A wholelife cost and carbon perspective of alternatives to septic tanks utilising nature-based solutions

Septic tank systems (STSs) are a widely utilised treatment flowsheet for decentralised wastewater treatment in the UK. With a growing consensus that STSs have a sizeable detrimental impact on the environment, there is a need for rural flowsheets with improved treatment capabilities. This study examines the lifetime cost and carbon emissions of using an enhanced septic tank nature-based solution (EST-NBS) to improve STS compared to a package treatment system (submerged aerated filter (SAF)). The wholelife cost (WLC) of the flowsheets and Scope 2 cradle-to-grave lifetime carbon emissions (LCEs) of the flowsheets were assessed. The EST-NBS flowsheets represent a lower cost improved treatment system than SAFs at population equivalents (PEs) from 5 to 1,000. An STS averages an LCE of over 4,000 kg CO2eq PE−1, with all other considered flowsheets having lower emissions. The EST-NBS flowsheets had lower carbon emissions than SAFs. Even at low populations upgrading from an STS to an EST-NBS is a competitive abatement strategy, with costs of £260 tCO2eq−1 emissions avoided, at 1,000 PE an NBS flowsheet has an abatement cost of –£17 tCO2eq−1. This shows the potential of using NBS flowsheets in rural wastewater treatment providing both a carbon and cost incentive against traditional designs.