Coupled hydraulic and geochemical performance assessment of passive mine water treatments in the UK

Abstract

Current design practice for aerobic wetlands treating net-alkaline mine water in UK applications of passive treatment is based on zero-order kinetics for pollutant removal; the commonly used area-adjusted removal formula. Lagoons are designed to allow 48 hours of estimated retention time. However, there is significant variation in performance between systems. Neither of these approaches takes account of the hydraulic factors that may influence treatment performance. Therefore, this study aimed to improve understanding of both hydraulic and geochemical factors that govern contaminant behaviour, such that future design of treatment systems is able to optimise treatment efficiency and make performance more predictable, and improve performance over the long-term. Assessment of the hydraulic behaviour (flow pattern) of the treatment systems was accomplished by means of tracer tests. The tracer tests and simultaneous sampling of mine water were undertaken at eight UK Coal Authority mine water treatment systems (lagoons and wetlands) within Northern England (main study areas) and part of southern Scotland. Analyses of mine water samples were also undertaken in the laboratory alongside the field tests for assessment of geochemical processes controlling iron removal in the lagoons and wetlands studied. Analyses of the tracer test results were performed using a residence time distribution (RTD) analysis to account for the different shapes of tracer breakthrough curves observed. There appear to be multiple influences that possibly affect the RTDs in lagoons and wetlands e.g. vegetation and seasonal variation (growing or non-growing season), system age, flow and geometry (length-to-width ratio and depth). The RTD analysis shows that lagoons generally have a more dispersed flow pattern, associated with a more pronounced short-circuiting effects and a long tail compared to wetlands. A modelling approach using a tanks- in-series (TIS) model was adopted to precisely analyse and characterise the RTDs, in an effort to account for the different flow patterns across the treatment systems. Generally, lagoon RTDs are characterised by a greater flow dispersion compared to wetlands (i.e. higher dispersion number, D and lower number of TIS, n). Consequently, the hydraulic efficiency, e for lagoons is much lower than wetlands (mean of 0.20 for lagoons compared to 0.66 for wetlands). This is attributed primarily to a much lower volumetric efficiency, ev in lagoons, meaning that a greater proportion of the total volume of the lagoon system is not being involved in the flow of water through them, with implications for design to optimise performance. In contrast, in wetlands a greater volumetric efficiency is evident, and there is therefore a longer relative mean residence time for retention and attenuation of iron. On the evidence of field data, in lagoon systems the iron removal processes are primarily controlled by ferrous iron oxidation, whilst in wetlands the removal is controlled by iron settlement. The time- and concentration-dependence of iron removal (oxidation and / or settlement rate) has also been investigated in the laboratory alongside the field data. The rates are faster in lagoons compared to wetlands due to higher concentration of iron available for the processes. General trends showed that efficient treatment performance for iron removal corresponds with greater system hydraulic efficiency in wetlands compared to lagoon systems. The greater hydraulic efficiency in wetlands was mainly attributed to a grea ter volumetric efficiency in the wetland systems. In contrast, shorter relative mean residence time was found in lagoons, thus a lower retention time for iron attenuation and lower removal efficiency as a consequence. For lagoon systems, performance can be optimised by ensuring greater volumetric efficiency (hence residence time), which can be achieved with a large length-to-width ratio system (up to a ratio of 4.7), but also a greater depth (up to 3.0 m), though only if systems are regularly maintained (dredged). For wetlands, the use of the area-adjusted removal rate formula appears to work well for the design of aerobic wetlands, despite the observed concentration-dependence of iron removal processes. However, use of first-order removal formula (TIS basis) would be a more appropriate approach to the design of mine water treatment systems since it takes account of the flow pattern effect on pollutant removal processes, in addition to the first-order kinetics (concentration-dependence) for iron removal. Regular sludge removal (yearly) is recommended in lagoons to provide longer residence time because lagoon depth and volume tends to rapidly decrease over time due to build up of ochre and debris (7- 49% depth reduction per year). Thinning of reeds is recommended whenever apparent channelisation would otherwise dominate the flow pattern, and therefore limit the capacity for adsorption and settlement of precipitated iron hydroxide.