Advanced phosphorous removal curbs eutrophication in Lake Tai

By Jos Frijns, Jianghua Zhang and Hillebrand Ehrenburg

Advanced phosphorous removal methods at municipal wastewater treatment plants along Taihu, or Lake Tai, situated west from Shanghai, China, could drastically reduce nutrient levels and help rehabilitate the large shallow lake that currently receives organic substances and nutrients from domestic and industrial wastewater discharges.

Figure 1: A/O process diagram

A study carried out by Grontmij Consulting Engineers in consultation with Jiangsu Provincial Environmental Protection Bureau and Zhejiang Provincial Environmental Monitoring Centre, examined the effectiveness of design modifications of advanced phosphorous removal methods used in the Chengbei and Lihua wastewater treatment plants, which serve the city of Changzhou. These methods include modified anaerobic/oxic (A/O) process and/or anaerobic/anoxic/oxic (AA/O) process systems equipped with contact tanks. This study is part of the Masterplan for the Rehabilitation of Lake Tai project, which provides for an integrated approach for sustainable lake management.

The study concluded that recommended modifications would increase the phosphorous removal rate result to 90%; proving that these promising designs should be employed in additional wastewater treatment systems within the Taihu basin.

Current pollution control efforts in China are extensive but hampered by economical, technical and institutional constraints. Only an estimated 30% of municipal wastewater treatment plants that are required for sustainable water management have been built, and of these, only a few apply advanced nutrient removal measures.

Lake Tai, the third largest fresh water lake in China with a surface area of 2,400 km2, supplies water for drinking and industry in the Taihu Basin and plays a key role in regional water management. Situated in the provinces Jiangsu and Zhejiang, the Taihu Basin is China's economically most advanced area and is populated by some 35 million people.

Rapid industrial and agricultural development in addition to high population growth during the last 20 years have resulted in a massive increase in pollutants being discharged into the lake. This has created serious environmental problems, including eutrophication, organic pollution and the deterioration of the aquatic ecosystem. The key to controlling eutrophication is limiting phosphorous inputs, hence, domestic wastewater treatment with phosphorous removal is likely to make an important contribution to restoring the water quality of Taihu.

The main pollutant load of wastewater discharged into Taihu originates from domestic wastewater. The organic (COD) load is mainly from domestic and industrial pollution sources in the urban area. The total nitrogen (TN) load is mainly from non-point pollution sources in the rural area. The total phosphorous (TP) load, the limiting factor for eutrophication in Lake Tai, is mainly from domestic wastewater of the urban areas. The annual TP load to Taihu is 2,970 tons. This waste load must be significantly reduced to rehabilitate Taihu; therefore more municipal wastewater treatment plants that reduce nutrient load, especially the phosphorus load that originates from urban areas, are required.

Currently, the main treatment processes include oxidation ditches, activated sludge, AA/O process, and SBR. In general, the performance of organic matter (COD, BOD) removal in existing plants is relatively good, in most cases adhering to the effluent standards. The performance of nutrient removal in existing plants is relatively poor. The effluent standard of TP is rarely met.

Table 2: Simulated effluent quality of the new AA/O design with and without contact tank for Lihua

Recently, Jiangsu and Zhejiang provinces prepared the Tenth 5-Year Plan for 2001-2005 of water pollution control for Taihu basin, which aims to improve the efficiency of wastewater treatment plants, and to install secondary treatment facilities with N and P removal in wastewater treatment plants.

Chengbei

Nutrient removal can be enhanced by modifying existing treatment plants and improving designs for new treatment systems. In the Chengbei facility, a conventional activated sludge system was modified to an anaerobic/oxic (A/O) process. This system is a single-sludge suspended-growth system that combines anaerobic and aerobic sections in sequence, resulting in biological P-removal.

The Chengbei wastewater treatment plant, the largest municipal wastewater treatment system in Changzhou city, was completed in two phases. The first phase was a conventional activated sludge system, constructed in 1997 with a capacity of 50,000 tons/day. The second phase involved an AA/O system, finished in 1999 with a capacity of 50,000 tons/day.

Sewage enters the plant through the main pumping station, which contains bar screens and flow-measuring equipment. It is then separated into two equal flows to the conventional system and the AA/O system that operate in parallel. Sewage entering the conventional activated sludge system is pumped to primary clarifiers and then flows into the aerobic basin. Each of the four basins is equipped with fine bubble aerators. The mixed liquor from the aeration basin flows to four final clarifiers.

Figure 2: AA/O process diagram with contact tank

The existing conventional activated sludge mode consists of two parallel trains of two aeration basins with each of the four tanks (1780 m3/tank) connected in series. Primary effluent and return activated sludge are added to the first tank and each series. The hydraulic retention time based on average influent flow conditions is 6.9 hours, which is not including the return activated flow (R=30-50%).

Modifying the system to include the anaerobic/oxic system (see figure 1) requires that the first tank in each series is expanded to 2,000 m3/tank. The aerator in the first tank of each series will be switched off. The A/O process has the advantage that a high removal rate of phosphorus can be achieved with a relatively short hydraulic retention time, and with a relatively simple operation process. The main disadvantage of the A/O process is that it is incapable of achieving high levels of nitrogen and phosphorus removal simultaneously.

The treatment performance of the above modification was modelled (HAS model), showing a significantly higher TP removal rate.

Lihua

Improved design features that increase nutrient removal rates should be considered for new and expanded treatment plants. In Lihua, the improved anaerobic/anoxic/oxic (AA/O) process introduced an anoxic zone for denitrification.

Expansion works with a capacity of 10,000 m3/d for the Lihua wastewater treatment plant is currently proposed to handle an estimated increase in wastewater discharged into the plant through the separated sewer system. The Lihua plant is a conventional activated sludge system, built in 1990, with a current design capacity of 10,000 m3/day.

Grontmij studied the effectiveness of the modified anaerobic/anoxic/oxic (AA/O) process that introduces an anoxic zone for denitrification. The anoxic zone is deficient in dissolved oxygen, but chemically bound oxygen in the form of nitrate or nitrite is introduced by recycling nitrified mixed liquor from the aerobic section. Effluent phosphorous concentrations of less than 1 mg/l can be expected.

The main advantage of the AA/O process is that it provides better denitrification capability than the A/O process. The main disadvantage of nutrient removal in the AA/O process is that the nitrate present in return sludge may affect phosphorus release in the anaerobic tank, resulting in lowering the phosphorus luxury uptake in the aerobic tank. An additional tank, which is called the anoxic contact tank, is introduced upstream of the anaerobic tank to overcome this problem. Return sludge from the final clarifiers will be mixed with part of the raw wastewater in the contact tank. Any nitrate in return sludge will be denitrified before entering the anaerobic zone to eliminate the effect of nitrate in the anaerobic tank from return sludge, and improve TP removal potential.

The treatment performance of the above new design is modelled (HAS model) with and without a contact tank and the results are presented in table 2. The results reflect that the process with the anoxic contact tank will have higher efficiency of TP removal and will meet the Class I effluent standard.

If, in theory, all planned treatment plants would be designed to have advanced phosphorous removal as described above, i.e., A/O process and/or AA/O process systems equipped with contact tanks, the phosphorous removal rate would reach 90%. This would imply that the TP load to Taihu would be reduced to 1,000 ton (a two-third reduction of the current load). The total achievable additional pollution load reduction can be even higher if the already existing activated sludge systems would be modified to A/O systems.