The effect of loading rate on tertiary wastewater filtration.

摘要

his study aimed to improve our scientific understanding of the role filter loading rate plays in filtration performance, through pilot-scale filter testing, followed by full-scale filter testing at several California water recycling plants, while also examining how coagulation and particle association affect virus removal through filtration.;The impact of loading rate on tertiary filtration of wastewater was studied using a pilot-scale, dual-media (sand and anthracite), rapid-depth filtration system. Loading rates of 5, 6.25, 7.5, 8.75, and 10.0 gal/ft2-min (12.2, 15.3, 18.3, 21.4, and 24.4 m/h) were tested on parallel filter columns treating the same coagulated secondary wastewater to determine the impact on removal of turbidity, particles (2-15 mum), total coliform bacteria, E. coli, and seeded MS2 bacteriophage, as well as on the particle deposition profile in the filter bed. For all conditions studied, loading rate had no observable impact on the ability to disinfect filter effluents with chloramines. The results of the pilot-study provided support and justification for testing loading rates higher than 5 gal/ft2-min at full-scale water recycling facilities in California.;Full-scale granular media filter studies comparing loading rates of 12.2 and 18.3 m/h (5.0 and 7.5 gal/ft2-min) were conducted at water recycling facilities in Monterey County and San Jose, CA. At the Monterey plant, 51% more coagulant was required at the higher loading rate to achieve equivalent 5-15 mum particle counts in the effluent (no difference between rates); with this coagulant dose the effluent turbidity and 2-5 mum particles were significantly lower at the higher loading rate (22 and 19%, respectively). At the San Jose plant, where no coagulant was added, increasing the loading rate from 12.2 to 18.3 m/h had minimal impact on the filter effluent quality (5.4, 3.1, and 11% increases for turbidity, 2-5 and 5-15 mum particles, respectively while operating at 18.3 m/h). No decrease in the ability to disinfect the filter effluent was observed at either plant.;The effect of coagulation on virus-particle association and removal of viruses through filtration was studied using three bacteriophages with varying isoelectric points. MS2, PRD1, and phiA174 bacteriophages were co-seeded into bench-scale jar tests and different coagulant types (aluminum chlorohydrate (ACH) and two different cationic polymers) and doses (0-100 mg/L) were applied. When properly coagulated, MS2 and PRD1 were associated with positively charged coagulant precipitates, which in turn were strongly attracted to the negatively charged filter media. In the absence of coagulant, the negatively-charged unassociated viruses still exhibited forces repulsive to the filter media as it passed through the filter. This is the first study to show that removal of viruses through tertiary wastewater filters was a result of association with particles formed during coagulation.;The results from this study have significant implications for California water recycling regulations and water reuse economics. Based on the findings, the Monterey Regional Water Pollution Control Agency was granted a permanent waiver from the 5 gal/ft2-min limit and are now able to operate water recycling filters at 7.5 gal/ft2-min, which has expanded plant capacity by 50% and saved approximately