Environmental Engineering Reference
In-Depth Information
taBle 32.3
environmental requirements
Parameter
hydrolysis/acidogenesis
methanogenesis
Temperature
25-35°C
Mesophilic: 30-40°C
Thermophilic: 50-60°C
pH Value
5.2-6.3
6.7-7.5
C:N ratio
10-45
20-30
Redox potential
+ 400 to -300 mV
Less than -250 mV
C:N:P:S ratio
500:15:5:3
600:15:5:3
Trace elements
No special requirements
Essential: Ni, Co, Mo, Se
Source:
Deublein, D. and Steinhauser, A.,
Biogas from Waste and Renewable Resources
. Willey-
VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2008.
taBle 32.4
regeneration time of microorganisms
microorganisms
time of regeneration
Acidogenic bacteria
Less than 36 hours
Acetogenic bacteria
80-90 hours
Methanogenic bacteria
5-16 days
Aerobic microorganisms
1-5 hours
32.3.2.2.1 Temperature
Anaerobic digestion can operate in a wide range of temperature, between 5°C and 65°C. Generally
there are three widely known and established temperature ranges of operation: psychrophilic (15-
20°C), mesophilic (30-40°C), and thermophilic (50-60°C). With increasing temperature the speed
of anaerobic digestion increases. For instance, with ideal substrate thermophilic digestion can be
approx. four times faster than mesophilic. However, using real waste substrates, there are other
inhibitory factors that influence digestion that make thermophilic digestion only approximately two
times faster than mesophilic.
The important thing is, when choosing a certain temperature it should be kept constant as much
as possible. In the thermophilic range (50-60°C) fluctuations as low as ±2°C can result in 30% less
biogas production (Zupancic and Jemec 2010). Therefore, it is advised that temperature fluctuations
in the thermophilic range should be no more than ±1°C. In the mesophilic range, the microorganisms
are less sensitive; therefore fluctuations of ±3°C can be tolerated.
For each range of digestion temperature there are certain groups of microorganisms present
that can flourish. In the temperature ranges between the three established temperature ranges the
conditions for each of the microorganisms are less favorable. In these ranges anaerobic digestion
can operate, however much less efficiently. For example, mesophilic microorganisms can operate
up to 47°C, thermophilic microorganisms can already operate as low as 45°C. However, the speed
of reaction is low and it may happen that the two groups of microorganisms may exclude each
other and compete in the overlapping range. All this results in poor efficiency of the process, and
therefore, these temperatures are rarely applied.
32.3.2.2.2 Redox Potential
In the anaerobic digester, low redox potential is necessary. Methanogenic microorganisms need
redox potential to be between -300 and -330 mV for optimum performance. Redox potential can
