Carbon footprint and energy footprint of activated sludge wastewater treatment plants

A novel model was developed and applied in this study to estimate carbon- and energy- footprint of wastewater treatment plants using activated sludge process (ASP) at variable sludge retention time (SRT) (in the range 2-50 d), temperature (10, 20 and 30 °C), and type of sludge digestion (aerobic digestion AD and anaerobic digestion AnaD). The reduction in CO2eq emission for unit of influent organic matter (kgCO2/kg bCOD) using low SRT (at 20°C) and AnaD was estimated between 33% (at SRT=2 d) and 22% (at SRT = 50 d) if AnaD is used instead of AD. Energy recovered from biogas when AnaD is used, is reduced from about 0.9 to 0.7 kWh/kg bCOD increasing SRT but the presence of primary sedimentation limit such reduction. Neverteless, when AnaD is used, energy which can be recovered from biogas is higher than energy demand required, regardless the value of the SRT. Specific energy required from combinations with AD range from about 0.7 to 0.88 kWh/kg bCOD with significant effect of ASP’s temperature. This analysis does not address restrictions on terminal biosolids disposal. 1