Minnesota Plant Finds Chemical Solution for Phosphorus Reduction

Minnesota Plant Finds Chemical Solution for Phosphorus Reduction
The pilot study setup includes two bulk chemical tanks for polyaluminum chloride. Peristaltic pumps and tubing enable discharge of the chemical over the basin walls and on top of the outfall weirs.

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Effluent from the Virginia (Minnesota) Wastewater Treatment Plant discharges to Manganika Lake, which is impaired by eutrophication. The Minnesota Pollution Control Agency ordered the plant to achieve 0.07 mg/L monthly average total phosphorus by March 2023.

In 2015, operators ran a 38-day study to see if adding polyaluminum chloride (PAC) would lower and maintain consistent TP levels. “It was one of multiple studies we will do to determine which solution is best for the plant,” says Jeff Frost, lead operator from PeopleService, contract operator of the facility.

Results from the study showed that injecting 70 ppm ferric chloride (FeCl) and 45 ppm PAC produced a sustainable effluent TP of 0.053 mg/L for an annual chemical cost of $168,217.

Treatment train

At the study’s outset, influent TP averaged 3.3 mg/L. An FeCl injection system removed phosphorus using a 10,000-gallon storage tank, a 405-gallon day-use tank and three peristaltic pumps feeding chemicals before the primary clarifiers and at the head of each aeration basin. Effluent TP was less than 0.50 mg/L.

A 2012 plant upgrade included four fine ROTAMAT mechanical bar screens (Huber Technology), a grit vortex removal system (WesTech Engineering), two 296,000-gallon primary clarifiers, two 250,000-gallon conventional activated sludge aeration basins, two 486,000-gallon secondary clarifiers, four sand/anthracite gravity filters and UV disinfection (TrojanUV).

A 267,000-gallon mesophilic anaerobic digester has dual alternating pumps that recirculate sludge at 1,000 gpm and send it through a heat exchanger maintained at 97 degrees F. From two 85,340-gallon storage tanks, material at 4 to 5 percent solids is sent to a Klampress 2-meter belt filter press (Alfa Laval Ashbrook Simon-Hartley). The resulting cake is land-applied.

Preliminary work

A chemist with Hawkins Water Treatment Group jar-tested different PACs on the plant’s mixed liquor, identifying AquaHawk 2192 as the best fit. The city budgeted $21,000 for it. “By the time we had gone through $11,000 worth of PAC, we had reached our objective and there was nothing else to learn,” says Frost. “Ending the study early also enabled us to stay under our annual budget for chemicals.”

Although the plant has its own laboratory, it uses Pace Analytical Services for certified tests. Pace’s normal phosphorus effluent reporting limit was 0.10 mg/L. That wasn’t accurate enough for the study, so the city paid for a test with reporting capabilities of 0.004 mg/L.

In May 2015, operators increased the aeration basin dissolved oxygen level from 1.0 mg/L to 2.0 mg/L to enhance conversion of nonreactive phosphorus to reactive phosphorus. They also relocated the FeCl influent feed point from before the grit vortex to the cascade point before the primary clarifier splitter box for additional mixing. The chemist recommended feeding FeCl at 70 ppm to remove as much phosphorus as possible before beginning the study. That dropped levels to between 0.15 mg/L and 0.19 mg/L.

One week before the study began, operators dewatered sludge in the holding tanks to provide 14 days of storage capacity. Hawkins Inc., the plant’s chemical supplier, set and filled two chemical storage tanks at the rear of the east and west aeration basins, then calibrated the peristaltic pumps (Stenner Pump Company).

To extend the feed tubes beyond the basin walls to drip chemical on top of the outfall weirs, operators bolted a 1-inch PVC pipe to each wall, inserted the tube and zip-tied the two together. They offset the screw caps on top of the chemical tanks to position the end of the suction tube on the bottom. “The offset left a gap, which we covered with a tarp to prevent rain from diluting the PAC or falling on the pumps,” says Frost.

Setting parameters

The study began on July 9 and ended Sept. 8, 2015. Three times a week, Pace chemists analyzed influent and effluent composite samples for TP, TSS and CBOD. They analyzed effluent composite samples for low-level TP daily.

Plant operators analyzed daily effluent composite samples for orthophosphate using PhosVer3 reagent powder pillows and a Hach DR/850 colorimeter. They also recorded influent flow, backwash water return and belt press filtrate return once a day, and recorded effluent turbidities and water height above the media filters throughout the day. The filters were backwashed automatically once a day.

“Our major concern was clogging the filters by adding too much PAC,” says Frost. “Our chemist suggested a beginning feed rate of 30 ppm, then increasing it and monitoring the beds.” Biosolids were dewatered three times each in June, August and September. On those days, operators increased the FeCl feed to the head of the plant by 13 gallons during the eight-hour press run.

Not a drill

For the first week, operators fed FeCl at 83 gpd and PAC at 30 gpd. “Two days into the study, our lab reported an orthophosphate result of 0.07 mg/L,” says Frost. On July 20, the PAC feed rate increased to 40 gpd. Two days later, the temperature probe on the digester spiked to 105 degrees F. Operators found the recirculation pump running hot, making a different noise, and the pressure check valve down. Switching pumps brought an identical result.

“We opened the inspection ports and discovered the recirculation lines clogged with up to 18.5 percent total solids,” says Frost. Operators stopped feeding PAC and injected 5,000 gallons of water a day into the digester to dilute the solids. They rerouted the recirculation lines onto the belt press and modified the wash troughs on the clarifier skimmers to add more water with each revolution. The pumps returned to normal. By Aug. 14, total solids had decreased to 4.4 percent, and operators resumed feeding PAC at 50 gpd.

Winding down

The feed rate increased to 60 gpd on Aug. 31. Operators also collected the final round of primary clarifier effluent, aeration basin mixed liquor, final clarifier influent and effluent, facility effluent, belt press filtrate return, filter backwash water, and secondary digester return water samples. These were tested for TP, dissolved phosphorus and orthophosphates.

Severe thunderstorms over the Labor Day weekend contributed to high flows and multiple power failures at the plant. “The feed pumps were powered by extension cords plugged into ground-fault circuit interrupter outlets,” says Frost. “We reset the outlets many times as electrical glitches tripped them.”

With less than 40 gallons of chemical remaining, the failures were insignificant. Operators drained the remaining 10 gallons of PAC into the aeration basins on Sept. 8. Hawkins refilled the tanks twice during the study.

“We couldn’t have done the study in winter because PAC freezes at temperatures in the mid-20s,” says Frost. “If we use it in the future, the city will have to add another heated building or retrofit a chemical supply building for the tanks, then bury the feed lines below the 7-foot-deep frost layer.”

Conclusions

The study proved that PAC concentrations of 69 ppm made the media filters perform better, rather than bind them. The amount didn’t affect TSS removal, total low-level mercury, dissolved low-level mercury or water heights above the beds.

“Operators noticed that even effluent in the final clarifiers looked cleaner the day after we began adding PAC,” says Frost. Effluent turbidity decreased from 1.50 NTU to 0.50 NTU during the study, while chloride concentrations increased by 18.1 percent. Other salty parameters (bicarbonates, calcium, hardness, magnesium, potassium, sodium, total dissolved solids, specific conductance and sulfate) were not affected.

The lowest effluent TP result was 0.018 mg/L, achieved when feeding 83 gpd of FeCl and 60 gpd of PAC. The study used 18,000 pounds (1,611 gallons) of PAC and 60,723 pounds (5,369 gallons) of FeCl. “We also learned that increasing the ferric chloride feed rate to the belt press filtrate as it returns to the head of the plant helps treat the additional phosphorus loading,” says Frost.

While Frost recommends PAC to those who must meet a limit quickly, he doesn’t believe it will help all plants, as Virginia’s are somewhat unique: a low phosphorus load and an influent pH averaging 8.3. “The high pH and alkalinity enable us to add ferric chloride at a higher amount,” says Frost. “The primary clarifiers remove 30 percent of phosphorus because they come before the aeration basins. The filters are designed for mercury and TSS, but also help with phosphorus.”



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