Long-term biosolids planning with an operational mega reservoir for combined sewer overflow-impacted stormwater capture.

The Metropolitan Water Reclamation District of Greater Chicago adopted the Tunnel and Reservoir Plan (TARP) to reduce combined sewer overflow (CSO) events in the Chicago region. The Thornton Composite Reservoir (TCR) became operational in 2015 providing an additional 30 million m of CSO-impacted stormwater capacity. In the United States, no other mega reservoirs are in operation to provide as a reference to study the long-term impacts of biosolids operations in water resource recovery facilities. The mean daily volume pumped from the Calumet TARP system to the Calumet Plant increased 144-238 m from 2012-2014 to 273-360 m from 2016-2018. Overall annual digester feed solids for the 2016-2018 post-TCR period were 28,182 Mg, which was 11 percent less than the mean for the 2012-2014 period of 31,745 Mg. Annual digester draw solids for the 2016-2018 post-TCR period were 19,422 Mg, which were 4 percent less than the 2012-2014 pre-TCR period mean of 20,190 Mg. This paper demonstrated a decrease in digester feed loading to the Calumet Plant and, ultimately, a decrease in digester draw solids with an overall increase in plant and TARP flow in the years following operation of the TCR for the capture and treatment of CSO-impacted stormwater. PRACTITIONER POINTS: Reservoirs capturing combined sewer overflow-impacted stormwater improve water quality of local waterways. Mega reservoirs may impact solids loading to water resource recovery facilities. Hydraulic loading to water resource recovery facilities may be substantial with mega reservoirs.