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Special
Publication SJ2000-SP5
An
Evaluation of the Periphyton Filtration System at Lake Wade,
Orlando, Florida
by Carol Fall
A
periphyton filter consists of a series of flat surfaces, inoculated
with algae, over which water flows.
The algae and captured pollutants are periodically scraped
off and harvested with a vacuum.
To determine the applicability of this technology for
renovation of urban lakes, the St. Johns River Water Management
District (SJRWMD) and its contractors constructed, operated, and
monitored a periphyton filter at Lake Wade in Orlando, Florida.
SJRWMD
collected both weekly water quality samples and three diurnal sample
sets from January through December 1998.
The city of Orlando collected monthly samples at the center
of Lake Wade. A
subcontractor, Azurea, measured the periphyton standing crop every 2
weeks during 1998.
For
the first 5 months, periphyton biomass was curtailed due to
predation by chironomids. In
late May 1998, a biological insecticide, Bacillus thuringiensis
israelensis (BTI), was applied, which allowed accumulation of
greater algal biomass and associated pollutant removal during the
last 7 months or the project.
Weekly
treatment efficiencies were calculated, based on concentration at
the intake and discharge points.
For total phosphorus, treatment efficiencies were generally
slightly higher in the afternoon than in the morning, with a mean of
17.2% and 12.5%, respectively, for the duration of the project.
During the first 5 months, the periphyton filter often
exported phosphorus and only averaged 1.4% removal (afternoon
sampling). Following the application of BTI, treatment efficiencies
for total phosphorus averaged 28.6% (afternoon sampling) for the
last 7 months.
Overall
treatment efficiencies for ammonia nitrogen almost doubled, form an
average of 42.2% before application of BTI to an average of 82.7%
after application. A
similar trend was observed for nitrate ammonia, which averaged 27.4%
removal before BTI application and 42.5% after application.
The periphyton filter consistently reduced cadmium, copper,
and lead levels following application of BTI.
For
the sampling period (50 weeks), the periphyton filter removed a
total of 5.8 kg of total phosphorus, based on concentration
differences and flow. Following
application of BTI, 98% of the total was removed.
For the last 7 months of 1998, the periphyton filter removed
an average of 62.2 mg/m2/day, based on concentration
differences and flow. In
comparison, phosphorus removal rates averaged 71 mg/m2/day
for the last 7 months, based on biomass data collected by Azurea.
No
significant change in trophic status or nutrient levels was found in
Lake Wade.
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