Dec 09, 2003

CMOM is Coming

What can be done to help water and wastewater professionals

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

*                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 is with Marsh-McBirney. For additional information contact Marsh-McBirney, Inc. at 800/368-2723 or www.marsh-mcbirney..