Inflow & Infiltration

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Inflow & Infiltration

The Town of Leesburg's Sanitary Sewer System is designed to carry wastewater from homes and businesses and convey that wastewater to the Water Pollution Control Facility. When storm and/or ground water enters the system through inflow and infiltration (I&I), the sanitary sewer system can become overloaded and potentially result in sewer backups or sanitary sewer overflows, which negatively affect residents and the environment. Typical sources of I&I include cracked pipes as well as sump pumps, downspouts, and foundation drains that have been directly connected to the sanitary sewer system. Most I&I is caused by aging infrastructure that needs maintenance or replacement. By working together, customers, residents, and Department of Utilities personnel can reduce sources of I&I and help protect the environment from sanitary sewer backups and overflows as well as keep fees, sewer system costs, and treatment costs down.


Inflow and Infiltration Image


Infiltration is groundwater that enters sanitary sewer pipes (interceptors, mains, manholes, or laterals) through holes, breaks, joint failures, connection failures, and other openings. Storm events can trigger a rise in groundwater levels and increase infiltration flows. See picture above for a graphical view of the sources of infiltration.


Inflow is surface water that enters the wastewater system from yard, roof, and footing drains, from cross-connections with storm drains, downspouts, and through holes in manhole covers. Inflow occurs because of storm events such as rainfall, snowfall, springs, or snowmelt that contribute to excessive sewer flows. See picture above for a graphical view of the sources of inflow.


Inflow and Infiltration is commonly referred together as I&I.

Private Sewer Lateral:

A private sewer lateral is a sewer pipe that connects a building's internal sewer plumbing to the public sanitary sewer system. The sewer lateral is part of the private plumbing system and is maintained by the property owner.

Sanitary Sewer:

The sanitary sewer is a system of pipes and devices that are designed to carry sanitary sewer, or wastewater, from approved drains in homes and businesses such as sinks, toilets, and showers, to a wastewater treatment plant.

Storm Sewer:

The storm sewer is a system of pipes and channels designed to carry storm and ground water from streets, parking lots, roofs, driveways, sidewalks, etc., directly to drainage creeks, rivers, and/or detention basins.


What is the difference? Sanitary sewer vs. Storm sewer

The Town of Leesburg’s sanitary sewer system is designed to carry wastewater from residences and businesses throughout the Town's Service Area to the Water Pollution Control Facility for treatment. The sanitary sewer system is designed with the appropriate sized pipes to handle the removal of wastewater only, and does not have the capacity to handle additional water from storm and ground water sources.

The storm sewer system is designed to carry storm and ground water directly to natural drainage features such as creeks, streams, rivers, and/or lakes, or to man-made detention / retention facilities. The stormwater system is designed with much larger capacity pipes and drainage features to handle large volumes of water from rain, snowmelt, and ground water sources. Storm and ground water is predominately clear water that does not need chemical or biological treatment before entering a natural or man-made drainage feature.


What problems do I&I cause?

During significant rain events, storm and ground water can find its way into the sanitary sewer system through Inflow and Infiltration. When this excess water enters the sanitary sewer system, it can create the following problems:

  • I&I can increase the flow in the sanitary sewer system to a point beyond the designed capacity of the pipes. When the capacity of the pipes is exceeded, the excess wastewater looks for the lowest release point, which can lead to basement backups, and/or sanitary sewer manhole overflows.
  • Storm and ground water that enters the sanitary sewer system is transported to the treatment plant and unnecessarily treated. This leads to higher treatment costs for the utility and its customers.
  • If I&I are not addressed, the sanitary sewer pipes may have to be upsized to handle these peak flows to reduce backups and overflows. These projects are expensive and do not completely solve the problem as the excess storm water in the system is still being unnecessarily treated at a cost to the Utility and its customers.


What are the benefits of removing sources of I&I?

Depending on the type of I&I connection(s), just a couple of properties with sources of I&I can overload a sanitary sewer line during a rain event. When the line becomes overloaded, the wastewater looks for the lowest point to release the pressure building up in the pipe. This can be a basement drain or a sewer manhole. Although a property owner may have I&I connections on their property and never have a problem, they could be contributing to a sewer backup in the basement of a neighbor or the overflow of wastewater from a sewer manhole. Removing sources of I&I can help to reduce the occurrences of backups and overflows. Any reduction in the amount of storm water entering the sanitary sewer system is a public benefit to all Leesburg customers. The benefits include:

  • Decreased occurrences of basement backups.
  • Reduced claims and claim related costs associated with backups.
  • Better protection of the environment from the impact of overflows.
  • Reduced treatment costs.
  • Increased capacity of the sanitary sewer system.
  • Reduced need for expensive projects to handle excess flow.


What is the Town Utilities Department doing to reduce I&I?

The public sanitary sewer system includes all pipes, manholes, pumping stations, excess flow holding basins, etc., in the system, continuing to the point where the customer's private sewer lateral connects to the public system. The Town of Leesburg Utilities Department is constantly evaluating and maintaining the public sanitary sewer system. This is accomplished by routing a television camera through the pipes to find defects as well as water-jetting cleaning, root removal, and visual pipe and manhole inspections. System repairs, replacements, and corrections are made from these evaluations to reduce sources of I&I in the public sanitary sewer system.


What can residents and businesses do to reduce I&I?

The private sanitary sewer system includes all the sewer plumbing in a property owner's home or business and includes the sewer lateral from that home or business to the point of connection with the public sanitary sewer system. The property owner is responsible for ensuring their private sanitary sewer system is maintained properly to avoid sewer problems on their property and/or avoid contributing to problems for property owners farther downstream on the public system.