Influences of Various Aluminum Coagulants on Algae Floc Structure, Strength and Flotation Effect

Floc structure and strength play important roles in particles separation in water treatment. In a jar test of coagulation/flocculation/dissolved air flotation (C/F/DAF) process, aluminium sulphate (AS) and polyaluminium chloride (PAC) were used as coagulants to treat the algal-rich water which had a high concentration of Microcysits aeruginosa (MA) cells. Two-dimension fractal dimension (D2) and strength factor (SF) were used to represent the floc structure and strength respectively. The experimental results showed: meeting the appropriate dose range (measured in Al3+) simultaneously, dose of PAC was less than AS; and treatment effect obtained by PAC was better than AS. Adding the moderate PAC, the algae flocs with highly branched structure and higher strength formed by bridging flocculation; this kind of algae flocs could firmly adhere to more tiny bubbles and be significantly more resistant to the hydraulic shear. So, compared with adding AS, the performance of flotation separation could be improved by PAC.