Thermo-economic analysis of a novel cogeneration system for sewage sludge treatment

Wastewater treatment plants are high energy-consuming systems, and their electric energy consumption contributes to 25-30% of the total operating costs. A significant part of thermal energy is needed for management of the sludge produced during the process. Sludge drying by Combined Heat and Power (CHP) systems is attractive to obtain substantial economic and energy savings, especially if all the waste heat can be used. Since biogas production, where available, is insufficient for sludge drying, the direct use of the exhaust gases of a CHP system fuelled with syngas derived from sludge gasification and waste vegetable oil, is proposed. The profitability of this system is comparable with that of the systems currently employed in advanced wastewater treatment plants. The economic convenience derives from the reduction of sludge to be disposed and the overall energy saving in the plant. The simple payback of the proposed system, equal to 6.8 years, is only one year higher than that found for of an analogue conventional system, while the Net Present Value is 30% higher than that calculated for a conventional system. In terms of environmental impact, the layout presented is more efficient as biomass-derived fuels are used instead of fossil fuels.