pH/volatile acids
The pH is the central parameter of the biochemical bacterial environment.
As soon as the pH departs from the optimum range, bacterial activity is seriously impaired, resulting
in lower gas yields, inferior gas composition (excessive CO2 content) and obnoxious odor (H2S -
like rotten eggs).
Table 5.5: pH ranges for biomethanation (Source: OEKOTOP, compiled from various
sources)
pH 7-7.2 optimum
pH < 6.2 acid inhibition
pH > 7.6 ammonia inhibition
Table 5.6: Substances with an inhibiting effect on biomethanation (Source: OEKOTOP,
compiled from various sources)
Substance
Copper
Calcium
Magnesium
Zinc
Nickel
Chromium
Cyanocompounds
Chlorinated hydrocarbons
Herbicides
Insecticides
Disruptive
effects
beginning (mg/l)
10-250
8000
3000
200-1000
350-1000
200-2000
25
traces
traces
traces
Toxins
Even a slight concentration of cytotoxins suffices to disrupt bacterial activity, with a resultant shift in
pH, lower gas yield, higher CO2 content and pronounced odor nuisance.
5.2 Design principles of simple biogas plants
The technical conception of biogas plants is determined by the aim of achieving optimal parameters
for the biological process (cf. chapter 5.1).
That being so, the following operating requirements/limitations must be given due consideration:
- type and composition of organic material, which determines the choice of process
- given demand for biogas and fertilizer, in addition to the available substrate quantities,
which determines the size of the biogas plant
- economy of labor input for building and operating the plants, including consideration of the
necessary mechanical equipment.
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