One Method to Identify Inflow and Infiltration (I/I): Flow Monitoring

They brave the depths of hundreds of miles of sewers beneath our cities. They may encounter rats, roaches, dark passages and can we talk about the smell? No, we’re not talking about a day in the life of the famous man who dons a rumpled brown fedora (a.k.a. the hat). We’re talking about a day in the life of SEH field expert Paul Kubesh and I/I technicians who setup flow monitoring instruments in your city’s sanitary sewer collection system.

One method used to identify inflow and infiltration (I/I) is to pinpoint where problems occur and isolate the areas that have the highest contribution of I/I and establish a flow monitoring program. The program should help to identify the presence, quantity and type of I/I issues which exist in the sewer system and provide information to assist cities with fulfilling the requirements of the federal and state Clean Water Act. 

And the time of year matters according to Paul Kubesh, who is an expert at conducting flow monitoring programs. “Ideally cities should install the meters in February or March to get dry-winter weather flow and meltwater. Then summer brings rainfall events and hopefully you get a few big rainfall events to capture enough flow data—the more data the better.”

What is inflow and infiltration

Inflow and infiltration are terms used to describe clear water, including stormwater and groundwater that enters sewer collection or wastewater system directly (inflow) or indirectly (infiltration): 

  • Inflow is stormwater that enters the sewer collection system through direct connections such as roof leaders, yard drains, catch basins, defective manhole covers and frame seals, sump pumps or indirect connections with storm sewers.
  • Infiltration is groundwater or rain-induced that enters a sewer collection system through defects in the sanitary sewer pipes, breaks, or manhole defects.

Why should communities monitor

I/I can result in negative environmental impacts, regulatory compliance issues, higher treatment costs, basement backups and excess wear and tear on the collection system. Additional water from I/I sources reduces the useful life, and the capacity of sewer systems and treatment facilities to transport and treat domestic and industrial wastewaters. Sewer flow monitoring helps communities determine whether their pipes are undersized in a certain location, what pipes need replacing and where leaks might cause problems, among other things. 

What is sanitary sewer flow monitoring

Sanitary sewer flow monitoring generally involves placing equipment, usually a sensor, into the sewer flow—or above the sewer flow depending on the type of equipment. The sensor measures the depth and velocity and uses those parameters, along with pipe size and shape to calculate the flow rate. Sensors are placed in the sewer pipes, and a data collector is hung near the top of a manhole. 

Example of sanitary sewer monitoring
Example of sanitary sewer monitoring

What is the goal 

The goal of sewer flow monitoring is to gain knowledge and collect accurate and current information on the flow characteristics of the study area. The information provided by flow monitoring will help to locate those areas that have excessive I/I and determine if those areas need further investigation. This task should be conducted at the earliest possible stage to minimize the survey costs. 

How to conduct a meter installation

A meter installation consists of four components:

  1. Map it out– identify the district areas and meter locations prior to the start of the project.
  2. Safety – confined space entry; traffic, safety and survey equipment; and appropriate safety gear are necessary throughout the meter installation process. 
  3. Flow monitoring/meter installation – from start to finish (varies with each site).
  4. Results –include data forms documenting the findings, photographs, climate conditions, and summary of final results. 

1.  Map it out

Whether your city has one district or 50, it’s important to map out the district location(s) before you begin. Geographic Information System (GIS) mapping of the sewer system includes sewer, water and storm drain system and provides the data necessary for analysis within each district. 

detailed list
Example of a detailed list consisting of the monitoring site(s), pipe size and location within a city.
flow meter locations map
Example of a flow meter locations map

2.  Safety

Confined space entry


A confined space is defined as any space that is large enough and so configured that a person can bodily enter and perform assigned work, has limited or restricted means for entry or exit and is not designed for continuous employee occupancy. In general, the atmosphere must be constantly monitored for sufficient levels of oxygen, and the presence of hydrogen sulfide gas, carbon monoxide gas and lower explosive limit levels. 

“Everyone doing this work is confined space trained—safety is first so we don't have any issues,” says Kubesh. We don't want issues for ourselves or our clients.” 


SEH field experts managing traffic

Traffic is another safety issue and SEH field experts manage traffic on most projects. “We may only be at one location for a half hour, or longer depending on the size of the pipe, so we take care of the traffic throughout the entire process,” says Kubesh. Because we are a moving system, we get traffic plans from the city and then manage the setup and tear down.” 


When SEH field experts are ready to install flow meters, they make sure they have the essential field safety gear and equipment. Here’s a check-list of the recommended gear to help them do their job safely, or just make it easier:


Additional field gear includes, but is not limited to:

  • Vehicles and traffic control equipment
  • Measuring tape
  • Manhole pick and shovel
  • Ladder
  • Jacket

Now that they’re prepared with the recommended safety gear and a plan, they’re ready to venture to the project site and begin the meter installation. When they arrive at the first project site, what’s next? These are the field procedures followed:

3.  Flow monitoring/meter installation procedures

At the job site, SEH field experts document existing and physical conditions of the collection system being evaluated. They identify and evaluate manhole(s) in the area and identify problem areas. Flow meter placement is similar to purchasing real estate—location is everything. The SEH field technicians select the manhole based on flow characteristics, identifying locations free of sediment and look for smooth linear flow conditions through the manhole. 

In addition, each site is selected for ease of equipment installation and the physical characteristics of the manhole to determine if it is suitable for flow meter placement.

After a thorough site evaluation is complete, the manhole cover is opened and an air monitor device is lowered into the manhole to test the levels of oxygen, and the presence of hydrogen sulfide gas, carbon monoxide gas and lower explosive limit levels. Once completed, they set up the safety and non-safety equipment needed for installation which includes:

  • Orange safety cones are placed around open manhole(s) 
  • A tripod and winch system is secured over the manhole
  • Equipment needed for the installation is prepped and ready (i.e. metal band, tools, parts, zip ties, etc.)
  • Computer to capture data
  • One field expert puts on safety gear and prepares to be lowered into the manhole
  • Another field expert (attendant) stays above ground and communicates with the field expert in the manhole

Communication is key between the field expert in the manhole and field experts above ground. 

Meter installation

  1. The metal band is lowered first into the manhole, followed by an SEH field expert 
  2. The probe is attached to metal band and sits in the pipe below the water line
  3. The metal band is secured in the sewer pipe
  4. The cord is cinched to rungs on the ladder in the manhole 
  5. The data collection box is secured at the top of the manhole and disconnected feeds the following information to the computer 
    • Level
    • Velocity 
    • Flow 
metal band
Probe is attached to this metal band.

“Depending on the pipe size and I have everything at my disposal, one installation can take as little as 15 minutes if all goes well,” says Kubesh.

4.  Results

A GIS-based application is used to document the following installation information: 

  • Pipe size
  • Pipe material
  • Placement of meter
  • Condition of the manhole
  • Silt level
  • Location
  • Traffic conditions
  • Type of flow 
  • Photos of the install 
  • Type of meter
  • GPS locations

The data is downloaded over two weeks and uploaded to a server where it can be reviewed and analyzed for I/I contribution based on wet weather events. “If we know a significant rain event is forecasted, we will go out and download data before the event,” says Kubesh. 

The flow data can be used to identify problem areas and establish additional investigation measures to target specific I/I sources.

Next steps

When a flow metering program has been completed, additional steps cities can take to reduce or stop inflow and infiltration include: conducting manhole inspections, smoke testing, dye testing, closed circuit television (CCTV) inspections or private property inspection. 

Wrap it up

Inflow and infiltration (I/I) can result in negative environmental impacts, regulatory compliance issues, higher treatment costs, basement backups and excess wear and tear on the collection system. Sewer flow monitoring is the first step in identifying where problems are occurring and isolate the areas that have the highest contribution of I/I by establishing a flow monitoring program.

About the expert 

Paul Kubesh

Paul Kubesh is a senior lead I/I technician with 16 years of helping cities find solutions for inflow and infiltration. Contact Paul

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