Field investigations on reactive settling in an intermittent aeration sequencing batch reactor activated sludge process

The overall reactions during settling in an intermittent aeration SBR activated sludge process have been studied on a full scale. Several field investigations were conducted during settling in different seasons. Nitrate, MLSS, phosphate and TOC profiles were obtained at specified depths. Mass balances of nitrogen were made in order to evaluate the effect of different MLSS and seasonal conditions on nitrogen removal during settling. Total nitrogen reduction of more than 30% was achieved during settling by maintaining high MLSS in order of 3000 mg/L. Residual DO plays an important role in causing delay of denitrification for lower MLSS sludge concentration. Phosphorus release was observed in the bottom of the reactor and its occurrence was well coincident with the pH decrease. Denitrification rates and their temperature dependency were determined. Temperature coefficient θ was found to be 1.083 (valid in the range of 17–27°C). Mass transfer limitation during batch sludge settling was discussed. It was found out from batch tests with and without mixing, that denitrification rate was reduced by 40% under mass transfer limiting conditions.