News / Blog

Combatting Inflow & Infiltration (I&I)

May 19, 2020

By Travis J. Long, Sr. Project Leader

With the end of winter, and spring upon us, those aging wastewater collection and conveyance systems are often subject to the direct effects of the costly “gremlin in the works,” Inflow and Infiltration (II). II is often called “clear water” (although it may be dirty) to distinguish it from normal sanitary sewage water (“gray water”) in the sewer system.

What exactly is Inflow and Infiltration?

INFLOW is stormwater that enters into sanitary sewer systems at points of direct connection. Various sources can contribute, including footing/foundation drains, roof drains, downspouts, drains from window wells, outdoor basement stairwells, and groundwater/basement sump pumps.

These sources are typically improperly or illegally connected to the sewer system via direct connections or discharge into sinks or tubs that are directly connected to the sewer system. An improper connection lets water from sources other than sanitary fixtures and drains to enter the sewer system. This water should be entering the stormwater systems or allowed to soak into the ground without entering the sanitary sewer system. As an example, a single sump pump can contribute over 7,000 gallons of water to sanitary sewer system in a 24-hour period, the equivalent of the average daily flow from 20-25 homes.

INFILTRATION is groundwater that enters sanitary sewer systems through cracks and/or leaks in sewer pipes caused by age related deterioration, loose joints, poor design, installation or maintenance errors, damage, or root intrusion. Groundwater enters through these cracks wherever sewer systems lie beneath water tables or where the soil above the sewer systems becomes saturated. Often sewer pipes are installed in susceptible wet, low-lying areas because they are the lowest point in the area to allow for gravity flow, and allow for more cost-effective installations. These sewer pipes are especially prone to infiltration when cracked or broken.

General sewer pipes are designed to last about 50 years, depending on what type of material is used. Often sanitary sewer pipes, along with the lateral pipes attached to households and businesses, go much longer without inspection or repair, and are much more likely to be cracked or damaged.

“Clear water” belongs in stormwater sewers or on the surface of the ground, and not in the sanitary sewers.

General sewer pipes are designed to last about 50 years, depending on what type of material is used. Often sanitary sewer pipes, along with the lateral pipes attached to households and businesses, go much longer without inspection or repair, and are much more likely to be cracked or damaged.

When sanitary sewer systems have reached capacity or become overloaded, wastewater flows at a much higher water level than normal. If sanitary fixtures or drains are below this overload level, water will flow backward through the sanitary sewer pipe, flooding basements or households and causing manholes to pop open releasing wastewater into the environment, creating a public health and safety risk, and violation of state and federal regulations.

We experience this dreaded increased flow typically during the spring and fall seasons. This extra flow can account for 50% or more of flows in our older systems, creating risks, disruptions to operations, and dramatically escalates the cost of operation/maintenance. This often leads to regulatory noncompliance issues and an onset of system improvement projects, which, if not properly studied and evaluated for best available practices, can be a large debt burden to the customer base. The cost of this disruption should not be underestimated, given that even small amounts of extraneous flow translates into substantial costs to the consumers. Remember, all water entering a wastewater treatment facility must be treated, and this extra flow causes a direct increase in operating costs proportional to the amount of “clean water” entering the sewer system. By reducing II, capital and operating costs can be lowered, and equipment and treatment system overall life expectancy can be extended.

In that exact situation, the question I hear often: “where should you draw the line on II? How much is too much?” Unfortunately, this answer is not straightforward. In broad terms, a system has to have a plan that evaluates what can cost-effectively be removed from the sewer system, based on a comparison of the cost of removal to the cost of conveying and treating the flows. This plan has to take into consideration the feasibility and cost-effectiveness of eliminating sources of inflow. Regardless of the numbers, if a plant experiences hydraulic overloads, the situation is excessive and requires corrective action, and left unchecked, will result in regulatory mandates.

Our team at JHA strives to work hand in hand with our municipal partners to be on the lookout the for sources of II and assessing the ways to reduce it, as well as educate the customer base to assist in this process of elimination. Documentation of the successes of corrective actions are equally important to continue the trends and report progress to the regulatory body and customer base.

So how do we identify whether you have an Inflow or Infiltration problem? We have to work as a team and communicate our thoughts, findings, and plans, and start by looking for the clues:

1. Trend pump run time (hour meter) and compare values during storm events and normal weather

2. Trend hydraulic loading at the wastewater treatment plant (metered flow) and the difference before/after a rain event

3. Look for localized overflows (manhole overflows) or simply the flows within the collection system network during heavy storm events

4. Increase in inflow during dry weather conditions compared to previous months. Additionally, more extensive assessments via in-system flow monitoring, closed circuit televising (CCTV), dye testing, etc. can be performed.

Source detection can be challenging, time consuming, and requires diligence in data acquisition and review, all of which comes as cost to the system and should be planned for accordingly.

Some of the most common ways we locate the sources of II:

Manhole inspections – Used to identify leaks from broken/deteriorated concrete, failing joints from ground movement, tree root intrusion or design issues. Additionally, manholes can contribute II through leaky covers/lid.

Smoke testing – Used to locate II sources by identifying stormwater drain cross-connections, broken pipes and lateral and unsealed manholes.

Dye testing – Used to identify leaks and confirm smoke testing results, via water mixed with a non-toxic dye. The colored water is conveyed through the ground and stormwater system and appears in the sanitary sewer collection system where leaks or illegal connections occur.

Flow Monitoring – Utilization of flow recording devices. Flow monitoring instrumentation must be placed in sanitary sewer systems at locations appropriate to obtain the data desired.

Periodically the system should monitor and measure their sanitary sewer system to maintain the integrity of the system and determine new sources of inflow and infiltration, as the system continues to age.

CCTV – Allows systems to visualize the inside of pipes, including laterals, by using a small camera that travels down the length of a pipe to produce a visual representation of its condition. CCTV inspections can reveal the need for rehabilitating leaky pipes or broken lateral connections and have proven to be a very effective tool.

Private property inspections – Consists of visual assessments of the stormwater and wastewater networks within a property for illegal connections.

We often need to utilize multiple investigative means from the ways listed above to thoroughly identify the issue to formulate corrective action plans. This step requires communication among the team members to implement effective solutions. Any reduction and control of II in sewer systems should be considered in regard to a disciplined, long-term monitoring and maintenance program. Remember, the first step to resolving any II problem is determining how significant the problem is.

Wastewater treatment infrastructure is an expensive investment for a community. Municipalities will be well served to seek to understand the enormity and repercussions of any II problems. A clear set of goals are important for keeping an II program focused, as sewers and treatment facilities are designed around expected average and maximum flows, not for the incorporation of excess II robbing the system of its valuable capacity. This puts burdens on operation and maintenance and reduces the life expectancy of the treatment facility.

Integrating II investigation and corrective action into a municipality’s normal public works budget can allow an incremental approach to continuous improvement. Don’t let unexpected II create a cost burden for your system. JHA Companies would be honored to offer our services to assist your utility in implementing a flow monitoring and II reduction plan to address those short and long-term needs of your system and help work as a team with you and your customer base.


Travis Long is a Senior Project Leader at JHA Companies, and works primarily with municipalities, communities, and plant operators to manage and improve water/wastewater facilities and systems. He is an avid outdoorsman , and enjoys spending time with his wife, 3 kids, and many animals, at their family farmstead in Pennsylvania. He can usually be found at his desk talking on the phone, with a smile on his face and asking how he can help, or chasing after loose cows in the pasture.

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