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Matthew O'Connor and Chris Bentley/Medill

You might not put a lot of thought into what happens to your water once it goes down the drain. But it takes a long journey, even making stops along the way to produce energy.

Energy down the drain: Making wastewater work

by Matthew O'Connor and Chris Bentley
March 11, 2011

primary treatment wastewater 252px

Chris Bentley/MEDILL

A skimmer scrapes film off the top of a primary treatment tank at Milwaukee's Jones Island wastewater treatment plant.

graphic: energy in wastewater

Chris Bentley and Matthew O'Connor/MEDILL

Heidrich's study valued the energy in wastewater at 7.6 kilojoules per liter, which means Stickney's 1.2 billion gallons could supply 134,000 homes with electricity each day.

milwaukee jones island pipeline

Chris Bentley/MEDILL

A landfill gas pipeline dives underground in Greendale, Wisc., 11 miles southwest of its destination: Milwaukee's Jones Island water treatment plant. During dry weather, Jones Island will be able to meet its energy needs entirely from landfill gas once construction is complete. 

secondary treatment

Chris Bentley/MEDILL

Air blowers at Jones Island churn vats of treated waste, encouraging aerobic bacteria to break down remaining organic material.

The wastewater that flows through Chicago’s Stickney Water Reclamation Plant each day contains enough energy to power almost 4,500 homes for a month. That’s according to figures from a recent study that shows sewage is an undervalued resource. The study was published in the American Chemical Society’s journal Environmental Science & Technology.

Worldwide energy demand continues to rise, adding to environmental impacts associated with its production: poor air and groundwater quality, rising costs and climate change.

Renewable energy sources such as solar and wind power have grown, but the vast majority of our electricity still comes from fossil fuels.

Wastewater remains a relatively untapped natural resource, but it is gaining more attention lately.

“I think the industry needs to change,” said the study’s lead author Elizabeth Heidrich, of Newcastle University, in the U.K. “We are currently using a lot of energy to get rid of something with energy in it, rather than using it.”

Windy City waste-to-energy

In Chicago, the Metropolitan Water Reclamation District has been capturing energy from its sewage for decades, and plans to expand current processes.

Stickney Water Reclamation plant, the largest wastewater treatment facility in the world, can process up to 1.4 billion gallons of wastewater per day. It generates enough sludge during treatment that it needs a 90-car rail transit system to move material and byproducts.

“I think we’re the only wastewater treatment plant in the world with a railroad,” said Reed Dring, engineer of treatment plant operations.

Dug into the side of a small hill at one end of the rail line, 24 anaerobic digesters await some 2.2 million gallons of sludge each. A community of microbes breaks down the waste in the absence of oxygen, a process known as anaerobic digestion. This produces biogas — a mixture of methane, carbon dioxide and trace gases.

Stickney has been burning its biogas in a series of boilers for 30 years, recovering enough energy to heat the entire 560-acre facility in cold weather and cool it in summer with gas to spare.

Soon, Stickney will rip out its 1930s-era settling tanks and replace them with tanks of a more efficient, circular design, Dring said. MWRD engineers estimate the old style may leak up to 40 percent of the volatile solids responsible for biogas generation before the sludge ever reaches a digester.

“It’s going to take a while, but once that happens we’re going to have a lot more digester gas,” Dring said.

Stickney could clean the gas and sell it to a natural gas utility, but that process is expensive. Instead, Dring said the district is thinking about burning it in a high pressure boiler to create steam, which will turn a turbine to produce electricity.

After leaving the treatment facility, the water can still generate electricity at the Lockport Powerhouse.

The powerhouse, operated by the MWRD, controls 77 miles of waterways in the Chicago area according to operator Phil Nieman.

The dam takes advantage of a natural 38-foot drop, running up to 160 tons of water per second through a pair of identical turbines to produce about 50,000 megawatt-hours of electricity — enough to power 5,000 homes, Nieman said.

Water flows downstream, under the dam, and spins a turbine. This rotates electromagnets around a metal coil, generating electricity.

The MWRD then sells that power to Commonwealth Edison, netting around $1.3 million a year, Nieman said.

Making the most of wastewater

Across the country, more facilities are setting their sights on greater energy efficiency.

The King County Water Treatment District operates two large facilities in Washington state. The South Plant is one of only a handful of facilities that cleans biogas generated in anaerobic digestion and sells it back through the natural gas pipeline, said Jessie Israel, head of resource recovery. Some of that gas is used to power the South Plant. In 2009, 23 percent of the electricity the plant used was generated with scrubbed methane, Israel said.

At the West Point Plant, which serves Seattle and the surrounding area, a new cogeneration system is on the way. The process uses methane produced during anaerobic digestion to power an engine which produces electricity and recycles its waste heat to offset on-site demand.

The system, expected to be online in 2012, should generate about one-third of the plant’s energy needs, Israel said. The district is looking to boost that number by adding fats, oils and grease to digestion tanks. Mixing in food waste and other kinds of organic matter can have a multiplying effect on the amount of gas produced — and on the overall energy savings.

“Right now people do pay to dispose of those things safely,” Israel said. “If they can pay to dispose of them in a way that helps us generate revenue, generate electricity and keep our rates low then it’s kind of a double win.”

In Gresham, Ore. a cogeneration system has been online at the city’s wastewater treatment plant since the summer of 2005. Veolia Water North America oversees the project that produces more than half of the plant’s energy needs.

Gresham hopes to make the plant energy independent by 2014 and is also looking to add grease to the equation, said plant manager Paul Proctor.

Construction on a receiving station is scheduled for completion by the end of the year. Area restaurants would truck grease to the station where it would be pre-treated and sent to the wastewater plant to be mixed into the digesters.

A landfill in Muskego, Wisc. will soon help treat the wastewater of 1.1 million people in The Milwaukee Metropolitan Sewage District. Veolia environmental services is under contract with the sewage district to clean and deliver gas through a 19-mile pipeline to Milwaukee’s Jones Island treatment plant.

“Right now they’re flaring off all that gas out at the landfill,” said Bill Graffin, spokesman for the Milwaukee Metropolitan Sewage District. “We said, ‘We’ve got all this landfill gas, so why don’t we use it for energy?’”

During dry weather, the landfill gas — which is essentially lower grade biogas — will power Jones Island’s entire operation. Wet weather means more wastewater, which bumps up the facility’s energy requirement.

“A lot of people don’t realize how much energy it takes to clean water,” Graffin said. “Anything we can do to be more sustainable and help our customers out is a good idea.” The district estimates it can save ratepayers tens of millions of dollars over the 20-year contract.

Wastewater treatment plants use about 1.3 percent of all U.S. energy. As energy demand continues to grow, getting these facilities off the grid could be a step in the right direction, Heidrich said.

But even if we could capture all the energy going down the drain, we would still be a long way from satisfying demand.

“I think where we’re going to be successful as a country reducing our carbon footprint is in doing lots of little things,” Israel said. “Having the ability to have a diverse suite of renewable energy or energy efficient options is really great and helping us move the needle faster.”