The Urban Water Cycle Needs an Energy Overhaul

It starts two and half miles offshore (1,2). Five cylindrical stone structures called “cribs” gulp water from the vast expanse of Lake Michigan (1,2,3). Swaddling an intake shaft, each one pulses water through a tunnel carved from the bedrock, bound for shore. Making landfall, the water whooshes through filters, leaving behind rogue fish that joined in the ride. Pumps lift the water three stories high before they begin a descent through thick mats of gravel and sand, stripping the water of the dust and microbes that sicken us (1,2). 

Pumping stations hum all day, thrumming 1 billion gallons of water each day through 4000 miles of underground pipes, where it finally reaches homes and businesses throughout the city of Chicago and the surrounding 120 suburbs (1,4). But these places are only pit stops along water’s long journey.

Wastewater, steeped with the dregs of city living, enters a maze of sewer drains, bound for the wastewater treatment center (5,6). Screens catch large debris, while an injection of air gurgles out dissolved gases and metals. In a settling tank, whatever bits remain sink to the bottom. The stripped water flows into the nearest riverway, completing its journey at the place where it started: nature.

In nature, everything connects, including waste. The waste products of one process become the starting blocks of the next, interweaving a lattice of circles (7). The urban water cycle mirrors nature’s lattice, gulping water in and returning it transformed. And like all natural cycles, this cycle demands energy: water is heavy, dirty, and not often where people are (8,9). In fact, in the US, the urban water cycle accounts for 1-2% of electricity demand annually (9), enough to power the entire world for a full day (10,11). But unlike the circle of urban water, energy is on a linear track: we extract it from nature, but once we use it, it can’t be returned to where it came from (12, 13).

We predominately make energy by extracting it from the bowels of the earth and burning it. We use it to transform lake water into clean water; wastewater into river water. But instead of returning energy to nature, we dispose of it by scattering the remains in the atmosphere. The carbon detritus of this take-and-dispose method has provoked a climate crisis that’s both increasing demand for clean water and dwindling water supplies (14), especially in the Great Lakes region (15).

We can tap new water supplies by pumping water from deep groundwater or by stripping salt from ocean water (16). But, these practices gobble energy and will double our energy-for-water needs over the next 25 years (16, 17). Continuing to spill carbon into the atmosphere to access clean water will only exacerbate the growing water problem.

Carbon-free nuclear energy emerged as a once-promising alternative in Illinois, and now accounts for over 50% of our state’s electricity (18). Unfortunately, spent nuclear rods create radioactive waste that must be managed. Burying the waste in containment tanks is often the method of choice, despite the potential for leaks. When leaks do happen, they destroy the surrounding environment (19, 20).

To produce an urban water cycle that can meet demand despite a mounting climate crisis, we need to rethink the way we make water.

We can start by looking at the energy contained within our wastewater. By simply living and using water, we infuse it with an energy-packed raw material that we can harness. Food waste, human waste, dirt, leaves, and whatever else finds its way down a sewer drain combine to produce sludge, a tar-like mixture whose rotten egg odor sticks in your throat like a fog (21). Other creatures salivate at the sight of this sludge.

Microscopic critters called anaerobic bacteria chew through sludge. Gorging themselves, they give off an energy-dense gas called biogas that we can capture and burn to generate electricity (21, 22). With enough sludge, we can stitch a new circle, infusing sludge-derived energy to power the urban water cycle (22). 

In Aarhus, Denmark, the energy circle is almost complete. In 2016, the city installed a sewer-sludge-turned biogas system that produces enough electricity to power 94% of the urban water cycle (23). Similar systems have popped up throughout Europe, with Germany serving as the world’s largest biogas producer (22).

In the US, biogas is gaining traction. In San Francisco, a wastewater treatment facility became the first in North America to produce more energy than it needed (24). Sliding over to the opposite coast, New York City produces enough biogas to power 5,000 homes (25). Returning back to the Midwest – and in our own backyard – Downers Grove, Illinois, uses a combination of sewer sludge and restaurant grease to cover 85% of the wastewater plant’s energy needs (26).

While these systems bend the flow of energy into an arc, closing the circle is not easy. The Industrial Revolution built the modern world on a foundation of linearity (12), so implementing innovative technology that bucks assembly-line structure often requires political and financial support (27). In Denmark, a strict set of water pollution regulations and $3.5 million in up-front costs spurred their sludge-eating system (23). The country now has one of the most sustainable water sectors in the world (28). Policy and money ultimately wove a renewed link to the lattice, revealing that everything – as in nature – is connected.

References

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