A Study On Treatment Of Hexavalent Chromium Containing Vastewater By Reduction-flocculation Process

With the commonly using of chrome in industrial production, and the emissions of chromium-containing waste water are gradually increasing, chrome has become one of maintain environmental pollutants. The existence form of chromium are mainly trivalent chromium and hexavalent chromium in the natural environment. Toxicity of hexavalent chromium is stronger than others, and it is recognized that is a kind of carcinogen, and having great harm for the health. The toxicity of trivalent chromium is not stronger as hexavalent chromium, and it is an essential trace elements for the human beings and animals.At present. there are a number of approaches including physical method, chemical method, physical and chemical method and biological method to deal with chromium wastewater, and among these methods the specific ones are osmosis process, floatation process, reduction and sedimentation process, activated carbon adsorption process, ion exchange method, electrolytic method and biochemical method, etc. This paper adopts reduction-flocculation process to deal with chromium-containing waste water. The main characteristics of this method are high efficiency, low cost, high processing speed, and suitable for dealing with serious pollution and emissions from big industrial wastewater. The former methods of chromium-containing waste water treatment usually adopt ferrous ion as reducing reagent. Because one ferrous ion can only provide an electron, so the effect is not quite significant through electronic ferrous ion itself to restore hexavalent chromium. The test uses FeSO4 and Na2S. two reducing reagents joint together, as well as utilize anionic polymers and cationic polymers combine with sodium sulfide (Na2S) and ferrous sulfate (FeSO4) to deal with hexavalent chromium containing wastewater. Compared with using only one flocculent, using two flocculents can not only through its chelating cooperation to purify hexavalent chromium wastewater, but also can use "bridging role" to reduce turbidity of waste material content. The advantages of this method are low cost, quick response, and water quality, and never produce secondary pollution.The experiment choose Na2S and FeSO4 as reducing reagent, and choose gelatin, PAM as flocculant, the experiment result lies on the two reducing reagents reaction concentration, the two of flocculant dosing reaction concentration and pH value. And the experiment use orthogonal experimental design method processing data, finally, we find the suitable reaction conditions for treatment of wastewater containing Cr6+ Through a series of experiment we get the research conclusion.Through orthogonal experiment, we qualitative analyse the influence of each experimental factor for the result of the experiment, find the several factors which can increase or reduce flocculation:FeSO4 dosing quantity, water pH value, Na2S dosing quantity, gelatin dosing quantity and PAM dosing quantity. Among them, through comparing FeSO4 dosing quantity, give the biggest influence of Cr6+ ion concentration in the experimental process, plays a key role; Na2S dosing quantity is just follow FeSO4 dosing quantity influence:Gelatin and PAM these two kinds of polymerization precipitation agent:the influence of both below Na2S, The influence of the pH value is the smallest.According to orthogonal experimental quantitative analysis, identified using reductive flocculation processing chromium-contained 20 mg/L wastewater the best reaction conditions:using Na2S as one step reductant, and dosing reaction concentration for 24.7 mg/L; For two step reductant concentration with FeSO4. dosing reaction for 25 mg/L; With gelatin and PAM for flocculating agent, dosing reaction concentration for 100 mg/L and for 10 mg/L respectively:PH value at nine or so.Flocculant with PAM collocation by gelatin, first add gelatin into the water, positive charge cationic of polymer gelatin can make the colloidal gelatin molecules take off instability, then, add shredded PAM, anionic polymer PAM anion make the negatively charged off the colloid particles, through strong bridge function and net catch function, the colloid particles quick trap flocculation, precipitation. So the alternate use of the settlement of the particle acceleration suspension, a very obvious accelerate solution clarification effects.Experimental results show that the good results of the flocculation treatment to processing wastewater containing Cr6+, according to the best conditions of processing data show that when containing Cr6+ for 20 mg/L concentration of waste water after treatment Cr6+ concentration of wastewater can reach 0.28 mg/L. removal rate was 98.6%. reach the national wastewater discharge standards.