Cost/benefit analysis has been the cornerstone upon which utility engineers base most retrofits of their infrastructure. In an era of rising maintenance costs and limited funding, it has become even more important for projected results to meet expectations. Management of Jacksonville (Fla.) Electric Authority (JEA) had no reservations about approving a recent conversion of the 3,700 gal per minute (gpm) (70 TDH) Oldfield Crossing Pump Station (OCPS) to a booster pump station. It was obvious this project would pay for itself and eliminate a chronic odor and ragging problem.
JEA ranks among the nation’s largest and most efficient community-owned electric, water and sewer utilities. It assumed responsibility for the municipal water and sewer system in 1997 and the sewer service area now has one of the largest geographic footprints in the nation, at approximately 900 sq miles, according to Bob Phillips, project engineer for JEA. The sewer utility serves 230,000 customers and handles a 78-million-gal-per-day average daily flow. The infrastructure includes 2,500 lift stations—1,252 of which are maintained by JEA—3,925 miles of collection lines, 34,000 manholes and 14 treatment plants. JEA also operates a biosolids pelletizing facility.
The sewer system’s maintenance budget was $125 million for 2012.
The engineering group within the utility goes beyond basic cost/benefit analysis to apply a sophisticated refinement of the method known as “root cause” analysis, which determines the best use of available funding to resolve the origins of a recurring problem. The utility also practices the Six Sigma industrial management strategy, which involves even more critical analysis for proposed projects every step through the budgeting stage. JEA’s proficiency was recognized by the International Productivity and Quality Center (IPQC) for the best Six Sigma project by a government agency in 2007. IPQC went on to recognize JEA in 2008 for Best Fast Track Project and Best Service Project the following year.
It should come as no surprise, therefore, that the projected savings from the OCPS conversion project delivered the expected results and much more.
A Common Problem
Phillips recently shared insight into the successful OCPS conversion from a traditional pump station and wet well into a booster pump station that has resolved chronic ragging and odor problems. The extended detention time for the force main’s flow within the 18-mile-long line creates ideal conditions for the buildup of hydrogen sulfide gas in the sewage flow collected from within a large portion of a growing part of the service area. This set the stage for the major odor problem when the flow reached the original pump station’s wet well. JEA elected to divert the force main’s flow off the wet well by building an above-grade booster facility in proximity to the original pumping installation. Designing the new station this way cut the required horsepower in half while still pumping the same flow. The existing 95-hp pumps and odor control equipment were replaced with 45-hp Flygt N pumps that provide the hydraulic muscle for the new companion booster facility
Without the discharge into the wet well, the retrofit eliminated the root cause of the odor problem. Instead, the sewage remains in the airtight modified force main and bypasses the OCPS wet well. The elimination of costs associated with odor-suppressing chemicals and biweekly wet well cleanouts more than offset the initial $300,000 cost of the conversion project, Phillips said. The ragging problem experienced with the old pumps also was eliminated, along with the contingent risk of the wet well overflows.
“JEA was spending $752 a day for chemicals in a futile attempt to control odor at that facility,” Phillips said. “Added to the nearly $275,000 a year spent on chemicals, we were spending an additional $3,400 a month for a vac-truck to clean out the blanket of floating rags and grease that accumulated in the original pump station’s wet well.”
Phillips emphasized that even more savings will accrue from reduced energy consumption attributed to the design of the Flygt N pump and lower horsepower. During the first six months of operation, energy use at the facility already had declined by 75% to 80%.
“Although some energy savings could be expected from the newer pumps, those savings were not included in the projected savings from the conversion project,” Phillips said. “We had enough for the project just knowing the actual costs for chemicals and vac-truck use.
“The actual energy savings achieved during the first six months were amazing,” he added. “Whereas we were consuming 22 to 24 kWh a day before the conversion, the usage dropped to within 4 to 6 kWh a day. Those savings will continue to add up, although the projected reduction in energy usage was not a known factor in the project proposal.”
OCPS was constructed in 1991 and underwent multiple upgrades during ensuing years. Prior to the recent conversion to a booster pump station, the 3,700-gpm facility had three 95-hp pumps in the wet well that repumped the combined 100-gpm flow off a local gravity collection system and 3,600 gpm from the 16-in. force main. The 18-mile-long force main receives flows from 88 lift stations throughout a large portion of the utility’s growing service areas in Duval and St. John’s counties. The length of the force main extended the detention time for the raw sewage in the anaerobic environment. This built-up hydrogen sulfide gas fueled the odor emissions upon reaching the wet well, resulting in understandable public complaints.
OCPS presented another ongoing issue in the form of ragging that could choke the earlier pump impellers. Ragging is becoming a major problem for many utility installations and is largely attributed to disposable cloth-like cleaning wipes and other fibrous debris flushed into systems due to the public’s continued misunderstanding that “disposable”does not necessarily mean “flushable.” The trash collects around and congests impellers of most pumps used throughout the industry. As the ragging builds up around the impellers, the ensuing drag chokes hydraulic efficiency, severely compromises energy efficiency and can eventually shut down a pump.
“Ragging became a real issue there,” Phillips said. “In addition to the biweekly cleanouts with the vacuum truck, we kept a $5,000-per-month pony pump on site for emergencies and had to use it on two earlier occasions, even with the proactive maintenance.”
Resolving the Odor Dilemma
Although upgrading the aging odor control system at OCPS would have helped to reduce the offensive emissions, the root cause of the odor—the high concentration of hydrogen sulfide in the force main’s flow—would have remained, Phillips said. Therefore, his engineering group pursued the more permanent solution of converting the pump station to a booster pump station.
JEA’s retrofit now diverts the force main’s flow away from the wet well and through a pair of new 16-in. below-grade header manifolds interfaced with the existing incoming and discharge force mains of the previously existing pumping station. These headers branch off to each of the three booster pumps and the low-pressure bypass with 12- and 8-in. above-grade piping.
The three 95-hp submersible pumps once used to repump the combined flows have been replaced with three 45-hp Flygt N submersible pumps mounted in a dry-pit configuration on the above-grade concrete pad. Valves can divert the pressurized flow back through the existing wet well in emergencies, but the original pumping station now normally carries only the existing localized gravity flow.
Significant additional savings were achieved by using the utility’s construction workforce to build the conversion. The work was accomplished for half the cost of an outsourced contractor, Phillips believes. Once again, this cost reduction for the project was excluded from the early cost/benefit calculations.
Overcoming Fibrous Flow
The costs associated with the ragging problem of the former pumps have been resolved by the design of Xylem’s Flygt N pump. The high-performance units employ a patented impeller that is energy efficient and clog resistant, and maintains unobstructed flow of stringy and fibrous material from building up and blocking the intake. The fibrous residuals that reached OCPS had become an even greater problem for the earlier pumps when combined with grease reaching the wet well. Instead, the pumps now avoid intake obstruction by allowing both types of debris to simply pass through the intakes. The pumps handle not only the materials flushed into the collection lines, but also achieve impressive energy savings.
Phillips said the conversion has proven successful and foresees the concept as applicable to other JEA installations and even other municipal operations. The investment in this retrofit was clearly justified by the numbers.