| Electrocoagulation(EC) is a kind of water purification technology which connected to an external power source, the sacrificial anodes will dissolve to produce metal ions to generate a series of polynuclear hydroxyl complex and hydroxide which can coagulate and remove contaminants in wastewater. EC has four functions of electroxidation, electroreduction, flocculation and flotation. EC methods offer advantages such as wide application ranges, high removal efficiency, much less secondary pollution, simple process and equipment, easy to control and operate,etc. It attracts much attention of researchers in world. However, the technology still possesses some disadvantages, such as low current efficiency and high energy consumption, so it limted to get a wide range of pulp and paper effluent applications. Discussing electrocoagulation mechanism for paper effluent and looking for suitable electrode materials, which are the problems to expand application in papermaking wastewater treatment.Three kinds of different wastewater from a papermaking factory in Shaanxi were studied by electrocoagulation treatments. The impact of electrode material, current density and electrolysis time on removal efficiency of CODCr and removal efficiency of chroma were studied. The electrolysis mechanisms of organic pollutant and removal process of inorganic pollutant were discussed. And processing cost was measured and calculated. The main contents and results are as follows:1. The cathode and anode electrode materials were optimized. This is the main influence factor of electrocoagulation in treating wastewater. The raw papermaking wastewater were treated by electrocoagulation, which graphite, pure aluminum and so on were used as the anode, titanium plate, stainless steel plate and so on were used as cathode.The removal efficiency of CODCr(Chemical Oxygen Demand) and the removal efficiency of chroma were comparativied and analysed, which adopts different combination of anode and cathode for the same raw papermaking wastewater. The results show: The removal efficiency of CODCr and the removal efficiency of chroma were 61% and 97.5% respectively when the pure aluminum and titanium plate were combining used, significantly higher than other combinations. So the electrode combination is selected.2. Electrocoagulation process was optimized. The different pollution loads of wastewater used in this study were obtained from a papermaking mill in Shaanxi Province and the wastewaters were treatmented by electrocoagulation with aluminum and titanium electrodes respectively. The effects of current density and electrolysis time on the removal efficiency of CODCr and the removal efficiency of chroma were analysed. The main resluts were as follows:(1)When current density is 12 m Acm-2, electrolysis time is 90 min and plate spacing is 20 mm for the raw papermaking wastewater,the removal efficiency of CODCr is 62.4%, the chroma removal efficiency is 99.2% and removal efficiency of SS is 100%. The consumption of aluminum electrode is 0.769 kg per m3 of wastewater and the power consumption is 21.4k W?h. The costs of Aluminum consumption and electric power consumption is about ￥25.5 per m3 of wastewater.The CODCr is 278 mg/L, which does not meet requirement of standard of GB3544-2008.(2)When electrolysis time is 60 min,current density is 13 m Acm-2, and plate spacing is 20 mm for the effluent of primary sedimentation,the removal efficiency of CODCr is 89.4%, chroma removal efficiency is 98.6%, and removal efficiency of SS is 100%. The aluminum electrode consumption is 0.569 kg percent m3 of wastewater and the power consumption is 16.3k W?h. The costs of Aluminum consumption and electric power consumption is about ￥20 per m3 of wastewater.These indicators meet the requirement standard of GB3544-2008.(3)Because wastewater standard of paper enterprises is raised, EC is used as an advanced treatment menthod to treat the effluent of secondary sedimentation. In electrolysis time of 45 min, current density of 3.17 m Acm-2, plate spacing of 10 mm, the removal efficiency of CODCr is 93.8%, the chroma removal efficiency is 82.4%, removal efficiency of SS is 100%, the removal efficiency of NH3-N is 23.9%, and the removal efficiency of AOX is 63%. The aluminum electrode consumption is 0.075 kg per m3 of wastewater and the power consumption is 0.44 k W?h. The costs of Aluminum consumption and electric power consumption is about ￥1.43 per m3 of wastewater. These indicators meet the standard of GB3544-2008.3. The reaction behaviors of the organics for EC process on the effluent of secondary sedimentation were studied. Using of electrooxidation and electroreduction reaction ability of certain radical group in the cathode and anode plate surface, the changes of organic components before and after wastewater treatment were analysised.(1) The organics of the treated wastewater were analyzed by GC-MS analysis for the effluent of secondary sedimentation before and after by electrocoagulation treatment. The type of phenol reduced from 4 to 0. So the chroma and toxic of effluent were reduced greatly. The type of alkanes changes from 29 to 30 and the relative content increased from 45.628% to 73.553%. The type of alkenes reduces from 9 to 5. The alkenes will cause addition reaction to transform alkenes into alkanes on the titanium cathode. So the relative content of alkanes increased. The type of esters reduced from 12 to 6. The type of alcohols increased from 6 to 9 because of the hydrolysis reaction of alcohols reacted.(2) The organic content of flocculation is about 35.78% while the organic content in the attachment on the titanium plate is only about 5.05% by TGA-GC-MS analysis for the flocculation and attachment on the titanium plate. Some radical group such as nitro group, amides group and azoic group can be adsorbed to cause reduction reaction then form the amine groups.These amine group are adsorbed on the surface of titanium plate and part of amine group fall off due to impacting of hydrogen gas bubbles on plate surface. The fallen amine group were captured by aluminum hydroxy complexes and constituted major part of organic matter in the flocculation, so that CODCr and content of ammonia nitrogen in effluent reduced. The halogen in the halogenated substances can not only be substituted by hydroxyl free radical through the anodic oxidation, but also can be replaced by hydrogen atom through the cathode reduction, so that the AOX value is reduced. It is shown that the titanium plate is not only prone to release and form a large number of tiny bubbles to remove suspended matter by electricfloating effect. Some organic pollutants can be absorbed to titanium plate surface and produce reduction reaction, which will cause degradation to small molecules or carbon dioxide and water. So that CODCr, content of ammonia nitrogen and color of water were reduced.4. The reaction model is established for the inorganic ions in electrocoagulation treatment process. When a large number of anions gathered in reuse wastewater, it will lead to the anionic trash pollution.The types and contents of inorganic ions in wastewater before and after EC treatment were determined by atomic absorption spectrophotometry, silver nitrate titration method or barium chromate spectrophotometry. Fe2+ which is in the wastewater can be deposited on the surface of cathode. Fe2+ and OH- can form hydroxy complex to purify water with aluminum complex compounds, too. Parts of the Ca2+ and Mg2+ in wastewater can bind CO32-, SO42- and OH- to formate deposition on the cathode. Parts of the Ca2+ combine SO42- and CO32- to form deposition which is flocculated by aluminum complex. Mn4+ and Cu2+ can be removed by co-coagulation of aluminum ion hydroxy complex. There are almost no metal ions such as Al3+, Fe2+, Mn4+ and Cu2+ in wastewater after treatment. So the residue problem of Al3+ which some researchers worries about will not happen.5. The effect of electrocoagulation process on the aluminum plate life was studied. Electrochemical behavior of aluminum is the most important factor for electrocoagulation treatment cost. FE-SEM of the aluminum plate showed that the electrolysis is not uniform on the surface of aluminum plate during EC treating paper effluent. Pitting corrosion formatted due to Cl- and the longer time lasts, the deeper corrosion pit develops. Cl ion is adsorbed on the defects of passivating film by convection and diffusion in the wastewater, then chemical reaction occurred between Cl ion and passivating film to generate soluble Al(OH)2Cl2-. So the passivating film is damaged to dissolve locally. The selective corrosion such as exfoliation corrosion can occur, too.Based on the above research results, the effluents of another paper mill in Shaanxi and university campus sewage were treated by this electrocoagulation experimental device. CODCr, content of ammonia nitrogen and color meet requirement of standard of GB3544-2008.In addition, Compared with the Fenton oxidation method which is used by an advanced treatment methods for paper effluent commonly now, EC has excellent characteristics of high removal efficiency of chroma, no chemicals, simple process, and the treated wastewater without any residues of Al3+, Fe3+, Fe2+. The cost of EC is lower than the traditional Fenton oxidation method in treating paper effluent.So EC can be applied as advanced treatment methods to treat the effluent of secondary sedimentation of paper mill, it has certain advantages regardless of the treatment effect and the treatment cost. |
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