1、PDF外文：http:/ State University,Delta Research and Extension Service, 1992外文文献翻译 Recirculating Aquaculture Tank Production Systems An Overview of Critical Considerations Thomas M. Losordo, Michael P. Masserand James Rakocy Traditional aquaculture production in ponds requires large quantities of
2、water. Approximately 1 million gallons of water per acre are required to fill a pond and an equivalent volume is required 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 pou
3、nd of fish production. In many areas of the United States, traditional aquaculture in ponds is not possible because of limited water supplies or an absence of suitable land for pond construction. Recirculating aquaculture production systems may offer an alternative to pond aquaculture technolo
4、gy. Through water treatment and reuse, recirculating systems use a fraction of the water required by ponds to produce similar yields. Because recirculating systems usually use tanks for aquaculture production, substantially less land is required. Aquatic crop production in tanks and raceways w
5、here the environment is controlled through water treatment and recirculation has been studied for decades. Although these technologies 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
6、 interest of prospective aquaculturists. In recent years, a variety of production facilities that use recirculating technology have 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 p
7、roduction. Prospective aquaculturists and investors need to be aware of the basic technical and economic risks involved in this type of aquaculture production technology. This fact sheet and others in this series are designed to provide basic information on recirculating aquaculture technology. Crit
8、ical production considerations All aquaculture production systems must provide a suitable environment to promote the growth of the aquatic crop. Critical environmental parameters include the concentrations of dissolved oxygen, un-ionized ammonia-nitrogen, nitrite-nitrogen, and carbon dioxide in the
9、water of the culture system. Nitrate concentration, pH, and alkalinity levels within the system are also important. To produce fish in 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
10、-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 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. A
11、s pelleted feeds are introduced to the fish, they are either consumed or left to decompose within the system. The by- products of fish metabolism include carbon dioxide, ammonia- nitrogen, and fecal solids. If uneaten feeds and metabolic byproducts are left within the culture system, they will gener
12、ate additional carbon dioxide and ammonia-nitrogen, reduce the oxygen content of the water, and have a direct detrimental impact on the health of the cultured product. In aquaculture ponds, proper environmental conditions are maintained by balancing the inputs of feed with the assimilative capacity
13、of the pond. The pond natural biological productivity (algae, higher plants, zooplankton and bacteria) serves as a biological filter that processes the wastes. As pond production intensifies and feed rates increase, supplemental and/or emergency aeration are required. At higher rates of feeding, wat
14、er must be exchanged to maintain good water quality. The carrying capacity of ponds with supplemental aeration is generally considered 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 f
15、or cost-effective fish production because of the higher initial capital costs of tanks compared to earthen ponds. Because of this expense 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
16、-products. Additionally, the oxygen concentration within the tank must be maintained through continuous aeration, either with atmospheric 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
17、 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 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 ammo
18、nia-nitrogen per day. (Approximately 3 percent of the feed becomes ammonia-nitrogen.) Additionally, if the ammonianitrogen concentration in 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 t
19、he tank would be approximately 5,600 gallons per hour (93 gpm) to maintain the specified ammonia-nitrogen concentration. Even at this high flow rate, the system also would require aeration to supplement the oxygen added by the new water. Recirculating systems design Recirculating production technolo
20、gy 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-through configuration or production system that uses water only once. In most cases, a flow-through requirement of nearly 100 gallons per minute to maintain
21、 one production tank would severely limit production capacity. By recirculating tank water through a water treatment system that “removes” ammonia and other waste products, the same effect is achieved as with the flow-through configuration. The efficiency with which the treatmentsystem “removes”ammo
22、nia from the system, the ammonia production rate, and the desired concentration of ammonia-nitrogen within the tank determine the recirculating flow 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 wat
23、er 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/0.5 = 186 gpm). A key to successful recirculating production systems is the use of cost-effective water treatment system components. All recirculating pro
24、duction systems remove waste solids, oxidize ammonia and nitrite-nitrogen, remove carbon dioxide, and aerate or oxygenate the water before returning it to the fish tank (see Fig. 1). More intensive systems or systems culturing sensitive species may require additional treatment processes such as fine
25、 solids removal, dissolved organics removal, or some form of disinfection. Waste solids constraints Pelleted feeds used in aquaculture production consist of protein, carbohydrates, fat, minerals and water. The portion not assimilated by the fish is excreted as a highly organic waste (fecal solids).
26、When broken down by bacteria within the system, fecal solids and uneaten feed will consume dissolved oxygen and generate ammonia-nitrogen. For this reason, waste solids should be removed from the system as quickly as possible. Waste solids can be classified into four categories: settleable, suspende
27、d, floatable and dissolved solids. In recirculating systems, the first two are of primary concern. Dissolved organic solids can become a problem in systems with very little water exchange. Settleable solids control: Settleable solids are generally the easiest of the four categories to deal with and
28、should be removed from the tank and filtration components as rapidly as possible. Settleable solids are those that will generally settle out of the water within 1 hour under still conditions. Settleable solids can be removed as they accumulate on the tank bottom through proper placement of drains, o
29、r they can be kept in suspension with continuous agitation and removed with a sedimentation tank (clarifier), mechanical filter (granular or screen), or swirl separator. The sedimentation and swirl separator processes can be enhanced by adding steep incline tubes (tube settlers) in the sedimentation
30、 tank to reduce flow turbulence and promote uniform flow distribution. Suspended solids control: From an aquacultural engineering point of view, the difference between suspended solids and settleable solids is a practical one. Suspended solids will not settle to the bottom of the fish culture
31、tank and cannot be removed easily in conventional settling basins. Suspended solids are not always dealt with adequately in a recirculating production system. 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
