2020 Theses Doctoral

Resource Recovery from Organic Waste through an Anaerobic Fermentation Platform

Pavlakis, Eirene

Resource recovery strategies have increasingly been employed nowadays due to the increasing waste generation, strict waste disposal regulations and increased public awareness. Anaerobic fermentation is a common method used for resource recovery of waste streams. This study focused on recovery of volatile fatty acids (VFA) from anaerobic co-fermentation of food waste and primary sludge and sequential conversion to polyhydroxyalkanoate (PHA). The ultimate objective was to maximize the VFA and PHA production by manipulating the fermentation operating conditions (substrate; food waste FW vs mixture of primary wastewater sludge and food waste PS+FW, organic loading rate OLR 10 and 25 kg-COD/m3/d, pH 7, 7.5, 8, and 9, hydraulic retention time HRT 2, 4, and 6 days) for VFA production and the substrate (feeding pattern; one-pulse vs multi-pulse, composition and source; fermentation-derived vs synthetic acids) for PHA production. The factors that were evaluated for the VFA and PHA optimization were the VFA yield, concentration and speciation and PHA yield and speciation.

Results showed that VFA production was higher at the higher of the two OLR, and the FW reactor showed higher concentrations of VFA than the PS+FW reactor, which showed that food waste are able to improve the fermentation process. Experiments showed that the preferred conditions for max VFA and PHA production were co-fermentation of primary sludge and food waste of OLR 25 kg-COD/m3/d, at alkaline conditions of pH 9 and HRT 6d and then feeding of fermentation derived VFA in one-pulse in PHA accumulating reactor. Maximum PHA accumulation achieved was 0.38 ± 0.05 g-PHA/g-VSS from fermentate produced at pH 9.0 and HRT 6d.

Polyhydroxyalkanoates (PHAs) are biopolymers synthesized by microorganisms. This study also focused on the microbial ecology of the two processes involved in PHA production; (1) acidogenic co-fermentation of primary sludge and food waste, and (2) enrichment of PHA accumulating microorganisms. The analysis regarding the fermentation reactor included comparison of microbial ecology between the different fermentation conditions tested (pH 7, 8 and 9 and HRT 2, 4 and 6 days). Most of the bacterial families present in the fermentation reactor belonged to the phylum of Firmicutes, Bacteriodetes and Proteobacteria. The microbial species in that condition were Tepidimicrobium xylanilyticum, Tepidimicrobium ferriphilum, Vagococcus lutrae, and Vagococcus acidifermentans. Regarding the PHA reactor, the analysis included monitoring of the swift in microbial ecology throughout the enrichment process. The maximum PHA accumulation achieved was 38% ± 5%, g-PHA/g-VSS, and the representative microbial species were Acinetobacter radioresistens, Pseudoxanthobacter liyangensis, and Xanthobacter agilis. The objective of this study was to connect the VFA and PHA production with the microbial ecology of each process. Study of the microbial ecology of the combined system of VFA production with further conversion to PHA through mixed microbial cultures could contribute to an efficient process design with possible application into Water Resource Recovery Facilities (WRRF) and decrease of the cost of PHA production, therefore increase their market value. Retrofitting of the existing anaerobic digestion infrastructure could create a biorefinery platform that uses wastes to produces biodegradable plastics instead of methane.