Automation Isn't the Sole Solution to Drive Down Turbidity

The team at the W.R. Wise Water Treatment Plant combines manual and automated processes to maintain high water clarity at Partnership Phase IV Excellence levels.
Automation Isn't the Sole Solution to Drive Down Turbidity
Treatment residuals are dewatered on drying beds on the plant property before being hauled to a landfill for use as cover material.

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If the water treatment plant team in Greenwood had a nickname, it might be The Turbidity Cops.

That’s because they do everything possible to eliminate turbidity from their produced water.

As a result, they’ve received the prestigious Partnership for Safe Water Phase IV Excellence in Water Treatment Award from the American Water Works Association for 10 years running.

“Turbidity is the key,” says C. David Tuck, superintendent of the W.R. Wise Water Treatment Plant in Greenwood, in northwestern South Carolina. “Typically, we’re right around 99.9 percent removal.” That’s from source water from Lake Greenwood with an average turbidity of 12 NTU based on the last three years of data.

“Extra capacity at the plant helps,” Tuck says. “Our sedimentation basins are designed for 33 mgd, and right now we’re at about 9 to 10 mgd. That gives us more time for settling.” The plant staff is also an important factor, springing into action with jar tests and keeping alum and caustic soda pumps operational and calibrated when the watershed gets heavy rain.

“We have good source water, a good plant and a good staff,” says Tuck. “In the settled water, we’re at less than 2 NTU 100 percent of the time, and less than 1 NTU more than 95 percent of the time, even during rare events of raw water turbidity over 100 NTU. Filtered water turbidity is less than 0.10 NTU 100 percent of the time.”

Conventional process

Lake Greenwood is an 11,400-acre reservoir formed by a hydroelectric dam built across the Saluda River in the 1940s. Two intakes 35 feet deep can draw water from three levels. Normally, water is drawn from the surface lake using variable-speed and fixed-speed pumps, which transport the water to the treatment plant, a quarter-mile away.

During summer, plant production averages 11 mgd but can fall to 8 to 9 mgd in winter. “We are only at 30 to 40 percent capacity,” says Tuck. “The plant was built for projected growth that never happened.” Most of the city’s textile mills have moved or shut down, but Tuck expects industrial development to resume: “We anticipate a new carbon fiber manufacturing facility coming online soon, and that should require 5 to 10 mgd.”

At the head of the plant, powdered activated carbon (PAC) is added to improve taste and reduce odors. Then, through an on-site generation system supplied by Eka Chemicals (now part of NALCO, an ECOLAB Company), chlorine dioxide is injected into the stream as a pre-oxidant ahead of the coagulation-flocculation-sedimentation process.

The chlorine dioxide oxidizes impurities but does not react with organic material in the raw water that might otherwise produce disinfection byproducts in post-treatment chlorination. “On average, our DBPs are around 20 ppb,” Tuck says. Alum is added in one two-stage flash-mix tank with a detention time of three to five minutes. Three-stage slow-mixing, horizontal paddle-style flocculators form floc that settles in a series of rectangular sedimentation basins.

Deep-bed filtration

Eleven deep-bed multimedia filters polish the settled water before disinfection and release into the storage and distribution system. The plant uses sand and anthracite media; some of the filter bottoms are a perforated block type supplied by Leopold - a Xylem Brand, and some use cone-type, ceramic-ball-design Wheeler bottom underdrains.

“Our filters don’t use an air scour feature,” says Tuck. “Instead, we use surface sweeps during the backwash. Each filter run is about 190 hours, or eight days between backwashes.” The plant’s excess capacity and excellent settled water quality enable such long filter runs. Some of the anthracite has been in place and functioning for 20 years and is being replaced. “It became a size issue,” Tuck says. “The effective size and uniformity coefficient of the media were getting out of specifications due to years of use.”

After filtration, the water is injected with aqua ammonia, which combines with free chlorine to form chloramines. Lime is added to bring finished water pH to 8.2 to 8.4 for corrosion control and optimal formation of monochloramine. Fluoride is added for dental health. Solids from the sedimentation basins and the filter backwash water flow to two sludge holding basins.

After additional settling, the solids are thickened and pumped to drying beds, where an anionic polymer facilitates dewatering. The water is decanted back to the lake under an NPDES permit. After six to eight weeks, the dried sludge is removed from the bed with a loader and transported to the local landfill for use as cover.

Until now, the plant has not had to add special chemicals for corrosion control, as its distribution system includes no lead service lines. “We have lots of ductile iron, galvanized and cast iron pipe in the system,” Tuck says. “We have been in compliance with the lead and copper rules since 1992. We’re not seeing any issues.”

In the future, Greenwood will install new RE-MIN Process and Cal Flo feed systems (Burnett Lime Co.) to add lime and carbon dioxide. This will increase calcium hardness and alkalinity, and make the water more stable and less aggressive to metal piping materials. The utility is also replacing pipes, some 120 years old, with new ductile iron lines. An automated water metering system is being deployed to track customer water usage more efficiently.

Keeping it clear

Tuck lauds the plant’s operations team members, most of whom have earned the Class A license from the state. The operators are Jack Able, Benny Webber, Eddie Brown, Jay Daniels, Shane Duncan, Darrell Green, Jimmy Nix and Neal Richard. Sam Abney and David Crawford handle maintenance, Class A-licensed chemist Charles Dunn and technician Brandon Lewis run the laboratory, and Vincent Price is the SCADA technician-electrician. Jeff Chapman is water department director and Steve Reeves is general manager.

Experience is the hallmark: Able and Webber came back to work after retirement. “In South Carolina, and as Greenwood Commissioners of Public Works allow, you can keep working after retirement,” Tuck says.

The team knows exactly what to do to protect the plant’s finished water from turbidity intrusions.

In heavy rains, remote turbidimeters (Hach) upstream of the intake feed data to the SCADA system. If the data shows that turbidity is increasing, the staff goes into the turbidity prevention mode, setting up the plant to handle the load.

“We do jar testing to determine optimal coagulation pH and alum dose, and make sure the caustic soda pumps are ready to run,” says Tuck. “Our alum dose is set for the optimal pH.” The coagulation pH is temperature-dependent: 6.9 to 7 in winter and 6.2 in summer.

“We need to find the sweet spot,” Tuck says. “When we get the coagulation optimized, it has a positive impact on our sedimentation and filtration.” Operator training sessions are held monthly or quarterly to review standard operating procedures. Team members share ideas freely within and between shifts.

In the plant’s state-certified lab, Dunn and Lewis handle all compliance monitoring quality control and lab training for the operators as required by the Safe Drinking Water Act and the Clean Water Act. Tuck believes some tasks should remain manual while others can be automated.

Filter backwashing has been automated since 2005. On the other hand, coagulation is not automated. “We think if it were, we might get too complacent,” Tuck says. “We check zeta potential through in-stream monitoring using a Micrometrix streaming current monitor at the flash mix tank. It monitors the net chemical charge in the coagulated water and alarms the operator of any setpoint excursions.

“It’s important to make sure we have the alum and caustic feed right at the correct dosages and keep it under control. Our people are experts at this. It’s a diverse staff, in a good frame of mind, and they know what to be ready for. We’re all on the same page.”

Do-it-yourself approach

The Greenwood team takes the hands-on approach to maintenance, as well, farming out as little maintenance as possible. “Stuff happens at night and on weekends,” Tuck says. “We do most of it ourselves, rather than hiring outside labor.” That approach extends to everything except large vertical turbine pumps, motors, specialty equipment and elevated tanks, where SUEZ Utility Services Co. handles clean-out and painting.

Plant staff members calibrate all instruments, do electrical and mechanical preventive and corrective maintenance, and practice predictive maintenance with vibration analysis on all rotating equipment. The SCADA system is by Allen-Bradley by Rockwell Automation. All the A-B Control Logix PLCs are programmed in-house, and the Ethernet communication network using fiber-optic cable is maintained within the plant. “It’s a welcome expertise to have on hand,” Tuck says. “We’re ready for just about anything that might come up.”

The staff shares pride in the continued Phase IV Excellence achievement. “It’s the first in South Carolina, and one of only 16 water treatment plants awarded across the country,” Tuck says.

“Before this, we had achieved the Phase III Directors Award and maintained it for 15 years.”

The goals of the Phase IV program are the same goals the staff shares in operating the plant:

“They’ve bought into the program. It has made us all better as operators. We are all about making excellent water for our customers, going beyond just meeting the regulations to be the best. It’s been very worthwhile.”

PAC for taste and odors

C. David Tuck recalls the day a customer called and complained that his water “tasted like dirt.”

“We have some nuisance algae in the spring, and sometimes we get an earthy and musty taste in our water,” says Tuck, Greenwood water plant superintendent. “It’s due to our warm climate and a long growing season for the algae, which produce organic compounds.”

That’s when the plant’s powdered activated carbon (PAC) system (Acrison) comes to the rescue. The carbon is added close to the raw water intakes on Lake Greenwood to improve taste and counter odors. Carbons approved for plant use include Aqua Nuchar (Ingevity), Norit and Hydrodarco B (Cabot Norit Activated Carbon) and MasterCarb (M.L. Ball Co.).

“PAC has a high adsorption rate because of large surface area,” says Tuck. “We get around a 60 percent reduction in taste- and odor-causing compounds. It reduces the concentrations to sub-threshold levels of 10 µg/L, a beginning level that can be detected by humans. Additionally, our finished water has less than 2 mg/L total organic carbon, which is attributable to the use of PAC and optimized coagulation.”

The PAC system generally operates from March through October: “We can pretty much predict when the problem will begin, based on historical data and warming water temperature. We start up the PAC system in advance of that.”


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