MESOPHILIC AND THERMOPHILIC SYSTEMS
Anaerobic digesters are normally operated at either mesophilic temperatures (30-40oC) or moderately thermophilic temperatures (50-60oC), allowing optimal growth of the bacteria involved in the breakdown of the organic matter. The main advantages and disadvantages of operating at each temperature range are described below.
Mesophilic Digestion Systems
Mesophilic bacteria have an optimal temperature for growth between 30-40oC and consequently mesophilic digesters are usually operated at temperatures around 35oC. It is essential for efficient operation to control temperature since reaction rates drop off considerably as temperature falls below 35°C and there is also a sharp drop off in activity at temperatures above 45oC, as mesophilic bacteria become inhibited by the heat.
Mesophilic digestion systems are generally more stable than thermophilic systems due to the fact that a wider diversity of bacteria grow at mesophilic temperatures and these bacteria are generally more robust and adaptable to changing environmental conditions.
Thermophilic Digestion Systems
Thermophilic bacteria have an optimal temperature range of 50-60oC. Thermophilic digesters are usually operated as close as possible to 55oC. Thermophilic digestion offers the advantages of faster reaction rates compared to mesophilic digestion, leading to shorter retention times. Thermophilic digestion also provides better pathogen kill due to the higher temperatures, although this is less important if the waste stream is pasteurised as part of the treatment process.
Thermophilic systems are usually more expensive to operate as they require additional energy to maintain the higher operating temperatures. Another drawback of thermophilic systems is the greater sensitivity to operational and environmental conditions e.g. greater temperature control. For feedstocks rich in nitrogen where ammonium/ammonia can result in inhibition of the digestion process, thermophilic operation is less recommended.
Thermophilic systems can be of benefit where high solid content feedstock with optimal C: N ratios are available.
Whichever thermal regime is used it is of great importance to keep the temperature as constant as possible as even small fluctuations in temperature can affect operating performance and the rate of biogas production. A sudden temperature drop can result in the inhibition of the methane producing bacteria (methanogens). Consequently, temperature control for the anaerobic digestion process is considered as one of the main design parameters.
Heat requirements to either the inflowing feed or to the digestion vessel are usually fulfilled on-site from the conversion of biogas to heat directly or via the recovered heat in a CHP unit. Digestion vessels should also be insulated since ambient temperature changes can also affect digestion performance.