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At present, the treatment of coking wastewater in China is usually the primary treatment including ammonia water dephenolization, ammonia distillation, final cooling water decyanation, etc., and the secondary treatment based on activated sludge method and its intensification method. With the increasingly stringent environmental protection requirements and the prominent contradiction of water resources shortage, it is extremely important to study the technical methods of advanced treatment of coking wastewater and the way of water reuse. At present, the advanced treatment technologies of coking wastewater mainly include membrane separation technology, coagulation sedimentation method, adsorption method, advanced oxidation (Fenton oxidation, O3 oxidation, catalytic wet oxidation, electrocatalysis, etc.) and biochemical method.
1. Membrane separation technology. The core of membrane separation technology is membrane. Its separation method mainly uses the selective permeability of membrane. The driving force mainly includes pressure difference, concentration difference and potential difference. Membrane separation is the general name of a series of membrane separation technologies such as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, gas separation, pervaporation, dialysis and electrodialysis. As a new type of fluid separation unit operation technology, membrane separation technology has the advantages of low energy consumption, obvious energy saving, no secondary pollution, high economic efficiency, high separation efficiency, small equipment volume, small floor area, less maintenance work, high reliability, and simple operation compared with traditional separation technology. In recent years, membrane separation technology has made great progress and has a wide range of applications. For coking wastewater, currently the double-membrane method (ultrafiltration+reverse osmosis) is mainly used for treatment, and the reverse osmosis produced water reaches the water quality standard of industrial circulating cooling water reuse. However, since the RO process only concentrates the pollutants rather than fundamentally removes them, it is still necessary to solve the problem of the direction of the RO concentrate, which has certain application limitations at present.
2. Flocculation sedimentation method. As a common physical and chemical method in the process of water treatment or purification, the principle of coagulation-sedimentation method is to add a certain amount of coagulant into the wastewater, so that the pollutants that are difficult to precipitate or filter in the wastewater can be aggregated into larger particles through physical or chemical action, so as to achieve the purpose of separation. At present, the commonly used inorganic flocculants include polyaluminum sulfate (PFS) and polyaluminum chloride (PAC), and the commonly used organic flocculant is polyacrylamide (PAM). At present, some new flocculants have been developed and utilized. For example, the effects of different flocculants on coking wastewater treatment have been studied. It is found that sodium ferrate has excellent decolorization and CODCr removal performance. For the advanced treatment of coking wastewater by flocculation, the removal effect varies greatly with different flocculants. Therefore, it is the key to develop cheap, efficient and secondary pollution-free flocculants.
3. Adsorption method. Adsorption method is a method that uses porous adsorbent to adsorb pollutants in wastewater and purify wastewater. At present, the commonly used adsorbents include activated carbon, zeolite, fly ash, fruit shell, etc. For example, the effects of zeolite and activated carbon on advanced treatment of coking wastewater were compared. The study shows that zeolite has a good removal effect on residual ammonia nitrogen, while activated carbon has a good adsorption and removal effect on refractory organic matter. In addition, the coking wastewater after biochemical treatment is treated with coal charcoal, fruit shell charcoal and coconut shell charcoal, which can make the effluent COD reach below 100mg/L. The coal charcoal is suitable for the reuse process, and the coking wastewater after secondary biochemical treatment is treated with (coagulation+activated carbon), which can achieve the effect of effluent COD ＜ 50mg/L. The author pointed out that coagulation pretreatment can reduce the organic load of activated carbon, facilitate the formation of biological activated carbon, and prolong the service life of activated carbon.
As well as using granular activated carbon and resin to treat coking wastewater after secondary biochemical treatment, mainly taking volatile phenol and COD as evaluation indicators, COD and volatile phenol removal efficiency is high, and after comparing activated carbon and resin, it is found that activated carbon has greater adsorption capacity and is more suitable for advanced treatment of coking wastewater biochemical effluent. Although the adsorption method has good treatment effect on coking wastewater, it is difficult to regenerate the adsorbent.
4. Advanced oxidation method. Advanced oxidation methods mainly produce hydroxyl radicals with strong oxidation through different methods（ ˙ OH), and then use its strong oxidizability to degrade the refractory organics in wastewater into biodegradable organic substances or small molecular substances, and even directly convert them into CO2 and H2O, mainly including Fenton oxidation, ozone catalytic oxidation, electro-catalytic oxidation, catalytic wet oxidation, etc. Fenton oxidation method uses hydrogen peroxide as oxidant and uses ferrous ion as catalyst to produce hydroxyl radicals. Many scholars have studied the advanced treatment of coking wastewater by Fenton oxidation technology, which can effectively reduce COD, and combine with flocculation sedimentation method to better remove COD and chroma. Ozone catalytic oxidation is the use of ozone as oxidant and the catalytic action of various catalysts (such as aluminum oxide as carrier loaded with metal ions) to produce hydroxyl radicals. Electrocatalytic oxidation process is to directly degrade organic matter through anodic reaction, or generate hydroxyl radical through anodic reaction（ ˙ OH), ozone and other oxidants degrade organics. This degradation pathway makes the decomposition of organics more complete and is not easy to produce toxic intermediate products. Although advanced oxidation technology has been paid attention to due to its high efficiency and no selectivity to COD, many studies have been done in the advanced treatment of coking wastewater, but its operation cost is relatively high.