Improvement of the digestion of cattle slurry via the process of co-digestion

Abstract

The use of maize (Zea mays) as a co-substrate with cattle slurry for the production of biogas was investigated in detail by running several long term digestion trials under different operational conditions in laboratory scale semi-continuous digesters. These conditions included varying the organic loading rate (OLR) from 2 to 6 g VS l-1 d-1, the proportion of cattle slurry from 25 to 100%, and the recirculating regime.
Results indicated that the co-digestion of cattle slurry and maize was viable at all loading rates tested with the greatest volumetric methane yield, 1.46 l l-1 d-1, produced at a 5 g VS l-1 d-1 OLR consisting of 40% cattle slurry; this corresponded to a specific methane yield of 0.26 l g-1 VS added. Successful digestion was shown at retention times as low as 15 days where a volumetric methane yield of 1.26 l l-1 was produced. Co-digestion had a pronounced effect on the volumetric methane yield with improvements of up to 355% when compared to the digestion of cattle slurry alone. Additionally, the OLR could be doubled by the addition of an equal quantity of maize, on a VS basis, with the volumetric methane yield increasing by over 200% without a great loss of the methane potential of the maize.
For each trial undertaken in this research the actual methane yield produced from co-digestion was compared to that calculated to be produced. Support for synergy was shown in the first two trials where the actual methane yield exceeded that predicted however, the method used to calculate the predicted yield was suggested to be an inaccurate determination. To address this inaccuracy a trial was designed testing the digestion of the two substrates alone and together under the same operational conditions and methodology. A comparison between the mono and co-digestion trials indicated that the addition of maize to cattle slurry produced a methane yield that more or less equalled that calculated from the sum of the cattle slurry and maize alone. This brought the early indications of synergy into doubt with suggestions that they were the result of an inaccurate determination of the predicted yield and of inhibition washout.
In the final part of the research an attempt was made to improve the volumetric methane yield by introducing solids recirculation to the co-digestion process with the objective of maintaining the slowly degradable fraction of the maize and cattle slurry in the digester for longer periods. This proved not to be a viable option with the methane production showing a decline; at an OLR of 5 g VS l-1 d-1, consisting of 50% cattle slurry, a decline of 0.31 l l-1 d-1 occurred. Solids recirculation was also introduced to the mono-digestion process to determine whether the differences shown in the co-digestion trial were a result of recirculating the solids of the cattle slurry, the maize or a combination of the two. Results showed that both substrates produced unstable conditions indicating that the co-digestion trial was not the result of just one substrate failing. Liquid recirculation was also tested on the co-digestion of the substrates and while no decline was observed no improvement was produced.
KEYWORDS: Anaerobic digestion, co-digestion, cattle slurry, maize, solids recirculation

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Identifiers

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

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

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