Anaerobic Digestion -
Mesophilic Vs. Thermophilic
In anaerobic digestion process, temperature is not only important for
microbial metabolic activities but also for the overall digestion rate,
specifically the rates of hydrolysis and methane formation. In general,
anaerobic digestion process can occur within a wide range of temperatures. This
temperature range has been broadly divided into two groups: mesophilic- 30 to 42 degree centigrade and thermophilic- 43 to 55
degree centigrade . In practice, most of the digesters are designed
to operate at mesophilic range, between 30 to 38 degree centigrade , and some of
them are designed for thermophilic temperature range of 50 to 57 degree centigrade (Metcalf
and Eddy, 2003). In general, thermophilic digestion
processes potentially allow higher loadings with reduced hydraulic retention
times, higher conversion efficiencies and pathogen disinfection while mesophilic
digestion is more stable, less at risk from ammonia nitrogen toxicity and requires
less process heat.
Mesophilic Digestion
The anaerobic digester that operates at the mesophilic temperature range (35-38 degree centigrade ) is known as mesophilic digestion. Mesophilic anaerobic digestion is most common system which has a more stable operation but a lower biogas production rate. Another disadvantage of mesophilic digestion is that it does not reduce the pathogen concentrations enough to produce Class A biosolids, a biosolids that contains no detectible levels of pathogens (WEF, 2004).
Thermophilic Digestion
The anaerobic digester that operates at the higher thermophilic temperature range (50 to 65 degree centigrade ) is known as thermophilic digestion. Interest in the thermophilic digestion developed based on the facts that higher temperatures reduce pathogens and thermophilic temperatures provide more rapid reaction rates than mesophilic temperature. Class A quality biosolids can be produced from thermophilic digestion when the time temperature criteria specified in the US EPA Part 503 are satisfied. Thermophilic anaerobic digestion in general are more efficient in biogas production but associated with higher maintenance cost.
The advantages of thermophilic digestion listed in the WEF white paper on the anaerobic digestion (2004) are:
- Increased volatile solids reduction
- Faster reaction rates for shorter retention times
- Higher capacity for a given volume
- Increased pathogen destruction
- Improved dewaterability of the digested biosolids
The most frequently cited disadvantages of thermophilic digestion are:
- Higher odor formation resulting from a higher volatile fatty acid (VFA) concentration
- Higher energy requirements for heating
- Increased sensitivity to thermal shock
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