If you said that mesophilic anaerobic digestion is “cool”, you’d be right compared with the other common temperature range used for AD plants, but probably in more ways than you would realize!
The term mesophilic when referring to a biodigestor has been adopted by biogas plant designers to distinguish the two best temperature ranges for operating biogas reactors for the highest efficiency of operation.
“The two conventional operational temperature levels for anaerobic digesters determine the species of methanogens in the digesters:
Mesophilic digestion takes place optimally around 30 to 38 °C, or at ambient temperatures between 20 and 45 °C, where mesophiles are the primary microorganism present.
Thermophilic digestion takes place optimally around 49 to 57 °C, or at elevated temperatures up to 70 °C, where thermophiles are the primary microorganisms present.” This is known as the mesophilic temperature anaerobic digestion plant range.
When compared with thermophilic anaerobic digestion, the cooler mesophilic temperature range is clearly inferior, because it is much slower than its higher temperature range equivalent. So much so, that at a glance the observer would surely express surprise to read that the most common type of biogas digester on farms is the mesophilic type.
Why don’t the operators of all mesophilic anaerobic digestion plants simply turn up the temperature and they will do much better, you might ask?
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The reason is that there are costs involved in raising the temperature, especially in the cool climates of the world. Once the temperature is raised, heat losses also rise, and therefore there is a decision of balance to be made. For many on farm feedstocks like farm slurries, for example, the high dilution/ low organic solids content would make the economics less favourable, despite the smaller fermentation tanks needed for thermophilic digestion. Just turning up the temperature would not help in such cases.
Thermophilic biogas plants are also less stable to operate according to most experts. They point out that the hotter the digester is the more susceptible the methanogens will become to high ammonia, and pH variations etc.
In other examples, such as the anaerobic digestion of the sludges produced at sewage (wastewater treatment) works) the higher temperatures of thermophilic digestion do make sense, because these types of organic material are much harder, and slower to digest at mesophilic temperatures.
In fact, sewage sludge may need to be preheated under pressure to break down a large proportion of the organic matter in any event, so the substrate is already heated after it leaves the additional (hydrolysis) stage.
From this it is clear that both temperature range types of anaerobic digestion will be likely to continue to be used, with each temperature range having its uses in individual cases, for the foreseeable future.
In many countries a stage of sanitization or pasteurisation is a legal requirement for certain digested wastes, before they can be disposed to the environment/ used as fertiliser. The usual requirement is that the output from the anaerobic digestion process must be heated to 70 degrees centigrade and held at that temperature (or above) for one hour.
It is clear that Mesophilic Anaerobic Digestion takes place well below this temperature, so it is necessary to add a stage during which the substrate (flow through the plant) is heated up to 70 degrees C for the required amount of time. This is usually carried out as the digestate leaves the process, but in some designs sanitisation occurs at the start of the process. Pasteurising the incoming feed can sometimes be useful in increasing biogas yield, because the pasteurisation stage performs a mild hydrolysis effect.
Where sanitization is necessary, the argument for mesophilic type AD Plants, and thermophilic plants that run at temperatures below 70, is weakened by the additional heat requirement of the sanitization stage, and thermophilic treatment is becoming more common in such cases. Nevertheless, many designers simply continue to use the mesophilic temperature range and only pasteurise the output. By using a heat exchanger system on the pasteurisation process module the heat balance of the plant overall may not necessarily be unduly affected.