In situ extraction and recovery of volatile fatty acids from biogas-producing anaerobic digestion

Morison, Storm Diana (2021-03)

Thesis (MEng)--Stellenbosch University, 2021.

Thesis

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.

AFRIKAANSE OPSOMMING: ’n Belangrike stel samestellings wat geproduseer word as intermediêre produkte in anaerobiese vertering (AD) -tegnologieë, al word hulle nie gewoonlik herwin as produkte in biogasaanlegte nie, is vlugtige vetsure (VFA’s). Bio-gebaseerde VFA-produksie vanuit AD deur ekstraktiewe fermentasie te gebruik, is ’n belowende benadering om te beheer teen drastiese pH-afname en onstabiele bedryfsdoeltreffendheid as gevolg van VFA-akkumulasie in AD-stelsels, terwyl ’n tweede waardevolle produkstroom geproduseer word. Hierdie werk ondersoek die lewensvatbaarheid van integrasie van VFA-ekstraksie en herwinning met AD deur ekstraktiewe fermentasie te gebruik sonder om die verteerder se biogas produktiwiteit te stuit. Vyf ekstraheermiddels (tri-n-oktielamien (TOA), tri-n-butielfosfaat (TBP), tri-n-oktielfosfienoksied (TOPO), Aliquat 336 en triheksiel(tetradektiel)fosfonium bis-2,4,4-(trimetielpentiel)fosfinaat([P666,14][Phos])) in kombinasie met olielalkohol, lampolie en kanola-olie as verdunners is ondersoek gebaseer op (i) ekstraksiekapasiteit by variërende pH, (ii) bioverenigbaarheid met die mikrobiese konsortium en (iii) uitvoerbaarheid van VFA terug-ekstraksie. Laboratoriumskaal vloeistof-vloeistof ekstraksie (LLE) -eksperimente met sintetiese VFA-oplossings het getoon dat die ekstraheermiddel Aliquat 336 die hoogste kapasiteit het om VFA’s by pH 3.9 — 6.8 te ekstraheer, wat ’n totaal van 50 — 70% VFA-ekstraksies bereik, deur die verdunners olielalkohol, lampolie en kanola-olie te gebruik. Ekstraksiekapasiteit het afgeneem bo die pKa van die sure vir die res van die ekstraheermiddels ondersoek. TOA-lampolie en TOPO-kanola-olie het 10 — 25% van totale VFA (tVFA) by pH 5.6 — 6.8 geëkstraheer, wat voorstel dat daar oplosmiddels is met die kapasiteit om sure te ekstraheer binne gepaste pH-bestekke vir biogas produserende AD, wat tipies bo die pKa van die geëkstraheerde sure is. Meeste oplosmiddelkombinasies, met die uitsondering van [P666,14][Phos], het soortgelyke of selfs verbeterde VFA-ekstraksies van afvalwaterstelsels getoon, wat hul potensiaal vir toepassing in nie-ideale stelsels beklemtoon. Biogasproduksie eksperimente op banktoetsskaal wat industriële afvalwater gebruik het gedemonstreer dat bioversoenbare ekstraksiemiddel-oplosmiddelstelsel koproduksie van biogas en VFA’s, met verbeterde biogasproduktiwiteit in sekere gevalle, toelaat. Stelsels wat TOA-olielalkohol, TBP-olielalkohol, TOPO-olielalkohol, TOPO-kanola-olie en [P666,14][Phos]-olielalkohol bevat, het twee tot vyf keer meer biogas geproduseer as die kontrole met gemiddelde metaanpersentasies van tussen 70 en 75% (in vergelyking met 55% bereik met die kontrole) en analoë produksie is waargeneem toe TOPO-lampolie en TOA-lampolie gebruik is relatief tot die kontrole. Die teenwoordigheid van Aliquat 336 het minimale gasproduksie tot gevolg gehad ongeag die verdunner wat gebruik is, en word daarom nie voorgeskryf vir toepassing in biogasproduserende AD nie. Totale terug-ekstraksie VFA-herwinning van 80 — 100% is bereik van TOPO, TBP, TOA en [P666,14][Phos] deur NaOH(aq) te gebruik om VFA’s te herwin en die oplosmiddel te regenereer. Aliquat 336 het laer potensiaal getoon vir terug-ekstraksie met herwinning tussen 40 en 50 %. Terug-ekstrahering met oplosmiddels wat kanola-olie bevat word nie voorgestel nie as gevolg van waargenome emulsifikasie in hierdie stelsels. Die eksperimente dui breedweg aan dat dit moontlik is om ’n bioversoenbare oplosmiddelkombinasie te kies wat gebruik kan word in AD met die vermoë om biogas en VFA’s te koproduseer, en selfs produktiwiteit in biogasproduserende verteringstelsels te versterk. Hierdie metodologie kan geïntegreer en gebruik word as ’n pH-beheerstrategie terwyl bestuur en reduksie van afval, hulpbronherwinning, en gebruik van hernubare energie, bevorder word. TOA-olielalkohol, TOA-lampolie, TOPO-olielalkohol, TOPO-lampolie en TBP-olielalkohol word voorgestel vir verdere ondersoek as potensiële oplosmiddels vir in situ VFA-ekstraksie van biogasproduserende AD-afvalwaterbehandelingstelsels.

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