cu2+, Zn2+, and the Fe2+ concentration during the fermentation process

Although the A/O two-stage biochemical treatment process has been widely used for organic matter degradation and denitrification in landfill leachate, the effluent performance is unstable. Therefore, based on mechanical filtration, the use of ultrafiltration membranes and A/O to form an MBR process has improved the removal rate of organic matter in landfill leachate.

The effluent, after sludge separation through ultrafiltration membranes, enters a nanofiltration (NFN) system. NF can effectively separate organic matter with a molecular weight of 200m to 2000m and can also be used to separate some high-value organic matter from MBR-produced water. Research was conducted on the properties of metal ions and the MBR+NF+RO three-layer membrane composite treatment process. Results demonstrate that the process operates effectively, and the effluent quality meets pollution control standards for municipal solid waste landfills.

During operation, if the NF effluent meets discharge requirements, it can be discharged directly. Otherwise, it can be further treated using the following RO system. Research on the Ar/O-MBR+NF+RO process for treating landfill leachate has shown that the addition of activated carbon to the A/O reactor not only improves the removal efficiency of organic matter and heavy metals but also reduces membrane fouling.

3 Application in high-quality recycling treatment

China has also established large-scale recycled water plants in Beijing, Tianjin, Hebei, Shandong and other places. Among them, several large recycled water recovery stations use the "UF(MF)+RO" double membrane process to treat two types of urban sewage into raw water, producing high-quality recycled water, which is used as part of the water for boiler thermal power plants and the other part is used as landscape rivers and domestic miscellaneous water .

A local wastewater treatment plant uses MBR combined with reverse osmosis to treat municipal wastewater for reuse. The water has achieved TOC levels <1.3 mg/L, NH3-N <0.03 mg/L, and TN <0.1 mg/L. Conductivity and turbidity are both less than 30 μS/cm and 0.12 NTU, respectively. The effluent quality meets drinking water standards. Reverse osmosis, as a core technology, has been widely applied to various water depths and high-quality water reuse applications.

To fully leverage the advantages of reverse osmosis technology, minimize membrane fouling, and lower water treatment costs, our company has developed a series of combined processes tailored to water quality characteristics. Traditional water treatment methods, such as coagulation and sedimentation, high-concentration, and high-efficiency filtration, are typically used to remove SS. NaClO, lime milk, and manganese sand filters are used to reduce RO, manganese salts, calcium salts, and silica salts in the influent Sargassum aeration and oxidation system. O3 oxidation is used to treat recalcitrant A/O organic matter. MBR, UF, and NF are used to retain fine SS and small organic molecules. High-valent ions are combined with UF+RO or MBR+RO dual-membrane systems, or even MBR+NF+RO triple-membrane systems, ensuring stable system operation and ensuring the RO treatment of recalcitrant organic wastewater.

Company Recommendations

"HID Membrane" focuses on membrane separation technology and membrane product development, providing customers with professional liquid separation, membrane concentration solutions and complete sets of system equipment from membrane manufacturing, application research and development, project consulting, system integration to integrated technical services.

The brand has been committed to industrial wastewater treatment technology and membrane product applications for more than 16 years. It has pioneered the world's first in multiple application fields of reverse osmosis membranes, accumulated rich application experience, and obtained many invention patents and utility model patents in related fields, filling many gaps at home and abroad , and has a high reputation in the industry.