Active treatment technologies
Elina Merta, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, elina.merta(at)vtt.fi
Treatment technologies for mining influenced water have traditionally been divided into active and passive treatment technologies, both of which can comprise physical, biological and chemical mechanisms. Combinations of several technologies are often needed to achieve the required treatment results. In active treatment technologies reagents and/or energy are added on a regular basis and the process usually requires routine maintaining and monitoring. Compared to passive systems, higher operational costs are associated with active treatment systems. While many post closure mine sites utilize passive water treatment technologies, in some cases active treatment might be the right solution due to site characteristics, e.g. very large or variable flow rates. In addition, it should be noted that in practise all treatment approaches, whether classified as passive or active, require some form of management and maintenance as well as considerations on the waste generated in the treatment system. (Johnson & Hallberg 2005, Taylor et al. 2005, Cooper 2014)
The main advantages of active treatment systems compared to passive technologies are listed in the following (Johnson & Hallberg 2002, Taylor et al. 2005, INAP 2009, Trumm 2010):
- can be designed to tolerate high total acidity loads (i.e. high water flow rate combined with high total acidity including hydrogen ion + mineral acidity)
- process controllability and adjustability for changing flow rate and properties
- effective removal of contaminants
- potential for metal recovery
- small area requirement
- not subject to seasonal variations
According to Trumm (2010) active systems should be considered when flow rates exceed 50 l/s and the acidity of water is > 800 mg/l as CaCO3. Thus, passive systems are mainly successful for low acidity loads (< 100- 150 kg CaCO3/day) (Taylor et al. 2005).
Active treatment can be carried out either at a fixed treatment plant or as in-situ treatment. Fixed plant treatment usually requires some pumping in order to gather the water to the treatment plant whereas in-situtreatment is performed in a system within or adjacent to the water body or stream. The pumping cost is typically the central factor in the choice between fixed plant and in-situ treatment. (Taylor et al. 2005)
The active technologies evaluated in CLOSEDURE project are listed below.
- Alkaline treatment
- Chemical precipitation
- Ion exchange
- Membrane processes
- Sulphate reduction in bioreactors
- Treatment/disposal of water treatment residues
Cooper, I. 2014. What miners tend to miss in choosing minewater treatment solutions. ME Online Exclusive. July 2014.
INAP 2009. The International Network for Acid Prevention. Global Acid Rock Drainage Guide (GARD Guide). Available: http://www.gardguide.com
Johnson, D.B. & Hallberg, K.B. 2002. Pitfalls of passive mine water treatment. Review. Re/Views in Environmental Science & Bio/Technology 1: 335–343, 2002.
Johnson, D.B. & Hallberg, K.B. 2005. Acid mine drainage remediation options: a review. Science of the Total Environment 338: 3–14.
Taylor, J., Pape, S. & Murphy, N. 2005. A Summary of Passive and Active Treatment Technologies for Acid and Metalliferous Drainage (AMD). Prepared for the Australian Centre for Minerals Extension and Research (ACMER)
Trumm, D. 2010. Selection of active and passive treatment systems for AMD flow charts for New Zealand conditions. New Zealand Journal of Geology and Geophysics. 53:195-210.
The table below gives a quick overview of active water treatment technologies applied for the treatment of mine effluents.
|Technology||Description||Advantages +||Disadvantages –||Applicability||Design considerations||References|
|Adsorption||Removal of metals in AMD can be readily achieved by activated carbon adsorption. However, the price of activated carbon is high and there is a need to find less expensive materials for AMD treatment. There are several studies suggesting alternative sorbents such as natural or synthetic zeolites, clay, natural clinker, fly ash, zero valent iron, agricultural wastes, microbial biomass, activated sludge, collophane, lignite. The regeneration (desorption) of the sorbent is possible for some sorbent materials to allow metal recovery and repeated usage of sorbent. However, less attention in the R&D has been paid on this aspect.
Adsorption can also be applied for removal of nitrogen and arsenic present in mine water. For arsenic removal sorption materials such as activated alumina (AA), granular activated carbon, granular ferric hydroxide, iron oxide coated sand and iron filings have been applied.
|+ Possibility for metal recovery
+ Suitable for dilute solutions
+ Some sorbent materials remove acidity from/produce alkalinity to the treated water
+ Design, scale-up and operation are relatively simple
|– May require disposal of spent sorbent
– Adsorbent loses its efficiency over time
– Material need for mining water treatment may be large, sorbent costs
|Applicable for mine effluents containing metals (nitrogen, arsenic) in low concentrations.||pH has a significant effect on adsorption process and the optimum for each sorbent/feed combination can be found. Sorption of metals is often declined at low pH. Other factors to be considered include temperature, initial metal concentration, presence of competing ions, sorbent dosage and particle size. Elevated ionic strength often reduces the metal sorption. The rate of adsorption varies by species.||16, 17, 18, 20, 21, 24|
|Alkaline treatment||The addition of alkaline agent, such as limestone (CaCO3), CaO, Ca(OH)2, NaOH, Na2CO3, or ammonia can be used to raise the pH and to achieve precipitation of metals as hydroxides, oxyhydroxides or carbonates. Some removal of sulphate as gypsum takes place when Ca-containing chemicals are used.
Resulting sludge in conventional “Low density treatment” (LDS) has solids content 2-7%. So-called High Density Treatment (HDS) can reach solids content > 30% by recycling sludge back to the neutralization tanks and applying more efficient flocculation. Thus, the volume of waste sludge and also the chemical usage are reduced.
|+ Well proven, state-of-the-art technology for mine effluent treatment
+ Wide range of different neutralizing chemicals available
+ Stable and easily controllable process
+ Adaptable to changes in water flow and quality
|– Vast amount of sludge with low solids content and poor dewaterability requiring appropriate disposal is generated (especially in LDS).
– The long-term stability of sludge and possible release of metals is a specific concern.
– Non-selective process, no possibility for metal recovery
-High operating costs; however, the sludge treatment and disposal costs may be ca. an order of magnitude higher than the capital and chemical costs
-Precipitation processes are relatively slow
-Required chemical dosage as well as sludge volume is difficult to predict; stoichiometric amounts of alkalinity are not enough for complete metal precipitation.
|Applicable for mine effluents containing a mix of metals with little or no commercial value.
Sufficient when relatively modest effluent quality standards need to be met.
Metals that can be precipitated by pH control include Cu, Pb, Zn, Ni, Cd, Fe, Mn, Al, Cr-III, Sb, As-V, Ag, Se, Th and Be. In contrast, some metals are virtually unaffected by pH control alone, e.g. Hg, Mo, Cr-VI and As-III.
Selective removal of certain elements, such as arsenic and molybdenum, can be achieved by e.g. multiple-stepped addition of reagents accompanied by pH control.
|The presence of multiple metals with different precipitation properties emphasizes the importance of pH control. Other important design factors include:
– solids content
– flow rate
– available land area
– ionic strength
– redox potential
– concentrations of suitable complexing agents (e.g. humic substances
– interactions of the precipitated solids
|1, 2, 5, 6, 7, 8, 11, 15, 21, 27|
|Biological N removal||Microbial processes converting nitrogen compounds to nitrogen gas by nitrification- denitrification or anammox process (anaerobic ammonia oxidation). Different reactor systems as well as wetland systems can be utilized.||+ Low operating costs
+ Proven technology
|– Low ammonia concentration, low temperature and pH as well as toxic compounds may limit the efficiency
– For wetlands, large areas needed for treatment
– Possible need for sludge disposal
|Applicable for nitrogen containing mine effluents, with no extensive toxicity effects.||5|
|Chemical precipitation||Chemical precipitation techniques can be used to solve a number of problems in AMD treatment.
Precipitation or coprecipitation can be used to remove metals as hydroxides, carbonates or sulphides.Selective metal precipitation by sulphide compounds (FeS, CaS, Na2S, NaHS, NH4S or H2S) can selectively recover metals at low pH. Biological process with sulphate reducing bacteria may be used to generate H2S for the precipitation process.Ettringite precipitation (addition of lime and Al(OH)3) can be used to remove sulphate. Precipitation with Ba salts is another option for sulphate removal.Coagulation by iron (ferrihydrite precipitation) or aluminium salts is a common method for arsenic removal from mine wastewater. Oxidation of As(III) to As(V) is often required in order to improve precipitation and the stability of resulting sludge.
|+ Some technologies are relatively simple and proven
+ Some technologies offer a possibility for selective metal recovery
+ Produce water with low metal/sulphate concentrations
|– Need for sludge treatment and disposal
– Uncertainty on the long-term stability of the sludges
– Reagent costs high in some processes
– Presence of toxic and/or flammable substances in some processes
|Sulphide precipitation is applicable for metal containing mine waters when very low effluent concentrations are required or when selective metal removal and recovery are needed. Precipitation with barium salts or ettringite precipitation can be applied for the reduction of sulphate from mine waters to low concentrations.||High reactivity of sulphide with metals emphasizes the need for process control and optimization.
In ettringite and barium precipitation processes constant water monitoring with regard to sulphate and pH is needed to allow optimized chemical dosages. High temperatures and corrosive gases may influence the choice of materials of process equipment. Due to possibility of toxic gaseous emissions, the occupational safety is to be carefully considered.
|1, 19, 21,25|
|Evaporation||Evaporation can be either natural or forced.||+ Simple and robust process||– Natural evaporation is slow and requires large ponds for treatment. Forced evaporation is more efficient but energy-consuming and thus expensive.||Evaporation can be used as a pretreatment step to increase pollutant concentrations.||14|
|Ion exchange||Ion exchange process utilizes a polymer resin packed column for the removal of target species. The resin contains active functional groups which capture target ions and release an equivalent ion to the solution. Usually the resin can exchange cations or anions, but some (amphoteric) are capable of exchanging both, depending on the pH. The loaded ions (such as metals) are removed from the resin by regeneration. The volume of regenerant solution is considerably smaller than the influent flow, thus resulting in effective concentration of target ions into regenerant.
Ion exchange can be applied for the recovery of different metals from AMD. Ion exchange can be also utilized to remove hardness, alkalinity, radioactive constituents and ammonia.
Suitable ion exchange columns can also be used to remove sulphate from mine wastewater.
|+ Removal can be targeted to specific contaminants
+ Suitable for dilute solutions
+ Possibility of metal recovery
+ Low temperature dependence
+ Not sensitive for toxic substances
+ Relatively simple treatment systems
+ Modular configuration possible
|– Need for pretreatment (e.g. pH adjustment, oxidation, solids removal)
– Need for the treatment and/or disposal of regenerant as well as spent resin
– High treatment costs
– Applications for AMD still at research/development stage
– Limitations in cold conditions
– Organic compounds, oxidants or high temperatures can degrade the resin
|Can be applied as a pretreatment step (concentration) for further metal recovery. Another aim could be the extension of existing water treatment capacity.
Conventional ion exchange processes are not applicable for high CaSO4 waters due to scaling. Modified processes, such as GYP-CIX are developed to overcome theses problems.
IX is best applicable to waters in the pH range of 4 to 8 containing low concentrations of suspended solids, iron and aluminium.
|Important design considerations include:
– type of resin
– volume and type of regenerant
– backwash water source and quantities
– need for pre-filtration of solids
– column configuration
– need for pH adjustment before and after ion exchange
– cycle lengthpH and temperature effects are important. Competing ions and their effect on removal must be considered.
|3, 4, 9, 11, 15, 19, 23, 26,28|
|Membrane treatment||Different types of membranes can be used to treat AMD. The separation is based on sieving effects as well as electrorepulsive forces due to membrane surface charge.
Nanofiltration (NF) has been considered as a preferred membrane process for the treatment of AMD at low temperatures because, compared to other membrane processes such as reverse osmosis (RO), it presents higher fluxes at lower pressure leading to lower capital and operational costs.
Conventional reverse osmosis process requires pretreatment (e.g. HDS) of mine water in order to reduce metals concentration to acceptable level considering membranes.
Membrane distillation is a thermally driven process. Its utilization in water recovery as well as acid and metal concentration has been recently demonstrated for mining wastewater. Sulphuric acid from mine water with high sulphate concentration can be recovered also by electrodialysis. However, this process has not been demonstrated on a large scale.
Membrane processes can be utilized also in sulphate and arsenic removal.
|+ Wide range of pollutants can be removed, both organic and inorganic
+ Can remove monovalent ions unlike other treatment methods
+ Possibility for recovery of valuable resources
+ Produces high quality discharge water for further use
+ Low chemical consumption
+ Small footprint area
+ Modular configuration possible
+ Ease of operation
+ Considered as established technology for mine effluent treatment
|– Need for pretreatment to reduce concentrations of metals, solids, hardness, sulphate etc. to acceptable level to protect the membranes
– High operating (electricity, chemicals) and maintenance cost
– Disposal costs for brine and pretreatment sludges
– Risk of membrane fouling, especially by gypsum formation
|In addition to producing high quality effluent, membrane processes can be used to concentrate and (with further processes) recover metals and sulphuric acid.
RO process always requires pretreatment such as pH adjustment and NF.
|The membrane type should be tubular to prevent fouling. With proper pretreatment and maintenance membranes typically last two to five years. Control of water temperature may be needed in cold/hot climates to minimize water viscocity.
Solution pH has significant impact on membrane performance. Important parameter is the membrane iso-electric point (IEP). At pH values below IEP the membrane is positively charged, and negatively charged elsewhere. Metal rejection has been indicated highest when operating at pH below IEP. For anions, such as sulphate, the pattern is reversed.
Operating pressure has an impact on the metal separation efficiency.
|3, 4, 9, 10, 13, 21, 26, 27, 28|
|SRB systems (sulphate reducing bacteria)||Bioremediation of AMD relies on the ability of some microorganisms to generate alkalinity and immobilise metals, thus reversing the reactions taking place in the formation of AMD. The most important alkalinity generating reactions are the reduction of ferric iron and sulphate as they are usually abundant in AMD.
SRB systems can be realized as passive systems or active sulphidogenic bioreactors. Active reactors are constructed and operated to optimize the production of H2S. SRB systems can also be divided to solid reactant and liquid reactant bioreactors. In solid reactant systems an organic solid substrate is used (e.g. manure, compost, sawdust etc.) whereas in liquid reactant systems alcohol is used as substrate.
|+ Compact systems
+ Considered technologically viable method for mine effluent treatment
+ Possibility for metal recovery
+ Commercial processes exist
|– High capital and operating costs
– Formation of microbial population is time-consuming
– Sensitivity to temperature and pH changes
|SRB systems are applicable for sulphate and metals removal.||Design parameters include:
· Sulphate loading
· Heavy metal loading
· Residence time required for the sequence of bacterial reactions to occur
|Sedimentation||Sedimentation is usually used as a subprocess of alkaline treatment or as a pretreatment step for membrane process or ion exchange. Coagulant or flocculent can be used to enhance settling.||+ Simple and robust process||– Dissolved species are not removed||Residence time, chemical dosage||1|
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