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Philiadelphia Water Department (PWD) is striving to re-invent the traditional Combined Sewer Overflow (CSO) control program. Instead of focusing on grey infrastructure (increased conveyance and storage capacity followed by treatment) with source controls added to further reduce CSOs, PWD is exploring the feasibility of making green infrastructure the primary contol measure, with grey infrastructure added only where required. That a green approach can be theoretically effective has been shown by extensive model simulations. Simulations show that the uncontrolled runoff, much of which becomes CSOs, can be reduced by more than 90% if all impervious surfaces are retrofitted. This would, however, require a complete retrofit of streets, parking lots and roof connections over a 20 to 30 year period. Although a great deal of this conversion could be accomplished as the City redevelops and renovates through enforcement of the newly instituted storm water regulations, it would also require a significant “economic push” through planned retrofits, aggressive storm water rates tied to impervious cover, and other incentives. This suggests that a careful weighing of costs associated with green infrastructure vs “grey infrastructure” is needed before the appropriate balance between green and grey is found.

Although initial cost estimates suggest that green infrastructure may result in slightly higher unit costs per gallon storm water captured than the lowest cost grey infrastructure alternative (current planning level estimates suggest an average cost of approximately 4 – 6 per gallon), this does not include the numerous other benefits associated with green infrastructure. With the considerable benefits associated with the green approach to storm water and CSO controls, Philadelphia is looking at setting a preliminary target for applying green storm water controls of between 50% and 60% of all impervious cover in the city. If this number can be achieved within a 20 to 30 year time frame, many of the costs required to increase storm water conveyance and storage facilities can be largely avoided and the city's urban environment will benefit.
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Document Type: Research Article

Publication date: 2008-01-01

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