The Metropolitan Sanitary District of Greater Chicago (MSDGC) has implemented an innovative and cost-effective method for drying the massive amounts of sludge generated by its wastewater treatment plants. This involves using auger tractors, nicknamed “Brown Bears,” to turn and agitate the sludge, speeding up the natural drying process.
This method was adopted after extensive testing of various other methods. Augering combined with land application proved to be significantly cheaper than alternatives like land reclamation, heat-drying, landfilling, and incineration. For example, compared to the previously used heat-drying method, augering saves MSDGC approximately $43.4 million per year in fuel costs alone.
The auger tractors are remarkably efficient, boasting a production rate 10 to 15 times greater than traditional loaders or graders. An auger can mix an impressive 3,000 cubic yards of sludge per hour, while a wheel loader manages only 200 to 300 yards. The continuous mixing action of the augers exposes wet layers to the sun and wind, creating natural drainage paths for rainwater runoff, which is then collected and treated.
The sludge, originating from both centrifuges and lagoons, is transported to drying beds. The centrifuge-produced sludge, averaging 175 dry tons per day, is transported by train from the West-Southwest dewatering facility to the primary drying basins. The lagoon material, initially at 3-4% solids, is pumped five miles from the West-Southwest plant to the same drying area.
The drying beds are strategically constructed on a 6-inch layer of asphalt. This foundation prevents the sludge from mixing with the underlying soil and groundwater, which would otherwise increase drying time, volume, and handling costs. The asphalt surface also increases drying efficiency up to six times compared to packed clay, as demonstrated by a study conducted by the City of Phoenix, Arizona. Furthermore, it prevents rocks from contaminating the final product, making it more suitable for land application.
The auger tractors play a crucial role throughout the drying process. They break up surface crusts formed during evaporation, ensuring continuous drying. The material is turned every day or two during the summer, which is the peak drying season. After 8 to 10 days in the primary drying area, the sludge reaches 16-18% solids and is transported to larger drying beds. Further augering in these beds for 2 to 5 weeks doubles the solids concentration to 30-40%, significantly reducing the sludge volume. Finally, the material is moved to satellite sites where an additional 2 to 4 months of augering results in a final solids concentration of 60-65%, sometimes even reaching 70%.
The dried sludge is then used for a variety of beneficial land applications, primarily by public entities. Approved uses include soil conditioning and topsoil replacement, with a maximum permitted depth of 6 inches. It is also used as a 5-foot-deep final cover for landfills, providing structural stability and preventing blowing trash. The success of this application is expected to lead to its implementation in other Chicago-area landfills over the next 30 to 40 years. Other applications include contouring expressway shoulders and backslopes for planting trees and shrubs.
To further enhance cost-effectiveness, MSDGC contracts the sludge augering work to private companies. This eliminates the year-round cost of employing dedicated personnel for a seasonal and weather-dependent task. Contractors can also utilize their equipment for other purposes during the off-season, such as snowplowing, ensuring their workforce remains employed and the equipment is not idle. Additionally, contracting eliminates the need for MSDGC to purchase, maintain, and store specialized equipment and spare parts. Moreover, contractors can take advantage of equipment depreciation, leading to reduced costs for both themselves and MSDGC.
The MSDGC’s approach to sludge management demonstrates a commitment to environmental responsibility and cost efficiency. By adopting the augering method and contracting the work, they have created a sustainable system that benefits both the environment and the taxpayers.
