A design study on the optimal water refreshment rate in recirculating aquaculture systems

Refreshment (make-up) water is used in recirculating aquaculture systems (RAS) mainly to purge off-flavors, to add alkalinity and sometimes for temperature control. Alternatively, alkalinity may be added by means of a chemical base and heat may be supplied by a heating system. The objective of this study is to show how the optimal (minimizing cost) mix of the three controls: water, base and heat, can be found for given temperatures and water prices. The optimal solution varies over the temperature space and also depends on the price of water. For conditions at Eilat, Israel (on the Red Sea), using supplementary heating to maintain a constant temperature may become prohibitively expensive. If heating is given up, the remaining choice is between the supply of alkalinity via the refreshment water and adding a base. The supply of alkalinity with the water requires ∼2.0 m³[water]/kg[feed], much more than the minimum refreshment rate required to purge off-flavors, which is thought to be ∼0.3 m³[water]/kg[feed]. If the price of water is more than ∼0.03 USD/m³, the use of sodium bicarbonate for alkalinity control is justified.