Food waste and waste activated sludge conversion into volatile fatty acids to produce bioplastics

Abstract

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.

El gran crecimiento poblacional está comportando una gran generación de residuos que necesitan ser tratados de la mejor manera posible. Por ello, la Unión Europea propone implementar nuevas estrategias de tratamiento de residuos dentro del concepto de economía circular mediante la transformación de las plantas de residuos en biorrefinerías, donde los residuos son concebidos como productos. En este sentido, el proceso de fermentación acidogénica emerge como una tecnología clave para valorizar los sustratos orgánicos en forma de productos de alto valor añadido como son los ácidos grasos volátiles (AGVs) que pueden ser usados para la producción de bioplásticos (polihidroxialcanoatos o PHA).

Los residuos alimentarios y los fangos activos de las depuradoras son los residuos orgánicos más generados en las zonas urbanas. Actualmente son tratados por separado, pero debería contemplarse su tratamiento conjunto ya que resultaría beneficioso debido a las sinergias que se formarían. Por este motivo, la presente tesis se basa en el estudio de la fermentación acidogénica de estos residuos por separado, así como su tratamiento conjunto (co-fermentación) con el fin de generar un efluente rico de AGVs para su posterior conversión en PHA.

Para ello, se llevaron a cabo estudios en discontinuo y en semi-continuo para comprobar el efecto del pH en la fermentación de los residuos alimentarios viendo que tiene un papel crucial. Seguidamente, se estudió la co-fermentación de fangos activados con los residuos alimentarios en diferentes proporciones. Este estudio demostró que la capacidad tampón del fango era suficiente para sostener el pH por encima de los niveles inhibitorios. Asimismo, se mostraron sinergias al co-fermentar estos residuos con producciones mayores que mono-fermentando, viendo que la proporción de cada residuo afectaba a la producción y perfil de los AGVs. Además, también se estudió la relación entre cada fracción de los residuos alimentarios y el perfil de producción de ácidos. También se comparó el efecto de trabajar en discontinuo y semi-continuo viendo que tenía importantes efectos debido a la inmigración y adaptación de los microorganismos. Finalmente, se estudió la viabilidad de producir PHA usando un efluente sintético rico en AGVs obteniendo unos buenos porcentajes de acumulación del biopolímero.