Greenwater Tank Culture of Tilapia with the Effluent Used as a Source of Water and Nutrients for Terrestrial Crops
Two harvests of fish were made producing 5,625 kg (12,375 lbs.) of tilapia. The selection of one farmer changed because the proposed site for the first participant farmer was determined to be unsuitable for construction of a fish tank. Permit applications have been submitted for construction of the two farmer tanks.
1. Discover if greenwater tank culture production characteristics revealed in experimental units will be duplicated on a commercial scale.
2. Determine quality, quantity and value of terrestrial crops produced with greenwater sludge as a water and nutrient source.
3. Ascertain the economic viability and environmental sustainability of this sound mergin of two very different forms of agriculture.
Production in the fish tank was 2,879 kg in the first production trial and 2,746 kg in the second. This is 14.4 kg/m^3 and 13.7 kg/m^3 respectively. In experimental trials a biomass of 17 kg/m^3 was achieved. High total suspended solids (TSS) and nitrate (NO3) could contribute to slower than expected growth. The range of these two water quality parameters in each trial is given below.
TSS mg/L (range) NO3 ppm (range)
Trial 1 220-1300 62-654
Trial 2 100-1960 182-707
Impacts and Contributions/Outcomes
Water quality factors contributed to the lower than expected production results. Nitrate levels increased throughout the trials, which have a slowing effect on fish growth. Removal can possibly be achieved with a denitrification tank that promotes growth of anaerobic bacteria that use nitrate. This tank will be constructed and tested in 2004.
TSS can be reduced with better clarification of the production tank water. The central cone in the tank does not function as expected. The use of 3 vertical prop aerators and 1 horizontal prop circulator keeps the solids in the tank in suspension and very little collects in the central cone. A 1,890-L (500-gallon) conical bottom clarifier was placed next to the production tank and a .5-HP pump was used to circulate water through it. This tank was installed one month before completion of the second trial and TSS was reduced from 1,744 to 600 mg/L. The removal of solids was, therefore, much more efficient with the external clarifier than with the central cone. This clarifier will be used through the entire production period in 2004.