Classification of Innovative Passive Treatment Methods

Passive treatment systems have been developed that do not require continuous chemical inputs and they take advantage of natural chemical and biological processes to cleanse contaminated mines (Skousen, 2002). Passive treatment systems are commonly, but not exclusively aggregate-carbonate based, with or without the inclusion of organic matter (Taylor et al., 2005). A variety of passive treatment systems have become the most predominate innovative technologies applied in the treatment of AMD solutions apart from traditional choices. These treatment technologies include constructed wetlands, anoxic limestone dams, permeable reactive barriers (PRBs), etc, and are mostly based on the same principle. The PRBs and constructed wetland technologies can all utilise alkaline agents and SRB to treat mine drainage. In low-flow and low-acidity situations, passive systems can be reliably implemented as a single permanent solution for many AMD problems. These passive treatment systems can provide a potentially long-term solution to the AMD problem although their success has been limited in cases where excessive volumes, high iron loadings and excessively low pH values are encountered (INAP, 2003). In addition, relative to chemical treatment, passive systems require longer retention times and greater space, provide less certain treatment efficiency and are subject to failure in the long term (Skousen, 1998).

The mechanisms of metal removal and retention in passive treatment systems are varied and include oxidation, precipitation as hydroxides and carbonates under aerobic conditions, precipitation as sulphides and hydroxy- sulphate under anaerobic conditions, complexation and adsorption onto organic matter, ion exchange with organic matter and uptake by plants

TABLE 7.1

Summary of the AMD Active Treatment Methods

Process

Process

Technology

Advantages

Disadvantages

Possible Improvements

HDS process

Chemical

neutralisation

  • • Inexpensive
  • • Low maintenance
  • • Removes trace metals
  • • Produces water quality suitable for irrigation or reuse in the mine
  • • Can be used as a cost-effective pre-treatment method to other processes
  • • HDS process proven technology
  • • Limited sulphate removal
  • • High amount of sludge produced
  • • Costs associated with handling and safe disposal of potential unstable sludge

• Reduction in the production of waste or recycling of sludge

MINTEK

SAVMIN

Chemical

neutralisation

  • • Reduce sulphate to very low levels <200 mg/L
  • • High quality water generated even at fluctuating feed sulphate levels
  • • It regenerates the required Al2(OH)3 reagent for reuse, which results in a significant cost reduction
  • • The waste products from the process can be disposed of either as a stable waste, or, in certain instances, constitute a usable by-product
  • • High amount of sludge produced
  • • Success depends on high level of gypsum crystallisation.
  • • Process can be complicated to control

• Reduction in the production of waste or recycling of sludge

BQE

ChemSulphide

Sulphide

precipitation

  • • Treated effluent meets regulatory compliance
  • • Saleable products that can generate revenue, reduce and eliminate waste
  • • The cost of sulphide precipitants is high
  • • Requires controlled addition of sulphide precipitant to maximise heavy metals precipitation with a minimum of excess sulphide to avoid the necessity of post-treatment

• Versatility in the case where AMD may contain significant alkalinity

Paques Thiopaq

Microbial

sulphate

reduction

• Can effectively treat solutions with low metal concentrations

BQE

BioSulphide

Microbial

sulphate

reduction

  • • Can tolerate high metals loading and higher sulphide demand
  • • Produces saleable metal products and clean water
  • • Can be integrated with other water- treatment technologies

Rhodes BioSure

Microbial

sulphate

reduction

  • • Effective in reducing sulphate in AMD
  • • Produces water that complies with the general standard of waste water.
  • • Generates stable bio solids

HiPRO

Reverse

osmosis

  • • Produces high quality water at variable water recoveries
  • • Applied commercially for water desalination

GYP-CIX

Ion exchange

  • • Can treat most waste water including scaling mine waters
  • • Low cost reagent used to regenerate the resin
  • • Produces very good quality water

BQE SULF-IX™ Process

Chemical precipitation + Ion exchange

  • • Low cost reagent used to regenerate the resin
  • • Produces very good quality water
  • • Saleable products that can generate revenue

Sources: Smit, 1999; INAP, 2003; Stedman, 2010.

  • • Sensitivity of SRB to high metal concentrations and low pH
  • • High process costs due to use of ethanol or butanol as carbon source and electron donor

• Explore microbial communities that can work at optimal process conditions, e.g., use of acid tolerant bacteria

  • • Hydrogen sulphide (H2S) produced as metabolic end
  • • Product requires proper management to avoid pollution and health risk to personnel
  • • Explore use of other microbial communities
  • • Explore alternative microbial substrates
  • • Limited to geographical locations with sludge/waste facilities next to AMD point source
  • • Hydrogen sulphide (H,S) produced as metabolic end product and released into the external environment can cause air pollution and be a health risk
  • • Explore easily available carbon sources
  • • Robust hydrogen sulphide capturing systems
  • • Membrane lifetime affected by fouling
  • • Mine water needs to be pre-treated.
  • • Relatively expensive

• Not suitable for scaling water-membrane extension

• Volume of gypsum sludge produced in the ion-exchange-resin regeneration process

  • • Reduction in the production of waste or recycling of sludge
  • • Reduction in the frequency of ion resin regeneration

• Requires controlled addition of sulphide precipitant to maximise heavy metals precipitation with a minimum of excess sulphide to avoid the necessity of post-treatment

• Reduction in the frequency of ion resin regeneration

  • (phyto-remediation) (INAP, 2003). However, the environmental conditions in the different passive treatment systems dictate the dominant metals removal mechanisms. Some of the passive treatment systems that have been employed for the treatment of AMD solutions are discussed in the subsequent sections.
  • 7.2.2.1 Constructed Wetlands

Constructed treatment wetlands are engineered systems, designed and constructed to utilise the natural functions of wetland vegetation, soils and their microbial populations to treat contaminants in surface water, groundwater or waste stream (ITRC, 2003). They can be considered as treatment systems that use natural processes to stabilise, sequester, accumulate, degrade, metabolise and/or mineralise contaminants and have the ability to remove organic and inorganic compounds and suspended solids (ITRC, 2003). During the past few decades, wetlands have been established as systems that have high potential for meeting wastewater treatment and water quality objectives in a controlled manner. Constructed treatment wetlands can be used alone or in conjunction with other technologies to extend the operational lifespan of the systems or enhance the removal performance of specific constituents during the treatment of AMD (Brodie et al., 1991; Hedin et al., 1994; Sheoran and Sheoran, 2006). This flexibility further makes the technology applicable to many types of contaminants in many types of situations (ITRC, 2015).

A wetland is usually composed of two distinctive zones, oxidative zone which is vegetated with aquatic plants and reducing zone which is the sedimentation zone rich in SRB (Kuyucak, 2006). The water treatment mechanisms are biological, chemical and physical, which include physical filtration and sedimentation, biological uptake, transformation of nutrients by anaerobic and aerobic bacteria, plant roots and metabolism, metal exchange reactions as well as chemical processes (precipitation, absorption and decomposition) that purify and treat the wastewater (WWG, n.d.; Hedin et al., 1994). Other beneficial reactions in wetlands include generation of alkalinity due to microbial mineralisation of dead organic matter, microbial dissimilatory reduction of Fe oxyhydroxides and S04 and dissolution of carbonates (Skousen, 2002).

Wetlands can be classified as either aerobic or anaerobic. The main difference in these systems is the biological and chemical processes promoted and the design of water flow direction (ITRC, 2015). Aerobic wetlands are essentially shallow ponds designed to precipitate metals from water under aerobic conditions usually in a horizontal flow system. The aerobic wetlands are designed with depths no more than 30 cm that lower suspended solids and provide a substrate and increased water retention times (due to reduced flow rates) for the reaction between influent alkalinity and acidity that is generated from AMD (ITRC, 2015). The reaction is via metal oxidation and hydrolysis, and oxygen infiltration is encouraged thereby causing precipitation and physical retention of metals as oxyhdroxides, hydroxides and carbonates within the wetland (Skousen, 2002; Taylor et al., 2005). Aerobic wetlands are often used for net alkaline waters and predominantly for just aeration and precipitation of metals (Skousen and Ziemkiewicz, 2005).

Anaerobic systems primarily rely on chemical and microbial reduction reactions to precipitate metals and neutralise acidity. They generate alkalinity through bacterial activity and the use of Fe3+ as a terminal electron acceptor (Fripp et al., 2000) and are most effective in the treatment of small flow acidic water. The water infiltrates through thick, permeable organic material that becomes anaerobic due to high biological oxygen demand (Skousen,

2002) . Since anaerobic wetlands produce alkalinity, their use can be extended to poor quality, net acidic, low pH, high Fe and high dissolved oxygen AMD (INAP, 2003).

The attractiveness of constructed wetland treatment lies in its ability to produce a near-neutral water product which can be readily discharged (Johnson and Hallberg, 2005). They also have significantly lower total lifetime costs and often lower capital costs than conventional treatment systems (ITRC,

  • 2003) ; once constructed they can operate for long periods of time with minimal operations and maintenance. However, constructed wetlands are limited by the metal loads they deal with; hence adequate pre-treatment is required especially when dealing with high volumes and/or highly acidic water. Furthermore, some of the metal precipitates retained in sediments are unstable when exposed to oxygen; hence, it is crucial that the wetland sediments remain largely or permanently submerged (Johnson and Hallberg, 2005).
  • 7.2.2.2 Anoxic Limestone Drains

Anoxic lime drains (ALDs) are well-known passive treatment systems that can be an effective and established technology for the treatment of acid mine water. The ALDs are buried cells or trenches of limestone engineered to intercept anoxic, acidic mine water and add alkalinity through dissolution of the limestone (Watzlaf et al., 2000). The water is constrained to flow through a bed of limestone gravel held within a drain that is impermeable to both air and water (Johnson and Hallberg, 2005). This creates an environment which is high in carbon dioxide and low in oxygen (INAP, 2003) and increases the dissolution of limestone while preventing the precipitation of iron hydroxide. Minimal iron precipitation is essential since iron hydroxides generated tend to inhibit limestone dissolution and clog the drain (Skousen, 1998). The ALDs can be used to treat AMD flows of various rates, alone or in combination with other treatment systems, and can be installed in a wide variety of locations with the use of commonly available construction equipment. They have been applied on a large scale for the treatment of acidic mine water from a number of abandoned mine plants in the United States (ITRC, 2015).

Although ALDs produce alkalinity at a lower cost than constructed wetlands, they are not suitable for treating all AMD waters. They are suitable to treat AMD that has low concentrations of ferric iron, dissolved oxygen and aluminium. In situations where the AMD contains significant concentrations of ferric iron or aluminium, the long-term performance of ALD is not good. When any of the three parameters (i.e., concentrations of ferric iron, dissolved oxygen and aluminium) are elevated, armoring of limestone can occur resulting in slow dissolution rate of limestone (Skousen, 1998; EPA, 2014). When the dissolution rate slows, there is a higher buildup of ferric iron and aluminium on the limestone, which eventually clogs the open pore spaces, resulting in abnormal flow paths that can reduce both the retention time of AMD within the ALD and the reactive surface area of the limestone (EPA, 2014). This may, in turn, cause failure of the drain within 6 months of construction (Watzlaf et al., 2000; Johnson and Hallberg, 2005). Moreover, problems also occur where ALDs are used to treat aerated mine waters due to iron oxidation. Hence, passage of AMD through an anoxic pond prior to the ALD may be necessary to lower dissolved oxygen concentrations to prevent this oxidation (Skousen, 1998). It is also important that the solution to be treated remains in an anoxic state prior to entering the ALD to prevent metals from precipitating out of the mine drainage and causing premature failure of the ALD (Cravotta and Trahan, 1999). This can be achieved by constructing the ALD directly on top of the discharge, allowing the acidic water to flow through the limestone, adding calcium carbonate to the water and increasing the alkalinity and pH while maintaining anoxic conditions (Skousen, 1996).

7.2.2.3 Permeable Reactive Barriers

The PRBs are barriers that are placed in the path of groundwater flow allowing the water to flow through easily and the barriers react with specific chemicals of concern (Blowes et al., 2000). The PRBs are often designed to provide a source management remedy or as an on-site containment remedy (ITRC, 1999). The treatment zone may be created directly using reactive materials such as iron or indirectly using materials designed to stimulate secondary processes, such as by adding carbon substrate and nutrients to enhance microbial activity. In this way, contaminant treatment may occur through physical, chemical or biological processes (ITRC, 2015). Other reactive media, such as limestone, compost, zeolites, granular activated carbon, apatite and others, have also been employed in recent years and offer treatment options for controlling pH, metals and radionuclides (ITRC, 2005). In PRBs that are designed to treat AMD, the barriers are generally composed of solid organic matter, like municipal compost, leaf compost and wood chips/ sawdust (Blowes et al., 2000). Organic matter encourages the proliferation of SRB that reduce sulphate to biogenic sulphide resulting in the subsequent formation of metal sulphides.

The PRB technology has been applied at a good number of sites worldwide, including some full-scale installations to treat chlorinated solvent compounds (ITRC, 2005). The advantages of using a PRB include a relatively low cost of operation and monitoring and the absence of above-ground structures. Although barriers often have very long theoretical treatment lifetimes when only the material and the contaminants of concern are considered, actual lifetimes can become considerably shorter if there are other reactive substances present in the environment (Blowes et al., 2000). Depending on several site- specific conditions, PRBs are expected to last 10-30 years before reactivity or hydraulic issues result in the need for maintenance. Disposal issues could also develop in the PRB treatment media after the contaminants are concentrated within the barrier system. This design is the most important in PRB systems that retain the contaminants, as opposed to PRB systems which degrade the contaminants as they flow through the system (ITRC, 1999).

 
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