Infiltration & Inflow

Definition of Infiltration/Inflow (I/I)

Infiltration occurs when groundwater enters the sewer system through cracks, holes, faulty connections, or other openings. Inflow occurs when surface water such as storm water enters the sewer system through roof downspout connections, holes in manhole covers, illegal plumbing connections, or other defects.

The sanitary sewer collection system and treatment plants have a maximum flow capacity of wastewater that can be handled. I/I, which is essentially clean water, takes up this capacity and can result in sewer overflows into streets and waterways, sewer backups in homes, and unnecessary costs for treatment of this water. It can even lead to unnecessary expansion of the treatment plants to handle the extra capacity. These costs get passed on to the consumer.

Determining I/I

Flow monitoring and flow modeling provide measurements and data used to determine estimates of I/I. Flow meters are placed at varying locations throughout the sewer collection system to take measurements and identify general I/I source areas. Measurements taken before and after a precipitation event indicate the extent that I/I is increasing total flow.

Both infiltration and inflow increase with precipitation. Infiltration increases when groundwater rises from precipitation, and inflow is mainly stormwater and rainwater. The city monitors rainfall with an automated rain gauge system. These rain gauges are placed throughout the city so that data about variations in rainfall can be monitored and correlated.

 Identifying Sources of I/I

The City of Springfield completed a Sewer System Evaluation Survey (SSES) in 2001. The SSES involved inspection of the sewer system using several methods to identify sources of I/I:

• Building Inspection - 2 person crews go door-to-door, gathering information from residents and businesses about any history of sewer backup and stormwater flooding in their buildings. Basements are inspected for possible illegal plumbing connections, including foundation drains and sump pumps connected to the sanitary sewer.
• Dye testing - Dye is used at suspected I/I sources. The source is confirmed if the dye appears in the sewer system.
• Smoke testing - Smoke is pumped into sewer pipes. Its reappearance above ground indicates points of I/I. These points can be on public property such as along street cracks or around manholes, or on private property such as along house foundations or in yards where sewer pipes lay underground.
• TV inspection - Camera equipment is used to do internal pipe inspections.
• Visual inspection - Accessible pipes, gutter and plumbing connections, and manholes are visually inspected for faults.

Sources of I/I are also sometimes identified when sewer backups or overflows bring attention to that part of the system. The purpose of the SSES is to proactively reduce these incidences by seeking out the sources before they cause a problem.

Repairing I/I Sources

 Repair techniques include:

• Disconnecting illegal plumbing, drains, and roof downspouts
• Insituform pipe relining
• Manhole frame and lid replacement
• Manhole wall spraying 
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Since 1995 the City of Springfield has engaged in an active rehabilitation effort to reduce the entry of I/I into the collection system. This has been achieved through a combination of contracted and in-house rehabilitation.