Decentralised Wetland Polishing for Pollutant Load Reduction in River-Dependent Water Supply System

Abstract

Urban water security in Malaysia is closely linked to river systems that function simultaneously as effluent receivers and raw water sources. In river-dependent supply configurations in which treated effluent is discharged upstream of potable abstraction points, concentration-based compliance does not fully represent the cumulative pollutant mass transported downstream. This study evaluates the mass-based pollutant interception performance of a pilot-scale mobile constructed wetland (MCW) system treating a portion of the effluent from a sewage treatment plant (STP) in Selangor, Malaysia. Using measured post-STP and post-MCW treatment concentrations, pollutant loads were quantified in kg/day and annualised under steady-state assumptions. The MCW achieved measurable reductions in total suspended solids (50%), biochemical oxygen demand (62.6%) and ammoniacal nitrogen (90.5%), translating into daily mass interception before river transport. Proportional scaling scenarios indicate that expanded polishing capacity could yield substantial cumulative load reductions upstream of the abstraction reach. Given that the STP discharge occurs upstream of the potable intake, upstream mass interception reduces the cumulative pollutant burden conveyed towards downstream treatment facilities. The findings highlight the analytical distinction between concentration compliance and mass-based management and demonstrate how decentralised polishing can function as an incremental buffering mechanism that enhances resilience within river-dependent urban water systems.