Overcoming Capacity Concerns
Surge tank helps Missouri city meet growing demand
The city of Branson, Mo., is known as the live music capital of the world. Currently, the city offers 50 theaters with seating for 64,057 guests, meaning Branson has more seating capacity than Broadway. This live entertainment, along with beautiful lakes, family-friendly activities, world-class dining, award-winning theme parks and down-home hospitality, attracts almost 8 million visitors from around the world every year. With a permanent population of slightly more than 10,000, Branson’s infrastructure is sized to handle the daily influx of an additional 70,000 people. One of the challenging aspects of the city’s wastewater collection system is the fact that the town is located in the rugged Ozark Mountains. Steep slopes and extreme elevation differences dictate the locations and methods for transporting wastewater to the city’s two wastewater treatment plants.
As part of its long-range planning efforts, Branson developed its wastewater master plan in 2007. Among the findings in the master plan were two critical issues: One involved the major interceptor sewer that served the city’s north wastewater treatment plant. A portion of the interceptor sewer was undersized. As commercial and residential growth in the watershed continued to increase, the biggest fear was that the sewer would overload, resulting in the wastewater surcharging in the manholes and overflowing into the adjacent pristine Roark Creek. This creek runs through one of the city’s most visited parks and directly into a cold-water trout lake. The cost to replace the interceptor was prohibitively expensive, so the city investigated alternative solutions.
The second critical issue identified in the master plan involved the capacity of the treatment plants. The north plant is landlocked and the capacity could not be expanded. The south treatment plant had ample capacity remaining and could be expanded 200% to handle additional growth. The problem was the high cost of transporting wastewater from the north watershed to the south watershed over a 260-ft-high mountain ridge. A large-capacity lift station would be necessary. The pumps would be expensive when sized to handle the maximum anticipated flow rate. The good news was that this future lift station would remove wastewater from the inadequate interceptor sewer, thereby permanently solving that particular issue. With the solution to both issues identified, the city began the process of finding an economical way to accomplish the goal.
Weighing the Options
The solution to both identified problems was the construction of a 1-million-gal prestressed concrete “surge” tank. The tank, with a 70-ft diameter, 35-ft standing water depth and domed roof, was equipped with an automatic wash-down system for cleaning after each use. During periods of heavy flow, a diversion structure would automatically send the excess flow into the surge tank. Until such time in the future when the lift station is constructed, that arrangement would prevent the inadequate interceptor sewer from overloading and surcharging wastewater. The surge tank could also be used as a peak flow storage tank so the lift station pumps could be reduced to smaller-capacity pumps that would operate longer, but at a reduced flow rate, because the excess flow would be held in the surge tank and slowly released. The capture and release of the peak flows also would help with the management of the processes at the treatment plants.
The city’s consulting engineers suggested three alternatives for the surge tank. A welded steel tank was a logical choice, but the long-term maintenance costs with corrosion and architectural or decorative coating would be expensive. A second alternative was a cast-in-place square concrete structure. This solution resolved the maintenance concerns, but was deemed to be prohibitively expensive. The final option was a circular prestressed concrete tank. This type of tank had never been constructed in the Branson area, so similar tank installations in other cities were reviewed and references were interviewed. It ultimately was determined that such a tank could be built quickly and easily, and would have low long-term maintenance costs, resulting in the lowest lifecycle costs.
During the design phase, the city expressed concerns with the tank being used to store raw sewage and the likely possibility of odor issues for the surrounding property owners. Therefore, an extensive odor control system was included with the project. The sewer maintenance staff had little desire to access the tank interior for cleaning and washing down, and there was concern of possible methane gas, requiring the need for adequate ventilation. The solution was the installation of a Butterworth automated interior wash-down system. After the tank was used in a peak flow event, the automated wash system would be activated and two nozzles would spray the interior with 150-psi rinse water, cleaning the entire interior in 45 minutes. There would be no need for any employee to actually enter the tank.
DN Tanks worked closely with the city and the consulting engineer, Crawford, Murphy & Tilly Inc., on the tank design and internal wash-down system during the design phase of the project. Four general contractors bid on the project, which included a diversion system, yard piping and the 1-million-gal storage tank. The low bid was received from Rosetta Construction and the tank was subcontracted to DN Tanks with a construction schedule of 180 days.
The project progressed well. The only significant challenge was the small site size available for the work. The contractor ultimately decided to negotiate with the adjoining cattle rancher for a temporary construction easement. With the easement obtained, the contractor and his subcontractors had ample room to maneuver on site during the construction.
The foundation of the tank was designed to be buried 10 ft below grade to lower the profile of the tank and make it less visually obtrusive to the surrounding property owners. The lower tank elevation would also be advantageous for the piping and onsite pumps by reducing the system head, thereby lowering power requirements for the onsite pumps. Fears of buoyancy issues due to possible elevated groundwater necessitated an underdrain system be installed with float-activated pumps.
The tank was painted an earth-tone brown to help blend into the nearby hillside in the winter. In summer, the surrounding vegetation nearly hides the tank from view.
The project was completed on time. Throughout the entire construction process, safety was a major concern. The success of DN Tanks’ safety program was evidenced by Roger Clark, engineering project manager for the city of Branson, who said, “One of the things I liked the most was [DN Tanks’] concern for safety. Many companies talk about safety, but the practice is lost in the field. I can honestly say that safety was the top priority … Even the OSHA inspectors who visited the site were very impressed with [the] job site and could not make any recommendation for improvement.”
Since coming online in May 2012, the surge tank has been used eight times. The goal of resolving the surcharging of the interceptor sewer has been resolved and the tank functions as designed. The interior wash system has performed well, and effectively keeps the tank clean and odor-free. The city of Branson is pleased with all aspects of the prestressed concrete tank.
http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/4..jpgThe tank has a capacity of 1 million gal.
http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/5B-1%20copy.jpgA temporary construction easement was obtained to allow the crew to move freely on the small site.
http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/7%20copy.jpgThe tank was painted brown to blend into its surroundings.