Anoxic limestone drains (ALD)
Emmy Hämäläinen and Kaisa Turunen, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND, e-mail: kaisa.turunen(at)gtk.fi
Anoxic limestone drains (ALD) are abiotic systems consisting impermeable soil or sediment covered channels, buried cells or trenches filled with crushed limestone. The acidic mine water flows through the limestone horizontally and dissolution of calcite (CaCO3) is used to raise pH, add bicarbonate alkalinity of the water and to neutralize acidity. Finally once neutralized water contacts the atmosphere in the outflow, Fe and Al precipitates and settles to the bottom of the down-gradient pond or wetland after the ALD.
ALD is suitable for net acidic mine waters with low dissolved oxygen level. In addition, Fe3+ and Al concentration of the water has to be low. ALDs are normally used before discharging the mine water to the constructed wetland or to the settling pond in order to reduce acidity and to increase alkalinity of acidic and anoxic mine waters. (e.g. Barton & Karathanasis 1999, Younger et al. 2002, Lottermoser 2007, Wolkersdorfer 2008)
Theoretically well-designed ALD could last 50-100 years. However, there are ALDs which failed already less than 10 years due to relatively high aluminium concentrations in the mine water. The lifetime of ALDs normally ranges between 15 to 20 years and it is based on the amount and quality of the used limestone. (Wolkersdorfer 2008)
ALD is suitable for net acidic mine waters with low dissolved oxygen level. In addition, Fe3+ and Al concentration of the water has to be low, as they would precipitate onto limestone in relatively short period of time. The precipitation of metals would result in coating of the limestone, plugging of the drain and failure of the system. To maintain the anoxic conditions throughout the system, the channels are covered and the limestone should be inundated at all times to avoid access of air into the system (Barton & Karathanasis 1999). In addition, burying of the cells allows the development of high CO2 partial pressures which can produce high alkalinity concentrations (MEND 1996). According to Younger et al. (2002), in many cases the clogging is limited to the first 10-15% of the total length of the ALD. In some cases, the first part of the drain is possible to replace every few months thus preventing the clogging, but this adds obviously the costs.
If mine water to be treated contains dissolved Fe3+, Fe2+ or Mn2+ which would oxidize in current conditions, ALD is not recommended, because clogging is likely. In addition, aluminium is a general problem, due to its existence in environment as trivalent form (Al3+). Trivalent Al can hydrolyze easily under anoxic conditions and in high-pH conditions due to the existence of limestone precipitates as Al(OH)3 thus filling of the pore space. If such conditions, it is unwise to treat the water containing more than 1 mg/l of dissolved oxygen or more than 2 mg/l of each Al3+ or Fe3+ by ALD. Aluminium can be removed from mine waters with pre-treatments. (Younger et al. 2002, Lottermoser 2007, Wolkersdorfer 2008) In addition, if iron is precipitating to ferrous iron or Al precipitates clogging the system due to too high oxygen level (>1 mg/l), ALD can also be used combined with RAPS-systems or an anaerobic cell at highly contaminated mine water sites. (Barton & Karathanasis 1999). According to Hedin & Watzlaf (1994), gypsum (CaSO4) may form within ALD if the sulphate (SO4) content of the influent is > 2000 mg/l. Formation of gypsum on the limestone or between the pores may reduce the dissolution of limestone and/or the flow through the system resulting in system failure.
The ALDs vary in size between sites but they are usually shallow in depth (1-2 m) and narrow in width (0.6-1m). However, also wider systems of up to 20 m wide have been effective (Hedin et al. 1994a). The length of the system is based on the required retention time to reach the chemical equilibrium, which in turn depends on the predicted flow regime (MEND 1996). ALD system includes limestone layer from 1 to 2 m of thickness, which is covered with minimum 0.6 m of compacted soil. The covering is usually accomplished by burying the ALD with clay or other impermeable soil. To prevent oxygen penetration and to maintain high CO2 pressure in the system a plastic layer is added between the limestone and soil. To prevent the plastic layer from breaking due to compaction and root penetration, a geotechnical fabric is often placed between plastic layer and soil. The soil is often revegetated with species that discourages establishment of plants with deep roots, such as trees. The ALD can also be equipped with a flushing system to prevent clogging, but the effectiveness of the flushing is controversial. The ALD system is usually followed by a pond or wetland to receive the metal precipitates. The ponds or wetlands should be designed based on the retention time, allowing sufficient volume of precipitates to settle. (Brodie et al. 1991, Barton & Karathanasis 1999, Younger et al. 2002, Lottermoser 2007.)
ALD relies heavily on the retention time of water in the system. Studies on ALDs have shown that after 8-14 hours of retention time the alkalinity generation decreases to a value of 10-20% of the theoretical calcite saturation (Hedin et al. 1994b). This results from the reduction in the rate of calcite dissolution as saturation is reached (Berner & Morse 1974). Additionally, several studies have shown that with retention times beyond 14 to 20 hours ALD will not achieve further alkalinity (Faulkner & Skousen 1994, Hedin & Watzlaf 1994, Hedin et al. 1994a). Therefore in order to obtain the maximum alkalinity generation, the ALD should be sized to ensure at least minimum retention time of 14 hours. To achieve sizing for a 14 hour retention time it is assumed that limestone aggregate homogenous in size will end up with 50% porosity. This allows free flow but also sufficient surface area for dissolution. According to Hedin et al. (1994b) the ALD systems constructed with fines and small gravel (<2cm) results in plugging of the system due to decreased hydraulic conductivity. Therefore, larger particle size (8-25 cm) is suggested (e.g. Hedin et al. 1994a, Faulkner & Skousen 1994).
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 ALD, 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).
As the limestone dissolves when it reacts with mine water, the equation is valid only in the beginning of the ALD treatment (Younger et al. 2002). Additionally, the calcium content of the limestone affects the dissolution and alkalinity production in the system. According to Watzlaf & Hedin (1994), high calcium limestone dissolved faster and produced higher alkalinity than dolomite samples with lower calcium content. Once the volume of limestone required is determined, the layout of the system can be designed and the dimension of the ALD system can be calculated (Watzlaf et al. 2000).
The ALD should be designed to accommodate the maximum expected flow (Watzlaf et al. 2000). Since the flow through the system follows Darcy’s Law, to ensure the ideal flow (Q) through the system the following three factors should be enhanced: the hydraulic conductivity (K), the cross-sectional area (A) and the hydraulic gradient (i) (Younger et al. 2002).
Darcy’s Law: Q = KiA
The hydraulic conductivity can be ensured by supplying well-sorted and coarse limestone. Related to the cross-sectional area, the greater the surface area, the greater the accommodated flow and thus the mine water should be directed to ALD from its broadest edge. Hydraulic head affects the retention time of the system and is therefore crucial for the function of the ALD (Younger et al. 2002). Since the limestone should be inundated at all times, the groundwater regime of the site (saturated/non-saturated) affects the operation of ALD. In saturated conditions more of the influent stream is contacted with the limestone and more alkalinity is generated. The saturated conditions can be achieved by constructing the drain at shallow depths, or if greater depth is needed, inundation can be ensured by constructing cay dyke within or at the toe to raise water level. In addition, the outflow can be controlled by pipes to ensure water level rise in the system (Hedin et al. 1994a).
Figure 1. The cross section of ALD system, a) without creating a pond before the dam, b) with addiotional pond before the dam. (Adapted from e.g. Hedin et al. 1994a, Skousen et al. 1998). © GTK
Maintenance and monitoring need
Since the little maintenance need of ALD considers mainly annual observation of root penetration or subsidence, ALD 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 main cause of system failure related to ALDs is oxidation of Fe, Al or SO4 due to oxygen, regular monitoring is needed before discharging water to the nature. Since the purpose of ALD 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 Fe, Al, SO4 and oxygen concentrations are within the limits of the treatment system (Watzlaf et al. 2000). Normally the ALD is followed by a settling pond or constructed wetland where the treated water can be monitored. The monitoring procedures after closure are presented in more detail in Closedure Monitoring part.
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