Secret City Conquers Clogs
When original sludge pumps began failing, a Tennessee town adopted new high-solids-handling technology
It became evident several years ago that the sludge pumps at the main wastewater treatment plant (WWTP) serving the city of Oak Ridge, Tenn., were near the end of their service life. The two, 15-hp centrifugal suction units had operated in their dry pit installation since the 24-million-gal-per-day (mgd) plant’s completion in 1983. Their link in the process chain involved pumping raw sludge off the primary settling tanks to the plant’s four 250,000-gal digesters.
“The initial problem involved persistent clogging by rags and other fibrous material in the thick sludge,” said Tom Roberts, the city’s director of maintenance. “Other incidental problems ensued with the pumps that began absorbing more and more operational and maintenance costs.”
A major upgrade five years ago included installation of rotating drum screens at the headworks and the addition of UV disinfection and other equipment retrofits.
Failures with the variable speed controls forced the plant operators to assign a worker to the dry pit for two to four hours to manually control the pump speeds as each shift drew raw sludge from the clarifiers to the digesters. Ear protection was also necessary because of the deafening noise level in the confined pit. An abandoned office chair and small rotating fan left in the pit are mute evidence of the workers once assigned there who endured strong odor and the unrelenting noise from the pumps.
This personal attention to the pumps was costly. The hours spent in the dry pit, related preventive maintenance and outright pump failures cost the city $55 per man-hour. Some repairs required 16 man-hours and even then involved a lengthy wait for delivery of replacement parts. The unreliability of the pumps became so acute that Roberts developed an emergency plan to use a portable submersible pump with enough bypass hose to carry the sludge around the pump pit and to the digesters in case both original units failed.
As most managers employed in various public and private sectors around Oak Ridge, Roberts had become accustomed to contingency planning and been receptive to applying advanced technologies. Fifteen of his 17 years with the city have involved oversight of the plant and other public infrastructure. The Public Works Department includes an additional five maintenance personnel besides himself, 11 plant operators and designated individuals for pretreatment, the lab and clerical administration (one employee each).
The Appalachian town and its original infrastructure originated with the Manhattan Project in 1942. Located at the eastern end of a linear valley some 25 miles northwest of Knoxville, Tenn., the town first emerged as a federal government initiative to provide housing for workers at the top-secret uranium processing complex that enriched uranium and produced the plutonium nucleus of the first atom bombs. The facility initially charaded as the Clinton Engineering Works, and the town was within a security fence.
Incorporated in 1949, Oak Ridge has been referred to since as “Secret City,” “The City Behind a Fence” and “Atomic City” after the plant’s mission became public knowledge. At peak activity, 70,000 people were busy working in the immediate area. That population has since downsized to approximately 3,000 residents in what has become a peaceful community with a mix of high-tech employment.
Because the Department of Energy nuclear plant still operates, warning sirens that alert the area of any threatening radiation leak still blare across the landscape on the first Wednesday of every month.
The environmental risks associated with these operations have shaped the award-winning WWTP into the most closely monitored in the world. The effluent is tested redundantly by plant, local, state and federal agencies for the presence of radioactivity, mercury or other toxins.
Deterioration, however, is finally eroding the integrity of the original infrastructure; it is evident in the inflow and infiltration (I&I) that supercharge the flow through the aging sewage collection system linked by 32 lift stations. Therefore, the plant’s engineering required a redundant safeguard to prevent bypasses or subpar treated effluent from reaching the discharge stream.
The WWTP normally handles 6 mgd during dry-weather conditions, but I&I can induce flows beyond the 10-mgd maximum capacity of the biological treatment process. Since the upgrade, any excess flow diluted by I&I is now pumped back above the headworks and diverted into a secondary facility with a 20-mgd chemical treatment system.
“Envision our facility as essentially two plants,” Roberts said. “The risk of bypasses or poor-quality effluent reaching the receiving stream can’t be allowed to exist here.”
In the past, clogging and other problems with the sludge-handling pumps could shut down the entire wastewater treatment process. Nedrow & Associates, Murfreesboro, Tenn., was asked for recommendations. The water/wastewater and mechanical treatment systems company carefully assessed the situation before recommending replacing the unreliable controls and the existing pumps with horizontally mounted ITT Flygt NT 3127 pumps.
The 7½-hp units have patented features and an advanced hydraulic end design that ensures efficient and reliable operation over long periods. Tim Nedrow, principal of Nedrow & Associates, was so confident that the city could use the replacements that he offered them on a trial basis for six months before asking the city for a purchase decision.
The N-Pump Series was engineered specifically to handle fluids with high solids and fibrous content. The replacement pumps feature a revolutionary self-cleaning impeller that is complemented by a special relief groove in the volute. The patented combination operates with a self-cleaning flow path through the pump, virtually eliminating clogging. Other proprietary features include the shaft, seal, oil housing and sensors.
After completing installation of the new Flygt N-pumps, the plant operators observed immediate improvement in the handling of the heavy sludge. Problems such as incidental clogging caused by rags and cavitation attributed to the thick sludge became things of the past. Although a nominal component of the utility’s overall electric consumption, the pumps’ improved efficiency also reduced energy costs. Another noticeable benefit was the significant reduction in noise level while the new pumps operated.
“Within a month, we were equally convinced of the reliability,” Roberts said, “and the new N-pumps were less expensive to put in than to buy replacement units of the previous type. We plan to upgrade our dewatering press system, and I intend to replace the progressive cavity pumps there with the N-pumps.”
The ITT Flygt pumps have been in place for a year, and hours once devoted to emergency repairs are now used for other tasks. On the operations side, workers who had to frequently manipulate the speed of the old pumps manually no longer find it necessary to enter the pit daily. The recurring problems are now fading memories. The only reminder of past problems exists in an empty chair and desk fan now pushed back against the wall of the dry pit.