Summer In Taihu Lake Algae Strengthen The Cod Coagulation Conditions <sub> Of Mn </ Sub> Removed

The enhanced coagulation effects of PAC/PDM composite coagulants prepared by polyaluminium chloride(PAC) and polydimethyldiallylammonium chloride(PDM),or AS/PDM composite coagulants prepared by aluminium sulfate(AS) on COD_Mn removal-rate to Taihu Lake algae-rich raw water in summer were studied.Two same pilot plants including the processes such as prechlorination,coagulation-sedimentation,sand filtration and disinfection units were conducted parallel in a water plant near Taihu Lake.One run under the condition using PAC or AS as coagulant,and the other run under the condition using PAC/PDM or AS/PDM stable composite coagulants.The COD_Mn of processed water in each treatment unit were analyzed and the COD_Mn removal rates for each treatment unit by using PAC or AS were compared with that by using their corresponding composite coagulants PAC/PDM or AS/PDM.At the same time,the enhanced coagulation effects of pilot tests were compared with that of jar tests,and the difference between those in pilot plant and in jar test were analyzed.The results showed that for the summer Taihu Lake raw water with COD_Mn of 6.36-14.9 mg/L and under the condition of prechlorination,when residual turbidity of 2-3 NTU for effluent water was achieved,the minimal dosage of using PAC as coagulant was 8-10mg/L. The total COD_Mn removal rate was 48.6%⁶0.4%and the COD_Mn removal rates for prechlorination,coagulation-sedimentation,filtration and disinfection unit were 8.35%, 51.2%,5.13%,3.59%,respectively.When composite coagulants were used,the total COD_Mn removal rate could raise 0%-2.9%and the COD_Mn removal rates for coagulation-sedimentation unit could raise 1.2%-4.7%compared with PAC used solely.The COD_Mn removal rates of filtration and disinfection increase hardly compared with PAC used solely.It could obtain similar effect when using PAC,i.e,2-3 NTU residual turbidity of effluent water when AS as coagulant with the dosage of 6.5-8 mg/L,and the CODMn-removal effect of using AS/PDM composite coagulant is better than that of using PAC/PDM composite coagulants.Otherwise,it was found that the enhanced coagulation ability of composite coagulants in pilot test was weaker than that of in jar test,which may be due to the weak coagulation intensity in pilot plant.Furthermore,the raw water for pilot test may contain the soluble reductive substances,which could be oxidized by KMnO₄ but not by chlorine,so the COD_Mnvalue after treatment could not be less than 3 mg/L.PDM composite coagulants could still work well under the weak coagulation intensity in the pilot plant.The main COD_Mn removal stage was the coagulation-sedimentation.To demonstrate the enhanced coagulation effects of the composite coagulants,stronger coagulation intensity in the pilot plant was required.