Studies On Effect Of Residual Chlorine On Coagu-flocculation In Municipal Wastewater Reclamation And Reuse

"Pre-chlorized secondary effluent→coagu-flocculation,sedimentation→filtration→disinfection" is the most widely used technics in Wastewater Treatment Plant,while chlorized disinfection is a indispensable part.In the pre-chlorination process,residual chlorine has a great effect on the coagu-flocculation process,thereby following process and stability are affected.Taking secondary effluent from BeiShiQiao wastewater treatment plant as study object,size distribution of colloid particles,molecular weight distribution,floc observation and Zeta potential as research content,we discussed the impact mechanisms of residual chlorine on coagu-flocculation and sedimentation technics. The main results are as follow:(1)In coagu-flocculation experiments,the best water quality indicators were obtained when PAC and CPAM were 50mg/L and 0.25mg/L.(2)By the research of size distribution,molecular weight distribution of colloid particles and floc observation,the results showed that when the dosage of residual chlorine was below 2.12mg/L,With the increasing concentration of residual chlorine,coagulation effect was significantly enhanced.This was due to a strong oxidation of HClO and[O]in sodium hypochlorite solution which oxidated the organic pollutants that made dispersion medium stable,and also oxidated Fe²⁺to Fe³⁺.Fe³⁺played a role of enmeshment,thereby enhancing the treatment effect of the coagulation and sedimentation process.When the dosage of residual chlorine was above 2.12mg/L,With the increasing concentration of residual chlorine,coagulation effect was weakened.This was due to the reaction of Hoffman rearrangement between NaClO and PAM which created water-fast amine compound,so that increased the concentration of SS and meanwhile decreaces the effective PAM,thereby weakening the treatment effect of the coagulation and sedimentation process.(3)The Zeta potential of secondary effluent is about-45mv～-30mv.Change the dosage of residual chlorine and coagulant,the Zeta potential absolute value of coagu-effluent was almost unchanged.So we can conclude that the mechanism of collode sedimentation in secondary effluent is bridging flocculation and enmeshment.