Greenwater Tank Culture of Tilapia with the Effluent Used as a Source of Water and Nutrients for Terrestrial Crops
Construction of tanks was delayed because of deficiencies in liner material manufacture. Establishing a 1% slope of the tank floor and construction of the concrete block walls was completed. The liner came from the manufacturer with significant defects and was replaced. Construction of one tank was completed by midyear 2002. This tank was filled with water and stocked with tilapia. They are fed twice daily and sludge removed once daily. They are growing at a rate of 2.0 grams/day. The construction and installation techniques learned to build this tank will facilitate the construction of other tanks for the SARE project.
- Discover if greenwater tank culture production characteristics revealed in experimental units will be duplicated on a commercial scale.
Determine quality, quantity and value of terrestrial crops produced with greenwater sludge as a water and nutrient source.
Ascertain the economic viability and environmental sustainability of this integrated production technology.
One of three greenwater tanks was constructed, filled with water and stocked with fish. The construction of this tank took 2 years. A long process of evaluating materials and methods for constructing the tank accounts for this long period. Now that the best materials and techniques have been selected the construction of the 2 tanks on participating farmer’s land will be much more quickly accomplished. Problems encountered related to the grading of the tank floor, installation of the center cone and the modification of the HDPE liner to connect to the cone and to lie flat on the tank floor.
The graded soil floor of the tank needed to be sloped to the center. Several materials were attempted to make a smooth sloping surface. Soil cement, sand and sifted soil were all used. Of primary concern was to make an evenly sloped bottom without dips or areas where water or solids could puddle. Soil cement was first used because it would create a firm surface that would not compact. However the soil cement itself was rough surfaced and it was apparent that stone chips in the cement could puncture the liner material. It was removed. Sand was used because it could be compacted easily but the cost to do the entire bottom became prohibitive. Finally, sifted soil was used and compacted by with a plate compactor. Soil is one-third the cost of sand but had additional labor associated with the installation as it needed to be sifted and compacted.
The HDPE liner came from the manufacturer with many defects in the seam between the tank floor and the side walls. The defects were repaired with tape supplied by the manufacturer. The liner was ordered with a center cone of HDPE which also had significant defects. The HDPE cone was removed and a fiberglass cone was purchased and used in its place. Special industrial tapes had to be identified and purchased which would adhere to both the fiberglass cone and the HDPE liner.
The tank walls were constructed with knockout blocks, rebar and poured concrete. This was the simplest component of the tank to build.
The tank was filled with both well water and acclimated greenwater from existing culture tanks. It was stocked with 4,000 62 gram male tilapia. They have been fed ad libitum twice daily and sludge taken once daily. Two surface aerator have been operating, one vertically to aerate the water and one horizontally to circulate the water around the tank. Sludge that accumulates in the center cone is removed once daily and is stored in a containment pond. This sludge can be used for fertilization of vegetable crops.
The nitrifying bacteria involved in the suspended growth processes, which maintain water quality, take some time to develop into sustainable cultures. The acclimation period to develop viable cultures of nitrifying bacteria in the water column and on the tank surfaces was completed and a production schedule initiated. Tilapia fingerlings (62 grams/fish) were stocked and have grown to 214 grams in 76 days. A growth rate of 2.1 g/day.
Two more tanks will be built and stocked with fish for commercial production. Vegetable plots will be established near each of the 3 tanks and irrigated with sludge removed from the systems. Evaluation of systems’ commercial potential will be made.
Impacts and Contributions/Outcomes
Local farmers will benefit from this system because the dual use of water for both aquaculture and horticulture reduces their costs. The reclaimed nutrients from fish waste reduce the need for inorganic fertilizers and reduce those costs. Application of fish sludge to vegetable crops eliminates their discharge into local streams and any associated environmental impact that would result. A local non-profit group “Farmers in Action” is anxious for the results of this work.