One day I pulled over on a main highway to watch a sewer crew fix a leaky pipe using what's called cured-in-place pipe: A long liner impregnated with resin is pushed into a pipe by water, then they pump steam through the pipe to harden the resin, and presto--the pipe is, though slightly narrower, all but new. The crew runs an auxiliary pipe while they fix the leaky one. Workers can find leaks by stopping up pipes with sandbags, pumping smoke down a manhole, and then seeing where the smoke starts creeping out of the ground. Clearly, people have thought about this stuff a lot.
"It gets in your blood," said a senior engineer and inspector named Dave, who managed a construction site near my house. I had stopped by a sewer truck to watch a couple of guys hose out a pipe, which they followed to a manhole at the backyard boundary of a few properties. "I don't like the looks of that," one had said when, spooning out grit from the hole, he unearthed pieces of clay pipe. Rather than dig on several people's private property, the city decided to abandon the line running between the houses and reroute the flow to the end of the street, where it could join a larger main and head downhill. But since the flow had to go downhill, the engineers had to get wastewater to flow away from the manhole in a different direction. That meant, simply, digging a deeper trench in the direction they wanted it to go. They had to do the same thing on the line that received the new flow, so they were digging on two streets for a few weeks, and I occasionally stopped by.
I watched them carefully lower new green PVC pipes and check the slope. They use a laser to measure, with a sort of bull's-eye target at the end. The red dot right in the center means the pipe has the right slope. "We used to run string lines, length by length," Dave told me. "This makes it easier. A lot easier." They lay pipe on a bed of gravel, supporting it with rocks and pieces of brick to maintain the slope, unlike water pipe, which, under pressure, can just be laid in a trench and buried. Any pipe below 12 feet has to be ductile iron, to support the weight of the earth on top; same with any pipe that crosses a creekbed, hangs under a bridge, or does anything but lie directly on the earth. Even though sewer pipes start out low and have to keep going lower, in a city with no subway system there's not much other infrastructure in the way; 12 feet is fairly deep for Raleigh.
The Neuse River Wastewater Treatment Plant, southeast of Raleigh, discharges most of the 40-plus million gallons of water it treats every day in a state bordering on potable. Spread over 300 acres (and surrounded by 1,200 acres of farm fields for application of biosolids), the plant is big enough that to see it you have to tool around in a van; you can't walk it like you can the water treatment plant miles upstream. Superintendent T. J. Lynch started my tour with a drive to the headworks, where screens and vortex filters remove the floating junk and grit from the outflow of the twin 72-inch trunks flowing in from the sewer system. The floating stuff is more obvious, but Lynch emphasizes the basins that settle out grit: In a process heavy on pumps and pipes, grit is anathema. "Think about sand," he says. "It's abrasive. It will literally wear out your equipment." They fill three Dumpsters a day with grit and the debris caught by screening, all of which goes to the landfill.
Remarkably, though the air has a certain tang, it doesn't stink. "Ninety-nine-point-nine percent of it is just water," Lynch says. "We've just got to get that 0.1 percent out."
Outside the headworks Lynch points to a concrete basin almost the exact size and shape of a baseball field, sloping downward from the outfield to a low point at home plate: a 32-million-gallon equalization tank. The sewer system does not have water towers or storage tanks to accommodate regular fluctuations in wastewater volume (flow peaks at breakfast time and again just after midnight), so it stores water in the equalization tank during periods of high flow and, especially, storms: Lynch says, "It's not uncommon for us during a heavy rain to see our flow double." The equalization tank gives Lynch a place to put that excess flow. When the flow slows down again, he uses four enormous screw pumps to push the water to the top of the slope on which the plant is built--the rest of the way through the plant, gravity does the work. He shows off the pumps proudly. They're called "vertical turbine solids-handling pumps," but he notes that they use technology roughly unchanged since the time of Archimedes: turning screws to lift water.