Figure 3: Specific investment costs per kilowatt of electricity depending on plant size

August 10, 2017 | General

Small-Scale Anaerobic Digestion For Agricultural Waste Management

A study in Germany assessed performance of 10 farm digesters that utilize different engineering designs producing under 75 kW of electricity.

Sabrina Eichenauer
BioCycle August 2017

The adoption and growth of renewable energy in Germany has been encouraged by the “Renewable Energy Sources Act” (Erneuerbare-Energien-Gesetz or EEG), introduced in 2000. The EEG, designed to encourage production of electricity and heat from renewable sources, has guaranteed fixed feed-in tariffs (by law) for producers and led to a remarkable growth of renewable energy installations in Germany. In 2012, an amendment to the EEG created a comparatively high feed-in tariff (23.14 ct/kWh = $0.26/kWh), for digesters producing up to 75 kilowatt hours (kWh) of electricity.
This higher feed-in tariff is only available to digesters that:
• Produce electricity from the biogas on-site (the methane may not be brought and used somewhere else)
• Have a maximum installed capacity of 75 kW at the location of the biogas generation unit
• Use at least 80 percent (wet/weight or w/w) liquid manure (besides poultry manure) for biogas generation (annual average)
There is no volume limit. Biogas plants that meet these conditions and get the high feed-in tariff are often called small-scale digesters or farm biogas plants, but neither are official terms in the German law.
A recent study conducted by researchers at the University of Applied Sciences’s Center for Energy and Environmental Engineering in Giessen, Germany assessed the performance of 10 anaerobic digesters producing less than 75 kW of electricity that utilize different engineering designs and were fed with agricultural substrates. Digesters were evaluated for two years based on degradation of volatile fatty acids (VFA) and organic dry matter (ODM), and on rate of capacity utilization (percent runtime) of the combined heat-and-power (CHP) unit. Different substrates and their relation to digester performance are characterized in this article; reasons for technical problems and their solutions are illustrated.
Various digester types are utilized at the facilities studied. These include: Mesophilic continuous stirred tank reactor (CSTR); Mesophilic digester with hydraulic mixing; Mesophilic digester mixed by injection of the substrate; Two-stage vertical thermophilic digester, mixed by pumping; and Thermophilic CSTR with a horizontal configuration.

Figure 1. Volatile fatty acids (VFA) before and after digestion

Figure 1. Volatile fatty acids (VFA) before and after digestion


Volatile Fatty Acids
Figure 1 shows the degradation of VFA in untreated liquid manure (grey bars) and in the digestate after treatment (green bars) in 10 different digesters using the example of liquid manure (measured in grams of VFA degraded per kilogram of substrate). Variations in the amount of VFAs at the different sites are shown. Sample 8 had a comparatively low level of VFAs in the untreated manure, possibly due to dilution by heavy rainfall. Because of this higher moisture content, the gas yield coming from this substrate was not ideal, even though the manure itself did not look different than other loads received.
The VFA in the digestate was analyzed to determine the efficiency of degradation. If the VFA in the substrate is not that high prior to digestion, gas yield will be rather low due to the quality of the substrate. But if the VFA in the substrate is comparatively high and the VFA in the digestate is still high, a lower than expected gas yield is probably caused by something else, e.g. retention time, temperature, lack of nutrients, etc. A high level of VFA in the digestate is an indicator for one or more system issues. Resolving those issues will lead to better decomposition and will help to use the substrate to its full capacity.
Organic Dry Matter
Degradation of ODM is a system-dependent parameter, and theoretically only valid for constant operating conditions in a particular digester. The higher the degree of degradation, the better the efficiency of the whole system. Figure 2 shows the differences in the degradation rate of ODM at the different plants. Usually, a poor ODM degradation rate cannot be traced back to a single source, but to a mix of problems, e.g. retention time, substrates with a high lignin ratio (which is not degradable by the microorganisms), poor mixing, etc.

Figure 2. Degree of degradation of organic dry matter (ODM)

Figure 2. Degree of degradation of organic dry matter (ODM)

CHP Units
Measurement of the efficiency of the 10 CHP units at different sites (based on percent runtime) ranged from a low of 53.3 percent to a high of 95.9 percent. Three of the ten CHP units exceeded 90 percent, four were above 80 percent, and three were below 70 percent. Poor utilization rates were caused by a lack of biogas, not by downtime due to defects or ongoing repairs in the units. Therefore the degree of utilization is a direct indicator for overall system performance and the biogas production process at the particular site.

Possible Errors And Defects

Gas-yielding quality of substrate: A common error in running a biogas plant is the high expectation regarding the gas-yielding quality of the substrate. All substrates used for biogas production consist of biomass and are therefore different depending on factors such as season, weather, environmental conditions, soil properties, forage (in case of manure), handling, and storage. Consequently, the same substrate used at the same digester but perhaps at different times of the year may have a different impact on biogas yield. In addition, some substrates will need a longer retention time than others.
CHP efficiency: Operating the CHP continuously at partial load versus operating in batches under full load produces less energy from the same amount of biogas than by operating with a full load. If there is not enough biogas to operate the CHP the whole time under full load, it can always be operated at regular intervals (essentially running in batch mode rather than continuous mode).
Optimum operating temperature: The temperature inside the digester affects the type as well as the activity of the microorganisms involved in biogas production. Mesophilic microorganisms have peak activity up to 40°C (104°F); thermophilic microorganisms have peak activity up to 60 °C (140°F). An increase in temperature generally leads to an increase in microbial activity. But beyond those two maxima, microbial activity is usually slowing down to the point of total inactivity at a temperature higher than 60°C (140°F).

Figure 3: Specific investment costs per kilowatt of electricity depending on plant size

Figure 3: Specific investment costs per kilowatt of electricity depending on plant size

Substrate impacts: Using substrates that the digester is not designed to process (e.g., solid waste vs. liquid manure) can create challenges with equipment (e.g., hydraulic pumps or mixers). A concrete-like scum layer can be formed inside the digester, which may require removal of the contents with heavy equipment. The financial loss caused by this situation can be extensive, and could lead to facility closure.

Economic Realities

As illustrated in Figure 3, the specific costs (investment/kw of electricity, or kWel) are higher with smaller-scale biogas plants, although the total costs for the facility itself are usually lower. Thus the actual amount of money required to purchase a plant is bearable even for small farms. However, almost all plant owners participating in the study said that they would rather invest more at the outset than spend a lot of money on repairs and retrofitting.
Second, most operators have little or no experience in running an anaerobic digester, e.g. expenditure of time, substrate input and technical needs (pumping, stirring, measurement, etc.). This can lead to the operator deciding to process different substrates than initially planned, which in turn requires more time to troubleshoot — and decrease performance of the biogas plant. If the anaerobic digester is operated as intended and planned carefully and realistically, the facility will have great ecologic and economic value.
Sabrina Eichenauer is a Researcher at the Mittelhessen University of Applied Sciences in Giessen, Germany. She can be reached at

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