Reducing and alkalinity producing systems (RAPS/SAPS)
Kaisa Turunen, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND. e-mail: kaisa.turunen(at)gtk.fi
The RAPS systems (originally called as SAPS systems) are used to treat net acidic mine water with high metal content. The RAPS systems were initially developed to overcome strict requirements of anoxic limestone drain (ALD) systems for low dissolved oxygen and element levels. Actually, the RAPS system combines ALD and compost wetland systems and includes both aerobic and anaerobic steps, but water flows vertically through the system. First the acidic water flows through the low hydraulic conductivity compost part of the RAPS, where dissolved oxygen is consumed and ferric iron (Fe3+) reduced to ferrous iron (Fe2+). As a result, the water becomes anoxic and will flow through the anoxic layer of limestone resulting in alkaline water without any iron precipitating on the limestone. (e.g. Watzlaf et al. 2000, Wolkersdorfer 2008, Matthies et al. 2010).
As in compost wetland, the first step in the RAPS system is to remove all dissolved oxygen from water by aerobic microbial activity in the upper organic substrate layer. If dissolved oxygen is not removed, iron oxides would oxidise, hydrolyse and precipitate resulting in clogging of the system. As the dissolved oxygen is consumed, forming anoxic conditions enhance the ferric iron to reduce to ferrous iron. Simultaneously, anaerobic microbes create bicarbonate alkalinity and aluminium is precipitated to Al(OH)3 and retained in organic layer. Once the water flows through the substrate it faces a limestone layer, where calcite dissolution results in bicarbonate generation and increase in pH. Additionally, the anaerobic microbial activity reduces sulphate to hydrogen sulphide which reacts further with chalcophilic elements such as Fe2+ to form mono and di-sulphides. Once the effluent gets in contact with atmosphere in outflow, iron and other elements precipitate.
RAPS is usually followed by aerobic pond or wetland that collects precipitate sludge. The removal capacity of RAPS system is approximately 20-40 g acidity /m2/d and estimated lifetime 20-30 years. However, due to tendency for clogging as in ALD, the longevity of the system is a bigger concern as in e.g. compost wetlands. And as in compost wetlands, the organic material may degrade and should be substituted every two to three years (Watzlaf et al. 2000, Wolkersdorfer 2008, Matthies et al. 2010). RAPS technology involves the establishment of sulphate reducing bacteria (SRB), which can be difficult in cold climates (Zaluski et al. 2000). However, since subsurface processes are protected from cold the anaerobic processes in RAPS systems can operate throughout the winter. Moreover, microbial processes are more sensitive to low temperatures in the beginning than during operation and should be started during warmer months to give the SRB and associated microorganisms a chance to build up population density before they encounter cold temperature conditions (e.g. Gusek 2002a, Gusek & Wildeman 2002). In fact according to Watzlaf et al. (2000) RAPS systems can handle cold climate better than other techniques. Moreover, despite of considerably lower concentration levels for most parameters during winter period in UK, some RAPS systems showed higher element loads during February to April due to the significantly higher flow rates (Matthies et al. 2010).
The vertical or downward flow direction greatly increases the interaction of water with organic material and limestone. Any shortcuts between inflow and outflow may result in malfunctioning of the system as well as metal precipitation on the limestone layer (Wolkersdorfer 2008). In addition, the hydraulic head has to be high enough the water to flow through the compost, since any overflowing of the system will result in zero efficiency of the method (Rees & Connelly 2003). Moreover, the water level should be maintained above the level of the organic material to prevent oxygen excess to the limestone. This might be a problem especially in arid and semi-arid areas where the incoming flow varies strongly (Watzlaf et al. 2000).
RAPS system is rather simple to construct, but the construction costs are slightly higher than for ALD and compost wetland (Younger et al. 2002, Wolkersdorfer 2008). The RAPS system is constructed of two overlapping reactive layers and the water flows vertically through the system. Typically, the reactive layer comprises of 0.1-0.3 m thick layer of organic compost which is underlain by a 0.5 to 1 m thick layer of limestone. The water depth in RAPS/SAPS system is usually 1 to 3 m. The RAPS system should be designed to attain approx. 15 h retention time. (e.g. Watzlaf et al. 2000, Wolkersdorfer 2008, Matthies et al. 2010). If the system is constructed with two separate layers of organic substrate and limestone, the hydraulic conductivity might be restricted. To overcome hydraulic issues with layered limestone and compost system, Fabian et al. (2006) suggested mixing of both layers together to enhance hydraulic conductivity of the system.
Due to the vertical flow system the RAPS system requires approximately 20% of the land area that compost wetlands would require. However, to be able to force the water through the system and attain sufficient driving head, RAPS systems require sufficient topographic relief on the site. Thus, it is suggested that RAPS system should have at least 2.5 m site relief after the point of water emergence and about 1m freeboard above the compost layer (PIRAMID consortium 2003). 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. 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, referred in Cohen & Staub 1992).
Figure 1. Schematic picture of a RAPS system. © GTK
The required amount of limestone can be calculated as for ALDs. Taking the design flow of Qd (m3/d) for the system, we can readily calculate the bulk void volume needed Vv (m3) to store a 14-hours (t) worth of water, as follows (Younger et al. 2002):
Vv = Qd •t
In order to calculate the minimum total volume (Vt) of the active part of the RAPS, the effective porosity (ne) of the aggregate needs to be taken into account (Younger et al. 2002):
Vt = Vv/ne
The needed amount of the limestone aggregates (M) can be calculated as follows (Hedin & Watzlaf 1994):
M = Q VL tr/Vv • Q C T/x
where: Q = flow (l/h), VL = bulk density of limestone (kg/l), tR = retention time (h), Vv = bulk void volume (% in decimal form), C = required alkalinity (mg/l), T = design life, x CaCO3 content of the limestone (% in decimal form).
Monitoring and Maintenance need
Since the maintenance need of RAPS is restricted mainly to substitution of the organic substrate layer and observation of subsidence, it is fairly maintenance free system. Additionally, the precipitated metal sludge may be needed to dredge and dispose from the pond or wetland to maintain the settling capacity and retention time. However, if the system is adequately designed, the dredging of the precipitates is not needed in after care. Since, the purpose of RAPS system is to add alkalinity of the water to enhance metal precipitation after discharge and not within the system, monitoring of in and outflow parameters is essential. Any major decrease in metal content and/or similarly in alkalinity between in and outflows, indicates precipitation of metals inside the system and possible clogging of the limestone pores. Moreover, the monitoring of inflow is needed to ensure that concentrations are within the permit limits (e.g. MEND). Normally the RAPS is followed by a settling pond or constructed wetland where the treated water can be monitored. The monitoring procedures after mine closure are presented in a more detail in Closedure Monitoring part.
The RAPS system has not been yet applied in Finland, but despite the challenges that low temperature provides, many passive treatment systems have successfully operated year-round in northern climates. The following case studies were carried out in Canada and United States in similar weather conditions as in Finland (the list modified from Kuyucak et al. 2006):
- The CANMET (Natural Resources Canada) carried out pilot scale system at the Halifax International Airport. The system operated for 900 days, maintaining anaerobic conditions and meeting discharge limits even at 0ºC (Bechard et al. 1995).
- Three full-scale successive alkalinity-producing systems were designed and implemented to treat coal mine AMD in Pennsylvania, United States, where winter conditions are comparable to Finland (Kepler & McCleary 1994). According to two year monitoring, the system removed about 200 to 350 mg/l as CaCO3 acidity.
- Bench- and two-year long pilot scale tests with 4.5 l/min flow rate was carried out at the Fran Mine site in Pennsylvania. The test demonstrated that a RAPS system could successfully treat AMD containing >200 mg/l Al without plugging even in cold climates (Gusek 2002b). The pilot system operated for over four years and despite being over-loaded through the winter of 2002 and 2003, it did not plugg up with Al. During the pilot scale SRBR testing period, about 99.2%, 93.5% and 99.8% of Ni, Zn, Cu were removed, respectively. Based on the successful results, implementation of a full-scale buried system was considered to handle about 190 l/min (Gusek 2004).
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