Assessing GHG emissions from sludge treatment and disposal routes: the method behind GESTABoues tool

In 2007, 1 100 000 tons of sewage sludge were produced in France. This figure is constantly increasing and sludges have to be eliminated. Four disposal routes are currently possible: land spreading (directly or after composting), incineration, incineration with household wastes and landfilling. These different disposal routes as well as the sludge treatments produce greenhouse gases (GHG). To help stakeholders to better understand the carbon footprint of sludge treatment and disposal options, we developed a tool called GESTABoues. This paper aims to present the underlying methodology used to quantify material and energy flows as well as GHG emissions all along the sludge treatment and disposal processes implemented in this tool. GHG emissions generated by our system are quantified for x tons of sludge produced by a wastewater treatment plant of x per-captia-equivalents (PCE) during one year. The carbon footprint method we developed is adapted to sludge treatment and disposal processes and based on the "Bilan Carbone®" method. The "Bilan Carbone®" method is a general method used to quantify GHG generated from all physical processes which are necessary for any activity or human organization (ADEME, 2009). In our method, three GHG are recorded: carbon dioxide, methane and nitrous oxide. Biogenic carbon was not taken into account but its sequestration was for two types of disposal routes (land spreading and landfilling). For each process involved in the sludge treatment and disposal routes system, three types of emissions are considered: direct, indirect and avoided emissions. (i) Direct emissions are generated by each process (storage, thickening, anaerobic digestion, composting, land spreading, incineration, incineration with household wastes, landfilling). (ii) Indirect emissions are due to energy and chemical consumptions (combustible or electricity) to operate each process. Transport emissions (for consumables, sludges and ashes) and civil engineering emissions were taken into account. The first ones were calculated for one ton of goods transported on one kilometre (t.km) and the second ones were the toughest to implement in GESTABoues tool. After a literature review, two main methods were identified. Renou (2006) considers that the most applicable methodology is to consider mass of all civil engineering and electrical/mechanical equipments whereas Doka (2007) considers that civil engineering emissions are defined by wastewater treatment plant for 5 classes of plants. We propose an intermediate methodology to assess these emissions : for each process, components (concrete, cast iron, steel…) of involved machineries and buildings were modelled for 3 sizes of wastewater treatment plants (<10 000 PCE, 10 000 – 100 000 PCE, >100 000 PCE). (iii) Avoided emissions are generated when products are not used and replaced by recyclable products (heat, electricity, fertilizer…). GHG data were collected through a literature review for each type of emissions and each process of sludge treatment and disposal routes. All collected data were implemented in GESTABoues, developed with VBA Excel to quantify GHG emissions generated by a wastewater treatment plant of x PCE.