Study On Compound Composition And Phosphorus Removal Of Domestic Sewage

In recent years, sewage emissions are increasing in our country, and the sewage of human waste, detergents, etc. contain large amounts of phosphorus. Research has shown that more than 0.02 mg/L of phosphorus can cause by eutrophication pollution. At present, the main methods of phosphorus removal are biological and physical chemistry method. But, biological methods do not meet the discharge standards, because of high phosphorus contents of sewage, However, the chemical precipitation has high phosphorus removal efficiency(75%~85%) and is stable, due to the use of coagulants which largely determines the innovation and development of water treatment technology, process simplification, cost of water and purification effect.Firstly, in order to provide reference for the coagulant used in wastewater treatment, phosphorus removal performance and CODCr removal effect of aluminum and calcium salt were investigated in this paper. Furthermore, coagulants were screened and compounded from the angle of removal effect, pH, economics, so as to reduce the cost of sewage treatment. Finally, orthogonal combined with step by step method was used to optimize three composite coagulant. The optimization of compound coagulant of phosphorus removal performance were carried on through Placket-Burman(P-B) experimental design and response surface method, in order to provides a new research ideas for coagulant compound.Coagulant performance test research shows that the optimum p H of Fe Cl3, Fe2(SO4)3, Fe SO4, PFS, AlCl3, Al2(SO4)3, PAC and Ca O were 5, 6, 8, 6, 7, 6, 7, 10 respectively. PAC dosage of effluent pH almost had no effect, but, AlCl3 and CaO had the most impact on effluent pH, the influence of the dosage of other coagulant on the effluent p H were listed successively as follows: Fe2(SO4)3>FeCl3>PFS>Al2(SO4)3>FeSO4; Under the same dosage and optimum pH, the effect of coagulant on phosphorus removal was as follow: FeCl3>AlCl3>Al2(SO4)3>Fe2(SO4)3>PFS>PAC>CaO>FeSO4; The CODCr removal effect of all coagulants was ranke d in the order of PAC>AlCl3>Fe Cl3>Fe2(SO4)3>PFS>Al2(SO4)3>CaO> FeSO4. The optimum hydraulic conditions of coagulation were mixing speed(150r/min), mix ed time(30s), flocculation speed(120r/min), flocculation time(15 min) and preci pitation time(30 min).The combination of two kinds of coagulant has showed that the optimum ratio of CaO and Fe SO4 was 3-2, based on which the CODCr and TP removal rate increased by 7.1%, 1.2%; 7.5%, 29.3% respectively than that of single coagulants. When PFS and AlCl3 4:1 were compounded, the CODCr and TP removal rate than single coagulant(PFS, AlCl3) increased by 10.4%, 16.7%; 12.1% and 0.6% respectively.Three coagulants were added into the samples in turn. The affected factors and the removal efficiency of the TP and CODCr for complex coagulant were investigated. The results showed that when the complex coagulant(AlCl3:FeCl3:Fe2(SO4)3) ratio was 5:6:7, the TP and CODCr removal efficiency were perfect, and pH was a key factor which affected the pollutants removal efficiency. When the dosage of the compound was 160 mg/L, TP and CODCr removal rate increased into 95.1% and 78.1% respectively, the effluent of TP concentration was 0.402 mg/ L, which achieved the level A standard of urban sewage treatment plant pollutant discharge standard, and the CODCr removal rate was 12.9~20.1% higher than the single coagulant(AlCl3, FeCl3 and Fe2(SO4)3) performed.Based on the results of single factor tests, three types of coagulants with promising effects on CODCr and TP removal were compounded. The P-B method was used to screen for the major factors influencing the performance of composite coagulants for CODCr and TP removal. The results demonstrated that: the primary influencing factors of TP removal were the initial pH of domestic sewage, AlCl3 dosage, and Fe2(SO4)3 dosage, while CODCr removal was significantly influenced by the initial pH and rotating speed. Furthermore, the method of steepest ascent was used to approach the optimal level of each factor. On the basis of the Box-Behnken model, response surface analysis was performed, thereby yielding the optimal level of each of the significant factors: an initial pH of domestic sewage of 5.2, AlCl3 and Fe2(SO4)3 dosages of 45 mg/L and 51 mg/L. At the optimal conditions, the experimental TP removal rate agreed with the predicted response value, thereby validating the feasibility of composite coagulant optimization based on a combination of P-B experimental design, steepest ascent design, and the response surface method. Compared to a single coagulant, the composite coagulant demonstrated more promising coagulation performance, and, compare with aluminum salt coagulants, reduced potential hazards to human health.The research results provide reference value in the field of sewage water treatment, and largely reduce cost. Furthermore, the result provides a new research idea for the compound coagulant as well as.