40. Community Need Increase Lime Kiln Untreated Residual Storage

The City of Dayton has two 96 MGD water treatment plants that use traditional lime softening for the treatment of groundwater. The two treatment plants produce approximately 700,000 gallons of residuals yearly. For many lime softening systems, the residuals created by the lime softening process are disposed of in landfills - this is not the case in Dayton. Dayton Water takes the normally unusable residuals and processes them through the kiln. Once processed in the kiln, the residuals become usable calcium oxide (lime) used in the water treatment process.

Dayton, Ohio, and Miami Dade, Florida are the only two municipal water systems in the country that operate a kiln to recycle calcium carbonate residuals into usable lime.

The purchase of lime for water treatment is expensive, and so is the disposal of the calcium carbonate residuals. When the Lime Reclamation Facility (LRF) is operating normally, the need to purchase lime for softening is eliminated.

The LRF also provides carbon dioxide used for pH adjustment at the Ottawa Water Plant. In 2012, the City of Dayton embarked on a trial project to import residuals from outside water systems to optimize the efficiency of the kiln. A 2017 LRF expansion allows the importation of outside residuals and the sale of pebble lime to outside systems. This increased production has the added benefit of optimizing energy use at the kiln.

The excess lime resulting from the higher production rate is then sold back to the other water systems for their use in water treatment. This recycling program is a regional asset that makes Dayton’s water treatment process more cost-effective and efficient and benefits other municipalities. Residuals are an essential commodity for Dayton Water.

The kiln needs to be taken out of service twice a year for planned maintenance and occasionally for emergency repairs. During the planned kiln shutdowns, residuals are still being made and due to a lack of excess storage, those residuals must be disposed of thus increasing the carbon footprint of the water system and decreasing the efficacy of the kiln. During times of very cold weather, residuals from other municipalities can be difficult to obtain due to frozen conditions at their calcium carbonate residual lagoons. An increase in storage could allow Dayton Water to reduce waste from the lime softening processing and increase the resiliency of the LRF due to weather conditions or operational disruptions.

Twice a year the treatment plant sedimentation basins are taken out of service for cleaning. The cleanings are to remove the residuals not conveyed to LRF during normal operations. Due to a lack of storage recyclable products are wasted. Increased storage would decrease wasted material and decrease the impact of weather-related shortages of residuals. Both decrease amount of reclaimed lime produced by LRF and impact the overall efficiency. An increase in residual storage would also allow better mixing before entering the kiln which would help with the final product quality and consistency of the lime produced.

To allow room for increased storage this project would include the demolition of an old storage building that has become a hazard due to its condition. New mixing pumps would be installed to replace the less-efficient, undersized, older pumps. Existing pipes, original to the lime plant’s construction, connecting the residual storage to the kiln would also be replaced. These pipes are corroded due to their age and the harsh conditions involved in processing lime residuals.