Low head oxygenator performance characterization for marine recirculating aquaculture systems

Abstract

This study evaluated the effect of temperature (20 and 25 °C), salinity (10, 15, and 20 ppt), and dissolved oxygen levels within low head oxygenator (LHO) outlet water on oxygen transfer efficiency (OTE) of LHOs for a planned marine recirculating aquaculture system (RAS). Test results indicated that OTE was generally greater at salinities of 10–15 ppt compared to OTE measured with freshwater and at 20 ppt salinity. Oxygen transfer efficiency in freshwater with LHO outlet oxygen saturation of 230% was only 58% compared to OTE in 10 ppt and 15 ppt salinity with an LHO outlet oxygen saturation of 230% of 79% and 72%, respectively. As expected, OTE declined as target dissolved oxygen levels in LHO outlet water increased from 150 to 230% saturation. Oxygen transfer efficiency at 15 ppt salinity and 150% dissolved oxygen saturation at the LHO outlet was 97%, while OTE dropped to 72% at 230% oxygen saturation. Increased OTE at higher salinities of 10–15 ppt was attributed to increased ionic strength of the water under saline conditions resulting in formation of smaller diameter bubbles, as opposed to larger bubbles formed in freshwater. However, at 20 ppt this effect may have caused the formation of small diameter bubbles with such a slow rise velocity that they were overcome by the water velocity leaving the bottom of the LHO and were carried out of the LHO, thereby reducing OTE. Improvements in performance for similarly-designed LHOs in intensive marine RAS can be realized by designing LHOs for lower hydraulic loading, taller fall height, and increased submergence.