Post-treatment and recirculation of undegraded fraction of digestate was investigated at a biogas Plant in Germany. The plant is equipped with a final separation step that separates solid and liquid phase of the digestate. Therefore a selective treatment of only the solid digestate and it’s subsequent recirculation in the digester seemed to be promising. Solid digestate from the final separation step was analysed with particular respect to fiber fractions (crude fibre XF, neutral detergent fibre NDF, acid detergent fibre ADF and lignin ADL) and micronutrients. Pre-tests with mechanical treatment (extrusion), thermal treatment (hot compressed water at 120 °C) and chemical treatment with NaOH were performed in order to investigate the influence of different treatments on fiber fractions. A strong decrease in XF and ADF fractions was observed for alkaline treatment. Mechanical treatment showed a slight decrease of XF, NDF and ADF while thermal treatment showed the least effects. According to these results, a mechanical and two versions of chemical treatment were tested in biomethane potential (BMP) batch-tests. Treated and untreated solid digestate were compared concerning specific biogas yield and kinetics of biogas formation. The results confirmed the disintegration effects found by fibre analytics. The specific biogas yield increased by factor 1,2 (mechanical treatment) respectively 2 (alkaline treatment). A clear increase of the reaction rate was observed as well. Furthermore, recirculation of (treated) digestate can contribute to micronutrient recovery and therefore save costs for trace element additives.
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