University of Wisconsin–Madison

Student Intern Column: Where Does Our Water Go?

The Office of Sustainability Interns took a trip to the Madison Metropolitan Sewerage District (MMSD) earlier in the summer.
The Office of Sustainability Interns took a trip to the Madison Metropolitan Sewerage District (MMSD) earlier in the summer.

Hannah Kasun is a junior studying Music Performance and Environmental Studies. She began her internship for the Office of Sustainability this summer as part of the Waste and Recycling Team, which provides education, tools, and resources to help campus groups improve their recycling and composting efforts. She enjoys working with other positive, passionate people interested in sustainability and is fascinated in gaining a broader understanding of sustainability at the level of institutions.

Water is a human necessity, and here in Madison most of us are lucky enough to have access to clean water with the twist of a faucet. But where does that water go after the brief time it has spent running through our hands, the washing machine, or a toilet? Dirty water can’t be introduced back into the hydrologic cycle without causing environmental damage, so humans have developed our own “cycle” so we can safely return the water we use back to nature. The Office of Sustainability Interns recently took a trip to the Madison Metropolitan Sewerage District (MMSD) to gain a greater understanding of the crucial and cutting-edge work being done with our wastewater.

The first stop on our tour was the district’s LEED Platinum certified Maintenance Building.
The first stop on our tour was the district’s LEED Platinum certified Maintenance Building.

The first stop on our tour was the district’s LEED Platinum-certified Maintenance Building. Part of the building’s impressive certification derives from its use of recycled and local materials. For instance, the structural steel used in the building is 90 percent recycled, and the front reception desk is repurposed wood from an 1800s Wisconsin barn. On the day of our visit, the building was well-lit thanks to “solar tubes” that concentrate sunlight from the roof and “beam” it into the building’s interior. On cloudier days when the sun is not so helpful, energy-efficient LED lighting picks up the slack. Other environmental design features include solar panels that provide 10 to 15 percent of the building’s energy needs, and a coat of white paint on the roof to help keep things cooler in the summer. We found one of the most unique design features in the bathrooms: to reduce the water demand of the building, the toilets are flushed with freshly treated water from the plant! The sustainability of the Maintenance Building immediately demonstrated the commitment which MMSD has to the environment, and was a great first impression on our tour.

An inside look at the treatment process

Prior to this trip, most of us interns didn’t realize the magnitude of Madison’s wastewater treatment operation. MMSD treats an average of 42 million gallons of wastewater per day, which is collected from homes and businesses in twenty-six cities and villages near Madison. The Sewerage District is designed to exclusively treat wastewater, since adding stormwater to their process would require a much higher capacity and would likely lead to overflow during heavy storms. Even so, the quantity of water treated by the district each day can increase dramatically during storms (more than doubling!) due to the inflow and infiltration of rainwater into the system.

The treatment process begins when the incoming water, called influent, passes through a screen with quarter-inch holes. This process filters out large items such as feminine hygiene products and flushable wipes. Next, smaller inorganic solids like sand and gravel are settled out using a vortex action in the Grit Chamber Tanks.

The wastewater continues on to the Primary Settling Tanks. These outdoor tanks are rectangular, up to 100 feet long, and about 9 feet deep. Each tank contains a slow-moving paddle that, along with an elevation change, helps move the water to the other side of the tank. The paddle also keeps in motion the organic solids that settle on the bottom of the tank, so they can be directed to a separate treatment process. On our tour, we couldn’t help but notice the fats and grease collecting on top of the water, as well as an unpleasant smell — definitely not the place to fall in!

Aeration Tank MMSD
Aeration tank at Madison Metropolitan Sewerage District.

In the next step, water moves into long, zig-zagging Aeration Tanks for Secondary Treatment. Here large quantities of air are blown into the water, encouraging microorganisms to consume the organic material and nutrients that contaminate the incoming water. This part of the process uses most of the energy — about 40 percent — since a lot of power is needed to keep the blowers running constantly. Fun fact: the water in this part of the process is known as “mixed liquor”!

Secondary Treatment concludes in the Final Clarifying Tanks. Although this step was only 50 or so feet from the previous one, it smelled dramatically different! There was even a duck swimming around in the circular tank as if to prove that the water there was almost ready to be returned to nature. In this step, the previously mentioned microorganisms are settled out, and most are returned to the aeration tanks to repeat their function.

The treated water, called effluent, proceeds through ultraviolet light chambers to kill disease-causing bacteria. After this, it is finally safe to return to the natural environment! Most of the treated water (about 90 percent) is discharged into Badfish Creek; the remaining amount is discharged to Badger Mill Creek. The effluent is not potable (meaning it isn’t safe to drink), but that is the intended result of the treatment process: since the water is discharged to natural streams, the goal is to get it as close as possible to natural stream water before discharging it.

In addition to their remarkable water treatment process, MMSD has several more forward-thinking programs. One of these involves the solid waste, also known as “sludge” which is removed from the primary and secondary treatment tanks. MMSD processes the sludge using nine digesters onsite; the eventual product is a liquid fertilizer called Metrogro, which MMSD provides free of charge to farmers. In addition, the gas produced by the digester process is captured to provide more than a third of the plant’s energy.

MMSD treats an average of 42 million gallons of wastewater per day
MMSD treats an average of 42 million gallons of wastewater per day.

Another cutting-edge program of the plant is its work on phosphorus harvesting. These locally-produced, phosphorus-rich pellets created in this process are used as fertilizer, further stretching the world’s dwindling supply of high-quality phosphorus, which would otherwise come from a mine. The district is also part of a groundbreaking initiative called Yahara WINS, which aims to reduce phosphorus in the Yahara watershed. This long-term reduction effort is intended to make the watershed a healthier place, and decrease harmful algae blooms like the ones which have been so prevalent this summer.

How to help out and Learn More

While on our tour, we also learned a few important things that we can do to make the treatment of wastewater easier for places like MMSD, and more cost-effective for all rate payers.

The plant doesn’t have the capacity to remove salt from the water they treat (a process known as desalination) because it requires extremely expensive technology. As a result, it’s crucial that we reduce salt contamination of water by using less road salt in the winter and replacing old home water softeners, which can use more salt than is needed.

It’s also important to note that no wipes are “flushable,” even if it claims to be on the package. Flushing any sort of trash besides toilet paper will eventually create buildups or blockages in pipes, which workers have to remove manually.

Also, medications or hazardous waste do not belong down the drain or in the toilet. Items containing mercury are especially of concern at MMSD. Instead, check out the Dane County Clean Sweep program for items you’re not sure how to dispose of.

If you’re interested in learning more about the process that treats Madison’s wastewater, check out the MMSD website, or take a tour! If you’re interested in wildlife, also consider planning a visit to the bird observation area maintained by the district—it features an amazing variety of birds.

It can be easy to take waste management systems for granted when they’re going well, and such is the case with the Madison Metropolitan Sewerage District. Next time you turn on the tap or enjoy clean waterways, take a minute to appreciate the work that places like MMSD do to make that enjoyment possible—and make sure you’re doing your part to help.