Abstract: An egg-shaped anaerobic sludge digester has upper and lower draft tubes, and an optional middle draft tube. The draft tubes are arranged so that liquid can be (1) simultaneously pumped downwards through the upper (or middle) draft tube and upwards through the lower (or middle) draft tube; (2) simultaneously pumped upwards through two or more draft tubes; or (3) simultaneously pumped downwards through two or more draft tubes. When a middle draft tube is provided, it can be used in place of the upper draft tube when the liquid surface in the digester falls below the top of the upper draft tube. Alternatively, the middle draft tube can work as a coordinated relay conduit to move liquid either in the same direction as the upper draft tube or to move liquid in the same direction as the lower draft tube.

Abstract: An anaerobic sludge digester has a top, a bottom, sloped sides leading to the bottom, and a draft tube with an upper end spaced downward of the top of the digester and a lower end spaced upward of the bottom of the digester. The draft tube may have a propeller, impeller, or jets that enable liquid to be transported through the draft tube from one part of the digester to another. The digester has a discharge mouth positioned within the digester to receive heavy material that is pumped from the bottom of the digester and which flows outward across the liquid surface. One or more spray nozzles are located above the liquid surface to break up foam and emulsions. The mouth may also be used to effectively remove scum and light solids from the liquid surface. A shutoff valve on a discharge pipe extending from the discharge mouth can be closed while sludge is added to the system, causing the liquid level to rise above the mouth.