Biorefining Sewage for Energy and Green Chemicals

Anaerobic digestion (AD) has become a well-established method for the treatment of organic wastes, such as sewage sludge, whilst recovering valuable products in the process. Typically, AD produces biogas in the form of carbon dioxide and methane but environmental concerns surrounding fugitive greenhouse gas (GHG) emissions has led to the exploration of alternative, higher value products. During the intermediate stages of AD, biohydrogen and volatile fatty acids (VFAs) are produced which are gaining increased attention due to their higher energy density and higher market value, respectively.<br/><br/>For their production to be adopted at industrial scale however, the yields of biohydrogen and VFAs need to be enhanced. This is often achieved through pretreatment methods and the optimisation of operational conditions to inhibit methanogenic activity and to ensure that conditions are favourable for hydrogen- and VFA-producing microorganisms. The in-situ recovery of fermentation products has also been reported to enhance yields of biohydrogen and VFAs through the alleviation of a process known as end-product inhibition. The main aim of this research was to therefore assess the effect of certain pretreatments, operational conditions, and recovery techniques, on the yields of VFAs and biohydrogen from sewage. These include using thermally hydrolysed sludge (THS), various hydraulic retention times (HRTs), a two-stage fermentation process and the use of a pervaporation-electrodialysis system for the recovery of VFAs from primary sewage (PS).<br/><br/>THS was found to be an unsuitable substrate for biohydrogen production, despite using operational parameters known to inhibit methanogenic activity including using heat-treated inoculum, short HRTs and a low pH. THS was also compared with PS as a substrate for VFA production and was found to produce 2.5 times less VFA than PS, indicating that PS is a more suitable substrate than THS for VFA production. This consideration is particularly relevant for the future development of biorefineries aimed at maximising VFA production from sewage-derived feedstocks.<br/><br/>PS was therefore used in a two-stage process which focused on enhancing VFA yields in the first stage and overall biomethane yields in the second stage. It was shown that a two-stage fermentation of PS successfully enhanced biomethane yields by up to 19.14%. Also, a first-stage HRT of 5 days achieved the highest mean total VFA yield of 95 gvfa kgvs-1 as well as the greatest increase in biomethane potential during the second stage. A two-stage configuration could therefore be considered at larger scale to optimise the production of both VFAs and biogas (or biomethane), requiring minimal adjustments to capital or operational expenditure. Before scaling up, additional optimisation and analyses, such as techno-economic and life-cycle assessments, should be conducted to ensure that both carbon and cost impacts are at least comparable to, or better than, those of current processes.<br/><br/>The findings from the previous experiments helped inform the design of the final experiment focused on VFA extraction. Specifically, the fact that PS is a more suitable substrate for VFA production and that a 5-day HRT is optimal for enhanced VFA yields. The final experiment therefore aimed to further enhance the VFA yields from PS at pilot scale by extracting them in-situ using a combination of pervaporation and electrodialysis (PTFE-ED). A 13% increase in mean total VFA yields was achieved during the VFA extraction phase compared with the control phase and VFA extraction via the PTFE-ED system altered the composition of VFA species produced, with the yields of acetic, propionic and butyric acid changing from 39 to 50%, 55 to 38% and 6 to 12% respectively, between the control and PTFE-ED phases. Employing a PTFE-ED system for VFA extraction from PS not only has the potential to increase overall VFA yield but may also influence the ratio of VFA species, which could be advantageous for downstream applications, such as industrial processes or as a feedstock for PHA-producing reactors.

Name dc:type.qualificationname

Doctoral Thesis

Level dc:type.qualificationlevel

Student thesis

Year dc:date.issued

2026

Author dc:creator

• Oram, Lucy

Advisors dc:contributor.advisor

• Guwy, Alan
• Massanet-Nicolau, Jaime

Language dc:language

eng

Identifier

oai:pure.atira.dk:studenttheses/057732ca-7f7d-4969-9259-bdcc179f9a1b

OAI identifier oai:identifier

oai:pure.atira.dk:studenttheses/057732ca-7f7d-4969-9259-bdcc179f9a1b