Abstract
Alarmingly, 48% of wastewater is estimated to be inadequately treated, posing significant environmental and public health risks. The industrial sector is a significant contributor and is often characterised by high concentrations of organic matter, requiring advanced treatment technologies. Conventional wastewater treatment processes are often energy-intensive and rely on fossil fuels, contributing to climate change. This realisation has prompted a shift towards more sustainable approaches. Anaerobic digestion (AD) is a biological process that converts organic matter to biogas and nutrient-rich digestate. Recovering biogas and nutrients can reduce environmental impacts and operational costs.Operational parameters play a crucial role in the performance and stability of AD. Microbial communities are subjected to fluctuating conditions in industrially scaled AD plants, including variable feedstock concentration and temperature. These fluctuations can lead to changes in community structure and function, potentially disrupting processes. Many studies have focused on lab-scale reactors using synthetic or real feedstock. However, synthetic feedstock is not representative of the variability present within real-world wastewater, and studies using industrial wastewater often lack detailed chemical characterisation when paired with microbiome studies. Furthermore, these studies are often limited to singular time points, failing to capture the dynamic nature of the microbiome response to fluctuating feedstock composition. Understanding the interactions between these parameters and the AD microbiome over time is key for enhancing process performance and stability.
Mycoprotein fermentation wastewater (MFWW) is a by-product of Quorn™ food production and represents a carbon- and nutrient-rich candidate for AD. This study aims to assess the resource recovery potential of MFWW and explore the impact of temperature and feedstock composition on reactor stability and microbiome structure over time. Two case studies were conducted using characterised MFWW as influent. In case study one, mesophilic and thermophilic reactors were compared to evaluate the effect of temperature on AD. In case study two, single- and two-stage reactors were assessed to determine the impact of reactor configuration. Reactors were also subjected to a temperature shock to explore the robustness of different reactors to operational perturbations.
MFWW was found to be a highly complex but balanced feedstock containing a diverse range of carbon and nitrogen compounds. Detailed chemical characterisation revealed high chemical oxygen demand (COD) levels (3.9 to 33.3 g/L). Protein and essential amino acid content were high, supporting the potential for nutrient recovery. Inhibitory compounds, such as ammonium and nitrites were present at low concentrations. Biomethane potential assays demonstrated high biodegradability achieving a biomethane of 92.0 ± 6.2% of the theoretical yield based on COD. These attributes make MFWW a promising candidate for AD, with a strong potential for resource recovery.
In case study one, mesophilic reactors produced more biogas than thermophilic reactors, but all reactors achieved high COD removal efficiencies exceeding regulatory thresholds. The lower biogas production in thermophilic reactors was attributed to issues with microbial community acclimation to higher operational temperatures. Acetic acid concentrations were also lower in thermophilic than mesophilic reactors, potentially limiting acetoclastic methanogenesis. Furthermore, elevated Methanomassiliicoccus, Methanospirillum and Methanobrevibacter populations were present in thermophilic reactors compared to mesophilic reactors. Notably, Methanomassiliicoccus is associated with novel non-acetate related methanogenesis pathways and lower biogas production. The difference in archaea population could also help to explain the lower biomethane yield of thermophilic compared to mesophilic reactors.
Case study two explored the effect of different reactor configurations (single-, first-, and second-stage) on reactor performance. First-stage reactors consistently exhibited the highest COD removal efficiency and biogas production. Second-stage reactors showed more variability in performance and often struggled to maintain COD removal above the regulatory threshold. A shift in taxonomic profile corresponded to increased organic loading rate and was associated with improved reactor performance. This result indicates that microbiome flexibility improves reactor stability in response to feedstock fluctuations. Despite having the same feedstock, operational conditions and starting inoculum, some reactors had distinct taxonomic profiles but comparable performance, indicating performance was decoupled from the microbiome profile.
Clostridiaceae, were associated with lower biogas production, and times-series network analysis revealed that Firmicutes had little conservation over time for reactors with high biogas production. Dissimilarity analysis revealed that influent COD was the most influential operational parameter on beta diversity, followed by OLR and temperature. Acetic acid and total sugar and sugar alcohol, significantly influenced beta diversity. Notably, individual VFAs appeared to have a more substantial influence on beta diversity than total VFA content, highlighting the importance of detailed chemical analysis for AD studies.
In conclusion, this research advances the understanding of anaerobic digestion for complex feedstocks, such as MFWW, providing valuable insights into the dynamic role of microbial communities and operational parameters in achieving stable and efficient AD. The findings have practical implications for wastewater treatment aiming to transition toward circular economy models, with AD offering a viable solution for energy and nutrient recovery from industrial wastewater.
| Date of Award | 1 May 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Miao Guo (Supervisor) & Peter Ellis (Supervisor) |
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