Norman Lofton does.

Lofton holds one of society's lower-profile jobs. Five days a week, he puts on a pair of chest-high rubber waders, a yellow raincoat, a hard hat and a pair of gloves and climbs underground. Lofton is a sewer-maintenance crew chief for the Water Department - a "sewer crawler," in the slang of his trade.

"I wouldn't say anybody likes it," said Lofton. "You'd have to be crazy to like it."

Lofton holds the record for the department's longest continuous sewer inspection. In 1987, he and his crew entered a pipe in a railroad yard near Queen Lane and came up for air around Henry Avenue, 5,745 feet later. Lofton measured it. "Sixteen hundred and forty feet was examined bent over," he said. "Some of the sewer was built out of Belgian block, and it was very slippery with waste water and feces. It was like walking on ice. It took us two hours and 15 minutes."

As Lofton spoke, he was standing in a chamber 26 feet below Wissahickon Avenue near Queen Lane. The light from the manhole above was so diffuse that the pits and ridges in the rough-cast concrete were barely visible. Though they wore yellow rain slickers, the three members of Lofton's crew, who were joining him at the bottom, moved about like gray phantoms.

Lofton tested a black box called a Dynamation meter, an electronic canary that shrieks if various gases in the sewer approach dangerous levels. The air can be so foul beneath oil refineries, slaughterhouses and chemical plants that the crawlers use scuba equipment. The hot sewage from laundries can block a pipe with steam and suds and scald the crawlers. On this day beneath Wissahickon Avenue, the air was dank but odorless. The box in Lofton's hand remained silent.

"People have no idea what it's like down here," said Lofton, 38, who has worked in the city's sewers for 18 years. "It's another city. It's a private city." Several small pipes entered the chamber from different directions. Sewage cascaded from each and swirled in a pool around Lofton's waterproofed ankles. Turbulence had rendered everything in the sewage beyond recognition. It looked like cafe au lait. The sewage flowed into a larger pipe that continued downhill.

Lofton aimed a flashlight into the darkness of the big pipe. "That's where we're going."

Nowadays, the sewage at Lofton's feet takes a much more circuitous route through a tremendously complex treatment system. The stuff is screened, floated, skimmed, separated, pumped full of air and then fed as lunch to a quintillion hungry bacteria. After that, it is tested, disinfected and released into the river.

In the last 15 years, Philadelphia's waste-water-treatment system has been overhauled at a cost of nearly $1 billion. When the work is completed this summer, it will rank as the largest public-works project in the city's modern history. The cost is twice that of the city's proposed convention center, three times the price of the Center City commuter tunnel and four times the cost of the Schuylkill Expressway reconstruction.

Now, sewage-treatment plants are called "water pollution control facilities," but a billion dollars has bought more than a euphemism. The overhauled system has dramatically improved the water quality of the Schuylkill and the Delaware. Fish have returned in large numbers. The cleaner waterways have, in turn, spawned waterfront development that would not have been possible 40 years ago, when the Delaware stank so badly it nauseated pedestrians at Broad and Chestnut Streets.

With much of the work buried, the project has escaped pageantry. Water Department officials are not naive about the capacity of sewage to inspire civic pride. They would be happy if nobody complained. "Success in sewage treatment is defined as not being noticed," said William J. Marrazzo, who was, until November, the city's water commissioner for eight years. But sometimes sewage can't be ignored.

When the winds are right, it is hard not to notice the city's three treatment plants. Each day, 466 million gallons of waste water flow into them from Philadelphia and parts of three surrounding counties, a volume equal to the daily flow of the Schuylkill in the summer. If One Liberty Place were an empty shell, the flow would fill all 62 floors in less than eight hours.

Of that load, only a tenth of one percent is solid matter - sludge. It is enough, however, to give sewage a bad name. But while New York dumps its sludge into the ocean, and Boston sends its muck right into its harbor, the Philadelphia Water Department composts its "sludge products" and markets them as fertilizer. The cutting edge of environmentalism, however, can cut both ways: The city has been hard-pressed to find enough large-scale users in places where people won't raise a stink.

In order to collect that sludge, the city routes its sewage - your sewage - through 2,933 miles of pipes beneath Philadelphia. In a system as long as the nation is wide, there are bound to be backups, leaks, clogs and overflows just about every day.

Norman Lofton knows that well.

The only noise was the splashing of Lofton's boots and the constant gurgling of sewage. Now and then, reminders of life on the surface penetrated the eerie world - the clang of an automobile running over a manhole cover, the sight of a steel I-beam piercing the pipe, the foundation of some structure overhead. Occasionally, small sewers protruded at head level from the sewer wall - the drains from nearby buildings. You may not think about your six flushes a day, but they are a matter of great concern to a sewer crawler. Fortunately, the unmistakable rumble of a flush is amplified in the pipes. Most of the time, the crawlers can move out of the way.

Lofton pointed to cracks in the sewer wall where clear water dribbled into the pipe. About 5 percent of the waste water enters the system through leaks. Some of it is groundwater; some of it is treated city water - pure, drinkable water - leaking from damaged mains. Sometimes, a leak in a main washes out so much soil that it forms an underground river between the water main and the sewer. Almost every year, one of the washouts collapses, sometimes swallowing a car in a sinkhole. Lofton and his crew make recommendations on major repairs and make smaller patches themselves. In a system as old as Philadelphia's, where many of the sewers were built in the 19th century, there's plenty of work to do.

As Lofton forged ahead in the slowly descending sewer, the sewage at his feet began to grow deeper and slow down. He aimed his flashlight ahead. Beyond the reach of the beam, the stream roared over an unseen obstruction that was damming the pipe. The blockage might be anything - a branch, a milk crate, a collapsed wall.

"It can be very dangerous down here," he said. An obstruction like the one ahead could dislodge without warning, unleashing the dammed sewage and sweeping away anyone standing in it. Lofton sloshed ahead a few feet, but when his flashlight showed the level of sewage reaching above the middle of the pipe ahead, he called it quits.

"You can't underestimate a sewer," he said. "There's always surprises." Since 1985, two people have died in sewers - a child who fell into a damaged manhole and a college student who clambered into an open sewer drain. Both were washed away. Crawlers fear cave-ins and flash floods. In May, Lofton was examining a 10-foot sewer under Third and Thompson Streets when the sewage surged from about a foot deep to more than three feet. Lofton fell down face first. The sewage carried him about 45 feet down the pipe before he could get a foothold. When he stood up, his rubber suit was filled with sewage. "That was the first time I was ever knocked off my feet," he said.

Inspecting sewers has few rewards - crawlers make about $21,000 a year. In 1971, however, two crawlers came upon some muddy bundles under Torresdale Avenue that turned out to be $92,400 in cash. Nobody claimed the dirty money, and the crawlers were allowed to keep it.

Besides cash, strange and awful things often show up in the sewers. Shopping carts and body parts have washed up at the treatment plants. Since the 1973 Supreme Court decision legalizing abortion, treatment-plant operators have noted with relief a dramatic decline in the number of fetuses. Some of the sewers are so large they could accommodate a two-lane highway. In fact, a Volkswagen body once was found lodged in one of the pipes. No one could account for it.

Even to the experienced, sewers can be unsettling. "If you turn out your light, it's dark," said Lofton. "I mean pitch dark. You can't see your hand in front of your face."

The conditions can twist even a sensible person's brain. When sewer crawlers apply for the job, they must be able to crawl through a pipe 18 inches in diameter. Some applicants have the physical dimensions but, during underground tests, freeze in claustrophobic panic. Doctors say it isn't true, but crawlers believe that panic leads a human to swell up. They said a crawler once became so frightened that he expanded like a cork and cut off the air to the man behind him. The trailing crawler passed out. Both had to be pulled out of the pipe by the safety ropes crawlers wear on the job.

Lofton is a stout man - he has a 36-inch waist - so he confines his inspections to larger sewers. Still, he occasionally finds himself in a tight squeeze. "A lot of times, I've gotten scared, I mean really scared. I started swelling up. And then I talk to the Creator. I talk myself through those sewers." The sewers have brought Norman Lofton closer to God.

There is considerable lore about the subterranean world. Some of it is true. In Paris, burglars once bored a hole from a sewer into a bank. In New York, the homeless have been found living in sewers.

Most of the stories, however, are greatly embellished. Contrary to common belief, Philadelphia's sewers do not surge during half time of the Super Bowl. The sewer system is so large that a citywide flush would be no different from a short-lived downpour. Half time at the Army-Navy game is another matter. The effect of 100,000 spectators' rushing for relief during intermission has overflowed the trunk lines leading away from JFK Stadium.

Many of the stories about underground creatures are also larger than life. Crawlers say they have seen cockroaches the size of mice and rats the size of cats, but reptilian life is no match for sewage. Somebody once dumped a 12 foot pet python into Philadelphia's sewers, but when it washed up at the Northeast treatment plant, not even biologists from the Philadelphia Zoo could revive it. Everybody has heard stories about alligators in the sewers, but nobody has ever seen one.

"I'm not saying my mind hasn't seen alligators," said Lofton. "There are a lot of strange noises here, and you spend a lot of time looking over your shoulder."

For most of its history, Philadelphia had no kidneys. The sewers of Ben Franklin's time were constructed of wood - they were open ditches designed primarily to carry away garbage, wash water and rain - and flowed toward the nearest creek. (Outhouses were built over pits, and human wastes remained in them.) As the city grew, the creeks that crossed Center City became choked with sewage. The city turned them into sewers, one by one. Dock Street was once such a stream.

In 1867, the city had only 67 miles of sewers. But then indoor plumbing replaced outhouses during the industrial age, introducing human waste to the system. By 1900, the city had 848 miles of sewers, many of which are still in service today. Most of the pipes are brick and shaped like the cross-section of an egg, with the narrow end pointing down. The shape forces even low volumes of sewage to flow fast.

Since the Mesopotamian Empire, when the first drainage systems were constructed, engineers designed sewers for one purpose: to transport sewage downhill as quickly as possible. Following the contour of the land, relying only on gravity for propulsion, sewers emptied in whatever stream was convenient.

Unfortunately, Philadelphia's sewage went into the same rivers from which it drew its drinking water - the Schuylkill and the Delaware. Thousands of people died each year from waterborne diseases, such as typhoid and yellow fever.

In the 1880s, a few years after the discovery of typhoid bacteria, the city built a giant sewer along the east bank of the Schuylkill to intercept the pipes that emptied into the river from industrial Manayunk. The interceptor channeled the sewage downstream and emptied into the river just below the Fairmount Water Works intakes, protecting Center City's drinking-water supply after a fashion. A few years later, a second interceptor was built along Wissahickon Creek.

Still, sewage discharges into the Delaware threatened the city's largest water intakes in Torresdale. "What was crazy was that raw sewage was being discharged into Pennypack Creek, and when it came out to the Delaware, it went right into the Torresdale water intakes," said William Wankoff, the city's chief of sewage treatment. "Talk about recycling."

In 1907, the state directed Philadelphia to treat its sewage. Seven years later, the city developed a plan to channel all of the sewage to three treatment plants. A network of interceptors would tie the old sewers to the new plants. Where gravity would no longer work, huge pumps would propel the sewage over hills.

The Northeast plant, at the junction of Frankford Creek and the Delaware River in Bridesburg, opened in 1923. It processed only about 15 percent of the city's sewage. The Depression, Philadelphia politics and World War II delayed construction of the other two plants for 30 years.

During the delay, it became plainly evident that Philadelphia's sewage was a serious environmental problem. In the 1940s, the Delaware River turned black. Hydrogen sulfide gas corroded metal on houses near the river, longshoremen fell sick, and ships' engines clogged from floating debris. Water quality is measured by the amount of dissolved oxygen it contains. A clean river has about 8 parts per million of oxygen. Fish labor to breath in a river with fewer than 4 parts per million. In the late 1940s, a 20-mile stretch of the Delaware River along Philadelphia contained no measurable oxygen. It was anoxic, like a septic tank.

After the war, Philadelphia finally built the two other treatment plants, the Southwest plant on Enterprise Avenue by the airport and the Southeast plant on Pattison Avenue, south of the Walt Whitman Bridge. They opened in the mid-1950s.

Finally, the city was treating its sewage. But the transition posed a new problem that has plagued the city to this day.

Until the advent of sewage treatment, sewers were designed to empty both sewage and rainwater directly into a stream. With the new design, everything would go to the treatment plants. During dry weather, when the only sewage in the pipes comes from houses, the flow is fairly predictable.

But even a one-inch rainfall in one day can send half a billion gallons down the sewers - as much as the city's treatment plants are designed to treat properly. Without some means of relief, a huge storm would fill the new sewers to the brim, overflowing into streets and houses and flooding the sewage treatment plants.

"I guess Noah saw something like that," said Jim Downs, a maintenance supervisor at the Southwest plant. "I don't want to."

The city's solution was to install 176 regulator gates where the old sewers had flowed into the rivers. Regulators are valves designed to remain closed during the first minutes of rain, forcing sewage that is already in the pipeline to be flushed to the treatment plant. The initial slug of sewage from a rain is called the "shock load" because it is heavily polluted with oil, dirt and trash rinsed from the streets. After the shock load passes to the treatment plant, the regulators open and discharge directly into the river. Theoretically, regulators allow only rainwater (and, inevitably, a small amount of sewage) to escape into the rivers.

In practice, however, regulators are finicky devices - some of them are 60 years old - and they sometimes open too soon. The shock load goes right into the river, where it lives up to its name. What's worse, the regulator gates often become jammed open with debris and do not close after the rain stops. Then they are like open wounds, bleeding sewage into the river until somebody notices, which may take days.

Last year, the Delaware River Basin Commission estimated that 30 percent of the oxygen-devouring pollution that Philadelphia discharges into the rivers is raw waste from sewer overflows and that more than half the leakage occurs during dry weather.

The most obvious solution is to build a separate sewer system for storm water. Almost all suburban areas and much of Northeast Philadelphia have separate sewer systems, but nearly half of Philadelphia's sewers are combined sewers. To build a separate storm water system would cost more than $4 billion.

Instead, the Water Department is spending about $2.5 million to overhaul some of the oldest regulators. The program includes installing computerized sensors to detect if waste water is leaking into the river so the city can dispatch a repair crew immediately, rather than discovering the leak days later. The project, however, is moving slowly.

The U.S. Environmental Protection Agency recently drafted new regulations that address storm-water pollution, but the improvements do not necessarily involve high-tech hardware. Some solutions are as basic as educating the public not to throw waste like old crankcase oil down the street inlets. The oil simply remains in the sewer inlet until the next storm, when it is washed into the river through a regulator.

"People may think that oil goes to the treatment plant," said Deputy Water Commissioner Patrick Cairo. "But it doesn't. It goes into the stream."

"I thought I would go into construction," said William Wankoff, 43, who graduated from Drexel University in 1968 as a civil engineer. "I was enamored with the idea of sitting on top of a bulldozer." But in the late 1960s, with the federal government pressuring local governments to clean up polluted streams, water-pollution control was the emerging growth industry for engineers. Wankoff found work with the Water Department. He returned to Drexel for a master's degree in environmental engineering in the early 1970s. Today, he is responsible for operating the city's treatment plants, where he bulldozes through paperwork.

Jim Downs was a 25-year-old Culligan Man when he saw a city want ad for somebody with experience in "water treatment." That's me, he said. Soon he learned the water he would be treating required more than softening. "You go home and your friends find out you work at the other end of the tunnel," he said. "They all kind of moved away from me." After nine years, he is now the head of preventive maintenance at the Southwest plant.

"We're not in it for the glamour," joked D. Robert Thompson, the head engineer of what is called the collector system - all the pipes between your house and the treatment plants. Thompson grew up in Doylestown tinkering with anything he could dismantle. At Lehigh University, preparing for a career as a structural engineer, he was introduced to hydraulics and sanitary engineering.

Thompson joined the department in 1978, when the water commissioner was Carmen F. Guarino. Guarino was considered a capable leader among waste-water treatment experts, but his relationship with federal regulators and environmentalists was antagonistic. Guarino left in 1980, and the next year, Mayor William H. Green appointed William Marrazzo, a 31-year-old boy wonder who had received his chemical engineering degree during the age of environmental awareness.

Marrazzo changed the Water Department's outlook. "What we don't want as managers in this department are people who can only understand what's going on underground," he said in 1981. "We want managers who understand the impact their work will have on the environment and the community."

Thompson fit the bill. Now, at the age of 33, he oversees 25 people - a position that would be difficult for someone his age to match at a private engineering firm. His car has one of those yellow road signs stuck to the window, only his says, "Sewer Expert on Board."

He joined Marrazzo's team of young engineers as it went about overhauling the system. Philadelphia's treatment plants had been considered state-of-the art in the 1950s, but over the ensuing years, the art changed radically.

In the old days, sewage received what is now called "primary treatment" - a settling out of the solids that removes less than half the oxygen depleting pollutants. Meanwhile, sewage was getting progressively more noxious. Industries poured all sorts of chemicals and organic waste down the drain. Water users paid the same rate whether they emptied the baby's bath or the rinse bath from an electroplater, loaded with heavy metals.

In the late 1960s, the Delaware still averaged less than 1 part per million of dissolved oxygen during the summer, when oxygen depletion is worst. To shad migrating upstream, the water off Penn's Landing could just as well have been a brick wall.

The Delaware River Basin Commission ordered the city to upgrade its treatment plants in 1968. The improvements would involve "secondary treatment" - biologically decomposing the sewage to remove 90 percent of the pollution. In 1970, the city agreed to make the improvements by 1977.

Congress put more pressure on the city in 1972 when it passed the Federal Water Pollution Control Act, the landmark legislation that has provided more than $40 billion to local agencies to finance sewage-treatment improvements. Enforcement fell to the EPA.

The EPA and the city have had a titanic struggle since. The EPA sued the city when it did not meet the 1977 deadlines, accusing it of dawdling. The city countersued, accusing the federal government of singling out Philadelphia, and contending that the EPA was "hindering and delaying" the project. After a decade of lawsuits and extended deadlines, the two sides were still using the same arguments last summer.

That may change under John Plonski, the former head of the Licenses and Inspections Department whom Mayor Goode has appointed to replace Marrazzo. Plonski, 41, the former city manager of Norristown, is the first non-engineer to lead the Water Department, and he said his priority is to clear its legal docket. "My coming in here offers these agencies and the city an opportunity to settle up and put the past behind them," he said.

One thing can be said about the lawsuits, however. They got results. The city is scheduled to finish the improvement project this summer, with the completion of sludge-handling facilities at the Southwest plant. The city says the troublesome plant, the target of the EPA's most recent suit, will then be able to meet its discharge standards.

And while the water quality in the Delaware has yet to reach its targets, it is getting close. Nowadays, dissolved oxygen rarely dips lower than 3 parts per million. The levels of fecal coliform have dropped enough so that most stretches of the river are considered "swimmable." The Delaware River Basin Commission found 36 species of fish in the Philadelphia stretch of the river in 1985, twice as many as were found at any time during the two previous decades. The change in the fish population was so dramatic, said David P. Pollison, the commission's head of planning, that officials resurveyed the river in 1986 to confirm the study.

"In the grand scheme of things, in the big picture," said Peter Ludzia, a compliance officer in EPA's Philadelphia office, "Philadelphia's improvements have had a pretty significant impact on the water quality in the Delaware."

Whether the investment has been sufficient, however, remains up to the federal government. The EPA is now considering whether to require sewage to receive a third level of treatment that would introduce chemicals to reduce levels of phosphorus, nitrogen and some of the pollutants that slip past secondary treatment. It could add hundreds of millions of dollars more to the project.

At the beginning of a course he teaches on waste-water treatment, James S. Nicolo, who was the manager of the Southeast plant last year, promises his students: "When you're done with this class, you'll really know your s-."

Nicolo is a clever teacher who enjoys describing the science and the nuances of waste water. Relaxed, cheerful and not one to take things too seriously, Nicolo discovered sewage as a second career. A graduate of Villanova's engineering school, he worked for the Navy until 1978, designing parts for F-14s and F-16s. But he was bored. When he was 30 years old, he took a $2,000 pay cut to work for the Water Department. He now says, "It was probably the best thing I ever did."

Rather than designing airplane parts, Nicolo helped redesign the Southeast plant, the city's smallest and newest. A few months ago, he was transferred to manage the Northeast plant, the city's largest plant and, because of its proximity to residential areas, its most problematic.

At Southeast, Nicolo learned sewage treatment from the ground up. "The most important thing about working at a sewage treatment plant is personal hygiene," he said on a tour of the plant. "When I first started working here, I went to the john after being out near the tanks: First, I went to the toilet, and then I washed my hands. One of the men said, 'You're new here, aren't you? Around here, we wash our hands before we go to the bathroom.' "

After that, Nicolo became familiar with every aspect of Southeast.

The plant is designed to handle 120 million gallons of sewage a day from a 21-square-mile area of Philadelphia and a sliver of Montgomery County. Sewage from as far as Chestnut Hill and the Wyndmoor section of Springfield Township takes up to 15 hours to travel to Southeast. In its final sprint to the plant, the waste water rushes through an 11-foot-diameter pipe that passes beneath Delaware Avenue.

The sewage enters Southeast's headworks at 37 feet below ground level, in the basement of a cavernous pump house. Nicolo led the way down a concrete stairwell to the basement where roiling, churning sewage was entering in a chamber called a wet well.

"That stuff comes in real fast, at 49,000 gallons per minute," said Nicolo. He had to shout to be heard over the droning electric pumps that elevate the sewage to ground level so it can flow through the rest of the treatment plant. During power outages, the pumps shut down, but the sewage keeps rushing into the basement of the pump house. It rapidly fills up the wet well. "All you can do is run like hell," said Nicolo. "It rises fast. It chases you up the steps." It happens often enough that the concrete walls of the stairwell are flecked in brown to a line about 12 feet above the top of the well.

Before the pumps draw the sewage to the surface, the waste water passes through iron screens that collect any debris wider than one inch, which might damage the pumps. A mechanical rake periodically clears off the trash. Most of the debris is street junk - lumber, beverage cans, rags and a fair amount of cash. "You'd be surprised," said Downs, "how many guys make it a part of their day to go walking through the screen room, especially after a big rainstorm."

Until this point, the sewer system has been designed to maintain a quick enough flow to keep all the solids afloat. The basic treatment process that follows - primary treatment - involves little more than slowing down the sewage to settle out progressively smaller particles.

The first step is the grit chamber, a large pool where gravel, sand and bits of glass settle to the bottom. If you dropped your wedding ring down a sewer, this is where it would stop. Grit is removed and sterilized in an incinerator before it is sent to a landfill.

Next, the sewage slows to a crawl in the primary settling tanks. The tanks are 12 feet deep and 250 feet long. During the two hours it takes sewage to pass through the primary tanks, suspended solids slowly drift to the bottom. Grease, tampon applicators and condoms - technically called scum - float to the top. Automated wooden boards slowly skim the scum from the top and then scrape the sludge away from the bottom. After it is collected, it is incincerated. Since a lot of it is grease, it burns easily.

Primary treatment involves only the physical removal of solids. In the last 15 years, Philadelphia has converted its three plants to include secondary treatment, or the biological decomposition of sewage. Southeast, which began secondary treatment only two years ago, was the last plant to undergo the improvements.

Secondary treatment essentially attempts to duplicate the cleansing process that naturally occurs in a river. Microorganisms in a stream feed o9converting the waste into carbon dioxide and solids. But the microbes also consume a stream's dissolved oxygen. Heavily polluted streams lose so much oxygen that even the microbes die and the cleansing action all but ceases. They become septic, like the Delaware River during the 1940s.

The key to this cleansing process are the microorganisms - bacteria, protozoa and algae. Treatment plant operators once were nuts-and-bolts kind of people, but secondary treatment has forced them to learn as much about microbiology as mechanical engineering. The microbes that consume sewage have a host of complicated names, but to treatment plant engineers, they are simply "bugs."

After primary treatment, the sewage flows into aeration tanks, where it is pumped full of oxygen - so much that the microbes not only survive, but also breed like crazy and gobble up the sewage. After this microbial feeding frenzy, the sewage, which is now called "mixed liquor," flows into a settlement tank, where the end product - sludge - sinks to the bottom.

Without the sludge, the water in the settlement tanks is ready to be released back into the river. One might presume that the sludge would be disposed of - "wasted," as they say in the trade. In truth, much of the sludge, teeming with bugs, is pumped right back to the aeration tanks to breed a new generation of bacteria. "You settle those bugs out and bring them back so they can eat again," said Tom Lauletta, the former manager of the Southwest plant.

The trick of the science is to judge the proper amount of sludge to send back for a second meal. Sewage treatment plant operators often characterize bacteria by their behavior. Bugs can be lazy, aggressive, old, young, hungry or fat. Adolescent, virile bugs get sent back to eat again. Old, indolent bugs get wasted.

"We're operating with a young sludge," Nicolo said. "We determine the age by a mean cell-retention time before we waste it. We waste 300,000 gallons each day from the two tanks. We like to keep our sludge at about 3 1/2 days old."

Nicolo walked along the tanks, passing a trough that contained mixed liquor. It looked like a bartender's practical joke. "Mixed liquor should be a golden brown and not have too much odor," he said. "The tests to measure the oxygen demand of sewage take five days to complete. By then the sewage is long gone. So you operate the plant through your five senses. This mixed liquor looks and smells just about right."

Often, a single source of sewage can make a great difference in the character of the waste water entering a plant. "This is not the normal color of our sewage," Nicolo said as he returned to the headworks. "This is funny. It's usually more gray, and not this brown." The department's investigators later determined that a building contractor had flushed a large amount of dirt down the sewers.

Philadelphia's sewage today largely reflects the city's decline as a manufacturing center. Industrial users accounted for 25 percent of the sewer system's flow three decades ago, but now contribute only 7 percent of the output. Today's sewage is more dilute. Not that the city misses the industrial sewage; it contained toxic metals or chemicals that strained the abilities of the treatment process.

The loss of other industries, however, was deeply lamented at the Southeast plant. The sewage from Schmidt's Brewery, the Domino Sugar refinery and Publicker's Distillery ended up at Southeast before they closed. "Those places had great waste - all sugars and carbohydrates," said Thomas Healey, the head of the industrial waste unit. "It was good food for the bugs."

Philadelphia's three plants produce about 175 dry tons of sludge a day, and all of it is composted at the sludge processing facility. Dry ton, however, is a misleading term; it refers to the weight of solids suspended in a liquid. Sludge is anything but dry. Straight from the settlement tanks, it is about 3 percent solid - very pourable. It solidifies somewhat while it spends a month in closed tanks, when the microbes continue to digest the waste. Afterward, it is wrung out in centrifuges and presses and becomes known as "dewatered sludge cake," which is about 20 percent solid. It has the consistency and the appearance of a fresh cow patty. A dry ton of sludge cake, including the water, actually weighs about five tons.

Sludge is William Toffey's business.

Two years ago, Toffey worked in the city planning office before taking on the monumental job as the Water Department's sludge utilization manager. When introduced to strangers, he does not hide the nature of his work. "I tell them I sell sludge," he said. "It generates all sorts of interesting conversations."

Selling sludge is nothing like selling encyclopedias. Toffey, 38, is equipped with an Ivy League education and bearing. He has a bachelor's degree in agronomy from Cornell University and a master's in environmental planning from the University of Pennsylvania. Slender and high-strung, he wears English-cut suits and wire-rim glasses. He uses such terms as "marketing utilization decisions." At other times, he uses plain English: "In order to maintain a consistent product, we screen out the solids that can't be squished or mished or pulled out."

With loping strides, Toffey marched around the 42-acre sludge-composting facility, the largest such operation in the United States. All around him were mounds of sludge cake and wood chips, smoldering from the 140-degree heat the compost generates. The heat kills most of the disease-causing bacteria and after seven weeks leaves the sludge looking at least something like earth.

Philadelphia produces several different sludge products, which it markets under the trade names of Mine Mix, Philly Mulch and Earthlife. The blends represent various mixtures of sludge and wood chips. "We didn't want to put all our sludge eggs in one basket," said Toffey. A plane approaching Philadelphia International Airport screamed overhead.

The original product - 15-year-old sludge that had been dredged from lagoons at the Southwest plant - was called Philorganic. The department gave it away in the late '70s. But as the department diversified its line, Philorganic was also used to refer to sludge composted with wood chips and then screened to achieve a fine loam. Confusion arose about Philorganic's true identity, so that name was discarded and the screened material was renamed Earthlife. "It is the finest, most value-added product we have to offer," said Toffey. Earthlife sells for about $12 a ton, if it's bought by the tractor-trailer load. Toffey said it is price-competitive with other soil conditioners.

About 25 percent of the city's sludge cake is applied straight to strip mines or farmland, just like manure. The rest is composted. A distributor sells the compost to landscapers and garden centers, through which it most often ends up beneath lawns or athletic fields. Despite assurances that little of the metallic content is transmitted to plants, the city maintains a conservative stand and recommends that the sludge not be applied to land where crops are grown for human consumption. The warning, Toffey acknowledged, is not a good marketing tool.

Sludge also has a short-lived odor when it is applied to the ground, an aroma that farmers regard as earthy. Suburbanites say it stinks. "It's not a popular program, especially when you get closer to suburban areas," said Toffey. "It's been very disappointing."

The city first started using the sludge mixtures to reclaim abandoned strip mines in Western Pennsylvania. Residents near the mines complained loudly about having to live with Philadelphia's waste. Because of the protests, fewer mines are being reclaimed today, even though studies of the sites showed no contamination, and plant life thrived on the once-barren land.

Because of the fear of sludge products, the city's program is struggling. As of January, the department had a four-month backlog of sludge products - about 120,000 wet tons of sludge cake and Mine Mix - stored at the processing facility.

Toffey views the sludge crisis as an irony of modern environmentalism: "As a nation, we've raised the fears in order to raise the environmental consciousness. But that fear has come around and bogged us down."

With New York and northern New Jersey cities on their way out of the ocean - Congress has banned sludge dumping starting in 1991 - there will be millions of tons of new sludge to deal with in the coming years. The disposal options are limited. Sludge can be incinerated or sent to a landfill, at considerable cost. The city once attempted to fuse sludge and incinerator ash into a vitreous roadbed material it called Ecorock, but the process was too expensive. Other processes to turn sludge into fuel pellets are considered experimental. And so the most environmentally conscientious - and cost effective - method remains composting.

Hence the city's focus on sludge. "If we can have a major success here," said Toffey, "I think it will have a lot of effect swaying people over to land application of sludge products." When the sludge-handling improvements are finished this year, people driving by on I-95 can gaze out on acre upon acre of composting sludge and feel proud for their small contribution to that great mound.

More likely, however, is that they'll see acres of concrete tanks filled with decomposing sewage. And, in the center of it all - hiding a complex of digesters, aerators, sludge thickeners and enough plumbing to reroute the Nile - rows of identical brick buildings. If you squint, they look like classrooms.

"We actually got some complaints," Marrazzo said, "from people who wondered how we allowed a junior college to get built in the middle of a sewage treatment plant."