Optimising Operational Reliability and Performance in Aerobic Passive Mine Water Treatment: the Multistage Westfield Pilot Plant

Abstract

AbstractA three-stage pilot system was implemented for passive treatment of circumneutral, ferruginous seepage water at a former opencast lignite mine in southeast Germany. The pilot system consisted of consecutive, increasingly efficient treatment stages with settling ponds for pre-treatment, surface-flow wetlands for polishing and sediment filters for purification. The overall objective of the multistage approach was to demonstrate applicability and operational reliability for successive removal of iron as the primary contaminant broadly following Pareto’s principle in due consideration of the strict site-specific effluent limit of 1 mg/L. Average inflow total iron concentration was 8.4(± 2.4) mg/L, and effluent concentration averaged 0.21(± 0.07) mg/L. The bulk iron load (≈69%) was retained in settling ponds, thus effectively protecting wetlands and sediment filter from overloading. In turn, wetlands and sediment filters displayed similar discrete treatment efficiency (≈73% each) relative to settling ponds and thus proved indispensable to reliably meet regulatory requirements. Moreover, the wetlands were found to additionally stimulate and enhance biogeochemical processes that facilitated effective removal of secondary contaminants such as Mn and NH4. The sediment filters were found to reliably polish particulate and redox-sensitive compounds (Fe, As, Mn, NH4, TSS) whilst concomitantly mitigating natural spatiotemporal fluctuations that inevitably arise in open systems. Both treatment performance and operational reliability of the multistage pilot system were comparable to the conventional treatment plant currently operated on site. Altogether the study fully confirmed suitability of the multistage passive setup as a long-term alternative for seepage water treatment on site and provided new insights into the performance and interrelation of consecutive treatment stages. Most importantly, it was demonstrated that strategically combining increasingly efficient components may be used for optimisation of treatment performance and operational reliability whilst providing an opportunity to minimise land consumption and overall costs.