Effect of lignocellulosic waste dry grinding on methanization and microbial populations

Municipal solid waste (MSW) methanization is promising as it enables mass reduction of waste and methane recovery. Methanization has been implemented at the industrial level for decades, in particular on liquid effluents. However, solid waste substrates raise specific issues; process and performance optimization is still required. In the present work, we focus on the MSW lignocellulosic fraction, representing in France ~50% of the MSW mass (papers, cardboards, sanitary textiles, green waste). It is a renewable and high energy potential fraction. It is however recalcitrant to degradation due to its insolubility and a highly complex and heterogeneous structure. Current MSW methanization processes probably do not yet fully exploit this energy potential.

Further authors:
X. Rouau - INRA, France;
L. Mazéas, T. Bouchez, A. Bize - Irstea, France

The work aims at assessing and understanding the effects of specific dry mechanical treatments applied to model lignocellulosic substrates (flat cardboard and organic wheat straw) on their subsequent methanization. Such treatments may induce contrasted effects by impacting the particle size and their other properties, including structural characteristics. The total accessible surface area may for instance be modified, as well as the release and accessibility of growth substrates and inhibitors. Model systems were established and degradation and methane production were assessed over time.
To test the effect of particle size and grinding method, the cardboard and the straw were finely grinded using distinct techniques (cutting -, ball -, impact -, and jet mills) with various settings and durations. Some techniques are expected to alter only the particle size, while others must in addition affect its crystallinity, such as ball milling. For the cardboard, a total of 3 distinct fine particle batches were obtained, with median particle sizes ranging from ~52 μm to ~170 μm. For the straw, 5 batches were obtained, with median particle sizes ranging from ~61 μm to ~170 μm. To study the methanization, samples of each particle batch were incubated in synthetic medium in replicated mesophilic anaerobic batch microcosms, inoculated with a rich biomass originating from a solid-waste mesophilic methanizer. Substrate degradation and methane production were measured over time by analyzing the biogaz production and composition and by characterizing the liquid phase (dissolved organic/inorganic carbon, volatile fatty acids). Bacterial and archaeal populations responsible for the bioconversion were analyzed by Automated Ribosomal Intergenic Spacer Analysis (ARISA), a commonly used fingerprinting technique.
Results showed that grinding methods tested had no influence on cardboard methanization performances. By contrast, a positive effect of fine grinding was observed for wheat straw methanization. In particular the ball milled fraction associated with the higher particle size reduction led to the highest methane yield. Besides, the structure of the bacterial and archaeal population was affected by the presence of the different pretreated fractions. This suggests that lignocellulose structure modifications lead to the selection of specific microbial communities able to degrade it. This work gives new insights into the solid waste methanization processes by studying the effects of substrates' structural modifications on the microbial populations, increasing the core knowledge required to establish more robust and efficient bioprocesses.

Copyright: © European Compost Network ECN e.V.
Quelle: Orbit 2012 (Juni 2012)
Seiten: 8
Preis inkl. MwSt.: € 8,00
Autor: Nelly Badalato

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