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Sewer School: An Intro to Our Pipes

Bill Graffin, Public Information Manager, Milwaukee Metropolitan Sewerage District

In one way, water is like electricity. It takes the path of least resistance. Underground, that journey often involves cracks in sewer pipes that open the door to enormous amounts of water leaking into the sanitary sewer system where it doesn’t belong.

Excess water in sewers is the main reason why we still have sewer overflows and basement backups. We’re talking about massive volumes of water that are not supposed to be in the pipes at all. So, where does it come from?

Reducing the risk of basement backups is our highest priority at the Milwaukee Metropolitan Sewerage District (MMSD). We’ve invested billions of dollars on our regional sewer system. However, many people do not realize that the regional system is only one third of the solution.

A Three Tiered System
When you use water in your home, it flows to a pipe called a lateral that runs from under your basement, out to the street, where it connects to your community’s sewer system. The community sewer pipes connect to MMSD’s regional pipes deeper in the ground. It’s the regional pipes that deliver all wastewater for 1.1 million customers and industry to one of two water reclamation facilities for the region.

MMSD serves 28 communities that all own and operate their own sewers. Additionally, homes and businesses account for thousands of miles of private sewer pipes in the overall system.

Compare numbers ownership by miles:
MMSD sewers - 300
Community owned sewers - 3,000
Private laterals - 3,000

These numbers do not account for any storm sewers that collect rain and melting snow from streets and parking lots. The vast majority of storm sewers are operated by municipalities. MMSD does not own or operate any storm sewers.

Combined vs. Separate
We hear a lot about combined sewers in the Milwaukee area. A combined sewer is one pipe that collects sanitary flows from homes and businesses and stormwater from streets. The only combined sewers in this region are owned by the City of Milwaukee and the Village of Shorewood. They make up five percent of MMSD’s total service area. You can find combined sewers in about one third of Milwaukee and roughly half of Shorewood.

Separate sewer systems have one pipe that transports sanitary flows from homes and businesses. A second pipe, storm sewer, collects stormwater from streets and delivers it to the nearest creek, stream or river untreated every time it rains or the snow melts.

Deep Tunnel System
Water reclamation facilities can only clean a certain amount of wastewater each day. For MMSD, each facility can process about 300 million gallons per day. When heavy rain hits and the reclamation facilities are operating at maximum capacity, more water continues to pour into sewers. MMSD’s Deep Tunnel System allows the region to store the excess water underground until reclamation facilities have time to clean all the additional flows.

The Deep Tunnel is 28.5 miles long and holds 521 million gallons of water. It ranges in size from 17 to 32 feet in diameter. Most of the tunnel sits 300 feet underground in bedrock. One section of the tunnel is about 175 feet underground in bedrock.

Before the first Deep Tunnel went into operation in 1994, the region used to have 50 to 60 overflows to waterways every year with an average annual volume of 8 billion to 9 billion gallons. With the Deep Tunnel in operation, we now average 2.5 combined sewer overflows each year with an average volume of 1.4 billion gallons over the past 17 years.

Where We’re Going
Since the 1980s, MMSD has invested $4 billion in the regional sewer system and water reclamation facilities. Municipalities have poured millions of dollars into improving their sanitary sewer systems. However, very little work has been done on private property and the laterals of homes and businesses, a significant portion of the three tiered sewer system.

A single downspout connected to the sanitary sewer system can deliver up to 12 gallons a minute of excess water during heavy rain. Homes without sump pumps typically have foundation drains that are connected to the sanitary sewer system. Those can deliver up to 10 gallons a minute of excess water. Cracked and failing laterals can allow many more gallons into the sanitary sewers when the ground becomes saturated from strong storms. All of this rainwater can add up fast.

We have a tremendous amount of work ahead to reduce the risk of basement backups in our region. In the next RiversReport, we’ll talk about ways to improve and a $150 million plan to address work on private property. 

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