Greywater recycling as a strategy for water conservation and addressing the water scarcity crisis: A review

Abstract

As global freshwater resources diminish due to climate change and urbanization, the transition from a linear "use-and-dispose" model to a circular water economy is critical. Escalating global water scarcity demands innovative strategies to decouple urban growth from freshwater depletion. Greywater recycling as a critical demand-side management tool for sustainable water conservation. Greywater, defined as domestic wastewater generated from washbasins, showers, and laundry but excluding toilet discharge, constitutes 50–80% of total household effluent. By adopting a "fit-for-purpose" approach, this underutilized resource can be treated and diverted for non-potable applications such as irrigation and toilet flushing, effectively substituting high-quality potable water. The analysis highlights that implementing on-site greywater recycling systems can reduce household freshwater consumption by 30–50%, while simultaneously alleviating hydraulic loads on centralized sewage infrastructure. Furthermore, the paper reviews various treatment technologies ranging from simple physical filtration to advanced biological disinfection required to mitigate health risks and ensure safe recycle. Ultimately, the study concludes that greywater recycling provides essential resilience against drought and infrastructure strain, positioning it as an indispensable component of integrated water resource management and sustainable urban planning in arid and semi-arid regions. This article provides a detailed technical classification of wastewater types, explores advanced and passive greywater recycling methodologies, and presents water quality standards for safe recycle based on WHO and EPA guidelines.