Limestone and Steel Slag Leach Beds
Clayton Larkins, Geological Survey of Finland GTK, P.O.Box 1237, FI-70211, Kuopio, FINLAND, email: clayton.larkins(at)gtk.fi
Alkaline leach beds have been recognized as one of the most efficient passive systems for acid reduction in acid mine drainage (AMD) impacted waters (Ziemkiewicz et al. 2003). This passive water treatment method employs a pond filled with alkalinity source material located up-gradient of an AMD source. The alkalinity source material, generally limestone aggregate or steel slag, buffers leach bed effluent, which neutralizes acidity and precipitates metals in AMD impacted waters (Black et al. 1999, Goetz & Riefler 2014). Leach beds are generally designed to receive acidic, metal free influent water and discharge effluent with high buffering capacity while sustaining minimal metals precipitation within the alkalinity source material (INAP 2009). Leach beds can be designed for in-situ AMD treatment by leaching buffered effluent directly to an AMD source, or by buffering water that will mix with AMD impacted waters downstream of the leach bed (Ziemkiewicz & Skousen 1998). Alkaline leach beds can also be used as a final, polishing step in a passive treatment system, to oxidize and remove residual manganese from treated water (INAP 2009). Under all applications, leach beds require influent with low pH and low metals concentration for optimal performance (Zipper & Skousen 2010).
Alkaline leach beds are advantageous because they are relatively simple to construct and maintain (Ziemkiewicz et al. 2003). As with other passive water treatment systems, monitoring and maintenance is required, though to a significantly lesser extent than for active treatment technologies. Alkalinity in a properly functioning leach bed will be slowly exhausted, but can be easily replenished by adding more alkaline substrate to the pond. Leach bed design and applicability is based on water quality, flow rates, topography, and other site specific characteristics (Ziemkiewicz et al. 2003, Zipper & Skousen 2010).
Description of the technology concept and methods
Leach beds treat AMD through neutralization and metals precipitation. By providing alkalinity from up-gradient of an AMD source, metals precipitation induced armouring and clogging within the alkalinity source is minimized. Therefore, this method provides a suitable initial treatment step for AMD impacted water with relatively high concentrations of ferric Fe and Al, the primary sources of flocculent in AMD. However, a low metals concentration water source must be available upstream. At some sites it may be possible to create a sufficient fresh water source from surface runoff and precipitation (Ziemkiewicz & Skousen 1998).
Leach bed alkalinity production depends on the alkalinity source material, influent water quality, and water residence time within the leach bed (Ziemkiewicz et al. 2003, Zipper & Skousen 2010). Limestone aggregate leach beds can generate 50 to 75 mg/l alkalinity, while steel slag, an alkaline waste product of steel manufacturing, can produce over 2,000 mg/l alkalinity (Simmons et al. 2002, Goetz & Riefler 2014). For optimal, sustained alkalinity production, influent water should be a pH of less than 3 with very low metals concentrations (Black et al. 1999). Further consideration of site-specific water quality allows for more accurate system design. For example, accounting for acidity due to manganese in solution, which does not contribute to limestone dissolution at circum-neutral pH, will allow for more accurate predictions of leach bed alkalinity generation (Zipper & Skousen 2010). Water residence time is a critical consideration because while longer interaction between water and alkalinity source increases total alkalinity production, the rate of alkalinity production exponentially decreases over exposure time due to increasing pH and the concentration of oxidation products in solution (Zipper & Skousen 2010). Recommended leach bed water residence time ranges from 1 to 3 hours (Black et al. 1999, Simmons et al. 2002).
The use of steel slag as an alkalinity source for AMD treatment is a relatively recent innovation (Ziemkiewicz & Skousen 1998, Simmons et al. 2002). Steel slag contains high concentrations of soluble alkalinity, primarily as Ca(OH)2 and Ca-(Fe)-silicates. The high neutralization potential of steel slag provides a feasible passive treatment option for AMD impacted waters that require buffering beyond the capacity of limestone (Simmons et al. 2002, Mack & Gutta 2009). The use of steel slag for AMD water treatment is particularly practical in regions of prevalent steel production, such as the Appalachian mountains of the United States (Ziemkiewicz & Skousen 1998, Goetz & Riefler 2014). However, the use of steel slag poses potential environmental concerns, including metals mobilization from the slag itself and the production of toxic levels of alkalinity, which must be considered during the design and monitoring of leach bed applications (Simmons et al. 2002, INAP 2009).
Alkaline leach beds are also used as a final, polishing step in AMD water treatment systems designed to remove manganese oxide (INAP 2009). Unlike the abiotic application of leach beds upstream of AMD sources, polishing leach beds depend on bacterial and algal growth to catalyze the oxidation and subsequent removal of Mn from solution. To facilitate oxidation of Mn, the limestone aggregate should not be fully inundated, and should breach the pond water’s surface. The aggregate provides a growth medium for bacteria and algae that accelerate Mn oxidation (INAP 2009). All iron must be removed from the water entering the polishing leach bed, as ferrous Fe acts as a reductant to Mn, causing it to re-dissolve and subsequently inhibits the leach bed’s effectiveness as a polishing step (INAP 2009).
Development stage, links to cases
Mack and Gutta (2009) present three case study applications of steel slag leach bed treatment of AMD at sites in West Virginia, USA.
Black et al. (1999) provide construction details and initial results from a limestone leach bed designed to buffer AMD waters entering Beaver Creek in West Virginia, USA.
Skousen and Ziemkiewicz (2005) assess the performance of passive mine treatment technologies across 116 sites in the south eastern United States, including 17 limestone leach bed and 2 slag leach bed applications.
Alkaline leach beds provide a passive AMD treatment option that can be applied to AMD material or AMD impacted waters when an upgradient, acidic, metals free water source is available. In contrast to anoxic limestone drains that require low dissolved oxygen (DO) and relatively low dissolved metals concentrations, as an upstream source of alkalinity, leach beds can be used to treat net acidic waters with DO, ferric Fe, and Al concentrations greater than 1 mg/l (Ziemkiewicz et al. 2003). Conversely, for manganese removal, the leach bed influent must have very low Fe content and is therefore applied as a final treatment step (INAP 2009). Table 1 summarizes the benefits and barriers of the primary leach bed applications.
Table 1. Advantages and disadvantages of leach bed applications (adapted from Gusek 2008, Goetz & Riefler 2014)
|Limestone leach bed as upstream source of alkalinity||
|Steel slag leach bed as upstream source of alkalinity||
|Limestone leach bed as downstream pond for Mn precipitation||
Ziemkiewicz et al. (2003) assessed the performance of four passive water treatment methods across 83 AMD producing sites in the eastern United States. Their review found limestone leach beds, applied primarily in Alabama and Tennessee, to be among the most efficient passive methods for acid reduction. Efficiency in this study was evaluated on a cost (price per ton of acid removed per year), and removal (the amount of acid removed per day per ton of limestone in the system) basis.
Steel slag leach beds produce significantly greater alkalinity than limestone leach beds and therefore have greater capacity to remove acidity (Skousen & Ziemkiewicz 2005). While performance of steel slag leach beds generally decreases exponentially over time, they are still capable of exceeding acid reduction performance of limestone leach beds (Goetz & Riefler 2014).
Leach beds assessed by Skousen & Ziemkiewicz (2005) ranged in size from approximately 414 to 6,250 tons of limestone and were observed to reduce acid loading from 1.2 to 60.1 tons per year. The average annual acid load reduction for the studied limestone leach beds was 17.6 tons per year. In a preceding study, which also included open limestone channels, vertical flow wetlands, anaerobic wetlands, and anoxic limestone drains, the acidity reduction achieved with limestone leach beds was second only to anoxic limestone drains, which were observed to remove an average of 22.2 tons of acidity per year (Ziemkiewicz et al. 2003).
As a relatively new innovation, only two steel slag beds were assessed as part of the study by Skousen & Ziemkiewicz (2005) and these had each been in operation for less than 2 years. However, steel slag leach beds were found to have the greatest acid treatment capacity of all reviewed passive treatment methods, removing an average of 76.3 tons of acidity per year. The two studied steel slag beds were smaller than limestone beds, containing approximately 75 and 200 tons of slag (Skousen & Ziemkiewicz 2005).
The maximum service life of alkaline leach beds has been estimated to be approximately 20 years, at which time either structural maintenance or alkalinity replenishment is required (Ziemkiewicz et al. 2003). Under ideal conditions of slightly acidic, metal free influent, an alkaline leach bed will hypothetically perform until the alkalinity source is exhausted (Skousen & Ziemkiewicz 2005). In practice, the armouring of limestone and flow reduction due to sediment clogging of pore space has been observed to reduce performance over time (Kruse et al. 2010, Goetz & Riefler 2014). Maintenance may entail replenishing or replacement of the alkalinity source material, or leach bed flushing (Mack & Gutta 2009, Goetz & Riefler 2014). Steel slag leach beds induce greater sedimentation resulting from intense alkalinity generation, and require more regular maintenance than limestone leach beds. For high flow slag leaching systems, maintenance may be required every one to two years, while in lower flow systems, slag leach beds have been observed to meet performance requirements without maintenance for up to 4 years (Goetz & Riefler 2014).
Maintenance needs can be reduced with adequate leach bed design planning and operation. Essential considerations include influent water quality, grain size of alkaline material, and factors that affect alkaline material reactivity (Goetz & Riefler 2014). Influent water quality considerations include the following:
- metals concentrations
- acidity source
- alkalinity concentrations
- exposure to atmospheric CO2 (noted to contribute to carbonate deposition in discharge piping)
Alkalinity source material grain size is a trade-off based on site specific water quality and flow, as smaller grain sizes generally provide greater reactivity, but are also more prone to sedimentation clogging. Additionally, Goetz & Riefler (2014) propose that the age of steel slag used in a leach bed is inversely proportional to its initial alkalinity production capacity.
Environmental and monetary cost aspects
Gravity fed, post-construction daily leach bed operations do not require energy inputs. Therefore leach beds are considered a relatively environmentally sustainable treatment option. The primary environmental and monetary costs of properly functioning passive systems are incurred during initial construction. A life cycle assessment study of different AMD water treatment methods found that the largest environmental impact for passive water treatment technologies was generally the transportation of alkaline material to the remediation site (Hengen et al. 2014). Both environmental and construction costs can be reduced if alkaline aggregate can be sourced near the site of remediation (Hengen et al. 2014).
Additional potential environmental costs associated with the use of steel slag as an alkalinity source include generation of toxic levels of alkalinity, or the release of heavy metals during alkalinity production. To minimize potential harmful effects from the use of slag, the chemistry of both the AMD impacted waters, and the slag proposed as an alkalinity source must be well characterized. The acidity loading and natural buffering within a stream system will dictate whether the use of steel slag is warranted, and can be applied without causing toxic alkalinity levels. For example, at the McCarty Highwall site in West Virginia, steel slag leach beds were utilized because acidity loading to the stream system surpassed the buffering capacity that could be achieved using limestone leach beds (Simmons et al. 2002). Although the slag leach beds produced effluent of greater than pH 9, effluent was quickly neutralized as it treated downstream acidic inputs, resulting in stream waters with near neutral pH (Simmons et al. 2002, Mack & Gutta 2009).
Ziemkiewicz and Simmons (2002) describe a mine water leaching procedure to assess neutralization potential and metal leaching risks from steel slag used in leach beds. This method, unlike other leach tests that may only account for relatively fast reactions, is intended to account for long term reactivity that will influence water quality in leach bed applications (Simmons et al. 2002).
In a review of 8 different passive mine water treatment methods across 116 different sites, Skousen & Ziemkiewicz (2005) found limestone leach beds to be, on average, the 5th most cost efficient treatment option, at 207 U.S. dollars per ton of acid treated per year. This average cost was based on the review of 17 different limestone leach bed sites. The review only included two, relatively young steel slag leach beds. Therefore the authors caution that the results from such a limited sample size may not be representative and should be interpreted as preliminary. However, these two steel slag leach beds provided the greatest average cost efficiency of all the methods reviewed, at 36 U.S. dollars per ton of acid treated per year (Skousen & Ziemkiewicz 2005).
Site specific data needs
The selection and design of a passive treatment system is based on water chemistry, flow rate, topography, and other site specific characteristics (Ziemkiewicz et al. 2003). For the traditional, upstream application of leach beds, water quality of both the AMD impacted water, as well as the un-impacted water providing leach bed influent must be characterized. Characterization of the AMD impacted waters is the first step in all water treatment design processes, and will dictate whether alkaline leach beds provide the most practical treatment option (Gusek 2008). Additionally, there must be a viable source of relatively un-impacted water that can be utilized to provide buffered discharge to the AMD source, or impacted waters.
A detailed evaluation of acidity and alkalinity sources in the un-impacted leach bed influent will enable leach bed design for greater performance. Accounting for acidity sources that will not dissolve leach bed limestone will enable more accurate predictions of alkalinity generation. For example, Mn in solution will not oxidize at pH less than 7, and therefore is not expected to contribute to alkalinity production in a limestone leach bed (Zipper & Skousen 2010). Similarly, ferrous Fe in solution will not contribute protons without the presence of oxygen (Zipper & Skousen 2010). Elevated Ca2+ concentrations will suppress alkalinity generation as a result of solution chemistry mechanisms (Zipper & Skousen 2010). Additionally, alkalinity in leach bed influent can produce carbonate precipitates in the presence of dissolved CO2, which like ferric Fe and Al derived metal precipitates, results in sedimentation induced flow and reactivity restrictions (Zipper & Skousen 2010, Goetz & Riefler 2014). Further, alkalinity in influent water may result in net acid production from steel slag leach beds, as observed at one leach bed in south eastern Ohio (Goetz & Riefler 2014).
Local weather and climate are important considerations during remedial design, as freezing temperatures impact the performance of above-ground passive treatment systems. Performance of a limestone leach bed at the Beaver Creek site in West Virginia was significantly reduced during periods when the bed was partially frozen (Black et al. 1999). Reduced alkalinity generation during periods of freezing temperatures were attributed to ice induced preferential flow through the leach bed, which caused reduced contact between water and the alkalinity source material, as well as slowed dissolution reaction rates under low temperatures (Black et al. 1999).
Seasonal variation in influent flow rate and water chemistry can also reduce leach bed alkalinity production. Seasonal low flows resulting in lower rates of alkaline leach bed effluent discharge have been identified to cause seasonal variability in leach bed performance (Ziemkiewicz & Simmons 2001).
Materials and appliances requirements
The primary advantage of leach beds over other passive systems is their simplicity. They can be implemented within existing water bodies, or constructed ponds. The materials required for construction generally consist of alkaline source material and piping (e.g. PVC pipe) for the development of a drainage system. Construction likely entails the use of earth moving equipment scaled to the size of the leach bed installation.
Monitoring and controls
As a passive treatment system, leach beds require significantly less monitoring than active treatment alternatives. None-the-less, leach bed monitoring is required to evaluate performance, inform the need for maintenance, and facilitates method advances. Monitoring generally includes flow rate and water quality measurements of influent and effluent waters. These data are used to assess the rates of sedimentation build up and alkalinity production within the leach bed. Kruse et al. (2012) identified the potential to use an analysis of alkalinity source in leach bed effluent as a means to predict impending failure. Carbonate dominated alkalinity in effluent was associated with impending sedimentation clogging, while effluent alkalinity derived from a well distributed combination of hydroxide, carbonate, and bicarbonate was associated with well functioning systems (Kruse et al. 2012). Using alkalinity source characterization as a proxy for long term performance, they identified potential measures for reducing clogging, which included mixing steel slag with river gravel, wood chips, or both. They postulate these steel slag amendments may increase long term performance by promoting microbial processes and increasing the hydraulic conductivity of the medium without contributing significant alkalinity to the system (Kruse et al. 2012).
Limestone leach beds have been widely implemented and studied in the eastern USA (Ziemkiewicz et al. 2003, Skousen & Ziemkiewicz 2005). Steel slag leach beds, while a younger technology, have come into common use and been the subject of numerous recent studies in south eastern Ohio and West Virginia, where steel production and AMD impacted waters occur in close proximity (e.g. Ziemkiewicz & Skousen 1998, Simmons et al. 2002, Kruse et al. 2012, Goetz & Riefler 2014, 2015). While the hypothetical operational life span for leach beds corresponds to the exhaustion of the bed’s alkalinity source, leach bed performance has been observed to decrease at greater than expected rates, especially with the use of steel slag (Ziemkiewicz & Simmons 2001, Simmons et al. 2002, Kruse et al. 2012). Failure mechanisms leading to reduced performance in alkaline leach beds over time have been investigated through site monitoring, laboratory testing, and geochemical modeling, and have led to refined design criteria (Ziemkiewicz & Simmons 2001, Kruse et al. 2012, Goetz & Riefler 2015).
In a series of limestone beds installed to treat AMD impacts at the Laurel Creek site in Tennessee, most leach beds were observed to treat similar acidity loads, and depending on loading rates, and provided acidity load reduction ranging from 23 to 78% (Ziemkiewicz & Simmons 2001). However, the development of preferential flow, or piping, at one of the leach beds significantly limited alkalinity generation and subsequently, that leach bed had no impact on acid loading (Ziemkiewicz & Simmons 2001). Piping due to aggregate drainage issues can easily be fixed, once identified through monitoring.
In steel slag leach beds the two primary causes for decreasing performance with time are sedimentation clogging and the inherent exhaustion of readily available alkalinity (Goetz & Riefler 2014). Sedimentation clogging has been the subject of multiple studies. Kruse et al. (2012) evaluated 6 steel slag leach beds, of which two were fully functional, three were operating below performance expectations, and one had been abandoned. This study found sedimentation clogging due to carbonate precipitation to be the main cause of reduced performance. They identified analysis of alkalinity sources in effluent as a means to monitor steel slag leach bed functionality and predict impending leach bed failure. They also proposed methods for extending the functional life of steel slag leach beds by amending slag with river gravel or wood chips to promote microbial processes and increase the medium’s hydraulic conductivity (Kruse et al. 2012).
Goetz & Riefler (2014) identified an approximately exponential decrease in alkalinity production rates over time from steel slag across 12 sites monitored in south eastern Ohio. On average, alkalinity production rates decreased more than 75% after 50 bed volumes of discharge (Goetz & Riefler 2014). Decreasing alkalinity production was attributed to the consumption of readily available alkalinity from slag surfaces. In addition to influent water chemistry and flow rate, this study identified the slag surface area and age prior to leach bed construction as controls for the initial alkalinity generation capacity (Goetz & Riefler 2014). Subsequently, aggregate grain size within leach beds is an important design consideration impacting both sedimentation clogging, and alkalinity generation. They predict the greatest alkalinity production to occur under low flow, low alkalinity conditions where fresh steel slag has been used as an aggregate (Goetz & Riefler 2014).
Although leach bed performance has been observed to decreases at greater than anticipated rates after implementation, ongoing research has provided insight that can inform system design and operation to optimize performance (e.g. Kruze et al. 2012, Goetz & Riefler 2014). Steel slag leach beds, in particular, have been observed to out-perform alternative passive remediation options with regard to alkalinity production and acid neutralization in AMD impacted systems (Skousen & Ziemkiewicz 2005, Goetz & Riefler 2014). While leach bed utilization is limited by site-specific factors such as the presence of an influent fresh water source, alkaline source material availability, and climate considerations, they are an attractive AMD treatment option due to treatment capacity, simplicity of design, and relative cost efficiency (Ziemkiewicz et al. 2003, Gusek 2008).
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