When Friendswood, Texas, a suburban area of Houston, began replacing or rehabilitating several large sanitary sewer lift stations, there seemed to be an obvious decision for the Crazyhorse lift station. This lift station is the largest in the city’s collection system and directly feeds its 9.25 million gallons per day (mgd) wastewater treatment plant. As the lift station aged, it became apparent that a solution was necessary. The lift station’s electrical control building, wet well, overhead crane and emergency power generator were all at the end of their useful life. The facility’s age, combined with unacceptable backups during heavy rainfall events, pushed the city into action.
Image 1. Existing wet well before design at Crazyhorse lift station (Images courtesy of Lockwood, Andrews & Newnam)Preliminary Design
The original intent was to use an adjacent empty property, owned by the city, to construct a new lift station. Multiple factors led to the decision to replace the lift station with a new one. These included:
- the importance and work of the lift station
- the city’s existing ownership of ample space for a new structure
- potentially deficient structural condition of the existing wet well
- significantly worn pumps with no consistency in size
- extreme corrosion of the overhead lift crane
- extreme corrosion of the existing electrical control building
- insufficient and unreliable emergency power generator
Per the original intent, the proposed lift station would tie into the discharge force main of the existing lift station. The adjacent property would also house a new electrical control building and emergency power generator.
The city contracted a national engineering firm to design the replacement lift station. The engineering firm began preliminary design of the proposed lift station and pumps with a hydraulic analysis of the existing force main, approximately 1.5 miles of 20-inch pipe. This hydraulic analysis included hydraulic modeling of the system’s response to various flow rates. The flow rates modeled were determined based on historical data from the existing lift station. The peak recorded flow rates occurred in late August and early September 2017 during Hurricane Harvey. During the hydraulic modeling of the force main, the engineer found the force main to be significantly oversized, even for the recorded peak flow rates.
Image 2. Depiction of proposed wet well with improvementsWhen the system hydraulics were analyzed, the velocities within the 20-inch force main were below the minimum requirements mandated by the Texas Commission on Environmental Quality (TCEQ). This discovery prompted the engineer to recommend a few alternatives to the city. These included increasing the storage volume of the proposed lift station to allow for higher discharge flow rates and velocities, slip lining the existing force main with a smaller diameter pipe and installing a new smaller diameter force main adjacent to the existing force main in conjunction with the proposed lift station.
The additional cost to modify or replace the force main was too great for the existing project’s budget. As such, the city tasked the design engineering firm to find a feasible solution, while still maintaining the original purpose of providing a more reliable lift station. After researching options and contacting the TCEQ, the engineer made a proposal that proved acceptable to the city.
An Altered Approach
The design engineer proposed a different approach—rehabilitating the existing wet well instead of replacing it with a new structure (the idea had been previously overlooked due to potential difficulties). This solution allowed the city to use the existing force main, because it would be “grandfathered” into previous TCEQ standards and, as such, would not require meeting the current minimum velocity standards. After the city accepted this approach and decided to rehabilitate the existing wet well, the engineer proceeded with a design that would alleviate the facility’s deficiencies.
Image 3. Lift station in city of FriendswoodWith the help of the design engineer, the city contracted a structural engineering firm based out of Skokie, Illinois, to provide a full evaluation of the existing structure. This evaluation included both destructive testing, such as concrete core sampling, and nondestructive testing, such as ground penetrating radar. The evaluation determined that the structure was salvageable, but extensive work and additional testing would be required on the existing structure before a satisfactory design could be attained.
The design engineer then proposed constructing structural walls within the existing wet well structure. While this would achieve the necessary design life, it would also reduce the storage capacity of the wet well. Once the required wall thickness was determined, the engineer reevaluated the hydraulics of the wet well based on applicable Hydraulic Institute and TCEQ standards.
To identify the proper pump configuration based on the resized wet well, the engineer worked closely with a local pump vendor to select the proper pump. The proposed lift station would use four of these pumps, with three duty and one standby.
Each pump was equipped with a variable frequency drive (VFD) and a firm capacity of 3,000 gallons per minute (gpm). The VFDs allows the pump station to operate using flow pacing, which enables the lift station to match the influent flow and maintain a constant volume in the wet well to accommodate the decreased size due to the additional wall thickness.
Per the city’s request, the lift station is also designed to operate all four pumps to accommodate increased flow during extreme weather events. This addition required increases in the size of the electrical equipment and emergency generator.
Conclusion
After starting as a straightforward project, the Crazyhorse lift station replacement quickly became a challenging job in need of an inventive, economical solution. By spending additional time and money upfront to investigate additional options, the city, with the help of the design engineer, was able to save an estimated $1.4 million in the design and construction of a new wet well and force main.