Life cycle assessment of decentralized greywater treatment systems with reuse at different scales in cold regions

Decentralized source-separated wastewater treatment systems offer an attractive alternative to conventional centralized wastewater treatment systems in various regions, yet few system analyses specifically address decentralized greywater treatment over different scales. Here we present a comparative life cycle assessment (LCA) and focus on global warming potential (GWP), eutrophication potential (EUP) and human health - carcinogenic potential (HHCP) of decentralized greywater management systems at different scales for a hypothetical community in a cold (winter) region. To provide a comparison between nature-based and engineered greywater treatment solutions, constructed wetlands (CW) and membrane bioreactors (MBR), respectively, were investigated at three different scales; community (3500 person equivalent [PE]), neighborhood (350 PE) and household (a single household [up to 5 PE]). Conventional centralized wastewater treatment was also included as a business-as-usual (BAU) scenario. In the MBR scenarios, greywater reuse was also considered for multiple non-potable applications due to its high-quality effluent and subsurface garden irrigation was considered for reuse in the CW scenarios. For scenarios with the same treatment technology, larger scales reduced GWP, EUP and HHCP up to 57 kg CO2-eq.PE-1.y(-1), 0.2 kg N-eq.PE-1.y(-1) and 5.3E-6 CTUh.PE-1.y(-1), respectively, despite the need for more extensive wastewater networks. The CW scenarios at community and neighborhood scales outperformed the MBR and BAU scenarios for greywater treatment, while the community-scale MBR scenario may be environmentally preferable when large amount of greywater can be reused. The scale of decentralized systems, quantity of water reused and mix of electricity technologies all played important roles in determining GWP, EUP and HHCP values.