Developing Solutions to Meet Regulatory Constraints and Long-Term Goals

摘要

With the multitude of factors affecting flow in a separate sewer system, developing solutions to satisfy regulatory constraints with a satisfactory degree of confidence is challenging. The thoughtful approach presented in this study is useful to municipalities and engineers as it demonstrates a means to build confidence in planning for system improvements dependent upon a wide range of variables. The Greater Peoria Sanitary District (GPSD) provides wastewater collection and treatment services to urban areas of Peoria County, Illinois. The GPSD has been reviewing the permitted discharges from the Paramount and Rutledge Remote Treatment Basins (RTBs) in Bartonville, Illinois. These permitted discharges, while infrequent, have demonstrated excursions of the effluent limits set forth in the GPSD's National Pollution Discharge Elimination System Permit. Two alternatives were identified - reduction of infiltration and inflow (I&I) through footing tile disconnections or the construction of a conveyance system that would effectively eliminate permitted discharges. An I&I reduction feasibility analysis was performed by comparing flows from subcatchments with variable quantities of suspected footing tile connections. It was determined that footing tile disconnections may not eliminate permitted discharges. The GPSD proactively led a footing tile disconnection and sewer lateral lining pilot program (Pilot Program). The data from the Pilot Program demonstrates footing tile disconnections can have a significant impact on reducing groundwater infiltration to a separate sewer system. Evaluation of inflow during wet weather flow confirmed footing tile disconnections would reduce, but not eliminate, permitted discharges. Based on the footing tile analysis, it was recommended to proceed with the implementation of the conveyance system. Using data from the flow monitoring effort, a XPSWMM model of the project service area was developed, calibrated, and verified. The variability of permitted discharges was evaluated based on season and antecedent moisture conditions. The model was calibrated to represent a range of storms under hydrologic conditions that have historically led to permitted discharges. The design storm was developed in a manner so as to generate suitable flow rates for the purposes of sizing the conveyance system that would effectively eliminate permitted discharges up to a 10-year level of control at any storm duration per Illinois State Water Survey's Rainfall Frequency Atlas of the Midwest. The final design of the conveyance system has since been completed and is currently under permit review for construction. The project demonstrates a pilot program can allow for a more informed decision when considering I&I reduction as a solution for permitted discharge control. Although I&I reduction may not be the best approach in all cases for the effective control of permitted discharges, implementation of an I&I reduction program can markedly reduce quantities of flow discharged from a subcatchment and can serve a role in a larger control solution. The study also demonstrates that not all models are developed equally. There are ways to calibrate models and plan for water, stormwater, or wastewater infrastructure that can be tailored to suit the purposes of the project and add confidence to a proposed solution that hinges on a multitude of factors to meet project objectives.