This research addresses gaps in current knowledge regarding process issues associated with long term semi-continuous digestion of food waste as a sole substrate, and the role of trace elements and biomass retention in digestion of food wastes. Source segregated food wastes were collected from a university catering facility and found, in characterisation studies, to have a total solids (TS) content of 28.1±0.25 %, a volatile solids (VS) content of 95.5±0.06% of TS and a chemical oxygen demand (COD) of 422±16 g kgwet weight -1. The total Kjeldahl nitrogen (TKN) and total lipid content were 22±1% and 3.8±0.24% of TS, respectively. The substrate was then processed during a number of digestion trials using mesophilic continuously-stirred tank reactors (CSTRs), to establish the suitability of this substrate for CSTR digestion. It was found that although good specific methane production of 0.36 l gVSadded -1 was obtained from the substrate, the process was unstable at a hydraulic retention time (HRT) of 25 days, with methanogenic failure occurring after 80 days or when the organic loading rate (OLR) was increased. Further digestion trials were initiated, therefore, to investigate the effects of trace element supplementation and extending HRT on process stability, areas for which there is little information in existing literature. Reactors with hydraulic retention times of 25, 30, 50, 100, and 180 days supplemented with a trace element solution showed stable digestion for longer periods than duplicate control digesters without supplementation. The time points of failure in the control digesters were shown to be related to washout time, as calculated using the HRT. Trace element supplementation allowed stable operation at an OLR up to 3.5 gVS l-1d-1, with specific methane production ranging from 0.41-0.47 l gVSadded -1 and VS destruction of 63-77%. Supplementation with trace elements did not, however, guarantee indefinite stable operation, as digesters at the shortest (25 days) and longest (180 days) retention time eventually showed methanogenic failure. A slow methanogenic biomass growth rate and accumulation of inhibitory substances, respectively, were hypothesised as possible reasons for these failures. Analysis of metal concentrations in the digestate showed that cobalt was the metal most likely to be responsible for the observed benefits of the mixed trace metal supplementation as the concentration of this increased in the supplemented digester whilst decreasing in its non-supplemented control. The relative importance of the liquid and solid fractions in maintaining stability were investigated in novel digestion trials in which solid and liquid retention times were uncoupled. Digesters with SRT of 25 days and HRT of over 150 days exhibited methanogenic failure after approximately 45 days. In contrast, reactors with SRT of over 150 days and HRT of 25 days maintained stable digestion, with specific methane production of 0.53 l gVSadded -1, and also showed recovery from a thermal shock applied during the experiment. Inhibitory compounds such as VFA were kept low by flushing through the system while alkalinity was regenerated by the action of biomass kept in the system. The retention of solids may also have facilitated the retention of trace metals.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:505589 |
| Date | January 2008 |
| Creators | Climenhaga, Martha Anne |
| Contributors | Banks, Christopher ; Heaven, Sonia |
| Publisher | University of Southampton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://eprints.soton.ac.uk/73611/ |
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