Improving production of volatile fatty acids and hydrogen from microalgae and rice residue: Effects of physicochemical characteristics and mix ratios

Dark fermentation may be hindered by insufficient bioavailable carbon and nitrogen sources as well as recalcitrant cell wall structures of substrates. Protein-rich microalgae and carbohydrate-rich rice residue with various mix ratios can optimise biohydrogen and volatile fatty acids production. Optimal pretreatment of the microalgae with 1% H₂SO₄ and the rice residue with 0.5% H₂SO₄ under hydrothermal heating (140 °C, 10 min) achieved reducing sugar yields of 187.3 mg/g volatile solids (VS) (hydrolysis efficiency: 54%) and 924.9 mg/g VS (hydrolysis efficiency: 100%), respectively. Multiscale physiochemical characterisations of solid hydrolytic residues confirmed considerable damage to both substrates. Co-fermentation of pretreated rice residue and microalgae at a mix ratio of 5:1 exhibited the maximum hydrogen yield of 201.8 mL/g VS, a 10.7-fold increase compared to mono-fermentation of pretreated microalgae. The mix ratio of 25:1 resulted in the highest carbon to volatile fatty acids conversion (96.8%), corresponding to a maximum energy conversion efficiency of 90.8%.