Decentralized greywater treatment and reuse for non-potable purposes in Southeast Asian Nations: Toward sustainable water management

Abstract

Rapid urbanization, population growth, and climate change are intensifying water scarcity across Southeast Asian Nations (ASEAN), while domestic wastewater management remains inadequate in many areas. Greywater (GW), accounting for approximately 50–80% of household wastewater, represents a largely untapped resource for non-potable reuse. Despite increasing interest, decentralized GW treatment in ASEAN remains limited, fragmented, and uneven across countries. This study presents an ASEAN-focused synthesis that integrates regional indicators, such as water availability, per-capita wastewater generation, and treatment coverage, with technological and institutional case studies to support a context-sensitive assessment of decentralized GW treatment and reuse. Decentralized treatment options, including biological, physico-chemical, nature-based, membrane, and hybrid systems, are compared in terms of cost, land requirement, operational complexity, governance feasibility, and reuse suitability across urban, peri-urban, and rural settings. The synthesis shows that while many decentralized GW systems are technically viable and sometimes cost-effective, large-scale adoption is constrained less by treatment efficiency than by regulatory gaps, limited monitoring capacity, institutional fragmentation, and public acceptance challenges. Emerging contaminants, such as surfactants, microplastics, pharmaceuticals, and personal care product residues, pose increasing risks to reuse safety, particularly in dense urban environments, yet remain insufficiently addressed in current system designs and regulatory frameworks. Based on the reviewed evidence, this study identifies priority research needs related to emerging contaminant monitoring, long-term system performance, and health risk assessment. It also outlines pathways for scaling decentralized GW reuse in ASEAN, including fit-for-purpose technology selection, harmonized reuse guidelines, strengthened institutional capacity, and targeted policy incentives.