Kaisa Turunen & Emmy Hämäläinen, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND. kaisa.turunen(at)gtk.fi
Open limestone channels (OLC), sometimes also called oxic limestone drains (OLD), are simple constructions for acidic mine water treatment. The idea is that as the acidic mine water flows through a channel filled with coarse limestone cobbles, CaCO3 dissolves and increases the pH and alkalinity of the effluent. The channel can be an existing ditch (Fig. 1), a larger constructed open channel, or a stream filled with limestone cobbles (Wolkersdorfer 2008). Since the channel is not sealed to prevent oxygen access to the system, metals readily precipitate, armouring the limestone. Therefore, the amount of limestone needed in an OLC is greater than in, for example, ALD or RAPS systems in order to overcome the loss of efficiency to armouring (EPA 2014).
Figure 1. Open limestone channel at the Luikonlahti Cu-Zn-Co-Ni mine in Finland, a) Toe seepage site before construction in May 2003, b) after construction in May 2008 and c) in May 2013. Photos © Marja Liisa Räisänen (a, b) and Kaisa Turunen (c), GTK
When designing and constructing an OLC, it is important to pay attention to the water velocity in the channel, channel slope and mine waters acidity. Metal precipitates coat the limestone quickly if the flow rate in channel is too slow. One the other hand, the contact time between the acidic mine water and the limestone must be long enough to allow the desired dissolution of CaCO3 .
In general, it is assumed that armoured limestone treats acidic mine water poorly. Ziemkiewicz et al. (1994, 1997) have studied the relative efficiency of armoured and unarmoured limestone for the treatment of acidic waters. For fully armoured limestone, CaCO3 is 20 to 50% as soluble as unarmoured limestone, though the dissolution depends on different variables such as pH and the thickness of the armour coating. Ziemkiewicz et al. (2003) studied the acid removal efficiency of 84 different types of passive water treatment technologies located in United States, of which 10 were OLCs. The OLC systems varied in size (40 to 2,700 t), years of service (1 to 6 years) and flow velocities (0.3 to 42 l/s). In most of the systems the efficiency was 15 to 20 g/day/t and treated, on average, 9.9 t of acid per year . The average cost per t/yr of acid removed was $138 and the average total construction cost was $27,500. The removal efficiencies of other treatment systems studied (Anoxic limestone drain, Vertical flow wetland, Anaerobic wetland and Limestone leach beds) were higher, but the average construction cost for OLCs was the lowest of the systems studied, and the cost related to acidity removal rate was the second lowest. In addition, OLC systems are fairly simple to construct. (Ziemkiewicz et al. 2003).
In titration studies armoured limestone was only 2 to 45% less effective in the treatment of acidic mine waters than unarmoured limestone. In a laboratory container study, armoured limestone was found to be 90% as effective as unarmoured limestone. Sizing the OLC is based on 90% of acidity being neutralized in one hour of contact time or 100% of acidity being neutralized in three hours of contact time (Ziemkiewicz et al. 1997, Telliard 2000). According to Wolkersdorfer (2008), the flow rate within an OLC should be over 200 l/min, and after the limestone treatment the water should be channelled to a settling pond before discharging to nature (Ziemkiewicz et al. 1997, Telliard 2000).
The well-defined advantages of OLC systems are their relatively low cost and simplicity. There are, however, some limitations for using an OLC system. The dissolution rate of limestone and the contact time between acidic water and limestone limit the system’s neutralization capacity. If metal flocculent (floc) fills the limestone pore space alkalinity production will decline, limiting the system’s effectiveness. For mine sites that discharge large volumes of acidic mine water, long channels (more than 900 meters) with thousands of tons of limestone would be required for suitable water treatment. Therefore OLC application may not always be feasible, especially at space-limited mine sites. Also the vicinity of the receiving water system and site topography must be considered with regard to OLC construction feasibility, as the recommended OLC slope gradient is 10 to 20% (Telliard 2000, Green et al. 2008).
Due to the tendency for armouring, the success of the OLC system is strongly dependent on the channel dimensions and slope (Ziemkiewicz & Brant 1996). The dimensions of the channel affect the turbulence and velocity of the water flow through the channel and thus also the tendency for armour build-up. The length of the channel needs to be sufficient for limestone to react with the effluent and for neutralization to occur. If the gradient is too great water flows too quickly, whereas if the gradient is too low metal precipitates accumulate in the channel thus inhibiting the neutralizing capacity and altering the flow characteristics of the system (EPA 2014). Green et al. (2008) recommend that the slope should be as great as 20%. A settling pond for OLC discharge is needed because the channels are designed to flush metal floc from the outlet point (Wolkersdorfer 2008).
Monitoring / control needs
If the OLC is designed and constructed properly, it should be maintenance free and treat acidic mine waters for decades (Ziemkiewicz et al. 1997). In some cases preferential flow of water through the OLC results in insufficient water treatment. Therefore monitoring is needed to ensure these conditions do not develop. Especially for long-term use, it is important to monitor the treated water regularly to ensure that the OLC is working properly. Monitoring can be conducted at the settling pond. Also the settling ponds should be cleaned regularly to maintain treatment efficiency (Telliard 2000, Wolkersdorfer 2008).
OLC dimensions are calculated mainly based on dissolution rate. Ziemkiewicz et al. (1994) developed a model for estimating limestone volumes and channel dimensions based on the following first-order kinetics:
ln Cf/Co = -kt
Cf = final acidity (mg/l)
Co = original acidity (mg/l)
k = rate constant (1/h)
t = time of reaction (h)
The rate constant (k) is for acid neutralization by limestone and is usually based on the studies of Pearson & McDonnell (1975), being -2.303/h.
Case examples of mine sites with open limestone channel in Finland:
The Luikonlahti Cu-Zn-Co-Ni mine in Kaavi. The Luikonlahti site was evaluated as a case study site in the Closedure project and a more detailed description of Open limestone channel as well as wetlands located in Luikonlahti site can be found in Case studies of Closedure articles: Constructed wetland – Luikonlahti mine site.
EPA, 2014. Reference guide to Treatment Technologies for Mining-Influenced Water, EPA 542-R-14-001. United States Environmental Protection Agency.
Green, R., Waite, T., Melville, M. & MacDonald, B. 2008. Effectiveness of an Open Limestone Channel in Treating Acid Sulfate Soil Drainage. Water Air Soil Pollut (2008) 191:293–304. Available from: http://anr.ext.wvu.edu/resources/295/1255464764.pdf
Pearson F. & McDonnell, A. 1975. Use of crushed limestone to neutralize acid wastes. Proc Paper 11131, J Env Eng Div, Am Soc Civil Eng 101, pp. 139-158.
Telliard, W. 2000. Coal Remining Best Management Practices Guidance Manual. Office of Water, Office of Science and Technology, Engineering and Analysis Division. U.S. Environmental Protection Agency. 2000. 522 p. Available from: http://water.ky.gov/permitting/Documents/CoalBMPManual.pdf
Wolkersdorfer, C. 2008. Water Management at Abandoned Flooded Underground Mines. Fundamentals, Tracer Tests, Modelling, Water Treatment. Springer. 465 p.
Ziemkiewicz, P.F. & Brant D.L. 1996. The Casselman River Restoration Project. In: Proceedings, Eighteenth West Virginia Surface Mine Drainage Task Force Symposium, 15-16 April 1996, Morgantown, WV.
Ziemkiewicz, P., Skousen, J., Brandt, D., Sterner, P. & Lovett, R. 1997. Acid Mine Drainage Treatment with Armored Limestone in Open Limestone Channels. Green Lands, NMLRC. Pages 1 and 1018-1024. Available from: http://anr.ext.wvu.edu/resources/295/1255464764.pdf
Ziemkiewicz, P., Skousen, J. & Lovett, R. 1994. Open limestone channels for treating acid mine drainage: a new look at an old idea. Green lands 24 (4), pp. 36-41.
Ziemkiewicz, P., Skousen, J. & Lovett, R. 2003. Mine Water and the Environment 22, pp. 118–129. Springer-Verlag.