Utilisation of waste-water sludge is one of the most difficult processes of environment protection because of the high moisture and contents of harmful substances. The waste-water treatment systems in Poland, especially in small towns, are often not separate from industrial sewage systems and this causes relatively large contents of heavy metals and other toxic substances in the sludge. Incineration is the only effective method of utilisation of such waste material. From the other side, coal is still the most popular fuel in Poland, and it is often used in many boilers producing hot water for the central heating of buildings or parts of the towns. Mostly they are stoker fired boilers of old construction, and they need urgent modernisation. The use of stoker-fired boilers for incineration of the sludge in small towns is the object of the analysis presented in this paper. There are two important points in the co-combustion of sludge with coal in a stoker-fired boilers: ecological and exploitation requirements. The important restriction of the sludge/coal ratio is the emission of harmful substances with the flue gas. There are substances originating from the components of the sludge: SO2, NOx, HCl, heavy metals and dust. Their concentrations in the flue gas should meet the environmental regulations. The other factor influencing the co-combustion process is the change of physical and thermal properties of the fuel: heating value, moisture content and ash composition. These influence the thermal output of the boiler, the amount of air required for combustion, the volume of flue gases and dust concentration and particle distribution. In this paper, the results of experiments performed with an experimental boiler are presented. The effects of the following parameters are considered: composition and thermal parameters of the sludge and their change during the year, emissions of SO2, NOx, CO and dust from the experimental boiler for various compositions of the fuel (sludge/coal ratio, moisture content). As a result of the analysis, the parameters limiting the amount of sludge in a mixture with coal are identified. (C) 2003 Elsevier Science Ltd. All rights reserved.