Abstract
A 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.
Highlights
Passive treatment is a rapidly spreading, eco-technological approach for the removal of various organic and inorganic contaminants from wastewaters through exploitation and amplification of natural biogeochemical and physical processes (Kadlec & Wallace, 2009; Vymazal, 2014)
Trivalent Fe and Al readily precipitate at circumneutral pH, forming particulatehydroxides (Stumm & Morgan, 1996) that are subsequently removed through gravitational sedimentation and/ or filtration in settling ponds and wetlands, respectively (Hedin, 2008)
Mn(II) oxidation and Mn(III/IV) precipitation in aerobic passive systems are relatively low unless ameliorated in a favourable environment that promotes distribution of Mn-oxidising bacteria and concomitant formation ofcatalytic surfaces (e.g. Luan et al, 2012; Neculita & Rosa, 2019; Tan et al, 2010; Tebo et al, 2004)
Summary
Passive treatment is a rapidly spreading, eco-technological approach for the removal of various organic and inorganic contaminants from wastewaters through exploitation and amplification of natural biogeochemical and physical processes (Kadlec & Wallace, 2009; Vymazal, 2014). This study is focussed on aerobic surface-flow systems that are commonly used for passive removal of hydrolysable metals (Fe, Al, Mn) from circumneutral, primarily ferruginous mine water (Sapsford, 2013; Wildemann et al, 1993). Aerobic systems predominantly include classic water treatment components such as aeration cascades, settling ponds, surface-flow wetlands and oxic sediment filters or leach beds where contaminant removal and water quality improvement are governed by naturally occurring physical and biogeochemical processes (Skousen et al, 2017). Removal of additional mining-associated metal(loid)s (e.g. As, Cd, Cu, Ni, Pb, Zn) is frequently observed in aerobic passive systems, it is important to note that this is predominantly attributable to either adsorption and complexation in wetland substrates (Opitz et al, 2021; Sobolewski, 1999) or to the omnipresence of Fe/Al/Mn (oxyhydr)oxides through scavenging, (ad) sorption or co-precipitation
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