In a digester tank with a floating or rising cover, it is made airtight by means of a(n):

In a digester tank with a floating or rising cover, the airtight seal is typically maintained using a water seal. This method involves creating a barrier of water that prevents gases from escaping while allowing for the necessary movement of the cover. The water seal is effective because it relies on the physical property of water to block the emission of gas, thereby ensuring that the digester operates efficiently and safely by containing odors and harmful gases.

Using a water seal is advantageous for maintaining anaerobic conditions within the digester, which are crucial for the biological processes that break down waste materials. Other methods of sealing, such as sliding rubber gaskets or leather bellows, may not provide the same level of airtight integrity or ease of operation in the volatile environment of a digester. A water seal is a well-established technique in sewage treatment operations and is widely recognized for its reliability in both sealing and facilitating the necessary movement of floating covers.