CLASS “A” SLUDGE FROM VACUUM DRYING FILTER PRESSES – THE CHATTANOOGA EXPERIENCE

The City of Chattanooga operates the Moccasin Bend Wastewater Treatment Plant, which serves as the regional POTW for the entire metropolitan area. The Moccasin Bend plant was recently upgraded to have a full secondary treatment capacity of 140 mgd with an additional 100 mgd of “wet-weather” treatment capacity.Beginning in the early 1980's, blended biosolids from the plant were dewatered using diaphragm filter presses supplemented with high-solids centrifuges. All dewatered solids were disposed of at a local sanitary landfill, which resulted in wasted volume and high disposal costs. Planning for the recent plant upgrade included an exhaustive evaluation of biosolids handling and disposal options. This led to the selection of a dual process biosolids handling approach. Primary solids are digested using two-stage (thermophilic/mesophilic) anaerobic digestion, and the digested solids are dewatered using high-solids centrifuges. Secondary waste-activated sludge (WAS) is dewatered and dried using vacuum drying filter presses. Both processes produce final sludges which meet Class A standards. Final biosolids disposal is by land application.In May 2001, the City received bids from two manufacturers of WAS dewatering equipment. An equipment procurement contract for 7,770,281 was issued to U.S. Filter Dewatering Systems for the vacuum drying filter presses. This contract included design of the process installation. CTI provided design for the ancillary systems as well as overall design coordination. In August 2003 bids were received, and a 6,144,000 construction contract was later awarded for the installation of the filter press and support components.The system was specified to dewater and dry 43 tons (39,000 kg dry wastewater solids weight) per day (based on 24-hour-per-day operation) of WAS with a minimum feed solids content of 1.5 percent dry wastewater solids. Lime and ferric chloride are used for sludge conditioning prior to dewatering. U.S. Filter's bid performance guarantees were a Class A final product with a minimum of 78 percent filter cake total solids content conditioned with a maximum of 20 percent lime (100 percent CaO) and 9 percent ferric chloride (100 percent FeCl3). The installed system consists of the following primary components:Six 2m × 2m, 110 chamber J-Vap filter pressesThree 9,600,000-BTU/hour gas-powered water heaters (boilers)Three 5,400,000-BTU/hour evaporative cooling towersFour 150-HP water-ring vacuum pumpsFour 500-gpm sludge transfer pumpsSix 500-gpm filter press feed pumpsTwo 2,000-pound/hour lime slaking systemsIt is believed that the Chattanooga installation is the largest vacuum drying filter press biosolids handling system in the world.The underlying technology of the system, lessons learned from the procurement/installation process, and operational performance of the installed system are described.