High mixing-energy demand
Mechanical agitators consume electricity and require maintenance due to wear. They also reach performance limits with highly viscous substrates.
Technical solution for wet anaerobic digestion
Culture-retaining and hydraulically circulated anaerobic digester
The anaerobic digester combines hydraulic circulation without continuously operating mechanical agitators with the targeted retention of microbial cultures adapted to the respective substrate. This supports reliable treatment of fibre-rich and nitrogen-rich organic residues.
Initial challenge
Conventional stirred digesters limit biological and energy efficiency
Loss of adapted microbial cultures
Intensive complete mixing makes it difficult to establish spatially specialised cultures for readily and slowly degradable substrate components.
Floating layers and sediment
Fibrous and mineral components can form floating layers or sediment, reduce usable volume and cause operating interruptions.
Fluctuating gas production
Periodic strong mixing pulses can lead to uneven gas flows and place additional loads on downstream gas-treatment systems.
Operating principle
Two connected digester zones with natural hydraulic circulation
Outer and inner digester
A concentrically arranged inner digester and an annular outer digester are connected at the tank floor by flow nozzles.
Gas buoyancy as mixing energy
Gas bubbles generated in the substrate overcome the hydrostatic load and continuously rearrange the highly viscous medium without permanently operating mechanical agitators.
Distinct microbial environments
In the outer digester, rapidly growing cultures mainly degrade readily available fats, proteins and carbohydrates. In the inner digester, slowly growing cultures for difficult lignocellulosic components can be retained.
Controlled gas flow
Gas overflow channels route gas from the outer to the inner digester. There it supports mixing near the overflow weir before being conveyed in a controlled manner to gas treatment.
On-demand fluidisation
Only when required is a gas-pressure equalisation initiated to generate a strong hydraulic mixing pulse for loosening and removing sediment. Additional fluidisation elements can break up floating layers.
Integrated safety and temperature control
Hydraulic and pneumatic safety devices protect against inadmissible overpressure and vacuum. Heat-transfer surfaces integrated into the outer wall control mesophilic or thermophilic conditions.
Design features
Robust design for large digester volumes
Concentric vessel geometry
Inner and outer digesters form two hydraulically coupled treatment zones with different biological functions.
Pressure-resistant steel roofs
Spherically shaped gas-tight roofs with reduced diameters lower loaded areas, anchoring forces and construction costs.
Sediment and floating-layer management
Flow nozzles, discharge connections, immersion sleeves and fluidisation elements enable corrective action during ongoing operation.
Flexible gas utilisation
A gas-pressure control valve enables adaptation to desulphurisation, gas storage, engine use or further biomethane upgrading.
Annular flat bottoms
The design of the fermentation vessels with annular flat bottoms is the decisive prerequisite for effective flushing of the vessel bottoms and thus for preventing sediments that can no longer be fluidised. Over time, such deposits would inevitably require the vessels to be opened and cleaned. The selected design is intended to avoid interruptions to operation throughout the entire technical service life of the fermentation vessels.
Client benefits
Higher biological performance with less mechanical equipment
Lower auxiliary-energy demand
Continuous circulation is driven mainly by the gas generated in the process. Permanently operating high-power agitators can largely be avoided.
Retention of specialised cultures
Different microorganisms can adapt to the prevailing substrate quality in each digester zone and be retained over the long term.
High dry-matter contents
Solids-rich biosuspensions and hydrolysates with approximately 15 to 20% dry matter enable high organic loading and long treatment times.
Less wear and downtime
Reduced use of mechanical mixing equipment and the ability to remove floating and settled material can reduce maintenance, repairs and unplanned downtime.
More uniform raw-gas flow
Avoiding regular large-volume pressure equalisation supports more uniform gas production and reduces loads on downstream gas treatment.
Broad feedstock range
The solution is particularly suitable for heterogeneous, fibrous and nitrogen-rich residues such as straw-based substrates, digestate and ethanol stillage.
Technical background
The technical solution presented is based on German published patent application DE 10 2020 004 136 A1. Specific sizing, material selection, pressure rating, safety systems and integration into the overall plant must be designed and assessed for the individual project, site and feedstock.
Technical project assessment