Many municipalities use lime to treat their water, but the Lime Reclamation Facility in Dayton, Ohio, is one of a kind. For almost 60 years, it has helped the city of Dayton efficiently manage its resources, and thanks to an impending expansion, its economic and environmental benefits will soon grow.
Taking It Back
For many years, Dayton has used quicklime to treat its water. In 1954, the city began to look into how it could best manage the large volume of residuals that were being generated by its 96-million-gal-per-day Ottawa Water Treatment Plant (WTP). Engineering studies suggested that a lime reclamation facility (LRF) would be the best solution to the problem, as it would allow the city to recover more than enough lime for all the city’s treatment needs; the excess lime would be sold to consumers.
To achieve this, though, engineers needed to overcome a logistical problem. “The major obstacle to efficient lime recovery was the removal of magnesium from the process,” said Shannon Zell, supervisor of Dayton LRF. “Pilot plant tests confirmed that a large part of the magnesium hydroxide could be removed by carbonating the residuals with carbon dioxide from the kiln gas. The carbonation of the residuals substantially improved settling, dewatering and quality of recovered lime.”
Construction of the plant began in 1956, and it began operation in November 1957. In 1964, the city added the Miami WTP to deal with the water demands of a population that was peaking at more than 250,000. The residuals from the Miami facility also are sent to the Dayton LRF.
Processing the Residuals
When the residuals arrive at the LRF, they first are processed through two carbonation basins, where carbon dioxide from the plant’s kiln exhaust is used to suppress the pH in order to solubilize the magnesium hydroxide and allow it to be separated from the calcium carbonate. The pH typically ends up in the 7.5 to 8 range.
The sludge then enters the plant’s two 65-ft gravity thickeners. The thickened residuals then are pumped to two 30-ft-diameter storage tanks, which provide equalization and mixing. Next, the sludge is pumped through an expansion chamber, where it is heated to approximately 100°F to help evaporate some of the water within the residuals.
The sludge then is pumped into centrifuges, where excess water is removed, resulting in a calcium carbonate “cake” that has an approximately 70% solids concentration. This cake is fed into the kiln, where it is baked into finished pebble lime that then is stored in onsite silos. The plant currently produces approximately 60 to 70 dry tons of quicklime per day.
Expanding Through Decline
When the LRF was created, Dayton was a growing manufacturing city. But in recent years, its population has dramatically declined, from a peak of 260,332 in 1960 to only 141,527 in 2010. This has created issues that have threatened the LRF.
“The kiln uses a large amount of natural gas to operate under normal conditions, and efficiency increases as production increased,” Zell said. “With the loss of major manufacturing in the Dayton area starting in 2008, water production dropped drastically, along with residuals production, which is directly related to lime production. This made the kiln less efficient because more natural gas is used per ton of lime produced at lower production rates.”
To combat this problem, the facility sought new sources of sludge. In 2012, the plant conducted a pilot project that involved residuals associated with lime lagoon cleaning in the nearby Ohio cities of Troy and Middletown. The project reclaimed 1,470 dry tons of lime and was deemed a success. The following year, the project was expanded to include residuals from WTPs in Troy and Middletown, and again, the results were favorable. Since then, the facility has continued to attract business from out of town, and as a result, it is eyeing expansion.
“Construction of a facility for long-term importation of residuals is scheduled to begin in 2016,” Zell said. “This will allow the city of Dayton’s LRF to receive NSF-approved residuals efficiently and, in turn, increase our production. The LRF provides these products and services—pebble lime and residuals handling—at a price that is competitive with regional markets for the same services while increasing the facility’s efficiency and reducing operating cost.”
The expansion is scheduled to be completed in the autumn of 2016.
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