Green infrastructure to combat sewage overflow in Philadelphia
How one city is taking an innovative approach to managing polluted stormwater from entering waterways.
Monday, August 15, 2011 - 15:36
Photo: Dave Tavani
In about 772 cities and towns across the country, raw sewage, industrial waste and other toxins pour into streams and rivers during heavy rainstorms.
While this may seem shocking given laws like the Clean Water Act, this kind of pollution is a regular occurrence.
However, cities like Philadelphia are rising to the challenge of controlling stormwater by creating green infrastructure that will not only help stem the tide of pollution, but also improve the aesthetic value of the city.
"This is the first combined sewer overflow program that really relies on green infrastructure for [stormwater] reductions," said Glen Abrams, watersheds planning manager for the Philadelphia Water Department. "Many cities are looking toward Philadelphia and seeing how successful this program will be. We really are out front with the scale that we're proposing."
Combined sewer systems are essentially designed to collect rainwater, sewage and industrial wastewater all in the same pipes. In Philadelphia in the 1800s, city leaders encouraged the burying of creeks and streams as a form of water management. So now, during a heavy rainstorm, wastewater treatment plants become overwhelmed and don't have the capacity to clean the increased volume of water.
The combined sewer systems have outflows that dump excess untreated water into rivers and streams.
One way of stopping the dirty discharge would be to construct and update the city's entire water infrastructure.
Abrams said the city looked into that possibility, but such a plan would require a $16 billion investment and would be cost-prohibitive.
Such a system would require the building of tunnels to house the combined sewage and rainwater until the water treatment plant could again handle the flow. Then, all the water would have to be pumped back up into the sewage system before flowing to the treatment plant for cleaning. Abrams said such a plan would not only be costly, but would require an incredible amount of energy to pump the water back into the system.
"It really doesn't help to solve the water quality problems," said Abrams. "Rainwater becomes polluted by picking up anything that's on the land. Unless you have practices in place to treat that water before it's released into the rivers and streams, it's polluted."
Philadelphia city planners have instead created a three-pronged approach to dealing with the combined sewer overflow, or CSO, problem involving green infrastructure, stricter stormwater regulations for developers, and a new billing process that encourages greening of properties.
The green infrastructure includes a variety of tools including the construction of stormwater tree trenches, rain gardens, stormwater wetlands, downspout planters and stormwater bumpouts.
"It's about trying to replicate the natural water cycle," Abrams said of the green infrastructure.
Also, any construction or redevelopment in the city needs to comply with stormwater regulations that require builders to manage the first inch of rain.
"Roughly 1 percent of the city is redeveloped on an annual basis," said Abrams. "Over time, we'll see a lot of benefit to our sewer system in terms of reduced flows from wet weather events just because of the redevelopment process in the city."
New billing system
The water department has also recently started a new billing system that includes a charge for stormwater management. While the charge isn't much for the average city resident, businesses are essentially charged based on a property's permeability — the more impervious the property, the higher the bill.
Abrams said that the city did a "triple bottom line" analysis with regard to the social, environmental and economic benefits of taking a green infrastructure approach and found a huge benefit over time.
He said that projecting out 40 years from implementation, for every $1 invested, the city and its residents will see a $2 benefit in terms of improved neighborhoods, air quality and property values.
The hope is that all these changes will encourage others to think green.
"As people need to make changes to their properties," Abrams said, "they'll begin to think about and be encouraged to manage the rainfall."