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Si us plau utilitzeu sempre aquest identificador per citar o enllaçar aquest document: https://hdl.handle.net/2445/223566
Engineering and microbiology insights into co-fermentation of waste activated sludge and food waste
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[eng] According to the circular economy principles, wastewater treatment plants (WWTPs) should be transformed into water resource recovery facilities. Urban organic waste, including sewage sludge from WWTPs, can be converted into valuable byproducts, such as volatile fatty acids (VFA), through acidogenic fermentation (AF). AF involves hydrolysis, acidogenesis, and acetogenesis, while limiting methanogenesis, especially aceticlastic Archaea that consume acetic acid.
One strategy to increase VFA yields from organic wastes is co-fermentation, which aims to ferment two or more complementary substrates simultaneously. The mixture of waste activated sludge (WAS) and food waste (FW) is one of the most studied combination for acidogenic co-fermentation, as WAS provides buffering capacity and indigenous microbial communities, while FW offers highly biodegradable organic matter. However, most studies (∼40 %) rely on batch tests, limiting insights into biomass migration and the effects of operational variables on the microbial community observed in continuous operation. Key operating variables, such as temperature, retention time, and organic loading rate (OLR) remain underexplored.
This thesis addresses key challenges in acidogenic co-fermentation, including enhancing fermentation yields, optimising product profiles, and preventing methanogenesis. This research evaluates the long-term co-fermentation of WAS and FW under varying operational conditions, including the effects of OLR shock, microbial source variability, operational temperature, and WAS collection period on fermentation performance and microbial dynamics, with the goal of achieving a consistent and reproducible co-fermentation process.
Three experimental studies on semi-continuous acidogenic co-fermentation have been conducted to evaluate different operational variables. In the first study, two key perturbations were evaluated in the acidogenic co-fermentation at 35 °C: an increase in OLR and changes in WAS sources (same WWTP, different collection periods). A control reactor was operated at 11 gVS/(L⋅d), while a test reactor experienced a sustained OLR increase to 18 gVS/(L⋅d). Despite the OLR increase, fermentation yields remained consistent (~300 mgCOD/gVS). However, the product profile shifted: at 11 gVS/(L⋅d), the fermentation liquid was enriched in acetic, butyric and propionic acids, while at 18 gVS/(L⋅d), acetic acid, ethanol, and caproic acid predominated. Reverting the OLR to 11 gVS/(L⋅d) restored the original profile. Changes in the collected WAS introduced acetic
acid-consuming methanogens, whose growth was temporarily suppressed by the higher OLR. This study demonstrated that microbial monitoring and post-fermentation tests are effective tools for the early detection of acetic acid consumption events.
The second study investigated long-term temperature effects on co-fermentation yields and microbial dynamics. Semi-continuous fermenters were evaluated at operating temperatures of 25, 35, 45, and 55 °C. WAS batches collected at different periods were used to evaluate their microbiota's influence on the fermenters' microbial communities. Results showed and improvement of the solubilisation at higher temperatures and the highest fermentation yield was at 55 °C (425 ± 28 mgCOD/gVS). Temperature also had a crucial impact on the fermentation product profile. At 55 °C, acetic and butyric acids were the dominated products. Then, at 35 °C, acetic, butyric, and propionic acids predominated, and at 45 °C, caproic acid was accumulated. Distinct microbial communities emerged at each temperature, also influenced by the WAS microbiota. Methanogens introduced with WAS were probably responsible for the acetic acid consumption detected in one of the fermenters operating at 35 °C.
In the third and last study, two long-term co-fermentation experiments evaluated the effects of temperature (35 °C vs. 55 °C), WAS origin, and collection periods. At 55 °C, solubilisation yields improved compared to 35 °C, although fermentation yields did not consistently higher. The product profile at 55 °C was dominated by acetic and butyric acids in 13 operated fermenters. Temperature had a greater impact on microbial community structure than WAS origin. Acetic acid consumption, observed only at 35 °C in fermenters fed with WAS_A, was attributed to Methanosarcina species (midas_s_9557), which were inhibited at 55 °C. Functional redundancy ensured stable metabolic functions among fermenters at 55 °C. These findings demonstrate the robustness and consistency of thermophilic co-fermentation as a biotechnological solution for managing WAS and FW.
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PÉREZ I ESTEBAN, Noemí. Engineering and microbiology insights into co-fermentation of waste activated sludge and food waste. [consulta: 5 de desembre de 2025]. [Disponible a: https://hdl.handle.net/2445/223566]