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Mine water treatment wetlands are the most important category of a wider suite of treatment technologies which are termed “passive treatment”. A passive treatment system is defined by the European Commission's PIRAMID project as follows: "A water treatment system that utilises naturally available energy sources such as topographical gradient, microbial metabolic energy, photosynthesis and chemical energy and requires regular but infre- quent maintenance to operate successfully over its design life".
The working definition of "infrequent" in this context is currently around six-monthly. Constructed wetland systems are currently the most widely-used passive mine water treat- ment technology, and are likely to remain so. There are several reasons for this, including:
Notwithstanding these virtues, the use of wetlands for passive treatment of mine waters has not been without its detractors. Most of the mis-givings which have been expressed in the literature arise from cases where the technology has been misapplied and /or the data mis- interpreted. For instance two instances may be cited from England alone in which disappointing early applications of wetlands technology gave rise to denigration of the technology as a whole. In both cases, the reason for the poor performance was that simple aerobic reed-beds were inappropriately constructed to receive extremely acidic spoil leachates, for which other technologies (compost wetlands or RAPS) are actually recommended. By contrast, 25 other UK systems were constructed around the same time in accordance with the guidelines of Hedin et al. (1994), all of which proved very successful (Younger, 2000a). These successes nullified the previous bad publicity, and both of the
For further details, see http://www.piramid.org, where thorough guidelines for passive system design can also be down-loaded free of charge.
These successes nullified the previous bad publicity, and both of the early, failed systems are now being retro-fitted with alkalinity-generating variants of wetland systems.
Despite the numerous success stories, some wetlands-based technologies remain less cer- tain in their applicability than others. In particular, wetlands treatment is still challenging for highly acidic waters, and may even be inadvisable for waters containing significant concen- trations of xenobiotic metals (such as Hg and Cd). Hence we do not wish to give the im- pression that the techniques described below represent a panacea, and we will attempt to highlight current limitations to the technology as we see them.
Simple cross-sections illustrating the three types of wetland used to treat polluted mine wa- ters (after Younger, 2000a). Mine water treatment wetlands in the UK
The development of passive treatment of mine waters in the UK up to September 1997 is documented in detail by Younger (1997; 2000a,b; 2001).There are currently about 30 con- structed wetland systems treating mine waters in the UK. These systems are of three basic types:
The degree to which each type of system can currently be considered to be “proven tech- nology” corresponds to the order in which they are listed above. This ranking of confidence is reflected in uptake rates (to 1-1-2002), with 15 full-scale reed-bed systems (of which 10 are operated by the Coal Authority, a national government agency), 5 RAPS and 4 compost wetlands.
Variants of the technology and their applicability
Each of the three types of wetland treatment system shown on Figure 1 are appropriate for a different kind of mine water, or for specific hydraulic circumstances. These are as follows: i)
Aerobic, surface flow wetlands (reed-beds) (Figure 1(a)): These are appropriate for removal of Fe (and to a lesser extent Mn) from net-alkaline mine waters
The mine waters may be naturally net-alkaline (this is the case for many deep mine dis- charges), or else previously acidic waters which have since been neutralised by conventional alkali dosing, or by use of an anoxic limestone drain (no full-scale ex- amples in the UK), or a RAPS. Simple aerobic reed-beds should not be applied to acidic waters - they will only lower the pH further!
Aerobic reed-beds are simply surface flow systems with shallow water (> 0.1m depth < 0.5m) densely planted with hardy wetland plants such as Typha latifolia,
Phragmites australis, and Juncus effusus. Fe removal occurs by oxidation of Fe 2+ to Fe 3+ , which then hydrolyses to form ferric hydroxide (ochre). The plants baffle flows, oxygenate the substrate to prevent Fe 3+ from becoming reduced again, and (particularly at low Fe concentrations, as in 'polishing' applications, by direct plant uptake of metals (Batty, 1999). ii) Compost wetlands (Figure 1(b)): These are appropriate for the treatment of net- acidic mine waters 3 on sites where there is insufficient relief to provide the head to drive water through a RAPS unit (which would be the preferred technology for such waters where practicable). They consist of a surface flow wetland with very shallow water (typically < 0.15m) over a thick ( 0.5m) substrate of compost which hosts anaerobic sulphate reducing bacteria (SRBs). The SRBs catalyse the reduction of SO 4 2- to sulphide, which then reacts with the pollutant metals (except Al 3+ ) to pre- cipitate sulphide minerals which accumulate in the substrate. pH rises and HCO 3 - alkalinity is generated as a by-product of the bacterial sulphate reduction, neutralis- ing the acidity. Typical substrates used in UK systems to date include composts de- 2
Waters are 'net-alkaline' where their total alkalinity (which mainly reflects bicarbonate content in these waters) exceeds their total acidity (principally due to the content of hydroxide-forming metals, such as Fe, Mn, Al, Cu, Zn etc).
In net-acidic mine waters, total acidity exceeds total alkalinity. The pH is not necessarily below 6.5 at the point of first emergence, but will typically drop below 4.5 if the water is aerated and left to stand. rived from horse manure, cow manure, municipal waste, tree bark mulch and spent mushroom compost. It is good practice to construct at least a small aerobic reed- bed downstream of a compost wetland, to remove residual Fe by aerobic proc- esses and to re-oxygenate the water before it enters a receiving watercourse. iii)
RAPS (Reducing and Alkalinity Producing Systems) (Figure 1(c)): These are the systems of choice for net-acidic mine waters, but they require a minimum relief of some 5 metres across the site if they are to be successfully constructed. This is because substantial losses of hydraulic head occur as water flows through the up- per layer of compost (see Figure 1 (c)) into an underlying bed of limestone gravel.
The addition of a limestone bed adds calcite dissolution as a major alkalinity- generating process, and the vertical subsurface flow through saturated compost ensures far more efficient sulphate reduction (and other anaerobic processes) than is achieved in compost wetlands. For this reason, a RAPS will typically have a much smaller 'footprint' than would a compost wetland treating the same water. (Younger et al., 2002, show that a RAPS will typically occupy only 15 - 20% of the total area occupied by an equivalent compost wetland). As with compost wetlands, it is good practice to construct at least a small aerobic reed-bed downstream of a RAPS unit, to remove residual Fe by aerobic processes and to re-oxygenate the water before it enters a receiving watercourse.
Design criteria
Settling a controversy
Most wetlands operating in Europe (Younger, 2000a,b), and many of those constructed within the last 8 years in North America (Younger et al., 2002), have been successfully de- signed using guidance given by Hedin et al. (1994) of the former US Bureau of Mines. Re- cent unwarranted controversy has surrounded these design criteria, arising from a claim by Tarutis et al. (1999) that aerobic reed-beds should be designed assuming that oxidation of ferrous iron is the dominant treatment process. Since Fe 2+ oxidation is known to be first- order in Fe 2+ concentration, Tarutis et al. (1999) proposed a first-order kinetic model should be applied to mine water wetland design. This is a somewhat reductionist stance, given that (as Younger et al. (2002) have noted) the internal functioning of aerobic reed beds depends on precipitation and sedimentation kinetics in addition to oxidation alone. Furthermore, simi- lar attempts to reduce the design of wastewater treatment wetlands to application of first- order BOD oxidation models have been trenchantly criticised by Kadlec (2000) on the grounds that they ignore variations in influent flow rates, variations in influent contaminant concentrations (which are rarely in phase with the flow variations), dispersion of flow and solute transport within the wetlands, and the likelihood that contaminant removal processes differ somewhat between the fast-flowing and stagnant zones within a treatment wetland.
The net result of these factors is that the overall rate of contaminant removal in a treatment wetland is unlikely to be well-modelled by comparison only with the first order kinetics of one of the main chemical reaction processes. There is no a priori reason why the situation should be any better in the case of mine water treatment wetlands.
A simple example should suffice to demonstrate why the US Bureau of Mines criteria (Hedin et al., 1994) provide a more sensible basis for design than those proposed by Tarutis et al. (1999): The construction of the Coal Authority's Edmondsley wetland (County Durham, UK) was completed in the summer of 1999. The Edmondsley mine water flows from an old coal CP-035 (2004) drift at a rate of about 10 l.s -1 , and contains some 30 mg.l -1 of Fe. The designers of the wet- land used the Hedin et al. (1994) criteria to estimate the minimum wetland area required, obtaining a value of 2505 m2 .
Fortunately, there was more than twice as much suitable land area available for purchase at the site. Hence, the system was constructed as four aerobic reed bed cells in series, totaling 4000 m 2 in area. This design allowed one or two of the cells to be taken out of operation at any time for maintenance purposes without compromising the treatment ability of the system as a whole. Monitoring of the system since it was com- missioned has shown performance in line with expectations, with virtually all of the Fe (down to a residual < 0.5 mg.l -1 ) being removed in the first two cells (i.e. the first 2000 m 2 of wetland). By contrast if the wetland had been designed using the first-order model proposed by Tarutis et al. (1999), the predicted area of wetland required would have nearly 2 ha! (Full details of the relevant calculations are given by Younger et al., 2002). Had the latter design figure been used, the costs of acquiring 2 ha of land in this scenic area would have pre- cluded wetland treatment as a serious option. An active treatment system would have been developed, with huge cost implications for long-term operation.
Recommended design criteria
Having thus satisfied ourselves that the US Bureau of Mines criteria (Hedin et al., 1994) re- main the most valid design rules currently available, we can now summarise these as fol- lows. The basic expression which must be evaluated is as follows values for other pollutants and various circumstances, as well as comprehensive guidance on the practicalities of wetland design for mine water treatment.
Ancillary benefits of mine water treatment wetlands
One of the principal attractions of wetlands as treatment systems is the possibility of integrating them into the surrounding landscape (Campbell & Ogden, 2000), and achieving healthy connections with the existing eco-systems in the area. Integration of wetlands into a landscape at the level of aesthetics is readily attainable, as a number of recent projects il- lustrate (see following case studies). Ecological integration is rather harder to achieve in practice, however, due to a number of factors (Younger et al., 1998), including the following:
Case Study: The Quaking Houses community mine water treatment wetland
The Stanley Burn is a small headwater tributary of the River Wear, one of the principal riv- ers of north-east England. Since the mid-1980s, the Stanley Burn has suffered conspicuous pollution by acidic drainage emanating from a perched water table within a superficially re- stored waste rock pile appertaining to the former Morrison Busty Colliery (abandoned 1975).
The leachate contains up to 200 mg.l -1 total acidity (as CaCO 3 equivalent), with elevated concentrations of Fe (? 30 mg.l-1), Al (? 30 mg.l-1) and Mn (? 15 mg.l-1). This polluted drain-age constitutes a classic ‘orphan discharge’ for which no legally responsible party could be identified. In 1994, when residents of the nearby village of Quaking Houses finally accepted that no remedial action was ever likely to be forthcoming from elsewhere, a "do-it-yourself" remedial programme was launched in collaboration with the mine water research team at Newcastle University. Drawing upon the inspiration of USA experiences (especially the work of the former US Bureau of Mines) passive treatment was soon identified as the most ap- propriate solution. As the water is acidic, a compost wetland system was the obvious choice, since these systems can generate alkalinity by means of bacterial sulphate reduc- tion (sulphate being present at high concentrations in the mine drainage). However, given that compost wetland technology had no previous track record in Europe at that time, the first step in the remedial program was to build and monitor a small-scale pilot wetland (Younger et al., 1997). This not only allowed the designers to gain valuable hands-on ex- perience which would prove invaluable later, but also proved crucial in building confidence in the efficacy of the technology amongst regulators and potential sponsors. In essence, the pilot system was a shallow pond, 45m in area, with a 0.3m substrate of horse manure and straw from the Quaking Houses Village Stables. It was designed to treat around 5% of the average leachate flow. After 18 months of monitoring, this pilot wetland yielded an average removal rate of 9 g.d
These encouraging performance figures, and the pleasant appearance of the pilot wetland, proved influential with potential sponsors, and by mid-1997 sufficient funding had been ob- tained from a range of charitable and philanthropic foundations to finance the construction of a full-scale system. While the pilot plant had been in operation, the seminal work of Ke- pler and McCleary (1994) had begun to influence passive system design in the UK, and pre- liminary plans were laid to construct the full-scale system at Quaking Houses as a RAPS (a vertical-flow system in which water flows in the subsurface, first through compost then through limestone). However, when the available plot of land was finally cleared of scrub vegetation and trial-pitted, it was found to be underlain by highly pyritic waste materials de- rived from a previously unrecorded coal washery finings pond. This meant that excavation of a suitable basin to install a RAPS would have entailed disposal of large volumes of highly reactive, acid-generating waste, the landfilling of which would have consumed much of the budget available for wetland construction. Without such excavation, there was a maximum of 1.0m of head available across the entire site, which is insufficient to drive water through a RAPS; hence the full-scale system was designed as a compost wetland, scaled-up from the original pilot-plant design.
Construction of the wetland commenced in August 1997 and took about 6 weeks of site work (Jarvis & Younger, 1999). The leachate was captured by construction of a concrete headwall across the outfall of the culvert from which the discharge emanates. This headwall gained some 0.5m of head to help drive water through the system. Two sections of 100mm diameter pipe were built into the headwall. The first carries water underground in an in- verted siphon to the influent point of the wetland, discharging into a basin from where the water is distributed across the width of the wetland. The second section of pipe allows over- flow back into the original watercourse when flow-rates exceed approximately 400 litres per minute. Because pollutant concentrations are lower at higher flow-rates due to dilution, and because of further dilution of the overflow water by the effluent from the wetland, the impact of this water on the receiving watercourse is minimal.
The heart of the Quaking Houses treatment system is a compost wetland unit occupying some 440 m This is enclosed by a bund composed of pulverised fuel ash (PFA), which is both strong and highly impermeable after mechanical compaction, yet costs less than half the price of the main alternative material (clay). To avoid toe drainage, which may have affected the integrity of the bund, the base of the embankment was sunk approxi- mately 0.2m into the in situ soil. The bund had a minimum crest width of 1.5 m, with inner slope angles (i.e. facing into the ponded area) of 1:3 or less, in order to encourage wildlife. (Outer slope angles were made to be not more than 1:2). Baffles and islands were also constructed from PFA, both to help minimise hydraulic short-circuiting within each wetland cell and to improve the appearance and habitat diversity of the wetland. PFA was also used to construct a central weir of about 0.4m height, which was incorporated into the design for four reasons:
Because the horse manure used in the pilot-scale wetland was not available in sufficient quantity, additional sources of compost were sought: the final compost wetland substrate comprises horse manure, cattle manure and municipal waste compost in the following pro- portions: 30:40:30. Additionally 30 tonnes of limestone were deposited at the far end of the wetland, to facilitate final pH adjustment if it should be required. The system was designed such that the compost depth in the wetland would be 0.3 - 0.5 m, leaving 0.30m of free- board for future accumulation of material on the substrate surface.
This comprises a circular, ornamental ‘willow pond’ and an appropriated area of natural Juncus stands, totalling some 100m in area. It should be noted that, for illustrative purposes, the contaminant removal rates and efficiencies discussed below relate only to the compost wetland unit (ponds 1 and 2), not to the passive treatment system as a whole, which achieves significantly more Fe removal than the compost wetland alone.
Layout of the Quaking Houses mine water treatment wetland (after Younger et al., 2002) Monitoring of the compost wetland unit within the Quaking Houses system over its first 27 months of operation (Jarvis, 2000) revealed a mean acidity removal rate of 5.6 g.d
One major benefit of the Quaking Houses passive treatment project deserves further men- tion here: once the long-term pollution from acidic mine site drainage had been abated, pressures mounted for the clean-up of other sources of pollution to the stream (particularly from combined sewer overflows and deicing salt store runoff). Previously, the organisations responsible for these two sources of pollution had justifiably claimed that there was little point treating their effluents to higher standards given that the acid drainage was clearly kill- ing all aquatic life in its path. With that excuse removed, the other sources of pollution soon became priorities for clean-up, with the result that the Stanley Burn has now been thor- oughly restored as a healthy stream ecosystem.
Acknowledgements
The concepts presented above are by no means all my own work; they have evolved through collaboration with a wide range of colleagues, most notably Dr Adam Jarvis (now at IMC Ltd), Dr Bob Hedin (Hedin Environmental Inc., Pittsburgh, USA), Mr David Laine (IMC Ltd), Dr Lesley Batty (Newcastle University) and the volunteers of the Quaking Houses En- vironmental Trust, who maintain the Quaking Houses wetland to this day.
References
Contact
Paul L. Younger
School of Civil Engineering and Geosciences
University of Newcastle
Drummond Building
UK - NE1 7RU Newcastle upon Tyne
Tel.: + 44 191 222 7942, Fax.: + 44 191 222 6669
Email: p.l.younger@ncl.ac.uk