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

190
USE OF SETTLING TANK CLARIFIERS TO MANAGE THE MICROBIAL BIOFLOC
COMMUNITY IN SUPERINTENSIVE, ZERO-EXCHANGE SHRIMP PRODUCTION
SYSTEMS
Beth L. Lewis*, John W. Leffler, Andrew Ray, Heidi Atwood, Jason Haveman, Alisha Lawson, Brad McAbee,
Andrew Shuler, Jesus Venero and Craig L. Browdy
SCDNR Waddell Mariculture Center
211 Sawmill Creek Road
Bluffton, SC 29910 USA
ThomasBL@dnr.sc.gov
Superintensive, zero exchange, greenhouse-based systems offer many advantages for U.S. shrimp farmers including multiple
crops per year, reduced pollutant effluents, and reduction of harmful shrimp pathogens. Water quality is dependent upon the
rich microbial community composing and living on the abundant particulate matter that dominates these systems. In both
raceways and large outdoor tanks, studies at the Waddell Mariculture Center have used settling tank clarifiers to manipulate
this “biofloc” community. This “cropping” removes the nutrient and organic-rich biofloc particles from the water column and
they are collected as sludge in the clarifier until periodic removal. The settling tank clarifiers use an air-lift to pump water into
a settling chamber where it’s flow rate slows, allowing suspended solids to settle. Low particulate water returns to the shrimp
tank by gravity.
Weekly measurements demonstrated that the clarifiers were effective in reducing significant amounts of total suspended solids
(TSS). Since most of the biofloc is biologically formed within the system, volatile suspended solids (VSS) followed a very
similar pattern as TSS. Although TSS may be the most direct measure of clarifier effectiveness, it requires at least half a day to
obtain results. For routine management of the clarifiers, a simple field parameter is required for daily management decisions.
Turbidity is a complex parameter that is difficult to interpret biologically and may not work with all biofloc communities.
However we found it to be an effective management tool for operating the clarifiers on a routine basis. Research is currently
underway using a terrestrial and an underwater sensor to simultaneously measure photosynthetically active radiation (PAR)
and to report percent penetration and light extinction coefficient almost instantaneously. This appears to an easily applied,
biologically interpretable parameter for managing clarifiers.
Cropping significantly reduces suspended solids. In some cases autotrophy increases dramatically as measured by total
spectrophotometric chlorophyll and by epifluorescent microscopy with computer assisted image analysis. This probably
results from increased light penetration. In other cases, cropping had no significant effect on total chlorophyll concentrations,
primarily due to an abundance of Oosystis sp. dominating the biofloc. Based on several studies we have conducted, proper
biofloc management through cropping tends to increase shrimp growth and leads to reduced dissolved oxygen demand, reduced
dissolved nitrate, and reduced bacterial and cyanobacterial densities.