HONOLULU – Sludge. The final, unwanted byproduct of a toilet flush. The semi-solid stuff that even wastewater treatment plants send packing to the landfill. Unseen in the steel pipes snaking high on the exterior of Honolulu’s H-POWER plant, sludge is injected into a massive boiler where it joins the city’s trash in a roaring inferno. From the gravel lot outside, it all seems very antiseptic and smells less than a stroll past the neighborhood dumpster. But the 70-megawatt waste-to-energy facility is a workhorse, processing as much as 2,000 tons of refuse each day from Oahu’s 1 million residents. All told, it generates up to 10 percent of the electricity needed to power this Pacific island.

Turning garbage into energy is nothing new for H-POWER – the plant has been doing so for more than two decades. The sludge tanks, though, were added last year during a $10 million upgrade the city commissioned to expand capacity at the facility. They allow the plant to dispose of 20,000 tons of sludge each year, as well as an extra 20,000 tons of bulky solid waste, like couches and mattresses, that was previously required to stabilize sludge in the city landfill. On an island one-third the size of Delaware and thousands of miles from the nearest continent, space is at a premium. So city managers made the upgrade for the same reason they built H-POWER in the first place: to cut the volume of waste headed to the dump.

“The mayor really wants to try and eliminate the need for an everyday landfill,” said Lori Kahikina, director of Honolulu’s Department of Environmental Services. “The next biggest thing besides our trash that was going to the landfill was our sludge.”

The H-POWER sludge project has also positioned Honolulu at the vanguard of a national movement to reevaluate waste – in particular, the waste we flush down toilets. What recycling did for solid waste management, sludge-to-energy technology now promises to do for wastewater management. Namely, it envisions sludge as a resource waiting to be reborn as electricity, fuel, fertilizer, and other useful products.

With more people living in urban areas than ever before, such efficiency is a growing priority around the world. Global Choke Point, a collaboration between Circle of Blue and the Wilson Center, documents how increasing demands for water, energy, and food, as well as changing climate patterns, are placing pressure on cities’ social and physical infrastructure. These choke points are especially acute in remote Honolulu. Here, as in China, the waste revolution is gaining steam – or rather, gas.

“There is a paradigm shift going on,” said Barry Liner, director of the Water Science and Engineering Center at the non-profit Water Environment Federation. “Wastewater treatment facilities are not waste disposal facilities, but rather resource recovery facilities that can recover nutrients, generate energy, and provide clean water.”

The Power and Promise of Sludge

The energy locked up in wastewater can be tapped in a staggering number of ways. In Paris, for example, the residual heat from sewer lines is being used to warm up pools and apartment buildings. In Sydney, the flow of wastewater through sewers and into treatment plants produces hydropower. Honolulu took another approach – burning the sludge alongside municipal trash. While the high water content of sludge makes it an inefficient fuel source, H-POWER offsets the inconvenience with extra energy produced by the bulky solid waste it now receives. An additional benefit, operators say, is the ability to use sludge injections to moderate temperature in the boiler, a crucial step for limiting harmful air emissions.

If the largest of America’s municipal wastewater treatment plants captured and burned their biogas through anaerobic digestion, a process by which microbes break down sludge in tanks without oxygen releasing a mixture of methane and carbon dioxide, they could generate 400 megawatts of electricity every year, according to the U.S. Environmental Protection Agency. In other words, only 20 percent of wastewater plants in the U.S. could generate 11 percent of the entire industry’s electricity demand each year, just from biogas. It would take 1.8 million tons of coal to produce an equivalent amount of electricity.

Converting sludge into energy also has the potential to more than offset the electricity required for wastewater treatment itself, which currently accounts for about one percent of all the electricity used in the United States. As treatment standards become more stringent to further prevent harmful nutrients and chemicals from reaching rivers and lakes, those electricity needs could grow.

Energy savings can be a strong incentive for cities to adopt sludge-to-energy technologies, both because they reduce costs and cut greenhouse gas emissions. Hawaii, for example, has some of the highest electricity costs in the nation. It also relies heavily on fossil fuels, though the state has set a goal to reach 100 percent renewable electricity by 2045. The H-POWER plant generates enough electricity to supply its own needs as well as about $67 million by selling excess to the grid. According to Kahikina, the electricity created from Honolulu’s sludge alone currently saves the city the energy equivalent of about 20,000 barrels of oil each year.

“We’re on an island, so we have to bring in fuel, we have to ship everything in and out – our trash, our recyclables, we have to ship everything,” she said. “It just makes economic sense for us because we are so secluded.”

Water savings are also possible. H-POWER uses 5.7 million gallons of recycled wastewater each month to create steam from its boilers, which is then recycled again in a closed-loop system. The plant also pumps about 50 million gallons per month from a salty, caprock aquifer to cool the boiler system. At those rates, H-POWER would have accounted for less than one percent of the water used for thermo-electric power generation on Oahu in 2010 – the latest year data is available – even though it generated more than four percent of the island’s electricity that year.

However, sludge-to-energy plants don’t make economic sense for every city, especially those with smaller wastewater treatment plants. As a general rule of thumb, it takes about 5 million gallons of wastewater a day for anaerobic digestion to be cost effective. Regulatory hurdles, difficulty transferring energy back to the grid, competition with fossil fuels, and the lack of a properly trained workforce can all deter cities from pursuing sludge-to-energy systems. Nonetheless, many small and mid-sized cities are willing to take the leap if they can get economic help, according to Lisa McFadden, director of Integrated Technical Programs at the Water Environment Federation.

“The biggest barrier is economic,” she said. “The others all just fall into place and can be overcome. But the economics, that’s the most significant. When you see incentives and funding coming forward, you will see quite a bit of change and a lot of these more innovative practices.”

For those cities that do decide to go all in for sludge, examining how a sludge-to-energy system fits into the big picture is key. That can mean troubleshooting potential technical challenges, like Honolulu had to do when it found that debris from the city’s wastewater plants was somehow getting through the screening process and clogging injectors at the H-POWER plant. It can also mean thinking ahead to prepare for issues such as nutrient recovery or water recycling operations.

“Integrated planning for all of this is critical,” said Liner of the Water Environment Federation. “You don’t operate in a vacuum; everything is tied together.”

The Leftovers

The process is efficient, but not completely so. Even if every scrap of energy is digested, burned, or otherwise stripped from sewage sludge, waste managers still have to deal with some material remains. Anaerobic digestion and incineration both reduce sewage sludge to about 10 percent of its initial volume, but the byproducts differ greatly.

The sludge and trash mixture burned at H-POWER leaves behind a residual ash that is trucked to Honolulu’s landfill. The ash from waste-to-energy plants could also potentially be used in concrete or other building materials, according to researchers at the University of Central Florida. This application is relatively unexplored in the United States, but has been widely utilized in Europe.

After the microbes in anaerobic digesters have had their fill, if the remains meet certain treatment criteria to reduce the concentration of metals and pathogens, they are classified as “biosolids” and can be used as fertilizer for agricultural or forest lands, or to reclaim mine sites. About half of the United States’ sewage sludge is repurposed this way, regardless of whether it was used to produce energy first.

A 2002 report by the National Research Council concluded that using biosolids as fertilizer is safe for human health, as long as regulations are followed (a rule that is not always followed). Nonetheless certain chemicals, pesticides, and other inorganic compounds persist in sewage sludge and may pose risks not addressed by current regulations that focus exclusively on toxic metals and biological hazards.

“Creating a circular economy is mandatory for sustainability, and biosolids are no exception to this rule,” says Rolf Halden, director of the Biodesign Center for Environmental Security at Arizona State University. “We need to reuse materials, energy, and nutrients in a circular fashion – and cannot continue feeding a linear pipeline that converts resources into waste. But if we subscribe to this more sustainable model, we also need to learn how to keep persistent toxics out of wastewater. Otherwise, these pollutants will come back to contaminate our drinking water, soil, and food.”

The National Sewage Sludge Repository at Arizona State University holds sludge samples from more than 160 cities across the United States, and researchers have been busy cataloguing the chemicals they contain and assessing how those chemicals break down in the environment. Depending on the “sewer-shed,” the area served by one wastewater treatment facility, the sludge can contain a wide variety of toxic metals and chemicals as well as precious metals and essential nutrients. And depending on where the treated biosolids are spread – whether in a cold environment or a warm one, wet or dry—hazardous chemicals can degrade at different rates, stay in one place, or leach into groundwater.

That’s why proper wastewater and biosolids treatment is critical, said Halden, noting that wastewater management is an underappreciated service.

“[Wastewater plants] can’t do magic,” Halden said. “They can’t undo mistakes that are made upstream in the process. One important lesson is that we cannot use wastewater as a disposal for everything we don’t want to see anymore.”

Not exactly magic, but at least Honolulu’s innovative waste-to-energy plant takes us one step closer.