In Iowa, rain can fall as a mixed blessing. In the right amount, the economy grows in lockstep with corn and other crops. During heavier storm events, however, even a 2-in. rain can spell trouble for public works officials with the city of Des Moines, Iowa.
As with so many other cities with aging infrastructure, inflow and infiltration (I&I) can overwhelm stretches of the sewage collection systems serving Des Moines and other communities comprising the Des Moines Wastewater Reclamation Authority (WRA). Des Moines functions as the operating contractor for the regional authority, whose consolidated lines from 16 neighboring municipalities reach the Des Moines Wastewater Reclamation Facility (WRF). Although Des Moines proper can escape a seasonal downpour, the potential still exists for outlying areas to be impacted by rain upstream from the city limits. When that happens, the flows could induce sanitary sewer overflows (SSOs) and backups into homes, businesses, rivers and streams. The events can prove costly, according to a recent report.
Des Moines has experienced 87 sewer overflows during 53 rain events in less than two years. When an SSO is attributed to a municipal utility’s combined or plugged sewer line, Iowa law allows citizens to collect for damaged property. Des Moines alone paid $311,000 over the past 18 months for SSO losses, according to information provided by the City Attorney’s office. Nine-hundred cities in the U.S. face this same problem as combined sewers discharge 1.2 trillion gal each year, according to the U.S. Environmental Protection Agency (EPA).
EPA-mandated treatment plant and sewer system improvements are projected to ultimately cost local residents served by the WRA more than $250 million, according to Jim Morano, pump station team leader with the Des Moines Public Works Department. Televised inspection, relining and other improvements are addressing the 850 miles of sanitary sewers—some built in the 1880s—and 400 miles of storm sewers. Of particular note are approximately 200 miles of combined lines tracing back to the 1930s; these are concentrated mostly near downtown and along both sides of the Des Moines River.
Several major pump stations have undergone significant upgrades because of WRA interconnected collection systems that extend beyond the boundaries of Des Moines. The largest serving the sanitary sewers, the Westside Equalization Basin & Pump Station, underwent a significant upgrade in 2007 to correct vulnerability to I&I-influenced flows. The project presented some challenging conditions, according to the project manager with the engineering firm.
“The improvements essentially doubled the station’s capacity within the existing footprint,” said Michael J. Halde, P.E., of Howard R. Green Co., which led the design and construction of the retrofit. “The construction sequencing was particularly challenging because the new larger pumps required replacement of the station’s piping, main switchgear, standby power and electrical starting equipment while maintaining normal pumping operations. The end result provided WRA with nearly twice the previous capacity at the facility, instilled the flexibility to limit peak flows into the station, divert any excess into an adjacent equalization structure and to synchronize the output with the needs of the plant downstream.”
Correcting a Bottleneck
The Westside Pump Station was developed about 100 yards from the Des Moines River and presented a bottleneck in the past for flows generated by heavy rains.
“It depended on the year, but we could expect the station to flood four to six times during some storms,” Morano said. “There was nothing we could do to prevent it if the basin filled and all four pumps were running but couldn’t keep up.”
The facility originated in the late 1980s as a 20-million-gal-per-day (mgd) dry pit station with four vertical long-shaft pumps, arranged with two each serving the station’s twin, 55-ft deep wet wells. An operable gate interconnected the adjacent wet wells with a similar isolation provision for controlled release back into them from the 4.8-million-gal equalization basin.
The pump station stands on a limited piece of ground the city leases along the Des Moines River from the U.S. Army Corps of Engineers. The natural terrain would also inherently limit any major physical expansion, which collectively ruled out building another equalization basin, the engineer noted.
The station’s inflow arrives through a 66-in. sewer line to the wet wells, where pumps draw from them to feed a 30-in. force main that extends uphill to the Westside Interceptor. An intersecting 48-in. gravity sewer, which is pressurized at high flow, then carries flows to the WRF. The capacity of the existing force main leading to the uphill connector presented another limiting factor.
An obvious concession to anticipated flooding was the high-pressure flushing and surface drain system provided in the initial design of the 4.8-million-gal equalization basin, according to Morano. A basketball court atop the equalization basin, an added recreational amenity for the neighboring Prospect Park, incorporated a drain system in the courts to redirect overflows back into the basin. The flushing system that provided the high-pressure water to eliminate residual buildup inside the basin was tapped to clean up the courts after an overflow event.
The WRA retrofit of this critical facility was the fifth pumping station upgrade since 1999, Morano noted.
Upgrading within Existing Footprint
Howard R. Green Co. drew upon a hydraulic study by subconsultant Black & Veatch. In addition to modeling the downstream effects of the intended capacity increase, Howard R. Green used the study to identify the maximum capacity increase within the station’s constructed footprint. The goal was to gain as much throughput as possible within the physical constraints of the limited site.
“I had always viewed the station as handling a combined system because the flow comes from several communities,” Morano said. “If we had a 2-in. rain in any of the WRA communities served upstream by the Westside station, I could generally count on the retention basin filling and overflowing.”
The engineering firm was able to upsize the station’s capacity to approximately 36 mgd by combining more powerful pumps with construction of a new, parallel force main from the station. The work replaced the original four 200-hp long-shaft pumps with 335-hp Flygt Model 3400 submersible pumps in the dry-pit facility. The contract also replaced the bar screens, increased the main switchgear size, added new pump variable frequency drive capability and upgraded the existing 750-kW to a 1,250-kW backup generator. The work was done in two stages so that the pump exchanges and other modifications could be done on one side, followed by isolating the other half of the facility to maintain normal pumping operations during construction.
The project added a 1,600-ft-long parallel 30-in. force main from the station to the uphill intersection, with the Westside Interceptor leading to the WRF. Modifications added larger roof hatches to install and service the new pumps, and created 6-by-6-ft openings through the at-grade operating floor and flushing room floor to accommodate the pump volutes. Both modifications required amending the structural steel supporting members. The wet wells received a sloped floor 2 ft higher on one side to minimize debris accumulation. A dispersion shelf was created along one wall to reduce turbulence and reduce cavitation at the powerful intake bells.
Milestone dates were set for completion of various phases of the work. Oakview Construction, Red Oak, Iowa, fully adhered to the schedule and completed the project at substantially less than the initial construction cost estimates.
The four Flygt pumps at the upgraded station embody a pump standardization program that Morano implemented approximately six years ago. Flygt was selected on the basis of competitively bid performance specifications. Electric Pump, a Flygt associate in Des Moines, supplies the pumps and performs maintenance on the larger units.
“I wanted reliability from one manufacturer and eliminated $300,000 in parts inventory we previously kept on hand for different brands of pumps acquired over the years,” Morano said. “Now we have no city investment in a stock of parts other than some spare impellers. Another advantage is that my pumps are now all interchangeable. If a pump is taken offline in a critical station, we can borrow one during the interim from another lower-demand facility.”
The WRA continues a steady stream of improvements to the system. The authority’s 160-mgd Des Moines WRF has had 70% of the $165-million cost funded by state and federal grants. The technologically advanced treatment facility replaced a 30-mgd primary treatment plant originally designed in the 1930s. Eight expansions over seven years have now converted the plant to an advanced treatment process facility.
It was more cost-effective to phase those projects over many years around the existing plant so that the new processes and equipment were woven into the old plant without interrupting operations of the existing infrastructure. Completed in 1991, the reclamation plant represents more than half of the WRA investment anticipated over a 15-year water pollution abatement program.