Kaisa Turunen (GTK). Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND. kaisa.turunen(at)gtk.fi
Anaerobic compost wetlands (AnWs) are based on microbial sulphate reduction for the generation of alkalinity, neutralization of the acidity of influent waters and precipitation of metals mainly as sulphides. Besides biologically meditated processes, mine effluent quality is improved by filtration of suspended solids and colloidal materials. In AnWs, as water flows mainly subsurface, the substrate becomes anoxic due to high biological oxygen demand resulting in emphasised reduction reactions and sulphide precipitation. AnWs are most effective at pH ~2.5 and since the processes may raise pH they are capable to neutralize acidity. In fact, compost wetlands were initially developed to overcome the acidification problems of aerobic wetlands. (Wildeman et al. 1993, Johnson & Hallberg 2005).
Metal removal in AnWs is based on a combination of mechanisms (Skousen et al. 1992, 2000), but the dominant mechanism is chemical and microbial reduction, which precipitates metals and neutralises acidity. Due to microorganisms and reducing conditions of the substrate, ferric iron is reduced to ferrous iron and sulphate to sulphide, resulting in precipitation of most metals mainly as mono- and di-sulphides (such as FeS and FeS2), but also as hydroxides (e.g. Al(OH), Cr(OH)) and carbonates (e.g. FeCO3) (Younger et al. 2002, Wolkersdorfer 2008). In AnWs water needs to flow through the compost layer so that substrate becomes anaerobic due to high biological oxygen demand. If the substrate is not permeable enough, the water flows above the substrate and the microbially catalysed reducing reactions will not occur resulting in decreased treatment efficiency or even system failure. The Fe oxidation and hydrolysis occur mainly in the aerobic surface layers, whereas Al hydroxides are formed on the surface of the substrate material. Yet, the sulphate reduction occurs only if all dissolved oxygen is consumed and Fe3+ is reduced to Fe2+. Reduction of iron can be enhanced by aerobic wetland treatment prior to anaerobic wetland treatment. The removal of metals from solution results in a lowering of the mineral acidity. In addition, the decomposition of the organic material consumes the protons and generates H2S in the mine water resulting also in removal of acidity, thereby raising the pH. (Hedin et al. 1992, Wolkersdorfer 2008).
Anaerobic wetlands function best in the pH range 6 to 9, but since bacteria produce bicarbonate, the treated water may be net acidic water and include high metal content. (Gusek & Wildeman 2002). According to several studies (e.g. Gusek 2002), successful sulphate reduction functions also at pH as low as 2 to 2.8. However, anaerobic wetlands are most successful when used to treat small flows and/or waters that have moderate water quality and higher net alkalinity than net acidity. In addition, AnWs perform better with low dissolved oxygen (DO) levels (Waters et al. 2003), but they can tolerate also high levels of DO (Skousen et al. 2000). In contrast to processes in aerobic wetlands, anaerobic processes can operate throughout the winter and subsurface processes are also better protected from cold. (e.g. Gusek 2002, Gusek & Wildeman 2002).
AnWs are similar in design to aerobic wetlands, except the thickness of organic substrate is much greater (0.3-0.6 m) and the thickness of free-standing water is much less (0-8 cm; Hedin et al. 1994, Skousen et al. 2000). Compost wetlands comprise of a 0.3-0.6 m thick layer of organic material with or without plants such as common reed (Phragmites australis), common rush (Juncus effuses) or reedmace (Typha latifolia). These plants have ability of metal take up and (unless planted in straight lines) reducing the velocity of the water flow, preventing forming of shortcuts, thus enhancing microbially catalysed oxidation, precipitation and co-precipitation (Younger et al. 2002, Wolkersdorfer 2008).
As aerobic wetlands, AnWs are typically sized based on acidity loading rates. According to several studies (e.g. Hedin et al. 1994, Younger et al. 2002) recommended loading rate would be 3.5-7 g acidity/m2/d.
Significant factor causing a failure of anaerobic wetland is the water depth. Too deep water layer will reduce the exchange between the water and the substrate thus resulting in decreased treatment efficiency. In addition, if the substrate is not permeable enough (K range 0.01 to 1 m/d), the water flows above the substrate and microbially catalysed reducing reactions will not occur resulting in decreased treatment efficiency. If the substrate is not flooded all the time, sulphide solids may oxidize. Suitable substrate should be sufficiently fibrous to retain a reasonable permeability. Fecal materials would also contain some sulphate-reducing bacteria (SRB) (Younger et al 2002). The organic substrate has to be alkaline and act as long-term easily degradable carbon source to microorganism to work. Following organic materials have been used for compost wetlands (Wolkersdorfer 2008):
- spent mushroom compost (e.g. Hedin et al. 1994)
- horse manure and straw (e.g. Younger 1997)
- cow manure and straw (e.g. Cohen & Staub 1992)
- composed municipal waste (e.g. Jarvis & Younger 1999)
- sewage sludge cake (e.g. Laine & Jarvis 2003)
- paper waste pulp (e.g. Laine & Jarvis 2003)
Due to the density and compactness, precipitating Fe mono- and di-sulphides may hinder the permeability of the substrate (Skousen et al. 2000).
Figure 1. The schematic picture of an anaerobic wetland. © GTK
AnWs are simple to design, have low operational costs, they are aesthetically pleasant and they serve as precipitate storage. In addition, due to reducing conditions within the substrate and high dissolution of limestone, the limestone will not become armoured with Fe oxyhydroxides. Organic matter may need to be added to the wetland if performance decreases, but otherwise AnWs require very little operation and maintenance (Waters et al. 2003). Therefore AnWs are suitable for remote locations without access to utilities.
Metal overloading due to the exhaustion of the sorption sites within the organic substrate and lowering of the permeability of the substrate are the key limitation factors of AnWs (Waters et al. 2003). In addition, high acid loads (>300 mg/l) may hinder the microbial activities (Skousen et al. 2000).
Case examples of mine sites with anaerobic compost wetlands in Finland:
1) The removal of metals and sulphate in a constructed passive system (combined system of aerobic and anaerobic wetlands and open limestone channel (OLC)) at Cu-Zn-Co-Ni Luikonlahti mine in Eastern Finland. The treatment system is evaluated as a case study site in the Closedure project. The evaluation can be found in the Luikonlahti case study part.
- M.L. Räisänen, 2009. Capability of natural and constructed wetland to mitigate acidic leakage from closed mine waste facilities – cases in Eastern Finland. Proceedings of Securing the Future conference, Skellefteå, Sweden, June 22-26 2009. (pdf)
- T. Karlsson & M.L. Räisänen, 2011. Care of mine districts and the environment after closure of mines. Excursion guide of 25th Geochemistry Symposium, Rovaniemi, Finland, 22-26 August, 2011. pp. 11-13. (pdf)
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