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An aerated waste stabilisation pond treating domestic effluent (15,000 inhabitant‐equivalents) after primary treatment was investigated from January 1994 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 stratification. During the studied period, the pond provided satisfactory primary and secondary treatments meeting most of the time the European requirements for COD, BOD5 and Total Suspended Solids. COD and BOD5 removal rates varied from 44 to 70% and from 69 to 81%, respectively. Decrease in germ density always reached more than 93%. Ammonium and soluble reactive phosphorus removal rates were most of the time lower than 40%. The lack of a significant nutrient removal rate was notably linked to the low phytoplankton development and to the globally low level of nitrification in the pond due to anoxic conditions. Moreover, a dense population of the large and efficient grazer Daphnia magna counteracted the ammonium and the soluble reactive phosphorus removal through feeding and remineralization of a significant part of the nitrogen and phosphorus assimilated by algae and bacteria. The benefits of an intensive harvesting of the zooplankton are discussed in relation to waste water treatment performance and to the potential commercial value of the harvested biomass.

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