Recirculating Aquaculture Systems

February 2022


Water re-use technology for fish culture began in the aquarium hobby, followed by early attempts to adapt municipal waste-water treatment methods. Over the past 50 years, a number of researchers and commercial entrepreneurs in North America, Europe and Asia have been involved in the development of technologies to allow profitable culture of aquatic species in closed, contained systems with continuous water re-use. Much of the interest in this type of production stems from the fact that many areas do not have suitable land, water resources, or climatic conditions for fish production in open ponds, cages or raceways. Additionally, these facilities can be strategically located near major markets or input suppliers.

Recirculating fish production systems use far less land and water than traditional methods, but they can be very capital and energy intensive, resulting in high production costs. Aquatic organisms like fish and shrimp produce ammonia-nitrogen, fecal solids and carbon dioxide throughout the production cycle, and each of these waste products must be rapidly removed or transformed in order to maintain optimal conditions for survival and growth within the production system.

Economic sustainability in recirculating aquaculture production requires a balance between the level of technology, the biological characteristics and environmental requirements of the species being produced, and market demand. When serious research was first undertaken to address recirculating aquaculture, the focus was on development of numerous, sequential processes to re-condition the water within a production system. Over the past three decades, however, more emphasis has been placed on utilizing fewer processes to accomplish multiple tasks, while increasing stocking densities.


Recirculation of culture water involves a series of system components designed to accomplish mechanical filtration (solids removal), biological filtration (nitrification), re-aeration, de-gassing, temperature control and chemical adjustments. Ideally, when the water completes the circuit, having passed through these various processes, it will once again approximate the optimal conditions for the species in question. Depending on the system configuration and the level of technology employed, the amount of replacement water required on a daily basis can range from less than1 percent to 100 percent or even more. The primary requirement for replacement water involves the removal of solids and dilution of accumulated metabolic by-products which treatment processes cannot entirely remove.

The removal of solids, such as fecal matter, uneaten feed, algal or bacterial muck, and proteins is the most fundamental process in water recirculation. Solids are typically categorized as settleable, suspended, floating and dissolved. A common approach is to design system water flow and aeration patterns to maintain settleable solids in suspension for removal when passing through granular- or mesh-based mechanical filtration units. Suspended solids typically will not settle out of the water column, and these solids must be removed mechanically – requiring filtration media which retain very small particles and in turn require frequent cleaning. Fine and dissolved solids are very difficult to remove from the water column other than by foam fractionation.

In spite of the inherent difficulties, mechanical filtration is usually economically feasible, with concentration and periodic removal of solids. A key consideration in the process is minimization of water loss. Make up water may require significant investment in terms of heating, cooling or chemical adjustments. Additionally, more environmentally compatible disposal options are available with concentrated waste streams.  

The second process in water re-use technology involves biological filtration, which focuses on the use of naturally-occurring nitrifying bacteria to convert ammonia nitrogen into nitrite and subsequently convert nitrite into nitrate. Ammonia is the primary waste product of most aquatic organisms, and is it toxic, especially in its unionized form which is prevalent at higher pH levels. Efficient biological filtration usually requires media such as granular materials, plastic rings or balls, or other materials which exhibit high surface area while maintaining sufficient spaces for water to flow through. Nitrifying bacteria grow on the media surface. One increasingly popular approach involves Moving Bed Biofilter Reactors (MBBR), with specific media (marketed as “MBBR media”) now available throughout the world.

The process of biofiltration requires sufficiently high levels of oxygen to allow the bacteria to thrive and adequate levels of alkalinity in the water, which can be supplemented by addition of bicarbonate of soda. Some additional examples of successful biological filter designs include trickling filters, fluidized bed reactors, rotating biological contactors, and floating bead filters. Essentially, in RAS production one must provide optimum conditions for both the fish being cultured and the bacteria in the biofilter.

With each pass through the treatment process, re-used water must be re-aerated to replace oxygen consumed by both the fish and the filter bacteria. Two principal approaches involve either mechanical aeration (increasing contact between the culture water and the atmosphere), or using pure oxygen to maintain sufficient levels for fish and bacterial metabolic demands. Either way, water leaving the production tank and entering the biofilter must contain sufficient levels of oxygen to support the bacteria.

Many devices used to re-aerate water also remove some of the carbon dioxide dissolved in the water as a result of fish and bacterial respiration, but in many situations more aggressive de-gassing is required because carbon dioxide is much more soluble in water than is oxygen. This situation is exacerbated when artificially high standing stocks are maintained through the use of pure oxygen and when buildings are poorly ventilated in an effort to conserve heat.

Equipment used in recirculating aquaculture systems must be suitable for the intended applications, including exposure to water (salt water in some instances). Structural considerations for facilities, such as slab strength, insulation, vapor barriers, drainage, and condensation and corrosion issues must be well thought out. Often it is more efficient to heat or cool the air within a production facility than the culture water itself. Ventilation must prevent accumulation of carbon dioxide over time, while minimizing heating or cooling costs. Electrical wiring and equipment must be rated for extreme outdoor conditions to accommodate high humidity levels and all circuits must be equipped with surge protection and ground-fault-interrupt capacity. Finally, waste stream disposal, including concentration and volume of effluents (as well as disposal of day-to-day mortalities) must be considered based on local standards and regulations.

Since RAS facilities are completely dependent on reliable, continuous electrical service, back-up emergency generators are essential. Automatic start-up and switching systems are almost always worth the investment because loss of power and/or equipment failures can result in catastrophic losses in a matter of less than 30 minutes. Multiple replacements must be on-hand for blowers, pumps, sterilizers, and all other critical equipment, and systems must be designed, plumbed and constructed to allow for failed or defective components to be swapped out in a rapid, straightforward manner. Additionally, RAS facilities require constant supervision or at the very least continuous monitoring systems that can alert nearby responders in the event of power outages, emergency failures or break-ins.    

The 2018 USDA Census of Aquaculture reported a total of 452 farms utilizing recirculating production systems, with a total of 20,889 tanks and a total capacity of 62,741,155 gallons. Forty-seven states reported at least one RAS operation, and Florida led the nation with 95, followed by California with 32, Ohio with 27, Texas with 26, Virginia with 25 and North Carolina with 21. Of these recirculating farms, 149 reported annual sales of less than $25,000 while 41 reported annual sales of $1 million or more. The remaining operations reported intermediate sales volumes between these extremes, with most (73) in the $100,000 to $499,999 category.

Marketing Considerations

Data available to define the total production harvested from recirculating aquaculture systems in the U.S. are probably incomplete, but tilapia, salmon, shrimp and several other species are being widely produced in such systems. Elsewhere in the world, recirculating aquaculture systems are used to produce eels, ornamental species, and a variety of marine fishes. From a business perspective, the main advantage of recirculating aquaculture production involves the ability to locate production facilities near large population centers, allowing for niche marketing of fresh (or even live), locally-grown, high-value fish or shellfish. In this way, competition from commodity-type products (primarily frozen fillets and frozen shrimp) can be minimized.

Numerous high-profile failures of recirculating aquaculture facilities have occurred over the past several decades. Factors contributing to these failures included under-capitalization, technically inadequate design, poor management, cash flow problems, disease problems and poor analysis and planning. Over-confidence and lack of expertise, of both system designers and operators, has been a common theme in most of these cases.

Although characterizing recirculating aquaculture production may be almost as difficult as defining aquaculture itself, many forms of recirculating aquaculture fit the concept of an “agro-industry.” For efficient use of facilities and equipment, as well as market development, production and harvests must take place year-round. A sufficient number of production units must be in place to accommodate multiple batches, or lots, of fish or shrimp in order to have harvestable product on hand on any given day.

In an RAS facility, inputs such as feed and fingerlings are continuously transformed into value-added products, be they live shrimp, or live, whole gutted or filleted fish. As a rule, the more complex the process, the greater the requirements for capital, technology and management. Whereas raw materials are typically the most important cost components of agro-industries, they may be eclipsed in an RAS enterprise by cost factors such as labor, energy, interest on borrowed capital and insurance fees.

To varying degrees, fingerling inputs for recirculating aquaculture systems exhibit seasonality, perishability and variability in quality. In contrast, the finished products being harvested are subject to more constant requirements and demand. This represents a fundamental difference between recirculating aquaculture and traditional manufacturing, and creates greater challenges in terms of inventory management, balancing supply and demand, scheduling production and coordinating internal acquisition, process and marketing activities.  

Other issues typical of agro-industries are also encountered in recirculating aquaculture. Storage space is often limited, and in reality, critical inputs such as feed and fingerlings must be considered highly perishable. Other critical inputs can also be highly variable such as feed quality and/or palatability, fingerling size and health status, chlorine/chloramine content of municipal water supplies, etc. Indeed, in many intensive aquaculture facilities even energy supplies must be considered somewhat variable, necessitating on-site generators and switching systems. In the future, water may also figure as a major raw material cost in some recirculating aquaculture businesses.

Less-standardized inputs result in additional pressure on production scheduling, cash flow and quality control than in traditional manufacturing. These challenges, in turn, require a much higher level of integration and information flow within the enterprise. In a recirculating aquaculture enterprise, these variations are compounded as various batches of fish move through the production cycle, requiring segregation, grading and maintenance until they are harvestable. Space, feed and labor must all be appropriately allocated to avoid undue stress on production stocks.

Evaluation of the production process must consider both financial and economic perspectives. While return on investment might be maximized through adoption of high levels of automation and process control, efforts to shift technology more toward the use of manual labor may make more sense if the overall goal is local or regional economic development. In most situations, these two needs must be balanced to some degree in order to secure both investment and local political support. In terms of facility location requirements, access to inputs, markets and labor are often overlooked or not given sufficient emphasis in recirculating aquaculture development. Similarly, utilities and water availability (potable, cooling/heating and process water) are often overlooked.

Financial analyses are generally essential for access to capital, while economic analyses are typically of greater interest to local authorities and communities where proposed recirculating aquaculture operations will be located. Financial analyses focus primarily on factors such as return on investment, while economic analyses address costs and benefits to local, regional and national economies. Each has its own format and key considerations, but neither can be considered reliable without a sound understanding of engineering considerations and the biology of the species being considered for culture. Again, these shortcomings are often the main causes when RAS operations fail.

Stocking and feeding rates determine (roughly) the age at harvest for whatever species is being raising. Having a number of tanks stocked at different densities may help generate regular harvests of uniform product. This, however, may also require a larger initial investment and the need to compete for market share with larger, established producers in order to justify investment costs. When planning an RAS facility it will be necessary to calculate the carrying capacity of the system in order to estimate capital costs, projected production, projected revenues and input requirements. At this point, an evaluation can begin in order to determine if the operation will be large enough to generate profits and if so, a market analysis can be undertaken. These projections will also help determine if settling ponds or other measures will be required to meet local and state effluent regulations and reduce environmental impacts from the farm discharge.


Markets must be available to accommodate production when it is ready for harvest because maintaining live animals beyond a harvestable size requires excessive feed, labor and energy costs as well as unproductive use of facilities and capital. Developing and maintaining committed relationships with both suppliers and buyers is crucial to the success of any aquaculture enterprise. Regardless of the scale of production, value must be provided through quality control, harvest scheduling and customer service. Customers must also be willing to provide reliable outlets for production in a timely manner.

Accurate and detailed cost projections are essential when establishing any fish farming business. Projected costs (variable, fixed, marketing and opportunity costs) must be compared to prevailing market prices for the sizes and quality of the product to be produced. Economies of scale must be considered – a smaller operation will have higher per-unit costs and will require a number of small-volume, local markets that are willing and able to pay higher prices. When a small operation loses a customer, alternatives may be limited or nonexistent. Input suppliers (feed, fingerlings and equipment) must be reliable and trustworthy. Consider overall feed requirements – who will supply it, at what cost, and in what quantities will it be delivered?  Bulk purchases will not make sense if the delivered amount (typically 10 to 20 tons per load) cannot be used before the quality begins to deteriorate. If buying in bags instead of bulk deliveries, costs of production will automatically go up. Bulk feed deliveries require adequate and accessible storage facilities, as well as a substantial daily drawdown to avoid spoilage.

Small scale RAS production may not be profitable without a competitive edge in local markets gained through accessibility, customer service and logistics. On-site processing and sales may improve revenues, but these approaches usually involve excessive focus on labor, permits, licenses and liability insurance.

There are distinct opportunities for profitable operation of recirculating aquaculture facilities in the U.S. and elsewhere, but this type of enterprise requires an understanding of the complex relationships between procurement, operations and marketing as they relate to the physical production systems in use, the aquatic species in question, and consumer preferences within target markets.  



Prepared by C. Greg Lutz, Louisiana State University Agricultural Center