Description and Assessment of the Surface Water Filtration and Ozone Treatment System at the Northeast Fishery Center
Steven Summerfelt, Julie Bebak-Williams, John Fletcher, Anthony Carta, and Duncan Creaser
Abstract.—A water filtration and ozone disinfection system was installed at the U.S. Fish and Wildlife Service’s Northeast Fishery Center in Lamar, Pennsylvania to treat a surface water supply that is used to culture sensitive and endangered fish. The treatment system first passes the surface water through drum filters operated with 60-m sieve panels in order to exclude the majority of debris, algae, and organisms larger than the sieve openings. After microscreen filtration, two variable speed pumps are operated in parallel to supply between 400 and 2,400 µmin to the ozone treatment system. Ozone contained within an approximately 95% oxygen feed gas is transferred in to the water (at 0.5–0.7 bar) through a downflow bubble contactor following each pump. The ozonated water is then collected and piped to a 15.1-m3 ozone contact column. The contact column provides approximately 20, 10, or 6.7 min of plug-flow contact time for water flows of 760, 1,500, or 2,270 L/min, respectively. A dissolved ozone probe at the outlet of the ozone contact chamber continuously monitors the dissolved ozone concentration discharged from the contact tank. A proportional- integral-derivative feedback control loop is used to adjust the concentration of ozone generated (and thus added) in order to maintain the dissolved ozone residual discharged from the ozone disinfecting contact tank at a preselected set point (nominally 0.2 mg/L). The water discharged from the ozone disinfecting contact tank then flows by gravity through a second 32.1-m3 contact tank, which provides additional time for the dissolved ozone to decompose. Any dissolved ozone remaining in the water exiting the second contact vessel is air-stripped, along with any large dissolved oxygen supersaturation, as the water flows by gravity through a forced-ventilated cascade column. This treated water then flows by gravity to the fish culture systems. The ozone system was evaluated during a start-up period from March through June of 2002. During this period, the ozonation and filtration system was found to consistently inactivate bacteria and exclude the majority of debris larger than the microscreen openings, even during extreme changes in surface water quality produced by storm events. Design and performance details are provided to offer insight into the strengths and weaknesses of the individual treatment processes.