Effectiveness of coagulants and coagulant aids for the removal of filter-clogging Synedra

The occurrence of Synedra spp. in the source water of the CheongJu water treatment plant (South Korea) decreased filter run times of rapid sand filters to below 5 hours. During the filter-clogging episode, full-scale Synedra removal by coagulation with polyaluminium hydrogen chloride silicate (PAHCS), flocculation and sedimentation ranged from 20 to 70%. To reduce filter clogging, strategies needed to be developed to improve the coagulation of Synedra. Jar test results showed that alum was more effective for Synedra removal than polyaluminium chloride (PACl), PAHCS and ferric chloride. At the optimum alum dose, Synedra removal in jar tests reached 88%. The addition of flocculant aids (Na- and Mg-alginate; nonionic and anionic polymers) in conjunction with alum did not improve Synedra removal. In contrast, the addition of cationic polymer in conjunction with alum improved Synedra removal to 99%. Analysis of floc with scanning electron microscopy (SEM) and light microscopy showed that the cationic polymer addition led to the formation of larger, stronger, and denser floc. More effective charge neutralization and the formation of interparticle bridges as a result of the cationic polymer addition can explain the improved incorporation of Synedra cells into settleable floc.