Basic Wastewater Treatment Terminology: What Is Coking Wastewater
Coking wastewater is a typical toxic and refractory organic wastewater. It is industrial wastewater generated during the high-temperature carbonization, purification, and by-product recovery of coking coal and coke oven gas. Its main sources include surplus ammonia water, gas purification wastewater, and refining process wastewater.
Characteristics: Coking wastewater has high pollutant concentration and is difficult to degrade. The presence of nitrogen in coking wastewater leads to an excess of nitrogen sources required for biological purification, causing considerable difficulty in meeting treatment standards. The wastewater contains volatile phenols, polycyclic aromatic hydrocarbons, and heterocyclic compounds containing nitrogen, oxygen, and sulfur; approximately 500 individual compounds have been identified, with characteristics of high chemical oxygen demand (COD, typically 1,200–1,300 mg/L) and high ammonia nitrogen (typically 200–700 mg/L). [16]
It is worth noting that the technologies and standards in this field are also continuously developing and improving.
Coking wastewater treatment technology has evolved from traditional activated sludge to improved A/O and A²/O processes, and then to advanced oxidation and other advanced treatment technologies. Traditional activated sludge has problems such as low efficiency and weak shock resistance; subsequent developments such as improved two-stage AO treatment—biofilm composite technology—improved system stability and treatment efficiency. [31]
In 2018, the project "Efficient Treatment and Resource Utilization Technology for Coking Wastewater with Whole-Process Optimization and Its Application" won the second prize of the National Science and Technology Progress Award. Following the concept of recovering valuable resources and comprehensive whole-process pollution control, the project invented core equipment and complete sets of technology for efficient and low-cost whole-process wastewater treatment. [12]
In 2023, Professor Chao-Hai Wei’s team developed an O/H/O process based on a multifunctional microbial fluidized bed water treatment reactor system integrating mixing-reaction-separation, for coking wastewater treatment, achieving efficient nitrogen removal and COD removal, and applied it in actual engineering. [10]
The pollutant concentration of coking wastewater is high; chemical oxygen demand (COD) is generally 2,900–4,100 mg/L, ammonia nitrogen mass concentration is 100–400 mg/L, and average phenol concentration is about 483 mg/L. [13] The ratio of biochemical oxygen demand to chemical oxygen demand (BOD₅/COD) averages about 0.3, indicating poor biodegradability. [13]
Coking plants mainly produce coke, commercial coal gas, ammonium sulfate, and light benzene and other chemical products. The plant’s tar recovery system adopts the ammonium sulfate process; tar processing uses two-tower continuous distillation with a tubular furnace; industrial naphthalene production uses dual-furnace dual-tower continuous distillation, washing, and refining. During coke oven gas cooling, washing, crude benzene processing, and tar processing, industrial wastewater containing phenol, cyanide, oil, ammonia, and large amounts of organic matter is generated.
The expansion project includes two parts: transformation of the original system and new construction. Based on the results of coking wastewater treatment and combined with the original wastewater treatment process, the new expansion project adopts the A1–A2–O biofilm process.
(1) Production wastewater from various workshops and domestic sewage enter the equalization tank uniformly. The main function of the equalization tank is to balance the quality and quantity of the wastewater, ensuring the stability of subsequent biochemical treatment facilities. Since the phosphorus content of the wastewater is very low, phosphorus nutrient salts are added to the equalization tank to provide the nutrients required by microorganisms.
a. Coking wastewater first enters the anaerobic acidification stage. In this stage, phenol, xylenol, and heterocyclic compounds such as quinoline, isoquinoline, indole, and pyridine in the wastewater undergo significant transformation or removal. The setting of the anaerobic acidification stage is very beneficial for the transformation and removal of complex organic matter. Therefore, after passing through the anaerobic acidification stage, the wastewater quality is greatly improved, the biodegradability is higher than that of the raw water, and an effective carbon source is provided for the subsequent denitrification stage.
b. The main reaction in the anoxic stage is denitrification. Wastewater from the acidification stage enters the anoxic stage, while a portion of the effluent from the aerobic stage is also returned to the anoxic stage, providing nitrate nitrogen for the anoxic stage. In addition, due to insufficient denitrification carbon source in the coking wastewater, methanol must be added to the anoxic tank as a supplementary carbon source. After anoxic treatment, nitrate nitrogen is converted to nitrogen gas, achieving the purpose of nitrogen removal. At the same time, most of the organic matter in the wastewater is removed, allowing the wastewater to enter the aerobic stage at lower COD, which is very favorable for the nitrification reaction in the aerobic stage.
c. After anoxic treatment, the wastewater enters the aerobic stage. In the aerobic stage, because the ammonia nitrogen in the wastewater is high and COD is low, the main reaction here is nitrification, and soda ash solution must be added to provide the alkalinity required for nitrification. After aerobic treatment, ammonia nitrogen is basically completely converted to nitrate nitrogen (the nitrate nitrogen is returned to the anoxic stage and finally converted to nitrogen gas for effective nitrogen removal), while organic matter is further degraded, so that the final effluent COD meets the standard.
(3) After biochemical system treatment, the wastewater passes through a coagulation-sedimentation tank for sludge-water separation. In the coagulation part, ferric polymer is added to enhance the settling performance of the sludge in the sedimentation part and further reduce the effluent COD.
(4) The excess sludge discharged from the secondary sedimentation tank is regularly discharged to the sludge thickening tank for thickening and stabilization. The supernatant of the thickening tank is returned to the equalization tank for re-treatment; the thickened sludge is discharged into the sludge storage tank and regularly dewatered by a sludge dewatering machine. Before dewatering, PAM must be added to the sludge for flocculation to improve sludge dewatering efficiency.
(1) Control influent quality and quantity
At the same time, the relevant supporting processes and equipment are also continuously optimized and upgraded.
Coal gas condensate wastewater and turbid water from various clear-turbid separation points are treated by gravity oil separation and air flotation oil removal (oil content below 30 mg/L), so that the oil content is below the concentration affecting the normal growth of microorganisms, and then discharged into the equalization tank.
In addition, from the perspective of industrial development, market demand is also driving technological progress.
Common coking wastewater treatment projects at this stage usually adopt treatment processes that can be divided into pretreatment, biochemical treatment, and advanced treatment. After biochemical treatment, most of the remaining organic matter in the wastewater is refractory, with low biodegradability; to achieve stable compliance or wastewater reuse, further advanced treatment is required. [13]
Currently commonly used advanced treatment processes include: advanced oxidation, adsorption, membrane methods, etc. Advanced oxidation mainly includes the Fenton method, ozone oxidation, electrochemical technology, etc. The Fenton method can effectively remove refractory organic matter in coking wastewater. Ozone catalytic oxidation has a COD removal rate of 64%–74%. Electrochemical process treating biochemical effluent yields effluent COD of about 80–100 mg/L, ammonia nitrogen less than 3 mg/L, SS less than 4 mg/L, and color less than 8 times. [13]
Commonly used membrane methods mainly include ultrafiltration, nanofiltration, reverse osmosis, etc., and different membranes can be combined in processes; currently the most common is the dual-membrane method, namely the "ultrafiltration + reverse osmosis" process. The combined process of ultrafiltration-resin adsorption-reverse osmosis for coking wastewater can make the effluent quality meet relevant reuse water standards. The combined advanced treatment process of Fenton oxidation + electrodialysis + ultrafiltration + reverse osmosis membrane method achieves a wastewater recovery rate of over 75%, with water quality superior to reclaimed water requirements. [13]
"Zero discharge" means treating and reusing all production wastewater; the concentrated brine generated in the wastewater treatment process needs to be concentrated and crystallized for disposal or resource utilization in solid form. The zero discharge of concentrated brine is the bottleneck and key step to truly achieve zero discharge. Zero discharge technology mainly includes reconcentration and crystallization of concentrated brine, such as membrane concentration, falling-film evaporation concentration, multi-effect evaporation, mechanical vapor recompression (MVR), freezing salt separation, etc. [13] Using the MVR + multi-effect evaporation combined process, industrial sodium chloride and sodium sulfate of very high purity are finally produced. [23]
BDD electrode electrolytic oxidation is a water treatment technology. [8] The O/H/O process has stable and efficient COD removal performance for coking wastewater. [10] The biofilm composite technology improves the number of microorganisms in the tank by adding fillers, constructing a biofilm, and significantly improves the shock-load resistance of the biochemical reaction stage. The AOP endogenous carbon cracking technology breaks the long-chain or polycyclic organic matter in coking wastewater into small-molecule organic matter through advanced oxidation. The Fenton fluidized bed integrated complete equipment can reduce agent cost and sludge production and simultaneously remove cyanide. [31]
In addition, actual engineering applications also need to consider multiple factors.
Coking wastewater has complex composition and strong toxicity; accurately monitoring its pollutant concentration and biological toxicity is key to evaluating treatment effects, ensuring compliance discharge, and environmental safety. [1] The detection content mainly includes routine pollutant indicator determination, specific pollutant special detection, and comprehensive biological toxicity assessment.
The industry standard "Determination of Thiocyanate Content in Coking Wastewater" (HG/T 6072-2022) has been implemented since April 1, 2023. The standard specifies two determination methods: the ammonium ferric sulfate spectrophotometric method (under strongly acidic conditions, thiocyanate reacts with ferric ions to form a red complex, measured at a wavelength of 456 nm) and the ion chromatography method (the sample is separated by an ion exchange column and then measured by a conductivity detector). Thiocyanate is a characteristic component of coking wastewater; its accurate monitoring is of great significance for evaluating COD contribution, controlling total nitrogen, and ensuring the stable operation of the biochemical system.
Routine detection indicators of coking wastewater mainly include chemical oxygen demand (COD), ammonia nitrogen (NH₃-N), phenolic compounds, cyanide, thiocyanate, etc. These indicators are core parameters for measuring wastewater pollution degree and treatment effect, and their discharge limits are clearly stipulated in regulations such as the "Emission Standard of Pollutants for Coking Chemical Industry" (GB 16171—2012). [31] Monitoring these indicators helps to grasp water quality in real time and guide treatment process operation.
To ensure the accuracy and timeliness of monitoring data, coking wastewater monitoring technology and management requirements include: online monitoring—installing online monitoring equipment for COD, ammonia nitrogen, etc. at key nodes such as the outlet of the wastewater treatment station, to achieve real-time data tracking, which is an important means of environmental supervision; manual monitoring and third-party testing—conducting regular manual sampling analysis, and entrusting third-party testing institutions with CMA/CNAS qualifications to issue test reports, ensuring fair and reliable data. [14][24][32-33] Strict monitoring requirements help eliminate illegal acts such as data falsification and illegal discharge. [24][33]
The national emission standard "Emission Standard of Pollutants for Coking Chemical Industry" (GB 16171—2012) stipulates the limits for direct and indirect discharge of coking wastewater, such as COD, ammonia nitrogen, cyanide, etc. [24] The concentrations of COD, ammonia nitrogen, and cyanide in the externally discharged sewage exceed the direct discharge standard in Table 2 of the standard by 0.5, 3.5, and 1.4 times, respectively. [24] The COD value is reduced to 108 mg/L, higher than the direct discharge concentration limit for COD of new enterprises in the standard (80 mg/L), close to the direct discharge concentration limit for existing enterprises (100 mg/L).
The project "Efficient Treatment and Resource Utilization Technology for Coking Wastewater with Whole-Process Optimization and Its Application" was selected as a landmark achievement of the Water Special Project and included in the "National Encouraged Environmental Protection Technologies (Water Pollution Control Field)" (2015) issued by the former Ministry of Environmental Protection. [12]
In May 2024, the Department of Ecology and Environment of Shanxi Province issued a document (Jin Huan Fa [2024] No. 8) requiring zero discharge of coking wastewater and improved governance, including adopting pretreatment + membrane concentration + evaporation crystallization salt separation processes and implementing full collection and treatment of initial rainwater. [32]
Environmental supervision inspections severely crack down on illegal discharge and require the installation of online monitoring equipment. [24][33-34] The Tianrui Coking Plant in Ruzhou, Henan, did not install online monitoring equipment at the outlet of the coking wastewater treatment station as suggested in the environmental impact assessment report, and the concentrations of COD, ammonia nitrogen, and cyanide in the externally discharged sewage exceeded the Table 2 direct discharge standard of the "Emission Standard of Pollutants for Coking Chemical Industry" (GB 16171—2012) by 0.5, 3.5, and 1.4 times, respectively. [24] The inspection team found that the company had prominent environmental violations. [33-34]
According to the "Emission Standard of Pollutants for Coking Chemical Industry" (GB 16171—2012), when new enterprises directly discharge wastewater, the concentration limit for chemical oxygen demand (COD) is 80 mg/L.
Enterprises must strictly implement the zero-discharge regulation for coking wastewater, re-treat the concentrated brine from advanced treatment and reclaimed water reuse devices, and achieve full recycling without external discharge. [32] If improperly treated, such as using non-compliant wastewater for coke quenching, pollutants will transfer from liquid to gaseous state and be discharged into the atmospheric environment through the quenching tower. The Ruzhou Tianrui Coking Coal Co., Ltd. in Henan Province had environmental violations such as abnormal operation of pollution control facilities and serious excessive sewage discharge. The Fenton reaction process in its coking wastewater treatment station did not add reagents according to specifications, and the ultrafiltration-reverse osmosis equipment was idle for a long time, making the control facilities virtually useless. The COD, ammonia nitrogen, volatile phenol, and cyanide concentrations in its quenching water exceeded the indirect discharge standard of the "Emission Standard of Pollutants for Coking Chemical Industry" by 2.0, 14.2, 22.7, and 13.2 times, respectively. Due to long-term discharge, the benzo[a]pyrene content in the bottom sludge of its plant drainage ditch exceeded relevant soil environmental risk control standards. [24]
Driven by policy, achieving "zero discharge" of wastewater has become an industry trend. [12] As a major coking province, Shanxi Province took the lead in promoting the quality improvement and efficiency enhancement of water pollution control in the coking industry, requiring full recycling and no external discharge of coking wastewater. [32] A typical case is the Qingxu coking industrial cluster in Shanxi, which achieved centralized treatment and resource utilization of coking wastewater by jointly building a centralized wastewater treatment plant—Qingxu Hongbo Wastewater Treatment Co., Ltd. The plant adopts the process of "biochemical treatment + reclaimed water reuse + evaporation crystallization," with an actual treatment capacity of about 1.7×10⁴ m³/d and a reuse rate as high as 92%. The treated reclaimed water is reused for enterprise circulating cooling water, and crystalline salt is sold as a by-product, realizing the transformation from "not a drop of wastewater discharged externally" and from "waste" to "product." [11][23]
At the same time, the relevant supporting processes and equipment are also continuously optimized and upgraded.
The concentrations of COD, ammonia nitrogen, and cyanide in the externally discharged sewage exceed the direct discharge standard of the "Emission Standard of Pollutants for Coking Chemical Industry" by 0.5, 3.5, and 1.4 times, respectively. [24-25][33] The volatile phenol concentration in quenching water exceeds the indirect discharge standard of the "Emission Standard of Pollutants for Coking Chemical Industry" by 22.7 times. [24-25][34]
On this basis, industry experts have also conducted extensive research and improvements.
The effluent after advanced treatment should meet the water quality standard for industrial circulating cooling water and be reused for circulating cooling water, boiler soft water make-up, plant greening, and even partially replace fresh water, achieving full recycling without external discharge. [32] The wastewater treated by independent coking enterprises can be used for wet coke quenching, coal yard dust suppression, dust removal and humidification water, and circulating water make-up. The coking wastewater of iron and steel enterprises after reuse treatment can be used for batching sprinkling, slag flushing, slag quenching, and circulating water make-up. [13] For example, the actual treatment capacity of Qingxu Hongbo Wastewater Treatment Co., Ltd. is about 1.7×10⁴ m³/d, with a reuse rate as high as 92%. [11]
Concentrated brine treated by the evaporation crystallization system can realize the resource utilization of crystalline salt. For example, using the MVR + multi-effect evaporation combined process, industrial sodium chloride and sodium sulfate of very high purity are finally produced and sold externally. [23] The average purities of sodium sulfate and sodium chloride products are 99.35% and 98.62%, respectively, reaching the first-grade industrial product indicators. [13]
At the policy level, in May 2024, the Department of Ecology and Environment of Shanxi Province issued a document (Jin Huan Fa [2024] No. 8) requiring improved coking wastewater governance and strict implementation of the zero-discharge regulation for coking wastewater. [32] In a practical case, Qingxu Hongbo Wastewater Treatment Co., Ltd. adopted the process scheme of "biochemical treatment system + reclaimed water treatment system + evaporation crystallization treatment system" to achieve "zero discharge" of coking wastewater. [23]
Resource utilization can also bring economic benefits; for example, in a coking wastewater retrofit project, the dry sludge generated daily is productively utilized in the form of coke/catalyst carriers, bringing an annual revenue of about 6.57 million yuan. [10]
