Current status of urban wastewater treatment in China

A research report of Sino-Africa wastewater treatment cooperation was released by Beijing Municipal Research Institute of Environmental Protection and Tongji University, to explore the possible wasterwater treatment technology transfer mechanism between China and African countries.

Urban wastewater consists of domestic and industrial wastewater, rainwater in combined wastewater systems and primary rainwater in the semi-triage system in China. In general, nitrogen and phosphorous concentration is relatively high with low content of harmful heavy metals and toxic substances.

Water quality and quantity fluctuation is large with good biodegradability. Wastewater in most towns is similar with BOD5 of less than 250 mg/L, COD of 250 to 300 mg/L and SS of about 200 mg/L. However, due to industrial wastewater often containing toxic and hazardous substances, urban wastewater component containing industrial waste is more complex, and shows regional differences.

Current status of urban wastewater treatment

It is estimated that 3513 large-scale municipal wastewater treatment plants have been built in China by the end of 2013 with a total treatment capacity of 149 million m3/day. Compared with the end of 2012, new wastewater treatment capacity has increased to about 6.8 million m3/day. By the end of 2010, urban drainage pipe network reached 370,000 km. National urban wastewater treatment rate is more than 60%. Cities wastewater treatment rate is not less than 70%. The utilization of reclaimed water accounted for only about 10% of wastewater treatment capacity with recycled water production of 1100 m3 / day and the total actual renewable water use of 920 m3 / day.

Currently, the main wastewater treatment process include oxidation ditch process series, SBR technology series, the conventional activated sludge process and associated A/O and A/A/O process, biofilter technology series and natural purification systems.

Oxidation ditch process makes use of continuous loop reactor pond and improves buffering capacity of the system with longer HRT to treat organic matter difficult to be biodegraded. In the application of municipal wastewater treatment, common processes include Carrousel oxidation ditch, double trenches (or three-channel) oxidation ditch, Orbal polycyclic oxidation ditch, integrated oxidation ditch. Compared to conventional activated sludge process, oxidation ditch process can save regulation tank, primary settling tank and sludge digestion tank with the advantage of simple operation, strong resistance to shock loading capacity, stable water quality, easy control and operation. However, the process is generally without primary sedimentation tank and sludge digestion tank, so the reaction unit is relatively large, and some costs will increase. Besides, it is required to maintain and replace the mechanical parts of the process aerator, so repairing workload is large.

Basic SBR consists of five basic processes, that is influent, aeration, sedimentation, effluent and stand chronologically, whose main deformation processes include cyclic activated sludge system (CASS or CAST process), continuous and intermittent aeration technology (DAT- IAT process), Alternating Circulating Activated Sludge Process (ACIS) and so on. Overall, SBR process can be run in the mode of orderly and intermittent operation. Stirring, aeration, sedimentation is achieved during different period to form anaerobic, anoxic and aerobic process with good sludge settling properties and high removal efficiency of organic matter, without secondary settling tank and sludge return. The process is simple, and the construction and operating costs is 20% lower than the conventional activated sludge process. However, SBR is prone to produce scum, which affects water quality, and requires automation and other issues.

The anoxic section of the A/O treatment process will increase nitrogen removal before the wastewater is being treated with conventional aerobic activated sludge treatment system. In aerobic section, organic matters are decomposed by aerobic microbial oxidation and nitration occurs. When nitrate returns to the anoxic zone, denitrification is achieved by organic matter in water. Because wastewater enters the anoxic tank first, which provides enough carbon source for denitrification; besides, a portion of the carbon source of organic matters are consumed by the denitrification process, which reduce the load for subsequent aerobic treatment process and improve the oxidation and nitrification rates in aerobic tank. The process has the advantage of good organic matter and nitrogen removal, simple process, easy operation and management. However, it is necessary to increase circulating reflux ratio and energy consumption in order to ensure the removal efficiency of TN.

A2/O (Anaerobic/Anoxic/Oxic) process can be adopted when it is necessary to achieve simultaneous nitrogen and phosphorus removal. Wastewater and sludge backflow from sedimentation tank is discharged into anaerobic tank to release phosphorus anaerobically, then enters into anoxic tank with the mixture from aerobic section to complete denitrification. Nitrification, oxidation of organic matter remaining and aerobic phosphorus uptake of PAOs is achieved in aerobic tanks, and then phosphorus is removed by the discharge of remaining sludge to obtain nitrogen and phosphorus removal synchronization. The process has the advantage of high contaminant removal efficiency, stable operation and good shock loading resistance. However, it requires more structures and large return quantity, high capital costs and operating costs. Besides, aerobic sludge return section may affect the efficiency of the anaerobic phosphorus. Other processes that may be considered to overcome shortcomings of A2/O are inverted A2/O process, UCT process, MUCT process and An/O process.

Biofilter media commonly used is gravel, slag, coke, etc., and for the latter, plastic filter is developed. Common biofilter biological filter includes low load biological filter, high load biological filter, tower biofilter and BAF. Biofilter has the advantage of high treatment capacity, good impact resistance, good water quality, small land occupation, low infrastructure investment, low operating costs, and modular structure, etc.; however, it requires high water quality demand of pretreatment and regular anti-wash, which can easily cause filter loss.

Natural purification system includes land treatment system and wetland treatment systems. Natural purification system has the advantage of low investment, easy operation and management, and low energy consumption; however it has the disadvantage of high land occupation and low contaminant removal rate, and is normally used as an advanced treatment technology after domestic wastewater is treated by other technologies.