In this study, the effect of the pretreatment of NH(4)-N by struvite precipitation on biological nitrogen removal was investigated in treating swine wastewater. Evaluation was mainly focused on nitrification which occurred in the activated sludge system after struvite precipitation. Laboratory experiments were performed at four different hydraulic retention times (HRT), i.e., 48, 32,24 and 16 h. Results of the long-term operation of systems showed that the struvite precipitation used as the pretreatment of raw swine wastewater enhanced the nitrification performance in activated sludge system by reducing the applied loading rates of NH(4)-N and TCOD in all operating conditions. The reduction of the applied NH(4)-N loading rate kept the levels of free ammonia (FA) concentration in biological reactors low and it prevented nitrite accumulation. In addition, the struvite precipitation elicited the biological denitrification reaction and PO(4)-P removal by increasing the ratios of carbon-to-nitrogen and carbon-to-phosphorus of wastewater after struvite precipitation. The struvite precipitation also enhanced the biological TCOD removal performance by reducing the toxic effect of FA. Triplicate INT-dehydrogenase tests clearly showed that FA inhibited the degradation of organic matter in activated sludge system. Finally, the struvite precipitation contributed to high TCOD, T-N and PO(4)-P removals of 83, 90, and 97% by facilitating biological reaction at a short HRT of 16 h. (C) 2009 Elsevier Ltd. All rights reserved.