Anaerobic digestion of catering wastes

Abstract

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.

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ORCID for Sonia Heaven: orcid.org/0000-0001-7798-4683

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