temperature regulation

Kinetics of the degradation of organique matter are temperature-dependent (Arrhenius' law) for optimum conditions; to obtain good performance with high loading and high efficiency, temperature regulation is essential: nor­mally 37 ± 2°C must be maintained.

However, depending on the characteristics of the effluents to be treated and for effluents from the agri-food industry for example, temperatures of 30°C are acceptable in order to ensure the good efficiency of the reduction in carbon pollution with optimised applied loads. A heating system is therefore always necessary; external heat exchangers, preferably ones with low sensitivity to suspended solids, are recommended.

When retention time is short, the raw effluent is often heated as it enters the tank. With longer retention time (more than a week), heat is best applied to the contents of the reactor itself, and concentric tube exchangers must be used (please refer to the contact methane fermentation unit). However, when loading or efficiency are not required to be maintained at their respective maximums at a lower temperature (> 25°C), the digester does not need to be heated.

Comment: at similar efficiency, loading is reduced by approximately 30 to 40% with a temperature drop of 10°C. When treatment is carried out on high temperature effluent, a cooling system may be required.

Comment: Thermophilic digestion (~ 60°C) is also an option although it is a more difficult process; however, frequent changes from the mesophilic to the thermophilic domains are not feasible (a different biomass has to be developed each time).