How to Use Condition Assessment Data to Improve Force Main and Lift Station Design and Operation

Lift stations and force mains are essential in wastewater collections and are required to operate in a highly corrosive environment and are typically subjected to widely fluctuating flows and pressure surges that affect force main integrity. Lift station and force main design seek to avoid common damage modes, including H2S-induced corrosion and solids accumulation, but rarely benefit from physical assessment of the installed and operating system to evaluate design robustness. Collection and evaluation of physical condition data for the City of Phoenix’s Lift Station 43 (LS 43) force main identified design and operational best practices that can be adopted to improve the service life of any force main. The LS 43 force main is constructed of three 24-in. ductile iron pipes that experienced two breaks associated with air accumulation. A comprehensive assessment was completed, including hydraulic modeling, closed-circuit television (CCTV), and remote field electromagnetic (RFEM) inspection. Hydraulic modeling utilizing different pump states identified areas of negative pressure and gas buildup. The results of hydraulic modeling were confirmed via CCTV assessment, which showed staining on the inside of the force main that indicates the presence of an air pocket. This permanent staining is called a Miscellaneous Water Mark per National Association of Sewer Service Companies (NASSCO) Pipeline Assessment Certification Protocol (PACP) guidelines. The presence of the air pocket indicated that the air release valves (ARVs) were not functioning effectively. This was also confirmed via hydraulic modeling which determined that incorrect placement of ARV caused air to accumulate in the force main. The study identified the factors contributing to air accumulation and remedial measures to address the concern. Additionally, RFEM assessment determined that internal corrosion below the springline was caused by solid accumulation, an infrequently considered damage mode for ductile iron pipe (DIP) 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 maximum future capacity in the initial design resulted in low velocities and solids accumulation. Cleaning tools can dislodge the accumulated solids but lift station and force main designs rarely allow for easy launch and retrieval of a cleaning tool. Furthermore, no clear guideline exists on cleaning frequency. The study identified the necessary velocities required to prevent solid accumulation and the frequency and essential design modifications necessary to support regular cleaning. In conclusion, the study has identified best practices to improve lift station and force main hydraulics and service life.