Why Our Next Fuel Source May Come from Our Own Waste
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The Metropolitan Wastewater Treatment Plant sits beside the Mississippi River, just downstream from St. Paul. The 10th-largest plant of its kind in the country, it treats sewage from three-fourths of the Twin Cities metro area -- more than 200 million gallons a day. Some of its low brick buildings, adorned by graceful Art Deco lettering, date to the plant's origin in 1938. From a rooftop, the 170-acre grounds is a warren of basins, tanks, stacks, and pipes.
Incoming sewage is screened for trash and chunks, then runs into settling ponds to remove solids. In aeration ponds, carefully managed populations of microbes break down organics. After more settling to remove dead microbes, wastewater is sterilized with liquid chlorine before being discharged into the river. The effluent is often cleaner and clearer than the river itself. Amazingly, there is barely a whiff of odor.
As clean as the effluent is, Minnesota is considering new standards that will most likely require further reduction of phosphorous and nitrogen. Excess phosphorus is a real concern in the Land of 10,000 Lakes because it causes unsightly algae blooms and fish kills. Nitrogen sluicing off farmland throughout the Midwest is blamed for the hypoxic dead zone at the Mississippi River's mouth. But meeting the new standards through conventional treatment could easily cost "hundreds of million of dollars" a year, says Willet. "We need to find some options."
So the University of Minnesota and the Met Council began research in 2007. Through its Initiative for Renewable Energy and the Environment, the university was looking for a way to produce a renewable fuel that wouldn't compete with food crops or tie up agricultural land. The Met Council wanted cleaner wastewater.
Early on, Ruan decided against growing algae on the raw wastewater streaming into the plant: The task of managing more than 200 million gallons a day seemed daunting. Instead, research focused on the "centrate," the millions of gallons squeezed from settling-pond solids by powerful centrifuges. The foul juice is so high in nitrogen and phosphorus that it kills most organisms.
Ruan's initial task was to screen thousands of species of algae to find one, or several, that would flourish in the harsh conditions of the centrate. He dispatched his assistants to scoop green, soupy water from ponds and rivers. Most perished in the concentrated nutrients, but Ruan eventually found several species -- greenish, spherical, single-celled plankton only 5 microns across -- that survived. By acclimating these survivors, Ruan was able to produce strains that thrived in the wastewater, while reducing the levels of phosphorus by 50 to 80 percent. They yielded 30 percent of their mass as oil and grew so fast they could be harvested daily.
So far Ruan and the Met Council have shunned genetically engineered algae, though they almost certainly could boost growth and oil content. "We are not interested because eventually on a massive operation like this, some of it is going to get loose in the river," says Willet. "And I have enough regulations."
Ruan also decided against open ponds to grow his algae. Ponds are inefficient, because algae blooms block light. Commercial-scale ponds would also require large acreage, and conditions are tough to control, especially in winter. "If you're talking about an open pond system, it's almost impossible in a northern climate," says Ruan. Finally, "if the algae is dilute, it's very, very expensive to harvest it."
Instead, Ruan began building dozens of different "photo-bioreactors"-- various configurations of tubes or plates that allow good exposure to natural and artificial light, as well as easy access for harvesting and cleaning. The current generation of reactors is operating in a shed in the plant's "solids building," not only to contain the stench of the centrate, but also to keep the equipment secret until the university secures patents.