An integrated approach for efficient conversion of Lemna minor to biogas

Highlights: • Biogas (CH₄ + H₂) production from L. minor by integrated approach has high potential. • Three-stage integration was effective for highest energy yield (38.7 ± 1 mol/kg TOC_R). • Maximum biogas recovered resulting in recycling of 530.7 dm³ of CO₂ per kg TOC_R. • Analysis of organic flux elucidated the effectiveness of each process on integration. -- Abstract: Aquatic weed, Lemna minor was evaluated for its potential as a feedstock for gaseous fuel production (biohythane) in an integrated strategy. Three approaches viz., acidogenic fermentation (H_AF), electrohydrogenesis (H_MEC) and methanogenesis (M_AD), were evaluated in single stage as well as in different combinations of two stage (H_AF → H_MEC, H_AF → M_AD) and three stage (H_AF → H_MEC → M_AD, H_AF → M_AD → H_MEC) to tap the maximum feasible energy. Compared to single and two-stage operations, three-stage operation evidenced higher biogas (H₂ + CH₄) yield with remarkable total organic carbon (TOC) reduction. Irrespective of the integration sequence, H_AF in first stage depicted the possibility of harnessing higher energy by accumulation of volatile fatty acids (VFA) along with H₂ production. Similarly, integration of M_AD in second stage showed the possibility of tapping higher energy rather than H_MEC due to higher carbon loss as CO₂ coupled to more H₂ fraction in biogas in case of H_MEC. Among, three-stage integrations, higher biogas yield and energy recovery was observed in H_AF → M_AD → H_MEC (38.77 mol biogas/kg TOC_R; 25,415 KJ/kg TOC_R) as compared to H_AF → H_MEC → M_AD (37.79 mol biogas/kg TOC_R; 15,416 KJ/kg TOC_R). Along similar lines, analysis of organic carbon flow exhibited significant substrate degradation in three stage integrations (72.5–81.4%) as compared to second (66.2–70%) and first stage (39.7–56.5%).