Environmental Engineering Reference
In-Depth Information
Operation conditions:
*Batch/Fed-Batch
*Continuous
*Mixing
*Feed rate
*Temperature
Physiological condition:
*Water content
*pH level
*C/N ratio
*Minerals
Bio-CH
4
Others:
*Microbial community dynamics
*Reactor types
*Heavy metals
*Antibiotics
Fig. 8.10
Main parameters affecting methane fermentation
Table 8.3
Comparison of mesophilic and thermophilic processes
Process operation
Mesophilic (35 ºC)
Thermophilic (55 ºC)
Process stability
Higher
Lower
Temperature sensitivity
Low
High
Energy demand
Low
High
Degradation rate
Decreased
Increased
Retention time
Longer or the same
Shorter or the same
Sanitation
No
Possible
of sensitivity depends on the temperature range. The pH optimum for methane
fermentation is between pH 6.7 and 7.4. If the process becomes acidified and the
pH drops below 6, the balance in the biomass is upset and acid-producing bacteria
will dominate the acid-consuming bacteria, so that the medium becomes inhibitory
or toxic to the methanogenic bacteria. In addition, strong ammonia production dur-
ing the degradation of proteins and urea may also inhibit methane formation with a
pH higher than 8.
For engineering applications, mesophilic and thermophilic methane fermentation
are the most commonly used methods. Mesophilic fermentation usually requires
over a 20-day hydraulic retention time (HRT), but is not so efficient in the reduction
of volatile solids and the deactivation of pathogenic organisms. To overcome these
limitations, interest has grown in thermophilic digestion, using the higher metabolic
rate of thermophilic microorganisms. Although better performance in the reduction
of volatile solids (VS) and deactivation of pathogenic organisms can be obtained
from thermophilic digestion, the effluent quality and ability to dewater the resid-
ual sludge are poor, and require additional energy to heat the digester (Table
8.3
).
