Anaerobic digestion of food and vegetable waste

Abstract

Food and vegetable wastes contribute a large percentage of the organic fraction of municipal solid waste (OFMSW), and anaerobic digestion potentially offers an ideal method for their management. Their chemical composition can, however, lead to unstable operation and in extreme cases complete process failure has been reported with this type of substrate. Semi-continuous trials on vegetable waste were carried out in laboratory-scale digesters with daily feed additions at different organic loading rates (OLR). At an OLR of 2g volatile solids (VS) 1-1 day-1 the methane yield was 0.345 1g-1 VS added, or 99% of that found in a biochemical methane potential (BMP) test. Higher OLRs led to reduced methane yield and energy conversion efficiency and to a drop in digester pH which could not be effectively controlled by alkali additions. To maintain digester stability it was necessary to supplement with additional trace elements including tungsten, which allowed OLR of up to 4g VS 1-1 day-1 to be achieved. Stability was also improved by the addition of yeast extract (YE). Co-digestion with card packaging and cattle slurry proved an effective means of restoring and maintaining stable operating conditions. Digesters fed on source segregated domestic food waste showed a long term pattern of failure as a result of the build-up of ammonia in the digester and the accumulation of volatile fatty acids, in particular propionic acid. The research gathered evidence to link this to an enzyme deficiency which was caused by a lack of selenium, or possibly other trace elements. Results from a fractional factorial designed batch experiment showed the importance of Se in the degradation of propionic acid, and indicated that Mo and Co might also play a role in this respect. Semi-continuous trials using laboratory-scale digesters also confirmed the requirement for Se which proved to be essential for both propionate oxidation and syntrophic hydrogenotrophic methanogenesis. Supplementation with Se allowed the digesters to operate at substantially higher OLR. At high loadings cobalt also becomes limiting, due to its role either in acetate oxidation in a reverse Wood-Ljungdahl pathway or in hydrogenotrophic methanogenesis. Critical Se and Co concentrations were established as 0.16 and 0.22mg kg-1 fresh matter feed at moderate loading. At this dosage the OLR could be raised to 5g VS 1-1 day-1 with specific and volumetric biogas productions of 0.75m3 kg-1 VS added and 3.75m3 m-3 day-1 at a standard temperature and pressure (STP) of 0°C and 101.325 kPa, representing a significant increase in process performance and operational stability. A tracer experiment using a radio-labelled isotope [2-14C] of acetate proved that under high ammonia concentrations, hydrogenotrophic methanogenesis was the dominant pathway leading to methane production in food waste digesters. A rapid non-derivatisation gas chromatographic method for quantification of palmitic, stearic and oleic acids was developed. These long chain fatty acids (LCFA) can accumulate in anaerobic digesters, and a simple extraction method was also developed to permit a more rapid sample turn-around time. The method was tested on digestate from food waste digesters. It was observed that the concentrations of stearic and palmitic acid in digesters routinely supplemented with trace elements were proportionally lower with respect to their relative loading when compared to those without supplementation. It was concluded that both Se and Co were deficient in source segregated domestic food waste collected in the UK, and that supplementation with these elements could improve digestion stability, allow higher OLR to be applied, and in doing so improve the energy yield of the digestion process.

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ORCID for Charles J. Banks: orcid.org/0000-0001-6795-814X

ORCID for Yue Zhang: orcid.org/0000-0002-5068-2260

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