Optimisation of nitrification-denitrification process in a sbr for the treatment of reject water via nitrite

An optimal sequencing batch reactor (SBR) strategy is proposed for Biological Nitrogen Removal (BNR) via nitrite of reject water (800-900 NH4+-N mg l(-1)) from mesophilic (35 degrees C) anaerobic sludge digester of a Spanish Municipal Wastewater Treatment Plant (WWTP). Two lab-scale SBR with control of temperature were studied with external COD addition for denitrification which was necessary due to the lack of readily biodegradable carbon source. Process kinetics were compared through the specific Ammonium Uptake Rate (sAUR) finding the appropriate operational sequences when working at 32 degrees C and 8 hour cycle length. Every operating cycle was carried out with a sludge retention time of 11 days, hydraulic retention time around 1 day and 2500+/-250 mg VSS l(-1). In order to avoid nitrate formation and thus save costs, the oxygen concentration was maintained below 1 mg V1 during aerobic periods and pH remained within an optimal range (7.5-9) alternating different aerobic-anoxic subcycles inside the operational cycle. With this strategy, the range of alkalinity could be controlled avoiding the addition of external additives and nitrite accumulation was prevented. Therefore, the reached sAUR was 22 mg NH4+-N g)-1) VSS h(-1) and the total nitrogen removal was 0.8 kg N(d m(3))(-1).