About the author: Cecil Coombs, P.E., is a project manager for Howard R. Green Company’s Sioux Falls, S.D., office. His project experience includes ground storage reservoirs, water distribution systems, water treatment, water pumping facilities, sewage collection systems, sewage pumping stations and wastewater treatment facility design.
Cecil Coombs, P.E.
undefinedThe Iowa Great Lakes area is located in northwest Iowa and is one of the most desirable summer recreation areas in the Midwest. The three major lakes in the region are West Okoboji, East Okoboji and Big Spirit. The impact of the recreation population can be recognized when one compares a permanent population of approximately 20,000 with a "4th of July weekend" count approaching 100,000. Two-thirds of the permanent population are located in seven communities while the great inflow of "tourists" inhabit the summer cottages, cabins, homes and condominiums that line the miles of lakeshore. Even in the 1930s the great contrast in numbers conflicted with the areas desire for a year-round pollution-free environment. When the people arrived, community treatment facilities as well as cesspools and septic tanks serving the lakeshore cottages overflowed, discharging their effluent into the lakes. The need for an area-wide collection and treatment system was recognized.
In 1949, the Iowa State Conservation Commission conducted a study of sanitary sewage needs for the Iowa Great Lakes region in Dickinson County, Iowa. Later that year an election was held proposing to create a Sanitary District to construct, operate and maintain a regional collection and treatment system. The residents of the area voted 985 to 125 in favor of the proposal. The District became a reality and the boundaries of the Iowa Great Lakes Sanitary District (IGLSD) were established.
Policies of the five-member Board are administered by the District’s superintendent, who heads a full-time staff of ten to operate and maintain the growing system. The current system consists of 95 miles of sanitary sewers, 63 sewage pump stations and one central wastewater treatment facility.
The total area within the IGLSD has grown to 23,450 acres with 11,550 acres covered by lakes.
Needs Identified
In 1996, a collection system study identified the IGLSD’s system current needs. The highest priority was the hydraulic overloading of the Wilsons Lift Station (LS 1-26) and Arnolds Park Lift Station (LS 1-25). LS 1-26 previously pumped into a gravity sewer serving a portion of Arnolds Park, Iowa (the influent sewer to LS 1-25). To increase the firm pumping capacities of each lift station, a new LS 1-26 and new 24-inch-diameter force main were recommended that would discharge into an existing interceptor that conveys sewage to the IGLSD’s Water Pollution Control Facilities (WPCF). This plan would divert flow around the LS 1-25 and result in both lift stations discharging into the same interceptor.
Projected Hydraulic Loadings
The new LS 1-26’s 20-year (2016) design peak hourly flow (PHF) is projected to reach 4,500 gpm. This flow is more than double the existing LS 1-26’s pumping capacity of 2,200 gpm. The new LS 1-26’s firm pumping capacity was designed to handle the present projected PHF. The PHF is defined as the highest flow that occurs in any one-hour period of a year and firm pumping capacity is the flow from a station with the largest capacity pump out of service. The design for the new LS 1-26 specified three pumps, each having equal pumping capacities. Therefore, any two pumps will meet the present PHF of 4,500 gpm. The ultimate PHF is projected to be 7,500 gpm.
Design
A dry well/wet well type pump was selected for the new LS 1-26. The wet well was designed for the top slab to be buried under part of Gordon Drive. While this made it relatively easy to tie into existing interceptor sewers, during construction it was difficult to keep this street open to traffic. Wet well access is through two pre-cast concrete square sections located next to the dry well masonry block building. IGLSD requested there be four access points, but this was not possible because of the buried fiber optics and their plastic conduits being located over part of the wet well. The interior surfaces of the wet well were prepped with a 4,000 PSI water blast, and then a calcium aluminate mortar was sprayed and troweled to smooth the surface. Inside the wet well an inlet structure receives flow from a 36 inch-diameter sewer on its north end and a 16 inch-diameter sewer on the south end. The inlet collects, distributes and directs the flow to each of the pump’s suction inlet, a 20-inch diameter 90-degree bell mouth fitting. A concrete "false" wall was cast in-place between the structural wall and the bell mouth fitting to provide the correct spacing from the wall.
The dry well is separated from the wet well by a common wall. The pumps are housed in the lower level (pump room) and the electrical equipment is located on the first floor (control room). The generator room is located adjacent to the control room and separated by a masonry wall with a door for access from the control room. An overhead door is provided in each of the rooms for equipment removal. Space has been provided in the control room for a truck to back in for loading/unloading. A 5-ton monorail, trolley and electric hoist allows the IGLSD to transport equipment from and to the truck.
Three vertical close-coupled non-clog sewage pumps with 60 hp electric motors are used to convey the wastewater through the 24-inch diameter force main. Each pump is rated at 3,000 gpm @ 59 feet. The pumps are controlled by three variable frequency drives (VFDs) that are paced by a PLC, which receives a signal from one of the two ultra-sonic sensors (one sensor is used as a stand-by). Each sensor is mounted inside a separate sealed 14-inch diameter pipe in the pump room. IGLSD wanted the ability to work on a sensor inside the dry well, so an aluminum platform was constructed for access.
The surge valve reduces the magnitude of the shock wave force created by flow that stops rapidly (e.g., a power outage). This valve opens when a pressure wave reflects back and releases the flow back into the wet well until the pressure is reduced below the pressure setting of the valve. A limit switch on the valve sends an alarm signal to the control panel in the control room, when the valve opens.
A 5-ton runway crane was placed in the pump room for handling pumps, valves and other equipment. This system contains two runway beams, trolley truck, monorail and electric hoist. Two three-phase submersible sump pumps cannot be reached with the runway system, so a cantilevered monorail, trolley and electric hoist was anchored to the concrete wall.
Each of the three non-clog sewage pumps is installed with their discharge rotated 45 degrees to the suction piping. Each suction pipe is designed for one-half of the ultimate PHF, and is 20-inch diameter with a shut off plug valve. Located on each pump discharge line are 14-inch diameter check valves and shut off plug valves. A 24-inch diameter discharge header collects flow from these valves and conveys it to the force main. Pump discharge lines are sized for one-half of the present PHF. The discharge header and the force main are designed for the ultimate PHF.
The force main also has an auxiliary 24-inch diameter connection
• to install a "poly-pig" for cleaning the force main, and
• in case of an extreme emergency, the ability to discharge into the force main with auxiliary pumps.
At the Wilson site, a 200 kW stand-by generator set is present in case of a power outage from the local utility. The IGLSD is able to operate
• two of three non-clog sewage pumps,
• two exhaust fans,
• two submersible sump pumps,
• 22 kVA electrical heaters,
• 3 hp exhaust fans, and
• 15 kVA miscellaneous lighting load.
In the event the generator and the local power source are out of service, an electric receptacle has been provided in the generator room. The IGLSD is able to plug into this receptacle with a portable generator and is capable of operating either Pump #1 or Pump #2, but not both.
The two submersible sump pumps are set in a 5-foot diameter PVC lined pre-cast concrete manhole section with a monolithic concrete base. The pumps are controlled with float switches in the sump and a wall mounted control panel at the sump. The major components in this panel are
• a main circuit breaker,
• alarm lights,
• alarm horn,
• pump starters and circuit breakers,
• H-O-A selector switches,
• automatic alternator, and
• elapsed time meter for each pump.
Sump pumps discharge back into the wet well that is covered with an aluminum grate that also serves as a floor drain. Most of the pump room floor uses trench drains and are piped to the sump.
The exit from the pump room to the first floor is by an aluminum stairway with an intermediate landing. In case of an emergency, a ladder leads to the platform and up to the first floor by another ladder. This path is intended for access to sensors and the surge valve.
The control room has a rectangle opening in the floor for removing/installing equipment in the poump room below. The MCCs and VFDs were installed so IGLSD personnel may manually operate the pump controls and still be able to see the pumps and their check valves in the pump room.
Construction
The site for LS 1-26 was selected because it
• was not a premium piece of developable property,
• was in close proximity to existing
interceptors,
• had convenient access from local hard surfaced street,
• was property given to IGLSD by the City of Okoboji, and
• did not take property off the county
tax rolls.
This site was very tight due to its close proximity to the City of Okoboji’s steel framed maintenance building, the large commercial boat retailer’s building, fiber optics in the street, a new brick home across the street and Gordon Drive and the local driveways.
Steel sheeting was driven to shore-up the excavation and was braced with interior steel beams. Several diagonal beams were cast into the wall when the concrete was cast in place. These beams later were cut off, leaving a recessed area on the inside and outside wall faces. The contractor grouted these areas so the wall faces were flush. All exposed interior concrete faces then were rubbed with a light coat of grout consisting of cement, silica sand, a latex bonding agent and a small amount of water. It was placed and finished with a rubber float.
During the time the steel sheeting was being driven, some damage appeared at Okoboji’s steel building. The contractor removed and replaced part of the east foundation wall, east steel siding sheets, several roof panels and the east portion of the interior concrete floor. The overhead doors were readjusted to provide an adequate seal.
It was decided to have the contractor leave the sheeting in place, rather than subjecting this building to more vibration. The sheeting was cut off three feet below the finished grade.
Wilson’s LS 1-26 was put on line in May 1999, slightly ahead of schedule. The only drawback to this project was that IGLSD started with 63 lift stations and finished with the same number.
