Inflow and Infiltration Studies in Your City

Posted: July 28, 2015
Category: Groundwater
Tags: Inflow, storm water

Groundwater and stormwater enter wastewater or sanitary sewer systems through inflow and infiltration.

• Inflow is stormwater that enters sanitary sewer systems at direct connection points. A poor connection lets water from sources other than sanitary systems (showers, toilets, sinks, etc…) into the pipes. Sanitary sewer systems are designed to overservice the homes they’re connected to. However, inflow from a small number of other sources can overload the system.
• Infiltration is groundwater entering sanitary sewer systems through cracks or leaks in the aging sewer pipes. Sanitary sewer systems have drained entire streams and creeks through cracked pipes and infiltration (the systems are often installed under flowing water bodies because they’re the lowest point in an area).

Sounds like a rare problem

It’s actually a very common problem. Especially in Canada, where snow melt is known to cause massive disruptions to sanitary sewer systems every year. Inflow and infiltration add “clear water” to sewer systems, increasing the likelihood those systems will overload.


Stormwater sewers are built to accommodate clear water, keeping it on the surface of the ground. When clear water enters sanitary sewer systems it must be transported away and treated like sanitary waste water. The sanitary system can only handle so much water, and an overload in the system will cause water to flow backward, flooding basements and fixtures along its path.

How to solve the inflow and infiltration issue in your region

To come up with a solution to the inflow and infiltration problem in your area, you first have to determine how significant the problem is. A sanitary sewer system evaluation can determine the quantity of inflow and infiltration, the sources, and provide a starting point to develop a cost effective plan.

You have to start by measuring the volume of inflow and infiltration. TO do this, information must be gathered from maintenance records, complaints, work orders, previous studies, maps and any other identifiable source.

When data has been analyzed, you must establish sewer flow monitoring points. Sanitary systems can often be separated into watersheds, and those watersheds further separated if necessary.
Flow monitoring equipment is then placed in the sanitary sewer systems at chosen locations.

• 1 flow meter for every 30k to 50k feet of sanitary sewer pipe
• Flow meter recording should be set at 10 minute intervals
• 1 rain gauge should be present for every 3 flow meters
• Monitoring should take place over the course of 2 months or more
• Accurate results come from measuring at least 6 separate waterflow events

You should glean two significant data points from monitoring the flow.

1. Comparative numbers between watersheds along the sanitary sewer system (to prioritize watersheds for future study and reduction)
2. Information to inform future relief or replacement sewer systems

Contact Heron Instruments today to talk about your water monitoring job and find the perfect tool for your job.

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