Improving removal of turbidity causing materials by using polymers as a filter aid

With recognizing that particles represent transport vehicles for undesirable chemical contaminants, and potentially disease causing microbial pathogens, the removal of particle materials becomes important to protect the public health. However, due to the complicated filtration mechanisms and interactions between the polymer and particles, the selection of polymers currently remains empirical. This study was initiated to investigate the feasibility of the use of polymers as filter aids for water containing lime softening particles; explore the roles of polymer characteristics in the filtration process; and examine major factors affecting the performance of polymer aided filtration. Seven representative polymers with different molecular weights and charges were tested using a declining rate filter pilot plant. The results show that at a starting flow rate of 15 m/h, the removal of turbidity particles could be significantly improved by using polymers as a filter aid. It was also found that the impacts of initial filter ripening could be substantially reduced and no turbidity breakthrough was observed after 3 days of operation. However, the use of polymer might significantly increase the filtration headloss, especially for the polymers with high molecular weight. To produce a high quality filtrate while ensuring the acceptable filtration productivity, low or moderately low molecular weight polymers are recommended. For a low molecular weight polymer, its optimum mixing intensity and polymer dose were found around 700 s−1 and 0.01 mg/l, respectively. On the basis of the results and particle properties, it is believed that interparticle bridging is the dominant mechanism underlying the interactions between polymers and lime softening particles.