Sanitary District Rises to the Challenge
Faced with an expanding customer base and increased commercial development, the Duckett Creek (Mo.) Sanitary District (DCSD) was in need of an expansion. The existing 5-mgd wastewater treatment plant never could have supported the service area’s continued growth, according to Tom Szilasi, the District’s executive director. He noted that the District’s customer base had risen from 14,619 in 1991 to 24,640 in 1998. Although still predominantly residential, the number of commercial connections had doubled during this decade due to a high-tech corridor under development along I-40/61. New housing starts now average nearly 3,500 per year in the rapidly growing area west of St. Louis.
The result was the building of a 6.25 mgd wastewater treatment plant in Missouri Research Park as part of a $32-million expansion of collection and wastewater treatment capacity needed to sustain economic development in St. Charles County. Now in its third full year of operation, the unmanned plant incorporates a variety of advanced technologies.
In addition, the facility was recently recognized as "Plant of the Year, Operations and Maintenance" by Region VII of the US Environmental Protection Agency. This is the ninth award the District has earned over the past decade. The District has an exemplary safety record and has operated with 100 percent compliance even during the Great Flood of 1993 that swept St. Louis and much of the Midwest.
Cut From the Bluffs
The nine-acre footprint for the plant was chiseled into the bluffs within Missouri Research Park overlooking the Missouri River. The challenging, 100¢ drop in elevation across the property was offset by a number of advantages, particularly the surrounding buffers that isolate the plant from any developed residential areas. The utility already operated a package plant within the research park that is owned and operated by the University of Missouri. The plant site is further isolated from residential areas by the Missouri Bluffs Golf Club and by state lands under the jurisdictions of the Department of Natural Resources and Department of Conservation.
A creek that flowed through the property provided the necessary channel to carry treated effluent into the Missouri River. Some treated effluent also could be diverted into a lake used to irrigate the golf course. Much of the treated biosolids has helped to revitalize farmlands affected by the 1993 flooding of the Missouri River.
In order to create enough flat land to accommodate the plant, three creek channels were modified and a 60¢ deep notch was cut into the rise in the rock bluff. A balanced construction scheme was pursued that utilized excavated material as surcharge and fill. More than 12,000 cubic yards of concrete subsequently went into the plant’s two oxidation ditches, two clarifier tanks, a solid waste handling building, operations center and a small lift station that captures sidestream water that also must undergo treatment.
An equally impressive amount of work was done upline from the plant. More than eight miles of 18* to 36* diameter gravity interceptor and trunk lines were built along the Dardenne Creek watershed and an emerging, high-tech commercial corridor. These lines link up with the Highway K Lift Station.
This powerful pump station extends 40¢ below grade and creates a 160,000-gallon wet well equipped with four 250-hp and one 100-hp ITT Flygt submersible pumps. The 100-hp pump is a dry weather unit rated to operate at 180¢ TDH, while the three wet weather pumps are rated to operate at 185¢ TDH. Normal lift station design capacity is 37,000 gallons per minute. The pumps transfer collected wastewater into a four-mile stretch of 30* force main linked to the plant located uphill from the lift station.
The pumps are controlled by an integrated, dual control system that senses wet well levels. The primary control system is an ultrasonic level sensor with secondary control achieved with a float system operating through intrinsically safe relays. This microprocessor-based system is capable of operating up to three pumps simultaneously in 16 interlocked stages as equipment combinations are determined by the software-programming matrix.
The force main’s transient pressures are controlled by slow operating air/oil type ball valves. The times for each setting are based on time in excess of the transient pressure wave occurring in the force main. The smaller, 100-hp pump is sufficient during dry weather and minimum flow but during higher flow rates the three large units operate individually in an alternating sequence that minimizes wear. The fourth pump is a reserve unit.
Meets High Standards
Through the plant, the influent is subjected to state-of-the-art biological treatment as well as an 800-tube ultraviolet disinfection section. The entire process is controlled and monitored by a Supervisory Control and Data Acquisition (SCADA) System. The monitoring capability alerts both the Districts’ main offices and transmits to alphanumeric pagers any incidence of equipment malfunction or failures. The Highway K Lift Station also is linked to the SCADA technology.
Key elements of the plant’s advanced design include phased isolation ditch technology, SCADA, dissolved oxygen (DO) monitors and controls in the oxidation ditches, sludge aeration with DO control, dewatered biosolids processing and storage, UV disinfection, chemical odor control and an automated clarifier scrubber system.
Phased Isolation Ditch Technology has energy-saving provisions for low-volume flow periods. Advantages include lower operating hours for aeration equipment, energy reduction, less manpower and maintenance requirement, extended life for equipment and sufficient aeration basin mixing.
The plant’s headworks perform the normal range of screening, grit removal and flow measurement with an odor control scrubber that also serves the solids handling building. The common scrubber lowered initial costs and is automated to provide only enough treatment as required for conditions. The unit is a constant volume, self-contained system responsive to variable loads that automatically balances the chemical requirement.
The oxidation ditches are equipped with rotors that mix and transfer dissolved oxygen used in the biological process. The ditches incorporate a feature that optimizes energy consumption. Three 9-hp submersible mixers permit the rotors to be turned off during low DO demand periods when the smaller horsepower mixers continue the required churn. Up to four rotors can be turned off, thus decreasing net energy use by 40 percent. This aspect of the process also is controlled by the SCADA.
Labor costs were reduced by the design of the clarifiers. Manual cleaning has been minimized by the inclusion of brushes on the skimmer mechanisms. These are changed on a scheduled basis.
The plant fully complies with the low limits of fecal coliform permitted during critical periods of the year. Ultraviolet disinfection was chosen based on safety considerations and the anticipated reuse of some treated effluent by the adjacent golf course. SCADA controls the number of bulbs energized at any one time to optimize energy consumption.
The biosolids storage scheme was initially designed with coarse bubble diffusers located at the bottom of its storage tanks and with variable frequency drive blowers controlled by a PLC to achieve DO control. This has since been changed out to use fine bubble diffusers for greater reliability. Enough additional energy savings resulted to offset the retrofit costs within a year. The new plant has proven to be 20 percent more energy efficient than its predecessor.
Archer Engineers served as the District’s design engineers for this facility. The 54-square-mile area served by the District continues to experience accelerating growth, averaging 8.5 percent over the past five years. By Year 2001, the county’s population is expected to reach more than 275,000.
Meeting the Challenge
With some unique planning, the District has met its responsibilities to have adequate wastewater utilities in place. Economic development is surging, with new projects ranging from a large headquarters for part of MasterCard International, to a planned, 1,100-acre mixed-use development known as Winghaven. The $550-million project will add 1,700 more housing units and 850 apartments. Some 600,000 sq. ft. of research space, light industrial and high tech assembly facilities also could materialize in the area served by the District.
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