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1、Production SystemsAn Overview of Critical ConsiderationsThomas M. Losordo, Michael P. Masserand James RakocyTraditional aquaculture production in ponds requires large quantities of water. Approximately 1 million gallons of water per acre are required to fill a pond and an equivalent volume is requir

2、ed to compensate for evaporation and seepage during the year. Assuming an annual pond yield of 5,000 pounds of fish per acre, approximately 100 gallons of water are required per pound of fish production. In many areas of the United States, traditional aquaculture in ponds is not possible because of

3、limited water supplies or an absence of suitable land for pond construction. Recirculating aquaculture production systems may offer an alternative to pond aquaculture technology. Through water treatment and reuse, recirculating systems use a fraction of the water required by ponds to produce similar

4、 yields. Because recirculating systems usually use tanks for aquaculture production, substantially less land is required. Aquatic crop production in tanks and raceways where the environment is controlled through water treatment and recirculation has been studied for decades. Although these technolog

5、ies have been costly, claims of impressive yields with year-round production in locations close to major markets and with extremely little water usage have attracted the interest of prospective aquaculturists. In recent years, a variety of production facilities that use recirculating technology have

6、 been built. Results have been mixed. While there have been some notable large-scale business failures in this sector, numerous small- to medium-scale efforts continue production.Prospective aquaculturists and investors need to be aware of the basic technical and economic risks involved in this type

7、 of aquaculture production technology. This fact sheet and others in this series are designed to provide basic information on recirculating aquaculture technology.Critical production considerationsAll aquaculture production systems must provide a suitable environment to promote the growth of the aqu

8、atic crop. Critical environmental parameters include the concentrations of dissolved oxygen, un-ionized ammonia-nitrogen, nitrite-nitrogen, and carbon dioxide in the water of the culture system. Nitrate concentration, pH, and alkalinity levels within the system are also important. To produce fish in

9、 a costeffective manner, aquaculture production systems must maintain good water quality during periods of rapid fish growth. To ensure such growth, fish are fed high-protein pelleted diets at rates ranging from 1.5 to 15 percent of their body weight per day depending upon their size and species （15

10、 percent for juveniles, 1.5 percent for market size）. Feeding rate, feed composition, fish metabolic rate and the quantity of wasted feed affect tank water quality. As pelleted feeds are introduced to the fish, they are either consumed or left to decompose within the system. The by- products of fish

11、 metabolism include carbon dioxide, ammonia- nitrogen, and fecal solids. If uneaten feeds and metabolic byproducts are left within the culture system, they will generate additional carbon dioxide and ammonia-nitrogen, reduce the oxygen content of the water, and have a direct detrimental impact on th

12、e health of the cultured product.In aquaculture ponds, proper environmental conditions are maintained by balancing the inputs of feed with the assimilative capacity of the pond. The pond natural biological productivity （algae, higher plants, zooplankton and bacteria） serves as a biological filter th

13、at processes the wastes. As pond production intensifies and feed rates increase, supplemental and/or emergency aeration are required. At higher rates of feeding, water must be exchanged to maintain good water quality. The carrying capacity of ponds with supplemental aeration is generally considered

14、to be 5,000 to 7,000 pounds of fish per acre （0.005 to 0.007 pound of fish per gallon of pond water）.The carrying capacity of tank systems must be high to provide for cost-effective fish production because of the higher initial capital costs of tanks compared to earthen ponds. Because of this expens

15、e and the limited capacity of the “natural”biological filtration of a tank, the producer must rely upon the flow of water through the tanks to wash out the waste by-products. Additionally, the oxygen concentration within the tank must be maintained through continuous aeration, either with atmospheri

16、c oxygen（air） or pure gaseous oxygen.The rate of water exchange required to maintain good water quality in tanks is best described using an example. Assume that a 5,000-gallon production tank is to be maintained at a culture density of 0.5 pound of fish per gallon of tank volume. If the 2,500 pounds

17、 of fish are fed a 32% protein feed at a rate of 1.5 percent of their body weight per day, then 37.5 pounds of feed would produce approximately 1.1 pounds of ammonia-nitrogen per day. （Approximately 3 percent of the feed becomes ammonia-nitrogen.） Additionally, if the ammonianitrogen concentration i

18、n the tank is to be maintained at 1.0 mg/l, then a mass balance calculation on ammonia-nitrogen indicates that the required flow rate of new water through the tank would be approximately 5,600 gallons per hour （93 gpm） to maintain the specified ammonia-nitrogen concentration. Even at this high flow

19、rate, the system also wouldrequire aeration to supplement the oxygen added by the new water.Recirculating systems designRecirculating production technology is most often used in tank systems because sufficient water is not available on site to “wash” fish wastes out of production tanks in a flow-thr

20、ough configuration or production system that uses water only once. In most cases, a flow-through requirement of nearly 100 gallons per minute to maintain one production tank would severely limit production capacity. By recirculating tankwater through a water treatment system that “removes” ammonia a

21、nd other waste products, the same effect is achieved as with the flow-through configuration. The efficiency with which the treatmentsystem “removes”ammonia from the system, the ammonia production rate, and the desired concentration of ammonia-nitrogen within the tank determine the recirculating flow

22、 rate from the tank to the treatment unit. Using the example outlined above, if a treatment system removes 50 percent of the ammonia-nitrogen in the water on a single pass, then the flow rate from the tank would need to be twice the flow required if fresh water were used to flush the tank （93 gpm/0.

23、5 = 186 gpm）. A key to successful recirculating production systems is the use of cost-effective water treatment system components. All recirculating production systems remove waste solids, oxidize ammonia and nitrite-nitrogen, remove carbon dioxide, and aerate or oxygenate the water before returning

24、 it to the fish tank （see Fig. 1）. More intensive systems or systems culturing sensitive species may require additional treatment processes such as fine solids removal, dissolved organics removal, or some form of disinfection.Waste solids constraintsPelleted feeds used in aquaculture production cons

25、ist of protein, carbohydrates, fat, minerals and water. The portion not assimilated by the fish is excreted as a highly organic waste （fecal solids）. When broken down by bacteria within the system, fecal solids and uneaten feed will consume dissolved oxygen and generate ammonia-nitrogen. For this re

26、ason, waste solids should be removed from the system as quickly as possible. Waste solids can be classified into four categories: settleable, suspended, floatable and dissolved solids. In recirculating systems, the first two are of primary concern. Dissolved organic solids can become a problem in sy

27、stems with very little water exchange.Settleable solids control:Settleable solids are generally the easiest of the four categories to deal with and should be removed from the tank and filtration components as rapidly as possible. Settleable solids are those that will generally settle out of the wate

28、r within 1 hour under still conditions. Settleable solids can be removed as they accumulate on the tank bottom through proper placement of drains, or they can be kept in suspension with continuous agitation and removed with a sedimentation tank （clarifier）, mechanical filter （granular or screen）, or

29、 swirl separator. The sedimentation and swirl separator processes can be enhanced by adding steep incline tubes （tube settlers） in the sedimentation tank to reduce flow turbulence and promote uniform flow distribution.Suspended solids control: From an aquacultural engineering point of view, the diff

30、erence between suspended solids and settleable solids is a practical one. Suspended solids will not settle to the bottom of the fish culture tank and cannot be removed easily in conventional settling basins. Suspended solids are not always dealt with adequately in a recirculating production system.

31、If not removed, suspended solids can significantly limit the amount of fish that can be grown in the system and can irritate the gills of fish. The most popular treatment method for removing suspended solids generally involves some form of mechanical filtration. The two types of mechanical filtratio

32、n most commonly used are screen filtration and granular media filtration （sand or pelleted media）.For more information on these devices see SRAC 453, Recirculating Aquaculture Tank Production Systems: A Review of Component Options.Fine and dissolved solidscontrol:Fine suspended solids （ 30 micrometers） have been shown to contribute more than 50 percent of the total suspended solids in a recirculating system. Fine suspended solids increase the oxygen demand of the system and cause gill irritation and damage in finfish. Dissolved organic solids （protein） can contribute significantly to the oxy