In this work, a modified swirling jet-induced cavitation has been employed for increasing anaerobic digestion efficiency of cattle manure. The hydrodynamic cavitation (HC) treatment improved the organic matter solubilization and the anaerobic biodegradability of cattle manure. The degree of disintegration increased by 5.8, 8.9, and 15.8% after the HC treatment at 6.0, 7.0, and 8.0 bars, respectively. However, the HC treatment at 7.0 bars had better results in terms of methane production. This result may be attributed to the possible formation of toxic and refractory compounds at higher inlet pressures, which could inhibit the methanization process. Further, total Kjeldahl nitrogen content was found to decrease with increasing inlet pressures, as the pH and the turbulent mixing favored the ammonia stripping processes. HC treatment decreased the viscosity of the treated cattle manure, favoring the manure pumping and mixing. Considerations on the energy input due to the HC pre-treatment and the energy output due to the enhanced methane yield have been presented. A positive energy balance can be obtained looking at the improved operational practices in the anaerobic digesters after the implementation of the HC pre-treatment.

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To achieve sustainable energy crop production, energy crops should not compete for land against feed and food crops. One option for sustainable energy-crop cultivation is the use of double cropping systems with minimum tillage use and digestate as natural fertilizer, where, in the same growing season, a second crop for biomethane production is planted after a first crop used for feed/food. Different sorghum phenotypes were evaluated in the present study as first and second crops in a double cropping system. A principal component analysis of the various sorghum phenotypes showed that starch content positively affected methane production. However, sorghum chemical composition did not influence the profitability of bioenergy production as much as the total solid biomass yields of the different sorghum phenotypes. The highest total solid biomass productive sorghum phenotype led to the highest methane hectare yield.

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