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3.3.4        Energy generation

Although UWWTPs use significant amounts of energy, the waste water theoretically contains between five and 10 times more chemical and thermal energy than needed for treatment. While only some of that energy can be recovered, it is possible for UWWTPs to be net energy producers (Riley et al, 2020).

Energy recovery of the chemical, thermal and hydrodynamic energy contained in sewage can provide electricity, biogas, steam and hot water. The energy content of sludge is typically similar to the energy content of low-grade coal (Stone et al. 2010). Energy can be generated from sewage sludge through pre-treatment by anaerobic digestion to produce biogas, and/or by incineration of the sludge. Biogas comprises of 60 – 70 % methane, and 30 – 40 % of carbon dioxide, trace amounts of other gases (e.g. hydrogen, hydrogen sulphide and nitrogen).  Recovery of methane from biogas allows use in applications such as gas engines, electricity and/or heat. Serious accidents can occur with biogas generation (e.g. ) and the ability to meet necessary safety standards as part of the transition to greater sustainability is an important consideration.

In a study considering the potential for sludge currently landfilled in the EU-27, it was estimated that 7 - 13 % of UWWT energy needs (1 800 - 3 200 GWh[1]) could be met if that sludge were pre-treated by anaerobic digestion, although data were hard to come by (Anderson et al, 2021). This value represents an upper limit, as it would not be feasible to implement anaerobic digestion at all UWWTPs.

Most focus on energy generation from UWWT has been on anaerobic digestion and incineration, but operators may have other options. For instance, solar and wind energy generation, and heat recovered from the waste water itself. Solar panels installed on the roofs of Viikinmäki UWWTP in Finland generate c.210 MWh per year, or approximately 0.5 % of annual electricity use. Heat recovery from effluent waste water shows can be a much more significant source of energy, with potential heat energy recovery at up to 500 % of a plant’s heat energy consumption (Heinonen, pers. comm.). Actual solutions for a particular plant depend on local circumstances. For instance, incineration facilities may not be available, while biogas generation may not be economic at some smaller sites.