Flow Rate vs Circulation Rate vs Exchange Rate

Recirculating Aquaculture Systems have become the go-to infrastructure choice for commercial aquaculture in many parts of the world and across a wide range of species, due to the many advantages they offer over alternative infrastructure types.  In essence they comprise fish tanks holding the fish at moderate to extremely high densities, supported by powerful filter systems to maintain optimal water quality.  In addition, they may include aeration, oxygenation, temperature control and/or sterilisation.  A RAS also includes a pumping system to recirculate the water continuously between the various components, bringing us to the subject of this eNewsletter.
 
 There is some confusion as to the use of specific RAS terminology, and in this article I aim to bring clarity to their usage.  These phrases are flow rate, exchange rate and circulation rate.  In essence these terms describe different components of the rate at which water moves within the system, but it is useful to distinguish between each of them as they describe different aspects of the RAS.
 
Flow rate refers to the speed at which water moves and is usually indicated in m/sec or cm/sec.  This can be used to describe the movement of water within the pipe network, and is only really relevant at the design stage to ensure that pumps, pipes, fittings and equipment are correctly scaled to ensure the technical effectiveness and energy efficiency of the installation.  With the industry moving to energy efficient, low head designs, pipes tend to be wide in diameter and replaced with gutters wherever possible, resulting in a desirably low flow rate which allows the water to move effectively whilst keeping frictional loss to a minimum.
 
The way in which the water enters the fish tank is profoundly important and introduces the second form of flow rate; that being circulation rate which describes the rate at which the water flows within the fish tank.  Circular tanks are used in recirculating fish farming, and these are fitted with a central, bottom drain.  Water returning to the tank enters along the wall and at an angle parallel to the side of the tank, introducing a current and causing the entire water volume to move in a circular motion.  Solid wastes are denser than water and so sink to the floor where the circular movement induces centripetal force, which carries them to the central bottom drain and out of the tank.  This is referred to as the self-cleaning effect of circular tanks and is massively important in maintaining high water quality within the fish tank.  The speed at which the water moves around inside the tank is also expressed in m/sec or cm/sec.
 
When rearing fish such as trout, which thrive in a current, then the circulation rate can be quite rapid, with the benefit of more efficient waste concentration at the central bottom outlet.  However, if the tank contains small fingerlings the same current would exhaust them so a slower water movement is desirable.  We set the current within the tank by changing the angle of the incoming water (see the photo below).  If the elbow on the incoming pipe is vertical the incoming water would not induce a circular current, but as we tilt it to one side it creates a current within the tank, the velocity of the current is directly related to the angle of incoming water.  This change can be made progressively as the fish grow within the tank between sorting events.

The third description of water movement within a RAS is exchange rate.  Whereas flow rate and circulation rate are descriptions of speed, exchange rate refers to volume movement in m3/hr or simply as a rate such as 2 cycles/hr.  Exchange rate is the cycling of water throughout the system and is a critical factor in the design of RAS as it describes the dilution power that water has as it moves through the fish tanks, thus impacting water quality and stocking density.  A high exchange rate is excellent for the fish as this rapidly introduces new, clean water into the fish tank and removes water containing faecal wastes, urine and carbon dioxide.  Unfortunately, the higher the exchange rate the larger the pump and wider the pipes need to be, and both increase the cost of the RAS.   Aquaculture systems designers generally settle for an exchange rate of between 1-2 cycles through each tank per hour.
 
Here is where things become interesting: a rapid exchange rate is desirable, especially in the hatchery where the fish are highly sensitive to adverse water quality.  However, tiny fry do not manage a current so the high exchange rate must be matched with a low circulation rate within the tank.  This can be achieved by ensuring an appropriate angle on the incoming water that does not induce much of a current within the tank.
 
Some people use the term ‘recirculation rate’ referring to exchange rate.  I discourage this because of the misunderstanding this then creates between the concepts exchange rate and circulation rate.
 
Aquaculture in a recirculating system is a synergy between optimal water movement efficiency and optimal fish health. The purified water is reused, rather than wasted, and the fish thrive and grow in the clean water. As RAS becomes increasing popular let’s stick to using the same terminology to avoid confusion.