phosphorous, potassium, calcium, magnesium and a number of trace elements such as iron,
manganese, molybdenum, zinc, cobalt, selenium, tungsten, nickel etc. "Normal" substrates
such as agricultural residues or municipal sewage usually contain adequate amounts of the
mentioned elements. Higher concentration of any individual substance usually has an
inhibitory effect, so that analyses are recommended on a case-to-case basis to determine
which amount of which nutrients, if any, still needs to be added.
Retention time
Batch-type and continuous plants
The retention time can only be accurately defined in batch-type facilities. For continuous
systems, the mean retention time is approximated by dividing the digester volume by the
daily influent rate. Depending on the vessel geometry, the means of mixing, etc., the effective
retention time may vary widely for the individual substrate constituents. Selection of a
suitable retention time thus depends not only on the process temperature, but also on the
type of substrate used.
Cost efficiency
Optimizing the process parameters retention time - process temperature - substrate quality -
volumetric load determine, among others, the cost efficiency of the biological processes. But
as each m3 digester volume has its price, heating equipment can be costly and high quality
substrates may have alternative uses, the cost-benefit optimum in biogas production is
almost always below the biological optimum.
Substrate
For liquid manure undergoing fermentation in the mesophilic temperature range, the
following approximate values apply:
• liquid cow manure: 20-30 days
• liquid pig manure: 15-25 days
• liquid chicken manure: 20-40 days
• animal manure mixed with plant material: 50-80 days
If the retention time is too short, the bacteria in the digester are "washed out" faster than they
can reproduce, so that the fermentation practically comes to a standstill. This problem rarely
occurs in agricultural biogas systems.
pH value
The methane-producing bacteria live best under neutral to slightly alkaline conditions. Once
the process of fermentation has stabilized under anaerobic conditions, the pH will normally
take on a value of between 7 and 8.5. Due to the buffer effect of carbon dioxide-bicarbonate
(CO2 - HCO3-) and ammonia-ammonium (NH3 - NH4+), the pH level is rarely taken as a
measure of substrate acids and/or potential biogas yield. A digester containing a high
volatile-acid concentration requires a somewhat higher-than-normal pH value. If the pH value
drops below 6.2, the medium will have a toxic effect on the methanogenic bacteria.
Nitrogen inhibition and C/N ratio
Nitrogen inhibition
All substrates contain nitrogen. Tabelle 1 lists the nitrogen content of various organic
substances and the C/N ratio. For higher pH values, even a relatively low nitrogen
concentration may inhibit the process of fermentation. Noticeable inhibition occurs at a
nitrogen concentration of roughly 1700 mg ammonium-nitrogen (NH4-N) per liter substrate.
Nonetheless, given enough time, the methanogens are capable of adapting to NH4-N
concentrations in the range of 5000-7000 mg/l substrate, the main prerequisite being that the
ammonia level (NH3) does not exceed 200-300 mg NH3-N per liter substrate. The rate of
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