Aerated Biofilters for Nitrification and Effluent Polishing

Abstract

To comply with new effluent discharge standards of 10 mg TKN/l, different upgrading methods for a highly loaded activated sludge plant were explored. As a conclusion, demonstration units were tested to assess process feasibility and performance data of an innovative technology. The Achères Treatment plant of the city of Paris is currently being extended to purify a flow of about 2 700 000 m3/d, corresponding to 8 Million population equivalents. Conventional activated sludge, loaded at about 0.6 kg BOD/kg SS d, delivers an effluent of 30 mg/l for both BOD and SS. To achieve nitrification, a considerable multiplication of basin volume and clarifier area would be required. In the densely urbanised Paris area, insufficient space is available for a such an extension. Therefore, new technology for plant upgrading was tested on industrial scale. Biological aerated filters combine aerobic degradation of pollutants with physical retention of suspended solids in one reactor. A high concentration of active biomass can be retained in the packed bed, and nitrifying bacteria can be attached to the filter media. Removal efficiency becomes thus independent of clarification and sludge settling, and ammonia oxidation can be achieved without sludge age requirements. Four parallel units were installed on the Colombes research platform, handling a total flow of 3000 m3/d. An extensive demonstration test program was carried out over a period of five years to assess the feasibility and performances of the process in line with a conventional activated sludge plant. The limits of loading to achieve different residual ammonia concentrations were studied, and the influence of temperature on biological and hydraulic parameters was verified. Backwash requirements and residual values of carbonaceous and suspended matter were explored in dependence on influent values and filtration velocity. At 13 °C, an ammonia load of 0.5 kg N/m3 d was completely oxidized. A concentration of 20 mg/l N-NH4 can thus be totally converted with an empty bed contact time of 1 hour. The Arrhenius temperature coefficient for nitrification was measured as 1.05. Biodegradable carbonaceous and suspended matter was completely removed at filtration velocities higher than 4 m/h, yielding an effluent of less than 5 mg/l for both SS and BOD. Backwash frequency was less than once per day, and a maximum of 5 % of the filter flowrate was used for backwashing.