Flush, p.23
Flush, page 23
Converting our unwanted matter into greener sources of energy and fuel that can reduce and even reverse the instability will require considerable buy-in. But modern poop-to-power strategies have built upon centuries of historical precedents and are picking up speed. By 2015, China alone had tens of millions of home-size digesters producing biogas and more than 110,00 larger systems. Germany, the European leader and world’s second-biggest installer of biogas plants, had set up nearly 11,000 by 2017. Wastewater treatment plants, which seem to have become regular tourist destinations for me (hey, some people love roller coasters and others gravitate toward sludge pools), offer particularly good examples of this transformation. At these plants, a disposal process once narrowly viewed as a way to minimize our damage as consumers is being reimagined as a means to expand our usefulness as producers. Some advocates, in fact, have begun calling them “resource recovery” facilities. We are the source of these resources: gases that can become heat or electricity or biomethane, solids that can become heating briquettes or compost or fertilizer, and liquids that can become biofuels or clean water.
In the patchwork of temperate forests, farms, and bedroom communities of western Oregon, Clean Water Services is responsible for safeguarding the eighty-three-mile-long Tualatin River. The name translates as “lazy river” in the language of the Indigenous Atfalati tribe of the Kalapuya, who lived here and who carved petroglyphs in the sandstone farther upstream. The river flows from a spring in the Coast Range east into suburban Portland, provides drinking water for about 200,000 Oregonians near its headwaters, and receives the effluent from Washington County’s four wastewater treatment plants before it empties into the larger Willamette River. Combined, the plants discharge about sixty-six million gallons of treated water every day, on average. Most goes back into the Tualatin; along with water from an upstream dam, it makes up about two-thirds of the river’s leisurely flow, which is critical during drier summer months. The replaced water keeps the river moving. It is life-giving.
I wanted to see it for myself, so I walked through the Tualatin River National Wildlife Refuge, one of the few located in a major metropolitan area. On a pleasant late summer day that could have been mistaken for any other except for the wind and distant smoke, only a handful of other visitors had ventured out and the refuge seemed unusually quiet; even the birds seemed to be holding their breath. I walked about a mile through pockets of woods and a sunny open meadow to an observation deck overlooking a very dry-looking expanse of restored wetlands. In the winter and spring, they are part of nature’s water-cleansing system, which can screen and pool and filter out and break down many of the contaminants before they reach the river. But after a parched summer, the expanse had reverted to a grassland. I lingered at other spots overlooking the slow-moving Tualatin. One sign read, “The River Is the Lifeblood of the Refuge.”
Most newer wastewater treatment plants have taken their cue from wetlands, rivers, and other parts of nature for some of their water-cleansing steps. After large screens that mimic the effects of cattails, rushes, and reeds remove bigger pieces of debris and gravity helps remove grit and gravel, settling tanks act like natural pools and let some of the heavier organic matter sink to the bottom. Filters commonly incorporate fine and coarse sand as effective ways to reduce algae, chemicals, and harmful microbes. Bubblers add oxygen to the water when needed, re-creating the turbulence and aeration that occurs when water flows over rocks and boulders. And at the center of advanced plants, beneficial microbes are the true stars: chomping away at the organic waste and nutrients to break them down. Some tanks are pumped full of oxygen to fuel the growth of aerobic bacteria; others are depleted of oxygen to favor the growth of anaerobes that cannot survive in its presence.
The Durham Water Resource Recovery Facility in Tigard is about five miles from the refuge, and a biogas generation system there yields a mixture of 60 percent methane and 40 percent carbon dioxide. To increase its heat and power generation, the plant has made particularly good use of the FOG waste that is normally landfilled. Instead, the plant solicits the waste from restaurant grease traps and pipes it right into two digesters to help feed the microbes. “Put it directly into the digester, and it can double or triple the amount of gas that you’re producing,” principal engineer Peter Schauer told me. This co-digestion process, as it’s known, is like supplementing the microbes’ regular meal with calorie-dense candy bars. “Or crack,” quipped senior engineer Pat Orr.
As I walked with a small contingent from the plant toward the receiving building that accepts the FOG deliveries, Orr said it would be more of an olfactory experience.
“Oh really?” I asked.
“Well, you’ll see,” Orr said, grinning. “Or smell.”
Inside, heating and grinding and recirculating elements kept a large tank of the stuff warm and well mixed so the liquid wouldn’t solidify and block the feed lines taking it to the digesters. A screen helped catch errant silverware from the restaurants’ grease traps. It smelled, well, like rancid grease. Not overpowering, but the kind of odor that tends to cling to your clothes. One of the biggest initial challenges, Orr said, was maintaining a happy medium between overly “hot” loads of incoming FOG and a scarcity on weekends. The microbes would get acclimated to the fattier food, he said, “and then you kind of take away all their sweets.” The plant began giving incentives to drivers to even out their deliveries, though those deliveries plummeted when many restaurants closed during the COVID-19 pandemic.
When the system is humming along, both of its engines fed by full loads of the biogas, Orr said the plant provides about two-thirds of its own electricity needs: most from the engines and a smaller fraction from an on-site array of solar panels. Heat recovered from the engines, in turn, supplies about 90 percent of the plant’s hot water needs, including the microbial digesters, workers’ showers, and other heating systems. Biogas that isn’t stored and can’t be used by the engines has to be flared; at some other treatment plants, the excess gas is regularly burned off.
Adding FOG to the mix at the Durham plant has increased its production variability, but a nearby dome-shaped building, which Orr called a storage bubble, acts like an accelerator pedal to keep the system in sync. As gas production increases and the volume inflates a liner inside the bubble, the engines go full tilt in powering an electricity generator. As the gas volume decreases and the liner deflates, the engines back off. “It’s matching our use to our production, which is a very elegant way to do it,” Orr said. He pointed to a nearby flare stack. “You don’t see anything coming out of that flare. We use every cubic foot of gas we make.” No excess gas at all? I asked. Not even a mouse fart, he replied, laughing.
Similar upcycling innovations have the potential to fundamentally overhaul disposal practices that have changed little in decades. In the United States, we still bury about 22 percent of our poop; doing so means carting it off by truck or train to vast landfills. In 2018, an uproar over the nauseating stench of a stranded “poop train” carrying ten million pounds of treated waste from New York to Alabama shone an uncomfortable spotlight on the disposal of urban waste in rural landfills—often hundreds of miles away. Coastal communities barred from dumping their sewage in the sea are finding that inland jurisdictions don’t want it either.
At landfills that still accept it, highly concentrated organic and inorganic compounds can leach away from the sewage and other decomposing matter with the help of percolating rainwater and contaminate surrounding land and water if not properly captured and treated. Within the decaying mass itself, the same anaerobic digestion that creates biogas in the human gut yields landfill gas that wends its way to the surface. In 2019, solid waste landfills accounted for 15 percent of all human-associated methane gas emissions in the US, equivalent to the greenhouse gases released by more than twenty million cars driven for a full year. Unlike the Durham facility that uses all of the biogas it produces, landfill operators often burn off the escaping methane in a process similar to the flaring of excess natural gas at thousands of oil production sites around the world. I like to think of it as a massive bout of pyroflatulence that needlessly sends hundreds of millions of tons of greenhouse gases into the atmosphere. Despite the 2020 economic slowdown caused by the COVID-19 pandemic, global carbon dioxide and methane emissions still jumped; the spike in methane was the largest since researchers began keeping records in 1983.
More advanced landfills are recapturing and using the gas instead of letting it escape; a massive one in Washington State’s Klickitat County that handles more than twelve million pounds of garbage every day is producing enough renewable natural gas to meet the daily needs of furnaces, kitchen stoves, and water heaters in 19,000 homes. Nearly everyone seems to agree that converting trash into gas is a good thing. But only about 25 to 30 percent of the Roosevelt Regional Landfill’s trash is organic matter capable of producing methane, meaning that efforts to improve composting and recycling could eventually squeeze off the gas supply.
We dispose of another 16 percent of our poop by burning it. Drying sludge in order to fully incinerate it often requires more energy than it produces. The resulting ash, maybe a tenth of the original volume, is free of pathogens but still needs to be disposed of or reused and the incineration still releases carbon dioxide into the atmosphere. Some engineers have demonstrated the potential to instead turn poop into the charcoal-like briquettes cited in the UN report through a high-temperature, oxygen-free process called pyrolysis.
In Kenya, the government banned logging and timber harvesting in all public and community forests in 2018 to curb deforestation blamed largely on the production of charcoal briquettes for cooking and heating. But with limited enforcement capacity and few alternatives, charcoal production continued. As a greener option, sanitation-focused companies Sanergy and Sanivation, both based in Kenya, are using poop to make fuel briquettes. Sanergy uses the biomass, collected from urban toilets, as a feedstock for a vast farm of larvae from the black soldier fly. A larval by-product, called frass, can be charred and pressed into briquettes. Sanivation, for its part, uses large parabolic mirrors as solar energy concentrators to dry and transform human feces and rose waste from flower farms into another form of fuel briquettes. The sun-drying process lowers costs though it also limits how much poop can be converted at once; each ton of resulting briquettes, the company has estimated, can replace twenty-two trees that would have been felled for firewood or charcoal.
Beyond their tangible products, the efforts may be creating some spillover benefits. Sanergy employee Sheila Kibuthu told me that the movement has helped people become more conscious of their waste and consider how they might recycle more of it or generate less of it. A combination of strategies may be needed to maximize the potential, but this much is clear: we can no longer do business as usual after doing our business.
In the US, the world’s top oil producer by 2017 and one of its top exporters, annual greenhouse gas emissions are second only to China. But a timely import from Norway has suggested another strategy for scaling up our own biogas production. In Washington, DC, the Blue Plains facility operated by DC Water is billed as the largest advanced wastewater treatment plant in the world and serves about 2.3 million residents. The sewer system collects wastewater through 1,800 miles of pipes that snake across Washington, DC and the surrounding region and directs it to the plant in the southwestern part of the district. A big part of the system, the Potomac Interceptor, conveys wastewater from two counties each in Maryland and Virginia. There are bigger districts, but Blue Plains has adopted the most stringent treatment process, said Chris Peot, the plant’s director of resource recovery, when I met him on a cold and cloudy winter morning. The 153-acre facility was treating roughly 300 million gallons of wastewater every day, on average, but had the capacity to treat up to one billion gallons during peak flows.
A self-described “geeky engineer” who wants to save the world, Peot happily dove into the details of the agency’s water treatment process. Unlike most US utilities, DC Water allowed its technical team to look abroad for biodigesters that would clean and reduce the wastewater’s organic matter while creating biogas in the process. Through a newly refurbished treatment plant in Dublin, Ireland, the team found what seemed like a perfect fit: a system manufactured by a company in Asker, Norway, called Cambi. These Cambi units specialize in thermal hydrolysis, which uses high heat and then a sudden difference in pressure to break open and kill bacterial cells in wastewater. In 2015, Blue Plains installed four of these units in what was the biggest such installation in the world.
Wastewater treatment plants tend to have their own unique design flourishes, and the eight stainless steel tanks in each Cambi unit, connected by a series of catwalks and extensive piping, make an impressive focal point. The first one, the pulper tank, thoroughly mixes and preheats the incoming slurry of roughly 85 percent liquids and 15 percent solids to a bit below the boiling point of water. Pumps send that mix into the six reactor tanks. At any one time, two are filling, two are heating, and two are emptying. When they’re full and sealed, these reactors stew the waste at 338 degrees Fahrenheit for at least twenty minutes at a pressure of ninety pounds per square inch (or about sixfold higher than the normal atmospheric pressure at sea level).
The reactors, Peot said, act like oversized, computer-controlled pressure cookers, fed with steam heat from a boiler. The thoroughly cooked slurry then empties into a shorter but fatter flash tank. With the opening of a valve, the intense pressure suddenly drops to normal atmospheric conditions, and the dramatic pressure differential causes the microbial cells to burst open. As a bonus to annihilating any pathogens that had survived the heat, the cells’ released nutrients become immediately available to other hungry microbes. If I ate a bag of dried pinto beans, Peot said, my stomach wouldn’t digest them very well. But if I first put the beans in a pressure cooker, the heat and pressure-aided breakdown would allow me to easily mash them up with a fork.
The pressure release also lets off a lot of steam, which can be fed back to the pulper to help preheat the next batch of incoming sludge. The outgoing, sterilized microbial mash, once cooled, becomes an ideal food source for other bacteria and methane-producing archaea microbes housed in four giant concrete digesters. The eighty-foot-tall digesters look like fat, flat-topped silos. Together, they can hold about 15.2 million gallons of sludge, or enough to fill twenty-three Olympic-sized swimming pools. Peot said the Blue Plains plant initially received its archaea strains from a sister plant in Alexandria, Virginia, in a process akin to seeding a batch of sourdough starter from a proven source. Over a three-week period, the bacteria and archaea within the oxygen-free tanks feast on the organic matter released by the burst microbial cells, efficiently producing biogas as a by-product of their metabolism.
In the plant’s nearby gas building, the captured biogas is purged of moisture and some chemical contaminants, and then compressed. When burned in a power plant, the gas turns three turbines the size of jet engines, though in this case the turbines turn the methane into electricity and heat. Within its first few years of operation, the process was generating roughly one-fourth of the plant’s power needs, with the potential to increase that production to about one-third of its consumption.
After the archaeal feeding frenzy, liquid resembling chocolate milk flows out of the digesters and into a dewatering building where belt filter presses squeeze out much of the water. Think of them as a series of giant rolling pins pressing down on a looping conveyor belt. The well-pressed organic matter that comes out the other end, a moist and pathogen-free by-product of the treatment process that the industry has dubbed biosolids, makes an ideal soil amendment. “There is no such thing as waste, only wasted resources,” Peot told me, repeating a common refrain in the industry. The plant no longer has to add lime to reduce pathogens and stabilize the organic matter before delivering it to farms or reclamation projects. By reducing the biomass and eliminating disease-causing microbes, the system has saved the plant millions in chemical and hauling costs. We walked past the bays where trucks load up the biosolids and Pamela Mooring, a communications manager for DC Water, recalled how the dewatering building used to make her gag. “It used to be a horribly smelly building,” she said. It smelled more like a farm on the day I visited, with a faint ammonia smell.
Flush with success from its gas production, Blue Plains has since added a new nitrogen removal process called anaerobic ammonium oxidation, or anammox for short. Researchers at Delft University of Technology in the Netherlands developed the process using bacteria that had been discovered in wastewater sludge in the 1990s. These microbes have the unusual ability to combine the problematic ammonium with nitrate or nitrite compounds to form nitrogen gas in the complete absence of oxygen. Peot said the nitrogen-removal shortcut, if a pilot test bears it out, could yield even more energy savings at Blue Plains.
For many of us, the most apparent impact of a treatment process that removes nitrogen and phosphorus and pathogens and fine particles may be what we don’t see. Algal blooms. Murky water. Dead fish. But along with the roughly 3,500 gallons returned to the Potomac River every second, some changes have begun attracting notice. “The stuff leaving the plant here is way cleaner than the river,” Peot said. Anglers in the know position their boats near where the discharge enters the Potomac, especially during fishing tournaments. The clear water allows the bass to see the bait more easily, leading to bigger hauls. It’s a not-so-secret tip, Peot said, chuckling. Eagles sometimes fly by, perhaps looking for a catch of their own, and workers are seeing more red foxes and woodchucks on the treatment plant’s property.
