CMOM is Coming

May 29, 2018
What can be done to help water and wastewater professionals

About the author: Susan McHugh is with Marsh-McBirney. For additional information contact Marsh-McBirney, Inc. at 800/368-2723 or www.marsh-mcbirney..

The U.S. EPA continues to develop proposed National Pollutant Discharge Elimination System (NPDES) permit requirements for capacity, management, operation and maintenance (CMOM) programs for municipal sanitary sewer collection systems.

Current EPA regulations have been modified with specific reference to Municipal Satellite Sewer Collection Systems, Municipal Sanitary Sewer Systems, Capacity, CMOM Programs, Prohibition on Municipal Sanitary Sewer System Discharges, and Record Keeping and Reporting for Municipal Sanitary Sewer Systems.

CMOM will affect all separate sanitary sewer systems including satellite collection systems.

Overview of CMOM regulations

The new regulations are establishing a framework for applying NPDES permits to municipal satellite collection systems: sanitary sewers owned or operated by a municipality that convey wastewater to a publicly owned treatment works (POTW) that has a treatment plant owned or operated by a different municipality. Poorly performing satellite collection systems often contribute to sanitary sewer overflows (SSOs) which go unreported to the NPDES authority and are a major contributor to peak flow problems in regional collection systems.

The new permit conditions include record keeping, reporting, public notification, CMOM, emergency response, and audit requirements for all municipal sanitary sewer collection systems.

There are four major documentation requirements of the CMOM permit. Documentation requirements vary based on the size and complexity of the municipal wastewater collection system and include:

1.             A written summary of the CMOM Program;

2.             An Overflow Emergency Response Plan;

3.             A Program Audit Report; and

4.             A System Evaluation and Capacity Assurance Plan.

Addressing prohibition of SSO discharges for CMOM

Within the new ruling, there is a general prohibition of SSO discharges. Essentially, if a SSO occurs, the permittee must demonstrate that the overflow was unavoidable, that the CMOM was in place and working, and that all reasonable steps were taken to stop and mitigate the impact of the discharge as quickly as possible. Under the new rules, owners must properly manage systems at all times by providing adequate capacity for peak flows in all parts of the system, taking steps to:

*                Mitigate SSOs;

*                Keep written records on SSOs;

*                Report on Discharge Monitoring Reports;

*                Provide 24-hour reporting of SSOs;

*                Keep written follow-up records;

*                Provide public notification of SSO or backup;

*                Prepare an annual report;

*                Develop a CMOM audit program; and

*                Make the CMOM available for public review.

This program will require owner certification that all appropriate measures are being taken to eliminate unauthorized SSOs and backups, and that the CMOM program is being implemented. In order to address the variety of SSO problems, EPA and the states will be targeting facilities where sewage overflows cause or have the potential to cause problems due to poor O&M practices. Facilities need to conduct a thorough self-audit and assessment of their entire sewer system and develop a plan to correct problems over a reasonable time period.

Why monitor flow?

The objective of the sanitary sewer is to convey only the sanitary flow into treatment plants. Unintended infiltration and direct inflow (I/I) of storm water are extraneous flows and should be minimized as they reduce the available capacity to transport wastewater. The System Evaluation and Capacity Assurance Plan, of which flow monitoring is the primary step, is needed to identify, characterize and address hydraulic deficiencies in the collection system. Generally the plan should address:

*                Collection and analysis of appropriate information on the management and performance of a collection system;

*                Development of management and performance objectives and goals of a collection system;

*                Clarification of management and performance objectives;

*                Selection and implementation measures;

*                Development and evaluation of alternatives; and

*                Continued monitoring, assessment, and adjustment of implemented measures.

Flow monitoring is a cost-effective method for maximizing the use of existing sewer infrastructure. Flow monitoring provides valuable information about the actual flows conveyed. Owners can be made aware of the variables in sewer flows due to unknown and improper connections, the conditions of joints, cracks, and the susceptibility to allow ground and surface infiltration and inflow.

Sources of flow in the sanitary sewer include domestic dry-weather flow and other extraneous (unintended) inflows. Identifying the sources of extraneous flows into the sanitary sewer is important. The hydrograph of sanitary sewers is made up with the following components:

*                Baseflow Source: The waste flow from normal residential, commercial and industrial operations, known as dry-weather flow;

*                Infiltration Source: Originates in part from groundwater leakage into the sewers and from rainfall-induced surface seepage through the soil cover; and

*                Inflow Source: The direct contribution of storm water through openings in manhole covers, directly-connected roof drains, sump-pumps or other storm-related elements such as catch-basins or drain-pipes.

How-tos of flow metering

A typical monitoring period varies from a minimum of three months to a maximum of six months. In most instances, it is critical that both the spring melt and a range of summer storm events are captured in the monitoring programs. In order to perform a detailed analysis of infiltration/inflow and pipe capacity, it is necessary to obtain flow-monitoring information during both dry and wet weather. Under ideal conditions, multiple rain events of varying intensities are monitored in order to accurately assess the inflow response for each event. Information obtained during the monitoring period can be used to determine the following:

*                Average daily flow--dry weather;

*                Peak flow--dry weather;

*                Average daily flow--wet weather;

*                Peak flow--wet weather;

*                Peak inflow rates; and

*                Total I/I volume.

Meter site selection can be accomplished after reviewing the collection system maps and preliminary field inspection of any SSO locations. Each monitoring site should be selected so that the footage of the collection system upstream of the meter can be isolated for the purposes of determining extraneous infiltration/inflow. Installation of rainfall meters across the study area to measure rainfall intensity and duration throughout the monitoring period can assist in establishing wet weather capacity for SSO analysis. Two basic sewer channel flow meter technologies are available:

1.             "Wetted sensors", or submersible sensors, where the level/velocity sensor is mounted in the flow stream and the sensor is secured to a mounting band that fits snugly in the pipeline; and

2.             "Non-contact" radar sensors, where the sensor is mounted in a level position above the flow stream ensuring that the radar signal is aimed at the flow and does not hit invert walls. A data logger is installed at the top of the manhole. For the most accurate data, flow direction should not change abruptly going through the manhole. The manhole should not have debris, brick or any other objects that might disrupt the flow.


After the primary discharge locations have been established, it is important that a flow balance calculation is completed to identify system problems and quantify flow volumes and contribution percentages. Flow balance calculations will indicate system constraints and problems at a glance. Tabular reports allow the system to be assessed using volumetric techniques, allowing for complete site-by-site quantification. Scatter plot analysis determines if capacity issues are due to upstream backwater conditions or downstream system hydraulics. Scatter plot shapes clearly indicate if base flows are present and how much. Scatter plots also indicate if the system is conforming to conventional gravity flow conditions or to a combination of open channel and closed pipe flow hydraulics. Flow Monitoring Program Reports should include site reports for each station, a summary of data collected at each monitoring station and discussions of the following:

*                Dry weather analysis (including calculated base flows and diurnal patterns);

*                Wet weather analysis (including locations impacted by I/I and to what degree);

*                Tables and figures necessary to explain the results and findings;

*                Conclusions & recommendations; and

*                Hydrographs and tabular data for each station for the monitoring period.

Develop flow monitoring and CMOM programs

Formal development of a CMOM program is critical for all communities that experience any SSOs no matter how rare. The EPA has set a standard of care that must be implemented. Any overflows that are associated with events that should not have occurred with proper O&M practice will now be considered a violation that is subject to monetary and legal fines.

With the proposed modifications to EPA regulations establishing NPDES requirements to develop and implement CMOM programs for POTWs and satellite collection systems, water and wastewater officials may be asking how to comply with the regulations and be efficient at the same time.

The answer: A Flow Monitoring Program could ensure accurate measurement of local hydraulics, base flows and capacity. Planning and establishing a Flow Monitoring Program in conjunction with an ongoing CMOM program could bring a wastewater collection system into compliance with EPA regulations and NPDES requirements. Remember that the CMOM program will act as the primary mechanism for potentially offsetting an enforcement action. As an added benefit, a CMOM program will result in a more efficient, better run collection system. Flow metering as a component of CMOM and ongoing sewer system investigations will assist municipalities in optimizing all aspects of the system's hydraulic performance. 

About the Author

Susan McHugh