| With the development of iron and steel industry, the scale of coke production has been increased, resulting in the large increase of wastewater. However, as coking wastewater contains many toxic and refractory pollutants, the effective treatment is rather difficult. If the wastewater could be treated and recycled with low cost and high efficiency, the discharge of wastewater and pollutants will be highly reduced and the environment will be significantly improved. In addition, the treatment and reclamation of the waste acid from benzene refining is an industrial problem in coking industry. Till now, there is no effective technology to treat the waste acid. If the problem was solved, the discharged pollutants would be reduced and considerable economic benefit would be acquired. Meanwhile, company image and competition will be upgraded.This study investigated the treatment of coking wastewater. Using the coking wastewater discharged from Wuhan Iron and Steel Company (WISCO) as the target, we analyzed the source and the quality of the wastewater, reviewed the technologies for the treatment of coking wastewater, and proposed the problem in the treatment. Herein, electro-Fenton and microwave catalytic oxidation, advanced oxidation processes (AOPs), were used to degrade the representative toxic pollutants. The feasibility of the processes on the treatment was researched firstly. The factors influencing the degradation, the degradation kinetics and mechanism and the engineering application were discussed. The industrial problem on synthetic treatment and recycling of waste acid from benzene refining was roundly solved in bench scale by the traditional chemical separation method, salting-out extraction process. The main conclusions are drawn as follows.(1) A comprehensive analysis on the production process in the WISCO coking plant and a detailed investigation on the source of individual wastewater were conducted in this study. By reviewing the treatment status in the field, we concluded the key problems, including the effective treatment of waste acid from benzene refining, the legal discharge of the wastewater from biological unit, and the impact of the high concentrations of wastewater from benzene and tar refining on the biological process.(2) The main organic pollutants in the wastewater from the key process and biological unit were analyzed by gas chromatography/mass spectrum (GC/MS). The wastewaters from rude benzene reclamation and tar refining contained high concentrations of nitrogen-heterocyclic compounds and polycyclic hydrocarbons, both of which were difficult to degrade. In the wastewater from biological unit, these compounds contributed to 72.64 percentage of chemical oxygen demand (COD).(3) Nitrogen-heterocyclic compounds, indole, quinoline and pyridine, could be degraded by anodic oxidation and electro-Fenton. For the 3 h degradation of indole, the removal efficiency by electro-Fenton was 20% higher than that by anodic oxidation. The same sequences of the degradation of the nitrogen-heterocyclic compounds by anodic oxidation and electro-Fenton were obtained as: quinoline > indole >> pyridine.(4) In the degradation of indole by anodic oxidation and electro-Fenton, the removal TOC was higher in the electro-Fenton process than that in the anodic oxidation process. The degradation mechanism in the electro-Fenton process was proposed as hydroxyl oxidation, anodic oxidation and anodic polymerization.(5) The COD of the coking wastewater from biological unit could be removed by anodic oxidation, electro-Fenton and three-dimensional electrode methods. In particular, the removal by three-dimensional electrode method is the highest. The NH3-N could hardly be removed by the three processes.(6) The COD of the coking plant wastewater after biochemical treatment could be effectively removed by microwave radiation assisted by Cu coated activated carbon. Compared with the adsorption by Cu coated activated carbon, microwave radiation assisted by Cu coated activated carbon could achieve 40% higher removal of COD. Results of orthogonal design demonstrated that the amount of Cu coated activated carbon had the greatest effect. The radiation power and time also influenced the efficiency.(7) The salting-out extraction process is used to treat the waste acid from benzene refining. When L/M is used as salting-out agent and the rude phenol is used as extracting agent, the waste acid from benzene refining could be effectively purified and reclaimed. The recovery of the waste acid and extracting agent attained 95%. The purified acid could be used as the raw material of LA production.(8) In the salting-out extraction process, the optimal separation conduction was investigated by orthogonal design. The turn of the effect was obtained as: the amount crude phenol > L/M > temperature.(9) In the salting-out extraction process, the main compositions of the recovered extracting agent had no significant difference from the crude phenol. The extracting agent could be reused.(10) Based on the experimental results, we proposed the solution to solve the problem in present coking wastewater treatment. The possible second pollution from the industrial application was also discussed. The operation costs were evaluated to be in the range of 0.3 - 1.8 $/m3 coking plant wastewater after biochemical treatment.
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