The nation’s capital is now powered by poop—at least partially anyway. Since flipping the switch in September, the Blue Plains Advanced Wastewater Treatment Plant of DC Water has been running a new installation that uses the solids generated by sewage treatment to produce methane, which is combusted on site to produce electricity.
While anaerobic digestion has been used to produce combustible methane from biomass for over a century, for the volumes of waste that has to treat, traditional methods would have taken an inordinate amount of room. General Manager of DC Water George S. Hawkins explains that they’re “the largest facility of its kind in the world” with an area of 157 acres. Still, “the typical standard anaerobic digestion process took too much space, and we didn’t have enough space for the number of digesters we would have needed.” To fit within the space available the facility used the Cambi System, which is a Norwegian technology. This is the largest installation of this method in the world.
The secret of the Cambi System is a unique “steam explosion” method which sterilizes the sludge and increases its biodegradability by rapidly decompressing sludge after it’s been boiled at high pressure. This method vastly reduces the number of digesters needed to accommodate a waste stream of any given size. The deactivated sludge then goes to a 3.8 million-gallon digester that has special microbes that eat the solids and produce methane, which is pumped to a refinery where moisture and contaminants are removed, and finally to furnace/generator which produces electricity.
The plant is expected to produce 13 mega-watts of electricity, enough for 10,500 homes. Importantly, this is a sustainable-energy technology that has an on/off button, so it’s not at the mercy of nature’s schedule like solar and wind (at least until energy-storage methods become substantially cheaper).
About a decade ago, DC Water was “facing a very significant effort to update their lime-stabilization process,” Hawkins says. But that would have generated 1,200 wet tons of class B biosolids, which because of the amount of bacteria they contain are subject to stringent regulations. This Cambi System now produces 500-600 wet tons of class A biosolids, which have less than 1,000 bacterial colonies per test. Hawkins reports they typically find values in “the single-digit-to-teens level, so it’s incredibly clean.”
Still the final product has a lot of nutrients. Says Hawkins, “We’re now looking at developing a soil-amending product that we’ll be able to put on the market. This is another way to turn what has only been a waste into a valuable revenue stream as well as something that’s good for the environment.”
This all started with an eight-year effort to find an alternative to the old method of lime stabilization. DC Water searched through 40 peer-reviewed studies to evaluate candidate replacement technologies. While the Cambi System has been used in other places in the world, this would be the largest of its kind. To make sure the process could be scaled up for Blue Plains, the team visited an installation in Scotland, testing the smaller-scale system with a waste stream that resembled what would be found back in DC. After it was demonstrated to work and became financed, construction took 3½ years, installing approximately 15,000 new pieces of equipment. Operations began last September.
When asked if he saw this technology being used elsewhere, Hawkins responded ,“I absolutely do! We have delegations from a number of cities already scheduled to come visit. The next big one is San Francisco, which has been very seriously considering a Cambi System.”
“I believe this will be a significant sector of power generation,” Hawkins says. “There are water reclamation facilities in every community and all of them can be generating power.”