A CONSTRUCTED WETLAND DESIGNED TO TREAT SECONDARY EFFLUENT: A REVIEW

Conventional secondary treatment reliably reduces biodegradable organic matter and suspended solids, but the effluent it produces still carries residual organics, nutrients, pathogens and trace organic contaminants at concentrations that frequently exceed the limits imposed for discharge into sensitive water bodies or for non-potable reuse. Constructed wetlands (CWs) have emerged as a robust, low-energy tertiary polishing option, particularly for small and medium communities, decentralised campuses and industrial estates where the capital and operating burden of mechanical tertiary units is difficult to sustain. This paper reviews the design basis, removal mechanisms, reported performance and operational constraints of constructed wetlands applied specifically to secondary effluent polishing. Free water surface (FWS), horizontal sub-surface flow (HSSF), vertical sub-surface flow (VSSF) and hybrid configurations are compared with respect to oxygen transfer, footprint, clogging susceptibility and pollutant-specific efficiency. Design methodologies are examined, from empirical areal loading rules to the first-order k–C* and tanks-in-series P–k–C* formulations, together with media selection, vegetation and hydraulic design. Performance data compiled from the literature indicate typical polishing efficiencies of 76–92% for BOD5, 68–86% for COD, 79–91% for TSS, 48–74% for total nitrogen and 42–62% for total phosphorus, with hybrid systems consistently outperforming single-stage beds for nitrogen. Temperature, hydraulic loading rate and hydraulic efficiency are identified as the dominant performance controls, while progressive clogging of the inlet zone remains the principal cause of long-term failure in sub-surface systems. An illustrative design example for a 500 m³/d secondary effluent stream is presented. The review concludes that constructed wetlands are technically sound and economically attractive polishing units, provided that loading rates are conservative, hydraulic short-circuiting is controlled and phosphorus retention is supported by reactive media.