EMPLOYING PRACTICAL METHODS OF MEASURING SOLIDS IN BIOFLOC DOMINATED SHRIMP PRODUCTION RACEWAYS IN A COMMERCIAL APPLICATION

Measuring solids in biofloc culture systems is important for several reasons. In general, high solids levels are indicative of poor culture conditions possibly resulting from overfeeding or inadequate solids removal. This excessive organic material can encourage the development of pathogenic bacteria and also increase oxygen demand. High solids concentrations can cause gill fouling, leading to respiratory distress or even death. On the other hand, reducing solids levels too low will inhibit nitrification.  It is therefore necessary to maintain solids levels within a targeted concentration. The two most common types of solids discussed relative to biofloc systems are: settleable solids (SS) and total suspended solids (TSS). Methods for the measurement of both are described in detail in the APHA Standard Methods for the Examination of Water and Wastewater.  However, as described both methods are beyond what may be practical on many farms. The purpose of this presentation is to (1) share one farm's solutions/adaptations for practical solids measuring in a biofloc system, and (2) hopefully stimulate discussion for further development of field expedient methods for the measurement of solids at the farm level.  

As previously stated the Standard Methods methodology for the measurement of SS and TSS are beyond what some managers would consider practical. In the case of SS the major constraint is time. As described this method requires one hour per sample.  We have assumed that the bulk of the solids we are interested in will settle within the first 10-15 minutes. In addition we may employ several methods to enhance the settling process, such as tapping, twisting, or slow stirring. As for TSS, the Standard Methods not only requires a lot of time but also some specialized equipment and materials not readily available. In our case TSS measurements are obtained indirectly using a turbidity probe (ProDSS, YSI Inc., Yellow Springs, OH). Using the instrument, turbidity readings are taken on at least six water samples from different culture tanks. Corresponding water samples are analyzed by a local laboratory (City of Cape Coral Environmental Resources Laboratory, Cape Coral, FL). The laboratory TSS values and measured turbidity values are entered into the instrument's software, which creates a correlation between the two data sets.  The instrument uses this correlation to derive TSS estimates in mg/L from turbidity readings.  The combined results of the two methods (SS and TSS) are used to manage solids concentrations of culture water within targeted levels.

The methods described in this presentation, while admittedly less accurate than those prescribed in Standard Methods, allow measurements to be taken more frequently. In other words we are less concerned with the absolute accuracy of the results as much as timeliness and the ability to detect and manage changes in the system from day to day. This presentation will include actual data to demonstrate the effectiveness of these two methods for controlling solids in biofloc dominated raceways.