Self Cleaning

The factors that make a tank “self cleaning” include the stocking density, activity level and size of fish, the distribution of fish around the tank, the water velocity and the slope on the base of the tank towards it’s outlet. High stocking densities, high fish activity levels, large fish size and high water velocities all serve to increase the turbulence of the water and maintain any solids particles in the water column rather than letting them settle. This is particulalry important in raceway and D ended tank design, where the wastes must be transported to the end of the tank before they can be removed. Round tanks have a natural vortexing effect on solids particles, drawing them to the centre screen. This makes the round tank more “self cleaning”, especially where there are small fish involved. It is for this reason that they are in common use in hatcheries. Some tanks are square with rounded corners. The “self cleaning” capabilities of these is dependant on the radius of the corners; in general the greater the radius of the corner, the more “self cleaning” the tank is. Such tanks have the advantage of being more space efficient than round tanks. The slope on the bottom of tanks has an important bearing on the self cleaning ability of a tank, especially when water velocities and stocking densities are low. A slope of between 1:20 and 1:40 towards the centre screen of a round tank or the end of a raceway can assist in the removal of faeces and uneaten feed and mortalities.

Water mixing (and water quality)

Tanks with well mixed waters (ie a more even water quality across the whole tank) are advantageous in that they give more consistent growing conditions (e.g. consistent oxygen concentrations across the tank) and feed distribution. Round tanks are generally well mixed systems, whereas raceways tend to be poorer mixed systems with good water quality at the start of the raceway, which gradually deteriorates along the length of the raceway. If the whole holding volume of the raceway is to be used, then some fish will inevitably occupy the lower ends of the raceway, where water quality may be poor. D ended tanks enable greater water velocities than would otherwise be allowed by a raceway, as only some of the water (often between 25 - 30%) is removed at the screen and the rest is allowed to continue flowing round the tank. This improves the mixing in the tank and provides a good compromise between raceways and round tanks.

Water requirements

The inflow rate to round tanks and D ended tanks is relevant mainly to the biomass of fish held in the tank (as the water velocities can be increased by means such as adjusting the angle that the water enters the tank and jetting the water in under pressure). Raceways however require a certain minimum flow to remain self cleaning regardless of the stock held. Therefore in a systems where the numbers of fish held in the tanks will vary, round tanks and D ended tanks are a more efficient option where incoming water is a limited resource.

The water velocity required for self cleaning of faeces and uneaten feed is between 2 and 4 cm/sec.