Coal gasification wastewater (CGW) contains a large number of toxic and refractory compounds, such as phenolic compounds and polycyclic and heterocyclic aromatic compounds. These toxic and refractory compounds are difficult to degrade if biological methods are the only ones used. In recent years, several novel biological coupling processes are used to treat CGW. In the study, this review attempts to offer a comprehensive summary regarding the biological coupling treatment technologies of CGW, including conventional biological processing arts, the combination of adsorption and biotechnology, biological enhancement technologies, co-metabolism technologies and the combination of advanced oxidation and biotechnology. Meanwhile, the treatment efficiency of different biological coupling processes was compared with each other. Co-metabolism and advanced oxidation with biotechnology are both highly effective and promising technologies for degrading toxic and refractory compounds. More research should be conducted on these two aspects in the future.
Keywords: Coal gasification wastewater; Toxic and refractory compounds; Adsorption and biotechnology; Co-metabolism; Advanced oxidation and biotechnology
In the face of a shortage of oil and gas resources and the rapid growth in demand, the coal gasification will quickly become one of leading directions in the traditional coal chemical industry (Pan et al. 2012; Zhou et al. 2012). However, coal gasification is a high water-consuming industry,the demand of water resources is great, and the problem of wastewater treatment has become a bottleneck that has restricted the development of the coal gasification industry.
In general, CGW is first processed by physico-chemical pretreatment, and it then enters the bio-chemical treatment stage and finally enters the advanced treatment stage. Currently, biological treatment is widely used to treat the CGW after the pretreatment that is composed of ammonia stripping and phenol solvent extraction (Ji et al. 2016). Even after the phenol and ammonia recovery, the concentration of residual pollutants still high, including phenols (about 1000 mg/L), PAHs such as naphthalene (around 5 mg/L), NHCs such as quinoline (about 30-50 mg/L), long chain n-alkanes (20-80 mg/L), cyanide (0.2-5 mg/L) and NH
In the periods of bio-chemical and advanced treatments, if only biological methods are used, these toxic and refractory compounds are difficult to degrade. Thus in recent years, some new technologies have been used to treat the CGW. For example, Xu et al. (2016) investigated a novel system integrating microwave catalytic oxidation (MCO) and MBBR processes on the advanced treatment of biologically pretreated Lurgi CGW. Catalytic ozonation of quinoline using nano-MgO was researched on real biologically pretreated CGW (Zhu et al. 2017b). Methanol was introduced to improve the anaerobic biodegradability of real CGW, and the effect of the addition of methanol on the performance was investigated in a mesophilic upflow anaerobic sludge bed reactor with a hydraulic retention time of 24 h (Wang et al. 2010). Zhuang et al. (2016a) investigated the effect of fine bubbles of pure oxygen on membrane bioreactor (O
Due to its highly recalcitrant nature, CGW has become a primary barrier that hampers further development of the new coal-to-chemical industry in China (Zhao and Liu 2016). Therefore, this review attempts to offer a comprehensive overview of the possible biological coupling treatment processes. In recent years, some novel biological coupling processes are used in the treatment of CGW, but in some other's reviews, these novel biological coupling processes are not contained. Some novel biological coupling processes are compared with each other and the research directions of each part are analyzed in the manuscript. Concept of industrial ecology taken into the coal chemical industry for the aim of zero liquid discharge is described. Industrial ecology as the promising strategy can provide efficient approaches for water utilization to reduce the environmental impact and realize the coordinated development with the environment.
In the influent of biological treatment technologies, the concentration of the COD, total phenol and ammonia is approximately 2000-4000 mg/L, 300-800 mg/L, and 100-300 mg/L, respectively. The concentration of the pollutants is closely related to the gasification technology, type of coal, and the phenol and ammonia extraction process (Ji et al. 2016).
Currently, CGW has been considered to be an emerging challenge in the sustainable development of the coal chemical industry. Therefore, extensive efforts have been devoted to developing various biological processes to enhance the removal of hazardous and refractory organics from the CGW with the ultimate target of zero liquid discharge (ZLD) (Tong et al. 2010). Aerobic and anaerobic processes are common applied processes for treatment of CGW. Currently, several new technologies are springing up, such as aerobic processes: moving bed biological reactor, submerged aerated biological filter, sequencing batch reactor, fluidized-bed bioreactor, hybrid fixed-film and anaerobic process: thermophilic anaerobic digestion.
Oberoi and Philip (2017) investigated the performance of a single stage moving bed biological reactor (MBBR) (Fig. 1b) and a submerged aerated biological filter (SABF) (Fig. 1a) to treat wastewater contaminated with heterocyclic and homocyclic aromatic hydrocarbons along with phenolic compounds commonly discharged from coal and biomass gasification plants. The SABF system showed better substrate removal efficiency (TOC removal ratio of 57%) during the initial start-up phase, but in formal operational stage MBBR system showed better stability against hydraulic and organic shock loads with TOC removal ratio of 61%. A laboratory scale sequencing batch reactor (SBR) was investigated to treat artificial pretreated CGW that primarily contained of ammonia and phenol (Liu et al. 2017). In the study SBR held the potential to simultaneously remove phenolic compounds and nitrogen through aerobic ammonia oxidation and anoxic denitrification with phenol as the co-organic carbon source, but in real CGW, high concentration of phenol would have strong inhibition on nitrification and denitrification bacteria. A fluidized-bed bioreactor was investigated by Donaldson et al. (2010) to treat CGW. In comparison to activated sludge process, biological activity in fluidized-bed bioreactor was stable, recovery from the upsets was rapid, and the reaction rates were high due to the high concentration of microorganisms retained on the support particles. Besides, the fluidized-bed bioreactor could save 50% of the capital investments and the operating costs. So far, the major challenges of fluidized-bed bioreactor were the acceleration of its fluidization and the optimization on its process (Zhu et al. 2018). A hybrid fixed-film bioreactor to treat CGW was utilized by Rava and Chirwa (2016) to remove the COD, phenols and ammonia-nitrogen. The advantages of hybrid fixed-film were that it was stable during fluctuating loading conditions and fixed micro-organisms were more resistant to changes in pH, nutrients and toxic substances. The bioreactor improved the quality of the CGW by removing 49% and 78% of the COD and phenols, respectively.Overall schematic diagram of a submerged aerated biological filter (SABF), b moving bed biological reactor (MBBR).Reprinted with permission from reference Oberoi and Philip (2017)
PHOTO (COLOR)
Wang et al. (2011b) used a thermophilic anaerobic digestion (55 ± 2 °C) of Lurgi coal gasification wastewater (LCGW) to investigate the UASB reactor. Compared with mesophilic anaerobic digestion, thermophilic anaerobic digestion could enhance the degradation of the phenolic compounds and other ingredients and improved both the anaerobic and aerobic biodegradation of the LCGW. However, the maintenance of high temperature in thermophilic anaerobic digestion would waste a lot of energy, which would hinder the development of this reactor.
The data in Table 1 shows that the COD removal ratios in most of the single biological processes were below 60%, and the total phenols removal ratios were different obviously with different single biological processes. The single biological processes described previously had some removal of phenolic compounds and nitrogen, but some polycyclic aromatic hydrocarbons, heterocyclic compounds, long chain alkanes and cyanide were still detected in effluents of these reactors. Some polycyclic aromatic hydrocarbons and other refractory compounds were difficult to degrade in aerobic conditions under limited hydraulic retention time. In an anaerobic environment, benzene ring cracking could occur due to the actions of hydrolyzing acidifying bacteria. Refractory compounds could be degraded completely in anaerobic conditions, but the degradation period after the benzene ring cracking would be much slower than during aerobic conditions. Thus, in the anaerobic/aerobic system, the anaerobic process acts as a pretreatment for the partial degradation of some refractory and inhibitory organic compounds (Suidan et al. 1983), while the following aerobic process plays the primary role in reducing the COD and nitrogen through the performance of predenitrification-nitrification (Melcer and Nutt 1985).
Biological treatment approaches are eco-friendly methods to mitigate industrial pollutants and involve the stabilization of wastes by degrading them into harmless substances either using anaerobic or aerobic processes (Chowdhary et al. 2017). Since some polycyclic aromatic hydrocarbons, heterocyclic compounds, long chain alkane and cyanide are still detected in effluents of single reactors, some combined biological processes have drawn researchers' attention.
A novel biofilm reactor developed by Kargi and Eker (2005) was used to remove 2,4-dichlorophenol and detoxify from synthetic wastewater. The COD and toxicity removal efficiencies in the rotating perforated tubes biofilm reactor were 96% and 79%, respectively. A fixed biofilm process was developed by Hsien and Lin (2006) to describe the biodegradation of phenolic wastewater. Similar to the result of Kargi and Eker (2005), the fixed biofilm reactor was capable of achieving very high phenol removal ratio of 94%. Experiments were conducted by Borghei and Hosseini (2004) to investigate the behavior of a moving bed biofilm reactor (MBBR) receiving a mixture of toxic (phenolic) wastewater. The similar conclusion was reached that the MBBR has good resistance to shock loads and returns to steady state conditions within two or three cycles of the retention time. However, the fluctuation of CGW quality presents a challenge for MBBR treatment. Typically, the sudden increase of influent NH
In addition to the biofilm reactor, the biocombination process is more about the combination between the reactors. Wang et al. (2011a) used a two-continuous mesophilic (37 ± 2 °C) UASB system with step-feed to investigate the optimization strategy for enhancing COD and total phenols removal of the system and the aerobic biodegradability of real CGW. Step-feed indicated that the influent raw wastewater was divided into two parts: one part flowed into the first reactor, and the other part which combined with the effluent of the first reactor served as the influent to the second reactor (Wang et al. 2011a). After the treatment of first reactor, the effluent had greater biodegradability, and it could enhance the other part of the influent in the second reactor. Thus, the step-feed method had better performance than the single-feed way in the two-continuous mesophilic UASB system. The step-feed supplied the system with a more reasonable distribution of organic compounds that increased the anaerobic degradation of refractory organic compounds with easily biodegradable organics in the second reactor (Diamantis and Aivasidis 2007; Fezzani and Cheikh 2010).
Xu et al. (2015a) used a two-phase anaerobic digestion of real CGW. In the study, two-phase meaned reactors 1 and 2 were connected in a series. Reactor 1 was enriched by acidogenic bacteria and reactor 2 was operated at pH 7.0 ± 0.2. Two-stage meaned reactors 1 and 2 served as commom anaerobic digestion reactors. After two-phase anaerobic digestion, the wastewater concentrations of the aerobic effluent COD could reach below 150 mg/L compared with 237.2 mg/L if two-stage anaerobic digestion was conducted and 328.5 mg/L if sole aerobic pretreatment was conducted.
Using a hydrolytic acidification-Anoxic/Oxic (AO) process to treat CGW, the COD and total phenols in the effluent and the removal efficiencies were studied (Xu et al. 2013, 2018). The start-up period was separated into two stages with influent COD concentration of 1500 and 2500 mg/L. The results from Xu et al. (2013) showed that after 59 days of operation, the COD and total phenol removal efficiencies were 80.3% and 72.9%, respectively. An anaerobic filter, an expanded bed, a granular activated carbon anaerobic filter, and an activated sludge nitrification system were evaluated to treat a synthetically prepared coal gasification wastewater (Suidan et al. 1983). Due to the ability that activated carbon could sequester some components of the wastewater that were toxic to the mixed cultures of anaerobic microorganisms, so the system showed fine COD and total phenols removal efficiencies. In another study, a lab-scale sequential anoxic moving bed biofilm reactor (ANMBBR) and aerobic moving bed biofilm reactor (AEMBBR) for pre-denitrification system were used to treat real coal gasi-fication wastewater (Li and Han 2014). The removal efficiencies of the COD, phenols, thiocyanate (SCN
The aerobic method generally applies only to wastewater with medium and low concentrations of organic compounds. The biodegradable organic matter can be degraded to a low level using aerobic methods. CGW contains a large number of toxic and refractory compounds, such as phenolic compounds and polycyclic and hetrocyclic aromatic compounds. The concentration of the COD, total phenol and ammonia is approximately 2000-4000 mg/L, 300-800 mg/L, and 100-300 mg/L. If this industrial wastewater goes directly into the aerobic reactor, high loading rates and toxic and refractory compounds would strongly inhibit the aerobic sludge. Therefore, in the first processing unit, the anaerobic reactor is more suitable than the aerobic reactor to treat real CGW. The anaerobic microorganisms have the function of detoxifying and decomposing organic matter in the presence of common metabolic materials. They can transform aromatic rings into an alkane ring structure or break the ring structure (Fuchs et al. 2011). After the anaerobic process, the typical polycyclic aromatic and heterocyclic refractory organic compounds, such as quinoline, indole and pyridine, all have different degrees of transformation and degradation (Meckenstock and Mouttaki 2011; Philipp and Schink 2012; Boll et al. 2014), and the wastewater degradation property can be significantly improved. This could provide a proper environment for the subsequent aerobic biological treatment. In fact, strong evidence showed that the operation eventually failed if anaerobic unit was not present or malfunctioned (Kuschk et al. 2010).
The novel system of EBA (based on external circulation anaerobic (EC) process-biological enhanced (BE) process-anoxic/oxic (A/O) process) (Fig. 2) was applied to treat the British Gas/Lurgi CGW in Erdos, China (Jia et al. 2016a). After a long time of commissioning, the EBA system demonstrated a stable and highly efficient performance. In particular, the concentrations of the COD, NH
PHOTO (COLOR)
The data in Table 1 show that compared with single biological processes, combined biological processes had a more efficient removal ratio of the COD and total phenols. Among these, the hydrolytic acidification-AO process, the ANMBBR-AEMBBR process and the EBA process all had a higher removal ratio of the COD and total phenols. EBA technology is a very mature and effectively method to treat CGW. Anaerobic external circulation (EC) can improve the biodegradability of toxic and refractory compounds and provide a relatively safe and stable environment. Most phenols and the intermediate products of toxic and refractory compounds can be degradated in biologically enhanced (BE) process. Nitrification and denitrification are conducted in anoxic/oxic (A/O) process for the removal of nitrogen.
In anaerobic process, genus Saccharofermentans composed the most relative abundance of 10.04%, the genus has been reported as a novelty genus isolated from anaerobic sludge treating brewery wastewater, a representative of this genus, Saccharofermentans acetigenes produced fumarate acetate, lactate and traced amounts of molecular hydrogen from several sugars (Chen et al. 2010). Bacterial genus Comamonas (8.79%) was thought to be a versatile aromatic degrader for polycyclic aromatic hydrocarbons and heterocyclic aromatics, such as indole, quinolone and carbazole (Ma et al. 2015). In the BE process, the genera Denitratisoma, Hyphomicrobium, Thauera and Thermomonas were indicated as the typical denitrifying bacteria under the anoxic condition (Wang et al. 2015), and the relative abundance of these four genera reached 11.87%. González-Martínez et al. (2013) indicated that genus Diaphorobacter was in close relation with partial-SHARON process and dominated the bacterial community, giving the relative abundance of 1.26% in BE process, which revealed the partial-SHARON process might occur in it. In addition, as previously reported, Thauera strains from a coking wastewater treatment bioreactor could degrade phenol, methylphenol and indole (Mao et al. 2010), giving the relative abundance of 1.36% (Jia et al. 2016a). In the anoxic gallery of A/O process, the genus Methylococcus was detected as the most abundant genus with the relative abundance of 12.27%, which was indicated as one typical methanotrophs, taking the metabolism using the methane as the only carbon source (Lee et al. 2015). Denitratisoma (2.43%) for denitrification and Nitrospira (1.19%) for nitrification were simultaneously detected in the anoxic gallery which revealed the possibility of simultaneous nitrification and denitrification (Jia et al. 2016a).
As shown in Fig. 3a, in anaerobic environment phenol was initially phosphorylated to phenylphosphate in the presence of phenylphosphate synthase (Schmeling et al. 2004; Narmandakh et al. 2006). After that, 4-hydroxybenzoate was generated due to carboxylation of the aromatic ring system in para-position (Schühle and Fuchs 2004). Then, thioesterification to 4-hydroxybenzoyl-CoA by a CoA ligase and reductive dehydroxylation by 4-hydroxybenzoyl-CoA reductase finally produced benzoyl-CoA as the central metabolite (Bonting and Fuchs 1996). Following the similar mechanism, some substituents such as methyl, ethyl, hydroxy and carboxy groups could be treated for anaerobic transformation of phenols to benzoyl-CoA (Zhu et al. 2018). For anaerobic degradation of nitrogen heterocyclic (e.g., quinoline, Fig. 3b), the initial hydroxylation of quinoline to 2-hydroxyquinoline that tautomerizes to 2-oxo-1, 2-dihydroquinoline in specific degrading bacteria was catalyzed by inducible enzyme (Licht et al. 1996). Then, 8-hydroxy-2-oxo-1, 2-dihydroquinoline was hydroxylated from 2-oxo-1, 2-dihydroquinoline (Zhu et al. 2018). Polycyclic aromatic hydrocarbons (PAHs) (e.g., naphthalene, Fig. 3c) could be biodegraded via the corresponding intermediates, e.g., 2-arylcarboxyl-CoA (Meckenstock and Mouttaki 2011). The catabolism of benzoyl-CoA, arylcarboxyl-Co esters and 2-arylcarboxyl-CoA indeed were driven by different dearomatizing reductases with VFA, CO
PHOTO (COLOR)
Phenolic compounds inhibited the activity of anaerobic microbes even at low concentrations. Although the sludge acclimatization to enhance the degradation of phenolic compounds had been reported, alkyl phenols were more persistent from biological treatment and inhibitory to anaerobic microbes than phenol (O'connor and Young 1996). Therefore, bioaugmentation methods, such as the addition of adsorption materials, had been used to enhance the removal of phenolic compounds.
PAC-activated sludge treatment was investigated by Luthy et al. (1983) for the treatment of CGW. With 5000 mg/L PAC in the mixed liquor under aeration PAC-activated sludge treatment gave better removal efficiencies than the activated sludge treatment for TOC, COD, phenolics, organic-N, NH
An et al. (2016) introduced a novel adsorbent lignite coke (LC), which is suitable for macromolecular contaminant removal in the CGW. Compared with the activated carbon, the LC had higher decolorization ability and removed more refractory compounds and could improve wastewater biodegradability (Aivalioti et al. 2012). As the LC had more mesopores than the ACs, which supplied enough area for high molecular weight pollutant removal (Mizera et al. 2007; Redding et al. 2009; Vassileva et al. 2009).
Powdered activated carbon and granular activated carbon are common adsorbent materials. With the combination of adsorption and biotechnology, reactors have a higher removal ratio of the COD and total phenols (Table 2) compared to the single biological process (Table 1). As a novel adsorbent material, if LC was used as a carrier of biological membrane, the degradation performance of the toxic and refractory organic matter would be much better. The characteristics of combination of adsorption and biotechnology include three parts. (
The fluctuated performance of the pretreatment determined that the phenolic concentration into the biological treatment is in the wide range of 500-3000 mg/L (Wang and Han 2012). Therefore, without biological enhancement technologies, the bio-reactors could easily deteriorate and break down (Fang et al. 2013). Fang et al. (2013) evaluated the performance of the biological contact oxidation reactor (BCOR) (Fig. 4) treating CGW after augmentation with phenol-degrading bacteria (PDB). Compared with traditional BCOR, the augmented BCOR exhibited strong resistance to phenolic compounds. Many of the refractory phenolic compounds were converted to easily biodegradable compounds in spite of the low TOC removal (Quan et al. 2004; Banerjee and Ghoshal 2010; Eker and Kargi 2010). In a study by Fang et al. (2013), phenol was primarily degraded by the PDB via the meta-cleavage pathway. The first step was the conversion of phenol into catechol by phenol hydroxylase. Then, cleavage of catechol was typically achieved, which decided components of the followed intermediate. The GC-MS analysis indicated that the relative amounts of acetaldehyde and ethyl, which were the intermediates in the meta-cleavage pathway (Polymenakou and Stephanou 2005), comprised 85.3% of the organic composition in the medium. Microbial community analysis revealed that the PDB presented as dominant populations in the bacteria consortia, which in turn determined the overall performance of the system. Studies have shown that specific strains can degrade specific refractory organic compounds. Comamonas strain JB had been demonstrated to be able to remove aromatic pollutants, such as quinoline, pyridine and phenol (Jiang et al. 2017; Tao et al. 2017). Strain JB could not use pyridine and quinoline as its sole carbon source, but could co-metabolically degrade pyridine and quinoline using phenol as the primary substrate, indicating that phenol was the real inducer for the degradation enzymes expression and degrading enzymes of phenol controlled the metabolic pathway. Ma et al. (2018) studied the integration of Fe-C materials with a biological reactor (MEBR) for strengthening removal of phenolic compounds in CGW. Compared with control reactor, MEBR with Fe-C materials achieved high efficiencies in the removal of the COD (removal ratio of 86.50%) and phenolic compounds (removal ratio of 88.30%), as well as improvement of biodegradability (risen from 0.25 to 0.46) of CGW under the micro-oxygen condition. The up-flow anaerobic sludge blanket (UASB) system with graphene assistance was developed by Zhu et al. (2018) for CGW treatment. Long-term results demonstrated that COD removal efficiency and methane production rate with assistance of the graphene achieved 64.7% and 180.5 mL/d (Table 2), respectively.Schematic diagram of BCOR.Reprinted with permission from reference Fang et al. (2013)
PHOTO (COLOR)
Biological enhancement technology is a more effective method for treatment of toxic and refractory compounds in CGW, which can improve the biodegradability of toxic and refractory compounds and provide a relatively safe and stable environment for the next process. In addition, biological enhancement technology can promote microbial accumulation and enzymes activities and improve the performance of sludge settling.
Co-metabolism refers to the process by which microorganisms degrade refractory organic compounds after obtaining most or all carbon sources and energy from other substrates. Some toxic and refractory material cannot be degraded directly by microorganisms, but the addition of biodegradable substances can promote the degradation of the refractory material. Common co-metabolic substances include methanol, glucose, sodium acetate, and phenol. In the presence of the common co-metabolic matrix, microorganisms can detoxify and decompose refractory organic matter. After co-metabolic processing, the typical polycyclic aromatic and heterocyclic refractory organic compounds, such as quinoline, indole, and pyridine, all have different degrees of transformation and degradation. The wastewater aerobic degradation of performance can be significantly improved, which could provide a safe environment for the subsequent biological treatment. Recently, co-digestion technologies have been developed for treating refractory wastewater (Youngster et al. 2008; Zhang et al. 2008).
The effect of the addition of methanol on performance was investigated by Wei et al. (2010) in a mesophilic upflow anaerobic sludge bed reactor with a hydraulic retention time of 24 h. Methanol can be easily obtained from the coal gasification industry (Kumabe et al. 2008), enabling the co-digestion of CGW with methanol substrate or its manufacturing wastewater to be a feasible option. In fact, the co-substrate is not unique, and other easily biodegradable substrates have been observed in laboratory scale studies (Cheng et al. 1998; Ramakrishnan and Gupta 2006). The presence of methanol could reduce the toxicity of CGW and increase the biodegradation of the phenolic compounds (Perez et al. 2006). Jia et al. (2016b) developed a long-term bio-augmented submerged membrane bioreactor (BSMBR) (Fig. 5) for the treatment of BPCGW with methanol as co-metabolites. Both prolonged HRT and the addition of GAC reduced membrane fouling, while the co-metabolic process with methanol sped up membrane fouling (Johir et al. 2013). Chen et al. (2010) studied the degradative effect of reactive black KN-B5 with glucose used as a co-metabolite. Tetrachloroethylene degradation was studied with Li et al. (2004) by acclimated anaerobic sludge with glucose, acetate and lactate as cometabolism substrates.Schematic diagram of MBR (1. influent tank; 2. influent pump; 3. bioreactor; 4. membrane module; 5. vacuum manometer; 6. effluent pump; 7. air pump).Reprinted with permission from reference Jia et al. (2016b)
PHOTO (COLOR)
With the deepening research on the metabolic mechanism, researchers have demonstrated that the co-metabolism of anaerobic microbes is one of the leading types of the degradation of refractory organic matter. Anaerobic co-metabolism can make up the shortage of the refractory organic compounds which are difficult to degrade due to the lack of carbon source and energy support by pure anaerobic microorganisms (Liu et al. 2009). The type and concentration of co-metabolic materials are the key factors that affect the decomposition of organic compounds (Su et al. 2008; Delgadillomirquez et al. 2011; Huang et al. 2011). When microbes in the use of easily biodegradable organics, the non-growth matrix gets close to the enzyme molecule. The enzyme protein is then induced by non-growth matrix and turns into a condition that can degrade the non-growth matrix. The enzymatic protein is combined with the non-growth matrix. Finally, the non-growth matrix is degraded with co-metabolism. For complex refractory organic compounds and biodegradable growth matrices, there must be a microbial community that can degrade them. The metabolic pathway of co-metabolism is complex and requires the interaction and multi-step combination to complete the degradation of refractory organic compounds (Aken et al. 2010). Methanogenic bacteria used cathodic hydrogen evolution to promote the co-metabolic degradation of chloroform, in which the resulting hydrogen matrix contributes to the reduction and dechlorination of chloroform (Weathers et al. 1997). In the degradation of refractory compounds, the selection of the growth matrix type by microorganisms was varied. Using naphthalene or methylnaphthalene as carbon and energy sources, sulfate-reducing bacteria could convert polycyclic and heterocyclic aromatics into corresponding carboxylic acids and isomers by co-metabolism, but the intermediate products would inhibit the activity of the bacteria to varying degrees (Safinowski et al. 2006).
In the studies described above, there was a lack of mechanistic research in the co-metabolism treatment of particular pollutants in CGW. As shown in Fig. 6, current studies of co-metabolic technologies are primarily focused on the following four aspects: (
PHOTO (COLOR)
Table 2 shows that adsorption and biotechnology, biological enhancement and co-metabolism technologies all have high degradation ratios of COD and total phenols. Adsorption and biotechnology could be used as a rapid recovery method in a crash running reactor. Biological enhancement and co-metabolism technologies were the leading types of the degradation of refractory organic matter.
A large amount of renovation and optimization for industrial CGW biological treatment processes has been conducted, but after biological treatment, most of the emulsified compounds in the wastewater will be discharged to the environment and result in entrenched contamination (Zhang et al. 2006). The COD and chromaticity of the effluent could not meet the discharge standards which still need advanced treatment to achieve minimal impact on the receiving ecosystems (Ji et al. 2016). As a result, advanced treatment technologies are necessary to degrade the residual COD and chromaticity. Currently, physical and chemical methods and advanced oxidation processes are most common techniques in the advanced treatment of CGW.
The advanced treatment of CGW by physical and chemical methods, such as coagulation precipitation (Yalou et al. 2010), adsorption (Lianqin et al. 2011), membrane separation (Ma and Jing 2009) and combined technologies (Lili et al. 2013) has been widely investigated and applied. The primary role of physical and chemical methods is to separate the pollutants, but not to degrade them, therefore it is necessary to pay more attention to the transformation of the pollutants and take measures to ensure their degradation (Li et al. 2016).
Advanced oxidation processes (AOPs) began to form in the 1980s in the processing of toxic pollutants. In the reaction of AOPs, free hydroxyl radical (
Three identical anoxic-aerobic membrane bioreactors (MBRs) were operated by Zhu et al. (2017a) in parallel for 300 consecutive days for raw (R1), ozonated (R2) and catalytic ozonated (R3) (Fig. 7) biologically pretreated CGW treatment. The results demonstrated that catalytic ozonation process (COP) applied as a pretreatment remarkably improved the performance of the unsatisfactory single MBR (Dai et al. 2014; Moussavi et al. 2014; Aghapour et al. 2015). In addition, typical nitrogenous heterocyclic compounds (NHCs) of quinoline, pyridine and indole were completely removed in the integrated process. In addition, a possible degradation pathway of quinoline (Fig. 8) in catalytic ozonation was proposed (Zhu et al. 2017b). Moreover, the COP could alter the properties of the sludge and reshape the microbial community structure, thus delaying the occurrence of membrane fouling (Zhu et al. 2017a). The advanced treatment of biologically pretreated CGW was investigated by Zhuang et al. (2014c) employing heterogeneous catalytic ozonation integrated with an anoxic moving bed biofilm reactor (ANMBBR) and a biological aerated filter (BAF) process. The effluent of the catalytic ozonation process was more biodegradable and less toxic than that of the ozonation alone (Kasprzyk-Hordern et al. 2003). In addition, ANMBBR-BAF showed efficient capacity of pollutants removal in treatment of the effluent of catalytic ozonation at a shorter reaction time, allowing the discharge limits to be met. Han and Zhuang (2013) investigated the heterogeneous catalytic ozonation of COD and quinoline from CGW secondary effluent with carbon-supported copper oxides as catalyst. Compared with granular activated carbon and single ozone, CuOx/GAC demonstrated a higher catalytic performance. Higher pH had a positive effect on the degradation of quinoline due to the formation of hydroxyl radical (Ma et al. 2005). A novel system of catalytic ozonation coupled with activated carbon adsorption was investigated by Fang and Han (2018) for removing the organic compounds treating in the RO concentrate from CGW. With the ozone dosage of 120 mg/L, catalyst dosage of 2.0 g/L, catalytic ozonation reaction time of 1 h, and modified activated carbon adsorption time of 1 h, the average TOC removal efficiencies were maintained at the stable level of 58% with the TOC concentration of 26 mg/L.Schematic diagram of the experimental set-up.Reprinted with permission from reference Zhu et al. (2017a)
PHOTO (COLOR)
Possible degradation pathway of quinoline.Reprinted with permission from reference Zhu et al. (2017b)
PHOTO (COLOR)
Previous studies have demonstrated that heterogeneous Fenton oxidation and heterogeneous catalytic ozonation could serve as alternative technologies by overcoming some drawbacks of traditional Fenton and ozonation processes (Zhuang et al. 2014b). However, they have challenges in terms of technical complexity, substantial equipment investments and harsh reaction condition, which limit their full-scale practical application (Xu et al. 2016).
Fenton oxidative process has proven to be effective in treatment of varied refractory-containing organic wastewaters (Oller et al. 2011). A novel system integrating heterogeneous Fenton oxidation (HFO) with anoxic moving bed biofilm reactor (ANMBBR) and biological aerated filter (BAF) process were conducted by Xu et al. (2015c) on advanced treatment of biologically pretreated CGW. HFO with the prepared catalyst (FeOx/SBAC, sewage sludge based activated carbon (SBAC) which loaded Fe oxides) played a key role in eliminating the COD and COLOR, as well as in improving the biodegradability of the raw wastewater. Another novel integrated process with three-dimensional electro-Fenton (3D-EF) and biological activated carbon (BAC) (Fig. 9) was employed by Hou et al. (2015) in advanced treatment of biologically pretreated Lurgi CGW. Similarly, 3D-EF demonstrated an excellent capacity at abating pollutants, toxicity and COLOR, as well as improving biodegradability (Anotai et al. 2006) by generating more H
PHOTO (COLOR)
Ultrasound is a sound wave at a frequency range from 20 kHz to 10 MHz that has a wide range of environmental applications (Oz and Uzun 2015). Ultrasound produces a hydrodynamic shear force in aqueous phase due to the rapid collapse of microbubbles during cavitation (Oz and Uzun 2015). The shear force plays an important role in the disintegration of coarse particles. Furthermore, the acoustic energy associated with ultrasounds initiates chemical reactions due to high temperatures and pressures created within a collapsing cavitation bubble, which forms free radicals (Velmurugan and Muthukumar 2011).
Jia et al. (2015b) investigated a novel system integrating catalytic ultrasound oxidation (CUO) with a membrane bioreactor (CUO-MBR) on the advanced treatment of biologically pretreated CGW. Compared with MBR, CUO-MBR with catalyst of FeOx/SBAC (sewage sludge based activated carbon (SBAC) which loaded Fe oxides) demonstrated higher efficiencies at eliminating the TOC, as well as improving the biodegradability (Duan et al. 2014). The enhanced hydroxyl radical oxidation (Velmurugan and Muthukumar 2011), the facilitation of substrate diffusion (Blaskovicova et al. 2007), and the improvement of cell enzyme secretion (Hasan et al. 2012) were the mechanisms for the CUO-MBR performance.
As one of AOPs, microwave (MW) technology has already been widely applied to household, industry, medical science, and the application of MW technology for wastewater treatment has been recognized as a cost effective, time saving, and environmental friendly technology in the mineralization of refractory pollutants and improvement of the wastewater biodegradability (Remya and Lin 2011; Yin et al. 2016).
Laboratory scale experiments were conducted by Xu et al. (2016) to investigate a novel system integrating microwave catalytic oxidation (MCO) and the MBBR process on the advanced treatment of biologically pretreated Lurgi CGW. The results indicated that MCO with catalyst of MOs/SAC (sewage sludge based activated carbon which loaded Mn oxides) demonstrated high efficiencies at eliminating bio-refractory compounds by generating
Xu et al. (2015b) examined the feasibility of using combined heterogeneous photocatalysis oxidation (HPO) and a moving bed biofilm reactor (MBBR) process for the advanced treatment of biologically pretreated CGW. The results indicated that the TOC removal efficiency was significantly improved (TOC removal ratio of 90.2%) in HPO. Gas chromatography-mass spectrometry (GC-MS) analysis indicated that the HPO could be utilized to eliminate bio-refractory and toxic compounds (Giri et al. 2008; Elmolla and Chaudhuri 2011).
Table 3 shows that the combinations of advanced oxidation and biotechnology have a high degradation ratio of the COD, total phenols and refractory materials. The treated wastewater from these combinations was more biodegradable. Advanced oxidation technology is technically feasible to manage toxic and refractory compounds, but the high cost of processing the complete mineralization is the bottleneck that restricts the promotion of these advanced oxidation technologies. Therefore, considering the combination of advanced oxidation technology and biotechnology, it will be the future research direction to treat toxic and refractory compounds in the CGW, as the combination of several advanced oxidation and biological methods described above. Some types of advanced oxidation processes described above are relatively novel and have a certain feasibility in laboratory scale, but the technology is still immature. It still has a long way to go before the application of the combination of novel advanced oxidation technology and biotechnology can be applied in engineering practice. An additional question needs to be researched. It is conceivable that during the pressing of the AOPs, additional toxic and refractory intermediate products could be generated, and these new generated intermediate products could have effects on the next biotreatment. It is necessary to determine the degree to which the organic compounds are mineralized and which combination is the most economical. While all of these potential issues need to be researched, there is no denying that this combination is a promising technology to treat toxic and refractory compounds in CGW.
According to the national energy development strategy, new coal chemical industries must develop and promote the high-efficiency and cleaner coal utilizing technology to realize the energy sustainable development (Xie et al. 2010). Industrial ecology (IE) is a promising strategy that can provide efficient approaches for water utilization to reduce the environmental impact and realize the coordinated development with the environment (Jia et al. 2016c). The concept of IE was popularized by using the analogy between natural ecosystems and industrial systems (Boix et al. 2015), and, as expected, a more recent definition for IE has been expressed as "a systems-based, multidisciplinary discourse that seeks to understand emergent behavior of complex integrated human/natural systems" (Allenby 2004, 2006). The key feature of IE relies on the integration of various components of a system to reduce the net resource input, as well as the pollutant and waste outputs (Despeisse et al. 2012).
A wastewater treatment plant (Fig. 10) was exclusively used by Jia et al. (2016c) for treating wastewater in the industry and the effluent was required to pump to the reuse sector as raw water with no wastewater discharge into the environment which satisfied the IE principles. This sector was one of the key techniques to realize ZLD. IE measures implemented in the coal chemical industry demonstrated an economy-environment win-win situation and provided scientific guidance for the designs and operations of water utilization for other coal chemical industries (Jia et al. 2016c). An inevitable development trend of the CGW treatment technologies could be the optimal combination of physico-chemical and biological treatment technology. It can provide the reference to the bottleneck of wastewater treatment leading to the accomplishment of zero discharge technologies in future in the coal gasification industries (Jia et al. 2016c).Diagram of water utilization in wastewater treatment plant (EC external circulation anaerobic process, BE biological enhanced process, AO anoxic-oxic process, AOP advanced oxidation process, BAF biological aerated filter).Reprinted with permission from reference Hou et al. (2015)
PHOTO (COLOR)
This review offered a comprehensive summary about the biological and advanced processes for treatment of CGW. Co-metabolism technology is a very promising method in treatment of toxic and refractory compounds, but the degradation mechanism of particular pollutants needs for further research; Some kinds of advanced oxidation processes described in review have a certain feasibility in laboratory scale but the technology is not mature enough in engineering application. EBA technology is a highly mature and effective method for the treatment of CGW, which also follows the concept of industrial ecology. Even after decades of effort, the treatment of CGW still be a big challenge considering the efficiency, economy and the environment. Meanwhile, the concept of industrial ecology should be taken into the coal chemical industry for the aim of zero liquid discharge. Application of industrial ecology in the treatment of CGW will provide the scientific guidance for designs and operations of water utilization for coal chemical industries.
This work was supported by National key research and development program-China (2017YFB0602804).
By Jingxin Shi; Yuxing Han; Chunyan Xu and Hongjun Han
