By Bill Stimeling, Disinfection Business Developer, America, Severn Trent Services

Since China's Contaminant Emission Standards for Urban Wastewater Treatment Plants were officially brought into effect in 2003, they have been strictly enforced by the country's environmental protection authorities. When a large wastewater treatment plant in China's largest city needed to improve its treatment capabilities to meet Class 1B discharge standards, it selected a system proven at facilities around the world to provide superior nitrification, BOD reduction and filtration of suspended solids in aerobic biological treatment applications.

Project background
The Shanghai Xinjiang Wastewater Treatment Plant is located in Shanghai's Jinshan District in eastern China on the shores of Hangzhou Bay, and its treated wastewater is discharged into the ocean. Covering an area of approximately 150,000 m² (37 acres), the plant is responsible for the treatment of wastewater from Jinshan's fast-growing urban area and the Petrochemical Industry Park. The park is one of four large chemical/petrochemical production bases in Shanghai that have contributed to the rising proportion of industrial wastewater in the city's sewage flow. The plant's design features a wastewater treatment capacity of 100,000 m³/day. The Shanghai Xinjiang Wastewater Treatment Plant needed to lower its water pollution levels to meet Class 1B discharge standards, which, for the Xinjiang plant, include BOD5 < 20 mg/L, TSS < 20 mg/L, TN < 20 mg/L, and NH4-N < 8 mg/L @ > 12 C, and NH4-N < 15 mg/L @ < 12 C.

After investigating a number of wastewater filtration systems, the Shanghai Xinjiang Wastewater Treatment Plant pilot tested and ultimately selected the TETRA® ColOX™ biological aerated filter system from Severn Trent Services for the plant's Phase II construction. The lead contractor for the project was Tongfang Co., Ltd.

Pilot testing the ColOX BAF technology 
Severn Trent Services designed an 11-cell ColOX BAF, with a filtering area of almost 1,400 square meters. Construction and installation of the system were completed in April 2011, and the pilot run was carried out in September of that year after two months of technical commissioning. During stable operation of more than one year, all indicators met the design standards, and the system became a model for large-scale use of the ColOX BAF by municipal wastewater treatment projects in China. The system was ultimately chosen because it features a small footprint and space-saving units that offer greater efficiency, operational reliability and versatility over other aerobic biological treatment processes.

The ColOX filter system uses a fixed-film biological process to provide superior nitrification, BOD reduction and filtration of suspended solids for wastewater applications where aerobic biological treatment is used. The ColOX system has a unique design, incorporating interlocking blocks that enhance block stability during backwash procedures and feature additional passive interlocking as well as positive row spacing for improved backwash distribution of both backwash air and water. Unlike traditional underdrain designs that rely on a high pressure differential across the block to distribute backwash water, there is virtually no pressure drop across the interlocking blocks.

Full-scale operation 
The plant operates as a single filter consisting of a number of cells into which the influent flow is distributed. The flow passes through media and suspended solids are removed. Filtered effluent is discharged through a clear well that retains a reservoir of water for cell backwashing. The size of the media used enables solids to penetrate deep into the bed. This use of depth rather than surface filtration gives the filters greater solids-holding capacity, enabling longer run times between backwashes. The filter is "regenerated" by taking one cell offline at a time and employing an air and water backwash to clean the media. The dirty backwash water generated is returned to the treatment works. While a cell is offline for backwashing, the influent flow is redistributed to the other cells, and operation of the filter continues with minimal impact on the quality of the effluent.

"As China's largest BAF, Shanghai Xinjiang Wastewater Treatment Plant has achieved continuous operation for as long as 18 months, and the treated water meets the national discharge standards for urban areas," according to Mr. Li Qiang, director of the wastewater treatment plant.

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