Technological Advancements and Prospects for Near-Zero-Discharge Treatment of Semi-Coking Wastewater

Abstract

This review examines the technological bottlenecks, potential solutions, and future development directions in the treatment and resource utilization of semi-coking wastewater (SCOW) in China. By comprehensively investigating the semi-coking industry and analyzing wastewater treatment research hotspots and existing projects, this study systematically explores the current status and challenges of each treatment unit, emphasizing the necessity for innovative wastewater treatment technologies that offer high efficiency, engineering feasibility, environmental friendliness, and effective resource recovery. This review highlights prospects and recommendations, including the development of novel extractants for phenol and ammonia recovery, a deeper understanding of biological enhancement mechanisms, exogenous bio-enhancement materials, and the creation of cost-effective advanced oxidation process (AOP)-based combined processes. Additionally, it underscores the potential for repurposing SCOW as a valuable resource through appropriate treatment, whether recycling for production or other applications.

1. Introduction

China is the world’s largest energy consumer [1]. By 2023, its energy consumption comprises 55.3% coal, 18.3% oil, and 26.4% clean energy sources, including natural gas, hydropower, nuclear power, and wind power [2]. Coal has always been and will continue to be a crucial energy resource for China’s industrial and economic development. Currently, the country is at a critical juncture in coal resource development. New coal chemical projects, notable for their green, efficient, and low-carbon technologies, have emerged as research and development priorities. The semi-coking industry is a significant part of this trend.

In 2008, the China Ministry of Industry and Information Technology included the semi-coking industry in the Energy Industry Development Guidance Catalog. This, combined with rising domestic demand, has driven rapid growth in the semi-coking industry of China [3]. However, this growth has led to substantial emissions of complex wastewater containing high levels of organic pollutants, which is normally regarded as semi-coking wastewater (SCOW) [4]. In 2022, China’s semi-coking production was estimated at around 65 million tons [5]. With a water-to-coke ratio of 2.0 per ton of coke produced, the annual discharge of SCOW is approximately 130 million tons [6]. The semi-coking industry is concentrated in provinces like Inner Mongolia, Shaanxi, and Shanxi, which face scarce water resources and fragile ecosystems. National policies like the “Water Pollution Prevention and Control Action Plan” and “Environmental Access Conditions for Modern Coal Chemical Projects” emphasize principles such as separating clean and polluted water streams, treating pollution separately, deep processing, and recycling by quality. These principles are critical for the safe treatment and resource utilization of SCOW.

Semi-coking production, a recent advancement in the coking chemical industry in China, involves medium-to-low-temperature dry distillation at approximately 600–800 °C. This process, characterized by lower temperatures compared to conventional coking, results in the production of significant amounts of gaseous low-molecular-weight organic compounds [7]. During the coal-to-semicoke conversion, these compounds are released into water during coal gas cooling and purification. Consequently, semi-coking operations generate SCOW with higher pollutant concentrations and more complex compositions compared to coking wastewater (COW). Figure 1 illustrates the comparative water quality characteristics of SCOW and COW [4]. SCOW contains highly concentrated and toxic pollutants such as phenols, cyanides, and polycyclic aromatic hydrocarbons, which pose significant risks to the environment and human health [8]. Additionally, ammonia nitrogen in SCOW significantly harms aquatic environments. Non-ionized ammonia can enter aquatic animals, reducing cell membrane stability and enzyme activity, and thereby decreasing metabolic activity [9,10]. Excessive ammonia nitrogen can also cause eutrophication in water bodies, leading to algal blooms [11]. Given the high toxicity and environmental impact of untreated SCOW, its discharge is strictly prohibited. Even treated wastewater requires stringent effluent quality control, and near-zero discharge (NZD) technologies must be vigorously promoted to minimize environmental risks.

The semi-coking industry initially adapted wastewater treatment technologies and processes from the COW treatments. However, SCOW has pollutant concentrations over ten-times higher and a more complex composition with lower biodegradability. Early methods, such as incineration and conventional biochemical treatments, failed to meet the discharge standards. Currently, an integrated pollution control process involving pretreatment, biochemical treatment, advanced treatment, and resource utilization is commonly used for semi-coke wastewater remediation, as depicted in Figure 2 [12].

The pretreatment typically includes oil and dust removal and the recovery of phenol and ammonia. These processes eliminate pollutants like COD, ammonia nitrogen, total phenols, and cyanide, enhance biodegradability, and facilitate the resource utilization of high-concentration oil, phenolic substances, and ammonia. The biochemical treatment stage, similar to coke wastewater treatment, uses anaerobic and aerobic microorganisms to oxidize and decompose residual organic matter. However, the COD in SCOW primarily consists of residual phenol, polycyclic aromatic hydrocarbons, and nitrogen–oxygen heterocyclic compounds, which cannot be easily utilized by heterotrophic microorganisms. This results in a low B/C ratio (the biochemical-oxygen-demand-to-chemical-oxygen-demand ratio, less than 0.16) and inadequate biodegradability. To improve this, a physicochemical pretreatment unit is often added before biochemical treatment. Despite this, the effluent still contains recalcitrant organic matter after biochemical treatment, necessitating advanced treatment techniques for removal. Historically, post-advanced treatment wastewater met discharge standards, but current NZD policies require further treatment, such as desalination and evaporation concentration, to meet recycled water quality standards for reuse in production.

Addressing resource scarcity necessitates a shift in wastewater management strategies. With appropriate treatment, the SCOW can become a valuable resource, either through recycling for the same purpose or repurposing for other applications. Key aspects of comprehensive treatment include phenol and ammonia recovery and water resource reuse. While previous studies have focused on the efficiency of these individual units or the effluent improvement by some combined process, there are very limited reviews on the overall process system and the fate of contaminants in the whole process of SCOW, as well as the interactions between different treatment units [13,14,15,16,17,18,19,20]. This study summarizes 15 years of research on SCOW treatment and resource utilization, systematically analyzing and summarizing the current status and challenges of each treatment unit. This comprehensive review also sharply forwards the requirement of NZD for SCOW treatment and proposes that more attention should be paid to the new combined wastewater treatment materials and technologies, which have the advantages of high efficiency, engineering feasibility, environmental friendliness, and resource recovery efficiency. This review is organized as follows: Section 2 discusses the pretreatment unit and the processes of phenol and ammonia recovery. Section 3 details biochemical treatment processes, while Section 4 and Section 5 explore advanced treatment techniques and resource utilization, respectively. Section 6 concludes with remarks on prospects.

2. Pretreatment Process

The pretreatment process for SCOW comprises an oil and dust removal unit and a phenol and ammonia recovery unit. This process is designed to eliminate pollutants and gradually recover pyrolysis oil, middle oil, acid gas, dilute ammonia water, and crude phenol from the wastewater. Effective pretreatment technology plays a crucial role in subsequently achieving efficient treatment and resource utilization of SCOW.

2.1. Deoiling

In the industrial sector, including COW treatment, common oil removal methods include sedimentation via gravity, centrifugal separation, and flotation [21,22]. However, these methods only partially separate floating, dispersed, and large particle emulsified oil, with poor efficiency for microscopic, emulsified oil. Particularly for SCOW, which contains complex oil components, traditional methods often fall short. Consequently, research on deoiling treatment for SCOW focuses on breaking emulsified oil, coalescence, flocculation, retention, and decomposition [18,23]. Currently, demulsification–air flotation technology has proven effective for recovering dispersed and emulsified oil from coal chemical wastewater (CCW) [24,25]. A study has shown that the oil removal efficiency of air and nitrogen in air flotation technology is similar. Conventional air is cost-effective but can reduce the biodegradability of wastewater, while nitrogen can enhance the biodegradability but is more expensive [26]. Therefore, it is essential to balance oil removal efficiency and economic input based on specific circumstances. For the SCOW, researchers are developing various new demulsification technologies. For instance, Zhao et al. [27] demonstrated that a demulsification–coagulation–electro flocculation method could serve as a pretreatment for the simulated SCOW, achieving a COD removal rate of 70.64% and an ammonia nitrogen removal rate of 41.38%. Moreover, choosing suitable demulsifiers is crucial in demulsification flotation technology. Currently, both organic and inorganic chemical demulsifiers are used, but high demand has increased costs and may cause secondary water pollution [28]. Therefore, future research should focus on developing environmentally friendly demulsifiers, including microbial ones. Kang et al. [29] increased the emulsified oil demulsification efficiency of oilfield wastewater to 63.5% using a microwave–O₃–biological filter method. Elmobarak et al. [30] achieved a recovery efficiency of 98.0% with Fe-MNP for oily wastewater at an oil concentration of 800 mg/L or lower, and the emulsifying agent Fe-MNP could be recycled. Advancements in physical processing technologies could facilitate the development of novel coalescence materials, benefiting emulsion-breaking technology with cost reduction and minimized environmental impact.

2.2. Phenol and Ammonia Recovery

After oil and dust removal, high-concentration phenol ammonia wastewater is sent to the phenol ammonia recovery unit to remove acid gas and recover ammonia nitrogen and crude phenol, which is normally regarded as a key step in SCOW treatment. The presence of acid in SCOW is indeed influenced by both coal quality and production processes, as well as the acidification treatment during the deoiling process to facilitate the removal of negatively charged oil droplets. Consequently, in the phenol and ammonia recovery process, the typical approach first involves deacidification and ammonia removal processes, followed by phenol extraction, and finally steam stripping of the extractant.

Deacidification is typically conducted through air oxidation, during which sulfur present in the wastewater is transformed into thiosulfate (approximately 90%) and sulfate (approximately 10%) [31]. Ammonia removal processes for high-concentration ammonia nitrogen in wastewater mainly include the stripping method [32], the adsorption method [33], and the magnesium ammonium phosphate (MAP) crystallization method [34]. Here, the focus is on the application and research of the stripping method, which is a popular method for recovering ammonia from SCOW, as the other two methods are often used to treat wastewater with much lower ammonia nitrogen concentration than semi-coking wastewater. Stripping methods include air and steam stripping. The former is to use Henry’s law to take the difference between the actual concentration of ammonia nitrogen in the CCW and the theoretical concentration as the power source and take air as the carrier to make it discontinue contact with the wastewater through the fan, to remove the ammonia nitrogen from the wastewater. Steam stripping uses water vapor as a carrier. Sun et al. [35] used air medium stripping to treat CCW wastewater with 1716.2 mg/L ammonia nitrogen from a carbon production plant. After 150 min under the optimal conditions, the ammonia nitrogen reduced to 8.28 mg/L, achieving a 99.52% removal rate, allowing effluent to meet national discharge standards. Compared to air stripping, steam stripping requires more energy and costs but is more efficient. At present, mainstream stripping technology is divided into single-tower and double-tower types, differing in ammonia nitrogen export. In terms of power consumption, according to a large number of field tests, the cost accounting shows that the single-tower type needs to use about 180 kg/t of steam (a ton of acid-mixed wastewater), while the double-tower type uses over 200 kg/t [36]. Comparatively, the single-tower ammonia recovery method is the mainstream choice because of its simple manufacturing cost and low power consumption in the operation process. In domestic and foreign engineering cases, researchers have devised a thermal integration-based process for phenol and ammonia recovery, aimed at minimizing energy consumption. In this innovative approach, the liberated ammonia gas from the steam stripping column serves as a heat source for conducting heat exchange with the extractive tower to facilitate phenols recovery, resulting in a substantial 34% reduction in operational expenditures [21,37].

Phenol recovery technologies in the CCW treatment include extraction and adsorption dephenolization. Extraction dephenolization involves transferring phenol from wastewater to an extractant, leveraging the solubility differences between phenol in wastewater and the extractant. This method is widely used for SCOW pretreatment [18]. The choice of extractant is critical, affecting both efficiency and cost. Common extractants include methyl isobutyl ketone (MIBK) and diisopropyl ether (DIPE) [38,39]. People have discovered that DIPE is effective for monophenol but less so for polyphenol, with low recovery energy consumption. MIBK is efficient for both, making it more commonly used [40,41]. Qian et al. [42] improved the conventional phenolic amine recovery process (desulfurization–dephenolation–ammonia stripping) by relocating the ammonia stripping unit to the front and integrating it with a stripping deacidification tower. They replaced DIPE with MIBK in dephenolization, increasing the total phenol extraction rate from 76% to 93%. This optimization positions the ammonia stripping unit upfront, allowing extraction under low pH conditions, significantly enhancing efficiency. Scholars suggest this improvement is due to the reduced ionization of polyphenols in low pH environments, allowing more phenolic compounds to be extracted in molecular form [43]. Others believe it is due to the increased distribution coefficient of solvents for phenolic compounds in weak acidic conditions, enhancing the mass transfer rate and distribution coefficient [18]. Further investigation into the specific mechanisms is imperative to optimize phenol recovery efficiency.

Although MIBK is effective, it also has high recovery energy consumption, cost, and water solubility issues. To enhance efficiency and economic and environmental friendliness, new extractants and methods are being developed [17,44,45]. For example, Zhang et al. [46] developed cyclohexanone as a novel extractant, with the extraction efficiencies of 91.65% for hydroquinone and 83.52% for phloraquinol, significantly higher than MIBK and DIPE. However, its cost-effectiveness requires further verification. Chen et al. [47] proposed a five-stage countercurrent extraction process using 2-pentanone (MPK) to remove the total phenol from CCW, reducing concentration from 12,700 mg/L to 313 mg/L with a feed ratio of 1:7.26. Guo et al. [39] investigated the synergistic effects between solvents to find efficient, low-loss mixed extractants. They found that combining alcohol hydroxyl groups with carbonyl, ester, or ether groups had a synergistic effect on phenol extraction. Comparing MPK + n-amyl alcohol, MIBK + n-amyl alcohol, propyl acetate + n-amyl alcohol, and DIPE + n-amyl alcohol, MIBK and n-amyl alcohol were the most effective co-extractants at a 6:4 ratio. Moreover, Zhang et al. [48] used a cationic cetyltrimethylammonium bromide (CTAB) microemulsion to enhance phenol extraction from wastewater. The enhanced extraction mechanism is shown in Figure 3. Figure 3a–c depict the distribution of phenols, CBTA, and other compounds in the organic phase and aqueous phase prior to, during, and after extraction, respectively. The phenol extraction process involves not only the principle of “similar solubility”, in which phenol molecules enter the organic phase but also the electrostatic adsorption of the negatively charged phenolate ion by the positively charged quaternary ammonium group in CTAB. This method, if widely applied, could effectively mitigate the aforementioned influence of phenol ionization on the extraction process. Meanwhile, the efficiency of dephenolization depends on the removal efficiency of both molecular and ionic phenols as evidenced by the above mechanism research. Therefore, future development of high-efficiency dephenolizing extractants should take both aspects into account.

Besides extraction, in some cases, adsorption can also be used to remove phenol. It involves using adsorbents to attract phenol through van der Waals forces, bond-like chemical interaction forces, hydrogen bonding, and other forces, offering high efficiency and no secondary pollution. Common adsorbents include resins [49,50], activated carbon [51], and zeolite [52], and their performance comparison in the recovery of phenols from COW is shown in

Table 1. Comparison of the performance of commonly used adsorbents for phenol recovery.

Adsorbent	Removal Efficiency	Advantage	Limitations	Regeneration Ability	Application
Macroporous Resins [49,50]	High (up to 95%)	High selectivity for phenolic compounds, flexible, long lifespan, narrow pore distribution	Sensitive to fouling (e.g., oils, colored substances), expensive regeneration agents	Good, possible chemical regeneration	Ideal for biochemical pretreatment and post-treatment; suitable for coal chemical wastewater (COW, SCOW)
Activated Carbon [51]	Moderate to High (60–90%)	Large surface area, effective for various pollutants, relatively low cost	Prone to saturation and fouling, difficult regeneration	Moderate, possible thermal regeneration	Widely used in industrial wastewater treatment, general use for phenolic waste removal
Zeolites [52]	Moderate (50–80%)	High thermal and chemical stability, reusable, low-cost	Lower adsorption capacity for organic pollutants compared to resins	Good, reusable after heat treatment	Suitable for initial treatment and multi-stage adsorption in conjunction with other methods

. The most promising adsorbent for high phenol wastewater recovery is a macroporous resin, which does not contain ion exchange groups and has a macroporous structure. This resin performs similarly to activated carbon but has a narrower pore distribution, higher mechanical strength, and easier regeneration. Wei et al. [53] identified NDA-99 resin as the best for treating phenolic substances in COW. Under optimal conditions (pH 4.0, flow rate 40 mL/h), volatile phenol was reduced from 1380 mg/L to 12 mg/L and COD was reduced from 15,500 mg/L to 650 mg/L. Yang et al. [54] have developed a novel aldehyde-based, ester-based hyper-cross-linked polar resin (DES-COOC-CHO) with a specific surface area of 627.2 m²/g and a micropore-specific surface area percentage of 29.94%. When applied to the treatment of actual CCW, the resin exhibited a selective removal rate of 90% for phenol. Lang [55] used a macroporous resin XDA-1G with stable regenerative performance to treat coal chemical industry wastewater, reducing phenolic organic content from 6143.41 mg/L to 1109.51 mg/L, with a removal rate of 81.94%. While adsorption has demonstrated efficacy in phenol removal from certain CCW treatments, challenges remain in its application to SCOW pretreatment: (1) some pollutants (emulsified oil and colored substances) have irreversible effects on the adsorbent material; (2) effective desorption often requires new chemical agents, which are costly and may cause secondary pollution; (3) lack of selectivity and limited pollution resistance make it unsuitable for raw wastewater treatment but better suited for biochemical pretreatment or post-biochemical treatment to ensure stable biochemical section operation.

In general, current phenol recovery through the extraction method faces challenges like high reagent consumption, high water solubility, and recovery difficulties. Therefore, exploring new extractants or co-extractants offers significant development potential [17,19,44]. With advancements in high molecular synthesis technology, specialized macroporous adsorption resin technology is being introduced for CCW treatment. While it has matured in laboratory preparation, its engineering application lacks extensive experience, making the selection of specialized resins for high phenol and ammonia wastewater treatment highly promising.

3. Biochemical Treatment Process

At this stage, the quality characteristics of SCOW are similar to those of COW. Although SCOW contains high levels of recalcitrant organic matter and has a low B/C ratio, it offers the potential for improved treatment efficiency through optimized biochemical processes.

3.1. Physicochemical Pretreatment to Enhance Biodegradability

After pretreatment, COD can be reduced to below 4000 mg/L, ammonia nitrogen to less than 300 mg/L, and phenol to under 500 mg/L [8]. At this stage, the main COD components include long-chain alkanes, polycyclic aromatic hydrocarbons, heterocyclic compounds, phenol, ammonia, and acidic gases, the majority of which exhibit toxicity towards activated sludge. Hence, physicochemical pretreatment is essential before biochemical treatment, aimed at improving wastewater characteristics, adjusting nutrient structure, reducing the inhibitory effects of toxic substances on microorganisms in subsequent biochemical processes, and providing sufficient energy materials.

Based on the existing literature, technical specifications, and engineering practices, the advanced oxidation (AO) method is the most effective method for biodegrability improvement of SCOW [56,57]. During the oxidation–reduction process, many difficult-to-degrade high-molecular-weight compounds are broken down into small, easily degradable molecules. Toxic substances are oxidized into low-toxicity compounds or completely oxidized, thus reducing toxicity while providing necessary nutrients for microorganisms. AO methods, such as Fenton oxidation [58], ozone catalytic oxidation [59], electrocatalysis oxidation [60], electrocoagulation [60], and chemical oxidation [61,62], offer fast oxidation speeds, high treatment efficiency, and convenient operation for the SCOW biodegradability pretreatment. However, challenges remain, particularly regarding the balance between oxidation–reduction potential and pollutant concentration. For instance, the Fenton method may produce a large amount of iron sludge, and it may also cause an increase in microbial toxicity in subsequent biological treatment units due to excessive iron input. Ozone oxidation processes may also produce hazardous and persistent intermediate by-products as a result of incomplete oxidation, subsequently impacting downstream treatment units [63]. Additionally, cost and energy consumption are critical factors. Ozone, combined with various oxidation technologies, has been proven effective in treating phenols and cyanides, as well as in decolorization and bleaching, but it is costly and requires complex maintenance. Electrochemical technology is regarded as highly efficient and environmentally friendly. Challenges such as high energy consumption and low mass transfer efficiency hinder its widespread adoption in large-scale industrial applications. Therefore, selecting AO methods for biodegrability improvement of SCOW should consider the accumulation of harmless and non-toxic products, high efficiency, low consumption, and non-stringent reaction conditions.

3.2. Conventional Methods

In practical applications, the biochemical treatment of SCOW follows the same protocols as COW treatment [64]. However, traditional biochemical treatment methods, such as the “hydrolysis acidification (HA)-Anoxic/Oxic (A/O) tank-two sedimentation tanks” process, have evolved into more efficient combinations. For example, a coal chemical factory in Henan, China uses a “grease separation-homogenization-gas flotation-anaerobic oxidation-sequencing batch reactor (SBR) (or multi-stage A/O)-electrolytic modification-contact oxidation” process, achieving effluent with COD ≤ 80 mg/L and BOD₅ ≤ 5 mg/L [65]. The Institute of Process Engineering of the Chinese Academy of Sciences adopted a new technology combining distillation and biological coupling for enhanced nitrogen and carbon removal [66]. This method achieved deep removal of ammonia, total nitrogen, and COD, reducing treatment costs by 20%, increasing impact resistance by 30%, and significantly improving water quality. Cheng et al. [67] introduced advanced oxidation into the biochemical treatment process, using “hydrolysis-acidification-two-stage A/O-ozone catalytic oxidation-membrane bioreactor (MBR)”, which improved the COD removal rate to 97.1%. Zhou et al. [68] proposed a step-feed three-stage integrated A/O bio-filter system (SFTIAOB) that effectively treated COW. The SFTIAOB reactor demonstrated strong long-term removal performance for aromatic organic compounds (AOCs). The above optimization processes primarily consist of combined conventional methodologies, as illustrated by the comparison of individual techniques in

Table 2. Comparison of several conventional biochemical treatment processes.

Process	Principle	Advantage	Challenges for SCOW
Hydrolysis Acidification (HA) [69]	Hydrolysis is the process where microorganisms complete biocatalytic oxidation reactions by releasing extracellular free enzymes or fixing enzymes. Acidification is a typical fermentation process where microorganisms produce mainly organic acids.	Converts difficult-to-biodegrade substances into easily biodegradable substances; improves wastewater biodegradability; widely used as a pretreatment for low-concentration, refractory wastewater; superior impact load resistance.	Inadequate removal of toxic pollutants in short-term processes; requires refinement of parameters or implementation of biological enhancement.
Anoxic/Oxic (A/O) [70]	In the anaerobic phase, heterotrophic bacteria degrade complex organic compounds into simpler ones, facilitating ammonification to break down proteins into free ammonia. During the aerobic phase, autotrophic bacteria metabolize and convert ammonia into nitrates and remove organic pollutants. Under anoxic conditions, denitrification occurs, reducing nitrates to nitrogen gas.	Widely used; simple process and low investment cost.	Poor removal of nitrogen, phosphorus, and refractory organic matter; not suitable for direct treatment of SCOW.
Anaerobic/Anoxic/Oxic process (A/A/O) [70]	Consistent with the A/O process.	Widely used; better nitrogen and phosphorus removal efficiency than A/O; robust against abrupt load changes.	Unsatisfactory removal efficiency for refractory organic compounds; inhibitory effect of toxic substances on nitrification and denitrification bacteria.
Sequencing Batch Reactor (SBR) [70]	A synchronized technology for nitrogen and carbon removal in activated sludge wastewater treatment, operating in an intermittent aeration mode.	Reduced number of processing equipment; simplified structure; easy operation and maintenance; process adjustable to water quality and quantity; impact load resistance.	Without a primary sedimentation tank, it is prone to floating sludge; and requires combination with other processes like co-digestion acidification to treat SCOW.
Up-flow Anaerobic Sludge Blanket (UASB) [71]	Forms good settling sludge floc and combines it with a sludge sedimentation system in the reactor to separate gas, liquid, and solid phases.	Widely used for CCW; high organic loading; long hydraulic retention time (HRT); and no need for stirring equipment or sedimentation tanks.	It is sensitive to sudden changes in water quality and load, and its shock resistance is slightly poor. It is necessary to enhance the tolerance of microorganisms to toxic pollutants.
Membrane Bioreactor (MBR) [72]	Combines high-efficiency membrane separation technology with the traditional activated sludge process; retains active sludge and large molecular organic substances in the biological reaction pool, eliminating the need for a secondary sedimentation tank.	Excellent organic matter removal efficiency, especially for phenolic substances and difficult-to-degrade organic matter; superior effluent quality compared to traditional sedimentation tanks.	Limited anti-pollution ability; prone to membrane pollution; suitable for deep treatment of effluent from biochemical treatment.
Biological Aerated Filter (BAF) [73]	Oxidation and decomposition of pollutants by microorganisms on filter media; adsorption and retention by filter media and microbial film; internal denitrification by microbial film microenvironment.	Simple process; good effluent quality; strong resistance to load increases; high oxygen transmission efficiency; reasonable bacterial community structure; effective denitrification.	Suitable for advanced treatment of effluent from biochemical treatment.
Microbial Co-Metabolism [74,75]	The addition of co-substrates provides a rich source of carbon and energy, enhancing microorganism activity and promoting the synthesis of corresponding enzymes, thereby facilitating the biodegradation of difficult-to-degrade substrates.	Significantly enhances microorganism activity; improves the efficiency of degrading difficult-to-degrade organic matter.	Tremendous potential for development.

. After comparison and analysis, it can be concluded that, for SCOW, the key to the effective application of any method is the microorganism’s tolerance to toxic pollutants in the wastewater and its resistance to containment load. Meanwhile, the antagonistic or competitive effect on the ring-opening, chain scission, and mineralization of cyclic compounds in SCOW, makes it difficult for a single process to achieve the complete removal of various pollutants. For example, refractory cyclic compounds have a significant inhibitory effect on methanogens and nitrifying bacteria. If the phenol concentration was higher than 280 mg/L, the inhibition of anaerobic microorganisms was irreversible [34]. Accordingly, there is an urgent need for exploring and developing new strategy for enhancing SCOW biochemical treatment.

3.3. Bio-Enhancement

At present, bio-enhancement technology is crucial and considered as an effective strategy for enhancing the biochemical treatment of SCOW. A common strategy involves introducing exogenous substances like co-metabolic substances, adsorbents, and conductive mediators into the biochemical treatment system. These facilitate the synergistic degradation of refractory organic compounds in SCOW by improving biomass, enriching microbial diversity, and modifying metabolic or electron transfer pathways [76].

Conventional co-metabolic substances like methanol, glucose, and phenol are widely used in the anaerobic co-metabolism of CCW. Studies [77,78] indicate that using glucose solution as a co-metabolic substrate offers significant advantages for rapid initiation and efficient phenol removal in anaerobic biofilm reactors. Xu et al. [79] found that methanol enhanced the settling performance and biodegradability of activated sludge when used as a co-metabolic substrate. Wang et al. [80] observed that adding phenol as a growth substrate to coal gasification wastewater (CGW) increased the removal rates of quinoline and pyrrole from 18.5% and 14.9% to 73.8% and 65.9%, respectively. Recently, novel co-substrates and mixed co-metabolic substrates have also been introduced into CCW bioprocesses, enhancing co-metabolism efficiency and facilitating some waste resource utilization. Li et al. [81] added potato starch wastewater as a biodegradable substrate to CGW influent, achieving 75% removal rates of COD and total phenol (TPh) and increasing methane production to 260 mL CH₄/g COD/day. Rea et al. [82] studied the impact of an acetic acid ester–butyric acid mixture as an additional carbon resource and energy source on the phenol conversion rate in an anaerobic membrane bioreactor treating CGW. The results showed that the ester–butyric acid mixture significantly improved phenol conversion compared to using only acetic acid ester as the sole co-metabolic substrate. Current research on co-metabolic substances focuses on discovering novel, efficient, and cost-effective co-metabolic substrates; optimizing co-metabolic conditions; exploring regulatory factors for enzyme activity to achieve efficient expression of key enzymes; extending substrate specificity of microbial enzymes through genetic engineering; and studying synergistic metabolic mechanisms among microorganisms during co-metabolism.

The effectiveness of bio-enhancement strategies in the SCOW treatment also hinges on selecting suitable carrier materials. Commonly employed adsorbents such as activated carbon [83], biochar [84], and conductive mediums like zero-valent iron (ZVI) [85], iron–carbon composites [86], and graphene [87] are pivotal choices. These materials not only facilitate microbial attachment and growth but also optimize microbial community structures. They promote synergistic degradation by diverse microbial consortia during biochemical processes, enhance microbial electrochemical activity, accelerate extracellular electron transfer mechanisms, and thereby augment the degradation of persistent organic pollutants. Among them, activated carbon has been extensively utilized to enhance the biotreatment of CCW. A recent study highlights its efficacy in anaerobic charcoal-based fluidized bed reactors, achieving significant removal rates for phenol (>96%), o-cresol (>91%), and 3,5-xylenol (>84%) [88]. And a smaller particle size of activated carbon was observed to enhance adsorption and low molecular weight pollutants removal as well as biological enhancement effects, while it is less efficient in adsorption of large-size pollutants. Comparatively, biochar- and lignite-based activated coke can offer higher specific surface areas and diverse chemical functional groups, enhancing adsorption capabilities for phenolic compounds and NHCs in CCW [89]. Moreover, polyurethane (PU), another innovative adsorbent carrier, not only adsorbs pollutants but also promotes microbial attachment and enzyme secretion through specific cationic and hydroxyl groups. Anaerobic reactors utilizing PU carriers have demonstrated enhanced degradation of phenolic compounds and nitrogen-containing heterocyclic compounds (NHCs) in coal slurry [90]. The practical effects of various materials clearly demonstrate the pivotal role played by the type and performance of the adsorption material in enhancing SCOW biological treatment. Simultaneously, exploring the interaction mechanism between adsorption and biodegradation, as well as understanding the intricate promotion and inhibition effects on pollutant degradation during the biological enhancement process, is essential. Moreover, special attention should be paid to some adsorption material that may cause environmental health risks.

Recently, environmentally friendly conductive mediators have gained attention for assisting biochemical processes. Magnetic activated carbon (MAC), for instance, has been shown to enhance the efficiency of moving bed biofilm reactor (MBBR) systems in treating chemical processing wastewater (CPW), achieving COD removal rates exceeding 92% and phenol removal rates exceeding 93% at concentrations up to 500 mg/L [91]. ZVI and graphene have been employed to enhance the degradation of organic compounds in CCW, facilitating improved degradation efficiency and methane production [85]. These materials create micro-galvanic cells and electron-conductive pathways in wastewater, promoting Fe(III) release as an electron acceptor for cyclic compound degradation. Graphene, specifically, enhances microbial aggregation and extracellular polymeric substance (EPS) production, benefiting sludge granulation processes [92]. Furthermore, nitrogen-doped sludge-based activated carbon (N-SBAC), derived from waste sludge, has significantly enhanced the anaerobic degradation of CGW by improving conductivity, enzyme activity, and EPS content in anaerobic sludge, promoting granulation and providing a conducive environment for microorganisms [93]. While an exogenous conductive mediator offers advantages in transfer rates, thermodynamics, and energy conservation, challenges remain such as nano-scale mediator loss, stability of refractory organic compounds, mechanisms for enhancing pollutant removal and methane production, and coordination between media in biochemical processes.

In general, the objective of bio-enhancement is to enhance the tolerance of microorganisms to toxic organic compounds and containment loads in semi-coke wastewater, as well as to explore metabolizable pathways for refractory organic compounds and facilitate synergistic degradation processes (as shown in Figure 4a). For instance, the presence of co-metabolic substances can directly supply microorganisms with carbon sources or energy, thereby expediting the production of EPS and utilizing these EPSs to convert refractory compounds into more readily degradable intermediates (Figure 4b). Subsequently, bacteria further metabolize these intermediates, transforming them into smaller molecules, thus achieving the degradation and removal of refractory organic matter. Furthermore, the import of adsorbents or conductive mediators not only enhances the EPS secretion effect as mentioned above but also offers additional benefits such as adsorbing pollutants, pre-oxidizing and decomposing organic matter, accelerating extracellular electron transfer, and promoting the growth of microorganisms with specific functions, thereby improving the removal of recalcitrant organic compounds (Figure 4c). These mechanisms offer essential guidance for selecting biological enhancement strategies, which are progressing towards low-energy-consumption, high-efficiency, environmentally friendly, and sustainable approaches. This represents the inevitable direction for meeting the NZD standards of SCOW. Subsequently, we will specifically explore potential development strategies for bio-enhancement under these requirements in the following discussion.

3.4. Development Tactics towards NZD

The evolution towards an ecological civilization and carbon neutrality poses new challenges and opportunities for bio-treatment modes in SCOW management, emphasizing cost reduction and efficiency enhancement. Future advancements must integrate engineering practicality, economic viability, and environmental sustainability. The trend towards combined optimization technologies signifies the future of biological enhancement strategies for SCOW treatment (as shown in Figure 4d). This approach demands deeper exploration of biological and genetic enhancement mechanisms, elucidation of synergistic effects from combined strategies, and iterative refinement of combination technologies to enhance operational control and achieve simultaneous removal of refractory organic matter and ammonia nitrogen. Such advancements aim to provide theoretical and technical support for new engineering applications in the field.

Biochemical treatment stages in SCOW management are crucial but energy intensive. Exogenous enhancement strategies often involve adding chemicals and materials to biochemical systems, thereby increasing energy consumption, carbon emissions, and operational costs. Moreover, substantial electricity is needed to sustain equipment for microbial metabolism, further inflating costs. To enhance economic efficiency, two key strategies are essential. Firstly, optimizing external material supply to conserve energy can be achieved through innovative approaches like “waste-to-treat-waste”, utilizing coal-based or carbon-modified materials as biological reinforcement agents [64]. This approach not only reduces solid waste generation but also yields additional economic benefits. Secondly, deploying fine control technologies can optimize biochemical treatment processes. Real-time monitoring through IoT and AI-driven (artificial intelligence-driven) analysis enables timely identification and resolution of issues, while big data facilitates rapid validation of processing schemes. AI-driven real-time adjustments of processing parameters ensure precise control over wastewater treatment processes, minimizing unnecessary energy consumption. In the context of China’s commitment to a peaking of carbon emissions and achieving carbon neutrality, adopting environmentally friendly processes and carbon reduction technologies is imperative for SCOW treatment.

Current bio-enhancement technologies predominantly rely on exogenous substance addition, which, if improperly selected or used, may pose environmental risks—such as iron excess leading to microbial cell membrane oxidation and acute toxicity [96]. Therefore, future research should prioritize the development of environmentally benign exogenous materials. Additionally, repurposing waste containing carbon sources (e.g., coal-based waste, active sludge, and discarded biomass) for bioaugmentation not only enhances sludge resource utilization but also mitigates carbon emissions. Concurrently, targeted enhancement strategies must address potential toxic pollutants in SCOW, ensuring the stability and safety of biochemical treatment systems and water environments.

4. Advanced Treatment Processes

Advanced treatment is essential to ensure the quality of treated effluent or recycled water. Various common physical and biochemical methods, such as flocculation sedimentation [97], adsorption [98,99], MBR, biological aerated filter (BAF), etc., have been previously introduced [100]. Subsequently, several advanced oxidation processes (AOPs) and combined processes with significant potential for application in the advanced treatment of SCOW will be discussed.

4.1. Advanced Oxidation Processes

AOPs have received various applications in wastewater treatment, and a few have been gradually used for treating the actual CCW, including SCOW. Herein, the application of common AOPs (Figure 5) in the actual treatment of CCW was summarized and compared [101,102].

As shown in Figure 5, the Fenton process primarily utilizes hydroxyl radicals generated by the reaction of hydrogen peroxide (H₂O₂) with ferrous iron under acidic conditions. Its efficacy is heavily influenced by the solution pH, with the optimal range typically between 2 and 4, where hydroxyl radicals predominate. At neutral pH levels, ferrous iron tends to precipitate as iron hydroxide colloids, capable of adsorbing pollutants. Moreover, the concentration of H₂O₂ significantly impacts the Fenton process performance; inadequate H₂O₂ limits hydroxyl radical production, while excess H₂O₂ can quench radicals, reducing COD removal efficiency. For instance, Peng et al. [103] demonstrated that with the initial COD levels of 800–900 mg/L and FeSO₄ at 1000 mg/L, the COD removal rate reached a peak of 61.96% with 6000 mg/L of H₂O₂. Additionally, ZVI can initiate the Fenton process, as shown by Chu et al. [104], achieving 65% COD removal with 3 g/L iron powder and 300 mM H₂O₂. Despite its effectiveness, the Fenton process generates iron-containing sludge primarily due to the slow conversion of Fe(III) to Fe(II). Modified Fenton processes like photo-Fenton and electro-Fenton have been developed to mitigate sludge formation. Photo-Fenton, however, has limited practical use due to poor UV and visible light penetration. Electro-Fenton, on the other hand, has gained prominence due to its implementational ease with electric current. For the CCW with an initial COD of around 2970 mg/L, the electro-Fenton method using Fe/AC/Ni cathodes at 10 V achieved COD removal rates of 88.91–96.65% [105]. Transition metal oxides, such as Fe₃O₄, serve as efficient Fenton-like catalysts, generating hydroxyl radicals without causing secondary pollution [106]. Moreover, Wang et al. [107] developed a novel three-chamber microbial electrolysis unit coupled with a Fenton reaction unit, which generated Fenton reagents through both biotic and abiotic cathodes. The bioelectrode system successfully produced 13 ± 3 mg/L of dissolved Fe(II) and 5 ± 0.4 mg/L of H₂O₂ for the Fenton reaction unit, achieving remarkable removal efficiencies of 99% for dissolved organic carbon and between 78% and 100% for specific recalcitrant organics.

In general, the Fenton process demonstrates high efficiency in treating SCOW. However, practical application necessitates addressing two key issues: expanding the pH range of the solution and reducing iron sludge generation. Numerous studies [108] have indicated that the introduction of reductive agents such as MoS₂ materials and boron can effectively facilitate the conversion from Fe(III) to Fe(II), thereby minimizing the production of iron-containing sludge. Nevertheless, caution must be exercised when incorporating chemicals into SCOW due to potential unknown impacts on the characteristics of the subsequent effluents or environment.

As shown in Figure 5, ozone oxidation relies on the oxidizing capacity of ozone and hydroxyl radicals derived from it to degrade organic pollutants. Catalysts can enhance ozone utilization efficiency and hydroxyl radical formation. Optimal ozone oxidation conditions for CCW (COD 183–235 mg/L) achieved significant COD reduction to about 100 mg/L with 60 mg/L ozone over 50 min [109]. An ozone catalytic oxidation technology and equipment developed by the Institute of Process Engineering of the Chinese Academy of Sciences was used for treating SCOW and comprehensive biochemical effluent (with COD approximately 350 mg/L and ammonia nitrogen 15 mg/L). After this treatment, the COD can be reduced to 50 mg/L, ammonia nitrogen is below 5 mg/L, and the removal efficiency of refractory organic matter reaches approximately 80% [66,110,111]. Combining ozone with ultraviolet light and H₂O₂ further enhanced pollutant removal, achieving over 90% fluorescence intensity reduction with PAH concentrations below 10 μg/L and minimal toxic benzo(a)pyrene (BaP) levels [112]. Catalysts like zinc ferrite have been shown to enhance ozone oxidation efficiency, particularly for phenol removal and increased mineralization compared to non-catalytic ozone processes [113]. Developing cost-effective and efficient ozone catalysts remains crucial for practical applications, along with the challenge of catalyst regeneration.

Persulfate-based AOPs, utilizing peroxy disulfate (PDS) and peroxymonosulfate (PMS), are easier to store and transport than H₂O₂. Activated persulfate generates sulfate radicals, superoxide radicals, singlet oxygen, and surface-bound complexes, offering high selectivity and redox potential for pollutant degradation. Common activators include thermal energy, UV radiation, and catalysts. Figure 6 presents a comparison of various catalysts used for persulfate activation, including both metal-based and non-metal-based materials. Metal-based catalysts, in particular, have been widely acknowledged and studied for their effectiveness in persulfate activation [114]. Zhang et al. [115] utilized sulfate radical oxidation combined with iron flocculation for upgrading biological effluent of COW, achieving 58.5% TOC removal with initial TOC at 153.8 mg/L and 2.9 g/L peroxydisulfate. Similarly, Ma et al. [116] synthesized Fe₃O₄–CuO@lignite activated coke catalysts, achieving approximately 60% COD removal in 180 min with 139 mg/L COD, 3 mM peroxydisulfate, and 0.1 g/L catalyst. Dong et al. [117] utilized persulfate and cyanide tailings (PS-CTs) for synergistic treatment of SCOW, which demonstrated a COD removal efficiency of up to 94.49% while simultaneously achieving oxidative de-cyanidation of cyanide tailings. The treated cyanide tailings had a total cyanide concentration of 8.21 mg/kg, with a leaching cyanide concentration of 0.12 mg/L, meeting emission standards. This process implemented the concept of “treating waste with waste”. Despite the unanimous confirmation of persulfate-based AOPs effectiveness by most researchers involved, persulfate-based AOPs alter solution pH, potentially increasing facility corrosion and requiring pH adjustment with NaOH. They also elevate sulfate ion concentrations in the solution. All these issues should be carefully considered in engineering applications.

In Figure 5, the electrochemical processes encompass electrochemical oxidation, electrochemical reduction, electrolysis, electrocoagulation, etc., which can remove refractory organic pollutants through the direct or indirect redox reaction on the electrode surface driven by an electric field [118,119,120]. Electrochemical technology is an environment-friendly technology due to its advantages such as no secondary pollution, simple equipment, easy regulation, etc. It has been used for the removal of antibiotics and herbicides, and for treating actual wastewater, such as landfill leachate and CCW [102]. Electrochemical technology’s efficacy hinges on electrode material reactivity, such as graphite, titanium, boron-doped diamond, titanium-doped IrO₂, and titanium-doped IrO₂–RuO₂. For CCW with 250 mg/L COD, Ti/Ti₄O₇ anodes achieved 78.7% COD removal, superior to Ti/RuO₂–IrO₂ anodes [121,122]. Electrochemical oxidation treatment for the CCW using vanadium-titanium magnetite particles and persulfate at 15 mA/cm² attained 94.15% COD removal and 57.67% dissolved organic matter removal, evidenced via FTIR analysis indicating phenol, ester, carboxylic acid, and derivative removal with formation of hydrocarbons, CO₂, and phenolic compounds [123]. Current studies [124] have demonstrated that anode material remains a critical factor in electrochemical process performance, underscoring the need for cost-effective mass production of advanced materials. Despite numerous advanced anode materials reported in the literature, achieving cost-effective mass production remains a significant challenge, necessitating further investigations to elucidate the relationship between the reactive activity of anode materials and the characteristics of SCOW.

Electron beam technology employs high-energy electron beams to degrade pollutants directly or indirectly by exciting water molecules to produce reactive species. Effective in treating COW, electron beams remove pollutants like phenol, nitrophenol, methylindole, indole, and thiophenol [125,126]. Initially used for wastewater disinfection, electron beam technology expanded to environmental applications such as flue gas and solid waste treatment (e.g., municipal sludge). Recent studies exploring electron beam technology for COW advanced treatment demonstrated COD reduction from 150 to 300 mg/L to 80 to 120 mg/L, showcasing its potential [125,127].

4.2. Combined Processes

Combined processes have been employed as advanced treatment strategies to address the complexity of biochemically treated SCOW and meet the demands for wastewater reuse.

The most fundamental combination processes involve integrating physiochemical and biochemical methods, as well as diverse physiochemical processes. For instance, the combination of MBR and membrane filtration processes, including ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), effectively treated COW. Experimental results showed that this approach achieved over 96.0% removal efficiency for COD and a water recovery rate of 70.7% [128]. Pan et al. [117] utilized the combined methods of intensified ZVI with an anaerobic filter (AF) and BAF for advanced treatment of biologically pretreated COW, achieving 76.28% and 96.76% removal efficiencies of COD and NH₄⁺-N, respectively. Li et al. [129] have developed a novel coagulation–adsorption integrated advanced treatment process for the biochemical pretreated CCW, which could effectively achieve removal rates more than 85.3% for COD and 99.40% for cyanide in all laboratory, pilot, and industrial-scale experiments. Xia et al. [130] used the combined method of polyaluminum chloride induced coagulation with sludge-based activated carbon (SAC) adsorption for advanced treatment of biotreated COW, effectively achieving COD removal from approximately 250 mg/L to below 80 mg/L. Moreover, SAC showed better performance in the hydrophobic components and fluorescent components than commercial powder-activated carbon. While these combination processes have been extensively tested and utilized for the advanced treatment of other COW or CCW, there is limited documentation regarding their application in SCOW.

To enhance the efficiency of pollutant removal and control, there is a growing interest in integrating effective AOPs, especially for the advanced treatment of SCOW. For example, combining the Fenton method with biological activated carbon adsorption for COW treatment achieved effluent COD and cyanide concentrations below 50 mg/L and 0.5 mg/L, respectively, over 70 days of continuous operation, starting with initial COD concentrations of 100–200 mg/L [131]. The combined Fenton and ozone process demonstrated superior performance compared to either single process, achieving a 40% removal efficiency of COD for the artificial COW with an initial concentration of 588 mg/L [132,133]. Qin et al. [134] utilized bituminous coal adsorption and advanced oxidation technologies to effectively treat CCW, achieving significant reductions in TOC, color, and UV₂₅₄ levels through lignite adsorption and subsequent electrochemical/UV/H₂O₂ treatment. Xie et al. [135] employed an electrocatalytic oxidation-activated carbon adsorption process to treat biochemical tailwater from COW, achieving a 99.7% COD removal rate and a 47.87% TOC removal rate under specific operational conditions. Hang et al. [136] developed Ti-Sn-Ce/bamboo biochar particle electrodes and established a three-dimensional electrochemical reaction system for treating COW with COD levels ranging from 420 to 480 mg/L. The system effectively removed dissolved organic matter (DOM), resulting in a remarkable 93% COD removal efficiency. For SCOW, the research on AOPs combined processes primarily focuses on the electrochemical coupling of flocculation [137], Fenton [138], ozone [139], and photocatalysis [140] methods. These electrochemical combination processes share a common feature of significantly enhanced radical activity compared to individual methods, enabling rapid removal of COD at rates exceeding 85%. However, due to considerations regarding cost and energy consumption, these combined processes have not yet been implemented for the advanced treatment of SCOW. Currently, the combination of coagulation and Fenton or ozone catalytic oxidation is widely used in the practical application of SCOW advanced treatment. The former has the problem of iron sludge discharge and the risk of secondary pollution. According to the research of Fu et al. [141], following colloidal removal through coagulation pretreatment, the effectiveness of ozone catalytic oxidation can be significantly enhanced. Nevertheless, it still encounters challenges in eliminating certain dissolved organic pollutants (e.g., polysaccharides).

It is worth emphasizing that electrochemical technology, a widely recognized environmentally friendly technology, has broad application prospects in the advanced treatment of SCOW. As for the problem of higher energy consumption, relevant optimization research has been conducted by scholars. Wang et al. [124] utilized plasma sintering (SPS) technology to prepare Ti₄O₇ reaction electrochemical membranes (REM) and coupled them with ozone for COW degradation. Under optimal conditions, the TOC removal rate reached 57.1%, with an energy consumption of 165.2 kWh/(kg·TOC), significantly reducing overall energy consumption. Electrochemical membranes have emerged as a novel material in advanced oxidation technology, garnering considerable attention in recent years [142,143,144]. By integrating electrochemistry with membrane separation technology, these membranes not only exhibit excellent separation capabilities but also effectively degrade recalcitrant pollutants through electrochemical oxidation–reduction reactions. Moreover, the application of an electric field helps mitigate membrane fouling (depicted in Figure 7), offering multiple advantages and demonstrating significant potential for diverse applications [145]. While research on the removal efficiency of recalcitrant organic compounds has yielded promising results across various sectors such as industrial oily wastewater [145], pharmaceutical wastewater [146] (e.g., sulfamethoxazole [147] and tetracycline [148]), and dyeing wastewater [149], studies specific to coal chemical industry wastewater remain limited. Furthermore, as China’s photovoltaic industry expands, addressing the high-cost issue in electrochemical industrial wastewater treatment may become feasible, given its high efficiency and environmental friendliness [150,151].

In general, advances in the treatment of biochemical tailwater from SCOW should consider developing highly efficient and cost-effective AOP-based combined processes. AOPs can degrade recalcitrant pollutants but typically require high oxidant concentrations, leading to elevated costs. Therefore, employing AOP-based combined processes is recommended for treating coking wastewater, aiming to maximize process advantages at manageable economic costs. Additionally, investigating the synergistic effects of advanced oxidation processes and membrane processes remains a crucial avenue for future research.

5. Resource Utilization

Efficient resource utilization and recycling technologies are crucial to comply with environmental mandates, counter water scarcity in the northwest, and address urgent water pollution concerns. These efforts are pivotal in achieving NZD and promoting sustainable growth in the semi-coking sector. SCOW contains recoverable resources such as oil, phenol, ammonia, recycled water, and salt, each essential for enhancing internal resource reuse and operational sustainability (Figure 8).

5.1. Tar

Medium- and low-temperature coal tar in SCOW, characterized by its high boiling point, primarily constitutes the recoverable oil fraction. Typically, tar recovery involves chemical demulsification and mechanical gravity methods. Zhao et al. [152] developed a coalescence-coupled hydrocyclone assembly that effectively treats oilfield-produced water while maintaining stable oil removal rates exceeding 95%. While effective for removing emulsified oils (~60%), the coalescence-coupled hydrocyclone assembly faces challenges with very fine emulsions (80–400 nm), often causing operational issues like channel blockages in SCOW systems. Membrane separation has emerged as another viable approach for oily wastewater treatment. For example, He et al. [153] fabricated reduced graphene oxide aerogel membranes achieving 100% retention of 0.37 to 0.92 μm oil particles, while Ikhsan et al. [154] developed halloysite nanotube-hydrous ferric oxide nanoparticle (HNT-HFO) composite membranes with 99.7% retention efficiency, yet facing stability and fouling issues in complex pretreatment setups such as SCOW. Solvent extraction is gaining traction in CCW treatment. Cao et al. [155] used IPE-PO extractants for phenol and sediment extraction from pretreated raw CGW, significantly reducing COD from 31,300–37,170 mg/L to 11,890–15,500 mg/L. Gai et al., [23] focused on the SCOW from a Yulin semicoke production plant and developed a separation method using tar fraction (F120) as an extractant, achieving an oil concentration of less than 300 mg/L with over 95% removal efficiency. With this new process, coal tar production can reach 1 kg/(a ton of wastewater), and enterprises can increase economic benefits by more than CNY 1. Despite its efficiency and cost-effectiveness, the stability and practical engineering feasibility require further validation. Accordingly, it can be inferred that the quest for a highly effective and economically viable oil removal technology for SCOW is still ongoing.

5.2. Phenol and Ammonia

Phenol and ammonia recovery is considered crucial in the treatment of SCOW. Widely utilized processes include acidification–extraction–dephenolation–desulfurization–ammonia removal (Lurgi Phenosolvan-CLL process), desulfurization–extraction–dephenolation–ammonia removal (Sedin process), desulfurization–ammonia removal (single- or double-tower)–extraction dephenolation process, as illustrated in Table 3. Among these, the Lurgi Phenosolvan-CLL process stands out as widely adopted in more than 30 facilities worldwide. This process uses sulfuric acid for acidification, recovering phenols in the Phenosolvan unit and liquid ammonia through the CLL ammonia recovery unit, employing DIPE as the extractant. The variation in coal quality and production processes in China leads to a diversity of water quality in coal chemical plants, resulting in suboptimal application effects of this process in the country. The Sedin process demonstrates acceptable deacidification and ammonia removal effects. However, the ammonia content in water remains relatively high due to separate deacidification processes, leading to suboptimal removal efficiency of phenolic substances in subsequent extraction systems operating at pH levels between 9 and 10.5.

Table 3. The recovery efficiency of each combined phenol–ammonia recovery process.

Process	Main Feature	Recovery Effect	Challenges	Application Examples
Lurgi Phenosolvan-CLL process	Extraction sequence: acidification–phenol extraction–acid gas stripping–ammonia recovery; DIPE as extractant; five-stage mixed-clarification tank for continuous countercurrent extraction.	Effluent: monophenol content < 20 μg/g; polyphenol extraction rate 85%; total phenol removal > 99%; free ammonia < 50 μg/g; COD < 3000 mg/L.	Effective energy conservation; verification needed for dephenolization under high acid gas concentrations; higher costs; no successful industrial examples in China.	Sasol Project, South Africa [156]; Great Plains Coal Gasification Project, USA [157].
Sedin process	Phenol recovery precedes ammonia recovery; extraction sequence: acid gas removal, extraction, ammonia removal under alkaline conditions; DIPE as extractant.	Effluent: v total phenol < 900 mg/L; total ammonia < 300 mg/L; COD < 4000 mg/L. Recovery rate: phenol about 84%; ammonia about 97%.	Extraction before ammonia distillation leads to alkaline extraction water impacting phenol removal; inefficient acid removal tower; and challenges with acidic gases and ammonia in CGW causing equipment fouling.	Yima, Henan Province, China [158].
Desulfurization–ammonia removal (double-tower)–extraction dephenolation process	Includes acid gas stripping tower, extraction tower, amine stripping tower, amine distillation tower, solvent distillation tower; DIPE as extractant.	Effluent: total phenol < 900 mg/L; total ammonia < 300 mg/L; COD < 4000 mg/L. Recovery rate: phenol ~84%; ammonia ~98%.	Acidic gas elimination leads to high ammonia concentrations and pH issues; and inadequate removal of phenolic compounds.
Desulfurization–ammonia removal (single-tower)–extraction deadenylation process	Ammonia recovery before phenol; acidic extraction; DIPE or MIBK as extractant.	Effluent (DIPE): COD < 4000 mg/L; total phenol < 600 mg/L; ammonia nitrogen < 300 mg/L. Effluent (MIBK): COD < 2000 mg/L; total phenol < 300 mg/L; ammonia nitrogen < 300 mg/L. Recovery rate: phenol 86–95%; ammonia ~99.8%.	Widely applied in the coal chemical industry and wastewater treatment in China; variations in extraction solvent affect treatment efficiency.	Harbin, Heilongjiang Province, China [159]; Ordos, China [159].

Table 3. The recovery efficiency of each combined phenol–ammonia recovery process.

Process	Main Feature	Recovery Effect	Challenges	Application Examples
Lurgi Phenosolvan-CLL process	Extraction sequence: acidification–phenol extraction–acid gas stripping–ammonia recovery; DIPE as extractant; five-stage mixed-clarification tank for continuous countercurrent extraction.	Effluent: monophenol content < 20 μg/g; polyphenol extraction rate 85%; total phenol removal > 99%; free ammonia < 50 μg/g; COD < 3000 mg/L.	Effective energy conservation; verification needed for dephenolization under high acid gas concentrations; higher costs; no successful industrial examples in China.	Sasol Project, South Africa [156]; Great Plains Coal Gasification Project, USA [157].
Sedin process	Phenol recovery precedes ammonia recovery; extraction sequence: acid gas removal, extraction, ammonia removal under alkaline conditions; DIPE as extractant.	Effluent: v total phenol < 900 mg/L; total ammonia < 300 mg/L; COD < 4000 mg/L. Recovery rate: phenol about 84%; ammonia about 97%.	Extraction before ammonia distillation leads to alkaline extraction water impacting phenol removal; inefficient acid removal tower; and challenges with acidic gases and ammonia in CGW causing equipment fouling.	Yima, Henan Province, China [158].
Desulfurization–ammonia removal (double-tower)–extraction dephenolation process	Includes acid gas stripping tower, extraction tower, amine stripping tower, amine distillation tower, solvent distillation tower; DIPE as extractant.	Effluent: total phenol < 900 mg/L; total ammonia < 300 mg/L; COD < 4000 mg/L. Recovery rate: phenol ~84%; ammonia ~98%.	Acidic gas elimination leads to high ammonia concentrations and pH issues; and inadequate removal of phenolic compounds.
Desulfurization–ammonia removal (single-tower)–extraction deadenylation process	Ammonia recovery before phenol; acidic extraction; DIPE or MIBK as extractant.	Effluent (DIPE): COD < 4000 mg/L; total phenol < 600 mg/L; ammonia nitrogen < 300 mg/L. Effluent (MIBK): COD < 2000 mg/L; total phenol < 300 mg/L; ammonia nitrogen < 300 mg/L. Recovery rate: phenol 86–95%; ammonia ~99.8%.	Widely applied in the coal chemical industry and wastewater treatment in China; variations in extraction solvent affect treatment efficiency.	Harbin, Heilongjiang Province, China [159]; Ordos, China [159].

To enhance phenol and ammonia removal efficiency, researchers have proposed several advancements. For instance, Yu et al. [160] introduced a single-tower process for acidic gas–ammonia–phenol removal (Figure 9a). This process strategically places the ammonia stripping step before phenol recovery, lowering wastewater pH to below 7 post-deacidification and ammonia removal. This acidic environment optimally facilitates solvent extraction, significantly improving phenol extraction efficiency using methyl isobutyl ketone (MIBK) as the extractant. Furthermore, to enhance efficiency and reduce energy consumption, Gai et al. [37] proposed two methods. One involves a low-pressure steam phenol–ammonia recovery process, where ammonia stripping and concentration occur in separate towers under low-pressure conditions (0.1–0.3 MPa), minimizing medium-pressure flow requirements (Figure 9b). The other integrates an acid removal stripping tower with a solvent recovery tower via heat exchange, using extracted ammonia gas from the acidic water-stripping tower as a heat source for solvent distillation and stripping towers. Compared to traditional single-tower phenol–ammonia removal processes, these innovations can reduce operational costs by 34% and annual consumption by 30.8%. Chen et al. [19] introduced an enhanced process for phenol and ammonia recovery in semi-coking plant wastewater, incorporating acidification pretreatment (acidification–deacidification–extraction–ammonia removal process) (Figure 9c). This method utilized a synergistic extraction agent (MIBK + n-PTL) to boost phenol extraction efficiency. A vacuum system in the solvent recovery tower minimized steam requirements for reboilers. Compared to conventional semi-coking processes, this technology achieved reductions in total phenols, ammonia, and COD to 270 mg/L, 50 mg/L, and 3050 mg/L, respectively, with phenol and ammonia recovery rates reaching 98.2% and 98.3%.

During the phenol–ammonia recovery process, concentrated ammonia gas (NH₃ ≥ 97 wt%) was obtained through three-stage flash distillation by extracting aqueous ammonia from the side of the stripping tower. Subsequently, stepwise separation and caustic soda washing cycles were employed to produce a diluted ammonium hydroxide solution (15~25 wt%) using an ammonia absorption tower, thus achieving efficient resource utilization of ammonia [18]. Following desulfurization, the acidic gas is subsequently fractionally condensed to produce a high-concentration acidic gas (CO₂ + H₂S ≥ 97 wt%), which is then directed to an acid gas flare, Claus sulfur recovery, catalytic wet oxidation, or wet sulfuric acid production unit to achieve the resource utilization of acidic gas. Given the current limitations in research and practical applications of SCOW treatment, the recovery of phenol and ammonia remains a secondary benefit alongside pollutant removal, with relatively low rates of recovery and purification. To ensure sustained, efficient operation of phenol and ammonia recovery, optimizing oil removal and stable, efficient extractant development are critical for sustained phenol and ammonia recovery efficiency.

5.3. Recycled Water

The utilization and recycling of SCOW are critical for the coking industry to achieve NZD, as well as for enterprises to conserve energy and reduce carbon emissions. Typically, SCOW has a high salt content (4500–8000 mg/L), necessitating desalination treatment to meet reuse water quality standards. Membrane treatment technologies, such as UF-RO, NF-RO, or UF-secondary RO, are commonly used for desalination. Wang et al. [161] successfully integrated these three membrane technologies to achieve compliant effluent quality in recycled COW. A significant challenge in desalination is membrane fouling caused by residual suspended solids, colloids, and refractory organic matter post-pretreatment. Hence, additional treatment methods are often employed before membrane filtration. For example, Ma et al., [162] achieved deep treatment of CCW using “Fenton oxidation + electrodialysis + UF + RO” with a wastewater recovery rate exceeding 75%, maintaining superior water quality compared to reclaimed water standards. The coking industry typically achieves a water recycling rate of 70–80% [163].

To achieve NZD, further treatment such as concentration and crystallization is necessary for the concentrated brine from RO. However, RO effectively removes difficult-to-degrade organic compounds and colloids during desalination, resulting in increased organic pollutant concentration in the concentrated brine. Direct concentration and crystallization may lead to corrosion, fouling, and impurities in the crystallized product. Therefore, pretreatment for organic matter removal is generally conducted before concentrating coal chemical brine. AOPs like ozone catalytic oxidation have become mainstream for pretreating high-salt wastewater due to their low investment cost, strong controllability, and stable operation, widely applied in engineering projects [164]. The brine concentration technologies include thermal evaporation and membrane concentration. Thermal evaporation comprises multi-effect distillation (MED), mechanical vapor recompression (MVR), and membrane distillation. MVR is prominent for its energy efficiency and is used in NZD projects for high-salt wastewater from coal chemical industries such as Zhongmei Tuke and Huinen Coal Gasification [165]. However, due to high energy consumption, membrane concentration technologies like high-efficiency reverse osmosis (HERO) and disc tube reverse osmosis (DTRO) have rapidly developed. HERO involves pretreatment such as softening, degassing, and alkalinizing incoming water before RO membrane concentration, operating under high pH conditions to clean membranes continuously and resist insoluble salt contamination, exhibiting strong anti-fouling properties. DTRO uses wide channels, short flow paths, and high-speed turbulence to mitigate fouling, achieving a water recovery rate of around 80%, lower than other processes reaching 90% [163]. Overall, reducing membrane fouling, extending lifespan, and lowering costs remain critical challenges in wastewater reuse for coal chemical industries. The treatment of brine solutions through crystallization has evolved from traditional evaporation ponds to more advanced methods such as MVR, MED, and multi-stage flash evaporation (MSF). This shift is driven by the higher environmental risks associated with evaporation ponds. These methods achieve water–salt separation by controlling operating temperatures and concentration ratios during the crystallization process, leveraging differences in concentration and solubility among inorganic salts. While these methods vary in heat efficiency and operational mode, they share the goal of effective salt recovery. High investment and operational costs are the primary constraints on the development and widespread application of evaporation crystallization technology.

5.4. Salt

The by-product of crystallization treatment is salt resources. However, conventional processes yield crystalline salts mixed with impurities such as heavy metals and organic matter, rendering them unsuitable for direct disposal by landfill or conventional means. Disposing of mixed salts as hazardous waste requires transportation offsite, leading to resource loss and increased operational costs for enterprises [166]. Consequently, salt separation crystallization has emerged as a research focal point in recent years for optimizing resource utilization from CCW brine solutions. Some approaches employ membrane separation processes to separate or enrich different salts based on differences in ion radius or charge characteristics before subjecting them to thermal crystallization. This method yields solid products. For instance, Dong et al. [167] explored dual-polar membrane electrodialysis as an alternative to the MVR process, successfully separating salts to produce sodium hydroxide and hydrochloric acid in laboratory experiments, although its large-scale engineering application remains limited. Additionally, some methods utilize the temperature-dependent co-saturation solubility properties of sodium chloride and sodium sulfate to extract these salts at specific temperatures: sodium chloride at 75 °C and sodium sulfate at 100 °C. Du [168] conducted experiments to investigate the influence of organic impurities on the solubility of these salts, examining liquid phase equilibrium density and crystal morphology in the NaCl + Na₂SO₄ + H₂O ternary system. These experiments provided preliminary insights into the nucleation mechanisms associated with sodium sulfate formation. Each salt separation technique has its own merits and drawbacks. The selection should be based on water quality requirements, the desired purity levels of the separated crystallized salt, and impurity rates, as well as investment and operating costs. Additionally, there is a challenge in disposing of and utilizing residual saturated mother liquor after salt crystallization. The low initial concentration pollutants increase with each cycle. The COD concentration of the evaporated mother liquor ranges from 7500 to 25,000 mg/L, exhibiting high COD, high salt content, and high viscosity [169]. Currently, there is no effective treatment process for evaporated mother liquor, which is usually temporarily stored in evaporation ponds, posing a significant risk in the treatment process of high-salt CCW [100].

5.5. Sludge

Biological sludge generated from the biochemical treatment of COW is categorized as general solid waste, rich in organic matter that can be recycled as a valuable resource. Pyrolysis of this sludge yields combustible gases, bio-oil, and semi-coking products, which hold significant energy and resource value. Zhao et al. [170] determined that employing the A/O/H/O process in conventional CCW treatment generates approximately 1.22 tons of biological sludge daily. Evaluating pyrolysis and incineration methods using a life cycle model revealed that sludge pyrolysis can recover approximately 1.55 × 10⁷ kJ·t^-1 energy with comprehensive economic benefits of 360 CNY/t—significantly lower than incineration costs (583 CNY/t). Additionally, converting biological sludge into activated carbon under suitable conditions presents another viable approach for advanced treatment and achieving waste-to-waste objectives. Yin et al.‘s research [171] demonstrated that incorporating coking sludge into activated carbon enhances its adsorption capacity for heavy metals such as Pb²⁺, achieving a maximum adsorption capacity of 111 mg/g with a 50% addition of coking sludge—a remarkable 146% increase compared to activated carbon without added sludge. Compared to CCW, SCOW, resulting from processing, contains higher levels of organic matter in its biological sludge, offering greater potential for resource utilization.

6. Concluding Remarks and Prospects

The production of semi-coke from various raw materials and processes creates complex, heavily contaminated wastewater that is challenging to treat. Replicating CCW or COW treatment processes is inadequate for achieving efficient, stable, and environmentally safe treatment, and it does not meet the NZD requirement. And the traditional approach of adding more treatment units to improve water quality is impractical. Consequently, SCOW treatment projects, both existing and upcoming, are transitioning towards whole-pollution control systems which include pretreatment, biochemical treatment, advanced treatment, resource utilization, and concentrated brine treatment. Notably, the key technologies for advancing SCOW treatment towards NZD involve phenol and ammonia recovery, bio-enhancement, effective integration of AOPs, and efficient brine handling. While current technologies can effectively remove pollutants and utilize resources, many challenges remain, and most research technologies are far from practical application. Treatment strategies should be tailored to specific wastewater quality characteristics, and collaboration between enterprises, universities, and research institutions should be strengthened to promote the fast and sustainable development of semi-coke wastewater treatment.

To achieve NZD and reduce carbon emissions in SCOW treatment, future efforts should focus on adopting clean and efficient short-process treatment technologies. Innovations are needed in the key technology aspects, with an emphasis on low-carbon emissions, low energy consumption, resource recycling, and water safety. For phenol and ammonia recovery, stable operation and pretreatment efficiency require novel green demulsifiers and mechanically–chemically coupled low-energy consumption processes and equipment. For bio-enhancement, research should focus on understanding enhancement mechanisms, clarifying the synergistic effects of coupled enhancement strategies, and improving system operation and control performance, considering both economic and environmental benefits. Developing environmentally friendly, efficient, and low-cost external materials, such as modified waste carbon sources, for biological enhancement is essential. For advanced treatment, employing AOP-based combined processes is recommended for treating SCOW, aiming to maximize process advantages at manageable costs. Additionally, addressing the reduction, minimization, and resource utilization of concentrated brine and impurity salts in the NZD process is urgent.