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China is a large coke producer and a major exporter of the world coke market. In recent years, with the rapid development of China's iron and steel industry, the coking scale supporting it has also expanded unprecedentedly. In 2013, China added another 43 coke ovens and added capacity of 26.6 million tons. The national coal output was about 3.7 billion tons. An increase of 8.1% year-on-year. As a result, coking waste water emissions from coal-based coke, gas purification, and coking product recovery processes will multiply. The large-scale discharge of coking wastewater will not only cause serious pollution to the environment, directly threaten human health, but also cause serious waste of resources. Therefore, the treatment technology of coking wastewater has attracted widespread attention from industry peers.
1 Coking wastewater sources and components Coking wastewater mainly comes from coking and gas purification processes and chemical product refining processes.The main sources are from three aspects: ammonia steam wastewater, gas cooling water, oil processing and crude benzene wastewater. The main source is the residual ammonia produced during the ammonia distillation process.
Coking wastewater has a complex composition and mainly contains dozens of inorganic and organic compounds. Sun Lingdong et al. Used a gas chromatography-mass spectrometer (GC / MS) to analyze the coking wastewater, and used liquid-liquid extraction and C18 and silicon de-microcolumn chromatography for pretreatment, and found that it contained 244 organic pollutants. Guoxin Song et al. Used the US Environmental Protection Agency's method-dispersed liquid-liquid microextraction-gas chromatography / mass spectrometry to analyze coking wastewater, and quantitatively analyzed 15 kinds of PAHs.
2 Research status of coking wastewater treatment methods 2.1 Physical and chemical treatment methods of coking wastewater 2.1.1 Coagulation method The efficiency of coagulation treatment methods mainly depends on the chemical properties of the coagulant. Common aluminum salts, iron salts, polyaluminum, polyacrylamide Wait. Fang Zhu et al. Used a composite coagulant (PAC and organic polymer coupling agent) to treat biochemical effluent from coking wastewater. When the PAC dosage was 400 mg / L and the organic polymer coupling agent dosage was 300 mg / L The removal rates of turbidity and chromaticity of coking wastewater reached 96.67% and 72.60%, respectively.
2.1.2 Adsorption method The adsorption method is often used in the advanced treatment of coking wastewater. The solute in the wastewater is adsorbed by a porous adsorbent to make the wastewater purified. MoheZhang et al. Used AC as an adsorbent and studied the analysis of activated coke adsorption in biochemical effluent from coking wastewater using UV-visible spectroscopy, gas chromatography-mass spectrometry (GS / MS), and scanning electron microscope (ESEM). The adsorption was performed at 40 ℃. After 6 h, the COD removal rate of the wastewater can reach 91.6%, and at the same time, the chroma removal rate can reach 90%. It can be seen that the adsorption of AC materials is stronger than that of activated carbon. Nan Zhang et al. Used electro-adsorption technology (EST) to treat coking wastewater and invented a new electro-adsorption device for desalination of coking wastewater. Under experimental optimized conditions, the salt removal rate reached 75% after electro-adsorption treatment. The effluent water quality can meet the industrial circulating cooling water standard (GB 50050-2007), and can be reused as circulating cooling water in coking plants.
2.1.3 Ozone oxidation method Because the oxygen atoms in the ozone molecules have strong electrophilic or protonic properties, the new ecological oxygen atoms generated by ozone decomposition also have high oxidizing activity, so ozone has strong oxidizing properties and short contact time. , High treatment efficiency, not affected by temperature, will not produce secondary pollution and other characteristics, usually used for advanced treatment of coking wastewater. Demin Yang et al. Used the ozone oxidation method to treat the biochemical effluent of coking wastewater. The ozone concentration was 150 mg / L, and the reaction was performed for 30 minutes at a pH of 10.5 and a temperature of 298 K. The COD and chroma removal rates could reach 69.65%, respectively. And 92.27%. This shows that ozone oxidation technology is an effective method for advanced treatment of coking wastewater.
2.1.4 Fenton's reagent method The main mechanism of Fenton's reagent method is the rapid reaction of Fe2 + and H2O2, which produces a strong oxidation energy. OH, OH radicals have a high electronegativity or electrophilicity, and can further react with the organic substance RH. The organic radicals R ¡¤, R ¡¤ are further oxidized, which causes the carbon bonds of organic structures to break, and finally oxidizes to CO2 and H2O. Peng Ruichao et al. Prepared an electric Fenton device with activated carbon fiber bound to the surface of stainless steel mesh as the cathode and titanium sheet as the anode, and used this device to treat the A2 / O effluent from a coking plant at pH 3, voltage 9 V, and anode The plate distance was 30 mm, the amount of Na2SO4 added was 5 g / L, the aeration flow rate was 600 mL / L, and the Fe2 + dosage was 0.2 mmol / L. After operating for 2 h, the COD of the wastewater decreased significantly, and the maximum removal rate was 82.5 %. Li Haitao and others used high-efficiency oxygen reduction cathode PAQ / GF and shape-stable anode IrO2-RuO2-TiO2 / Ti as anion and anode for advanced treatment of biochemical effluent from coking wastewater. Under optimized conditions, the pH was 5-6 and the current density was 10 mA / When the air flow rate was 0.5 cm / cm2, the coking wastewater with an initial COD of 192 mg / L was treated at a reaction time of 1 h. The COD removal rate was above 50%, and the TOC removal rate was 25% to 30%.
2.1.5 Electrochemical Oxidation Method Electrochemical oxidation method is the use of an external electric field to degrade organic pollutants in water by high-potential oxidation in a specific electrochemical reactor. Electrochemical oxidation technology mainly depends on the electrode material.
Shujing Sun et al. Used modified electrodes with carbon nanotubes and PTFE as coatings to treat the biochemical effluent of coking wastewater. The results were analyzed using UV-Vis, GC / MS, and COD analyzers. The MWNT-ME electrode was degraded for 2 hours and then coked. The amount of organic pollutants in the wastewater was reduced from 107 to 49, and the COD removal rate reached 51%. Compared with the IrSnSb / Ti electrode, the MWNT-ME electrode showed better results. Xuwen He et al. Used saturated coke as a filling material and used a three-dimensional electrode fixed bed reactor to deeply treat coking wastewater. The results show that the coke powder can be used as a catalytic electrode. Under the optimized conditions of electrolysis time of 60 min, current of 8 A, particle size of 10 to 20 grids, dosage of 400 mL, and plate spacing of 1 cm, the COD removal rate reached 70. %, And by scanning electron microscopy (SEM) analysis, it can be seen that the active coke is ideal for electrode treatment due to its compact structure, high crystallinity, and suitable porosity.
2.1.6 High-strength supercritical water oxidation technology Supercritical water oxidation technology (SCWO) uses water as the medium, and uses the absence of gas-liquid interface mass transfer resistance under supercritical conditions (temperature> 374 ℃, P> 22.1 MPa). Increase reaction rate and achieve complete oxidation. This technology was proposed by American scholar Modell in the mid-1980s, and the United States and Japan are ahead of China in industrialization research in this field. Yuzhen Wang et al. Studied the treatment of coking wastewater with supercritical water oxidation technology, and showed that temperature and oxygen ratio (OR) enhanced the molar fraction of H2 and the removal efficiency of COD. At 465 ℃, 25 MPa, and OR is 0.2, H2, CO, The molar fractions of CH4 and CO2 are 56.88%, 1.17%, 7.82%, and 34.13%, respectively. Meanwhile, the removal efficiency of TOC, VP (volatile phenol) and NH3-N reached 81.37%, 86.09% and 47.63%, respectively.
2.1.7 Flue gas method The composition of flue gas is mainly nitrogen, carbon dioxide, oxygen, water vapor and sulfide. The flue gas is used to treat coking wastewater. SO2 in the flue gas and NH3 and O2 in the wastewater react to form ammonium sulfate (NH4) 2SO4, so as to achieve the purpose of waste treatment. Jianjun Dong et al. Used a spray tower countercurrent device, and installed automatic flue gas detectors at the inlet and outlet to detect the SO2 concentration in the flue gas, and to change the SO2 concentration during the test and the initial SO2 concentration in the sintered flue gas. The effect of desulfurization rate was studied, and it was shown that the sintered flue gas after treatment reached the air pollutant emission standard of the steel industry.
2.2 Biological treatment method of coking wastewater 2.2.1 Activated sludge method Activated sludge method is used to treat coking wastewater, which uses the role of activated sludge to agglomerate, adsorb, oxidize, decompose, and precipitate in the wastewater to achieve the removal of organic pollutants in the wastewater. the goal of. This method continuously introduces air into the wastewater. Due to the proliferation of aerobic microorganisms, sludge-like flocs are formed after a certain period of time. Microbial groups dominated by bacterial micelles have a strong adsorption and oxidation of organic matter. ability. Activated sludge process is mainly used for secondary treatment after pretreatment of coking wastewater. Y. Lu et al. Used upflow anaerobic sludge bed (UASB reactor) to degrade organic matter in coking wastewater. The pH was 6.8 ～ 7.2, the stirring speed and temperature were 2 r / min and (30 ¡À 18) ℃. Under the experimental conditions, the UASB reactor was started for 133 days and the COD removal rate could reach 54%. At the same time, GC / MS analysis shows that the UASB reactor can basically remove more than a dozen organic compounds such as aniline, phenol, o-phenol, p-cresol, benzoic acid, indole, and quinoline in coking wastewater, which is an effective Feasible method for degrading organic matter in coking wastewater.
2.2.2 Biological denitrification technology Traditional biological denitrification technology can be divided into AO, AAO, OAO and other processes.The new biological denitrification technology mainly includes semi-nitrification process (SHARON), anaerobic ammonia oxidation process (ANAMMOX), and semi-nitrification-anaerobic Oxy-Ammonia Oxidation Process (SHARON-ANAMMOX), Automated Nitrogen Removal Process (CAUON). Among them, compared with the traditional nitrification-denitrification process, the semi-nitrification-anaerobic ammonia oxidation process significantly reduces the oxygen consumption, does not require the addition of a carbon source, and generates a small amount of residual sludge.
Haibo Li and others used the AOO process to treat coking wastewater. The mass concentrations of NH4 + -N, phenols, and COD in the coking wastewater were 200 to 500, 250 to 300, and 1 700 to 2 200 mg / L. After passing through the anoxic process, NH4 + The removal rates of -N, phenols, and COD were 17.84%, 41.78%, and 82.63%, respectively; the temperature in the aerobic reactor was (35 ¡À 1) ℃, and the dissolved oxygen was 2 to 3 L-1. The nitrite accumulation rate remained above 85%. At the same time, analysis by GC / MS showed that most organic pollutants were decomposed in the denitrification stage, and the AOO process had good prospects for coking wastewater treatment.
Xin Zhou et al. Studied the treatment of coking wastewater with the OOAA biofilm method and conducted a pilot test. The OOAA biofilm system was operated for 239 days and the hydraulic retention time was 116 h. The removal rates of COD and NH4 + -N reached 92.3% and 97.8%, respectively. The effluent was stable and reached the first-level standard for sewage discharge.
Mingjun Shan et al. Applied the short-range nitrification-anaerobic ammonia oxidation-nitrification coupling technology to the treatment of coking wastewater. Through continuous optimization of denitrification technology, the effluent water quality can reach the "level 1 standard for comprehensive wastewater discharge standards" (GB 8978-1996). . The removal rates of ammonium nitrogen and COD reached 99.5% and 96.1%, respectively.
2.2.3 Biological fluidized bed technology Biological fluidized bed technology is a new type of biofilm process.The carrier is in a fluidized state in the fluidized bed, so that solid (biofilm), liquid (wastewater), and gas (air ) Full contact between the three phases, severe collisions between particles, continuous renewal of the surface of the biofilm, microorganisms are always growing vigorously, maintaining a high concentration of biomass, extremely high mass transfer efficiency, short hydraulic retention time, and operating load than normal active sewage The mud method is 10 to 20 times higher and has strong impact load resistance. Therefore, in recent years, it has become more and more widely used in the treatment of refractory organic wastewater.
Na Li et al. Studied the degradation of COD and NH4 + -N in coking wastewater using a three-phase aerobic biological fluidized bed in combination with a new ultra-structured biological filler. The pH was 7.5 and the DO was 2 to 5 mg / L during 20 h operation. Under these conditions, the removal rates of COD and NH4 + -N can reach 82% and 87%, respectively. Feng Wang et al. (22) used magnetically stabilized fluidized bed (MSFB) in combination with magnetic mesoporous silica particles to immobilize laccase to treat phenol in coking wastewater. The degradation rate of phenol can reach more than 99%, which is a very promising development. Prospect approach.
2.2.4 Bio-enhanced treatment technology Compared with traditional bio-treatment technology, bio-enhanced technology uses special-effect microbial flora and biocatalysts to maintain the activity of the flora, which can greatly shorten the treatment process and engineering investment, without secondary pollution, and can suppress Swelling of sludge increases the stability of the operation of wastewater treatment systems, so more and more attention is paid to organic wastewater treatment. Shengnan Shi et al. Used bio-enhanced technology to treat coking wastewater. After 120 days of operation, the removal rates of pyridine, quinoline, and TOC were 99%, 85%, and 65%, respectively, and the removal rates of COD and NO3--N were above 95%. . Through the analysis of 16SrDNA by terminal restriction fragment length polymorphism, the diversity of the bacterial community in the bioreactor showed an increasing trend.
2.2.5 Sequential Batch ReactorSequential batch reactor (SBR) is a batch-injected reactor system, including an independent fully mixed reactor, in which all steps of the activated sludge process take place, typical processes include The five processes of water inlet, reaction, precipitation, drainage, and idle are an intermittent operation wastewater treatment process that integrates biological degradation and nitrogen and phosphorus removal. E. Mara? ¨®n and others used SBR to treat coking wastewater. The reaction was carried out in a CSTR (continuous stirred tank reactor). The ammonia stripping efficiency was 96%, the hydraulic retention time was 115 h, and the thiocyanate and phenol in the effluent were removed. The rates are 98% and 99%, respectively.
2.2.6 Biological aerated filter The biological aerated filter (BAF) process has the functions of removing SS, COD, BOD, nitrification, denitrification, phosphorus removal, and removal of AOX (harmful substances). Aerated biological filter is a new process that integrates biological oxidation and trapped suspended solids, which saves subsequent sedimentation tanks (secondary sedimentation tanks). It has a volumetric load, large hydraulic load, short hydraulic retention time, less infrastructure investment, and effluent water quality. Good, low energy consumption and low operating costs. Yaohui Bai et al. Used a zeolite aerated biological filter (Z-BAFS) to treat coking wastewater, and zeolite was used as a filler. Analysis of the clone library showed that the growth of microorganisms in the biofilm was enhanced. This method has a good development in the treatment of refractory organic wastewater. prospect.
2.3 The latest progress of coking wastewater treatment research
2.3.1 Application of multiple carriers in biological treatment technology Wufeng Jiang et al. Used fixed-bed reactors (MPHC) with high carbon metal balls as carriers to treat coking wastewater and remove phenols, cyanide, COD, and ammonia nitrogen from wastewater The effect was studied, and the results showed that MPHC had a good degradation effect on the degradation of phenols and cyanides, the removal rates were 99.88% and 99.81%, respectively; the COD degradation rate was 70.61%. According to FI-IR analysis, organic pollutants were not adsorbed, but degraded after MPHC treatment.
Yue Cheng et al. Studied the use of porous ceramsite modified by magnetic materials as a carrier for the treatment of coking wastewater in biofilm reactors. Compared with the traditional activated sludge method, porous ceramsite modified by magnetic materials was used as a carrier. The carrier used in biofilm reactor to treat coking wastewater can increase the removal rate of COD and NH3-N by 25% to 30% respectively; compared to the biofilm reactor without magnetic carrier, the porous ceramsite modified by magnetic materials As a carrier used in biofilm reactors to treat coking wastewater, the removal rates of COD and NH3-N can be increased by 15% to 20%, respectively. When the aeration rate is 1.5 mL / h, the aeration time is 10 h / d, and the temperature is 25-30 ℃, the removal rates of COD and NH3-N can reach more than 90%.
2.3.2 Modified organic bentonite treatment technology Bentonite is a hydrated clay mineral based on montmorillonite. Because of its special properties, such as swelling, adhesion, adsorption, catalytic, suspension, etc., it has been widely used in refractory wastewater.
Haixia Guo et al. Used modified organic-inorganic bentonite as an adsorbent to further treat coking wastewater. The results showed that under the conditions of 30 min, pH 9 and 50 g / L dosage, aluminum sulfate and cetyltrimethyl Ammonium bromide modified bentonite can effectively reduce ammonia nitrogen and COD in coking wastewater.
Zhenhua Wu et al. Used organic bentonite coking wastewater for pretreatment. The results show that the adsorption capacity of organic bentonite for organics is proportional to the cation-exchange surfactant and the octanol-water (KOW) partition coefficient of the solute. The removal rates of 0.75 g / L bentonite and 180 mg / L (60% of bentonite's cation exchange capacity) cetyltrimethylammonium bromide, except for naphthalene, reached 16 types of polycyclic aromatic hydrocarbons (PAHs). More than 90% of the US Environmental Protection Agency's regulations on coking wastewater treatment, of which benzo (a) pyrene reached more than 99.5%. At the same time, the removal rates of COD, NH3-N, volatile phenol, color, and turbidity were 28.6%, 13.2%, 8.9%, 55%, and 84.3%, respectively, and BOD5 / COD increased from 0.31 to 0.41, effectively improving coking Biodegradability of wastewater.
2.3.3 Preparation of Coking Wastewater Coagulant from Rare Earth Waste Residue Miaomiao Bao et al. Studied the use of rare earth waste slag and NaOH to prepare coagulant for coking wastewater treatment, and determined the best conditions for the preparation of coagulant: 7 g rare earth residue, boiling time 2 h, ripening for 3 h, reaction temperature is 60 ℃, the treatment effect of coagulant coking wastewater was studied by orthogonal test, the results showed that the turbidity removal rate reached 93%, the chroma removal rate reached 98.68%, and the COD removal effect Very high.
3 Research on the combined process for treating coking wastewater 3.1 BF-BFB combined process Wenpeng Wu et al. Used the BF-BFB (biological filter-biological fluidized bed) combined process with a special carrier to treat coking wastewater. The pretreated coking wastewater contains 1 460 mg / L COD, 360 mg / L NH4 + -N. In the biofilm formation stage, the biofilm formation time is the key factor affecting the treatment effect; in the formal operation stage, the treatment effect is mainly affected by the hydraulic retention time, reflux ratio, pH, and aeration rate. The BF-BFB treatment system achieved a removal rate of 87.6% and 97.5% for COD and NH4 + -N, respectively, and the NH4 + -N in the effluent reached the national first-level emission standard.
Yingjun Hao et al. Studied the treatment effect of coking wastewater from the BF-BFB combined process, and the results showed that the biofilm needs to be mature and stable for 25 days, and the removal rates of COD and NH4 + -N should reach more than 95%, respectively. Proteobacteria is the largest dominant flora, which is 55% of the total bacteria.
3.2 A1-A2-ZB-MBR combined process Xiaobao Zhu et al. Used A1-A2-ZB-MBR (anaerobic / hypoxic / zeolite biofilter / membrane bioreactor) combined process to treat coking wastewater, which is obtained by pyrosequencing Microbial community and kinetic composition of the treatment system. The A1-A2-ZB-MBR composite process treats coking wastewater, and the effluent COD and total nitrogen are relatively stable. At the same time, 66,256rRNA gene sequences were obtained in the system, and the microbial diversity and abundance of 5 samples were determined. The types of microorganisms in the five samples were different, but Proteobacteria and Flavobacterium were common and accounted for the largest proportion of all microorganisms. Pyrosequencing analysis showed that the microbial community was transferred within ZB-MBR. In addition, during the treatment, nitrosating bacteria and nitrifying bacteria gradually became the dominant bacteria for ammonia oxidation and nitrite oxidation, which enhanced the stability of ammonia nitrogen.
3.3 IE-UASB-A / O2 combined process Pan Luting et al.  used the IE-UASB-A / O2 (internal electrolysis / upstream sludge bed / anaerobic / aerobic / aerobic) combined process to treat coking wastewater and raw water The COD and phenol mass concentrations were 2 500 and 320 mg / L, respectively. After pretreatment with internal electrolysis, the effluent COD and phenol mass concentrations decreased to 150 and 0.1 mg / L, respectively. According to gas chromatography-mass spectrometry (GC-MS) analysis, the internal electrolytic pretreatment can effectively degrade heterocyclic compounds, and the UASB reactor can effectively degrade phenol and quinolone. After the coking wastewater is treated by the IE-UASB-A / O2 combined process, the organic pollutants in the wastewater are greatly reduced.
3.4 ADEC-SCWG-SCWO combined process Yuzhen Wang et al. Studied and evaluated the combined process of ammonia evaporation and evaporation-supercritical water gasification-supercritical water oxidation (ADEC-SCWG-SCWO) to treat coking wastewater. Some ammonia was removed during the evaporation and concentration stage; after that, the wastewater was concentrated into the SCWG stage to produce mixed gases, such as H2, CO, CH4, etc .; then the liquid wastewater entered the SCWO stage, and the organic pollutants in the wastewater were basically oxidatively degraded. The ASPEN PLUS software was used to simulate the investment and operating costs of the combined process. It was found that the ADEC-SCWG-SCWO combined process was used to treat coking wastewater, not only with high efficiency, but also a profit of 5.1 yuan per ton of wastewater. Economic benefits.
3.5 MBR-RO combined process Wang Ye and others aimed at the treatment and reuse of traditional coking wastewater, and studied the treatment effect of pollutants in coking wastewater using a short-sequence sequential batch MBR-RO composite system, indicating that the sequential batch MBR-RO composite The system can be successfully applied to the secondary treatment of coking wastewater. The COD removal rate is relatively stable, above 93%. The COD of the reverse osmosis water is 28.7 mg / L, and the total nitrogen removal rate is stable above 96%. The effluent mass concentration of MBR-RO system is 0.24, 0.02 mg / L, and the concentration of reverse osmosis is 3.85 times and 4.53 times, respectively. In addition, in order to alleviate membrane fouling, the MBR ultrafiltration membrane critical flux of 35.44 L / (m2 ¡¤ h) and reverse osmosis membrane critical flux of 10.68 L / (m2 ¡¤ h) were used as initial conditions for continuous operation.After 60 days of operation, MBR The specific membrane flux loss was 65.3%, and the RO specific membrane flux loss was 73.2%. The exacerbation of RO membrane pollution may be attributed to the pollution of a large amount of dissolved organic matter during continuous concentration operation of the membrane element for 2 hours per day.
3.6 A1-A2-O-MBR-NF-RO combined process Xuewen Jin et al. Studied A1-A2-O-MBR-NF-RO (anaerobic-hypoxic-aerobic-membrane bioreactor-nanofiltration-reverse osmosis ) The combined process was used to treat coking wastewater. The results showed that the removal rates of COD, BOD, ammonium nitrogen, phenol, total cyanide, thiocyanate (SCN), and fluorine were 82.5%, 89.6%, 99.8%, 99.9%, 44.6%, 99.7%, 8.9%; in the A1-A2-O stage, the fluorine removal rate reached more than 86.4%; the MBR process reduced the turbidity to below 0.65 NTU, and most of the organic pollutants were basically degraded at this stage .
4 Conclusions and prospects (1) The coking wastewater contains a large amount of organic pollutants such as phenols, oils, cyanides, etc., and its COD and ammonia nitrogen contents are high, and the water quality of coking wastewater is complex and changeable. If separate physical and chemical or biochemical treatment is used, It is difficult to make wastewater meet discharge standards. IE-UASB-A / O2 and other combined processes have a broad application prospect for coking wastewater treatment, can achieve reuse purposes, and achieve zero discharge of coking wastewater.
(2) Treatment of coking wastewater should be closely combined with prevention and treatment. In the early stage of construction, when selecting the coking plant site, full consideration should be given to wastewater treatment schemes, gas purification processes, and wastewater treatment schemes for each process in order to reduce the burden on the final wastewater treatment.
(3) Coking wastewater should be recycled for coking production as much as possible after treatment. For example, it can be used as coal yard sprinkler water, cooling water, dust removal supplementary water, etc. to protect the environment and save energy.
(4) Research on new technologies for coking wastewater treatment is bound to be obtained. The combination of new technology and traditional methods to ensure that the three requirements of treatment effect and treatment cost and no secondary pollution are met is an urgent problem.
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