Determination of Hydraulic Residence Times in Several UK Mine Water Treatment Systems and their Relationship to Iron Removal

Abstract

In the UK, the Coal Authority has more than 40 mine water treatment systems, most of which are wetland systems with settlement lagoon pretreatment. The purpose of treatment in wetlands is the oxidation of ferrous to ferric iron and the subsequent hydrolysis and precipitation of ferric hydroxide within the wetland. It is generally accepted (Hedin et al., Passive treatment of coal mine drainage, 1994, p 35; Skousen and Ziemkiewicz, Acid mine drainage control and treatment, 1996, p 362; Younger et al., Mine water: hydrology, pollution, remediation, 2002, p 442) that this process proceeds by a first-order rate law, although most systems are designed based on an areal removal rate (10 g/m 2 /day) developed by the U.S. Bureau of Mines (Hedin et al., Passive treatment of coal mine drainage, 1994, p 35); this design guideline inherently assumes a constant removal rate. Given the actual kinetics of iron removal in wetlands, it follows that residence time will control iron removal; given the wide range of system geometries and aspects, it is logical to ascertain the actual hydraulic residence time of wetlands and settlement lagoons and determine the effect this has on iron removal. To make a preliminary assessment of this link, hydraulic residence time of two Coal Authority wetlands (Lambley and Whittle) and two Coal Authority settlement lagoons (Acomb East, Acomb West and Whittle) were measured using bromide tracer tests. Water samples for iron analysis and flow measurements were taken during each tracer test. The Lambley wetland performs well in terms of residence time, and, as reeds become established and adsorptive processes increase, its iron removal performance (currently 58% removal) may improve, but the low influent iron concentration appears to be a significant impediment to meeting the original performance target. In contrast, the hydraulic performance of the Whittle wetland system is poor, which appears to be due to accumulation of dead plant material coupled with a high length to width ratio. However, performance in terms of iron removal is good (92% removal), which appears to be due to the higher influent iron concentration, and especially the fact that the iron enters the wetland largely in particulate form. The longer residence time of water within the Acomb lagoons (&12 h) resulted in far more effective iron removal (72% in the east lagoon and 85% in the west lagoon) than the shorter residence time at Whittle (24% iron removal, &5 h residence time). Performance (in terms of iron removal) of the settlement lagoon systems appears to be far more closely related to the hydraulic residence time (albeit this conclusion must be tentative, given that only three systems have been investigated, and the Acomb system receives chemical addition). Based on this study, treatment system sizing using 100 m 2 lagoon area per 1 L/s flow appears to be a more appropriate basis for design rather than an areal iron removal rate.

The Role of Macrophytes in Constructed Surface-flow Wetlands for Mine Water Treatment: A Review

“…Badly maintained wetlands with spatially irregular vegetation distribution or silting up can have opposite effects, such as flow channelling or plugging (Brix 1994;Jadhav and Buchberger 1995;Klerk et al 2016;Persson et al 1999;Wahl et al 2012). Field tracer tests of various settling ponds and wetlands in UK passive mine water treatment systems showed better hydraulic efficiency for wetlands, whereas the effective volume of settling ponds was often considerably reduced due to dispersed flow, poor mixing, and short-circuiting (Kruse et al 2009;Kusin et al 2010;Sapsford 2013).…”

Natural pretreatment and passive remediation of highly polluted acid mine drainage

“…As the processes involved in the remediation of net-alkaline and netacidic metal polluted water in aerobic wetlands and lagoons are very similar, it was decided to use both of them to compare with the results obtained in the NFOL in Monte Romero (Table 3). As can be observed, the highest Fe removal reported in lagoons treating net-alkaline waters is 26 g/m 2 /day (Hedin, 2008;Kruse et al, 2009) while aerobic wetlands treating net-acidic waters only achieved 6 g/m 2 /day (Whitehead et al, 2005). The NFOL pretreatment showed a mean Fe removal as high as 100 g/m 2 /day over the study period, 4 to 16 times higher than the Fe removal observed in other conventional treatments.…”

Hydraulic residence time and iron removal in a wetland receiving ferruginous mine water over a 4 year period from commissioning

“…Goulet et al 2001;Kruse et al 2009). Therefore a higher removal of iron would be anticipated for system with a more hydraulically efficient performance.…”

“…longer hydraulic residence time) should be reflected in effective treatment performance. However, assessment of actual residence time within the UK coal mine water treatment systems has not been widely investigated (Kruse et al 2007). It is therefore the aim of this study to assess the effect of system hydraulic efficiency (i.e.…”