I would like to get your advice about performing 10-year maintenance on a digester: are there aspects that require particular attention? Do you perform this type of operation in-house? Do you use specialized subcontractors? What do you do with the sludge while the digester is off line?
Emptying a digester is a complex undertaking in terms of both operational procedures and safety concerns: you absolutely have to consult with the person in charge of safety to formulate a detailed plan.
Below is a partial list of the steps to take, which other forum members will be able to add to:
• Fill up a settling tank and save the sludge while the digester is stopped, so its contents can be used to restart the digester.
• If you have not already done so, make sure there is a mesh filter on your centrifuge in order to dewater thick sludge upstream from the digester.
• You should also have a mobile dewatering unit to process the sludge directed through a tapped connection on the digester.
• About 2 weeks in advance of maintenance operations, slow down the production of methane by cutting the supply of sludge to the digester and stopping preheating in the sludge/water heat exchanger; and use a flare to burn the excess biogas.
• You need to ensure the biogas pipes and the digester expansion space are inert: basically this is done by injecting nitrogen in order to expel methane from the digester, after which it will be burned using the flare and dispersed in the open air.
- The operation is complete when the digester expansion space contains less than 4% CH4. Since the combination of nitrogen and oxygen does not produce an explosive mixture, the digester can then be safely opened.