Not so fast. The trend in stormwater treatment best management practices (BMPs) may be headed toward passive treatments such as rain gardens, but a study just released by the Minnesota chapter of the American Public Works Association questions the cost-effectiveness of those types of BMPs.

Passive stormwater treatment is gaining traction as a BMP, but if you're looking for cost-efficiency, it's worthwhile to stand back, take a look at your options, and analyze all the BMPs you have available to you, says Michael Eastling, public works director for the city of Richfield, Minn. For the past year the Environmental Committee of the Minnesota chapter has partnered with consulting firm WSB and Associates to evaluate the life-cycle costs and the benefits of six common BMPs:

  • National Urban Runoff Program basins: For an estimated 8,000-cubic-yard pond, the capital cost was estimated at $336,000 with annual operations and maintenance costs at $38,840. This type of BMP typically removes just half of the phosphorous, but is cost-effective.
  • Rain gardens: Used to treat stormwater runoff from small tributary drainage areas, they consist of shallow vegetated depressions that capture runoff and allow it to soak into ground, so annual maintenance costs are only reflected in a fully vegetated area. The benefits are increased reduction of phosphorous and total suspended solids (TSS). With an estimated capital cost of $210,000 and annual costs of $15,250 in this example, it would cost about $8,715/pound to remove phosphorous (rain gardens remove about 70% of the total amount of phosphorous).
  • Spray irrigation/water reuse systems: These BMPs work well because they return solubles to the ground. The practice removes stormwater runoff volume as well as 100% of both phosphorous and TSS. In this example, the study estimated that a 1-acre area with an electric pump on area (such as a golf course) would cost about $10,000 in capital and about $3,150 in annual costs. It removes 100% of the phosphorous, and at $533 spent per pound of phosphorous removed, it is cost-effective, "plus you're irrigating something," Willenbring says.
  • Wastewater treatment plants: Since most plants have infrastructure that is currently in place and operate 24 hours a day, there are no capital costs, and annual costs are only about $10,430, according to the study's estimates. The systems offer 95% removal efficiency of phosphorous and cost just $220/pound of phosphorous removed.
  • Onsite systems: They require pumping water from a water body, treating it, and returning it to that water body to improve the basin's overall quality. Treatment can achieve greater removal efficiencies and significantly higher water quality: Capital costs for a 300-gallon/minute plant are $310,000, and annual operations and maintenance costs are about $15,700 in this example. With 95% removal efficiency, it would cost just $280/pound to remove phosphorous.
  • Underground systems: Designed to remove sediment from stormwater, these BMPs typically only remove about 40% of the phosphorous. On a 500-acre drainage area with $1.2 million in capital costs, it would cost about $830/pound to remove phosphorous.

"The cheaper systems are the active systems, those that run even when it's not raining," says Pete Willenbring of WSB and Associates. "It's cost-effective to operate the ones that run all the time. The cost-effectiveness of a BMP is determined by the pollutant loading that is directed to it."The utilization of active systems (irrigation, water reuse, wastewater treatment plants) is cost-effective and can remove phosphorous to low levels, he says.- Michael Fielding

Session: Results of Cost-Benefit Analysis of Various Stormwater Best Management Practices
Michael John Eastling
Public Works Director, city of Richfield, Minn.
Pete Willenbring
WSB and Associates Inc., Minneapolis
Sun., Aug. 18, 2008
3-3:50 p.m.