DESIGN OF AN ACID/GAS DIGESTION PROCESS FOR BALTIMORE'S BACK RIVER PLANT

The existing anaerobic digestion facilities at Baltimore's 180-mgd Back River wastewater treatment plant include two 3.0 million-gallon egg-shaped digesters (ESDs) and six 1.3 million gallon cylindrical digesters. The process is single-phase mesophilic anaerobic digestion. The current facilities have just enough volume to meet regulatory requirements under average conditions, but do not provide any reserve capacity to handle peak loadings. In addition to the limited digestion volume, the most recent rehabilitation of the cylindrical digesters occurred more than 25 years ago and these tanks and their equipment are in need of significant repair Upgrading of the existing facilities is needed to provide sufficient capacity to treat current maximum month loadings and projected future loadings. An evaluation of alternatives for upgrading the anaerobic digestion facilities considered several digester shapes and digestion processes. The alternatives were evaluated based on construction cost, total present worth of the capital and differential O&M costs, and non-monetary factors. The recommended alternative was a two-phase acid-gas mesophilic digestion process using a new tall cylindrical acid reactor followed by four egg-shaped gas reactors (two new and two existing). Two of the existing cylindrical digesters would be converted to digested sludge storage tanks and the other four cylindrical digesters would be abandoned in place. Detailed design concepts were developed and reviewed in multiple meetings with the City staff to assure consideration of all plant impacts including operability, constructability, and coordination with other plant facilities.