The anaerobic digestion of sugar beet pulp

Abstract

World-wide there are substantial quantities of sugar beet pulp, which arises as a residue after the processing of whole beet to extract sugar for refining as a foodstuff or for use in fermentation, in particular for the production of ethanol for the biofuel market. In both cases the resulting pulp residue is still rich in pentose sugars and fibre, and the research considered anaerobic digestion (AD) as a potential technology for the conversion of this material into renewable energy in the form of biogas. To determine the best operational conditions for biogas production both mesophilic and thermophilic digestion options were considered. Both were tested using 4-litre working volume mixed digesters operated with semi-continuous feed over a minimum of three hydraulic retention times (HRT). The first long term trial used mesophilic temperatures (37 oC±0.5 oC) at applied organic loading rates (OLR) from 2-5 g volatile solids (VS) l-1 day-1. This resulted in a specific methane yield of ~0.31 l CH4 g-1 VS day-1 with a biogas methane content of 51.05%. VS destruction was ~90% at all loadings, and increasing the loading resulted in an increase in volumetric biogas and methane production without significant loss in specific yields. The major limitation found was not in the biochemical conversion but in dewatering of the digestate, the characteristics of which were assessed using capillary suction time (CST) and frozen image centrifugation (FIC). At the higher loading there was also the appearance of a stable foam which made the digesters difficult to operate as this could block the gas outlet, leading to pressure increases and the loss of digestate by ‘blow out’.
In the same digesters at mesophilic temperatures antifoam was tested to assess if this could offer a solution by suppressing foam formation. In practice this required unusually high doses of the reagent and, in continued use, these appeared to have an inhibitory effect on the digestion process. Dilution of the feedstock to the digester was also tested but showed no beneficial effects on dewaterability or foaming. As a post-treatment alternative cellulolytic enzymes were added to the digestate, but had no effect on improving dewaterability. Trace element (TE) supplementation to the digesters was, however, shown to eliminate the occurrence of foaming and also gave a slight improvement in dewaterability. TE supplementation reduced the polymer dose required for dewatering as determined by the CST test, and eliminated polymer dosing when dewatering was by centrifugation. Digestate dewaterability could also be improved in a post-digestion one- and two-stage chemical treatment with the use of chemical coagulants/flocculants alone or combined.
The second long-term trial compared mesophilic (37 oC±0.5 oC) and thermophilic (55 oC±0.5 oC) digestion over 3 HRT using duplicate digesters fed at OLR of 4 and 5 g VS l-1 day-1. The digesters were operated without water addition. The thermophilic digesters gave higher biogas and methane productivity and were also able to operate stably at the higher OLR, whereas the mesophilic digesters showed signs of instability. Digestate dewaterability was assessed by the CST and FIC tests and the likelihood of stable foam forming was assessed using a foaming potential test. The results showed thermophilic operation performed better even at the higher loading and gave a digestate with superior dewatering characteristics and with very little foaming potential. Using a combination of CST tests, filtration tests, Frozen Image Centrifugation, SEM and grading centrifugation it was concluded that the poor dewaterability seen in mesophilic digestate was due to the presence of extracellular polymer substance (EPS) leading to blinding of the filter by fine particulate materials.
The carbon, energy and nutrient (CEN) footprint was estimated for mesophilic and thermophilic digestion in which the process was coupled with combined heat and power (CHP) and biogas upgrading to biomethane. The results showed that the energy input for thermophilic digestion was higher than for mesophilic although this could be compensated for by the increased specific methane yield at the higher loadings modelled. There was also no significant difference in the emissions savings or in the quantities of nutrients recycled in the digestate. The model indicated that the use of CHP gave a higher net energy yield compared to biogas upgrading, but this of course is dependent on there being an economic use for the heat produced.
Keywords: anaerobic digestion, sugar beet pulp, biogas, trace elements; digestate, dewatering, foam formation, carbon footprint, energy footprint, nutrient footprint

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Identifiers

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

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

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