The wet fermentation technology widely used up to this point, which was designed essentially for fermentation of liquid manure, reaches the limits imposed by the process at DS (dry substance) contents greater than 15%. The reason for this is essentially that the substrate is increasingly difficult to agitate and pump. In addition the wear out of aggregates increases with higher dry substances. By contrast, there is no upper limit for DS content in dry fermentation based on our process. The substrate and fermentation residue are brought in and removed in solid, stackable form. Our plants consist of two main plant sections in which organic substances are simultaneously converted into biogas, dry fermenters and a process water tank. There is in addition a delivery area and a part of the plant designed for use of the biogas that is produced (for example a combined heat and power unit (CHP) or gas supply) as well as further plant sections.

Dry fermentation also requires a moist environment for fermentation. In our process, this environment is created and maintained by controlled sprinkling of the substrate in dry fermenters with conditioned process water, which is guided through a circuit. A specially developed nozzle system on the fermenter cover ensures even distribution of water over the entire surface, thereby thoroughly and homogeneously moistening the substrate pile. The process water percolates through the substrate, moving down and to the side through baffle plates with vertical perforations out of the substrate pile. Then it can run off freely out of the dry fermenter through inclined gutters and lines. The percolate running out of the dry fermenters is captured in a collecting container and temporary reservoir and is then directed to the built-in high-volume process water tank, which is constructed as a gas-tight tank.

The dry fermenters as well as the process water tank conduct to the biogas production. The intensive percolation rate and the high-volume of the process water tank are the effective regulation items of the biogas production. With the help of these items the bacteria and the needed energy is brought into the substrate in the beginning of the process. Hence the hydrolysis is strated.
As the fermenter "matures" progressively (i.e. as the organic substance initially present is continuously converted) hydrolysis activity decreases. The drainage system ensures effective removal of acids from the dry fermenter, in order to realize most favourable conditions for methanogenic bacteria.

The process water tank is an integral component of the process water circuit system and serves to dilute and buffer the acids that are removed from the dry fermenters. Nutrient movement out of the dry fermenters continuously supplies the process water reservoir with fresh organic acids, which in turn act as nutrients to methane-forming microorganisms. Because of this, while biogas is being produced in the dry fermenters, it is simultaneously and continuously being produced in the process water reservoir as well. In order to supply continuously the CHP or gas cleaning/feeding system with sufficient biogas, multiple dry fermenters must be operated in parallel but staggered in time. When the previously described biogas production curves are superimposed, the result is continuous availability of biogas. The more dry fermenters are operated in parallel, the smaller are the deviations in availability. Further levelling is achieved by biogas production which occurs continuously in the process water reservoir.

For minimizing the emissions and to optimizing the usage of the produced biogas we have developed a system, which secures the collection and treatment of the weak gas in the end of each fermentation cycle. The biogas with a low methane content is collected separately and directed to a weak gas storage. Out of this storage a flare can treat the weak gas in an optimized and effective way. A mixing with biogas (higher methane content) is possible. In addition to that it is avoided to come to the explosion areas which secures a optimized security of our facilities.

Our process allows an optimum processing of organic substances with high DS content. This makes plants designed according to our process especially efficient.


Herhof GmbH

Kalkgraben 2
35606 Solms – Germany

Phone: +49 (0) 64 42 / 207 - 0
Fax: +49 (0) 64 42 / 207 - 233


The advantages of dry fermentation compared to wet fermentation may be summarized as follows:

  • Significantly lower water requirement and no mashing necessary
  • Lower process energy requirement (no mixing mechanism necessary)
  • Less material wear due to fewer movable machine parts
  • Less susceptibility to detrimental substances and overacidification
  • Usually lower sulphur content in the gas, which has a positive effect on the service life of the CHP
  • Smaller digesters due to higher energy contents of the substrates that are used
  • Synergy in the use of agricultural equipment (e.g. wheel loaders, tractors, mixing carriages, manure spreaders, etc.)
  • Simpler (stackable) storage of fermentation residue.

Our process offers the following advantages compared to other dry fermentation processes:

  • Control and measurement system for every single fermenter
  • Weak gas management system
  • Optimum biogas yield with short delay time
  • Less investment costs (no mixing of substrates needed) Minimum internal electrical and thermal energy need (no heating of dry fermenters, weak gas management system, no excess water and no mixing of substrates required
  • Homogeneous percolation and optimised process conditions through percolation and high-volume process water tank
  • Low methane emissions and homogeneous aerification of fermentation residue pile with special aeration system
  • Low variations in gas production through simultaneous production of biogas in process water tank

Process sequences are represented in diagram format in the flowchart below. The essential elements of the process are dry fermentation for fermentation of solid organic substances by batch and continuous fermentation in the process water reservoir through nutrient movement in the process water reservoir circuit.

Steps of the process:
Preparation of substrates if necessary (e.g. pulverization, mixing etc. depending on the substrates)

  1. Placing the substrates in the empty dry fermenter
  2. Optional pre-aeration in the closed dry fermenter to increase the temperature
  3. Anaerobic processing by means of percolation with process water
    • Primarily hydrolytic phase
    • Primarily methanogenicphase
  4. Post-aeration
    • Expulsion of biogases, aerification of digestate, odour removal
    • Optional aerobic post-processing to reduce water content
  5. Removal of digestate
    Post-treatment of fermentation residue (e.g. hygienisation in Herhof boxes or maturation, screening etc. depending on the application)

Our diagram shows you a schematic sequence of our biogas process.

» Show diagramm