Payne is concerned about this street grime and air pollution because it’s all destined to become water pollution when the snow melts in the spring. According to Payne, meltwater from snowbanks and the city’s various glaciers has become one of the city’s most toxic — and least regulated — threats to Casco Bay’s ecosystems.
Of course, all of this pollution still exists when it doesn’t snow. Rainstorms also transport air pollution and street grime into the bay, but they do so invisibly, through Portland’s underground sewers.
Suppress your gag reflex for a moment and imagine a few million gallons of bathwater, feces, and kitchen garbage mixed in with all the rest of the glaciers’ frozen filth. If you then imagine these hypothetical glaciers washing away in a rainstorm and floating around Casco Bay like enormous toilet icebergs, you’ll begin to have an idea of Portland’s combined-sewer overflow problems.
Like many older cities, Portland has a combined sewer system: stormwater runoff from the city’s streets goes into the same pipes that handle the more polluted sewage from all of the indoor drains in Portland’s homes and businesses. During dry weather, this works well enough: everything that goes down your drain gets sent to the sewage-treatment plant on the Eastern Prom.
The treatment plant is designed to handle a peak load of 80 million gallons of sewage per day, and Portland residents and businesses typically generate about 20 million gallons of sewage per day, according to fact sheets from the Portland Water District, which operates the plant.
But a rainstorm can quickly overwhelm the sewers’ capacity. An inch of rain falling on the entire three square miles of the Portland peninsula can generate about 50 million gallons of rainwater. If that water falls on city parks or forests, it will slowly sink into the soil. But much of that rain falls on rooftops, streets, and parking lots, from where it takes a short trip to the nearest storm drain and into the city’s combined sewers. Along its way, it picks up the same sort of street grime and garbage that’s currently frozen inside the glaciers.
Millions of gallons of raw residential and commercial sewage mixing with tens of millions of gallons of stormwater runoff can quickly overtax the capacity of the sewer pipes. When this happens, the sewers are designed to overflow the toxic mixture of excrement and runoff into the nearest body of water — which could be Capisic Brook, Back Cove, or under the wharves on the waterfront — before the sewage begins to back up into bathtubs and streets.These “combined-sewer overflow” sites act as relief valves for Portland’s sewers. They’re marked by small green signs (and sometimes, by foul smells as well) at various points along Commercial Street, Baxter Boulevard, and the Fore River.
Even when sewage manages to make it to the East End, the treatment plant is sometimes forced to dump semi-treated wastewater into the Bay if the pipes are delivering more than the plant can handle. These overflows, which usually occur during or after sustained periods of wet weather, can sometimes cause the city to shut down the East End Beach due to dangerous levels of fecal coliform bacteria (see “What If ...” by Alex Irvine, January 9, 2004).