Effects Of Chemical Phosphorus Removal On Decontamination Efficiency And Activated Sludge Performance In CASS Process

The survey results in the past 20 years showed that water pollution caused by eutrophication in water bodies in China has developed rapidly,which caused increasing damage to the water environment.In actual operation of the municipal wastewater treatment plant,the concentration of total phosphorus(TP)in the effluent of CASS process is generally in the 3.0-4.0 mg/L.It is impossible to control the concentration of TP in the effluent under 0.5 mg/L only by biological phosphorus removal without combining with other methods.Hence,to ensure that the effluent quality can reach class A in the standard of urban sewage treatment plant(GB18918-2002),chemical phosphorus removal is added together with biological phosphorus removal to further improve the removal rate of TP,thereby reducing the concentration of pollutants and reaching the standard requirements.The research object in this study was the CASS process.By studying the influence of the type and concentration of coagulant,the temperature of the reactor and process parameters on the conventional effluent index and activated sludge,the best type and concentration of coagulant as well as operating parameters at normal and low temperatures were determined.By applying economic analysis on different chemical phosphorus removal schemes,appropriate coagulants and dosages at different reaction temperatures were also obtained to ensure that the TP concentration of the effluent under this dosage was below 0.5 mg/L.The test results showed that at room temperature(20-30?),80 mg/L aluminum sulfate or 70 mg/L ferric chloride had the best phosphorus removal,with the effluent TP below 0.5 mg/L and other conventional indexes all reaching Level 1A requirements in standard GB18918-2002.When the concentration of aluminum sulfate was increased from 60 mg/L to 90 mg/L,the sludge volume index(SVI)changed from 80.5 mL/g to 358.2 mL/g,the specific oxygen consumption rate(SOUR)changed from 3.279 mg(O2)/(g(MLSS)·min)to 3.588 mg(O2)/(g(MLSS)·min),and Zeta potential changed from-2.35 mv to 0.87 mv,indicating that microbial metabolism in sludge increased and sludge sedimentation performance became worse after adding coagulant aluminum sulfate.When the concentration of ferric chloride increased from 30 mg/L to 70 mg/L,the SVI value of the sludge changed from 82.5 mL/g to 104.4 mL/g,the SOUR changed from 3.158 mg(O2)/(g(MLSS)·min.)to 3.488 mg(O2)/(g(MLSS)·min)and the zeta potential changed from-2.05 mv to-2.90 mv,indicating that the microbial metabolism of the sludge increased while sedimentation performance didn't change significantly after adding coagulant ferric chloride.The microbial community analysis of the sludge in the main reaction zone found that the abundance of microorganisms increased after dosing,and the proportion of dominant bacteria-proteobacteria increased while the proportions of Bacteroides and Greenbeads decreased.All these results above showed that coagulant FeC13 had less impact on CASS process compared with Al2(SO4)3Using a constant temperature cooling bath to reduce the temperature of the CASS reactor to 5?10?,the test results showed that although the low temperature would affect the microbial activity of sludge,it didn't affect the TP effluent quality significantly in low temperature since most of the TP was previously removed by chemical methods.Results showed that when the aluminum sulfate dosage was 90 mg/L and the ferric chloride dosage was 80 mg/L,the effluent TP could reach Class A standard(0.5 mg/L or less).Low temperature had no obvious effect on the sedimentation performance of microorganisms.The analysis of microbial communities also revealed that the abundance of microorganisms was almost unchanged compared to that at room temperature,and the proportions of proteobacteria and green Bentebone which did not adapt to low temperatures decreased while the proportions of thick-walled bacteria and acid bacteria which adapted to the sludge environment increased.In the new environment,those microorganisms that were not suitable for low temperature were eliminated and those adapted to the environment grew into microbial flora,which was beneficial to the stable operation of CASS reactors.