Remediation of ammonia-rich minewater in constructed wetlands

A three-year study of ammonia removal from minewater was carried out employing constructed wetland systems (surface flow wetland and subsurface flow wetland cells) at the former Woodey Mine in West Yorkshire, UK. The 1.4 Ha surface flow wetland (constructed in 1995) reduced the ammonia concentration from 3.5 - 45 mg l(-1) to < 2.3 mg l(-1) during the first half of the study and to essentially zero in the last year (2000 - 2001). About 25% of contained ammonia was converted to nitrate, about 10% was consumed by the plants and up to 30% was converted to nitrogen gas. This maturation effect was attributed to increased depth of sludge from sedimentation of ochre, providing increased surface area for immobilisation of ammonia oxidising bacteria. The surface flow wetland finally removed 2.3 g m(-2) day(-1) ammonia in comparison with 3.8 g m(-2) day(-1) for the subsurface flow (pea gravel) wetland cells, constructed for the present work and dosed with ammonium salts. Removal of ammonia by both systems was consistent with well-established mechanisms of nitrification and denitrification. It was also consistent with ammonia removal in wastewater wetland systems, although the greater aeration in the minewater systems obviated the need for special aeration cycles. The general role of wetland plants in such aerated conditions was attributed to maintaining hydraulic conditions (such as hydraulic efficiency and hydraulic resistance of substratum in subsurface flow systems) in the wetlands and providing a suspended solids filter for minewater.