Inflow & Infiltration

One rainy day last month, a wastewater treatment plant designed for 9 million gallons per day of flow received 36 million gallons of flow in a 24-hour period. Did I mention it only rained about an inch that day? When this occurs, you know you have a problem. But the question remains, what can you do about it?

Inflow and infiltration (I&I) is one of the most pressing problems that face wastewater treatment and collection facilities every day. Average sewer pipes are designed to last about 20 to 50 years, depending on what type of material was used to make them.

Inflow is an illegal tap or connection into the sanitary sewer system. These can be footing/foundation drains, roof drains or leaders, downspouts, drains from window wells, outdoor basement stairwells, drains from driveways, groundwater/basement sump pumps and many others.

Infiltration is the entrance of ground water into the sanitary system. This occurs through leaks, cracked pipes, broken joints, leaks at stream crossings, leaking manholes and lift stations (above and below ground), poor design, installation or maintenance errors, damage or root infiltration. Groundwater can enter these cracks or leaks wherever sanitary sewer systems lie beneath water tables or when the soil above the sewer systems becomes saturated.

When trying to assess flow issues, the extent of both these problems needs to be determined. You know how much is coming into your system, but you don’t know from where. How can you find out where the problems are in your system?

The answer to this is to assess your system. Now you may be thinking, “Where do I even start?”

Visual inspection of the observable areas of your collection system is the easiest way to start. Look at manholes. Is there staining on the walls, which would indicate a leak? On rainy days, is there standing water over a manhole? Where does the water go? Is there water leaking into the manhole at the pipe joint or at the collar joints?

You may also have specific concerns if your system has a manhole near a creek or stream. What happens to the water level in the stream during rain events? Does it cover the manhole? Do you have overflowing manholes? Make sure to look upstream and see where all the water is coming from. Keep moving upstream from mains to branch sewers. Try to determine where the largest amount of water is coming from. Yep, this means getting wet.

This method may be useful in solving your I&I issues. In fact, the estimated 8 million manholes in the United States may contribute up to 50 percent of the measured I&I problems for collection systems nationwide. There is an estimated $8.5 billion in repair or replacement that needs to be spent to help combat this problem. The good thing is that manhole repair or refurbishment is normally the first thing done, and you can realize a major reduction in inflow and infiltration after completing the work.

As you complete the visual inspection of your manholes, you should document the information on a form that gives the number, location, depth, diameter, influent and effluent pipe sizes, condition of the frame, rings, a visual assessment of the material the manhole is made of (concrete, brick lined, wood), approximate age and general condition. This documentation can be written on paper or accomplished electronically.

After you have taken pictures and documented the information you collected (Remember, if you didn’t write it down, you didn’t do it.) and correlated all the information on a map, you can begin to assess the manholes in your system. This will allow you to determine where to make improvements to get the biggest bang for your buck.

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