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Investigating Inflow & Infiltration with Sewer Flow Monitoring

As communities face the burden and costs of increasingly stringent wastewater treatment standards, the need for sanitary sewer system maintenance and flow monitoring couldn’t be stronger. Through regular observation, sewer system deterioration, defects, and flow issues are identified. As a result, communities can take a proactive approach to keep their sanitary sewer system functioning properly and avoid unexpected costs.

Sewer system inspections uncover signs of structural fatigue, corrosion, damage, root intrusion, and other issues. Equally important is wastewater flow monitoring, which helps identify flow problems due to undersized sewer piping or inflow and infiltration.

Sources of Inflow & Infiltration

Inflow and infiltration (I/I) occur when stormwater or groundwater leaks into a sanitary sewer system. Stormwater that enters a sanitary sewer system is known as inflow. Direct connections between sanitary sewer systems and downspouts, storm sewer systems, sump pumps, or basement floor drains are common sources of inflow. Infiltration occurs when groundwater leaks into a sanitary sewer system through cracked or damaged manholes, pipe joints, or service lines.

An illustration of a residential diagram of infiltration and inflow sources to a sanitary and storm sewer system.

Problems Associated with Inflow & Infiltration

Reducing and eliminating I/I helps prevent sanitary sewer system overflows (SSOs) that result from system overloads. SSOs can cause water quality problems, property damage, and put public health at risk from back-ups in homes and the release of untreated flow into rivers, lakes, and streams. I/I reduction also prevents wastewater treatment costs from rising due to higher electrical and chemical usage as well as increased wear and tear on equipment.

“Regardless of whether a sanitary sewer system flows to a treatment plant or lagoon system, there’s a cost to handle and treat the flow,” explains Steve Klocke, PE, Civil Engineer for Snyder & Associates. “Eliminating I/I allows the system to function at the capacity it was designed for to protect public safety and environmental health while keeping treatment costs in check.”

Using Flow Monitoring to Right-Size Treatment Facility Design

For communities that are looking to upgrade, expand, or replace their wastewater treatment facility, flow monitoring is also beneficial to the design and construction process. High flows from stormwater I/I means that any upgrades or changes to a community’s wastewater treatment system must be designed with that flow and capacity in mind. And with larger facilities comes higher planning, design, construction, operation, and maintenance costs.

Klocke points to the growing need for many communities to add disinfection to their wastewater treatment facility as an example. He says that disinfecting not only the incoming wastewater but also any added stormwater or groundwater that’s diluting it, “can substantially increase the size of the infrastructure needed and the costs associated with it.”

Taking steps to reduce I/I before the design and construction of wastewater treatment upgrades, will save a community money long-term because the facility will be appropriately sized. Flow monitoring identifies where I/I issues exist and quantifies them, which empowers communities to make informed decisions.

“Budgets are always limited, so community infrastructure decisions need to be made with the big picture in mind,” shares Klocke. “Before making changes or upgrades to your wastewater treatment facility, it’s wise to investigate the sewer system leading to it because you can save money and avoid headaches later on.”

What is flow monitoring?

Flow monitoring uses equipment to read and record flow depth and rate to determine if issues are resulting from inflow, infiltration, or both. Using the data gathered, it’s possible to plan and design a sanitary sewer rehabilitation approach to correct I/I issues at targeted locations. Once sewer rehab is complete, another round of flow monitoring is often beneficial to illustrate how effective the improvements are.

In addition to I/I investigations, flow monitoring is also helpful for:

  • Signaling overflow alarms that alert wastewater facility staff to issues within the sewer network
  • Determine the capacity of an existing sanitary sewer system and whether or not upgrades are needed to accommodate new development
  • Billing for communities that pump their wastewater to another community for treatment

Flow Monitoring Equipment & Methods

Flow monitoring equipment consists of two parts: a monitor/logger and a sensor. The monitor/logger is installed below the manhole cover to record and store information gathered by the sensors that read the sewer flow. Information can be downloaded in the field directly from the monitor using a laptop or uploaded to the cloud via satellite or cellular connection for viewing on a desktop computer or through a smartphone app.

Based on a community’s needs and constraints, there are three sensor types to choose from. Two are in-flow sensor types and the other is a long-range sensor that doesn’t come in contact with wastewater.

In-Flow Sewer Monitoring

One type of in-flow sensor mounts on the bottom of a pipe. Velocity, depth, and temperature measurements are gathered as wastewater flows over the unit. An ultrasonic depth sensor within the unit bounces a radar beam off the water surface of the flow and back down to down to gain depth information. If the pipe is full due to surcharging, a surface isn’t available for the ultrasonic depth sensor to read. In these instances, a pressure depth sensor reads what’s going on in the pipe.

Using this type of sensor results in the most accurate velocity reading, because it averages velocity throughout the entire depth of the flow and accounts for surcharging. Man-entry is required for installation, which is a disadvantage. The sensor also requires a minimum flow depth to work, and sensors can be fouled by debris.

A cross section illustration of a water filled sanitary sewer pipe with an inflow sensor mounted to the bottom of the pipe.









The other type of in-flow sensor mounts on top of the pipe. It shoots a beam down into the flow that bounces off the surface to measure velocity. Not every sensor comes equipped to measure pressure, but it’s an option to measure surcharge conditions. Because the sensor installs on the top of the pipe, it’s less susceptible to debris. However, man-entry is still required for installation, and the sensor is slightly more costly. Velocity readings may not be as accurate as those provided by the bottom-mounted sensor because measurements come from the surface only.


A cross section illustration of a water filled sanitary sewer pipe with an inflow sensor mounted to the top of the pipe.










Long-Range Depth Monitoring

Mounted directly to a manhole lid or just below it, long-range depth monitors look down into the manhole invert below. It gathers an accurate reading of flow depth, is the least costly, doesn’t required confined space entry, and is a great option for operators that would like flow alarms. The inability to measure velocity makes it best suited for instances where depth is the primary concern. For flow rate estimation, the manhole should ideally have a uniform and well-formed invert at the bottom, so not all manholes are ideal for this type of technology.

Solutions for Inflow & Infiltration

Data gathered from flow monitoring is translated into charts depicting changes in pipe flow over time. Comparing the charts with storm and precipitation information allows the engineering team to identify areas of I/I and determine what solutions are appropriate. There are a variety of ways to address I/I issues. The most expensive option is main and manhole replacement, but according to Klocke, that solution is only for cases of extreme infrastructure deterioration. Depending on the severity of I/I and other factors, sewer system rehabilitation is likely, which may include:

Eliminating cross-connections and disconnecting residential sump pumps and roof gutters from the sanitary sewer system may also be advised.

Flow Monitoring: Our Approach & What to Expect

The first step of any I/I flow monitoring project is to develop a plan that guides the investigation. This begins by dividing the system into catchments or smaller regions of infrastructure that are the same age or material. This approach allows the engineering team to focus on areas that are relatively similar and narrow down areas where I/I is occurring.

In terms of investigating I/I, flow monitoring study can take anywhere from three to six months on average, with the weather playing an important role in the monitoring process. Dry weather establishes base flows, while wet weather helps the engineering team understand when and where flows are surging. Sharp increases during storms are likely from stormwater inflow, while long durations of increased flow after a storm indicate groundwater infiltration.

“If your community is experiencing backups, overflows, or planning any sort of treatment system upgrade, flow monitoring is a good place to start. It’ll help you understand where you are today, quantify the value of rehab, and plan for the future,” concludes Klocke.

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