The energy advantage of wastewater biomethanation is twofold: the conversion of carbon in the wastewater into biogas; and the savings in kilowatt hours of electrical energy which would otherwise be needed to remove this carbon by aeration. In addition, biomethanation eliminates volatile fatty acids, which are responsible for the odors of these liquid waste products. Besides the environmental aspect and waste valorization, it also increases the processing potential of the plant.
Biomethanation (an anaerobic process) is carried out by bacteria present in underground pre-insulated tanks. At the end of this process, the digestate is sent to a settling tank where it is partially recycled and re-injected into the cycle. The treated water leaving the biomethanation reactor is sent to sprinkler heads in the aerobic system to complete the purification process. When the digestate leaves the biomethanation reactor, an air pocket ensures its final degassing and the storage of the biogas.
The biogas thus produced and stored in the air pocket is used to supply a cogeneration unit with 55 kilowatts of electrical power. This cogeneration unit includes a gas engine driven alternator. The electricity it produces is used by the waste treatment plant, and the engine heat serves to warm the biomethanation tanks.
The electricity and heat that are produced are self-consumed by the waste treatment plant. Such an approach is appropriate for a circular economy model at the local level, with other installations such as heat pumps which use river water issuing from Mathilde's Fountain.
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