Each time you pull the lever on your toilet, the water disappearing in that comforting swirl begins a long, underground journey through the engineering marvel that is the sewage system of the metropolitan area. The waste first travels through a dark labyrinth composed of everything from 10-inch pipes to cavernous, 10-foot culverts large enough to be traversed by a speedboat; in all, 550 miles of sewer ducts--the distance from Minneapolis to Rapid City, S.D.--lace the ground under the Twin Cities.
Down there the sound of rushing water, as your trickle gradually merges with the waste of 2.2 million other metro-area residents, grows to a Niagara-like roar. By the time the torrent reaches the Metro Sewage Treatment plant near Pig's Eye Lake in St. Paul, where 80 percent of Twin Cities wastewater goes, it reaches tsunami proportions. Some 226 million gallons of liquid waste arrive at the plant every day.
As your household offal travels the septic byways, it becomes involved with sewage of dubious character. Lead, cadmium, mercury, and other heavy metals, along with cyanide and more than 100 organic chemicals, are found in the combined residential and industrial wastewater. Half of those pollutants, according to Metropolitan Council Environmental Services, which manages the plant, are removed in "primary treatment" ponds, through a series of screens, settling tanks, and skimming devices. In "secondary treatment," bacteria break down most of the organic waste materials. Next, the water is separated from the remaining solids, chlorinated, then dechlorinated. Finally it's put back into the Mississippi River.
In the sewage business, what's left after the water is gone is known as "biosolids"; a less charitable term is "sludge cake." By either name, it's a big load--some 225 tons every day. The material contains everything that comes through the sewers and hasn't been destroyed by treatment, including most heavy metals and a wide variety of microorganisms. Most of it is incinerated at the plant, generating $1-million worth of electricity annually.
But technological progress is about to come to the sewage system. Last July the Metropolitan Council voted to proceed with a $200-million overhaul of its flagship plant, to be completed in 2004. Most of that money will go to replacing the old burners with high-tech incinerators, which the council says will reduce emissions and save energy. But the plan also calls for a growing share of the plant's sludge to be spread on area farm fields, to fertilize everything from lettuce and tomatoes to corn and soybeans. And though the process has the blessing of federal regulators, some environmentalists are worried: "By spreading this stuff on farmlands, they are turning fields into toxic landfills," charges Jackie Hunt Christensen, Food Safety Project Director for the Minneapolis-based Institute for Agriculture and Trade Policy.
Since the U.S. Environmental Protection Agency created regulations for "land spreading" of sewage sludge in 1993, the practice has become increasingly popular in municipalities across the state and the nation. Some 200 Minnesota municipalities now give all or part of their sludge to farmers. Although the Pig's Eye plant currently doesn't use land spreading, sludge from two smaller Met Council plants is used on fields, including about 2,000 acres near Eagan. Steve Stark, a soil scientist who manages the biosolids program for all nine Twin Cities sewage treatment plants, says farmers are interested in the lime- and nitrogen-rich sludge. "We used to promote biosolids actively at the State Fair," he says. "But now we've got a waiting list of farmers who want to use it for fertilizer."
Most agricultural uses involve what's known as "Class B" sludge, the residue at the end of standard wastewater treatment. But the Met Council's Seneca plant near Farmington, Stark says, uses an additional treatment process in which sludge is mixed with lime dust to create heat that destroys pathogens; the result is a friable material called N-Viro soil, a name created by the Ohio firm that licensed the technology. N-Viro soil meets and usually exceeds EPA metal and pathogen standards for "Exceptional Quality" sludge, according to records supplied by Stark. According to EPA regulations, Exceptional Quality sludge is clean enough to be used on vegetable crops like lettuce or broccoli. A similar treatment will be used for the 10 percent--up to 20,000 tons--of Pig's Eye sludge slated for land spreading under the Met Council's plan.
But critics are not impressed with the "Exceptional Quality" standard. They cite an August '97 Cornell University report that warned that the EPA allows significantly higher levels of heavy metals in sludge than do regulators throughout Europe and Canada: Ontario sludge, for instance, can have no more than 34 milligrams of cadmium per kilogram, and Denmark allows less than a milligram of the metal in a kilogram of land-applied sludge. By contrast, the EPA permits up to 39 milligrams of cadmium per kilogram of biosolids.
"The term 'Exceptional Quality' is hogwash," charges Charlotte Hartman, executive director of the Sludge Alliance, a national consortium of organizations opposed to land spreading. In addition to toxic heavy metals, Hartman notes, land-applied sludge--particularly the nonpasteurized "Class B" material--may carry all manner of pathogens into farm soils. According to the Cornell report, "Class B sludges contain significant levels of pathogens... Few viruses have been studied in regard to sludges, and no monitoring is currently required for viruses in sludge products."