Comparison of Different Mechanical Pretreatment Methods for the Anaerobic Digestion of Landscape Management Grass

The aim of this study was to use landscape grass from species-rich orchards for biogas production, thus preserving these very valuable areas for future generations. Since these grass clippings have high lignocellulose content, the substrate has to be pretreated before being fed into the biogas digester. In this study, three different mechanical treatment processes (cross-flow grinder, ball mill and a mounted mower) were investigated and compared with untreated grass clippings. Chemical composition, specific methane yield, degradation kinetics and microscopic images were analyzed. In order to derive recommendations, the harvesting and pretreatment processes were examined in terms of energy demand, additional methane yield, and suitability of the substrate for use in biogas plants, taking into account conservation aspects. Within the pretreatment process, ball milling leads to the highest significant increase in specific methane yield of up to 5.8% and the fastest gas formation kinetics (lag time λ BM : 0.01 ± 0.0 d; duration to reach half of total gas production ½M(x) BM : 5.4 ± 0.2 d) compared to the untreated variant (λ UT : 1.02 ± 0.2 d; ½M(x) UT : 6.5 ± 0.2 d). A comparison of the energy required for the mechanical disintegration of the substrates with the increased yield of methane during the digestion process shows that the mechanical processing of these substrates appears to be useful. A positive energy balance was achieved for the cross-flow grinder (12.3 kWh t VS −1 ) and the ball mill (21.4 kWh t VS −1 ), while the Amazone Grasshopper left a negative balance (−18.3 kWh t VS −1 ), requiring more energy for substrate pretreatment than was generated as methane surplus. In summary, the pretreatment of landscape management grass is a suitable approach for utilizing agricultural residues efficiently in a biogas plant and thus contributing to sustainable energy production.