Abstract: The results of previous studies indicate that anionic polyacrylamide (APAM) and cationic polyacrylamide (CPAM) do have bonding and bridging functions. A comparative and analytical study on the effects of APAM, CPAM and PAC (aluminium polychlorid) on adjusting the turbidity and organic matter of overlaying water of the of Erhai lakeshore sediments was conducted. The results of the study show that the turbidity removal rates by using CPAM and APAM were 53.7% and 36.55%, respectively, and the removal rates of dissolved organic matter (DOC) were 9.75% and 10.20%, respectively. High molecular weight substances (PAM) tend to increase the viscosity of water, which is not conducive for the destabilization and sedimentation of colloidal particles. With the increasing of the dosage of PAM, the turbidity and the organic matter concentrations did not decrease but increased. After conditioning the overlying water with PAC reagent, the removal rates of turbidity and dissolved organic matter were 85% and 20.87%, respectively. Results of three-dimensional fluorescence spectrum show that the overlying water had a high content of humus. After conditioning with PAC reagent, its characteristic peak intensity was reduced by 42%, which is much higher than the reduction rate of 8% with PAM reagent. The correlations between DOC and turbidity, COD and three-dimention excitation emission matrix fluorescence spectroscopy (EEM) (R²=0.905, 0.905 and 0.881) were much higher conditioned by PAC than by CPAM and APAM, indicating that charge neutrality has a better conditioning effect on DOC, such as humus, in overlying water. Study on the flocculation mechanism of the three coagulants/flocculants in the overlying water conditioning process can be a basis for future researches on the treatment of low turbidity water and the subsequent process.

Key words: overlying water, coagulant/flocculants (PAC,APAM and CPAM), turbidity, organic matters