This paper presents a quantitative investigation and analysis of anammox activity in sludge taken from biological swine wastewater treatment plants. An incubation experiment using a (15)N tracer technique showed anammox activity in sludge taken from 6 out of 13 plants with the rate ranging from 0.0036 mu mol-N(2)/g-VSS/h to 3.1 mu mol-N(2)/g-VSS/h, and in a biofilm with the highest activity at 25.8 mu mol-N(2)/g-VSS/h. It is notable that 9 out of 11 sludges in which the pH was maintained between 6.6 and 8.1 retained anammox activity, while those with either a lower or higher pH did not. Moreover, anammox-positive sludge had a significantly higher concentration of NO(2)(-)-N plus NO(3)(-)-N than did anammox-negative sludge. A significant difference was not observed between anammox-positive and negative sludge regarding BOD/NH(4)(+)-N in the influent, DO concentration in aeration tanks, and the concentrations of NH(4)(+)-N, free nitric acid, and free ammonia. (C) 2009 Elsevier Ltd. All rights reserved.