Performance of a single-cell aerated waste stabilisation pond treating domestic wastewater: A three-year study

An aerated waste stabilisation pond treating domestic effluent (15,000 inha bitant-equivalents) after primary treatment was investigated from January 1 994 to December 1996 for hydraulic regime, performance in nutrient removal, decrease in oxygen demand and microbial pollution. The pond has a complete mixed hydraulic regime with virtually no short-circuiting or stratificatio n. During the studied period, the pond provided satisfactory primary and se condary treatments meeting most of the time the European requirements for G OD, BOD, and Total Suspended Solids. COD and BOD, removal rates varied from 44 to 70% and from 69 to 81%, respectively. Decrease in germ density alway s reached more than 93%. Ammonium and soluble reactive phosphorus removal r ates were most of the time lower than 40%. The lack of a significant nutrie nt removal rate was notably linked to the low phytoplankton development and to the globally low level of nitrification in the pond due to anoxic condi tions. Moreover, a dense population of the large and efficient grazer Daphn ia magna counteracted the ammonium and the soluble reactive phosphorus remo val through feeding and remineralization of a significant part of the nitro gen and phosphorus assimilated by algae and bacteria. The benefits of an in tensive harvesting of the zooplankton are discussed in relation to waste wa ter treatment performance and to the potential commercial value of the harv ested biomass.