Carbon and nitrogen mass balance during flue gas treatment with Dunaliella salina

The biotreatment of flue gases with algae cultures is a promising option to sequestrate CO2, yet the emission of other greenhouse gases (GHG) from the cultures can hamper their environmental benefit. Quantitative data on the sequestration potential for CO2 and NO x in relation to the direct production of CH4 and N2O are urgently required. The present study assessed the flows of carbon (C) and nitrogen (N) through cultures of the green alga Dunaliella salina, supplied with biodiesel flue gas, by means of mass balancing. D. salina was grown in artificially lighted, field- (42-L bubble column reactor) and laboratory-scale cultures (23 °C, pH 7.5). In the bubble column reactor, algae grew with an average specific growth rate of 0.237 day-1 under flue gas supplementation (6.3 % (v/v) CO2, 1.2 ppmv NO x ), and CO2 was retained to 39 % in the system. The specific sequestration rate for CO2 was low, with 0.13 g CO2 L-1 day-1. Cultures emitted up to 13.03 µg CH4 L-1 day-1 and 4261 µg N2O L-1 day-1. The moderate retention of NO x -N was outweighed by emissions of N2O-N, and total N in the system decreased by 15.48 % during the 9-day trial. Results suggest that GHG production was mainly the outcome of anaerobic microbial processes and their emission was lower in pre-sterilized cultures. Under the tested conditions, up to six times more CO2 equivalents were emitted during flue gas treatment. Therefore, the direct GHG emissions of algae culture systems, intended for flue gas treatment (i.e. open ponds) need to be reviewed critically.