CO2-neutral wastewater treatment plants or robust, climate-friendly wastewater management? A systems perspective

•The energy efficiency of three wastewater treatment configurations are compared.•Decentralized heat recovery from greywater has a higher energy saving potential.•With heat recovery, wastewater temperature may decrease substantially in winter.•Robustness of configurations is thus more important than energy efficiency.•Nutrient recovery from urine is robust at low temperatures and climate-friendly.

CO2-neutral wastewater treatment plants can be obtained by improving the recovery of internal wastewater energy resources (COD, nutrients, energy) and reducing energy demand as well as direct emissions of the greenhouse gases N2O and CH4. Climate-friendly wastewater management also includes the management of the heat resource, which is most efficiently recovered at the household level, and robust wastewater management must be able to cope with a possible resulting temperature decrease. At the treatment plant there is a substantial energy optimization potential, both from improving electromechanical devices and sludge treatment as well as through the implementation of more energy-efficient processes like the mainstream anammox process or nutrient recovery from urine. Whether CO2 neutrality can be achieved depends not only on the actual net electricity production, but also on the type of electricity replaced: the cleaner the marginal electricity the more difficult to compensate for the direct emissions, which can be substantial, depending on the stability of the biological processes. It is possible to combine heat recovery at the household scale and nutrient recovery from urine, which both have a large potential to improve the climate friendliness of wastewater management.

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