February 15-18, 2009 Washington State Convention Center Seattle ...

TREATMENT OF RECIRCULATING WATER USING UV IRRADIATION ONLY OR A
COMBINATION OF OZONATION AND UV IRRADIATION
Steven T. Summerfelt, Mark Sharrer, Scott Tsukuda and Michael Gearheart
The Conservation Fund’s Freshwater Institute
1098 Turner Road
Shepherdstown, WV 25443 USA
s.summerfelt@freshwaterinstitute.org
Obligate and opportunistic fish pathogens can accumulate in recirculating aquaculture systems (RAS) that do not use a continuous
water disinfection process, especially during a disease outbreak when the micro-organism is propagating and shedding
from its host. Ozonation and ultraviolet (UV) irradiation processes have been used separately or in combination to treat water
in RAS before it returns to the fish culture tanks. The objective of the present study was to compare bacteria inactivation and
overall water quality produced in a RAS that includes either full-flow UV irradiation, a combination of ozonation and UV irradiation,
or no disinfection process (i.e., control). These respective test conditions were maintained for 12, 5, and 7 weeks in
a full-scale recirculating system located at the Freshwater Institute. System replication was not practical. During the O3 + UV
trial, a proportional integral feed back control loop was used to automatically adjust O 3 generator output to produce an oxidation
reduction potential (ORP) value of 447 ± 5 mV in the full recirculating flow exiting the 1.5 min contact tank, i.e., immediately
before the water entered the operational UV irradiation unit. The UV channel unit irradiated 100% of the 4,640 L/min
recirculating water with a dose of approximately 100 mJ/cm 2 just before the flow returned to the fish culture tank. Results
are tabulated below. The combination of O3 + UV produced complete inactivation of total heterotrophic bacteria plate counts
(i.e., 0 ± 0 cfu/ml), and nearly complete inactivation of total coliform bacteria counts (i.e., 1 col/100 ml) before the water was
returned to the culture tank. Achieving this level of treatment required adding a mean ozone dose of approximately 29 ± 3 g
O 3 per kg feed and dosing approximately 0.39 mg/L of ozone gas into the recirculating flow. Applying UV irradiation alone
reduced total heterotrophic bacteria plate counts to 5 ± 2 cfu/ml and total coliform plate counts to 112 ± 85 col/100 ml, which
was not as effective as O3 + UV. The best water quality was produced in the O3 + UV treatment. The TSS of water exiting the
UV channel averaged 8.2 ± 1.4, 4.4 ± 0.9, and 3.7 ± 0.8 mg/L, respectively, for the control, UV only, and O3 + UV treatments.
For the same three treatments, water color (and %UVT) exiting the UV channel averaged 12 ± 1 (89 ± 1%), 16 ± 2 (85 ± 1%),
and 3 ± 1 Pt-Co units (94 ± 1%), respectively.
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