This work aimed to investigate the influence of sand layer depth on partial nitritation performance as a preparative step for anaerobic ammonium oxidation (anammox) process in treating anaerobically digested effluent of swine wastewater. A lab-scale biological sand filter system was constructed and partial nitritation was successfully maintained with nitrogen loading rate (NLR) of approximately 50 g NH4+-N m(-3) d(-1). An average NH4+-N removal efficiency of 61.34% and conversion efficiency of NH4+-N to NO2--N of 79.77% were achieved with a sand layer depth of 32 cm. An effluent with a NH4+-N concentration of 242.52 mg L-1 and a NO2--N concentration of 306.39 mg L-1 was achieved when the sand layer depth was 32 cm, giving a NO2--N/NH4+-N ratio close to 1.32, as required by anammox. Overall, using a biological sand filter system to treat anaerobically digested effluent of swine wastewater by partial nitritation pretreatment prior to anammox is feasible. (C) 2011 Elsevier Ltd. All rights reserved.