Articles and Information

"Greenwater Tank Culture of Tilapia"

by John Martin
Research Specialist, Aquaculture
University of the Virgin Islands
Agricultural Experiment Station

There are several ways to culture fish. Methods range from manured, unaerated ponds (extensive) to the most advanced recirculating systems (hyper-intensive) that use pure oxygen (oxygenation). Greenwater tank culture lies near the lower end of this continuum but represents a significant advance over pond culture.

Greenwater tank culture is an appropriate method for producing commercial levels of tilapia in locations that have environmental constraints such as limited land and water (e.g., U.S. Virgin Islands) or suboptimal temperatures where a greenhouse would be used to control temperature. The University of the Virgin Islands (UVI) has developed a system that consists of a 20-ft. diameter rearing tank and a 375-gallon clarifier from which solids (sludge) are removed twice daily. The rearing tank is continually aerated with 13 air stones and a 1/20-hp vertical lift pump.

Greenwater tank culture is a very appealing form of aquaculture because of its numerous advantages. First and foremost is the high productivity of the system. Channel catfish (Ictalurus punctatus) pond culture is the most important type of aquaculture being practiced in the United States. Well-managed farms produce up to 7,000 lbs./acre (0.78 kg/m3). In a recent greenwater experiment, over 1,100 lbs. (17.6 kg/m3) of Nile tilapia (Oreochromis niloticus) were produced in a single tank. That is equivalent to 156,981 lbs./acre, which is 22.4 times more productive than ponds. Conversely, production from a 1-acre pond (i.e., 7,000 lbs.) could be achieved in a 50-ft. diameter tank using only 4.4% of the water required by pond aquaculture. This is excellent water use efficiency.

Water exchange is not practiced with the greenwater systems. The only losses are from evaporation and sludge removal. Not including initial tank filling, small-scale systems require less than 50 gallons per day. That’s about ½ % of system volume!

In the greenwater system some nutrients are recycled back to the fish. The vertical-lift pump and air stones keep detritus, feces and plankton in constant circulation. Nitrifying bacteria (Nitrosomonas and Nitrobacter spp.) colonize this floating "substrate" creating a "suspended growth treatment process". These bacteria oxidize toxic total ammonia nitrogen (TAN) into relatively harmless nitrate (NO3-) and heterotrophic bacteria (bacteria that consume organic matter) proliferate. The suspended growth treatment process maintains adequate water quality for the fish while recycling waste nutrients into plankton and bacteria. Tilapia graze on these and receive supplemental nutrition, thus lowering feed conversion ratios and feed costs. An added advantage of this process is the elimination of the need for a fixed-film biofilter, which would not only increase capital costs but also increase management and maintenance needs.

A secondary advantage is the production of concentrated solid waste (sludge) for associated land agriculture. In the greenwater system, the culture water is constantly circulated through the clarifier at a rate of 6.6 gallons/min (retention time ˜ 1 hour). This produces one complete circulation of the culture volume every 24 hours. As the water moves under the baffles within the clarifier, its flow becomes laminar, enabling detritus, feces and dead algal and bacterial cells to settle to the bottom. Twice daily this sludge is drained from the clarifier reducing the biochemical oxygen demand (BOD) of the culture water and encouraging continued algae and bacterial population growth, which further improves water quality. The sludge is stored in a sump until it is applied to a field crop. Upon application, greenwater sludge provides immediate nutrition to plants with the nitrate and orthophosphate that are in solution. In addition to this, the solid portion of the sludge (»1%) provides a form of slow-release fertilizer with the nutrients becoming available as the solids mineralize. In past experiments, greenwater sludge produced yields equivalent to traditional inorganic fertilization regimes. The crops evaluated were pak choi, green bell peppers and guineagrass, a forage crop used in the Virgin Islands livestock industry.

The greenwater tank culture system is easy to manage. Other than stocking and harvesting, a greenwater tank generally requires less than 1 hour of management per day. The configuration of the system is simple and maintenance needs are minimal. This is important when promoting the technology to potential aquaculturists.

There is a high potential for profitability. High production capabilities, combined with reasonable capital and operational costs, permit greenwater tank culture to be profitable on both a small and a large scale. This small-scale greenwater tank system was recently evaluated economically. Accounting for all fixed and variable costs, barring owner/operator daily labor, the break-even price to produce a pound of Nile tilapia was calculated to be $1.38. This cost is reasonable if producing for the West Indian whole-fish market where tilapia sell for $2.50/lb. However, future market expansion is believed to be in the product form of fresh fillets. This is because 80% of the seafood consumed on St. Croix is imported by hotels/restaurants to support the Island’s tourist industry. If the break-even price can be lowered to $1.00/lb., local farmers can use this technology to enter the fresh fillet market.

Here is how that figure was determined. Nile tilapia dress-out (no bones, viscera, or skin) at about 35% of live weight. This brings production costs to $2.86/lb. After labor, marketing and distribution, the price increases to approximately $4.00/lb. On St. Croix, fresh fillets will fetch $5.00-6.00/lb. if marketed directly to restaurateurs. Therefore, at a whole fish production cost of $1.00/lb., an attractive profit margin will be realized. We feel this is an attainable goal when considering economies of scale and slight increases in production efficiency, as the technology is fine-tuned.

There are disadvantages with greenwater tank culture. The first is risk. As with any agribusiness, there is risk involved with raising tilapia in this system. Poor management, hurricanes, disease or theft may destroy an entire crop.

With greenwater tank culture you are limited in your choice of fish species. Although the suspended growth treatment process maintains adequate water quality for tilapia, many fish species would not be able to tolerate the water quality extremes that may occur in the greenwater system. Of course, lowered stocking and feeding rates could prevent the development of water quality extremes.

Power failure is also a potential disaster with this system. In the event of a power outage, all of the fish in the system would asphyxiate within hours. This is because the suspended growth treatment process creates a very high BOD. Therefore, it is essential to have a backup generator.

A final disadvantage is the difficulty of sludge application. Because of the viscous nature of the greenwater sludge, crop application can be problematic. Sludge may clog drip irrigation lines or leave a crust on the ground if applied too heavily. Therefore, a high-pressure sprinkler or a truck with spreader bar or furrow irrigation should be used.

Greenwater tank culture of tilapia is practiced on a relatively small scale in California, Louisiana and Israel using various tank sizes and shapes, filtering methods and feeding strategies. All greenwater tank culture systems have the same goals: high production levels, maximum nutrient utilization and minimal water discharge the latter becoming increasingly important.

UVI built its first greenwater tanks in 1990 and began experimenting with them in 1993 after completing six systems. In past experiments we have explored various stocking rates, water exchange rates, the necessity of solids removal and fish species (Nile and red tilapia). We have learned that:

· It is most appropriate to stock between 600 and 750 fish per tank (20-25 fish/m3). With less fish the carrying capacity of the system is not reached. With more fish the suspended growth treatment process cannot neutralize the metabolites produced from the amount of feed necessary to sustain the desired growth rates.

· It is not necessary to exchange water beyond that which is used to offset sludge removal and evaporation losses. No increase in production was realized when 5% of the water was exchanged per day. This is also wasteful.

· It is necessary to remove solids. Without solids removal, BOD builds and limits carrying capacity. With solids removal by a clarifier, carrying capacity was increased by 24%.

Our future plans call for a seven-times scale-up of our current experimental systems. We will build a 200-m3 round "tank" with an in-tank "clarifier" for solids collection. There will be no diffused aeration. Instead, three 3/4-hp Kasco aerators will be used for aeration and circulation. If our current production characteristics are maintained, this tank should produce 7,500 lbs. of tilapia per crop.

Here in the Virgin Islands we raise Nile and red tilapia to enter the whole-fish and fillet markets. We are able to do this because we have year-around warm weather and a market that finds these fish and product forms acceptable. However, most people in the continental United States do not live under these circumstances. Is greenwater tank culture appropriate for you? Maybe. There are several production options, the economic feasibility of which will all be dictated by local market conditions.

First, fish may be grown only during the warmer months of the year. Under normal conditions, tilapia will attain market size within six to eight months. Second, greenwater systems have been shown to perform well in greenhouses. This may be an option applicable to you if you are willing to pay heating expenses for part of the year. Third, greenwater systems have not been tested for species other than tilapia. A fish with high market value (i.e., yellow perch) may eventually provide a niche where greenwater technology can be exploited. Considerable research is still needed under different climatic conditions to assess these possibilities.