Submerged fixed-cover design, introduced in 1993, gives conventional-shaped digesters the same key features that have made egg-shaped digesters successful in controlling digester scum and debris accumulation. Specifically, both designs minimize the liquid-solids surface by submerging the cover so the liquid level is in the gas dome and digested solids are continuously removed from the liquid surface via an overflow standpipe. This limits the liquid surface area and the tendency for scum and debris buildup. Digested solids also are removed continuously from the steep-sloped bottom of the digester to prevent grit accumulation.