Nitrification of high -strength ammonium wastewater using fluidized -bed reactors

Nitrogen contamination of surface waters can lead to adverse ecological and human health effects including depletion of the dissolved oxygen, eutrophication, toxicity to aquatic life, and reduction in the suitability of water for reuse. One of the severe problems facing most wastewater treatment plants is the handling of the high-strength ammonium-polluted wastewater, which is generally produced during anaerobic sludge digestion. The ammonium concentration of the sludge decant, estimated between 800 and 1000 mg N/L before mixing with dewatering wash-water, is about 50 times greater than that usually found in typical municipal wastewater. This high-strength ammonium wastewater, usually returned to the head of the treatment plant, creates a burden on the biological processes and can cause non-compliance with current regulations for ammonium effluent discharge. The use of biological nitrification to remove high ammonium concentrations has not been implemented. In this study, a novel type of fluidized-bed reactor was extensively investigated for nitrification of high-strength ammonium synthetic and actual sludge decants through operation of lab-scale and pilot-scale physical models. The relatively high oxygen demand required for the nitrification of such high ammonium concentration was supplied using an externally coupled tall aeration column. A mathematical model, which fitted the observations of the lab-scale setup quite well, was also developed to understand and optimize the process. The results demonstrated that the fluidized-bed reactor system is capable of handling exceptionally high ammonia concentrations (up to 500 mg NH 4+-N/l) with high removal efficiency (more than 90%). The specific removal rates in this system (up to 2.5 kg NH4 +-N/m3·d) are one order of magnitude higher than that for conventional systems. Such a high-performance system can reduce the supplementary reactor volumes required for nitrification to less than 10% of that needed in conventional activated sludge systems. Complete nitrification can be achieved in the proposed system with a dissolved oxygen concentration as low as 1.0 mg/L. Most of the biomass was retained in the reactor as the volatile suspended solids in the effluent was relatively low (10–25 mg VSS/L). No sludge wasting was needed. Thus, the exclusion of the sedimentation step and solids recycling can lead to great savings. These results clearly point to the potential economic gains to be realized from implementation of this technology