The co-treatment of sewage and mine waters in aerobic wetlands

A trial aerobic wetland in NE England is the first passive system ever designed to treat both a polluted mine water and a secondary sewage effluent. Both of these discharges currently enter a small third-order stream (the River Team), significantly degrading its water quality. As the total mine water and sewage water discharges to the river are ∼ 300 and 100 L/s respectively, the pilot-scale wetland (25 × 25 m) has been designed to treat a small portion of each discharge in the same 3:1 ratio. The main drivers for remediation are Fe (∼ 3 mg/L in the mine water), BOD (∼ 14 mg/L in the sewage water), N–NH3 (∼ 2 mg/L in the sewage water), suspended solids (∼ 23 mg/L in the sewage water) and PO4 (∼ 7 mg/L in the sewage water).The combined treatment has many potential advantages over separate treatment of the discharges. Besides the mutual benefits of mixing these two wastewaters (which each tend to be low in pollutants which are high in the other), the biogeochemical properties of the wastewater types can be expected to yield real synergies in treatment. For instance, suspended solids in the sewage water should encourage iron flocs to form by Fe entering in the mine water, expediting the precipitation of oxyhydroxides. Similar processes may also accelerate manganese removal. Phosphate, which is generally difficult to remove using either active or passive treatment can be removed via sorption onto iron oxyhydroxide precipitates. The same oxyhydroxides are also likely to provide numerous ideal sites for the attachment of nitrifying and denitrifying bacteria.Although the wetland is still immature, initial results suggest that co-treatment is highly successful. Effluent concentrations have consistently been lower than Environment Agency effluent design standards and removal rates for all parameters are likely to improve with time as both biological and microbiological communities become established.