Research On The Treatment Of Electroplating Wastewater COD By Chemical Method

Electroplating wastewater not only contains heavy metal ions, but also contains anamount of COD (chemical oxygen demand). High COD will affect the self-purificationcapacity of water, lead to lakes and wetland degradation and the massive death ofaquatic life, and break the water ecological balance, which will affect the energy flowand material cycle of the whole ecosystem. At present, the biochemical method is thebest methods which can make electroplating wastewater COD meet emissions standardsat home and abroad. Biochemical method needs a lot of upfront investment, and thebiological species is sensitive to heavy metals, and biological bacteria culture conditionsis strictly, and Biochemical method is suitable for continuous wastewater treatment, it’sdifficult to widespread generalize this method in small and medium-sized electroplatingenterprises. Now, there is no other mature and effective method can treat electroplatingwastewater COD effectively at home and abroad. In order to solve the problem oftreating small and medium-sized enterprises electroplating wastewater COD, in thispaper, the chemical treatment of electroplating wastewater COD has been researchedfully and comprehensively. We chose simulate electroplating wastewater and the actualelectroplating wastewater as the research object, and used the flocculation process,chemical oxidation, chemical catalytic oxidation on COD removing, and explored theremoval efficiency of each process, influencing factors and mechanism of each methods,and found a suitable method for small and medium-sized enterprise electroplatingwastewater COD removing.Simulative electroplating wastewater(COD200～300mg/L)treatment research foundsome basic date of flocculation, chemical oxidation method and oxidation-flocculation.The results showed that the best dosage of polymeric ferric sulfate(PFS), polyaluminiumchloride(PAC),Polyacrylamide(PAM) was200mg/L,200mg/L,10mg/L, and the best pHwas8,7,7,respectively. Compared with PAM, PFS and PAC have better performance inCOD removal, but the amount of sludge was much higher than the former, so as theprocessing cost and secondary pollution. Using PAC and PAM as a combined flocculants(mass ratio(1:5)～(1:10))and treating with the own designed two-steps process (adjustpH8～9-flocculation-adjust pH10～10.5-flocculation), the COD removal efficiencyincreased signally, and the quantity of sludge and moisture content of sludge decreased alot. As a consequence, the costs of sludge treatment fall off, and so as the secondarypollution. At the same time, we solve the problem of the heavy metal removal can’t reach standard stability by traditional process. Flocculation method could only degrade a littlepart of organics which can be flocculated easily, and has no obvious effect on the restorganics. Chemical oxidation method can oxidize organic matter and the reducing agents;it can make up the defects of flocculation method. As we all know, Hydrogen peroxide(H2O2) and sodium hypochlorite (NaClO) were widely used in wastewater treatment.The experiments showed that hydrogen peroxide is weaker of removing COD thansodium hypochlorite in simulated electroplating wastewater treatment process. For thewastewater containing heavy metal ions, after oxidation process, a flocculation processwas needed. It’s proved that when the dosage of sodium hypochlorite was30mg/L, theoxidation reaction pH6, oxidation assisted with two-step process flocculating systemcould guarantee the simulated electroplating wastewater emission standard(GB21900—2008Table2,COD80mg/L).Actual electroplating wastewater treatment study found that the COD of copperwastewater and chromium wastewater, nickel wastewater, acid and alkali waste waterand comprehensive wastewater were not high. They concentrated within250mg/L.Degreasing wastewater and a variety of surface active agents, additives were the mainsource of COD. Compared with the simulated wastewater, the pollutant composition ofactual electroplating wastewater was more complex, the pollutants mutual influencemade the treatment processing more difficult, and therefore, there were some differencein heavy metal ions and COD removal efficiency. Experimental results showed that afterthe four kinds of actual electroplating wastewater (copper, chromium, nickel, acid andalkali, and comprehensive wastewater) treated by oxidation-flocculating method, theheavy metal content were lower than the national emission standard completely, andonly the COD of acid and alkali wastewater was slightly higher than the emission values(GB21900—2008Table2,COD80mg/L).In order to solve the problem of actual electroplating wastewater COD treatment,this experiment used chemical catalytic oxidation deal with the COD of electroplatingwastewater. The oxygenant was NaClO, and the activated carbon-metallic oxide(C-MOx)catalyst has been prepared by the dipping-calcination method with active carbon used asa carrier. The experimental results show that when the composition of the dippingsolution was20～25%manganous nitrate and5～10%Copper nitrate–trihydrate and1～2%nickel nitrate-hexahydrate and0～0.5%Cerium nitrate-hexahydrate and1%Cobaltous nitrate-hexahydrate, the catalyst made by Segmented heat curing was highlyactive and has long service life. The chemical oxygen demand (COD) removal efficiency by catalytic oxidation was86.13%at the condition of electroplating wastewater’s CODwas175.5mg/L, the pH was6, the dosage of NaClO was30mg/L, the C-MOxcatalystwas2.5g/L by reacting3h. The COD of electroplating wastewater after treatment was24.34mg/L, which was far below the National emission standard minimum value (GB21900—2008Table3,COD50mg/L).The C-MOxcatalyst can be used several times, whichindicated that this catalyst is practical.