A systematic review of greywater properties and variability that govern decentralised off-grid treatment feasibility

Greywater reuse is increasingly promoted to enhance water security in decentralised off-grid systems; however, treatment sustainability remains constrained by pronounced variability in influent characteristics. Existing reviews predominantly assess treatment technologies by pollutant removal performance, offering limited guidance for technology selection under variable operating conditions. This PRISMA 2020-guided systematic narrative review synthesised evidence from 131 empirical studies (2016–2026) to evaluate how source-specific greywater variability governs treatment feasibility in decentralised off-grid systems. The evidence revealed substantial heterogeneity in greywater quality, with COD ranging from 100 to 2,000 mg L⁻¹, TSS from 30 to 1,000 mg L⁻¹, anionic surfactants frequently exceeding 100 mg L⁻¹ during laundry discharge, and microbial indicators commonly exceeding recommended reuse thresholds. Methodological appraisal showed that 41% of the included studies were of high robustness, 38% moderate robustness, and 21% limited robustness. Across the literature, temporal variability, household behaviour, source mixing, and storage-induced transformations consistently emerged as the principal drivers of influent variability, influencing fouling propensity, biological treatment efficiency, operational robustness, maintenance requirements, energy demand, and health risk. Building on these findings, this review develops a Property-Driven Framework that translates greywater characteristics into engineering constraints and preliminary treatment feasibility criteria, enabling systematic screening of technically compatible treatment options before detailed process selection. The proposed framework shifts technology assessment from pollutant removal performance towards variability-informed feasibility, providing an evidence-based foundation for resilient, sustainable, and context-appropriate decentralised greywater treatment.