Inflow & Infiltration – What’s the difference?

“I keep hearing about I/I and wondered what people were talking about? I asked someone and they said that it stood for infiltration and inflow of water into a sewer pipe and it wasn’t a good thing but they didn’t know why. What exactly is the difference between the two and why is it not a good thing? I thought it would help wash solids down the drain and keep them cleaner.”

Let’s start with the difference between inflow and infiltration. Inflows are planned dumping into a sanitary sewer. These planned events are things like piping a roof drain directly into the sanitary sewer piping. In several cities, years ago, storm water was combined with wastewater and both flowed together in the pipe. The curb drains in the street led directly into the collection system. You found these conditions in cities where the combined flow was dumped into waterways and referred to as outfalls. The idea was that storm water would dilute the wastewater and would be beneficial. Other inflows included sump pumps that moved groundwater that was coming into a building out to the sanitary sewers. All of these were planned.

Infiltration, on the other hand, is groundwater that enters the sanitary or combined sewer system that isn’t planned. Leaking pipe joints, missing sections of pipe and eroded piping where the pipe wall is missing completely is the cause of infiltration or letting water into the collection system in an unplanned way.

Wastewater Treatment Process

Why isn’t it considered a good thing to use extra water to wash solids down the line? As our world grew, the demand to treat wastewater became a focus as opposed to dumping it back to mother nature to absorb the effluent. Cities grew, and the collection systems ended up carrying more and more wastewater which by then was transported to wastewater treatment plants. At the treatment plant the wastewater or influent passed through a bar screen to catch any debris that isn’t considered wastewater such as clothing, wood, trash, etc. The remaining influent is dumped into a Primary Clarifier settling tank where solids settle to the bottom and water floats to the top. The solids then pass through a digester and remains collected and transported away to landfills. Grit is the material that biologically won’t break down into a liquid such as sand, gravel, jewelry, etc. Air is then added to the water and further solids are broken down or collected and hauled away what the liquid moves to a secondary clarification tank. Chemicals and coagulants are added to the process to further cleaning the remaining water with any remaining solids pumped back to the digester. Chlorine is added as it moves to tertiary treatment, the final stage in the process. Many treatment plants have cleaned the water to bring it back to a drinking water state. All of these processes cost money. The more flow, the higher the treatment costs. By adding in groundwater that is clean to begin with to wastewater that isn’t adds volume to the treatment process.

A few years ago, the Hyperion treatment plant in Los Angeles processed 400,000,000 gallons of wastewater per day during a dry cycle. When it rained that total jumped to 800,000,000 gallons per day. This means that all of that rain water that either came in through inflow or infiltration cost taxpayers to pay to clean it. Treating already clean water seams counter productive.

There are the answers to your questions. CIPP lining can help reduce the infiltration side of the equation and the reason many sewer agencies foster programs to encourage people to have their sewer problems fixed. If we can help you work with your local sewer agencies to develop a program for I/I reduction, call us at 888-354-6464 or write

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