Supernatant from settled sow-manure was purified at 10-degrees-C in a bench-scale activated-sludge plant with separate reactors for nitrification (NR) and predenitrification (DR) and recycle of mixed liquor from NR to DR. More than 99% of the ammonia was converted to nitrate. The necessary amount of BOD for full denitrification was measured in relation to the DR/NR ratio (0.25-1) and the recycle factor (6.6 to -30). Full denitrification proved to be impossible with supernatant from manure as the only carbon source under the circumstances tested. Therefore, molasses was used as an extra carbon source. The minimum amount of molasses decreased with increasing DR/NR ratio and decreasing recycle factor. In the range explored the minimum amount of BOD from manure and molasses combined per gramme of reduced nitrate-N was 3.4 g at DR/NR = 1 and a recycle factor of 30.