Greenwater Tank Culture of Tilapia with the Effluent Used as a Source of Water and Nutrients for Terrestrial Crops
Construction of a circular 200 m3 tank was completed with the installation of a HDPE liner (30 mil) and central fiberglass cone for the collection of solids. The tank was stocked with 4,000 tilapia fingerlings in June 2003. The fish were fed daily with a floating pelleted feed. Water quality was maintained with acceptable levels of dissolved oxygen through aeration and acceptable levels of ammonia and nitrite by reducing feed input when water samples measured showed upwards trends. The fish were harvested in February 2003 and had a production rate of 14.4 kg/m3, 29 times higher than typical pond production rates.
- 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.
Construction of the first fish culture tank was completed at the UVI site. This has taken several years as different material and methods of construction were evaluated and revised. The best materials and method of construction is now known. 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. (The fish were harvested on February 28, 2003. Final biomass was 2,879 kgs or 14.4 kgs/m3. This is 29 times higher than typical production from pond systems. Survival, 79%, was lower than expected due to bird predation. Better survival would have yielded even higher production values.)
The pH of the water was monitored daily and adjusted to 7.5 with the addition of calcium hydroxide as needed. Other water quality parameters (ammonia, nitrite, nitrate) were monitored weekly. Surface aerators (1 to 3) were installed to aerate the water. Initially one aerator was installed. As the fish grew and the biomass increased a second and then a third were added. The use of multiple aerators was used to reduce the amount of electricity required. One horizontally oriented aerator was used to circulate the water around the tank. The circulation concentrates solid waste into the center cone, from which it can then be removed. The circulation also enhances mixing of all the water in the tank, ensuring that there are no zones with low dissolved oxygen or concentrations of waste. The mixing and circulation of the water by the aerators created an environment where nitrifying bacteria, involved in the suspended growth processes of water quality management, was optimized.
Work continued on obtaining VI Department of Planning and Natural Resource Earth Change and construction permits on the property of two farmers. Azyaza Shabazz of St. Croix and Hugo Roller of St. John both remain committed to the installation of tanks at their farm sites. The HDPE tank liner material and the fiberglass cones are purchased and on hand for installation once the permits are issued.
Impacts and Contributions/Outcomes
The tank was stocked and operational during the annual “Aquaponics and Tilapia Aquaculture Short Course. Twenty-five students from the US, Europe, Middle East and Africa attended the class and learned about the “Greenwater Tank Culture” technology. The development of minimal discharge aquaculture systems is an important advance and is now being used with other species including shrimp.
The EPA will soon release the final version of “Effluent Limitations Guidelines and New Source Performance Standards for the Concentrated Aquatic Animal Production Point Source Category.” Greenwater tank culture will fall within the limits because of the minimal discharge and the land application of the effluent to field vegetable crops.