TECHNICAL AND ECONOMICAL EVALUATION OF A THERMAL, AND TWO OXIDATIVE TECHNIQUES FOR THE REDUCTION OF EXCESS SLUDGE PRODUCTION E. PAUL 1 , P. CAMACHO 2 , M. SPERANDIO 1 and P. GINESTET 2 1 INSA-Toulouse, Laboratory of Environmental Process Engineering (LIPE), Toulouse, France 2 Suez Environnement - Dore, Cirsee, Le Pecq, France M unicipal wastewater treatment is closely linked with the production of sludge. This production may be reduced by combining a sludge disintegration technique with a conventional biological reactor, such as an activated sludge. The disintegration techniques are based on mechanical, electrical, thermal, thermo-chemical, biological and oxidative treatments. This work tries to give an evaluation of the efficiency and the economics of disintegration techniques that have demonstrated their potential to reduce the excess sludge production when combined to an activated sludge plant fed with a real urban wastewater. A high ESP reduction rate is achievable (more than 40%) by using disintegration technique such as a thermal (958C), an ozonation or a hydrogen peroxide treatment. These three dis- integration techniques were studied in details in our laboratories. The experiments carried out provided the technical data in order to make an assessment of the economics of the three selected combined processes. The ozone appeared to be the most interesting route for sludge reduction but the heating route is also economically competitive with conventional sludge treatment and disposal especially if stringent constraints on the sanitary quality of the sludge are imposed. Keywords: biological wastewater treatment; excess sludge reduction; thermal and oxidative treatments; economics. INTRODUCTION In Europe, major ways of disposing excess sludge from wastewater treatment plants are subject to more and more legal and social constraints. Moreover, disposal of the excess sludge production (ESP) may account for up to 60% of total plant operating costs. Therefore, reducing ESP instead of merely treating it appears to be a very appealing solution to this social and economic issue since the problem would be treated at its roots. Processes can be applied in combination with the existing biological treatments to reduce ESP. The chosen combined process aimed at sludge disintegration can be placed either on the wastewater treatment train or in the sludge treatment train (e.g., combined to anaerobic digestion). In all cases, treated sludge is sent back to a biological reactor for further degradation of the organic material. Thus, the use of the dis- integration techniques mainly aims at improving the sludge COD biodegradability while solubilizing the sludge mineral matter. A scheme of the different configurations that can be applied in a wastewater treatment plant is given by Paul and Salhi (2003). The disintegration techniques already studied are based on mechanical, electrical, thermal, thermo- chemical, biological and oxidative techniques. Various review papers (Weemaes and Verstraete, 1998; Liu and Tay, 2001) described the potential use of these very different techniques for solubilizing the sludge COD but there is a lack of data on the real performances in terms of reduction of ESP for systems fed with real wastewaters and on the process economics (Weemaes and Verstraete, 1998). In this paper, considering only techniques that are placed on the wastewater treatment train and so combined with an activated sludge reactor (techniques used to pre-treat sludge prior to anaerobic digestion are not considered in this paper), we underline what the objectives and constraints for a combined disintegration technique should be and also the main results obtained from applying these types of treatment. Following this, we select three disintegration techniques that are able to reach more than 40% reduction of the ESP when combined to the activated sludge reactor working on real wastewater. These techniques are oxidation by ozone, oxidation by hydrogen peroxide at 958C and thermal treatment at 958C. Performances of the whole  Correspondence to: Professor E. Paul, INSA-Toulouse, Laboratory of Environmental Process Engineering (LIPE), 135 Avenue de Rangueil, 31077 Toulouse Cedex 4, France. E-mail: paul@insa-toulouse.fr 247 0957–5820/06/$30.00+0.00 # 2006 Institution of Chemical Engineers www.icheme.org/psep Trans IChemE, Part B, July 2006 doi: 10.1205/psep.05207 Process Safety and Environmental Protection, 84(B4): 247–252