The 1950s-era concrete pipe sanitary sewer system of Seattle’s Broadview neighborhood has experienced repeated wet weather sanitary sewer overflows from both manholes and into basements during extreme storms. Over time, the pipes have deteriorated, allowing excessive amounts of infiltration through separated joints, fractures, and other defects.
As a result, Seattle Public Utilities (SPU) undertook a pilot project to address two poorly understood aspects of sewer rehabilitation:
- An intensive asset management-based process known as a business case evaluation.
- The cost effectiveness of an innovative sewer rehabilitation process on a large targeted area.
Simulations revealed that the project reduced the peak hour flow. SPU intends to continue using this technology in select locations.
The project earned a 2013 Water Environment Research Foundation (WERF) Award for Excellence in Innovation. The award recognizes improvements to wastewater and stormwater collection, storage or treatment operations, facilities, or processes by applying WERF research (Project No. INFR5R11, Flood Grouting for Infiltration Reduction on Private Side Sewers).
As collection systems age and deteriorate, groundwater infiltration into both mainlines and laterals increases. Municipalities seeking to reduce wet weather flows have been limited to structural repairs or replacement. Pipes are often in satisfactory condition, but only structural rehabilitation methods are available; grouting isolated points has been conducted with varying success.
Utility managers decided to investigate a nonstructural solution: flood grouting, a two-step trenchless process that seals all parts of a gravity pipe section between two manholes (see sidebar at right).
Four options—flood grouting, joint grouting, pipe bursting, and cured-in-place pipe (CIPP) lining—were evaluated to identify a preferred alternative and validate the technology. The higher cost and disruptions eliminated open cut pipe replacement from being included.
Estimates for each method were developed and then compared to the benefits of reduced infiltration: fewer insurance claims, lower storage costs at a regional wet weather treatment facility, lower conveyance and treatment costs, the cost of installing cleanouts on side sewers, and inspecting privately held sewer assets.
The evaluation identified flood grouting using the Sanipor process developed in Germany as having the greatest benefit-to-cost ratio.
To maximize infiltration reduction, it was important to include the maximum length of side sewers possible. For the first time, the utility had to obtain permission to enter and work on private property.
SPU conducted an extensive education. This included holding several community meetings, mailing informational fliers, developing a website, following up with telephone calls, and holding an onsite meeting with the contractor to show the equipment expected to be used.
This outreach resulted in a 95% signup rate among affected residents. The remaining 5% of houses had a cleanout installed on the side sewers within SPU’s right-of-way and the portion of the sewer within the right-of-way was rehabilitated.
Sanipor was applied to a 31-acre residential sub-basin. All manholes and mainlines were sealed, but only 30% of total side sewer length was addressed for reasons including multiple side sewer branches on each house, landscaping, elevation differences, and homeowner approval of the cleanout location. In all, about 56% of the entire sewer basin was sealed.
Based on measured exfiltration rates of the flood grouting chemicals, the sections that were sealed had a 99% improvement in their exfiltration rates. The average total construction cost per foot of sewer sealed was $77.
SPU maintains a network of flow meters and rain gauges in the Broadview area that was augmented with additional flow meters to capture before-and-after information. The recorded depth, velocity, and flow rate was used to calibrate two sewer models—one for before the project and one for after the project. Long-term simulations show the project reduced the peak hour flow rate coming out of the pilot basin by 41% and reduced storm volumes by 66%.
Managers intend to continue flood-grouting in locations where infiltration has been determined to contribute significantly to wet weather flow issues.
Carita Parks is communications manager for the Water Environment Research Foundation, an independent nonprofit organization that focuses on wastewater and stormwater issues. Visit www.werf.org; email [email protected]. This article originally appeared in the Spring 2013 issue of Progress, a quarterly foundation publication. Reprinted with permission.
CONTINUED: "What is flood Grouting?" and "Cities test business case software"