The production of biogas as a renewable energy source is currently very important, which is why the number of agricultural biogas plants with fermentation of organic substrates (renewable raw materials, slurry, etc.) is steadily increasing. Although fermenters in agricultural biogas plants are dimensioned with longer retention times, the speed-determining hydrolysis step also has to be overcome here. Sonication of the active bacterial biomass from the fermenter or secondary fermenter with ultrasound has proven to be an effective solution. The result is intensified anaerobic degradation with increased biogas production and in general higher methane content.

Technical limits of fermentation and overcoming these with ultrasound

Anaerobic fermentation of organic substrate (biomass) is a perfected, mature technology. The biomass is decomposed by bacteria in the fermenter at temperatures of around 37°C. Biogas is produced as a metabolic product, whose main component and actual energy carrier is methane. However, the biological anaerobic degradation process takes place very slowly, as the solid biomass is difficult for the bacteria to break down. The first step in the degradation chain is the so-called hydrolysis, which causes reduction of the particle or molecular size and there- fore determines the speed of the entire degradation process.
The effect of our ultrasound aims at overcoming this limiting hydrolysis step. The ultrasound oscillations not only affect the structure of the substrates (breakdown of the cells) but also the active microorganisms (stimulation through fluctuating sound pressure and cavitation, prevention of agglomerate formation, release of enzymes). Biological hydrolysis is therefore supported or entirely substituted by use of our ultrasound system, so that accelerated and enhanced degradation takes place.

Intensification of the fermentation through ultrasound

Ultrasound intensifies fermentation on biogas plants. The fermenting microorganisms produce the enzymes (exoenzymes) necessary for degradation of the organic substrate. When our high-power ultrasound equipment is used, even a small energy input results in enhanced release of these enzymes so that the activity of the microorganisms is stimulated. Sonication also causes an increase in the interface between the liquid and solid phase (particles and flocks disintegrate), which facilitates enzymatic attack. Further energy supply also destroys the walls of plant and bacterial cells and therefore results in the release of the dissolved organic cell contents, which are in turn very readily degraded by fermentation. The end effect is an intensified digestion process with further degradation of the organic substrates and increased biogas production.
Therefore, by using ultrasound, on the one hand more biogas can be produced from the same biomass feed; on the other hand, it is also possible to reduce the feed quantity with unchanged biogas production. In addition, the quality of the biogas is improved by an increase in its methane content. Due to the difficult to degrade agricultural biomasses used, the methane fraction produced by conventional biogas plants is sometimes less than 50%. In addition, sonication of the volumetric flow added reduces the viscosity of the fermenter content and therefore makes it more fluid. This reduces the power consumption of the agitators and pumps.

Design and easy integration

Due to the difficult structure of the raw biomass fed we prefer to sonicate a partial flow of the fermented biomass from the main or secondary fermenter. The sonicated sludge in a loop is then returned to the main fermenter. This volumetric flow usually equals 30 to 50% of the substrate quantity added daily. The standard Ultrawaves ultrasound system (connected load 5 kW) is very compact with a footprint of 1.45 m x 0.25 m and therefore requires little space. On-site, it is only necessary to provide inlet and outlet connections as well as an electrical and water connection. The ultrasound systems themselves usually operate 24 hours a day.

Integration of ultrasound technology into biogas plants

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