By Uli Merkle
Ever since it was commissioned in 1974, the Echallens wastewater treatment plant in the Swiss canton of Vaud has been generating power from the recovery of biogas.
In 2019, a new biogas cogeneration unit was built, which generates 150,000 kilowatt-hours of power annually. Warm water recovered from the cogeneration unit is mainly used for the plant’s own needs, for instance, to warm up the sludge in the digester or heat indoor areas.
To make the process more economically efficient, a decision was made to optimize the circulation of biogas within the digester. A portion of the biogas generated in the digester is fed back into the sludge. To do this, biogas is suctioned out of the top part of the digester, compressed and fed back into the sludge at the bottom of the digester.
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The biogas flows through the sludge and then collects back up at the top of the digester, where it is fed through a line and into a gas tank as an interim reservoir. From the gas tank, the biogas then goes straight to the cogeneration unit. This currently runs for 20 hours a day and is only switched off for a few hours at night.
Circulating biogas has the following benefits:
Concentrated sludge is heated to 38° Celsius in the digester. Flooding the sludge with biogas helps to ensure that the heat is evenly distributed.
Mixing the sludge also helps to avoid deposits collecting and compacting at the bottom of the digester.
The rising of the gas bubbles up through the sludge improves the microbiological degradation processes in the sludge, which ultimately leads to a higher gas yield.
Circulating biogas through the sludge also means that it does not have to be mixed mechanically.
In May 2020, two old oil-lubricated piston compressors from the original facility used to mix the sludge in the digester were replaced by one MINK claw compressor from Busch Vacuum Solutions. This enabled the amount of power required for this process to be reduced by up to 40%.
MINK claw compressors are ATEX-certified, which means that flame arresters are no longer required. Furthermore, they are equipped with a variable speed drive so that the volume flow can be adapted precisely to the requirements in the digester.
These units compress biogas without using any oil at all. This is made possible by the compressors’ contact-free operating principle, which means that the internal moving parts do not come in contact with each other or the housing. As a result, no oil changes are needed, unlike the original oil-lubricated piston compressors which required two each year.
Plant personnel also noted that the claw compressor is much quieter than the original piston compressors.
Uli Merkle is with Busch Vacuum Solutions. For more information, email: danie.lemay@busch.ca
