Pulp and paper plants produce large amount of sludge. Environmental problems connected with this waste were not solved satisfactorily in the past. Since land-filling is not a suitable solution from environmental point of view, thermal treatment process proved itself to be the most appropriate solution. A unit for thermal treatment with capacity more than 100 tons of wet sludge per day had been built in one large pulp and paper plant some years ago, located in a mining area. When built, this unit was quite modern. However, because of an increasing number of more and more sweeping environmental laws affecting the process industry, the unit required a complete retrofit. The retrofit has been realised in three stages. The first stage provided mainly means to cut down the emissions below the limits given by environmental legislation. Second stage can be characterised as "waste-to-energy" one. The third stage of retrofit consisted in substituting the currently used fuel by mining gas. The unit for thermal treatment of sludge after retrofit can be considered as one of the most up-to-date incinerators with a waste-to-energy system at present. Economic profitability, positive environmental impact and other beneficial features of the retrofit are clearly shown. (c) 2004 Elsevier Ltd. All rights reserved.