Integrating Collection System and Wastewater Treatment Plant Hydraulic Modeling for Wet Weather Control

Wet weather collection system facilities planning projects are frequently undertaken by system owners and operators to comply with regulatory requirements, such as those stipulated in a National Pollutant Discharge Elimination System (NPDES) permit. Facilities plans are typically focused on the combined sewers, separate sewers, infiltration and inflow or flooding issues that need to be addressed by the project. Significant effort is spent investigating, documenting and modeling the collection system. An important aspect that needs to be focused on, when addressing the collection system, is the impact of the recommended plan improvements on the wastewater treatment plants (WWTP).Integration of the dynamic response of the WWTP to rising water levels with a collection system model is difficult. However, not integrating these two systems can diminish the ability to predict overflows or bypass flows near the treatment facility. Hydraulic modeling of the plant processes improves the understanding of the relationship between the collection system and the WWTP, which leads to higher levels of confidence in facility planning results. Simulating wastewater treatment plant processes using dynamic models is challenging because channel and tank configurations can cause stability problems. Simplification of the system can minimize stability issues, but results need to be reviewed carefully to ensure that the fundamental hydraulics are intact. Steady state models offer stable environments to analyze wastewater treatment process performance. These types of models do not require much simplification of the system. They do, however, require that the modeler understand and specify the system operations and can not be used to assess system transients.The Northeast Ohio Regional Sewer District (the District) has conducted five major wet weather facilities planning projects since 1995 covering the various sewersheds that deliver wastewater to their three WWTPs. The District has recognized the importance of the wastewater treatment plants in collection system facilities planning. Steady state hydraulic models were developed for two of the wastewater treatment plants, the Easterly WWTP and the Southerly WWTP, and integrated into the dynamic collection system model. Treatment plant flows, combined sewer overflows and general collection system performance were affected by hydraulic capacities in preliminary and primary treatment systems. Plant modeling aided in understanding several aspects of the plant and collection system interaction including:How the plant currently responds to rising flowsHow the interceptors respond to the plant treatment capacitiesHow the plant would respond to increased flows resulting from collection system facilities planning alternativesHow flows change for various operating conditions