Abstract: The dissolved oxygen plays a key role in Aquaculture. A better understanding of the principles behind the transfer process and how to improve efficiency has a positive impact on the performance of the culture species. The foundation of salmon health and performance lies in understanding the principles of water oxygenation and how oxygen is captured and metabolized by the salmon. The talk will discuss the application of Oxygenation (gas to liquid interface) to improve the dissolved oxygen in different water conditions. The seminar will include some studies related to determining SOTE (standard oxygen transfer efficiency), kLa (volume mass transfer coefficient) and providing some practical examples.
Methodology: The experiment was carried out in clean, fresh water and salt water at a temperature around 15 °C and a pressure of 1 atm. A cylindrical acrylic tank with a final net volume of 0.8 m3 was installed and filled with a 1-meter water column. The initial oxygen was removed by controlled injection of gaseous nitrogen (N2), until it reached near zero and the rest oxygen was removed with sodium sulfite. During oxygenation through each hose diffuser, hundreds of dissolved oxygen measurements were taken, recorded by four optical sensors (FireSting, PyroScience) every five seconds.
Result: The differences in performance between diffuser hoses are directly related to attributes of the materiality of each hose, such as wall thickness, number of perforations, and distribution of the perforations, which ultimately determine the bubble size. Consequently, a smaller bubble size is what determines efficiency, since, if for a given volume there is a greater contact or transfer area, a greater exchange is achieved, and this improves efficiency. Meanwhile, the volume mass transfer coefficient (kLa), expresses how fast oxygen (gas) is transferred to the liquid and the gas/liquid exchange area per unit volume of the liquid. Mass transfer requires the presence of two regions with different chemical compositions and refers to the movement of chemical species from a region of high concentration to a region of lower concentration.
In this study, different hose diffusers were evaluated to determine under controlled conditions the two key parameters (OTE and kLa) at different mass flow rates to determine the performance of each one of them. In this way, the correct correspondence between the oxygen demands of the fish in a culture unit and the required diffuser capacity can be established, considering their efficiency or inefficiency to correctly size the length of these diffusers. Therefore, more efficient and responsive diffusers allow savings in oxygen consumption and the diffusers can be used when oxygen conditions are critical, such as emergency or back-up systems, or used on a more permanent basis to optimize oxygen levels and achieve better performance in fish production.