Comparison of three coagulants by online turbidity monitoring

The removal of suspended algae by coagulation with polyaluminium chloride, polyaluminium silicate chloride and polyaluminium ferric silicate chloride was investigated by online turbidity monitoring. The experimental results indicate that although a smaller (compared with polyaluminium silicate chloride and polyaluminium ferric silicate chloride) polyaluminium chloride dose is required for coagulation, polyaluminium chloride is less efficient in producing large and settleable flocs. Thus, for both polyaluminium silicate chloride and polyaluminium ferric silicate chloride, which are hydrolytic co-polymeric products between metal ions and polysilicic acid, the efficiency of algal removal is enhanced by activated silicic acid. However, polysilicic acid may also decrease the positive charges carried by polyaluminium silicate chloride and polyaluminium ferric silicate chloride. Therefore, increased doses of polyaluminium silicate chloride and polyaluminium ferric silicate chloride are needed to flocculate the algal suspension effectively. Additionally, a new online nephelometric turbidimeter connected to a data-acquisition unit was used in this study for continuous measurement of the coagulated sample turbidity every second during rapid mixing, slow mixing and sedimentation. The experimental results thus obtained provided much better and more detailed information than conventional jar test results.