As St. Louis has grown, so has the Missouri River Wastewater Treatment Plant, one of seven treatment plants in the Metropolitan St. Louis Sewer District (MSD). Yet, over two decades, the district has implemented each upgrade, capacity increase and structural addition without taking the plant offline for significant periods of time.
The first phase of construction, completed in 2009, saw a rebuilding of the headworks facility. The second phase, wrapping up this year, increases the plant?s capacity from 28 million gallons per day (MGD) to 38 MGD and also upgrades treatment capabilities to meet new regulations stipulated by the U.S. Environmental Protection Agency (EPA).
The expansion began with removal of the existing secondary treatment process and four trickling filters, which were replaced with an activated sludge process followed by an ultraviolet (UV) disinfection facility. Contractors have installed a new low lift pump station, six aeration basins, a blower building, six 125-ft diameter final clarifiers, a sludge pumping station, a solids handling building, a primary digester and two splitter structures. The facility also is shifting its solids treatment process from gravity belt thickeners to rotary drum thickeners and centrifuges, a change that will allow for higher treatment volumes.
Throughout the course of the project, engineers were able to implement each phase without taking the plant offline, due in part to planning and in part to the spaciousness of the property. But construction amid the existing facilities and pipelines still required attention and care. For example, installation of new 66- and 72-in. pipes next to the existing trickling filters required contractors Goodwin Brothers and Corrigan Mechanical to install an elaborate sheet pile system, ensuring the filter remained in service and didn?t settle.
Playing A Key Role
Connecting each of the new buildings are two types of concrete pressure pipe: 3,324 ft of prestressed concrete embedded cylinder pipe (ECP) in 54- to 108-in. diameters, and 3,926 ft of prestressed concrete lined cylinder pipe (LCP) in 30- to 42-in. diameters.
ECP pipe consists of a steel cylinder embedded in a concrete core, then helically wrapped with high-tensile-strength steel wire and coated with a layer of Portland cement mortar to electrochemically protect the pipe against corrosion. ECP is available in 54- to 144-in diameters and laying lengths from 12 to 24 ft. ECP is ideal for treatment plants, distribution and transmission mains and industrial piping.
LCP features a concrete-lined steel cylinder with the application of helically wrapped high tensile strength steel wire followed by a Portland cement mortar coating. The pipe is supplied in diameters from 16 to 48 in. and laying lengths from 16 to 32 ft. Along with water and wastewater, LCP can also be used for industrial process piping.
Prestressed concrete cylinder pipe combines the tensile strength of steel with the compressive strength and corrosion-inhibiting properties of concrete and mortar, making it ideally suited for water and wastewater treatment plants. It can safely carry aggressive flows for water and wastewater treatment plants, including aggressive natural waters, highly septic sanitary sewage, and a wide range of chemicals and industrial waste materials (In some cases, it may be necessary to protectively line the interior of the pipe to handle some flows).
Furthermore, unlike ferrous pipe materials that may undergo atmospheric corrosion in humid environments, the opposite is true for concrete pressure pipe: The cement-rich mortar that coats the concrete pipe and fittings continues to gain additional strength in such humid environments.
Along with corrosion protection, PCCP?s inherent strength provides for economies on the jobsite. Prestressing wire wrap places the steel cylinder and concrete core in a state of circumferential compression, allowing the pipe to withstand internal hydrostatic pressure and external loads. Because of its rigid design, PCCP does not rely on backfill for lateral support, which reduces installation time and cost versus the requirements of flexible piping materials.
Combined with these inherent performance characteristics, concrete pressure pipe can be designed and engineered to navigate the often complex and confined installations required at treatment plants, particularly those retrofit applications that call for expansion and enhancements to existing infrastructure.
Rick Deremiah, P.E., is technical resources engineering manager for Hanson Pressure Pipe.
