Minimax-Based Evaluation of Layered Natural and Waste-Derived Media Filtration for On-Site Greywater Treatment and Reuse

Abstract

Passive filtration through granular and fibrous media is the treatment route most often proposed for household-scale greywater reuse, and a recurring design proposition holds that arranging complementary media in layers delivers performance beyond that of the constituent media. This review tests that proposition rather than restating it. A structured search of an open scholarly index and the Crossref registry, executed without a relevance ceiling or date restriction and supplemented by backward and forward citation chasing of every included primary study, returned 16,879 records that reduced to 9,566 unique records after deduplication. Two-stage screening yielded 73 primary studies, classified into four evidence tiers by matrix and system type and into seven comparator types according to the causal effect each design can estimate. The central finding is negative and specific: no study compares a layered configuration with its constituent media at matched bed depth, media mass and contact time, so the benefit attributable to layering, as distinct from added depth and residence time, has never been estimated. What the evidence does support is narrower: adding an adsorptive carbonaceous stage to a straining bed improves removal of dissolved organics, surfactants and micropollutants, consistently in direction across independent groups; substituting a pyrogenic carbon for sand improves organic and, more modestly, microbial removal; and pyrolysis rather than botanical origin accounts for most of the performance of lignocellulosic residues. Against this, mixing media degraded most endpoints in the one study that tested mixtures, and several organic media leach dissolved organic carbon. Loading effects are endpoint-specific: organic removal is robust in generously sized beds, whereas microbial removal declines consistently as loading rises. Faecal-indicator reductions span roughly 0 to 3 log₁₀ in granular beds, viral data are sparse, and risk-based studies find residual viral risk after filtration alone. Media filtration should therefore be specified as one barrier within a multiple barrier train, and the depth-matched factorial experiment required to resolve the field's central design claim is set out.