More than 250 million gallons of raw wastewater rushes down sewers every day across the Twin Cities. There, it flows with all the liquid waste and by-products from breweries, dairies and other industries into one of a number of treatment plants. Then it literally hits the fan.
Giant fans, more precisely, that require enormous amounts of energy to continuously pump oxygen into the wastewater. For decades, these fans and this aeration process has made wastewater plants the biggest energy consumer in local government.
The city of St. Cloud and researchers at the University of Minnesota believe they're close to finding far more efficient ways to treat waste. One of the answers, they say, comes from the human stomach.
Researchers are working to divert the thickest, most concentrated industrial sewage into treatment that doesn't require any oxygen at all. That would drastically cut the amount of energy consumed by sewage plants, as well as reduce the amount of carbon dioxide they produce.
A change is needed because running those fans in the aeration process accounts for about half of all the power used at a wastewater plant, said Paige Novak, a researcher with the U.
The oxygen keeps helpful bacteria alive so they can break down the solids and leave behind cleaner water, which is then further purified until it is safe enough to discharge. This aeration process is used as the first step in treating sewage at virtually every plant in the country, but there are two major problems with it, Novak said.
The first is cost. Every month, taxpayers spend more than $1 million to pay the energy bill to run the nine wastewater treatment plants in the Minneapolis and St. Paul metro alone. The city of St. Cloud's treatment center uses enough energy to power 520 homes for a year. Nationwide, wastewater treatment plants account for about 2% of all the electricity used in the country at a cost of nearly $3 billion, according to a study from the Water Research Foundation.
The second problem, Novak said, is that these waste-eating bacteria exhale carbon dioxide. Minnesota's roughly 600 wastewater plants produce the same amount of greenhouse gas emissions as about 72,000 cars, according to data from the Minnesota Pollution Control Agency.
"When you look at the big picture of what we're doing, it's kind of ridiculous," Novak said. "We're taking this stuff, this wastewater, that could be an energy source and we're using a bunch of energy to treat it and turn it into carbon emissions."
There are other kinds of bacteria that eat sewage, however, and don't need added outside air at all. Many of these bacteria live in our own digestive tracts or in deep swamps and can provide the same treatment to wastewater anaerobically, getting all the oxygen they need through the materials they eat.
The U has been testing processes that use these bacteria to treat some of the highest-concentrated wastewater produced in the state — from the by-products of milk and beer manufacturing.
In order to work, the bacteria need to be left with high-strength sewage in tightly controlled containers, called digesters, that are sealed off from outside air, Novak said. The sewage must remain relatively undiluted.
The goal, she said, is to treat industrial waste, which accounts for up to 85% of wastewater running through the sewers of the Twin Cities, through this oxygen-free process at the industrial sites before it is ever pumped into a sewer.
"When you can take these high-strength streams out of the system, it all becomes much more effective and efficient," Novak said.
The U has been testing this process with Fulton Brewing Co. to treat the spent yeast and other by-products on site. The early results are promising, researchers said. Rather than releasing carbon dioxide, the bacteria that work without aeration ferment the waste until they produce methane. That methane is captured and used to power the on-site treatment process. Meanwhile, the water is being cleaned to state standards, the brewery is saving money on sewage discharge fees and the residue left behind by the bacteria is turning into a useful fertilizer, the U has found.
It's unclear how much it might cost or if it's even possible to scale up this kind of industrial treatment to serve the entire state or a population as large as the Twin Cities.
The Metropolitan Council, which runs the nine treatment plants in the area, has started to test the waters and is offering a handful of industries financial incentives to pre-treat their waste to prevent it from entering the sewer system.
The city of St. Cloud, which has drastically cut its energy bill over the past four years, has gone further than any other in applying larger-scale anaerobic treatments. The city started using both anaerobic and traditional aeration processes in 2017. It allows dairies, microbreweries and other food processors to avoid big sewage or disposal charges by letting them haul in their waste and pump it directly into the city's digester tanks.
Those tanks are designed to act as closely to human stomachs as possible, said Tracy Hodel, St. Cloud public services director.
Their temperatures are even kept at 98.6 degrees, just how the bacteria like it. The only raw waste allowed inside comes from food and beverage industries, to make sure the material going in doesn't have metals or other highly toxic pollutants that might disrupt the system, Hodel said. The city also pumps in some of the waste that has already been aerated.
It's hard to say exactly how much energy the anaerobic treatment has saved the city since 2017, Hodel said. The city started testing the treatments at the same time as it installed an extensive solar panel array that can produce more than 5 million kilowatt-hours of energy.
Between the solar panels and the methane, the wastewater plant, which serves more than 120,000 people, now typically produces nearly 90% of the energy it needs every day. The city took out a multimillion-dollar bond to pay for the renewable energy upgrades, and the plant owes about $390,000 a year on that bond, Hodel said.
The energy savings alone, which Hodel estimates at roughly $600,000, are more than enough to cover that bond, she said.
"It's a big point that I don't think many people realize or ever think about, but cities really have two services — supplying drinking water and cleaning wastewater — that alone are 40% of the city's energy bill," Hodel said. "Cutting that bill is how we've been able to stabilize rates for customers."