Evaluation of carbon dioxide and nitrate utilization from recirculating aquaculture system for microalgal production

Abstract

This study evaluated carbon dioxide and nitrate utilization from tilapia cultivating system by means of microalgal cultivation. The first part of this study evaluated the feasibility of using nitrate-rich effluent from aquaculture to cultivate Scenedesmus armatus in comparison with the cultivation using BG-11 media. Comparable biomass productivity and pigment content of S. armatus from the cultivation indicated that nitrate-rich effluent from aquaculture could be used as substitute for a more expensive BG-11 growth media. The second part of this study evaluated the feasibility of utilizing carbon dioxide concentrated air from roughly 3 kg/m3 tilapia cultured tank to grow S. armatus. It was found that the maximum productivity of S. armatus at 110 mg/L·day was associated with using BG-11 media and carbon dioxide from fish tank although biomass dried weights and maximum specific growth rates were insignificant different as compared to other treatments using ambient air and BG-11 media without Na2CO3. The final part of the study examined the effect of fish biomass on carbon dioxide and nitrate production and their utilization by means of microalgal cultivation. Approximately 3, 5 and 10 kg/m3 of fish (tilapia) biomass were introduced into the recirculating aquaculture system and cultivated for 8 days and measured for carbon dioxide and nitrate production. Nitrate concentrations were relatively constant at 45.3 mg N/L for the range of fish biomass considered. The highest carbon dioxide production was found in 10 kg/m3 fish cultivation, measured at 1,605 and 915 ppm from outlet of fish tank and solid separating unit, respectively. Outlet air from this particular fish cultivation (i.e., 10 kg/m3) also yielded significantly higher biomass of S. armatus as compared to those using ambient air. Carbon and nitrogen mass balance calculation from tilapia cultivation indicated that 7.99% to 16.42% of carbon and 7.58% to 15.58% of nitrogen from feed were accounted in fish biomass while significantly larger fraction of carbon in feed from 50.67% to 73.46% was converted to carbon dioxide. Only 2.2% to 3.4% of carbon dioxide produced were accounted into the biomass of S. armatus, whereas 33.40% to 74.63% of input nitrogen, mostly nitrate, from solid separating unit were transferred into microalgal biomass. The results from this work clearly demonstrated that microalgal cultivation was able to utilize wastes, namely carbon dioxide and nitrate, from recirculating aquaculture system to produce valuable products, and moreover, reduced adverse environmental impacts. In the broader picture, this work followed closely the concept of bio-, circular- and green (i.e., BCG) economy, which is one of the flagship policies of Thai government to propel the country from middle-income trapped status.