Mechanisms and enhancements on harmful algal blooms conversion to bioenergy mediated with dual-functional chitosan

Dark fermentation (DF) with notorious algal blooms (ABs) is a promising approach to simultaneously resolve water pollution and energy shortage with extra benefit of carbon neutralization, but it is challenging due to inefficient biomass harvesting and fermentation. To conquer these problems, a strategy using chitosan as dual-functional mediators was proposed, in which ABs were first efficiently harvested with chitosan as flocculant and then fermented into bioenergy with chitosan as C:N regulator. For 1.0 g/L of ABs density, harvesting efficiency of 97.58 % was achieved with 0.50 g/L chitosan dosage under pH of 2. ABs harvesting mechanisms were illuminated from kinetic perspective. More importantly, C:N ratio of the harvest flocs were improved to 11.82 by mixing C-rich chitosan with N-rich algae biomass, which endowed the feedstocks with better property for bioenergy production. As results, the chitosan- microalgae flocs with up-regulated C:N ratio product a higher hydrogen yield of 49.99 mL/(g VS) and improved volatile fatty acids yields with butyric acid (1672.35 mg/L), acetic acid (1170.49 mg/L) and ethanol (361.93 mg/L) as major products. At this time, the highest energy yield of 70.1 kJ/L with an energy conversion efficiency of 59 % was obtained. Together, conversion mechanism of DF indicated that active fatty acids conversion occurred during DF. This study will inspire the eco-friendly ABs disposal with extra benefits of bioenergy production.