Recirculating aquaculture system(RAS)

Designing Innovative Aquaculture SolutionsRecirculating Aquaculture Systems (RAS) are land-based, closed containment rearing systems with full water treatment and recycling technologies employed, that offer economic and fish husbandry benefits to the farmer. RAS systems have been used for fish farming for over 20 years in hatcheries and grow out systems for a wide variety of species. Increased demands for farmed fish combined with the challenges of access to new open water culture sites for traditional fish farming are now causing this market to explode.

ATI is developing RAS technologies for applications where water access is restricted or where limited water resources are available for land based farm consumption. RAS systems allow water quality of the rearing environment to be controlled, thus accelerating fish growth and crop turnover, while the entry of pathogens is minimized, as only small make up water flows need be treated.

The discharged water carrying nutrient goes through a series of purification and regulation treatments before being partially or totally reused. Water treatment processes include solids removal (waste and uneaten feed), biofiltration, gas balancing, oxygenation, and disinfection. For example, the high water quality is achieved by using drum filters, bio-filters, ultraviolet sterilization for pathogen control, as well as ozone filtration. Recirculating aquaculture systems at the Virginia Tech Department of Food Science and Technology
Recirculating aquaculture systems (RAS) are used in home aquaria and for fish production where water exchange is limited and the use of biofiltration is required to reduce ammonia toxicity.

Other types of filtration and environmental control are often also necessary to maintain clean water and provide a suitable habitat for fish.

The main benefit of RAS is the ability to reduce the need for fresh, clean water while still maintaining a healthy environment for fish. To be operated economically commercial RAS must have high fish stocking densities, and many researchers are currently conducting studies to determine if RAS is a viable form of intensive aquaculture.

A series of treatment processes is utilized to maintain water quality in intensive fish farming operations. These steps are often done in order or sometimes in tandem. After leaving the vessel holding fish the water is first treated for solids before entering a biofilter to convert ammonia, next degassing and oxygenation occur, often followed by heating/cooling and sterilization. Each of these processes can be completed by using a variety of different methods and equipment, but regardless all must take place to ensure a healthy environment that maximizes fish growth and health.

All RAS relies on biofiltration to convert ammonia (NH4+ and NH3) excreted by the fish into nitrate.[4] Ammonia is a waste product of fish metabolism and high concentrations (>.02 mg/L) are toxic to most finfish.[5] Nitrifying bacteria are chemoautotrophs that convert ammonia into nitrite then nitrate. A biofilter provides a substrate for the bacterial community, which results in thick biofilm growing within the filter.[4] Water is pumped through the filter, and ammonia is utilized by the bacteria for energy. Nitrate is less toxic than ammonia (>100 mg/L), and can be removed by a denitrifying biofilter or by water replacement. Stable environmental conditions and regular maintenance are required to ensure the biofilter is operating efficiently.

Key Features

  • Fish products can command premium pricing

  • Improves feed conversion ratios

  • Increases bio security

  • Optimizes growth rates

  • Provides consistent biomass once steady-state is reached

  • Controls water temperature and salinity levels

Reoxygenating the system water is a crucial part to obtaining high production densities. Fish require oxygen to metabolize food and grow, as do bacteria communities in the biofilter. Dissolved oxygen levels can be increased through two methods aeration and oxygenation. In aeration air is pumped through an air stone or similar device that creates small bubbles in the water column, this results in a high surface area where oxygen can dissolve into the water. In general due to slow gas dissolution rates and the high air pressure needed to create small bubbles this method is considered inefficient and the water is instead oxygenated by pumping in pure oxygen.[8] Various methods are used to ensure that during oxygenation all of the oxygen dissolves into the water column. Careful calculation and consideration must be given to the oxygen demand of a given system, and that demand must be met with either oxygenation or aeration equipment.

In all RAS pH must be carefully monitored and controlled. The first step of nitrification in the biofilter consumes alkalinity and lowers the pH of the system.[10] Keeping the pH in a suitable range (5.0-9.0 for freshwater systems) is crucial to maintain the health of both the fish and biofilter. pH is typically controlled by the addition of alkalinity in the form of lime (CaCO3) or sodium hydroxide (NaOH). A low pH will lead to high levels of dissolved carbon dioxide (CO2), which can prove toxic to fish.[11] pH can also be controlled by degassing CO2 in a packed column or with an aerator, this is necessary in intensive systems especially where oxygenation instead of aeration is used in tanks to maintain O2 levels

All fish species have a preferred temperature above and below which that fish will experience negative health effects and eventually death. Warm water species such as Tilapia and Barramundi prefer 24 °C water or warmer, where as cold water species such as trout and salmon prefer water temperature below 16 °C. Temperature also plays an important role in dissolved oxygen (DO) concentrations, with higher water temperatures having lower values for DO saturation. Temperature is controlled through the use of submerged heaters, heat pumps, chillers, and heat exchangers.[13] All four may be used to keep a system operating at the optimal temperature for maximizing fish production.