Influence Factors And Mechanisms For Coagulation Removal Of Microcystis Aeruginosa From Drinking Water

Algal pollution has become a major problem in drinking water treatment inrecent years. As a result, it lead to the difficulty of water treatment and have harmfuleffect on human health. As a conventional unit in water supply treatment process,coagulation has been known as an efficient method in removal of suspended matternand macromolecular substances, e.g., algae and their extracellular organic matter(EOM). In the coagulation process of algae and EOM, removal efficiency andmechanism will been influenced by water quality and technological conditions. Thispaper chooses Microcystis aeruginosa, a common unicellular cyanobacteria ineutrophic water body, as a target alga in invstigation. The effect of different waterquality conditions on coagulation efficiency was studied with using polymericaluminium (PAC) as coagulant. Moreover, the effect of humic acid (HA), a mainnational organic matter, on coagulation efficiency and mechanisms was highlightedresearched. Furthermore, the effectiveness of three modified bentonite in coagulationremoval of the algae was compared. The main results of this paper are as follows:(1) Using low dosage of PAC (<30mg/L), raw water with high turbidity (13.8NTU) could promote the removal efficiency of chlorophyll-a (Chl-a) of M.aeruginosa than that with low turbidity. A highest removal efficiency of87.1%couldbe achieved when the dosage of PAC was20mg/L. The Chl-a removal efficiency ofalgae containing EOM was higher than that not containing EOM. A maximal value of66.6%for algae with EOM was obtained when PAC dosage was20mg/L. Chl-aremoval efficiency reduced gradually with the increase of ionic strength. The lowestremoval efficiency of Chl-a was only28.2%when the ionic strength of raw water was0.085mol/L.(2) An optimal removal efficiency of Chl-a was obtained at pH7.0-8.0by chargeneutralization with a low concentration of HA (2mg/L), while was achieved at pH6.0-7.0by charge neutralization, gathering and bridging process with a highconcentration of HA (8mg/L). The floc structure was more compact and bigger-sizedand the calculated fractal dimension (Df) was larger at a more optimal coagulationefficiency, and the variation of Dfconsisted with that of Chl-a removal efficiencyunder the same coagulation conditions. The maximum value of Dfwas obtained(1.853) at pH8.0with a low concentration of HA, while was achieved (1.704) at pH6.0with a high concentration of HA. (3) Compared with raw bentonite, sulfuric acid modified bentonite andmicrowave modified bentonite, the promotion of hydrochloric acid modified bentonitein coagulation removal of M. aeruginosa was most significant. The optimalpromotion conditions of f hydrochloric acid modified bentonite could be gainedthrough changing the dosage of PAC, the dosage of hydrochloric acid modifiedbentonite, the initial pH, the addition sequence and the sedimentation time. Theremoval efficiency of Chl-a and UV254could achieve94.0%and88.0%respectively,when30mg/L of the modified bentonite was fed first, and then20mg/L of PAC wasfed, the initial pH was7.0and the sedimentation time was15min.