Time for a re-examination of sewer design flow criteria

Each year, many miles of new or rehabilitated sewer lines or pumping stations are designed and constructed based on flow criteria that may be unrelated to the desired level of system performance. Because of the need to reduce the risk of sanitary sewer overflows and sewer backups into buildings, it can be beneficial to re-examine the basis of sewer hydraulic design. Traditional approaches such as the 10 States Standards, individual state sewer design criteria, and sewer agency in-house design criteria may need to be updated. This paper reviews design criteria, design flow curves, and methodologies used to develop updated design curves. It is important because it provides guidance to utilities relative to review of their design flow criteria and can result in improved long-term system performance.

In many systems, inflow is the most critical flow component and may represent 60 to 95 percent of the total peak flow rate under “peak” or design flow conditions. Little guidance is provided in design manuals regarding the development of design curves when inflow is a significant part of the total flow. Many agencies use standard curves to develop design flows that have been empirically developed, or based on the literature, which show ratios of peak and minimum flows to average daily wastewater flow, population or tributary area. These curves are useful guides, but every collection system (and subsystem within it) has its own characteristic flow curve.

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