Browsing by Author "Morison, Storm Diana"
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- ItemIn situ extraction and recovery of volatile fatty acids from biogas-producing anaerobic digestion(Stellenbosch : Stellenbosch University, 2021-03) Morison, Storm Diana; Pott, Robert William M.; Van Rensburg, Eugene; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: An important set of compounds which are produced as intermediates in anaerobic digestion (AD) technologies, although they are not widely recovered as products in biogas plants, are volatile fatty acids (VFAs). Bio-based VFA production from AD using extractive fermentation is a promising approach to control against drastic pH reduction and unstable operational performance due to VFA accumulation in AD systems, while producing a second valuable product stream. This work explores the viability of integrating VFA extraction and recovery with AD using extractive fermentation without arresting the biogas productivity of the digester. Five extractants (tri-n-octylamine (TOA), tri-n-butyl phosphate (TBP), tri-n-octylphosphine oxide (TOPO), Aliquat 336 and trihexyl(tetradecyl)phosphonium bis-2,4,4-(trimethylpentyl)phosphinate ([P666,14][Phos])) in combination with oleyl alcohol, lamp oil and canola oil as diluents were investigated based on (i) extraction capacity at varying pH, (ii) biocompatibility with the microbial consortium and (iii) feasibility of VFA back-extraction. Laboratory scale liquid-liquid extraction (LLE) experiments with synthetic VFA solutions revealed that the extractant Aliquat 336 had the highest capacity to extract VFAs at pH 3.9-6.8, attaining total VFA extractions of 50-70% using the diluents oleyl alcohol, lamp oil and canola oil. Extraction capacity decreased above the pKa of the acids with the rest of the extractants studied. However, TOA-oleyl alcohol, TOPO-lamp oil and TOPO-canola oil extracted 10-25% total VFA (tVFA) at pH 5.6-6.8, which suggested that there are solvents with the capacity to extract acids within suitable pH ranges for biogas-producing AD, which are typically above the pKa of the extracted acids. Most solvent combinations, with the exception of [P666,14][Phos], exhibited similar or even improved VFA extractions from wastewater systems, highlighting their potential for application in non-idealised systems. Bench-scale biogas production experiments using industrial wastewater demonstrated that biocompatible extractant-solvent systems allow for co-production of biogas and VFAs, with enhanced biogas productivity in some cases. Systems containing TOA-oleyl alcohol, TBP-oleyl alcohol, TOPO-oleyl alcohol, TOPO-canola oil and [P666,14][Phos]-oleyl alcohol produced two to five times more biogas than the control with average methane percentages of between 70-75% (compared to 55% achieved with the control) and analogous production was seen using TOPO-lamp oil and TOA-lamp oil relative to the control. The presence of Aliquat 336 resulted in minimal gas production regardless of the diluent used, and is therefore not recommended for application in biogas-producing AD. Total back-extraction VFA recoveries of 80-100% were achieved from TOPO, TBP, TOA and [P666,14][Phos] using NaOH(aq) to recover VFAs and regenerate the solvent. Aliquat 336 exhibited lower potential for back-extraction with recoveries between 40-50%. Back-extraction with solvents containing canola oil is not recommended due to observed emulsification in these systems. The experiments outline that it is possible to select a biocompatible solvent combination that could be used in AD with the ability to co-produce biogas and VFAs, and even enhance productivity in biogas producing digester systems. This methodology could be integrated and used as a pH control strategy while promoting management and reduction of waste, resource recovery, and utilisation of renewable energy. TOPO-lamp oil, TOPO-oleyl alcohol, TOA-lamp oil, TOA-oleyl alcohol and TBP-oleyl alcohol would be recommended for further investigation as potential solvents for in situ VFA extraction from biogas-producing AD wastewater treatment systems.