Comparative LCA of sewage sludge valorisation as both fuel and raw material substitute in clinker production

A life cycle assessment to evaluate the environmental impact of urban sewage sludge use as alternative fuel or raw material in clinker production was carried out. In order to quantify in detail the overall environmental impact of both scenarios, the sewage sludge treatment process and the transport to cement facilities for both alternatives were considered. The substitution ratio of fuel (petcoke) and raw material (limestone) was fixed between 5 and 15% according to the cement production plant limitations. Both scenarios show CO2 savings when compared to the clinker production without substitution. The mid-point and end-point analysis were favourable to the fuel substitution with savings ranging from 3 to 7% compared to the raw material substitution and also to base case without substitution. The influence of the amount of sewage sludge used for both scenarios indicates that fuel substitution reduced the CO2 emissions when the amount of substitution is increased, while other mid-point and end-point categories were proportionally favourable to the fuel substitution scenario. Additionally, the influence of the substituted material characteristics showed that low heating value (fuel substitution) and CaO addition in lime stabilized sludge (raw material substitution) are critical parameters in terms of environmental impact in clinker production. The fuel substitution represents a significant environmental improvement compared to the raw material substitution scenario and clinker production without substitution.