Evaluation of Mass and Energy Balances in the combined microalgas growth digestion process

Abstract

The fundamental mass and energy balances of the combined growth-digestion process are necessary to quantify the potential for energy and nutrients recovery in the overall biofuel production process from microalgae. This study quantified the C, N, and P mass balances and the energy balance in the integrated process of microalgae photosynthetic cultivation (under autotrophic and mixotrophic conditions) followed by anaerobic digestion. Under fully photoautotrophic growth, the biomass stoichiometric formula was CH1,63N0,14O0,43P0,006S0,005, while the maximum photosynthetic efficiency (PE) and microalgae production rate accounted for 7,6% and 71,3 g/m3·d, respectively. C. sorokiniana was capable of photoautotrophically assimilating 1,8 ± 0,1 g of CO2 per g of TSS of microalgae and released 1,0 ± 0.0 g of O2 per g of CO2 consumed. The stoichiometric formula of microalgae mixotrophically grown was CH1,68N0,16O0,48P0,006S0,008, with only 13,8 ± 1,2% of the initial C-glucose present in the MSM assimilated, which suggested a partial growth inhibition by glucose in the particular MSM here used. Mixotrophic conditions supported a specific growth rate at the exponential growth phase () of 1,6 d-1, similar to 1,5 d-1 under autotrophic conditions, and reached a maximum PE of 7,4% and a maximum production rate of 142,3 g/m3·d. The anaerobic digestion of microalgae cultivated autotrophically, showed that 54,6% ± 0,6 of the initial C present in the biomass was hydrolyzed, resulting in 14,5 ± 0,7% as C-CO2 and 33,2 ± 0,5% as C-CH4. The potential recovery from N and P present in the biomass was 59,2 ± 1,9% as N-NH4+ and 88,8% ± 2,3 as P-PO4-3, respectively. During the anaerobic digestion of mixotrophically grown microalgae, 45,5 ± 0,6% of the initial C was hydrolyzed, 13,5 ± 0,9% as C-CO2 and 35,7 ± 0,5% as C-CH4. The potential recovery from N and P was 69,9 ± 2,9% as N-NH4+ and 77,3 ± 1,9% as P-PO4-3, respectively. The energy recovery from the chemical energy fixed as biomass under photoautotrophic and mixotrophic conditions was 47,9% and 48,9%, respectively, while these values decreased to 3,3%, when referred to the total energy supplied