Nitrogen control in AO process with recirculation of solubilized excess sludge

In order to establish a sludgeless process with a nitrogen-controlled effluent, batch and continuous experiments in a lab scale anoxic–oxic (AO) process were carried out to investigate the possibility of ozonized sludge (OS) usage as a denitrification energy source. Through ozonation at an ozone dose of 1.2 g O3/g MLVSS, 63.2% of treated MLVSS was solubilized, 12.7% of COD was lost (probably due to complete oxidation to CO2), and soluble COD/TN ratio of OS appeared to be only about 10.78 because ozonation released cellular proteins and other nitrogenous substances. In oxic conditions, incubation of OS supernatant with activated sludge generated nitrate without significant ammonia accumulation, which meant that rapid nitrification occurred following ammonia generation from heterotrophic degradation of nitrogen-bearing cellular substances. In anoxic conditions, externally supplied nitrate was removed at the expense of organic carbons in the OS supernatant. However, ammonia was accumulated as anoxic incubation proceeded probably because of heterotrophic degradation of nitrogenous materials as in oxic conditions. Thus it was appeared that solubilized excess sludge acted as a reducing power for denitrification but also as a nitrogen source. In addition, 24–41% of COD contained in OS supernatant were found to be consumed for denitrification. But the remaining COD was not assimilated further even in the presence of nitrate. It was concluded by a nitrogen balance analysis that the energy source contained in OS was not sufficient to completely reduce the nitrogen that was originated from OS itself to nitrogen gas.