Incineration is one of the major methods for the disposal of sewage sludge. Currently, several plants are incinerating mechanically dewatered (wet) sludge (20-40 wt.% d.m.) or semi-dried sewage sludge (30-55 wt.% d.m.), although some plants burn dry sludge (with more than 80 wt.% d.m.). Whereas significant information is available on NOx and N2O emissions characteristics of wet and dry sludge, not much has been reported on semi-dried sludge. This paper presents some of the results obtained from the combustion of semi-dried sludge in a semi-pilot scale fluidised bed combustor (150 mm in diameter and 9 m high) together with some measurements from a large-scale FBC incineration plant (7 m(2) bed area, 9 m high and a capacity of 3 t/h dry sludge). The investigations have shown that semi-dried sludge exhibit emission characteristics which are similar to those of wet sludge. NOx decreases slightly whereas N2O remains more or less the same with increase in oxygen concentrations. Just like wet sludge, staged combustion was not effective for the reduction of NOx and N2O. However, increasing the freeboard temperature led to rapid reduction of N2O and some NOx reduction was achieved using flue gas recycling technique. Comparison shows that the results from the test rig a ere more or less similar to those obtained from the large-scale plants. (C) 2000 Elsevier Science Ltd. All rights reserved.