Discard Coal as Filter Bed Material in Horizontal Subsurface Flow Constructed Wetlands: a Preliminary Study

Abstract

Constructed wetlands (CWs) are engineered systems that use the natural functions of vegetation, substrate and microorganisms to treat wastewater. In coal mining regions, low calorific coals are dumped as discard. Left unattended, discard and slurry ponds contaminate surface and groundwater, cause erosion and sedimentation of particulates into nearby rivers and dams and contribute to atmospheric pollution and landslides. This study sought to investigate the use of South African bituminous discard as filter bed material for CW. A laboratory-scale horizontal subsurface flow (HSF) CW was supplied either nutrient-poor tap water (TW) or nutrient-rich advanced facultative pond (AFP) effluent, and quality of the treated water monitored over 6 months. Additionally, residual material from the discard coal filter bed was assayed after 6 months to establish substrate stability and to assess the contribution of phyto-biodegradation. Results showed successful establishment of P. australis on discard coal, better plant performance (measured as PSII quantum yield and biomass accumulation) and greater nutrient removal when fed AFP effluent. Discard coal filter bed material had greater ash content, sustained fixed carbon and C/N ratio with unchanged electrical conductivity (EC) and sulphate and phosphate concentration, indicative of balanced ion exchange. This, along with a > 70% reduction in NH4+-N concentration, yielded a final effluent within the general limit set by the South African authority for either irrigation or discharge, into a water resource that is not a listed water resource, for volumes up to 2000 m3 on any given day.