asked 20 March 2017 asked by gram (420 points)
Hi, 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.