Food waste and waste activated sludge conversion into volatile fatty acids to produce bioplastics
[eng] Rapid population growth is leading to many environmental problems, among which a large waste generation should be highlighted. More than 40% of these wastes correspond to organic wastes that are mainly treated by composting, anaerobic digestion, incineration, or even landfilled. Hence, it is i...
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| Formato: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2022 |
| País: | España |
| Recursos: | Universidad de Barcelona |
| Repositorio: | Dipòsit Digital de la UB |
| OAI Identifier: | oai:diposit.ub.edu:2445/187541 |
| Acesso em linha: | https://hdl.handle.net/2445/187541 http://hdl.handle.net/10803/674750 |
| Access Level: | acceso abierto |
| Palavra-chave: | Fermentació Àcids grassos Compostos orgànics volàtils Plàstics biodegradables Polièsters Fermentation Fatty acids Volatile organic compounds Biodegradable plastics Polyesters |
| Resumo: | [eng] Rapid population growth is leading to many environmental problems, among which a large waste generation should be highlighted. More than 40% of these wastes correspond to organic wastes that are mainly treated by composting, anaerobic digestion, incineration, or even landfilled. Hence, it is important to implement new treatment strategies within the circular economy concept transforming organic waste treatment plants into biorefineries. In this way, acidogenic fermentation emerges as a key technology to valorise these substrates producing high value- added products such as volatile fatty acids (VFAs), that could be further transformed into bioplastics (i.e., polyhydroxyalkanoates or PHA). Consequently, acidogenic fermentation and co-fermentation of waste activated sludge (WAS) and food waste (FW) were studied to achieve a VFA-rich effluent for its subsequent conversion into PHA. The pH effect on the acidogenic fermentation of FW was studied in batch mode (pH range from 4 to 11) and in semi-continuous fermenters working under acidic (6) and alkaline pH (9.5-10). Batch tests revealed that pH between 6-9 could lead to a higher VFA production, with a higher acetic acid content when pH increased within this range. In the semi-continuous fermenters, a maximum VFAs yield of 503.1 mgCOD/gVS was obtained when working at pH 6 with acetic, butyric and caproic acids as majoritarian VFAs, while at pH 10, a lower yield (315.1 mgCOD/gVS) was obtained with acetic as the main acid. WAS and FW co-fermentation was explored (50%, 70% and 90% WAS on VS basis), demonstrating the benefits, since a higher fermentation yield was obtained in WAS/FW co-fermentation assays compared to WAS or FW mono-fermentation. In all mixtures tested, the buffer capacity of WAS was enough to maintain the pH above inhibitory levels without reagents addition. Moreover, when the proportion of FW in the WAS/FW was raised, more butyric and less propionic acid was produced achieving a maximum fermentation yield of 480 mgCOD/gVS when the mixture was 50%FW+50%WAS. Since the effect of each FW fraction has been barely studied, discontinuous assays using diverse FW fractions were also performed. The principal components analysis (PCA) revealed the relation between each fraction and each fermentation profile. Furthermore, these assays were essential to understanding the importance of balancing the protein-to-carbon ratio to achieve a maximum VFA yield of about 500 mgCOD/gVS. WAS and FW co-fermentation was studied under long-term conditions in semi-continuous fermenters at several organic loading rates (OLR). This study demonstrated the importance of microorganisms’ immigration with the feed substrates and their adaptation in semi-continuous processes. The stages carried out at OLR 9 and 11 gVS/(L·d) obtained lower VFAs yields probably due to the methanogenic activity favoured at the neutral pH. However, when higher OLR were applied (14 and 18 gVS/(L·d)), the pH started to decrease with a concomitant increase on the VFA yield until achieving a maximum of 475 mgCOD/gVS, with butyric as the main acid. Moreover, this study demonstrates that not only the FW properties and its proportion affect the obtained VFA yield and distribution on co-fermentation, but also the WAS characteristics are important, especially when the pH is low, and the buffering capacity could avoid a sudden drop in the pH. Finally, the start-up and operation of a sequencing batch reactor (SBR) to select PHA-storing microorganisms was performed using a VFA-rich feeding with an OLR of 2.0 and 2.8 gCOD/(L·d). The biomass selection was carried out with a double growth limitation strategy (feast/famine and uncoupled carbon and nitrogen feeding). The successful selection of PHA-storing biomass was confirmed in batch accumulations assays where VFA pulse feeding strategy was used obtaining a PHA content between 44 and 46% (on SS basis) with polyhydroxybutyrate (PHB) as the main component. |
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