Defining Collection System Performance Criteria to Meet Level of Service Goals

Defining collection system performance criteria is a critical step in the master planning process because it sets the metrics by which existing collection system infrastructure will be evaluated to meet level-of-service goals by which future facilities will be designed. It is important to recognize the difference between planning criteria and design criteria for judging the performance of collection system infrastructure. Planning criteria relate to those metrics that are used to analyze the adequacy of existing facilities and to project future infrastructure needs for financial planning purposes. These criteria are not intended to provide the same expected levels of safety and protection that design criteria provide. It is generally inappropriate to use standard design criteria as planning criteria, especially when significant wet weather flows impact an existing collection system (as is the case with an aged combined sewer system). For example, new sewers are designed to convey flow under non-surcharged conditions, while surcharging may be permissible during the analysis of existing sewers, especially during wet weather flows. Wet weather flow may dramatically affect the performance of collection systems. Master planning collection system infrastructure requires an assessment of the performance criteria used to size the systems during wet weather flows. Determining how to measure the performance of existing facilities and deciding on a design event or condition, or level of service, for wet weather planning may not be straightforward. This is especially true when basing the performance on an expected flow duration series. Literature and regulations offer little guidance on the design condition or design flow event frequency to use. Choosing a large event can incur significant capital costs, while too small of an event may lead to unacceptable overflow frequency and volume. This paper examines collection system performance criteria for both planning and design purposes that can be used for completing modeling analyses and identifying capital improvements. The planning and design criteria help define the type, location, and extent of the facility deficiencies that should be corrected to maintain service reliability. Criteria for dry weather flow as well as wet weather flow conditions caused by infiltration and inflow are included. Examples are included to illustrate how results can vary from one approach to another, and affect the extent of capital improvements as well as the level service of the collection system.