SRAC Publication No. 455

VI November 1997 PR

Pond Recirculating Production Systems

Andrew M. Lazur and Deborah C. Britt*

Water conservation and reuse has working knowledge of these bio- Water circulation within become a major issue in aquacul- logical processes and their effect ture in recent years. Concern over on water quality. Understanding increased demand on aquifers, the amount of nutrients, primarily Circulating water within a costs of operating , environ- feed, that can be safely applied to impacts water quality in several mental impact of a system is important in ensuring ways. A major objective of water effluent, and the desire to increase that the algae and bacteria will circulation is to destratify, or mix, production efficiency continues to not be overloaded in their ability the deeper, cooler, -defi- drive advances in technology and to filter wastes. cient waters with the shallow, management practices. Several of Recirculating tank system research warmer waters rich in dissolved the management strategies devel- has provided major leaps in oxygen. This practice improves oped to address these issues culture intensification, technology, water quality throughout the include enhancing water circula- and the understanding of water depths of the pond, reducing the tion in ponds and developing quality management. Much of this risk of low dissolved oxygen due intensive, recirculating tank sys- progress is necessitated by the to turnovers. This process is espe- tems. The combination of these desire to maximize profit by cially important in ponds deeper two technologies is referred to as increasing production, lowering than 6 feet which often stratify pond recirculating systems. costs, and conserving water. during warmer months. Pond culture is the most widely Circulating water also increases Recirculating systems require a the suspension of nutrients which used production technology in significantly greater input of world aquaculture. The lower can stimulate plankton growth specifically engineered compo- and increase microbial activity. stocking and feeding rates per nents including water pumps, unit of water and lower invest- Fry growout or nursery ponds can supplemental oxygenation, and benefit by continuous but gentle ment and production costs have biofilters to replicate a pond’s bio- contributed to the present expanse water circulation which aids in logical processes. However, the maintaining a more even distribu- of pond production worldwide. A need for water conservation, pond’s natural , includ- tion of nutrients. This disperse- lower feed conversions, reduced ment prompts healthy plankton ing photosynthetic algae and ben- disease treatment costs, and closer eficial bacteria, drives important populations which are important proximity to markets makes recir- food organisms for fry growth. biological processes that impact culating tank systems attractive in the daily oxygen cycle and pro- certain areas (see SRAC publica- The benefits of water circulation vides a natural biofilter which tions 451, 452, 453 and 454 for in ponds depend on existing breaks down harmful nitrogenous more information on recirculating water quality and production wastes. Successful fish production tank systems). Pond recirculating levels. Pumping water from a in ponds requires a thorough production systems utilize certain nearby source such as a bayou, advantages of both pond and tank river, or is a common production technology to achieve practice to replenish oxygen and *University of Florida Fisheries and flush ponds of excessive nutrients, Aquatic Sciences Department increased production with reduced inputs. such as ammonia, when growing crawfish and shrimp. In many Use of mechanical aeration has water quality management tool. cases, however, flushing ponds increased the amount of fish pro- Paddlewheel aerators placed in does not improve water quality duced in ponds. Oxygen concen- each pond corner create a unidi- significantly, particularly in large trations are increased during the rectional flow (see Figure 2). normal, low, nighttime period ponds. Additionally, the cost This circulation tends to force when algae respiration is signifi- associated with this practice is solids to collect to a central drain. cant. Aeration also contributes to often not economically feasible. Aeration rates of 5 to 10 horse- water circulation. Some produc- Earthen or plastic baffles can power per acre are necessary to tion systems, such as intensive assist water flow throughout the provide both aeration and water shrimp culture, depend on aera- pond thus improving water circulation depending on stocking tor-driven circulation as a key quality (see Figure 1). density and daily water exchange. Permanent crawfish ponds with Water Out earthen baffles use aeration and water circulation to direct water flow. Proper placement of aerators in the pond is essential to maxi- mize water circulation (see Figure 3). In ponds without baffles, stud- ies have shown that placing a paddlewheel aerator at the middle Earthen Baffle Levee of the pond’s longest levee maxi- mizes circulation within the pond (see Figure 4). Diffused air systems, such as air- lift pumps, are often used to sus- pend nutrients in fry ponds and to destratify deep ponds (see Figure 5). They are also effective in moving water both vertically and horizontally. Water In The use of specifically designed Figure 1. Common water exchange design using baffles in pond. water circulators or blenders has received attention in recent years for use in catfish, crawfish, and shrimp ponds. These propeller

Aerator Aerator type units pump or move high volumes of water with low power. Water flows of 1,700 to 11,000 gal- lons per minute are attained with 1 and 3 hp units, respectively (see Water Current Figure 6). Water circulators can be used in Solid combination with aerators in pro- Collection duction systems where both aera- and tion and water circulation are Drain desired. Water circulators are often operated during afternoon hours coinciding with peak photo- synthesis. Consequently, oxygen- supersaturated surface water is mixed with deeper waters which increases the total pond oxygen budget prior to the normal, late

Aerator night, and early morning low oxy- Aerator gen period. Water circulation may reduce the time and cost needed Figure 2. Paddlewheel aerator-driven water circulation used in shrimp production for nighttime aeration which uses ponds. aerators with greater horsepower ratings. Aerator tainability by using the biological processes of phytoplankton and other aquatic plants. These sys- tems vary in design according to production goals and topography. One common practice involves using a pond or series of ponds as a green water reservoir to supply water to fish production in race- Earthen ways or tanks (see Figure 7). Some Baffle systems incorporate a ditch or Levee wetland system using algae or wetland plants as filters to reduce nutrient levels in water prior to returning it to the pond. These types of pond recirculating systems allow for polyculture of herbivorous species such as tilapia or bighead carp which can be con- tained separately in raceways or other ponds (see Figure 8). This segregation reduces handling Aerator stress associated with separating the fish at harvest. Another poten- tial advantage is the increased Figure 3. Water recirculation pond design using baffles and paddlewheel aerators marketing control due to being to move and direct water flow. able to more easily harvest the fish from raceways rather than Another impact on water quality Pond recirculating seining an entire pond. However, from water circulation is an systems since water is pumped continu- increase in microbial activity ously through raceways at 0.5 to because of the mixing of nutrients A variety of pond production sys- 4 exchanges per hour, increased within the pond and over the tems that incorporate complete electricity inputs are required as pond bottom. This effect may be water recirculation have been compared to traditional pond cul- beneficial in enhancing the recy- developed in recent years. Funda- ture and, therefore, fish produc- cling of nutrients in the pond mental to this technology is reduc- tion costs may be higher. Cost which may reduce levels of cer- ing water use, increasing produc- considerations should be conduct- tain toxic metabolites such as tion efficiency, and attaining a ed to evaluate whether the advan- ammonia and nitrite. greater level of production sus- tages of these systems are prof- itable. Paddlewheel Aerator Aquatic plant treatment components The use of aquatic plants to treat or assimilate nutrients from aqua- culture production water is a rela- tively new concept, and specific recommendations for the ratio of production to plant treatment area are limited. This ratio, whether a green water reservoir or wetland plant system, to fish production unit volume, will vary depending on the type of aquatic plant and its ability to assimilate nutrients, the fish species, feeding rates, and Figure 4. Recommended placement of a paddlewheel aerator in a pond to water circulation rates. Phyto- plankton are relatively efficient at maximize water aeration and circulation efficiency. assimilating nitrite, ammonia, and phosphorus as food sources. fill with solids which will horsepower, hydraulic-driven require periodic removal. paddlewheel type circulator. The Use of marketable plant rotation speed of the circulator species which require peri- and, hence, flow rate through the odic harvesting can reduce system, is regulated electronically solids accumulation and by water quality devices that provide an income to cover monitor dissolved oxygen. The costs of the wetland system water flow averages 0.1 foot per Water Surface (see SRAC Publication No. second but can be increased dur- 467, Cost of Alternative ing periods of low oxygen to pro- Effluent Treatments for vide maximum oxygen levels in Catfish Production, for infor- the water column. Water retention mation on economics of times can be controlled to range effluent treatment). from 1 to 10 hours. Other advan- tages include controlling the Intensive automative water flow and optimizing algal technology or phytoplankton growth. The fish production raceways in Another pond recirculating the PAS offer advantages similar Air system design, being devel- to other raceway systems. These oped by Clemson Univer- benefits include increased control sity, utilizes computer con- over water quality, predators, and trolled technology to regu- feeding; reduced disease treat- late water circulation 1 ment costs; and reduced labor through a /3 acre pond input. One main raceway in the reservoir with baffles and PAS is used for production of a into fish production race- principal fish species such as Water ways (see Figure 9). channel catfish. An additional, Figure 5. Airlift pump. Termed the Partitioned parallel raceway is used for a her- Aquacul-ture System, or bivorous fish or shellfish species PAS, the unit’s water circu- which feeds on phytoplankton. However, in a static pond or pond lation is regulated by a low Water flow rates in the raceway recirculating system, a daily limit of nutrient loading exists. Even with intensive aeration, a point will be reached where phyto- plankton or wetland plants cannot assimilate all of the nutrients. This excess could result in toxic con- centrations of nitrogenous wastes. Wetland plant species such as bul- rushes, cutgrass, maidencane, and rice have been shown to improve water quality. Preliminary studies have shown that wetlands are effective in reducing BOD, phos- phorus, suspended solids, and nitrogen using hydraulic residence times of 4 to 10 days for catfish production pond effluent. However, the area of wetland required would approach or exceed the fish production area, therefore, significantly increasing costs. Future studies may provide needed information on cost effec- tiveness of using wetland systems for natural nutrient filtration. Figure 6. Example of commercially available water circulator. Over time, wetland systems can Several potential advantages include: ■ reduced water use and produc- Reservoir tion effluent Pond ■ improved water quality ■ easier accessibility to fish from raceways ■ increase in species diversity Gravity ■ increased sustainability Supply Growout Several possible disadvantages of Pond these systems include: ■ need to pump or mechanically move water Raceways ■ higher fish production costs compared to open pond culture due to increased pumping costs ■ unproven economic feasibility in some cases Recirculating pond systems will continue to develop in the future Solids as water reuse becomes more Settling/Collection Growout important. Despite the positive Pond impact on water quality, there has been little research to show that pond recirculating systems have significantly greater fish produc- tion over traditional open pond aquaculture. Specific design criteria will vary Figure 7. Example of multiple pond/raceway recirculation system. depending on individual site, pro- duction objectives, and regulatory requirements. Farmers should carefully consider the costs and can be regulated by electronically production. Reduced water benefits prior to developing a controlled gates. requirements, control over algal pond recirculating system. Supplemental aeration is supplied populations and oxygen produc- tion, raceway production benefits, to the raceway by floating, verti- Additional Reading cal pump aerators. A sump to col- and partial recovery of nitrogen lect solid fish waste is located for use as fertilizer makes this Constructed Wetlands for Treatment immediately after the raceway system attractive and compatible of Channel Catfish Pond Effluents. and can be cleaned periodically with present and future conserva- M. F. Schwartz and C. E. Boyd. by mechanical vacuuming. tion concerns. Progressive Fish-Culturist 57:255- Removal of solid fish wastes aids 266. 1995. Summary in significantly reducing oxygen Cost of Alternative Effluent demand, and the waste can be Circulating water in ponds is an Treatments for Catfish Production. used as a supplemental source of effective tool in destratification SRAC Publication No. 467. nitrogen and phosphorus fertilizer that improves oxygen levels for terrestrial plant crops. Nutrient Recovery and Reuse for throughout the pond and increas- Water Quality Control in the Research results of the PAS are es microbial oxidation of organic Partitioned Aquaculture System. D. promising. Fish production is matter. Pond recirculation system E. Brune, J. A. Collier, and T.E. being improved as field studies technology is relatively new with Schwedler. Sustainable Aquacul- continue. Present studies have many potential designs. Key to ture 95. Pacon International, achieved production levels of the systems is mechanical water Honolulu, Hawaii. 1995. 14,000 pounds of catfish and 2,000 circulation (pumps, aerators, or pounds of tilapia per acre with circulators) within a pond or Recirculating Aquaculture Tank new tests planned to increase /raceway combination. Production Systems. SRAC Publication Nos. 451-454. Growout Pond Optional Pump Raceway for Plankton- Feeding Species (e.g., Tilapia, Carp)

Ditch

Drain to Water Inflow Ditch to Pond

Water Supply Line

Figure 8. Example of pond/ditch recirculation polyculture system. Paddle- Wheel

Tilapia Tilapia

Catfish Catfish Catfish

Standby Aerator Drain Oil Pump

Figure 9. Overview of Partitioned Aquaculture System (PAS) (Clemson University). The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service.