Carrying waste water, sewage and everything in between, more than 11,000 kilometres of sewers lie beneath Toronto’s surface, a snaking network that shuttles millions of litres of liquids to treatment plants, storage tanks or straight out to Lake Ontario every day.

Here’s a look at the system essential to keeping the city sanitary and (mostly) dry, but rarely seen by the millions it services.

A tale of two sewers

Toronto has two kinds of sewers — combined and separated. In combined sewers, an archaic design that makes up about 23 per cent of Toronto’s system, household waste and whatever comes in through storm drains travel in the same pipes to one of the city’s four treatment plants. In separated sewers, the contents never mix; one series of pipes, which can range from 15 centimetres to three metres in diameter, carry household sanitary waste to treatment plants, while another sends storm drain contents straight out to local streams, rivers and Lake Ontario.

“We’re not different than some other North American cities . . . even other cities kind of like Ottawa and Hamilton where you have some older cores,” explained Toronto Water general manager Lou Di Gironimo, who’s been in charge of the city’s drinking, sewage and storm water for 11 years now.

Most combined sewers are found in Toronto’s old core, where development dates back to the 1800s. Separated sewers became standard in the ’50s and ’60s, meaning the city’s younger suburbs are outfitted with the newer design.

For the most part, sewers tilt downhill so their contents flow via gravity, but, due to hills and dips in Toronto’s geography, that isn’t always possible. That’s where pumping stations come in — 84 of them are scattered around the city to get sewer contents where they need to go.

The flooding challenge

Combined sewers can be a headache when heavy storms hit — the extra volume can overwhelm pipes and cause floods. Toronto Water has built reliefs in the form of storage tanks and holding ponds, but, like separated sewers, these are more likely to be in newer and more spacious regions like Scarborough (this is why North York and Etobicoke are more prone to flooding — no ponds).

Sometimes, those reliefs still aren’t enough.

Di Gironimo likes to use a bathtub comparison.

“If a bathtub fills up, you’ve got the little relief thing where it stops it from spilling over, right? So that’s if it’s raining normally and not too heavy, you get a little bit that drains over the side, the tub hold most of it,” he said.

“The challenge that we’ve been seeing lately is, as we get heavy, heavy rainfall, it’s essentially like using a fire hose to fill the bathtub.”

When the system gets overwhelmed, drainpipes still discharge combined sewers directly into local bodies of water; most of them, Di Gironimo said, are along the lower Don River, the Humber Creek and the Inner Harbour.

“That’s not great for the environment and we do have large plans in place to deal with that, but they’re long-term,” Di Gironimo said — think 25 years, and to the tune of $2-3 billion.

Not that nothing’s been done yet — enter the Western Beaches Tunnel.

The Tunnel

Sunlight flits through the dust-speckled, heavy air from the metal grates above, providing just enough light to illuminate the concrete walls of the 24-metre-wide, 46-metre-deep concrete shaft — the last chamber in the trio making up the Western Beaches Tunnel.

Completed in 2002 at a price tag of roughly $80 million, the structure consists of three underground concrete shafts, each deeper than the one before, connected by a tunnel that starts around High Park and winds its way down to just beside Ontario Place.

During heavy downpours, the shafts serve as holding tanks for combined sewers — about 10 per cent sewage and 90 per cent rain, the mix is held until solids settle to the bottom. The remaining water is UV-treated to kill off bacteria and released into Lake Ontario while the sludge is pumped to a treatment plant.

The last shaft, when empty, also serves as a fascinating study in things that aren’t supposed to go down drains but do anyway — plastic tampon applicators, condoms and wet wipes litter a rickety metal staircase to the bottom; a wool sweater that might have been white at some point is draped on a hand rail, and a still-intact parking ticket lays crumpled on a landing.

Although 2016 was an unusually dry year, in general, heavy rainfalls are happening more frequently, said Bill Shea, Toronto Water’s director of district operations.

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The tunnel, he explained, was built with a capacity for a two-year storm — 85 million litres — but in the past five years, the city’s already seen two or three 50-year storms. And a heavy rain, Shea said, can fill up the shafts in as little as 20 minutes.

Counterfeit cash and ‘fatbergs’

Sewer contents are made up of more than water and waste. You won’t find alligators or irradiated testudines, but you will come across “weird insects” in deep trunk sewers like “giant albino centipedes,” Shea said. It’s also not uncommon to find objects that, at first glance, should be too big to fit into any entryways, like 2” x 12” pieces of lumber or foot-long Tonka toy trucks.

“I’ve been getting into Breaking Bad lately, but that episode where they dissolve a body in acid and flush it down the toilet?” Shea said. “All I could think was, ‘No! We’d find all of that!’”

Pump stations can also be another source of wonder — at one station, a crew once found a flotilla of what turned out to be counterfeit bills float through.

“We figured some guy heard the cops knocking on his door and just started desperately flushing all his fake cash down the toilet,” Shea said with a chuckle.

After some prodding, Shea confirmed that the Rogers Centre station catches an unusually high number of condoms (he declined to offer a theory on why), and, rather reluctantly, also recalled that at an open area in Scarborough, crews once stumbled across an apparent animal sacrifice when they found five goat heads impaled on posts and surrounded by flowers (but only four goat bodies).

But it’s a few everyday items that wreak the most havoc — fat, oil, grease and wet wipes. Although they go down the drain as liquids, fat, oil and grease harden when they cool in pipes and sewers, creating blockages known as “fatbergs.” And although wet wipes are “flushable” in the sense that they go down the toilet, they don’t break down and end up clogging pipes.

People flushing medications or dumping liquids like paint down storm drains is another complication, Di Gironimo said.

“The organic processes, we can deal with, that’s why we have our treatment plants,” he said. “But when you put things like pharmaceuticals, chemicals or things we can’t treat for down the drain, well now we have a problem because it’s getting into the environment. Our plants aren’t designed to deal with that.”

Breaks and backups

Toronto Water deals with around 1,600 water main breaks a year, many of them in the colder months when the ground surrounding pipes begins to shift.

Sometimes, the fix can be as simple as patching a crack; other times, crews may have to shut off water to a large area in order to cut out a badly damaged pipe and replace it.

The introduction of PVC piping, Di Gironimo said, has helped with reducing breaks; unlike steel, PVC is flexible and handles shifts to the dirt, and also doesn’t corrode.

Backups can be caused by a few things — pump stations may get jammed by dental floss, or pipes, clogged by the aforementioned fatbergs and wipes. When the latter happens, crews bring in a truck armed with a high-pressure water hose and vacuum to break through and suck up the blockage; the same truck also does preventative cleaning in areas where blockages are known to happen, like sewers “downstream” from restaurants.

The takeaway

“It’s a connected system, at the end of the day,” Di Gironimo said. “They are not separate. What you drink from the tap, what you put down, it’s important to remember to protect both of them . . . we have to be vigilant and continually invest in them, not just for ourselves, but for future generations.”

Correction- Dec. 28, 2016: This article was edited from a previous version that missstated the surname of Bill Shea.