REDUCE FLOW OR INSTALL BIGGER PIPES – A UTILITY DILEMMA

Utilities maintaining wastewater collection systems are taking proactive approaches to better prepare for future regulatory requirements such as the Capacity Assurance, Management, Operations and Maintenance (CMOM) program. Infiltration/inflow (I/I) in wastewater collection systems is a common problem, resulting in inadequate capacity under peak flow conditions. Deterioration, aging, and poorly constructed manholes and pipelines are some of the major contributing factors to this problem. In certain geographic regions, such as the mid- Atlantic, the peak flow is comprised mainly of inflow. The ratio of peak flow to Average Daily Dry-weather Flow (ADDF) can range anywhere from 2.5 to 10 or greater depending on the age of the sewer, deterioration, groundwater table, storm intensity, and a number of other factors.To better serve the existing but growing collection system areas, it is necessary to plan for adequate capacity by either reducing the extraneous I/I flow or by providing additional pipe capacity through relief sewers. A relief sewer is defined here as either a parallel pipe to an existing one or a replacement pipe of larger diameter. The choice between providing relief sewers or implementing I/I reduction programs is not always clear, however, it is possible to evaluate the alternatives in light of cost as well as achievability.This paper discusses a methodology that was used in developing a master plan of the McDowell Creek Basin for the Charlotte Mecklenburg Utilities (CMU), in Charlotte, NC. The McDowell basin is expected to grow from an existing population of approximately 30,000 to 160,000 between 2000 and 2050. The infrastructure needs are imminent given the projected growth of over five folds. During this project, hydraulic analysis of the system was performed for years 2010, 2020, 2030, 2040, and 2050 using dynamic hydraulic modeling. System deficiencies were identified for peak flows when the system was subjected to a 5-year, 24-hour design storm. Using the simulated peak flows, relief sewer pipes were sized and priced and potential I/I reduction areas were identified. A cost-Benefit analysis of 13 comparable projects revealed that I/I reduction programs could provide cost savings of up to 1.6M.