- A recently developed and experimentally validated model was used to delineate the effects of various design and operation parameters on the performance of single-sludge systems. Model simulation resulted in charts which illustrate the mutual effects of raw feed COD/ammonia ratio, aerobic volume fraction and recycle rate. The most significant phenomenon, observed experimentally and supported by the simulation, is that increasing recycle may lead to increased effluent nitrate. This situation may occur under low raw COD/ammonia ratios, whereas for high COD/ammonia ratios, as in most domestic wastes, an opposite phenomenon occurs. This means that the recycle has an optimum value which depends on feed composition and on aerobic/anoxic fractionation of the system. The presence of viable cells in the feed in significant numbers affects the composition of biomass in the system and the removal of nitrogen compounds.