Constructed wetlands and denitrifying bioreactors for on-site and decentralised wastewater treatment: Comparison of five alternative configurations

Abstract

The performance of five alternative treatment trains receiving primary domestic wastewaters was compared in side-by-side trials over an annual period after 2 years maturation. One of the systems was a passive horizontal-flow wetland. The other four hybrid systems comprised different configurations of subsurface horizontal- and/or intermittently-dosed (24/d) vertical-flow constructed wetlands (single pass or recirculating), and attached growth and carbonaceous (wood-chip and coconut husk) denitrifying bioreactors. The scale of the systems ranged from ∼20 to 40% of that required for a single household. Mean biochemical oxygen demand (BOD5) and total suspended solids (TSS) were reduced by over 94% in all systems. Mean ammonium-N (NH4-N) concentrations were reduced to between 0.05 and 0.20gm−3 (98–99.8% reduction) in the hybrid wetland and bioreactor systems, compared to ∼13gm−3 (61% reduction) in the horizontal-flow wetland. Mean total nitrogen (TN) removal ranged from 49% in the horizontal-flow wetland to between 58 and 95% in the hybrid systems. Apparent nitrification rates of 3.8–7.3gNm−2d−1 (based on ammonium reduction) were recorded in the vertical-flow wetlands and apparent denitrification rates of 2.8–12.4gNm−3d−1 (based on nitrate reduction) in linked denitrifying bioreactors and horizontal-flow wetlands. Mean total phosphorus (TP) removal ranged from 36 to 65%, while faecal indicator bacteria were reduced by 2.5–4.7log units in the different systems. The results of this study show that simple hybrid systems combining wetland and denitrifying bioreactor components are capable of achieving advanced effluent quality with low energy inputs. The areas required for these hybrid systems were generally half or less of those required for horizontal-flow wetlands.