The treatment of the sludge digester effluent taken from a full-scale municipal wastewater treatment plant (WWTP), in Istanbul, Turkey using a pilot-scale partial nitritation (PN)/anammox (A) reactor was examined and maximum nitrogen removal rate (NRR) of 0.65 kg N/m(3)/d was achieved in this study. The PN, which was carried out in a continuously stirred tank reactor, successfully converted 56.4 +/- 4.8% of NH4+-N to NO2--N. The effluent NO3--N concentration kept below 1 mg/L with 0.4-0.8 mg/L of dissolved oxygen (DO) and pH of 7.2 +/- 0.1 at 25 +/- 3.3 degrees C. Partially nitrified sludge digester effluent was further treated in a continuously stirred tank (CSTAn) anammox reactor, where high ammonium removal efficiency (ARE) achieved with a NO2--N:NH4+-N and NO3--N:NH4+-N ratios of 1.28 +/- 0.06:1 and 0.29 +/- 0.07:1, respectively. The results show that ARE up to 94 +/- 0.05% and total nitrogen removal of 75.6 +/- 12.6% were achieved. This study also illustrated that in CSTAn reactor, better NRR achieved when pH and DO were 7.72 +/- 0.04; 0.02 +/- 0.02 mg/L, respectively. However, the NRR was tending to rise below 10 mg/L of NO2--N. The biomass concentration in the CSTAn reactor was 1075 +/- 330 g VSS/L with the SAA 0.38 +/- 0.06 kgN/kgVSS/d in the whole study. The anammox activity increased exponentially and correlated with the observed growth in volatile suspended solids.