Challenge
A shortfall in treatment capacity left the northeast Tennessee community of Mountain City under a state-imposed sewer-connection moratorium that hindered efforts to attract new housing, tourism and industry. Class B Biosolids produced on drying beds restricted options for beneficial use. Limited drying capacity on the drying beds influenced activated sludge waste rates, making process control a challenge.
With Class-B material, they faced a shortage of available land within cost-effective hauling distance. Runoff was a concern as well on many farms in this area, where slopes exceeded the limits set in federal and state regulations.
Solution
Siemens Water Technologies supplied a complete Dewatering/Drying J-VAP® system. The system included:·
- J-Vap filter press 40cubic feet expandable to 50 cubic feet
- Feed pumps
- Chemical Feed pumps
- Sludge conditioning tank with mixer
- Dried solids conveyor
- LP-gas-fueled energy conversion module supplying heated water for pressurization and drying
- Vacuum system
- Automatic controls
The plant wastes sludge to the digesters at 7,000 GPD at one percent solids. Centrifugal pumps transfer material from the digesters to a mixing tank for chemical treatment with a ferric chloride polymer blend. The blended slurry then enters the J-Vap® unit via diaphragm pumps.
During the batch process, the filter plates are clamped tightly together in a specially designed filter press. Slurry enters the filter chambers, where initial filtration takes place and free liquid is removed. Filtrate flows through the filter cloths, leaving a filter cake.
After filtration, the vacuum/drying cycle begins. The feed pumps stop, the feed and filtrate valves close, and the reduction chamber diaphragms are pressurized to 100 psi to squeeze the filter cake and remove as much water as possible.
Hot water at 180 degrees F is circulated in a closed loop behind the diaphragms to heat the filter cake. At the same time, a vacuum is introduced to the filter chambers, causing water remaining in the filter cake to boil off at a low temperature. Cycle time can be adjusted to reach the desired moisture content.
At the end of the drying cycle, a pathogen kill cycle begins. The vacuum is turned off, raising the boiling point within the unit, thus raising the material temperature to meet U.S. Environmental Protection Agency time and temperature requirements.
By drying the material at low temperature, the J-VAP® unit conserves energy. Air exhaust from the unit is minimal, requiring no air-quality permits. Mountain City cools the vapor from the unit using plant effluent passed through a heat exchanger. Condensate returns to the head of the treatment plant.
Results
The J-Vap® system proved more cost-effective than other drying systems and had significant advantages over belt presses, another option the city considered.
More efficient solids handling has also led to process improvements. Before the facility upgrade, solids residence times in the plant's aerobic digester was brief, and the digester essentially served as a storage tank. Since the upgrade, digester residence times have increased to 15 days, and in the future Mountain City hopes to achieve the 30-day residence time required for complete digestion.In the end, more efficient solids processing with the J-Vap® system has helped Mountain City improve its treatment plant operations and facilitated the community's opportunity for growth and prosperity.