Co-production of volatile fatty acids and biogas from an anaerobic digestion system using in situ extraction

• Published in: Separation and purification technology 2021-02, Vol.257, p.117891, Article 117891
• Main Authors: James, Gerard, Görgens, Johann F., Pott, Robert W.M.
• Format: Article
• Language: English
• Subjects: Anaerobic digestion; Biogas; In situ extraction; Liquid-liquid extraction; Volatile fatty acids
• ISSN: 1383-5866; 1873-3794
• DOI: 10.1016/j.seppur.2020.117891

•Anaerobic digestion can be used as a source of volatile fatty acids.•In situ liquid-liquid extraction is capable of extracting volatile fatty acids.•Co-production of biogas and volatile fatty acids is possible in anaerobic digestion.•Trioctylamine and tributyl phosphate are biocompatible in anaerobic digestion. Trioctylamine (TOA) and tributyl phosphate (TBP) along with three diluents (canola oil, lamp oil, and oleyl alcohol) were identified as potentially suitable solvents for direct comparison for the application of in situ volatile fatty acid (VFA) extraction from anaerobic digestion (AD) systems. The highest degree of extraction at pH 5.0 was observed for TOA/oleyl alcohol (50.5 ± 0.1%) in liquid-liquid equilibrium experiments. Biochemical methane potential (BMP) tests were conducted for solvent toxicity on the biogas producing consortium, using the three best solvents. From these experiments, the samples extracted using TOA/canola oil and TBP/lamp oil performed better than the control in total gas production (168 mL ± 26 mL and 146 ± 5 mL) and methane percentage (12.6 ± 2.8% and 14.7 ± 6.7% respectively). The control produced 115 ± 39 mL of gas (9.7 ± 1.3% of methane). Over a 28 day digestion period, 2.40 ± 0.30 g/L and 5.84 ± 0.36 g/L of VFAs from 10 mL of solvent were successfully recovered from TBP/lamp oil and TOA/ oleyl alcohol, respectively, in these BMPs. To demonstrate applicability at a larger scale, a novel 17 L AD-bioreactor was modified by placing an in situ extraction tube inside the reactor, connected to a circulator and back-extraction unit. 0.078 g of total VFAs were extracted out with TOA/oleyl alcohol using a pH-swing back-extraction unit. Additionally, the AD bioreactor produced 6.71 L of biogas with a methane percentage of 43% over a 22 day digestion period. This implies that in situ LLE extraction may be industrially applicable as a potential co-production process for biogas and VFAs, significantly building from previous VFA extraction work.