Fulford may just be looking for the wrong sort of precedents. Biogas, for instance, is quite familiar to city engineers, just not as a standalone source of energy. More than 1,000 wastewater-treatment plants use the process of anaerobic digestion to reduce the amount of sewage they must eventually haul away or burn. But until recently, most cities viewed the biogas that results as a by-product, not a commodity. Wastewater-treatment plants used it to heat the sludge digesters (the microbes like warmth) and to run some part of the energy-intensive water-treatment process.
Now, though, a few U.S. cities, prompted by high energy costs and new greenhouse-gas emission targets, are experimenting with ways to generate excess biogas. The New York City Department of Environmental Protection built eight 145-foot-high egg-shaped digesters that tower over the Brooklyn neighborhood of Greenpoint. The shiny silver eggs are a part of the Newtown Creek wastewater-treatment plant, where a million gallons of sewage arrives from a pumping station across the East River every day, and a million more flows in from parts of Brooklyn and Queens.
The waste feeds into a series of covered holding tanks, and the heavier materials settle to the bottom. That sludge passes through a centrifuge, where more liquid is removed, before moving on to the new digesters. Each one holds three million gallons of sludge for 15 days, mechanically stirring and heating it to a microbe-friendly 100°F.
The digesters were part of a $5-billion upgrade that began in the late 1990s and will take two to three more years to complete. The idea wasn’t to produce energy--it was to reduce maintenance. The old digesters didn’t generate much gas, but they did accumulate 10-foot-thick layers of grit that required manual removal every three years. The new digesters are better at mixing the sludge, so they create less buildup and are able to hold far more gas-producing material. When those went online, engineers discovered that they also produced twice as much methane, which is far more than the plant can use in its daily onsite operations: It’s enough energy to heat 2,500 homes. By next year, the city will be running that excess biogas through a cleaning system and then into the natural-gas pipeline. New Yorkers’ fecal matter and ground-up food waste will help cook their food and warm their homes, all while annually releasing 16,650 fewer tons of CO2 into the atmosphere.
The regional sewage-treatment agency in Milwaukee is working with researchers at Marquette University to determine which types of local high-energy waste can best be used for biogas without having to upgrade their system at all. For the past five winters, the agency has been vacuuming up de-icing fluid--propylene glycol, an organic compound--from the airport and trucking it to a water-treatment plant, where engineers put it directly into the digester to generate more gas. “When we started,” says Peter R. Topczewski, the director of water-quality protection for the Milwaukee Metropolitan Sewerage District, “you could see an instant spike. We were seeing two and a half times as much gas production.” The agency has also recently made a deal with a local Coca-Cola bottling plant to take its rejected ingredients.
Swedish Biogas International, meanwhile, has opened a North American headquarters in Flint, Michigan, where it is building a plant that will run on sewage sludge and waste from the processing of local products such as meat and salad dressing. The plant will generate about 1.6 megawatts of electricity for the city and also produce a steady supply of fertilizer. The essence of the project, as with all biogas systems, is tapping into what’s nearby. “We want to work on collecting local waste,” says Tom Guise, the company’s CEO.single page
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.