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Post-treatments and recirculation of agricultural solid digestates: impact on full-scale methane yield

Abstract : In order to enhance agricultural biogas plant efficiency, impact of solid digestate post-treatments and recirculation on methane yield were evaluated at lab scale. Several full-scale solid digestates were characterized to determine their residual methane potential. Subsequently, effects of thermo-alkaline or short-term aerobic post-treatments on solid digestate methane potential were measured. Results show that direct recirculation of 50% of agricultural solid digestate leads to a low increase on plant methane yield (less than 3%). Therefore, post-treatment cost has to be carefully taken into account as potential gains are weak. Thermo-alkaline treatment leads to a 30-45% increase of the solid digestate methane yield but its cost at full scale is not profitable. On the contrary, short-term aeration post-treatment can be performed at low cost at industrial scale but it leads to methane losses since the remaining easily biodegradable fractions are degraded instead of lignocellulosic ones. Session-Post-treatments anaerobic effluents/digestates Introduction At full scale, Hydraulic Retention Time (HRT) and feedstock type are the two key parameters affecting biogas plant efficiency (Ruile et al., 2015). To ensure a high efficiency, HRT has to be adapted as a function of the feedstock (Muha et al., 2015). However, current co-digestion processes and empirical selection of the HRT in agricultural biogas plants often lead to remaining methane potential in the digestate. Therefore, a good management practice of digestates has to be identified in order to enhance efficiency and environmental footprint of agricultural biogas plants. For liquid digestates, coverage and heating of the storage tank were identified as good practices (Bacenetti et al., 2016; Lijo et al., 2017) and they are often implemented at the industrial scale. However, for solid digestate, there is currently no good management clearly advised. This study focuses on increasing agricultural biogas plant efficiency by recirculating part of the solid digestate. Additionally, two post-treatments were tested to enhance biodegradability and methane yield of solid digestates: (i) a thermo-alkaline post-treatment using soda, as it was proven to be an efficient pretreatment to degrade lignin (Carrere et al., 2016), (ii) a short-term and controlled aerobic post-treatment, as long-term composting of solid digestate leads to low methane yields (Menardo et al., 2010). Impact and interest of such practices are evaluated regarding potential economic gains at full scale.
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https://hal.inrae.fr/hal-02944599
Contributor : Hélène Carrere <>
Submitted on : Monday, September 21, 2020 - 3:30:18 PM
Last modification on : Thursday, April 8, 2021 - 2:30:03 PM
Long-term archiving on: : Saturday, December 5, 2020 - 1:29:00 AM

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Ulysse Brémond, Aude Bertrandias, Jean-Philippe Steyer, Hélène Carrère. Post-treatments and recirculation of agricultural solid digestates: impact on full-scale methane yield. 16th IWA world conference on anaerobic digestion, Jun 2019, Delft, Netherlands. ⟨hal-02944599⟩

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