How to use condition assessment data to improve force main and lift station design and operation

Purpose: Lift stations and force mains are essential to any wastewater collection system. Lift stations and associated force mains operate in a highly corrosive environment, typically subjected to widely fluctuating flows and pressure surges that affect force main integrity. Lift station and force main design seeks to avoid common damage modes, including H2S-induced corrosion and solids accumulation, but rarely benefits from the physical assessment of the installed and operating system to evaluate design robustness. The current study outlines the design and operational enhancements of lift stations and force mains as identified during the condition assessment of Lift Station 43 (LS 43) in the City of Phoenix (City). The City owns and operates 28 lift stations and approximately 72 miles of force mains, including approximately 62 miles of 12-inch and larger force mains. A comprehensive condition assessment of the LS 43 force main was performed as part of the city's ongoing efforts to understand and minimize the risk associated with buried infrastructure. The LS 43 force main conveys wastewater approximately 1.5 miles from LS 43 on 75th Avenue south of Southern Avenue along 75th Avenue to Broadway. The force main has three barrels constructed of 24-inch diameter ductile iron pipe referred to as Barrels 1, 2, and 3 and experienced two breaks associated with air accumulation. A comprehensive assessment was completed, including hydraulic modeling, CCTV, and electromagnetic (EM) inspection. The study's results can be adopted to improve the service life of not just LS 43 but any future lift station and force main. Benefits: The LS 43 assessment identified best practices to improve lift station and force main hydraulics and service life. By utilizing hydraulic modeling with different pump states, the assessment identified the areas of negative pressure and gas buildup. The results of hydraulic modeling were confirmed via CCTV assessment, which showed the presence of a miscellaneous waterline in the force main. This revealed that incorrect ARV placement obstructed the efficient functioning of the lift station and force main. This presentation will benefit that industry by outlining the factors contributing to air accumulation and remedial measures to address the concern. The presentation will also discuss design modifications such as the location and elevation of air release valves (ARVs) and the type of ARVs the industry should adopt to improve the life of the force mains. Additionally, EM assessment determined internal corrosion below the springline caused by solids accumulation, an infrequently considered damage mode for ductile iron pipe in force main service. Suspended solids in wastewater can settle in the force main and develop areas of microbially induced concrete corrosion (MICC). Efforts to allow for maximum future capacity in the initial design had resulted in low velocities and solids accumulation. Cleaning tools are regularly employed to dislodge accumulated solids, but the lift station and force main designs rarely allow for easy launch and retrieval of a cleaning tool. Additionally, there is no clear guideline on the cleaning frequency to improve the system's service life and reduce operational costs. The study aims to discuss the velocities needed to prevent solid accumulation, as well as the frequency and essential design modifications necessary to support regular cleaning. The recommendations presented in this study can be applied universally to any lift station and help the industry develop designs with cleaning and maintenance requirements in mind. Status of Completion: The investigation work to address ongoing air accumulation and internal corrosion issues is 100% complete. Results indicated the flow velocities and cleaning methodologies the City should utilize to prevent further internal corrosion and location for ARVs installation to prevent air accumulation. The City is in the process of procuring a consultant to design the recommended improvements. Conclusion: The City will now be able to allocate necessary funds for regular force main cleaning, identify locations of concerns, conduct regular maintenance, improve lift station efficiency, and improve service life. In conclusion, the study has identified best practices to improve lift station and force main hydraulics and service life. These best practices can be used to improve future force main designs.