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/m2/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 m2 lagoon area per 1 L/s flow appears to be a more appropriate basis for design rather than an areal iron removal rate.