Akaligenes faecalis strain No. 4 which is capable of heterogeneous nitrification and aerobic denitrification, was used to remove high-strength ammonium (approximately 1 g NH4+ -N/1) from digested sludge, the product of an anaerobic digestion reactor, in which methane was produced from excess municipal sewage sludge. Repeated batch operations were conducted at 20 degrees C and 30 degrees C for 550 h, using a jar fermentor. The removal ratios of high-strength ammonium reached 90-100% within 24 h, and the average ammonium removal rate was 2.9 kg-N/m(3)/day, more than 200 times higher than that in conventional nitrification denitrification processes. During these operations, the cell density was maintained at 10(8)-10(9) cells of A. faecalis strain No. 4/ml. At 3% NaCI in the digested sludge, strain No. 4 exhibited an ammonium removal rate of 3 kg-N/m(3)/day. (C) 2013, The Society for Biotechnology, Japan. All rights reserved.