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 tr eatment plant (South Korea) decreased filter run times of rapid sand filter s 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 c logging, strategies needed to be developed to improve the coagulation of Sy nedra. Jar test results showed that alum was more effective for Synedra rem oval than polyaluminium chloride (PACI), PAHCS and ferric chloride. At the optimum alum dose, Synedra removal in jar tests reached 88%. The addition o f flocculant aids (Na- and Mg-alginate; nonionic and anionic polymers) in c onjunction with alum did not improve Synedra removal. In contrast, the addi tion of cationic polymer in conjunction with alum improved Synedra removal to 99%. Analysis of flee with scanning electron microscopy (SEM) and light microscopy showed that the cationic polymer addition led to the formation o f larger, stronger, and denser floc. More effective charge neutralization a nd the formation of interparticle bridges as a result of the cationic polym er addition can explain the improved incorporation of Synedra cells into se ttleable floc.