Deblina Maiti, Iqbal Ansari, Mohd Ashraf Rather, Arukula Deepa; Comprehensive review on wastewater discharged from the coal-related industries characteristics and treatment strategies. Water Sci Technol 1 June 2019; 79 (11): 20232035. doi: https://doi.org/10.2166/wst.2019.195
Wastewaters discharged from various coal-related activities deteriorate fresh water quality and inflict possibilities of groundwater contamination. Their characteristics mostly depend on the parent coal properties, though some of the pollutants are cyanide, thiocyanate, ammonia, phenol, heavy metals and suspended solids. This paper has reviewed the treatment techniques along with the characteristics of all such kinds of wastewater and also identified the challenges and future perspectives. Primarily, demineralization of coal can attenuate and control release of pollutants in wastewaters if implemented successfully. Mine water from non-lignite mines can be purified using simple techniques, for its reutilization. Acidic mine water and leachates can be treated using passive bioreactors with microbial activity, different organic substrates and limestone drains. Additionally bio-electrochemical systems, membranes, macrocapsules, zeolite filters, ores, physical barriers, and aquatic plants can also be used at various stages. Coal washery wastewater can be treated using natural coagulants obtained from plant extracts along with conventional coagulants. Nitrification and denitrification bacteria fixed in reactors along with activated carbon and zero-valent iron can treat coke oven wastewater. Some other sophisticated techniques are vacuum distillation, super critical oxidation, nanofiltration and reverse osmosis. Practical use of these methods, wisely in an integrated way, can reduce freshwater consumption.
Rapid increase in the usage of coal has resulted in release of coal-related toxic pollutants through wastewaters (WWs) originating from coal mining sites, washeries, coke plants, and thermal power plants (TPPs). The quality of these WWs depends on the coal properties and the process involved in coal processing. Discharge of these WWs gravely impacts the quality of nearby water bodies, aquatic life and the food chain along with the groundwater (Pal & Kumar 2014).
The above coal-related industries mainly release four kinds of WW. The first of is mine water (MW), which is generated in ample amounts during non-lignite coal mining. It is mostly affected by hardness, suspended solids and bacterial contaminants (Soni & Wolkersdorfer 2016). Comparatively, one other kind of MW also called acid mine drainage (AMD) originating from lignite mines is highly contaminated with metals. Oxidation of pyrite present in the mine overburden dumps in presence of moisture is the actual cause of AMD generation, which also dissolves many toxic metals such as zinc, copper, iron, manganese, cadmium, nickel, chromium, lead, cobalt, and mercury during its passage from the rocks (Equeenuddin et al. 2010). Seeping of the AMD into water systems leads to 21,200 times more metal concentration in them compared to the world average metal concentration in water bodies. The metals are non-biodegradable and some of them such as lead, cadmium, and mercury, are highly toxic to living organisms. Metals bioaccumulate when passed from a lower trophic level to the next higher trophic level and seriously deteriorate metabolic functions of organisms. Natural reduction in acidity and subsequent metal concentration in AMD is only possible if there are sufficient carbonates in the coal rocks (Miranda et al. 2010).
The second kind of WW is that which is generated from coal washeries during coal washing. The mined-out coal is sent to coal washeries through a conveyor belt, wherein its impurities are removed through specific gravity-based processes, to increase its heating value (Ghose 2001). The process generates a huge amount of coal slurry having large amounts of solids, chemical oxygen demand (COD) and metals. This effluent is often disposed of in surface waters which affects aquatic life by having a blanketing effect. Optionally it is also injected into abandoned underground mines (Yan et al. 2012). In the event, the process also results in loss of coal fines, usually 610 tonnes per day from a single washery plant (Tiwary & Dhar 1994; Ghose 2001).
The third and fourth kind of WW is generated from steel plants and TPPs respectively, which are responsible for steel production and electricity generation from the washed coal. The former makes metallurgical coke from coal through carbonization for further use in blast furnaces, which ultimately generates coke oven WW having highly hazardous nitrogenous, phenolic and other organic pollutants. The contaminants have the property to migrate to long distances from disposal sites and are genotoxic as well as carcinogenic to the living organisms (WHO 1997; Pal et al. 2011; Sharma & Philip 2016). Presence of these pollutants in surface water largely increases the oxygen demand and rapidly destroys the aquatic life. Eutrophication of the water bodies are observed due to rapid phenol degradation (Yu et al. 2016). TPPs generate fly ash (FA) through coal combustion. FA contains many heavy metals attached to its alumino-silicate matrix, which are often 410 times higher than the parent coal and are readily leachable in aqueous medium (Maiti & Prasad 2016). These metals find their way through wet and dry disposal sites of FA. Wet or slurry form disposal is done in ponds, from which the leachate left after FA settling often overflows to surface water bodies during the monsoon or leaches into groundwater through unlined ash ponds; also the ash itself can spill out through breached earthen walls (Yao et al. 2015). Dry disposal of FA creates huge unstable dumps which are the primary cause of slurry erosion during the monsoon. The dumps also lead to blowing of fine particles of FA which can also deposit on water bodies causing turbidity and metal pollution (Stefaniak et al. 2015).
In view of the above problems various laws have been formulated, which instruct the related industries to treat the WW up to a standard quality before discharge (EPR 1986; WHO 1997; BIS 2012). In this regard an extensive literature survey has been done to advocate possible treatment strategies for the WWs based on their individual characteristics, recent developments, and eco-friendly nature, while also addressing their benefits and challenges.
Opencast mining encounters a permanent flow of water into mine pits along with subsequent loss of water from the nearby aquifers; as a result the communities living near the mine sites suffer from water scarcity. Annually only 13% of MW is reutilized for sprinkling and dust suppression in sites and a majority amount of the water is disposed of (obtained from an unpublished report of Mines of India). The amount of water depends on the size of mine. For example, different coal mine subsidiaries in India release 0.070.41 Mm3 of MW every day and the total amount of water released per day is 1.24 Mm3 (Debnath 2013). The mine water is characterized by dissolved solids, hardness, alkalinity, calcium, magnesium, iron and manganese (Table1), at concentrations which are comparatively higher than the standard drinking water limits (BIS 2012). However, these contaminants can be easily removed by simple cost-effective techniques to achieve the purity level of drinking water (Figure1).
cZhang et al. (2009), Zhao et al. (2009), Zhang et al. (2010), Wei et al. (2012), Pal & Kumar (2014), Ozyonar & Karagozoglu (2015), Yu et al. (2015), Sharma & Philip (2016), Yu et al. (2016) and Zhao et al. (2017).
The technique includes removal of floating impurities, followed by storage, and sedimentation of coarser materials in horizontal flow tanks. The dimensions of the tanks would depend on the capacity of the water treatment plant. The settled particles can be removed periodically after formation of a sludge zone of >0.8 m, followed by aeration to remove odours and precipitation of iron as well as manganese. Hardness can be removed by rapid dissolution of lime-soda. Coagulants such as alum conjugated with FeSO4 or lime can be added to remove turbidity. Final disinfection can be ensured through chlorination or ozone treatment. A few highly sophisticated techniques such as nanofiltration (NF), reverse osmosis (RO) and ion exchange are practised in some mines across the globe, which remove salt ions, total dissolved solids, hardness, alkalinity, ammonia and metals (USEPA 2014). However, they extend the budget of the mining industry and are unfeasible for the developing countries.
On the other hand, pollutants present in AMD (Table1) are remediated through biological procedures (Table2) such as passive remediation using sulphate reducing bacteria (SRB) in bioreactors and wetlands. Metabolically the bacteria utilize the carbon and nitrogen source of a substrate and produce hydrogen sulphide in anaerobic conditions, which in turn reduces dissolved metals to sulphides. The later precipitates are ultimately absorbed in sedimentary deposits. Industrial utilization of this process can treat AMD and induce alkalinity in it. Although the process requires minimal energy, it is less effective at high flow volumes and metal concentrations (Wieder 1989; Greben et al. 2009). This technique can be supported through installing anoxic limestone drains (ALDs), which increase the metal removal efficiency by 30%, add bicarbonate alkalinity and also have a lifetime of >20 years (Hedin et al. 1994; Ziemkiewicz et al. 2003). These ALDs are also affected in presence of highly acidic AMD with a coating of reddish iron oxyhydroxide within 48 h of contact. This can be reduced by using CO2 (Hammarstrom et al. 2003). Coupling of ALD with SRB (or zinc-tolerant SRB) is known as successive alkalinity-producing systems, which can also be installed as vertical passage systems to reduce area requirement (Kepler & McCleary 1994). This integrated technique has been effective in reducing high sulphate and zinc in AMD (Castillo et al. 2012). Macrocapsules having a pH-sensitive polymer and a phosphate buffer are better than limestone for AMD treatment (Aelion et al. 2009).
Substrates which have been studied in artificially constructed anaerobic reactors for efficient AMD treatment are spent mushroom compost (Dvorak et al. 1992; Christensen et al. 1996), degradation products of grasses to remove sulphate (Greben et al. 2009), mixture of cow manure, mushroom compost, sawdust, gravel, limestone and sediment (Vasquez et al. 2016), mussel shells (Uster et al. 2015) and coagulants obtained from chitinous material (Daubert & Brennan 2007). More than 95% of the activity of the substrates can be regained by adding reducing equivalents such as methanol to the substrate (Tsukamoto & Miller 1999). Macroalgae can treat AMD in hybrid systems, which in turn also increases the macroalgae chlorophyll content (Oberholster et al. 2017). Metals in AMD can be removed by aquatic plants (Eichhornia crassipes, Lemna minor) which accumulate 10 times more metals in the roots, without showing any toxicity symptoms (Mishra et al. 2008), zeolites synthesized from coal FA (Cardoso et al. 2015), and bio-electrochemical systems (Pozo et al. 2017) without any extra chemical dosing and with reduced sludge formation. Earlier field applications in this aspect includes installation of an artificial permeable reactive wall perpendicular to the groundwater flow, which helped in treating AMD in a time period less than a year while staying effective for at least 15 years (Benner et al. 1997). Overall, AMD can be treated by combined strategies (Figure2) such as wetlands with SRB, macroalgae, aquatic plants, different organic substrates, and ALDs, and can be further treated with macrocapsules, natural coagulants, membranes or zeolites.
The huge amount of WW released by coal washeries (approximately 300500 m3 of effluent each day per plant) can be treated for reuse in coal washing and to reduce fresh water consumption, because, as per estimates, every 100 tonnes of coal requires 6 105 to 2 106 gallons (2,2707,570 m3) of water for washing (Ghose 2001). Various parameters in the effluents (Table1) have values above standard limits (EPR 1986; WHO 1997; BIS 2012). The pollutants that are mainly coarser solids can be removed through sedimentation, and dissolved solids by coagulants (Table4), wherein oppositely charged ions destabilize stable colloids and result in settleable flocs (Menkiti & Onukwuli 2011a, 2011b).
COD, chemical oxygen demand; TN, total nitrogen; TOC, total organic carbon; A1-A2-O, anaerobicanoxicaerobic biofilm system; ZVI, zero-valent iron; MBR, membrane bioreactor; BDD, boron-doped diamond; BOD, biochemical oxygen demand; TCN, total cyanide.
Poly-electrolytes such as alum, dioctyl-sodium-sulpho-succinate, N-cetyl-N,N,N-trimethyl ammonium bromide and synthetic flocculants such as Morarfloc-A or True-floc are mostly used (Folkard & Sutherland 1996). Although synthetic flocculants are more effective than alum as they remove more solids in less time and require a much smaller dose (1 mg L1) than the latter (3050 mg L1), they are linked with many health effects (Ghose 2001). Therefore natural products or coagulants should be considered, which not only remove pollutants, but produce less sludge, ensure potability, and are biodegradable and cost-effective. Literature has reported use of bark resins, ashes, and seaweed extracts by villagers for water treatment (Folkard & Sutherland 1996). Coagulants can be prepared from seeds of Moringa oleifera, M. stenopetala, Hibiscus sabdariffa and Strychnos potatorum (Kapse et al. 2017), or from the byproducts obtained during oil extraction from the seeds (Ghebremichel 2004). Due to presence of amino acids such as arginine, Moringa seed preparations can outperform Al2SO4 to remove 95% of the pollutants in coal washery effluent (Menkiti et al. 2011). Pilot scale studies have also proved the efficiency of coagulants from Mucuna sloanei and Afzelia bella seeds for treatment of WW (Menkiti et al. 2010; Menkiti & Onukwuli 2011a, 2011b). Coagulants from natural starch and chitin also show similar results (Table 3) and can be prepared by electrolyzing a solution of cassava starch (obtained from milled cassava tubers) and calcium hypochlorite in 1:2 ratio followed by mixing with FeCl3 or mixing the extracted chitin coagulant with alum and FeCl3 (Menkiti et al. 2008, 2009; Menkiti & Onukwuli 2011a, 2011b; Nnaji et al. 2014). FA-based coagulants were also studied by Yan et al. (2012). The macrofungus Pleurotus ostreatus showed removal of manganese, zinc, nickel, copper, cobalt, chromium, iron, and lead from 50% diluted WW (Vaseem et al. 2017). Thus a hypothetical coal washery treatment plant can be proposed which should incorporate an adequate sedimentation time followed by a wise use of coagulants. The process can also ensure a possibility for recovery of coal fines.
Every 1,000 tons of coke production requires 4,000 m3 of freshwater and generates 1,000 m3 of coke oven WW during cooling of hot coke over different coolers (Sharma & Philip 2016). This WW contains high amounts of ammonia, phenol, cyanide, thiocyanate, total nitrogen, suspended matter and COD (Table1) at a concentration higher than the standard permissible limits (EPR 1986). Pollutants removal from the WW is either done by concentration or destructive processes (Figure3). In the concentration process the pollutants are recovered by dissolving them out by organic solvents which have a high distribution coefficient for the particular pollutant; some of the solvents are methyl isobutyl ketone, di-isopropyl ether and butyl acetate which extracts phenol (Lu et al. 2009). Similarly, cyanide is often precipitated as valuable iron complexes using ferric solutions which also remove suspended matter, oil and grease. Ammonia is removed and recovered through steam stripping, wherein alkaline ammoniacal liquor is reacted with steam. Synthetic zeolites can also remove ammonia through ion-exchange technique and advantageously can be regenerated with low-cost brine (Chu et al. 2012; Cardoso et al. 2015). The destructive treatment procedures are listed in Table4, and mainly degrade the pollutants by various agents. For example advanced Fenton oxidation process with H2O2 can remove COD and phenol; activated coke can remove maximum COD and color at 40 C (Zhang et al. 2010); zerovalent iron (ZVI) having carbon and iron can also remove COD through coagulation, precipitation and oxidationreduction processes compared to ferric sulphate or activated carbon alone (Hong et al. 2003; Lai et al. 2007); electrochemical oxidation in presence of lead dioxide coated titanium anode can remove COD and ammonia (Chiang et al. 1995).
However, boron-doped diamond anode systems are more efficient than lead dioxide anodes as the energy consumption is only 60% (Zhu et al. 2009). Manganese and magnesium ores can oxidize COD, ammonia, phenols, and sulphides under acidic conditions (Chen et al. 2009); supercritical oxidation of water with H2O2 and 300% excess oxygen in a continuous-flow reactor can remove 99% of pollutants (Du et al. 2013); vacuum distillation with caustic soda can remove 99% of COD (Mao et al. 2010). Bio-augmentation is another method which is used in removing recalcitrant organic compounds from WW. Aerobic bacteria usually degrade pollutants at optimum pH, temperature, oxygen, nutrients, biomass concentrations and residence time (Chu et al. 2012). Jianlong et al. (2002) used a quinoline degrading bacterium, Burkholderia pickettii, for bioaugmenting the oxic reactor of an anaerobicanoxicaerobic (A1-A2-O) system. Nitrosomonas bacteria can oxidize ammonia to nitrite while Nitrobacter oxidizes nitrite to nitrate (Jianlong et al. 2002). They can be attached to fixed media to form fixed-film bioreactors or hybrid biofilms to remove pollutants from WW passing through them (Rong et al. 2007). WW passed through the immobilized fungus Phanerochaete chrysosporium reduced phenol and COD (Lu et al. 2009). Biofilm reactors with ZVI have also been used in practical applications (Lai et al. 2009). Ammonia can also be oxidized by activated sludge treatment process as well as oxidation ditches, which have a longer residence time than the former (Schroeder 1977; Ning et al. 2005), whereas lagoon systems have 57 days residence times and higher temperature (Kargi & Uygur 2002). Activated sludge plants generally involve three reactors which remove COD, followed by phenols, ammonia and thiocyanate, and finally nitrate. Methanol can be added as an external carbon source in such systems (Vazquez et al. 2006). Sequencing batch reactors (SBRs) also have three reactors which undertake simultaneous clarification along with treatment; the reactors are namely stripping tank, neutralization tank and SBR tank (Maranon et al. 2008). Membrane bioreactors (A1-A2-O-MBR) include an extra anoxic basin where the screened raw WW is collected, and then passed to pre-aeration as well as MBR basins to remove pollutants (Zhao et al. 2009). However, these reactors require frequent membrane cleaning processes to prevent membrane fouling. The propensity for membrane fouling can be reduced by coupling membrane distillation with pre-coagulation for biologically treated WWs, which can remove salts, and volatile and non-volatile organic pollutants through coagulation with poly-aluminum chloride (Li et al. 2016). Submerged biofilm-activated sludge hybrid system can remove maximum ammonia and COD from WW (Junxin et al. 1996). Additionally, ultrasonic irradiation can also be coupled with catalytic oxidation or activated sludge process, which shows more efficiency in treatment (Ning et al. 2005). Anammox reactors are a type of system which undertakes anaerobic ammonium oxidation using planctomycete bacteria (Aksogan et al. 2003). Down-flow hanging sponge reactors have a series of hanging polyurethane sponges with diverse microbial biomass upon which ammoniacal WW is trickled. The diverse microbial biomasses are for establishing ecosystems with long food chains and reduction in sludge production during treatment (Zhang et al. 2008). Practically, A1-A2-O systems can be coupled with MBR, NF and RO systems for WW treatment (Jin et al. 2013). However, the disadvantage of such systems is further treatment and disposal of the concentrates from NF-RO units.
FA leachates contain varying concentrations of metals, as shown in Table1, which often surpass the limits of Indian drinking water standards (BIS 2012). Some management practices for these contaminated WWs include mainly prevention of their generation through covering dumps with topsoil and reducing contact of FA with rainwater. The buffering capacity of soil attenuates the metals in organic matter. Transportation of FA as high concentrated slurry during disposal can also minimize leachate generation and water consumption. Demineralization of coal before its burning is a topic of recent research, which can control release of many pollutants in all the WWs. Even if the leachates are formed, their migration to groundwater can be reduced by an adequate liner below the ash ponds, and a leachate collection system wherein the leachate is collected. Along with these the surface water flow should also be controlled (Maiti & Prasad 2016). Attenuating barriers composed of sand, iron, and bentonite can reduce chromium from 25 to 0.0025 mg L1 in alkaline leachates (Astrup et al. 2000). Flue gas desulphurization gypsum can precipitate 40100% of metals in acidic leachates as sulfides (Jayaranjan & Annachhatre 2012). However, metals in acidic leachates can be removed by similar procedures as mentioned for AMD, viz. use of coagulants, bio-electrochemical technology, etc. Extensive research is required for large scale treatment of leachates using latest developments in the field of water treatment.
Despite the usefulness of the above treatments, none of them if used alone can be outstanding in terms of amount of wastewater to be treated, contaminant removal potential and treatment cost. The specific challenges of the above techniques are as follows.
Although advanced oxidation and membrane techniques are primary processes for advanced treatment of coke oven WW, they are expensive and undergo membrane fouling. Studies should be focused on reducing treatment cost along with techniques to control membrane fouling.
Very few field studies have been found which could uphold the wide applicability and sustainability of the treatment procedures. Hence, future studies should be focused towards scaling up the pilot scale studies with the actual coal-related effluents.
A wise analysis of advantages and challenges suggest that the treatment methods can potentially purify water to the drinking level stage when used collaboratively. After further validation studies they can be practically implemented in areas where fresh water is scarce. Advantageously simple treatment techniques can remediate mildly polluted WW with less expense. There is a growing inclination towards WW treatment through safer eco-friendly techniques; hence biological approaches can be used in an integrated manner with conventional techniques to reduce use of chemicals and formation of sludges, and ensure potability, cost-effectiveness and regeneration of treatment materials.
Authors are grateful to Dr Pradeep Kumar Singh, Director, CSIR-CIMFR, Dhanbad, for continuous support and motivation. The preparation of the manuscript did not involve the contribution of funding sources.