Integrated management of municipal solid waste and sewage sludge: Optimization criteria

In many countries, lack of areas for municipal solid waste (MSW) landfills has been increasing the cost of this option. Moreover, problems related to biogas collection and leachate treatment suggest minimization of putrescible material to be landfilled. Therefore, a number of alternative technologies considered anti-economical in the past, deserve to be reconsidered. Target of this paper is to provide optimization criteria of overall energetic recovery from anaerobic digestion of sewage sludge and municipal solid waste organic fraction (MSWOF) combined with incineration of the remaining fractions. This work is based on the hypotesis that MSWOF is obtained by separate collection (SC). Therefore, an assessment of the extent to which SC affects potential energy recovery and emissions from incineration processes is also performed. Different technologies for energy production are compared, with special focus on co-generation (electric power and heat). Separate anaerobic digestion of MSWOF and sludge is compared with cc-digestion in terms of biogas generation, and possible modifications of existing sludge digesters are evaluated. Optimization of MSW water balance positively affects co-digestion, since separation of the OF (higher umidity) favours both incineration and anaerobic digestion processes by increasing the combustible fraction's lower heating value (LHV) and improving mixing rates repectively. Gross evaluation of environmental indicators, such as CO2 emissions due to biogas and solid waste combustion, are compared to the case of all MSW landfilled. Economical analyses (based on current Italian costs) are also performed considering the following options: co-digestion of MSWOF and incineration of the remaining fractions, separate sludge/MSWOF digestion and incineration, sludge digestion and MSW incineration, sludge digestion and MSW landfill disposal. Feasibility of the different options is evaluated considering the city of Milan as a case study. Peculiarity of this city is the absence of a centralized wastewater treatment plant and the lack of substantial alternatives to MSW landfilling. This situation stimulates treatment/disposal planning activities with little constraints.