GESTABoues, a decision tool to assess greenhouse gases of sewage sludge treatment and disposal routes

Sewage sludge production increases continuously reaching almost 20% (946 700 tons of dry matters in 2003 to 1 118 795 tons of dry matters in 2007) during the last decade. In 2007, 70% of the produced sewage sludge was spread (directly or after composting). The remaining 30% was incinerated (with or without household wastes) or landfilled. Nowadays, sludge reduction is a major concern. This activity has to become more sustainable and stakeholders have to be careful to the environmental impacts of sludge treatment and disposal routes. To help stakeholders in that way, we developed a decision tool called GESTABoues. GESTABoues is a tool based on the "Bilan Carbone®" method (ADEME 2009). It was developed for stakeholders dealing with wastewater treatment plants (plant manager, public administration…) to quantify greenhouse gases (GHG) for each type of emissions and each process of sludge treatment and disposal routes. The tool was developed with VisualBasic programming language. This tool can be used in a four step procedure: (i) The user built as many wastewater treatment plants (WWTP) he wants. He should specify the WWTP capacity, the type of sewage network and the water treatment system. (ii) Then the user creates all the sewage sludge treatment processes used for each WWTP. He has to choose different parameters for each step of sludge treatment and disposal. The user has the opportunity to choose either its own data or data collected through a literature review and implemented as "default value" in the tool. (iii) Once each WWTP and the treatment and disposal routes of each WWTP created, the user can choose different graphic presentations to assess the impact of greenhouse gases emissions (Figure 1). GESTABoues calculate all emissions (direct and indirect) of carbon dioxide, methane and nitrous oxide for all steps of sewage sludge treatment (storage, thickening, anaerobic digestion, aerobic digestion, dewatering, alkaline stabilization, composting, drying) and sludge disposal route (land application, incineration, incineration with household wastes, landfilling). Emissions from infrastructures and transports are also considered. These graphs present emissions for each step (thickening, dewatering, land spreading…), each gas (dioxide carbon, methane and nitrous oxide) and each origin of greenhouse gases (combustible, electricity, direct emissions, avoided emissions, infrastructure, chemicals and transport). The graphs can be presented either as values or percentages. (iv) Finally, the user can compare different sewage sludge treatment processes and disposal routes options and create reports with Microsoft Word through an export button. Each report summarizes the mass and energy balance as well as the selected graphs to be exported. In this study, this tool is used to compare 3 systems on a same WWTP to help stakeholders to identify which processes have the worst environmental impact all along the treatment and disposal route, which emission is overwhelming and to help them selected the most interesting system from an environmental point of view.