SEWER FLOW MONITORING CRUCIAL TO EPA'S CMOM COMPLIANCE

Flow monitors have been used extensively for conducting inflow and infiltration (I⊘I) studies in sewage collection systems over the past 28 years. These studies are part of the United States Environmental Protection Agency's (EPA) requirements for properly sizing new facilities, and identifying excessive I⊘I in pipes that may require rehabilitation. However, properly planned networks of flow meters are capable of providing much more information about sewer system operation than estimates of I⊘I and input parameters for sewer simulation models. In particular, the advent of instruments designed for gravity sewers which calculate flow using the continuity equation from depth and velocity data, allow analysts to evaluate operating conditions and to diagnose some capacity problems, unrelated to I⊘I. These additional capabilities have assumed greater significance in light of EPA's pending sanitary sewer overflow (SSO) regulations which give emphasis to proper system operations and maintenance (O&M). Presently, promulgation of the proposed regulations has been delayed due to the change of political administration. However, sewer system operators should still recognize that the practices outlined in the proposed regulations primarily represent good operating procedures, and only have a secondary regulatory function.This paper discusses experiences in three cities with the application of flow monitoring networks that follow the strategy intended by the CMOM program. The City of Chattanooga installed a permanent network of flow monitors for revenue billing applied to its regional customers. In addition to improving the accuracy of bill calculation, the city also discovered capacity problems at several locations. The City of Nashville has maintained a long-term network of 70 flow monitors in its trunk sewer system and 19 meters in chronic, wet-weather overflow locations. Analysis of major trunks using multiple meters revealed previously unsuspected problems with potential I⊘I in the upstream sewer tributaries. Data from both permanent meters and over a thousand temporary meters installed before and after sewer rehabilitation have allowed system managers to document over 10 million m3 (2.7 billion gallons) of I⊘I removal annually. Additionally, Nashville has documented a reduction of 64% of annual duration of SSOs using a combination of 23 long-term and 53 short-term meters. Finally, Jackson, TN saved significant costs and avoided urban disruption when data from a flow monitoring network revealed that a major sewer trunk line with overflows was actually not overloaded. Lining rehabilitation was sufficient to recover capacity and eliminate back-ups and overflows.