A digester tank has a servicing device that allows access to the interior of the digester tank. The sealing device may have a movable retention wall adapted to resist the pressure of the gas inside the digester by one or more of a) increasing the bending resistance of the retention wall, for example by inflating pockets in the wall, b) forcing the lower edge of the retention wall downwards, for example by weight, a mechanical arm or inflating pockets, and c) providing a mechanical structure within the retention wall, for example a non-moving cage-like structure. A sealing device with a fixed wall has a pipe with an isolation valve between the headspace of the tank and the inside of the sealing device. In operation, a seal may be formed across the separation curtain by raising the level of the liquid contents and closing the isolation valve.