| Microbial flocculant has the excellent characteristics of being biodegradable and non-toxic harmless, however, its large dosage of flocculants and expensive costs make it being restricted in the mass production and application in industry. The combination of microbial flocculants and chemical flocculants works remarkably better than only using one kind of flocculants, which can also reduce the quantity of reagent, while saving costs, it can also eliminate or decrease the harm to water environment caused by the chemical flocculants.In order to make the microbial flocculants widely used in sewage treatment, this paper studies the nature and best test condition of microbial flocculants(MBF) and poly aluminium chloride(PAC) when used alone and together in secondary effluent. The software Design- expert 8.0.5 was used to make model fitting on the results of DOC removal rate when using combination of coagulants dealing with secondary effluent, the coagulation mechanism of combination of coagulants is discussed, and the influence of coagulation and ultrafiltration after using different coagulants is primarily studied. The following conclusions are made through the above test:(1) Under the circumstances that the raw water’s pH value is 7, the dosed MBF quantity is 20 mg/l, quickly stirring it with the intensity of 200 r/min for 60 s and slowly stirring it with the intensity of 40 r/min for 10 minutes,the removal rate of the secondary effluent DOC is 14%, the removal rate of UV254 is 17%, and removal rate of turbidity is 30%. When PAC is dosed in quantity of 30 mg/l, quickly stirring it with the intensity of 200 r/min for 60 s and slowly stirring it with the intensity of 30 r/min for 10 minutes, it can reach the optimal treatment effect, while the DOC removal rate is 25%, the removal rate of UV254 is 30% and the removal rate of turbidity is 41%. The quantity in the combination of coagulants of PAC and MBF are 20 mg/l and 10 mg/l respectively, whose intensity of quick stirring is 200 r/min for 60 s and that of the slow stirring is 40 r/min for 5 min, where the coagulate effect can reach the best, the DOC removal rate is 33%, the UV254 removal rate is 43%, and the turbidity removal rate is 52%.(2) This test got the quadratic response surface model of DOC removal rate, which was significant. In this model, the dosage of MBF and PAC as well as the slow stirring time are all the significant factors in the one degree term, while the initial pH value is the non-significant factor. In the quadratic term, the significant factors above mentioned are still significant. In the interactive term, the dosage of MBF and PAC as well as the slow stirring time are all significant.(3) It further proves that the combination of coagulants has the functions of absorbing bridge and sweeping flocculation by scanning the electron microscope, and the adding of MBF is helpful to promote the small flocs to grow into the big flocs, which make the combinations have a better coagulating effect. In the process of combination of coagulants, the MBF and secondary effluent components were subject to the role of physisorption.(4) According to the effect of coagulation- ultrafiltration on the secondary effluent treatment and the influence of coagulation- ultrafiltration on membrane pollution, when adding MBF separately, the DOC removal rate is 22%, the removal rate of UV254 is 33%, and the turbidity removal rate is 80%; When adding PAC separately, the DOC removal rate is 40%, the removal rate of UV254 is 49% and the turbidity removal rate is 87%; When adding the combination of coagulants, the DOC removal rate is 48%, the removal rate of UV254 is 53% and the removal rate of turbidity is 85%. When adding the MBF only, the membrane flux declined fastest during the ultrafiltration phase, followed by the combination of MBF and PAC, and the declining is slowest when adding the PAC alone.(5) The compound of MBF and PAC in this experiment can reduce the dosage of PAC under the same treatment condition and effect. |
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