Comparative LCA of decentralized wastewater treatment alternatives for non-potable urban reuse

Municipal wastewater (WW) effluent represents a reliable and significant source for reclaimed water, very much needed nowadays. Water reclamation and reuse has become an attractive option for conserving and extending available water sources. The decentralized approach to domestic WW treatment benefits from the advantages of source separation, which makes available simple small-scale systems and on-site reuse, which can be constructed on a short time schedule and occasionally upgraded with new technological developments. In this study we perform a Life Cycle Assessment to compare between the environmental impacts of four alternatives for a hypothetical city's water-wastewater service system. The baseline alternative is the most common, centralized approach for WW treatment, in which WW is conveyed to and treated in a large wastewater treatment plant (WWTP) and is then discharged to a stream. The three alternatives represent different scales of distribution of the WW treatment phase, along with urban irrigation and domestic non-potable water reuse (toilet flushing). The first alternative includes centralized treatment at a WWTP, with part of the reclaimed WW (RWW) supplied back to the urban consumers. The second and third alternatives implement de-centralized greywater (GW) treatment with local reuse, one at cluster level (320 households) and one at building level (40 households). Life cycle impact assessment results show a consistent disadvantage of the prevailing centralized approach under local conditions in Israel, where seawater desalination is the marginal source of water supply. The alternative of source separation and GW reuse at cluster level seems to be the most preferable one, though its environmental performance is only slightly better than GW reuse at building level. Centralized WW treatment with urban reuse of WWTP effluents is not advantageous over decentralized treatment of GW because the supply of RWW back to consumers is very costly in materials and energy. Electricity is a major driver of the impacts in most categories, pertaining mostly to potable water production and supply. Infrastructure was found to have a notable effect on metal depletion, human toxicity and freshwater and marine ecotoxicity. Sensitivity to major model parameters was analyzed. A shift to a larger share of renewable energy sources in the electricity mix results in a dramatic improvement in most impact categories. Switching to a mix of water sources, rather than the marginal source, leads to a significant reduction in most impacts. It is concluded that under the conditions tested, a decentralized approach to urban wastewater management is environmentally preferable to the common centralized system. It is worth exploring such options under different conditions as well, in cases which new urban infrastructure is planned or replacement of old infrastructure is required.