In order to meet the requirements for total nitrogen removal in sensitive area, as specified by the EC Standards, existing high-rate or medium loaded activated sludge plants treating only carbonaceous pollution have to be upgraded. Two attractive technical solutions are proposed. Depending on both the design and operational conditions of the activated sludge plants, all existing treatment facilities will be reused for nutrient removal with either an immobilized nitrifying system implemented directly in the existing activated sludge tank or with an additional up-flow nitrifying biofilter. Both technical solutions enable the effluent total nitrogen concentration to be as low as 10 mg TN/l. When a high or medium loaded activated sludge plant with primary settling tanks is designed for a HRT of more than five hours, the Pegazur immobilized carrier system can be implemented directly in the biological reactors. The existing aeration tank is divided into anoxic and aerated zones, and the nitrifying Biocubes are introduced in the aerated one. Such an upgraded activated sludge system (which did not nitrify before the upgrading) can reach a maximum eliminated loading rate of 0.5 kg NH4–N per m3 of aerated volume per day at 12°C. This solution was used to upgrade the 120,000 p.e. Bellozanne Sewage Treatment Works, Jersey, Channel Islands, UK. When an existing high rate activated sludge plant is designed for a HRT of less than five hours, the activated sludge tank can be transformed into a high-rate denitrifying anoxic reactor, coupled to a nitrifying Biofor up-flow filter. The high-rate anoxic sludge is continuously controlled by an ORP probe in order to ensure both the denitrifying conditions and recycle ratios are at their optimum level. This controlled operation and high water velocities applied to the nitrifying up-flow Biofor enable the operational mode to switch from dry weather conditions to wet weather conditions where the recycle of nitrified liquor is switched off and the high loaded anoxic sludge is temprarily aerated.

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