During anaerobic digestion of biosolids, the nitrogen fraction in the reduced organic matter is converted to soluble ammonium, which can reach concentrations of 1000 mg N/L (before mixing with wash water) in the sludge dewatering discharges. A pilot-scale fluidized-bed reactor with
a novel external aeration loop was used for biological nitrification of anaerobically digested sludge decant. Due to the extensive aeration requirement for nitrification of such high ammonium concentrations, the aeration column was designed to operate under pressure in order to increase the
oxygen transfer efficiency. The results demonstrated that the system is capable of handling ammonium removal rates of up to 2.25 kg NH4+-N/m3.d, while removal efficiencies were as high as 90%. Ammonium loading rates higher than 2.25 kg NH4-N/m3.d
resulted in decreased ammonium removal efficiency while removal rates did not exceed 2.5 kg NH4+-N/m3.d. The high specific removal rate in this system is higher than other fixed-film systems and is one order of magnitude higher than that of suspended-growth
systems. This can reduce the supplementary reactor volumes required for nitrification to less than 10% of that needed in conventional activated sludge systems. These results clearly indicate the potential economic gains that could be achieved through implementation of this technology.
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