Study On Removal Of Low Concentration Chromium And Nickel From Wastewater And ORP In Chromium Reduction Process

The rapid urbanization and industrialization have caused serious water pollution which attracted great attention,especially the contaminants from the metallurgy,electroplating and other industries.Among the above mentioned industries,the heavy metals presented in wastewater,such as copper,zinc,chromium and nickel,are especially serious for water pollution.This subj ect focused on the treatment of workshop sewage that provided by an automobile factory in Shanghai.The sewage composed of 8 types of water sources,i.e.degreasing wastewater,defat waste liquid,electrophoretic wastewater,electrophoretic wastewater,spray painting wastewater,film wastewater and domestic sewage.In view of this problem,the removal of chromium and nickel from certain automobile factory was investigated in this study,the process of coagulation and sedimentation was optimized,and the existing technology was improved.The main factors of investigation including:1)The in:fluence of coagulating sedimentation process pH,coagulant dosage and coagulant types on the removal of turbidity,chromium and nickel.2)The application of oxidation-reduction potential(ORP)in controlling the reduction of Cr(Ⅵ)in real wastewater,and quantify the ability of target wastewater to restore Cr(VI).The pH is correlated with the ORP,thus the pH,as a main factor,is investigated.3)Studying the effect of membrane technology on further removal of chromium and nickel.4)Using heavy metal additives to treat chromium and nickel,the main factors are dosage and interference ions.5)In combination with the experimental results of the actual wastewater,the reduction process of Cr(Ⅵ)in the simulated wastewater was studied by the ORP,and two factors of the reducing agent and pH were mainly investigated.The data of simulated water and actual sewage were compared with theoretical calculation.The experimental results show that the effluents of the factory have a certain reduction ability,which can completely remove the Cr(Ⅵ)at initial concentration of 1.5mg/L and maintain a reaction redox potential of+50mV.The existing coagulating sedimentation process has a good removal effect on turbidity.Using coagulation tests of Polyferric and PAM,the optimum pH is 9.5,and the Cr(Ⅵ)of 1.5mg/L in the influent can be precipitated in the form of Cr(Ⅲ).Ni can hardly precipitate.At the initial Ni concentration of 1.0mg/L,the combined coagulation,precipitation,and membrane filtration can hardly meet the Ni effluent standard(Ni<0.1mg/L).And the effluent can meet the standard if only the influent Ni is less than 0.8mg/L.When the heavy metal capture agent(HMC-M2)was added at the dosage of 11mg/L,the effluent nickel concentration reached the effluent standard,but it could not meet the requirements of the project(<0.05mg/L).Copper did not affect the effectiveness of complementation agent,but zinc has great influence.The addition of heavy metal capture agent(5.0mg/L)with followed membrane filtration can achieve the effluent nickel concentration of 0.03～0.04mg/L when the influent nickel is 1.Omg/L.The subtitution of PAC for poly iron in coagulation sedimentation process could reduce the effluent nickel concentration to 0.05mg/L at the initial nickel of 1.0mg/L，which also slightly reduced the mud production.In experiments with simulated wastewater,Cr(Ⅵ)was reduced by three reductants,i.e.,sodium sulfite,sodium bisulfite and ferrous sulfate.As an optimal reducing agent,sodium bisulfite could maintain the pH at 2-3.The addition of 1.6 times stoichiometric dosage of sodium bisulfite could stabilize the redox potential to the range of 450～550mV,and completely reduce Cr(Ⅵ).For the reduction of Cr(Ⅵ)in sewage,the lower pH is favorable,the optimum redox potential is at 474～544mv,and pH is 2～3.The ORP technique has been used abroad for many years in controlling the oxidation and redox process of chromium and cyanide in sewage.However,there are not much reports and practical engineering applications in China.ORP automatic control technology is a simple and fast monitoring method to ensure the quality of redox treatment,and effectively reduce monitoring time and cost.In this study,the redox potential control method was used in the actual production of wastewater.It was verified by simulated water experiment and theoretical calculation,which was proved to be more accurate.It can provide reference for the application of ORP automatic control technology in engineering.