Many United States cities are about to embark on the design of combined sewer overflow (CSO) and sanitary sewer overflow (SSO) control projects to comply with their consent orders / decrees. The capital cost of these projects is estimated in hundreds of millions of dollars. Both conventional gray and green wet weather solutions are generally required but many communities are interested in a “green-first” approach that requires maximizing the full potential of green solutions before considering the gray solutions.
A green-first approach requires understanding the strengths and limitations of both the gray and green technologies. Comparing the life cycle cost (capital + O&M) and hydraulic performance (overflow reduction) of green and gray solutions is essential for implementing a green-first approach. Green stormwater infrastructure is not a panacea! Recent studies have shown that despite the growing interest in green solutions, information is limited on its design, implementation, operation and performance. The approach is still relatively new and, although it is being widely adopted, there are few studies in the peer-reviewed literature that report and contrast the performance of different green infrastructure technologies. For example, using decades of gray infrastructure education and experience, and a vast library of handbooks, manuals, and computational tools engineers can quickly design a detention basin to reduce the CSO volume at an outfall by 85%. They can confidently estimate the construction cost and prepare a maintenance manual. Unfortunately, designing green solutions to meet the same target is more challenging due to shortage of experience, cost estimating databases, and proven design tools. For best results, green solutions must be planned and designed with the same level of effort as their gray counterparts who have received most of the planning attention in most wet weather projects.
This presentation will discuss the green vs. gray cost and performance comparison approaches and tools such as LEED, Envision, and AutoCASE. Cost analysis tools and models will be summarized. A step-by-step methodology will be identified to compare the gray and green solutions using a triple bottom line approach, viz-a-viz social, economic, and environmental benefits. Examples and lessons learned from the recent projects and studies will be included.