Abstract
The effect of pasteurisation and co-pasteurisation on biochemical methane potential values in anaerobic digestion (AD) was studied. Pasteurisation prior to digestion in a biogas plant is a common hygienisation method for organic materials which contain or have been in contact with animal by-products. Tests were carried out on food waste, slaughterhouse waste, animal blood, cattle slurry, potato waste, card packaging and the organic fraction of municipal solid waste (OFMSW); pasteurisation at 70 °C for 1 h was applied. Pasteurisation had increased the methane yields of blood (+15%) and potato waste (+12%) only, which both had a low content of structural carbohydrates (hemi-cellulose and cellulose) but a particularly high content of either non-structural carbohydrates such as starch (potato waste) or proteins (blood). With food waste, card packaging and cattle slurry, pasteurisation had no observable impact on the methane yield. Slaughterhouse waste and OFMSW yielded less methane after pasteurisation in the experiments (but statistical significance of the difference between pasteurised and unpasteurised slaughterhouse waste or OFMSW was not confirmed in this work). It is concluded that pasteurisation can positively impact the methane yield of some specific substrates, such as potato waste, where heat-treatment may induce gelatinisation with release of the starch molecules. For most substrates, however, pasteurisation at 70 °C is unlikely to increase the methane yield. It is unlikely to improve biodegradability of lignified materials, and it may reduce the methane yield from substrates which contain high contents of volatile components. Furthermore, in this experimental study, the obtained methane yield was unaffected by whether the substrates were pasteurised individually and then co-digested or co-pasteurised as a mixture before batch digestion.
Highlights
Anaerobic digestion (AD), which yields both biogas as energy carrier and digestate as valuable soil amendment, is a suitable and frequently implemented valorisation pathway for food waste and organic by-products occurring along the food supply chain
Among the substrates under study, only potato waste and animal blood showed higher methane yields after pasteurisation, namely an increase in methane yield of 12% for potato waste and of 15% for animal blood. Both materials achieved an experimental biochemical methane potential (BMP) value which was more than 95% of the theoretical BMP, which indicates that in the pasteurised material, the biodegradable constituents were nearly completely available for their conversion into biogas
The positive impact of pasteurisation may be explained by the occurrence of gelatinisation during pasteurisation, when heating causes starch molecules to be released into the liquid phase
Summary
Anaerobic digestion (AD), which yields both biogas as energy carrier and digestate as valuable soil amendment, is a suitable and frequently implemented valorisation pathway for food waste and organic by-products occurring along the food supply chain. Organic residues which embody meat or meat products or have been contaminated by such materials, or which originate from livestock breeding, can contain pathogenic microorganisms that are of sanitary concern when applying AD digestate to agricultural land [1,2]. Through contamination of plants grown on agricultural land, infectious microorganisms in AD digestate can cause outbreaks of human or animal diseases [3,4]. To prevent such spreading of disease, hygienisation is required. Thermal hygienisation is most commonly applied [5]; it is Processes 2020, 8, 1351; doi:10.3390/pr8111351 www.mdpi.com/journal/processes
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