Managing Infiltration Inflow in Wastewater Collection Systems

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Managing Infiltration Inflow in Wastewater Collection Systems

Reference Number: MTAS-587
Reviewed Date: 08/19/2026

What Is Infiltration and Inflow (I/I)?

Groundwater infiltration and stormwater inflow—commonly referred to as I/I—are among the greatest operational challenges facing wastewater collection systems. Excessive I/I consumes valuable collection and treatment capacity, increases operating costs, contributes to sanitary sewer overflows (SSOs), and may accelerate the need for expensive system expansions. Reducing I/I is often one of the most cost-effective ways to improve the performance of an existing wastewater system.

Infiltration is groundwater that enters the wastewater collection system through defects in the system, including:

  • Cracked or deteriorated pipes.
  • Defective joints.
  • Leaking manholes.
  • Faulty service laterals.
  • Improperly sealed connections.

Because infiltration originates from groundwater, it often increases during periods of elevated groundwater levels and may continue long after rainfall has ended.

Inflow is stormwater that enters the wastewater collection system through direct connections or openings, including:

  • Roof downspouts.
  • Foundation drains.
  • Yard drains.
  • Area drains.
  • Cross-connections with storm drainage systems.
  • Manhole covers located in flooded areas.

Unlike infiltration, inflow enters the system rapidly during rainfall events and typically subsides shortly after the storm ends.

Why Is I/I Important?

Excessive I/I can:

  • Cause sanitary sewer overflows.
  • Contribute to basement and building backups.
  • Increase wastewater treatment costs.
  • Reduce available capacity for future growth.
  • Increase pumping costs and energy consumption.
  • Shorten the life of pumps and treatment equipment.
  • Increase the frequency of regulatory violations.
  • Lead to unnecessary capital expenditures for larger pipes, pump stations, or treatment plant expansions.

In many communities, reducing I/I is significantly less expensive than expanding treatment or collection capacity.

Does Your System Have I/I?

Nearly every wastewater collection system experiences some degree of infiltration and inflow. Older systems, systems with vitrified clay pipe, aging concrete manholes, deteriorated service laterals, or extensive pipe joints often experience higher rates of I/I than newer systems.

Utilities should routinely evaluate system performance during and after rainfall events. Sudden increases in pump station flows or treatment plant influent during wet weather often indicate excessive inflow or infiltration.

Since 2018, Tennessee's renewed National Pollutant Discharge Elimination System (NPDES) permits have included expanded reporting requirements for sanitary sewer overflows, backups, and collection system performance. These requirements encourage utilities to better understand their collection systems and prioritize I/I reduction efforts.

Is It Really I/I?

Not every overflow or backup is caused by infiltration or inflow. Before investing in an I/I reduction program, utilities should determine whether operational or maintenance problems are contributing to the issue.

Common causes include:

  • Grease accumulation.
  • Tree roots.
  • Debris or blockages.
  • Pipe sags (bellies).
  • Protruding service taps.
  • Collapsed or deteriorated sewer lines.
  • Undersized sewer mains.
  • Pump station equipment failures.
  • Leaking check valves.
  • Worn impellers.
  • Air locking in force mains.
  • Corrosion or sediment accumulation in force mains.

Many of these problems can be corrected through routine operation and maintenance without major capital improvements.

Evaluating an I/I Problem

Utilities should avoid assuming that every wet-weather problem requires system-wide rehabilitation. Instead, begin by collecting data to identify where excess flow is entering the system.

Common investigative tools include:

  • Flow monitoring.
  • Rainfall correlation.
  • Closed-circuit television (CCTV) inspection.
  • Smoke testing.
  • Dye testing.
  • Manhole inspections.
  • Sewer system mapping using GIS.
  • Pump station flow trending.

Collecting accurate information allows utilities to prioritize rehabilitation projects where they will produce the greatest benefit.

Managing I/I

An effective I/I reduction program should be continuous rather than a one-time project. Successful utilities typically:

  • Maintain accurate GIS mapping of the collection system.
  • Routinely inspect manholes and sewer lines.
  • Clean sewer mains on a scheduled basis.
  • Repair defective manholes and pipe joints.
  • Replace deteriorated sewer mains as part of a capital improvement program.
  • Eliminate illegal inflow connections.
  • Track sanitary sewer overflows and customer backups.
  • Evaluate system performance after significant rainfall events.

Reducing I/I is rarely accomplished through a single project. Instead, long-term success results from combining routine maintenance, targeted rehabilitation, capital improvements, and ongoing system evaluation.