Sludge Dewatering
Aside from effluent, MSD’s second biggest output is sludge. Sludge is a semi-solid by-product of the wastewater treatment process. The options for disposing of sludge include incineration, landfilling, composting, or application to agricultural lands as fertilizer. MSD uses incineration, because the benefits outweigh the costs and concerns. For example, incineration does not have the same public safety issues, costs, or fuel, air emissions, and landfill space concerns associated with the landfilling option. The figure at right shows how much sludge was produced by each plant in 2009.
For MSD, the primary environmental issue associated with sludge is finding the best balance between energy and chemical consumption. Dewatering processes apply polymer chemicals to thicken the sludge, and energy is required to run belt presses and centrifuges that remove the water. The resulting product (called “sludge cake”) requires less energy for efficient incineration. In addition, because sludge cake is more compact, MSD uses less fuel to transport it from the treatment plants to the incinerators. The challenge is to adjust the system so that both energy and chemical consumption are minimized in both the dewatering and incineration processes. Optimization efforts at the Polk Run treatment plant have dewatered sludge further, thereby reducing sludge hauling by about 40 percent and saving approximately $34,000 per year.
In 2009, MSD had a target range of solids in sludge between 22.5 percent and 25 percent. At the Mill Creek and Little Miami facilities respectively, the average solids contents in 2009 were 26.8 percent and 22.0 percent. With the installation of the new fluidized bed incinerators at Mill Creek, MSD is working with industry experts to determine the optimal operating range to achieve quality outputs at the least costs for energy and chemicals.