LARGE EGG-SHAPED DIGESTERS ISSUES AND IMPROVEMENTS

Egg-shaped digesters have been used to stabilize municipal sludge for over 40 years. The first installations were in Germany where the technology was first developed. Egg-shaped digesters were first installed in the United States (U.S.) in the early 1980's at Los Angeles' Terminal Island treatment plant. As experience and familiarity with egg-shaped digesters has grown, the design has been modified to reduce or eliminate some of the early operational and maintenance issues.This paper discusses the District of Columbia Water and Sewer Authority Blue Plains Advanced Wastewater Treatment Plant egg-shaped digester project. Key issues include vessel procurement and vessel design to control foam and lower corrosive potential.After initial concerns that only one vendor was available, several vendors provided positive bids. The large outlet box used on most egg digesters has been replaced with a continuous top sludge removal system that helps selectively continuously remove foam and scum. Deletion of this box eliminates a potential corrosion and maintenance feature.