4. Optimisation of digester performance with increasing organic loading rate for
4.3.8 Mono-digestion of grass silage (R6)
The OLRs of 3.0, 3.5 and 4.0 kg VS m-3 d-1 were higher than previously reported in literature for mono-digestion of grass silage. It is thought the higher OLRs were achievable through substantial recirculation of separated liquor effluent which, in general, kept the FOS/TAC values below the threshold of 0.300. It is postulated that the deterioration in SMY and corresponding drop in biomethane efficiency suffered by R6 at 4.0 kg VS m-3 d-1 was a consequence of the HRT becoming too short at 19 days. Maximum effective degradation of such a high lignocellulosic content
substrate was not possible in such a short time.
4.4 Conclusions
Continuous long term digestion trials illustrated the requirement for high grass silage input to maximise the potential biomethane output. Optimum conditions were
assessed for 100 % grass silage at an OLR of 3.5 kg VS m-3 d-1 generating a SMY of 398 L CH4 kg-1 VS at a biomethane efficiency of 1.0. Increasing this to 4.0 kg VS m- 3
d-1 caused a drop in SMY of 12 %. With addition of 20 % dairy slurry the optimum condition was at an OLR of 4.0 kg VS m-3 d-1 generating a SMY of 349 L CH4 kg-1 VS at a biomethane efficiency of 1.01.
Acknowledgements
D.M. Wall was funded through the Teagasc Walsh Fellowship scheme.
Eoin Allen was funded by Science Foundation Ireland (11/RFP.1/ ENM/3213). Jerry Murphy was funded by the Science Foundation Ireland (SFI) centre MaREI (12/RC/2302).
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