Effect of long retention time in the settler on phosphorus removal from communal wastewater

The objective of the paper is to illustrate the solution of a chronic problem occurring in a wastewater treatment facility with a capacity of 12,000 inhabitant equivalent. A drastic phosphorus concentration increase (two or three times higher than the Hungarian limit of 1 mg/l) was observed in the wastewater treatment facility for a long time (12-24 h) with changing time periods indicating malfunctioning in the operation of the facility.

Computer-aided simulation technique was used to develop a solution for the treatment of the problem using a software developed by the Department of Environmental and Chemical Engineering, University of Veszprém, Hungary.

The simulation studies show that if the nightly inflow is less than 200 m3/h at least for two hours, the system doesn't then receive enough fresh nutrients which can cause a deficiency in the nutrient uptake of the PAOs in the anaerobic zones. This can result in the fact that the PAOs accumulate less phosphorus into the aerobic zones.

Long retention time (10 h) is the reason for the problem, namely--under special conditions--the phosphorus in the sludge of the settling tank of the wastewater treatment plant. The problem was caused by phosphorus dissolution from the sludge of the settling tank during the undesirably long retention time.