Partial nitrification in a high-load activated sludge system by biofilter backwash water recirculation.

The South-Budapest Municipal Wastewater Treatment Plant (SBWWTP) based on the high-load activated sludge process (AS) was upgraded into nutrient removal in 1998-1999 in Hungary. Biofor type nitrifying (NP) and denitrifying (DN) biofilters were implemented for nitrogen removal downstream of the (AS) stage in order to meet the required effluent standards characterized by 50 mgCOD l(-1), 10 mgBOI5 l(-1), 10 mgTN l(-1), 1 mgTP l(-1) and 35 mgTSS l(-1). Phosphorus removal is obtained by chemical addition. The study presented sums up the performance of the upgraded plant throughout 25 months. Besides the efficient pre-denitrification obtained in the AS basin, significant ammonium oxidation was observed in the aerated zone, upon the introduction of recirculation of nitrate rich water and backwash water. The introduction of biofilter backwash water, containing nitrifying biomass, has generated significant ammonium conversion in the aerated basins, where nitrification was observed previously to a very limited extent. Process parameters of the AS stage were: aerobic sludge age of 1.5-2.0 days, 1.5 hours hydraulic retention time, specific sludge load of 1.1 kgCOD kgMLSS(-1) d(-1). The nitrification in the AS tank has significantly reduced below 16 degrees C, however, denitrification efficiency was not as highly dependent upon temperature during the investigated period. The paper investigates the nitrogen balance of the plant in two aspects: (i) the effects of seeding on nitrification in the high load AS reactor and (ii) pre-denitrification potential in the anoxic zone of the AS tank of the pre-settled wastewater.