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.
RAS water treatment processes:
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.
Ammonia is a waste product of fish metabolism and high concentrations (>.02
mg/L) are toxic to most finfish. 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.
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
In addition to treating the liquid waste excreted by fish the solid waste must also
be treated, this is done by concentrating and flushing the solids out of the
system. Removing solids reduces bacteria growth, oxygen demand, and the
proliferation of disease. The simplest method for removing solids is the creation
of settling basin where the relative velocity of the water is slow and particles can
settle at the bottom of the tank where they are either flushed out or vacuumed
out manually using a siphon. However, this method is not viable for RAS
operations where a small footprint is desired. Typical RAS solids removal involves
a sand filter or particle filter where solids become lodged and can be periodically
back flushed out of the filter. Another common method is the use of a mechanical
drum filter where water is run over a rotating drum screen that is periodically
cleaned by pressurized spray nozzles, and the resulting slurry is treated or sent
down the drain. In order to remove extremely fine particles or colloidal solids a
protein fractionator may be used with or without the addition of ozone (O3).
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. 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.
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. pH can also be controlled by degassingCO2 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. All four may be used to keep a system
operating at the optimal temperature for maximizing fish production.
Disease outbreaks occur more readily when dealing with the high fish stocking
densities typically employed in intensive RAS. Outbreaks can be reduced by
operating multiple independent systems with the same building and isolating
water to water contact between systems by cleaning equipment and personnel
that move between systems. Also the use of a Ultra Violet (UV) or ozone water
treatment system reduces the number of free floating virus and bacteria in the
system water. These treatment systems reduce the disease loading that occurs
on stressed fish and thus reduce the chance of an outbreak.
*Reduced water requirements as compared to raceway or pond aquaculture
systems. Reduced land needs due to the high stocking density.
*Site selection flexibility and independence from a large, clean water source.
*Reduction in wastewater effluent volume.
*Increased biosecurity and ease in treating disease outbreaks.
*Ability to closely monitor and control environmental conditions to maximize
production efficiency. Similarly, independence from weather and variable
*High upfront investment in materials and infrastructure.
*High operating costs mostly due to electricity, and system maintenance.
*A need for highly trained staff to monitor and operate the system.
Special types of RAS:
Combining plants and fish in a RAS is referred to as aquaponics. In this type of
system ammonia produced by the fish is not only converted to nitrate but is also
removed by the plants from the water. In an aquaponics system fish effectively
fertilize the plants, this creates a closed looped system where very little waste is
generated and inputs are minimized. Aquaponics provides the advantage of being
able to harvest and sell multiple crops.
Home aquaria and inland commercial aquariums are a form of RAS where the
water quality is very carefully controlled and the stocking density of fish is
relatively low. In these systems the goal is to display the fish rather than
producing food. However, biofilters and other forms of water treatment are still
used to reduce the need to exchange water and to maintain water clarity. Just
like in traditional RAS water must be removed periodically to prevent nitrate and
other toxic chemicals from building up in the system. Coastal aquariums often
have high rates of water exchange and are typically not operated as a RAS due to
their proximity to a large body of clean water.
Source : BAJAJ ENTERPRISE India.