An Epic Flood Meant A Long Recovery For The Plant Team In Clarksville, Tennessee

An epic flood in 2010 meant extensive destruction and a long but successful recovery process for the treatment plant team in Clarksville, Tenn.
An Epic Flood Meant A Long Recovery For The Plant Team In Clarksville, Tennessee
Construction crews work on the treatment plant’s new secondary clarifiers.

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Thirty feet of water and sewage covering the treatment plant grounds and structures. Building roofs lifted off. Clarifier vacuum units afloat and drifting free. Water inside every electrical wire and component. Major pumping stations out of commission.

If you can picture all that, you can begin to imagine the caliber of disaster caused by the flood of May 2010 at the Clarksville (Tenn.) Wastewater Treatment Plant.

Days of heavy rain ultimately sent Cumberland River floodwaters over the top of a protective berm and into the plant’s “bowl.” It took most of a week just to empty the site of water and three months to restore full secondary treatment. More than four years later the plant is still in recovery, though fully functional and in compliance with its effluent permit.

Through it all the plant team has performed heroically, according to Pat Hickey, general manager of Clarksville Gas & Water, the city department in charge of the facility. “The staff was exemplary in keeping the plant in operation,” says Hickey.

“Initially, everybody had a deer-in-the-headlights look about them. We stressed that this was going to be an amazing experience — that not too many people in our profession have an opportunity to go through something like it. They worked safely with no accidents. Everyone knew what their jobs were and performed with very little supervision.”

An upgrade now in progress will boost wet-weather design capacity from 45 mgd to 75 mgd and correct some preflood deficiencies in the existing plant.

Silver lining

Hickey notes that the Clarksville plant, designed for 25 mgd dry-weather flow, had constraints that limited performance. Average flow was 10 mgd, but when flow exceeded about 12.5 mgd there was a risk of solids escaping the secondary clarifiers and of other issues leading to potential permit violations. “We’re using the fact we have to rebuild the plant anyway to change some operations and procedures and upgrade the facility to make it more efficient, so we’ll have the true capability to treat 25 mgd within our permit parameters,” Hickey says.

Before the flood, the headworks consisted of four drum screens, two vortex grit removal units, three primary clarifiers, three aeration basins fed by turbo blowers and ceramic fine-bubble diffusers, nine rectangular secondary clarifiers with top-mounted vacuum units, and UV disinfection before discharge to the river. Primary and secondary sludges were mixed and dewatered on plate-and-frame presses, and the biosolids were lime stabilized and land-applied.

From late April into early May 2010, the area saw torrential rains. “We are downstream from the Old Hickory Dam, the Wolf Creek Dam and several other dams on the Cumberland River,” says Hickey. “Throughout the rainy period we had no flooding. We were getting regular estimates of where the river was going to crest and everything looked pretty good.

“Our plant sits at an elevation of 378 feet. It’s protected by a berm with an elevation of 392. As late as Monday morning [May 3] at 8 o’clock, reports from the U.S. Army Corps of Engineers and others said the river was going to crest below 392 feet, which meant we would be OK.”

Kevin Buchanan, public utilities director, Water/Wastewater Division, was at the plant relaying his observations to Hickey. Concerned that Corps of Engineers data might be understating the risk, Buchanan and his team began moving equipment, computers and documents out.

Longtime submerged

As it turned out, on Sunday afternoon the Corps had significantly increased the rate of release from Old Hickory Dam, about 50 miles upstream. That meant a surge of water was heading for Clarksville. “Monday was a beautiful May morning — sunny, blue sky,” says Hickey. “Kevin and his group were taking a break and sitting on a bench on the berm when the water started to seep over the top. He called and told me they were evacuating.”

The water crested at 394.5 feet, putting all plant structures underwater except the headworks building and part of the primary clarifier pump building. While the major pump stations that feed the plant were inoperable, flow to the facility continued at 6 to 8 mgd. Meanwhile, the plant staff mobilized and began following emergency procurement procedures to arrange for pumps, lighting, generators and other emergency equipment needed to restore at least basic operations at the plant and lift stations.

The river crested on Tuesday, and the next day the water dropped below the level of the berm so that site dewatering could begin. By then the Clarksville team had engaged Allied Technical Services for recovery assistance; pumps were on site and were started immediately. All water pumped from the plant was disinfected with chlorine tablets and filtered through a 60- by 40-foot geosynthetic bag to provide primary treatment.

The entire bowl had been dewatered by Sunday, May 9, and restoration began. “We soon realized that we would be in some form of emergency operations for an extended time,” says Hickey. “We did have one motor for a primary clarifier that had been repaired and had been returned and was sitting on a hillside awaiting installation. That allowed us at least to get some primary treatment re-established.”

The long road back

By May 12, the screens, grit system and primary clarification were functioning and bulk sodium hypochlorite was providing disinfection. By week’s end, through emergency RFPs, the city had engaged the Hazen and Sawyer engineering firm for plant restoration and Shermco Industries to rebuild the electrical system.

“We found that while not all of our motor control centers had been totally flooded, wastewater had wicked up through the wires and into the equipment,” says Hickey. “Every electrical component and all the wiring had to be replaced.”

Shermco brought on the BELFOR disaster recovery firm for site cleaning and disinfection. “The 10-acre plant site inside the berm had been under 30 feet of sewage for five days and everything had to be disinfected and cleaned,” says Hickey. “That took several months.”

Restoring secondary treatment was critical: That took until August. The motors from the three 1,000 hp Turblex blowers (Evoqua) had to be removed and rebuilt by the manufacturer. The ceramic fine-bubble diffusers, destroyed by the flooding, were replaced with membrane fine-bubble units (Sanitaire). Then the secondary treatment process had to be reseeded. “That took time as well,” says Scott Woodard, P.E., a senior associate with Hazen and Sawyer. “It was just like starting up a new facility.”

Meanwhile, to restore final clarification, the Clari-Vac (Leopold) units had to be repaired. “Another contractor came in to gather up the Clari-Vacs, which were turned upside down, off track and bent up,” recalls Chris Lambert, public utilities senior director, Water/Wastewater Division. “They had to refabricate six out of the eight.” Solids dewatering was restored by way of a trailer-mounted belt press and centrifuge.

Through it all, the team had to relearn how to operate the plant manually — the flood had ruined the SCADA system. “We didn’t have VFDs where we used to have VFDs,” says Hickey. “We didn’t have DO sensors where we used to have them. They had to go out and get a sample, run it, come back, make adjustments and see what the results were. The staff handled it extremely well. Even today during the new plant construction, we’re still running manually.” Many of the on-line analyzers have already been replaced since the flood in order to more efficiently operate the plant, but the SCADA system has not been restored at this point.

A fresh start

During the recovery, the Clarksville team and Hazen and Sawyer moved straight into design of the new facility, to be complete in March 2016.

The upgrade includes improvements to the existing aeration basins, including a full-width effluent weir to eliminate short-circuiting and an anoxic zone at the front end. “The anoxic zone was added to limit filamentous bacteria and improve the settleability of activated sludge in the secondary clarifiers,” says Woodard. “That zone also provides a secondary benefit if nutrient limits are included in a future permit.”

Other upgrades will include:

  • A headworks relocated for better hydraulics and equipped with dual-entry drum screens (Ovivo) and a Eutek Headcell grit removal system (Hydro International).
  • Replacement of the rectangular secondary clarifiers with four 140-foot-diameter circular units (Walker Process Equipment).
  • A centrifuge (Andritz) for dewatering; biosolids will be landfilled.
  • Replacement of UV disinfection with chlorine contact.

A new administration and lab building was completed last September, and the berm around the plant has been raised to 398.5 feet elevation — 6 feet higher than before. The total cost of flood recovery and plant redesign and upgrades will be $120 million.

For the Clarksville team, the memory of the flood won’t soon wash away. “This wasn’t your run-of-the-mill 100-year event,” says Hickey. “I don’t know if it has been finally classified, but this was either a 500- or 1,000-year storm.”

Afterward, the team’s first “office” was a 10-foot-square awning over a table. Through much of 2014, the staff still worked out of temporary office trailers and with a prefabricated laboratory (CPM Labfab). In the early stages, long days were spent scrambling for equipment and for diesel fuel to power generators. In some respects the plant is still in a recovery mode.

Says Hickey: “It’s been one heck of a ride.” 



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