Anaerobic digestion of catering wastes

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

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Climenhaga, Martha Anne

Advisors dc:contributor.advisor

• Banks, Charles
• Heaven, Sonia