2. Determining the Applicability of a Biochemical Reactor 

Figure 2-1. Decision flow process for determining the applicability of a biochemical reactor.

As the influent is characterized, consider seasonal fluctuations in chemistry and flow in response to climate. Characterizing the MIW may require seasonal monitoring of flow and chemistry for a year or more. Peak flow and loading conditions should be well understood, since these are factors that affect the size and required components of the BCR system. Since peak flows and loading may change year-to-year, gather as much information on annual changes as is practical. Evaluation of historical regional stream flow data and climate data (such as that collected and published by United States Geological Survey (USGS), National Oceanic and Atmospheric Administration (NOAA) or others) may be useful in assessing overall trends and predicting peak flows and seasonal flow fluctuations if actual site data is not available.

The overall goals for a specific treatment system depend on the geographical location and receiving water body, as well as whether the site is an abandoned or active mine site. Generally it is assumed that a treatment system discharges effluent to surface water rather than groundwater. Surface water discharges to streams, wetlands, and oceans are typically regulated by the Clean Water Act of 1972 (CWA) or state or tribe equivalent. Discharges to groundwater may be considered as an alternative and may have different regulatory requirements. In most cases, active mine sites must comply with provisions of Surface Mining Control and Reclamation Act (SMCRA) for coal mining and the National Pollutant Discharge Elimination System (NPDES); see the USEPA NPDES Permit Program Basics web page (http://cfpub.epa.gov/npdes/home.cfm?program_id=45) for more information. For hard rock mining, sites must secure permits for discharges to surface waters. Allowable contaminant concentrations under NPDES permits are determined by USEPA's technology-based standards, maximum contaminant levels (MCLs) or toxicity-based aquatic life standards established for stream uses, or the state or tribal equivalent. The permits are generally site specific and may include total maximum daily loads (TMDL) to determine the treatment effluent goals. A specific point of compliance (POC) is normally established during the permit process (Section 6.2.3).

Regulatory flexibility depends on the purpose of a treatment system, including whether the system is intended as an interim water treatment measure or a long-term treatment solution. Regulatory flexibility also depends on whether the system is intended to provide at least some level of treatment in an abandoned, remote location that is otherwise impractical for more active water treatment systems (see ITRC MW-1 2010 Regulatory Issues , https://projects.itrcweb.org/miningwaste-guidance/reg_issues.htm and Section 6 of this document).

Often when BCR systems are used as an interim water treatment measure, (while a long-term plan is being formulated) certain water quality parameters may be waived that would otherwise be applied to treatment plant discharges. Often only pH and metals are targeted for regulatory compliance in these interim or very remote situations (see Appendix B.8, Leviathan Mine Case Study and Appendix B.2, Mayer Ranch Case Study). Depending on the setting and goals of the system it may be possible to target percent contaminant removal for the system. However, when a BCR system is planned as the long-term solution in an accessible area, regulators may target water quality parameters that are more comprehensive and may require it to meet tribal, state, or local water quality criteria or MCLsmaximum contaminant levels.

Regulatory treatment goals normally apply to the final effluent of the sequential treatment system, which includes the BCR. The treatment system may contain pretreatment such as a settling pond to reduce the sediment load (Section 2.4) prior to a BCR, or posttreatment unit such as an aerobic wetland (Section 2.5). However, each of the components of the treatment system, including the BCR, must have an established treatment goal in order to accurately design the BCR and required pre- and posttreatment units for successful installation and performance. Establishing treatment goals for individual process units for the treatment system is done in the design phase. Well-defined expectations also enable the operator to develop an appropriate intersystem monitoring program to identify and resolve potential problems before damage to the system occurs.

Figure 2-2. A periodic table of passive treatment for MIW.

Source: Reprinted with permission and additional modifications from author (Gusek 2009).

TDS are not removed in BCRs and in some cases certain TDS components may increase due to alkalinity addition or other amendments in the BCR or pre- and posttreatment units. While calcium is also conserved in BCRs and may actually increase if limestone is a reactor substrateEither (a) a chemical which reacts or (b) a solid surface or (c) an electron donor. component, calcium is depicted in green because its beneficial contribution to hardness diminishes the toxicity of some metals to aquatic life.

BCRs generally are designed to reduce sulfate to sulfide to promote the formation of metal sulfide precipitates, which are removed as amorphousHaving no crystalline form. solids from the MIW. Sulfate in the influent water is a necessary component for metals removal by BCR treatment. The minimum concentration of sulfate required to remove the desired concentration of metals can be calculated stoichiometrically from the sulfide reduction and the metal sulfide precipitation equations (Table 1-2 and Gusek 2002). A contingency is then added to account for sulfate or sulfide lost to gaseous hydrogen sulfide formation, adsorptionNon-covalent bonding of a chemical to a solid surface., and inefficient transformation. In most MIW waters, sulfate is stoichiometrically in excess of metals and is not removed by a significant percentage. Certain sites may only see a 10 to 20% decrease in sulfate concentration in the effluent (see Appendix B.8, Leviathan Mine Case Study). However, in certain MIW waters, low sulfate concentrations may limit the metals removal capacity and preclude treatment by a BCR. Additionally, biological sulfate reductionThe stripping of oxygen atoms from sulfate (SO₄²⁻), most often yielding sulfide (S²⁻) as an ultimate product. to sulfide can be the rate-limiting step in the process if conditions are not favorable for this biological activity.   

As shown in Figure 2-2, a BCR can remove both aluminum and iron by aerobic and anaerobic processes. Often these metals are in very high concentrations in proportion to other metals in the MIW. Therefore, calculate the loading of these metals because they can contribute to rapid clogging of the BCR, require a larger size BCR, require pretreatment, and shorten the life span of the system. When high concentrations of iron or aluminum are present in the MIW, consider pretreatment steps to reduce this load to the BCR. Pretreatment to reduce metal concentrations can allow smaller sizing and longer design life for the BCR. Various pretreatment technologies are described further in Section 2.5.

BCRs can also treat other compounds in MIW. Cyanide, from gold leaching processes, is degraded biologically in the anaerobic conditions of a BCR. Nitrate and nitrite residuals from blasting are biologically reducedIn chemistry, having gained electrons. Often gaining electrons is accompanied with gaining protons (hydrogen). As an example, when O₂ reacts with H₂, the oxygen is reduced, forming H₂O. to nitrogen gas in BCRs. Ammonia, also a residual from blasting, may be taken up by the bacteria or plants if they are present in a BCR, but is most effectively removed by oxidation to nitrate in aerobic conditions prior to or within a BCR. Also, total suspended solids (TSS) may be removed to varying degrees by interception within the BCR, although TSS removal may be greater in BCR with solid phase substrates (see Section 3).

Effluent from BCRs typically contains residual soluble organic substrate and reduced compounds (sulfur forms), and dissolved oxygen (DO), that can adversely affect water quality. Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) can substantially increase from BCR treatment because of the dissolution and transport of the organic material added as microbial substrate. Additionally Butler et al. (2011) demonstrated toxic BCR effluent by aquatic toxicity testing from four different treatment systems. The effect of discharging effluent with these characteristics should be weighed against the benefit of treatment for the other constituents and the possible ecological receptors in the receiving water body, which could be impacted. Posttreatment steps should be considered to remove these compounds and raise dissolved oxygen levels prior to final discharge for most sites.

Hydrogen sulfide may be generated in a BCR and can present occupational health and nuisance concerns. Hydrogen sulfide can accumulate to toxic levels in certain circumstances such as under a BCR cover. The Occupational Safety and Health Administration (OSHA) has set an acceptable ceiling limit at 20 parts per million (ppm); other organizations recommend varying levels. Concentrations in air above 500 ppm may result in death. At very low concentrations no health effects have been found in humans, so at low concentrations the gas is considered only a nuisance due to the rotten egg odor. See the hydrogen sulfide fact sheet from the Agency of Toxic Substances and Disease Registry (ATSDR) for more information (http://www.atsdr.cdc.gov/tfacts114.pdf). The design process must address the generation of hydrogen sulfide gas since proper design and siting of the BCR can mitigate this problem. Methods of H₂S control include aeration, blending untreated water with effluent, chemical oxidation, and pH control (neutral). Hydrogen sulfide gas can also create a corrosive environment in the immediate proximity to the BCR and corrode metal components of equipment.

Figure 2-3. Decision tree for pre- and posttreatment units for BCR systems.

Source: Adapted from Gusek (2008).

Aeration processes can be active or passive. Active aeration can be completed within an MIW storage pond, tank, or pipe using a mechanical mixing device or by injection of air or oxygen. Passive aeration can be completed by routing MIW over cascades, riffles, or along roughened channels. This type of aeration requires little energy, but is only applicable to sites that have the appropriate topographical conditions. The energy can often be supplied with wind or solar energy generation on site (Appendix B, Mayer Ranch Case Study). INAP’s GARD Guide (2010) provides examples of numerous oxidation processes for MIW, as does PIRAMID (2003). Aeration can also be completed using trickling filters or by chemical oxidation methods. Trickling filters operate by feeding water through the top of a tank packed with high surface area media (Younger et al. 2002). An active commercial treatment system that aerates water with the use of a rotating drum is the Rotating Cylinder Treatment System™ (RCTS™) described in ITRC MW-1 (2010).

Because a BCR operates in a reduced oxidation state, oxidation of MIW prior to BCR treatment may not be desired. However, oxidation of MIW may be a component of other necessary pretreatment approaches such as alkaline addition, sedimentation, or aerobic wetland treatment. In some cases, the MIW may already be in an oxidized form due to site-specific conditions. When the oxidized MIW has high iron concentrations at or near saturation, further aeration can convert more of the iron to the oxidized form and maximize the formation of ferric iron precipitates. BCR effluent can be oxidized using methods similar to those mentioned above, but oxidation is most commonly achieved with constructed aerobic wetlands and settling ponds.

Constructed aerobic wetlands typically consist of shallow excavations filled with flooded gravel, soil, and organic matterStrictly defined, compounds in which carbon is bonded to hydrogen. Generally describes decomposed biological residues and other organic compounds synthesized by organisms. to support wetland plants such as Typha, Juncus, and Scirpus sp. (Skousen et al. 1998). Aerobic wetlands promote oxidation and hydrolysis in the shallow surface water, which causes precipitation of metal oxyhydroxides and carbonates that form under aerobic conditions. This process results in uptake of metals by plants and physical filtration of precipitates by plant root matter and biomass. Unlike typical BCRs, aerobic wetlands are most often horizontal flow systems. Extensive published literature on aerobic wetlands documents decades of successful treatment of certain types of coal-mine drainage. ITRC WTLD-1 (2003) provides a detailed analysis of the application, contaminant removal mechanisms, and regulatory guidance of constructed wetlandA man-made treatment system using saturated soils or sediment beneath standing water to remove contamination. Constructed wetlands almost always treat waste water of some type and almost always contain wetland plants. systems, while ITRC MW-1 (2010) provides additional guidance on the technology as applicable to MIW treatment. The term "constructed wetland" as described by ITRC MW-1 (2010) and others (such as Hedin et al. 1994 and Skousen et al. 1998) includes aerobic and anaerobic processes, which are processes that are now defined as BCRs according to this guidance. This guidance avoids the use of the term "wetland" to describe a BCR system because of regulatory issues that can be associated with this term.

Two applications of aerobic wetlands include pretreatment of MIW prior to a BCR unit or posttreatment of MIW after a BCR unit. As a pretreatment, aerobic wetlands are typically used to treat net-alkaline water to promote removal of iron, aluminum, and manganese as oxy-hydroxide precipitates. The system is designed to form these precipitates and sustain plant growth that provides retention of the precipitates.

As a posttreatment to BCR units, aerobic wetlands can be used to polish the BCR effluent. The aerobic wetland provides an increase in DOdissolved oxygen, uptake of BOD, and retention of solids from the BCR effluent. Plant uptake can also help to remove trace metals in the BCR effluent. This posttreatment step is also called an aerobic polishing cell (APC)A shallow pond which allows for aeration and settling of particles, typically following a BCR.. In conjunction with proper sedimentation processes, the aerobic polishing unit can produce a final effluent water quality that is acceptable for discharge to receiving streams or other water bodies.

Conventional active alkaline treatment for MIW is typically implemented by addition of reagents such as hydrated lime, limestone, or sodium hydroxide. This process is considered active because both the chemical processing and mixing require controlled addition of mechanical energy. The reaction rate at which metal precipitates form depends on numerous factors such as the MIW chemistry, type of alkaline reagent and dosing rate, reaction temperature, and degree of mixing and oxidation. Oxidation is implemented throughout the alkaline processes to improve the precipitation rate. Sedimentation is implemented after the precipitation and mixing process to settle the precipitates from solution.

An important component of application of alkaline addition is the solubility of various metal precipitate forms. The solubility is specific to each metal and controls the stability of the metal either in the oxidized solid precipitate form or in the dissolved form. Because many MIWs contain trace metals that are highly soluble at neutral pH (such as cadmium, lead, nickel, manganese, and zinc), conventional alkaline addition may require a pH increase to 9.0 or greater in order to ensure that the targeted metal removal efficiency is achieved.

Semi-passive alkaline addition can be implemented by passing the MIW through a solid reagent (such as sodium hydroxide pellets or lime-based solids) or addition of a solid or liquid reagent to a MIW stream or pond. Semi-passive approaches have also been implemented that use micro-hydropower (water wheels) to feed solid and liquid neutralization reagents at set rates into acidic MIW. Two commercial systems that feed alkaline reagents at a rate defined by the water flow are Aquafix^TM solid pebble lime dosing and Wheeltreater^TM sodium hydroxide dosing (Gusek and Conroy 2007).

Semi-passive alkaline addition to a high endpoint pH (9.5) to remove metals can be difficult to implement due to factors that affect the efficiency of the process, as well as the difficulty of managing the very large volume of sludgeA watery semi-solid. produced. Alternatively, semi-passive approaches can raise pH of acidic MIW to a moderately acidic pH at which iron and aluminum precipitate (such as a pH of 4.5). This approach removes a majority of the iron and aluminum that otherwise might cause plugging in a BCR unit. However, regular operation and maintenance must be performed to manage the metal precipitate sludge produced.

Passive alkaline addition is generally implemented by allowing the MIW to flow through a bed of limestone or dolomite; at most sites considered for passive treatment systems the low cost and ready availability of limestone or dolomite are unmatched. Some low-cost waste products can also provide alkalinity, such as fly ash, lime kiln dust, and slag. Waste products should be evaluated for unanticipated contaminants prior to use.

Examples of passive treatment units are the anoxic limestone drain (ALD, https://projects.itrcweb.org/miningwaste-guidance/to_anoxic.htm), open limestone channel, limestone bed, and a reducing alkalinity producing system (RAPS). As acidic water contacts limestone, the limestone dissolves, releasing calcium and bicarbonate, which raises the pH. Because the reaction works by dissolution, the limestone eventually becomes depleted and requires replacement. However, a coating of limestone by ferric iron, aluminum, and manganese precipitates can occur in oxidized MIW very quickly, rendering the limestone ineffective.

MIW can be highly oxidized, so a RAPSreducing alkalinity producing system system prior to a BCR unit may be appropriate. This type of system reduces the oxidation state of the water using an organic layer and provides some neutralization using a limestone layer. A RAPS can also be used after an oxidation or neutralization pretreatment process, both of which create a highly oxidized MIW. Approaches for design and examples of RAPS are provided in Younger et al. (2002), INAP (2009) and Gusek and Figueroa (2009), Kepler and McCleary (1994), and Watzlaf et al. (2000).

A downflow RAPSreducing alkalinity producing system unit, also called a vertical flow pond (VFP)An engineered treatment system that uses an organic substrate to drive microbial reactions to reduce the concentration of free oxygen, followed by a carbonate source to increase alkalinity in mining-influenced water. Also called a successive alkalinity producing system and also called a reducing alkalinity producing system., consists of a layer of limestone overlain by a layer of compost or other organic substrate material. The purpose of the organic layer is to strip dissolved oxygen from the water and to reduce any Fe³⁺ to Fe²⁺prior to the water contacting the limestone aggregate (Younger et al. 2002). If a RAPS system were designed to be upflow, then the layers of organic material and limestone would be reversed. A SAPSsuccessive alkalinity producing systems uses the same principles, except the term "successive" indicates that several alternating layers of organic material and limestone are used to treat the MIW (Younger et al. 2002) or that more than one RAPS is used in series.

Sedimentation ponds or conventional clarification processes must provide adequate residence time, and therefore low enough overflow velocity, to allow settling of suspended solids. Basic design sizing for a sedimentation pond to remove inert suspended solids is provided by Younger et al. (2002). Sedimentation ponds can be operated passively, however, sufficient space for operation of the system must also include sludge storage volume.

An important component of sedimentation processes is collection and management of the settled solids or sludge. Design sedimentation systems to provide for ongoing maintenance and removal of settled solids as needed. Treatment sludge may be a hazardous waste under RCRAResource Conservation and Recovery Act (40 CFRCode of Federal Regulations Part 261.24 Table 1) and should be tested using the toxicity characteristic leaching procedure (TCLP) and any applicable tribal or state required tests to determine the disposal requirements. Recycling and reuse options may be available for the sludges (ITRC MW-1 2010). On-site disposal of nonhazardous sludge may be possible, depending on site-specific conditions (such as dewatering, space requirements, and costs).

Sedimentation of MIW as a posttreatment addition to a BCR unit reduces the suspended solids content of the effluent. As part of the overall passive treatment system, sedimentation of BCR unit effluent can be implemented using an open pond or aerobic polishing unit. The pond provides the retention time to settle suspended metal sulfides or other sulfur precipitate forms (for example, elemental sulfur) that form within the BCR and are present at some quantity in the effluent stream. The sedimentation pond also provides oxidation of the effluent MIW at the surface. Deeper ponds can be used to provide adequate retention time and as a control to limit re-oxidation of reduced sulfur forms. A sedimentation pond may also be used as pretreatment.

Gravity flow BCRs are preferred, although for small flows solar powered DC pumps may be acceptable. In order to avoid pumping water, the site topography must accommodate the design. If the water source is an underground mine, a bulkhead might be used to partially flood the workings to provide enough static pressure headA specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation expressed in units of length. to deliver the water to a treatment site topographically above the mine portal. If space is limited immediately adjacent to the mine portal, consider installing a gravity pipeline to a flatter and more accessible area for a solid-phase substrate BCR. Alternatively a liquid substrate BCR may be used to overcome space constraints. The design of a liquid substrate BCR must address generated solids retention.

Initially BCRs were not used in areas with severe winter climates. However, by including covers in the design, BCRs have been used at high elevations and in areas with long and cold winters such as Colorado (Reisman et al. 2008), and Montana (Reisman et al. 2003). The covering should be vented, as it can cause gas build-up underneath. Additionally, a cover can keep rainwater and snowmelt from introducing higher electron acceptors and from diluting the BCR influent. Covering also can minimize evaporation in hot and dry climates (Buchanan and Gusek 2011).