Nitrification in Constructed Wetlands Treating Ochreous Mine Water
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A survey of ochreous discharges from former coal mines in the UK indicated ammonia contamination at 2–5 mg/L in water from flooded shafts of depths >400–800 m. Although significant, this was much less than historically observed in working mines. No correlation was observed between ammonia and iron concentrations. However, ammonia was removed to some extent in constructed wetlands designed primarily to remove iron.
A mechanistic study of wetland removal of ammonia from mine water indicated the main process to be bacterial nitrification, similar (despite great differences in operating conditions) to that occurring in many wastewater treatment works. The study was based on water containing 4–5 mg/L ammonia and some 12–27 mg/L iron from the abandoned Woolley mine in Yorkshire. Notwithstanding relatively high salinity and short residence time, most of the ammonia entering the wetlands was, at least initially, converted to nitrate. Field measurements showed that the conversion efficiency was increased at lower flow rates, higher temperature, and longer flow stabilisation, which are all consistent with bacterial action. Subsurface flow conditions were simulated in column studies, using pre-sterilised gravel and mine water taken from the wetland cells; two strains of bacteria commonly associated with nitrification in domestic wastewaters, Nitrosomonas europaea and Nitrobacter agilis, were able to reproduce the 89% ammonia oxidation observed in the wetlands. It was concluded that the high degree of aeration, neutral pH, and nutrient content of the mine water greatly favoured nitrification. Although more saline and lower in biochemical oxygen demand than organic wastewater, nitrification was not inhibited.
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