Here's How a North Carolina Plant Team Deals With Variable Source Water Turbidity

Charge analyzers help a North Carolina plant team deal with big weather-driven variations in source water turbidity.
Here's How a North Carolina Plant Team Deals With Variable Source Water Turbidity
A Chemtrac DuraTrac 4 Streaming Current Sensor downstream of the flash mixer.

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The Yadkin River produced wide and rapid turbidity swings that regularly challenged the city of King Water Treatment Plant operators. Turbidity on dry days averaged 6 to 8 NTU, but heavy rains could raise levels to 800 NTU or higher within 30 minutes.

The operators at the plant in northeastern North Carolina ran six-beaker jar tests and then waited for the results, which could take an hour or so. “By then, the water we were testing had passed through the flocculators and was on its way to the filtration basins,” says Ben Marion, plant supervisor. “During storms, we fed alum as hard as we could (100 ppm), but the settled numbers never fell below 6 to 10 NTU.”

Looking for solutions, operators in 2014 switched to feeding DelPAC 2000, a polyaluminum chloride (USALCO). “It worked well, but we still lacked a way to jar-test samples quickly,” Marion says. “We were in the middle of a plant upgrade, and I wanted my operators to have the best technology possible to run it.”

The facility had an outdated streaming current monitor; Marion discussed options with Chemtrac representative Mark Vandiver, who brought a benchtop LCA-2 Laboratory Charge Analyzer that measured the negative surface charge of particles and predicted through a titration process the amount of PAC to feed to neutralize the charge.

“That was the tipping point,” Marion says. “We manually titrated PAC with a pipette and five minutes later had the result. When the demo was over, we bought the machine.” Its addition to the laboratory influenced the entire treatment train. Later, the plant team deployed more advanced online charge analyzers, further improving process consistency.

Conventional process

The 3 mgd (6 mgd final design) plant renovation opened in November 2015. Two 2,100 gpm vertical turbine pumps from Aurora pumps (Pentair) in the raw water pump station deliver Yadkin River water to a static flash mixer (Lightnin, an SPX Brand) at the front of the plant. Treatment includes feeding of DelPAC 2000 and adjusting pH as necessary.

Water flows from the mixer through eight 5,200-gallon flocculation tanks (four per train) with Lightnin mixers before entering two 280,000-gallon sedimentation basins. Operators add sodium hypochlorite to remove iron and manganese before the water flows to four FilterWorx dual-media filters (Leopold - a Xylem Brand), passing through 34 inches of anthracite over 6 inches of sand with a Leopold IMS cap porous plate support.

After filtration, the water is treated with caustic soda and poly-orthophosphate and then stored on site in four above-ground tanks totaling 2.5 million gallons. Sodium hypochlorite is fed to the water before two 350 hp, 4,800 gpm vertical-turbine high-service pumps from Aurora pumps (Pentair) and to a booster pump station for storage in three in-ground tanks totaling 3.5 million gallons.

From the booster station, water is pumped to three elevated storage tanks (1.05 million gallons total) and an in-ground storage tank before distribution to 25,000 customers through 12-inch mains. The system stores 8 million gallons — a three-day supply of water.

Boosting performance

Once the charge analyzer was deployed, “The changes in the plant were amazing,” Marion says. “Now, it took only minutes to adjust chemical doses during heavy rain, and we were feeding 400 pounds of PAC per day instead of 800 pounds.” Reducing the chemical feed lightened the load on the filters, improved filter times, and extended media life.

“Heavy rain used to push the filters almost to our limit of 0.30 NTU and reduce filter run times to six or eight hours,” Marion says. “Better flocculation upstream has enabled operators to air scour the filters at intervals of 84 hours versus 48 to 60 hours previously and to replace the anthracite in five or six years instead of annually.”

Removing more organics and particulates in the sedimentation basins increased the volume of sludge discharged to the waste well daily. Operators dewatering the material weekly on a Klampress 1-meter belt filter press (Alfa Laval) watched polymer usage drop from 150 pounds per session to less than 100 pounds. “Cake went from 18 to 30 percent solids,” Marion says. “Every two weeks, we haul it to a landfill. Even with drier material, our volume rose from 3.5 to 8 dry tons.”

Enhancing optimization

In May 2016, Vandiver suggested pairing the LCA unit with a Chemtrac HA4 online Streaming Current Charge Analyzer (HydroACT 4 analyzer and DuraTrac 4 sensor) to confirm online whether enough chemical had been added to neutralize the charge. “The thought of seeing it go through the flash mixer in real time was intriguing,” Marion says. “When the operators learned they could base dosages solely off the streaming current monitor, two threatened to quit if I returned the demonstration unit.” He didn’t.

After six months, the operators had learned what optimization settings worked best in the analyzer. Marion first called other area operators with streaming current monitors in their plants and asked what settings they were running. “It varied widely,” he says. “One town was running a plus 30 charge, while another was running a negative 10.”

The operators closely tracked system data to determine the streaming current charge range targets for the most effective turbidity reduction. “We learned that running a positive 10 to 15 charge on the upside of turbidity gave us optimal settled numbers, and a negative 10 on the downside kept us one step ahead of it,” Marion says. “Then, the storm hit in late April 2017.”

Mud and mayhem

The Yadkin River, fed by seven days of downpours, surpassed the 16-foot flood stage and crested at 22 feet. The average river level at the facility is 2 to 2 1/2 feet. “When we came in at 7:30 Monday morning, source water turbidity was around 40 NTU and we were feeding 28 ppm PAC,” Marion says. “Within 30 minutes after the rain began, turbidity jumped to 400 NTU. We ran the LCA and got 70 ppm PAC. We then fine-tuned the doses using the online streaming current analyzer. That was huge for us.”

Source water turbidity rose to 900 NTU, and runoff held it there for two weeks. Operators maintained a settled number of 3.0 NTU or less without seeing an increase in disinfection byproducts. “Decreasing our chlorine feed in the sedimentation basins — combined with the removal of 99 percent of organics — was leaving nothing on which total trihalomethanes and haloacetic acids could form,” Marion says.

Nevertheless, the experience convinced him to upgrade the HA4 analyzer to an HA8, allowing for the addition of two pH and three chlorine sensors. “They all run through the same box, and the best thing is that reagents aren’t required to test chlorine,” Marion says. “The $200 we save per month on reagents covers the increased tipping fee at the landfill.” Most important to the operators: They now have the technology and equipment to meet their top priority of providing the best water to every customer.

An additional outcome of the team’s efforts could include recognition by the state Area Wide Optimization Program. Marion says, “We are now meeting the AWOP requirements and hope that by the end of the year we’ll be an optimized plant.”



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