A “Swiss Army Knife” Approach: Condition Based Assessment of a South Florida Sewage Force Main

The City of Lake Worth and Palm Beach County (FL) share operating ownership of a 14.1 mile wastewater regional force main (RFM) serving six additional municipalities and over 475,000 residents. Installed in 1972–79, the FM extends from the Master Pump Station within Lake Worth 4.45 miles as 36” pre-stressed concrete cylinder pipe (PCCP) and from the city limits 9.65 miles into the county as 42” – 54” ductile iron pipe (DIP) terminating at the East Central Regional Water Reclamation Facility (ECRWRF) within the City of West Palm Beach. Flow is characterized primarily as domestic and commercial with an average daily flow (ADF) of 20 MGD; pressure averages 20–25 psig with periodic peaks of about 45 psig. Because the RFM is manifolded it serves as the primary conduit of wastewater to the treatment plant – there is are no redundant or bypass lines to resort to if the line fails, it cannot be taken out of services for more than a few hours, and incoming flow would be forced to spill to an ocean outfall. Recent failures of the DIP portion resulting in spills and regulatory action from FL DEP heightened the awareness of the served jurisdictions of the potential for a catastrophic failure and led to an investigation of the integrity of the entire line. Defects that included deterioration of the pipe crown and at joints were found in 48” pipe along with a failure in an adjoining 16” line which ultimately ties into a 30” force main that connects with the RFM. Using state-of-the-art assessment technologies including ultrasonic and broadband electro-magnetic (BEM) scanning, measurements were made of the existing DIP thickness including a profile of the wall at selected locations. Similarly, acoustical emission technology (AET) was applied to the PCCP segments to “listen” to active distress signals from tensioning wires stretching or breaking within the pipe wall. Using historical operating and maintenance records and a field study of air release valves, a determination was made regarding useful life of the pipeline, likely failure points, remaining wall thickness of the DIP segments, and what areas should be rehabilitated or replaced. A phased program of “renewal” was then developed for the RFM to be implemented by Lake Worth and the Palm Beach County Water Utilities Department that reduced forecasted potential replacements costs from 24 million to 1.9 million.