August 3, 2016The Cape Fear Public Utility Authority (CFPUA) manages the public water distribution and wastewater collection systems for the City of Wilmington, N.C., and surrounding New Hanover County. To handle a service area of 328 sq miles and a population of approximately 200,000, CFPUA owns, operates and maintains three wastewater treatment plants and more than 800 miles of sewer line.
In 2008, CFPUA came to an agreement with the City of Wilmington, New Hanover County and the U.S. Environmental Protection Agency (USEPA) to minimize sanitary sewer overflows (SSOs). Under the agreement, CFPUA was required to complete more than $40 million of specific capital improvement projects within a certain timeframe to address their aging infrastructure.
One project that CFPUA engaged in was to replace a section of force main sewer line that was susceptible to sanitary sewer overflows. The project was identified as the Northeast Interceptor Rehabilitation Phase 2 Project (NEI Rehab), which included the replacement of iron force main between the Bradley Creek and Hewlett’s Creek Pump Stations. This section was installed in 1977 and inspections showed it had eroded from corrosive soils and sewer gases, causing extensive deterioration that could potentially lead to failure. It needed to be addressed immediately.
Bringing in the Best
CFPUA hired State Utility Contractors (SUC) out of Monroe, N.C., to manage and execute the NEI Rehab Project, which ended up being a complex and multifaceted project that ultimately won SUC the prestigious CAGC Pinnacle Award. (Presented annually by the Carolinas Association of General Contractors, the Pinnacle Award recognizes the State’s best construction projects.)
The rehab project called for the replacement of approximately 5,140 lf of 20-in. DIP force main, which would require a 100 percent redundant sewer bypass system – capable of handling peak flows of as much as 13.5 million gallons per day (MGD). SUC brought in engineers from the local Xylem branch to work closely with SUC and Kimley-Horn consulting engineers to assess the complexities of the project and to develop a bypass plan.
Mapping out the Plan
As the Xylem engineers began to assess the bypass and establish a plan to execute, they determined that there were three primary components of the bypass operation: the two major pump stations (PS); the eight smaller pump stations; and the emergency back-up plan.
The first part of the bypass plan was to consider how the bypass would impact flow at the two primary pump stations, PS 34 and PS 35. The flow rates during wet weather peak flows at PS 34 and 35 were 4.7 and 7.9 MGD, respectively, and the existing pumps couldn’t handle the additional head requirements of the bypass. Xylem engineers recommended the rental of a Godwin Electric Drive CD300M pump at both the PS 34 and 35 stations.
They chose electric drive pumps to save diesel fuel costs during the seven-month project. Each of the Godwin rental pumps would be designated as the primary bypass pump, and they were equipped with a Godwin Variable Frequency Drive (VFD), allowing the pumps to change speed according to flow. The electric pumps were tied into an onsite PLC with a built-in modem, allowing SUC personnel to see the status of the electric pumps from anywhere with internet access.
The existing diesel pumps at PS 34 and 35 were designated as back-up pumps for the bypass, and were outfitted with the Godwin Field Smart Technology (FST), which provided 24/7 automated monitoring of the bypass pumps and system, eliminating the need for onsite manual pump watch duty.
The second component of the bypass was to address how to handle the eight smaller pump stations feeding into the primary bypass, and how to get the remaining flow off the line. There were no inline valves in the 15,000 lf of 20-in. force main, which could be used to isolate or redirect the flow from the smaller pump stations. Xylem engineers recommended to SUC that it consider installing valves as part of the bypass, suggesting that inserting temporary valves (line stops) at the ends of each section would be a good solution. The SUC team had originally considered a massive pump-and-haul operation, using vac trucks placed at each of the eight stations along the force main to handle the flow while the 20-in. force main was cut and capped in each section. SUC ultimately agreed that the line stops would be the best option to reduce the risk of a spill, and the line stops were installed and then removed as each section was tested and reconnected.
The third and final component of the bypass plan was to put contingencies in place in case of an emergency. Any spill during the bypass operation would be unacceptable, negatively impacting the environment and potentially costing SUC significant fines by the USEPA. But the teams knew that the best way to mitigate the impact of a spill is to plan for it, so Xylem, SUC and Kimley-Horn worked closely to design an emergency response contingency plan for any potential mishaps. The contingency plan included information about how an emergency would be handled should one occur, as well as information about how back-up equipment would be provided and action steps for implementation.
Saving Time and Money
For sewer bypass operations like the NEI Rehab project, utilities or contractors will typically engage in “pump-watch” patrol, to keep an eye on the system 24/7 to be sure that the pumps are handling the flow as required to help avoid an environmental disaster. With the Godwin Field Smart Technology (FST) providing the 24/7 automated monitoring, onsite manual pump duty wasn’t necessary. The implementation of the FST technology, combined with the use of electric versus diesel pumps as the primary pumps, ended up saving SUC thousands of dollars in man-hours and $1 million in fuel costs, significantly reducing the cost of the overall project.
“Engaging Xylem in the planning and execution of this project gave us the reassurance that we were addressing our needs,” says Dustin Wagner of State Utility Contractors. “Their team of engineers was able to identify some of the challenges upfront, saving us time, money, and headaches throughout the project. They have the expertise to know what it takes to get the job done, and that put our minds at ease.”