Combining Diatomite With Polyaluminum Chloride(PAC) For Enhancing Coagulation Performance And Reducing Contents Of Residual Aluminum

With the aggravation of the drinking water pollution in many areas,the traditional coagulation method can not fully meet the requirement of feed water treatment,to seek the safe,efficient and economic enhanced coagulation technology is imminent.However,there still exist some limitations in the studies and applications about enhanced coagulation technology.For instance,the most studies focus on improving coagulation performance and controlling disinfection by-products.The research on reducing residual aluminum content in the treatment of drinking water through enhanced coagulation technology recieves little attention.Aluminum is harmful to human body health,so the studies on residual aluminum in treated water attract more and more attention.However,most water plants and researchers only pay attention to the contents of total aluminum or dissolved aluminum at present,rather than the studies on other speciation distribution of residual aluminum.Therefore,we can't adequately understand the perniciousness from speciation and each component.This study selected the enhanced coagulation which diatomite combined with polyaluminium chloride(PAC)to treat the micro-polluted raw water.The feasibility and influence factors of the combining technology were mainly investigated: the content and speciation distribution of residual aluminum were analyzed in water after PAC coagulation;the removal efficiencies of turbidity,algae and organic matter in water were contrastively studied by combined coagulation and alone coagulation;the contents of total aluminum,total dissolved aluminum,dissolved organic aluminum,dissolved monomeric aluminum and dissolved organic monomeric aluminum were mainly discussed;the effects of initial pH value,addition sequence and hydraulic mixing conditions on coagulation performance and reduction of residual aluminum by the enhanced coagulation were investigated.The main conclusions were summarized as follows:(1)Using the enhanced coagulation combining diatomite and PAC can effectively improve coagulation performance and increase the reduction of residual aluminum.Diatomite played a role of coagulant aid.Coagulation performance significantly enhanced after adding diatomite.The best collocation of the combined coagulation was 15 mg/L PAC + 40 mg/L diatomite.(2)Compared with PAC coagulation alone,the peak of flocculation index(FI)was higher(about 1.3 and 0.8,respectively)when used the enhanced coagulation.It appeared faster floc growth rate and larger floc size in the process of the combined coagulation.(3)Initial pH value not only had a very large effect on coagulation performance,but also seriously affected the contents of residual aluminum in treated water.The optimal initial pH value was 7.99 or so for removing turbidity and algae,but it was about 5.97 for removing organic matter.When the initial pH value was 6.95 or so,namely near neutral environment,the reduction of residual aluminum was the biggest.(4)Addition sequence had an obvious effect on coagulation performance and reduction of residual aluminum when used the combined coagulation.The most appropriate addition sequence was when diatomite was prior to PAC.It can not only significantly improve coagulation performance,but also vastly reduce the content of residual aluminum.(5)Hydraulic mixing conditions had a huge influence on coagulation performance and residual aluminum.The optimal hydraulic mixing conditions for achieving the best coagulation performance and reduction of residual aluminum were: mixing intensity 200 rpm and mixing time 120 s in the rapid mixing phase;mixing intensity 50 rpm and mixing time 600 s in the slow mixing phase.