Q5. What's green, 80 feet below ground and up to 12 feet above? (P12)
Why are there Strange Green Pipes growing from the ground?
Every once and awhile, there they are in front of you, on a street or beside a park – green pipes about 8 inches in diameter and 3 to 12 feet tall, rising out of the ground mysteriously. They don’t make a sound, and most people don’t think they can detect an odour, but do you ever wonder why they are there, and what they do?
The taller green stacks are oftentimes paired with shorter, candy cane–shaped ones, but eventually these green pipes all lead to one place. Those that are part of a ventilation stack for Toronto’s complex sewer system known as the Mid Toronto Interceptor (MTI), all end up at an underground chamber of sewage almost 100 feet underground.
Buried eighty to ninety feet, the MTI runs between High Park and the Ashbridges Bay Treatment Plant. But because the MTI is an enclosed sewer system, it requires street level venting for non-toxic methane gas that collects underground. Methane is a colourless, odourless, non toxic gas that is lighter than air, so it’s easy to collect and direct into the atmosphere.
It is a natural by-product of the decomposition of organic material by bacteria, in the absence of oxygen. Methane production may occur wherever there is decaying organic matter, for example in landfills, swamps, and even in organic soils.
Exposure to methane gas does not pose a direct health risk. However, this kind of passive venting system and monitoring for methane gas is a safe and necessary way that cities today control underground gases. Such gases collect somewhere before they leak into our air or worse still, accumulate and possibly cause bigger problems.
The green stacks connected to the MTI are fitted with a series of carbon filters, permit the circulation of fresh air into the system and allow for fetid air to exit. The filters are changed regularly.
Built in the late sixties and early seventies, the MTI was constructed as a further means to prevent untreated sewage from ending up in Lake Ontario. The MTI collects sewer flows from across Toronto, including from local municipal combined sewers, flows diverted from two other interceptors (High Level Interceptor HLI - and Low Level Interceptor LLI) as well as sludge from the Humber Treatment Plant. The MTI drains to the Ashbridges Bay
Treatment Plant Flows are controlled by sluice gates (which act like valves) throughout the system. Gates are located in underground chambers along the MTI and along branches connecting to the HLI and LLI. It gets confusing because there are additional green pipes, which also aid in underground air circulation and the ventilation of non toxic methane gas that is being slowly released from underground landfill sites – where there is decaying organic material.
In some US cities, you can find examples of “passive venting” where a city has developed over a former farm. In Toronto, you can find it along the edges of various parks or green spaces that have been created on a former landfill site or filled-in ravine. Riverdale Park, for example, was built on a landfill site last used in the 1920s. Many years later, around 1978, a passive venting system – with those green pipes – was installed along the eastern edge to safely vent the continuous methane gases.
There are an estimated 15 green “pipe” stacks connected to the MTI in Toronto, in addition to the pipes placed along various parklands in the city. Beaches Living invites anyone to go out and count them!
Reference: City of Toronto,
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