Inhibition of thermochemical treatment on biological hydrogen and methane co-production from algae-derived glucose/glycine

[Display omitted] •Algae-derived glucose/glycine was thermally treated in water, acid and alkaline.•Untreated glucose/glycine gave the highest H2 and CH4 yield of 171.9 and 200.9 mL/g.•Alkaline treatment exhibited the lowest H2 and CH4 yield of 5.9 and 112.9 mL/g.•Alkaline treatment led to formation of 2,3,5-trimethylpyrazine (49.0% of peak area).•Reactions between carbonyl (CO) and amino group (NH2) led to byproducts formation. Algae have emerged as a sustainable feedstock for gaseous biofuel production (such as hydrogen and methane). Fermentative sugars and amino acids can be obtained after suitable pretreatment and hydrolysis of algae. However, binary interactions between the carbonyl group (CO) in sugars and the amino group (NH2) in amino acids possibly occur during thermal pretreatment, resulting in deficient hydrolysis and fermentation performance. In this study, algae-derived glucose and glycine as model substrates were subjected to thermochemical treatment (135 °C for 15 min) under neutral, acid and alkaline conditions to assess their decomposition routes and the associated implications on sequential biohydrogen and biomethane fermentation. Acid treatment mainly resulted in direct decomposition of glucose into 5-methylfurfural (C6H6O2, 34.4% of peak area). While thermal treatment with deionized water and alkaline led to the formation of nitrogen-containing Maillard compounds, namely 1-azido-4-dimethylaminobenzene (C8H10N4, 33.1%) and 2,3,5-trimethylpyrazine (C7H10N2, 49.0%), respectively. Untreated glucose/glycine yielded a biohydrogen production of 171.9 mL/g, while alkaline treatment exhibited a biohydrogen yield of only 5.9 mL/g due to the great loss of fermentable substrate. The total energy conversion efficiency (ECE) of 71.1% was achieved through the second-stage biomethane fermentation of untreated glucose/glycine. Comparatively, alkaline treatment significantly inhibited the total energy recovery with an ECE of 31.9%. The findings of this study suggested that optimised pretreatment strategy for algae needs to be developed to avoid fermentable compounds loss and achieve a higher ECE.