Research Of Carbon Dioxide Accumulation And Its Impacts On Biological Aerated Filter In Marine Recirculating Aquaculture Systems

Recirculating aquaculture system (RAS) is one of the most important directions offuture development of the aquaculture industry. However, high-density recirculatingaquaculture is also accompanied by the accumulation of CO2which inhibits fishgrowth or causes other negative issues. This research focused on the influencingfactors of CO2accumulation and its effects on biological aerated filter (BAF) inAtlantic salmon (salmo salar) industrialized RAS. Firstly, research was performed toinvestigate the24-h variation of CO2concentration, the dynamic changes of CO2between water treatment units and the influences of stocking densities, daily feedingrates, discharge management, gas-water ratios and fish sizes on CO2accumulation inAtlantic salmon (salmo salar) industrialized RAS. Meanwhile the reasons for theaccumulation of CO2were analyzed. Then the research compared the effects ofdifferent CO2concentrations on BAFs, including COD removal rate, TAN removalrate and the amount of microorganisms on the medias.According to the research that focused on the Atlantic salmon (salmo salar)industrialized RAS, when daily feeding rate was in the range of0.2%-0.75%, therewas a linear correlation between daily feeding rate and CO2concentration, and thecorrelation coefficient was0.9881; under the condition of the same day feeding rate,CO2concentrations in the system increased with average stocking density, and therewas the linear correlation between them (correlation coefficient was0.9836), butdifferent densities within the same system did not lead to significant differences on CO2concentrations when daily feeding rate was0.20%to0.75%; system gas-waterratio, discharge management, fish sizes and removal efficiencies of CO2in facilitywould affect the concentrations of CO2. The results also revealed that poor removalefficiency of CO2in facility was the direct cause of CO2accumulation in the system.COD removal rates, TAN removal rates and the amounts of microorganisms onthe medias of BAFs with different CO2concentrations were analyzed. The resultsshowed that CO2could promote the removal of TAN at concentration of17-22mg/L,while it would inhibit the removal of TAN as its concentration rose to39-44mg/L.However, the inhibition was weakened over time. At the concentration of17-22mg/Land39-44mg/L, CO2could inhibit the removal of COD by BAF in the early days.But after a short adaptation, the COD concentrations in effluents of BAFs with thesetwo levels of CO2were nearly the same. And compared to the situation when CO2concentration was0-2mg/L, they were much less. Under the three levels of CO2, theamounts of culturable heterotrophic bacteria were106-107on per gram of medias inBAFs, and the value decreased as CO2concentration increased. The amount ofammonia-oxidizing bacteria (AOB) was one order of magnitude higher than that ofnitrite-oxidizing bacteria (NOB). And the amounts of AOB and NOB were maximumwhen CO2concentration was17-22mg/L, followed by0-2mg/L, then39-44mg/L.But the amounts of AOB and NOB on medias in BAFs with different levels of CO2were in the same order of magnitude, respectively.