Inflow and Infiltration

Manhole Inspection
Manhole Inspection

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The purpose of a water reclamation plant is to collect and treat wastewater from homes and businesses. Sometimes, stormwater and groundwater can get into the sanitary sewer system where it is cleaned unnecessarily. Reducing clear water coming into the sewer system by inflow and infiltration can save money while protecting human health and the environment.

What is inflow and infiltration?

Inflow and infiltration (I&I) is clear water from groundwater and stormwater sources that flows into the sanitary sewer system.

More specifically, infiltration refers to groundwater, or groundwater that is influenced by surface water, that seeps into sewer pipes through holes, breaks, cracks, joint failures, and faulty connections. Infiltration typically has seasonal variation in response to groundwater levels. Storm events can trigger a rise in groundwater levels and increase infiltration flows. The highest infiltration flows are typically observed following significant storm events or following prolonged periods of precipitation.

Inflow is surface water that enters the sewer system from yard, roof, and footing drains, from cross-connections with storm drains, downspouts, and through holes in manhole covers. This also includes improper sump pump discharge into the sewer system by discharge or direct connections into the wastewater system. Inflow occurs as a result of storm events such as rainfall, snowfall, and spring or snow melts that contribute to excessive sewer flows. Peak inflow can occur during heavy storm events when storm sewer systems are surcharged, resulting in backups and local ponding.  

Inflow Sources Image

Why is I/I a problem?

Additional water in the sewer system can be problematic for several reasons. It takes up capacity in the sewer pipes and ends up at the water reclamation facility where it must be treated like sewage, resulting in higher treatment costs. Additional water requires new and larger treatment facilities to convey and treat larger volumes of flow, resulting in higher capital expenses. And it can also contribute to sanitary system overflows (SSO), which are harmful to public health and the environment.

I/I is also costly. The District charges communities for the amount of flow that arrives at the water reclamation facility. In addition, I/I results in more energy usage to pump the flow and the unnecessary treatment of groundwater and stormwater.

What is being done to reduce inflow and infiltration?

While it is virtually impossible to eliminate all I&I from the sanitary system, WalCoMet began an I/I Reduction Program in 2008. The initial program goals included investigation, preparing a reduction plan, addressing private property issues, plan implementation, and designing construction standards. Since that time, projects have been completed to reduce clear water flow. In addition, annual inspections and cleaning of sanitary lines and manholes are completed. The sanitary line cleaning is completed using a high-pressure water jetting system. Once cleaning is complete, a lighted television camera is used to inspect the top, bottom, and sides of the sanitary lines, as well as lateral connections, manholes, and any possible defects. Manholes are inspected by WalCoMet staff using specialized camera equipment. The digital video and photos are then reviewed to determine if any defects are present.

Additional methods used to identify I/I can include smoke testing, dyed water tracing, and internal building inspections.

How do we remove inflow and infiltration?

There are many methods used to remove and reduce I/I. Modern techniques allow for the sewer lines to be inspected, cleaned, and even replaced or rehabilitated without the traditional open-cut excavation. These technologies, such as cured-in-place-pipe lining (CIPP), spray lining, and slip lining, allow for repair of sewer lines without the expense and interruption of conventional excavation.