How Kemira PAC Product Eliminates Microthrix Blooms

Operators at a Colorado plant beat back winter filamentous bacteria outbreaks by feeding a polyaluminum chloride solution.
How Kemira PAC Product Eliminates Microthrix Blooms
After 72 days of feeding Kemira FEX-208, the anoxic reactors are back to normal with no traces of Microthrix bacteria.

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Winter blooms of Microthrix parvicella at the James R. Dilorio Water Reclamation Facility caused extreme slowing of the 30-minute solids settling rate. The increase from 100 to 300 sludge volume index (SVI) led to additional wasting and more solids to dewater.

The tangle of brown, matted foam formed a scum cap on the center ring of the secondary clarifiers at the plant in Pueblo, Colorado. As proteins in the bacteria degraded, they emitted an odor operators found revolting. “We tried chlorination, but it didn’t cause a die off,” says John Millard, the plant’s assistant superintendent. “That’s partly because much of the bacteria floats and doesn’t come in contact with the chlorine.”

Operators also installed water spray bars in the mixed liquor return box on either side of the eight anoxic basins. The spray broke up the foam, which released trapped air and prevented the foam from escaping. Then superintendent John Lindstrom read a case study done by Denver Metro Wastewater Reclamation District on FEX-208, liquid polyaluminum chloride (PAC) from Kemira. The chemical controlled the foam.

In a 72-day trial of the product in 2014, Millard’s laboratory staff watched the filament count go from “excessive” (present in all flocs, appears more filaments than flocs, or filaments growing in high abundance in bulk solution) to “some” (filaments commonly observed, but not present in all flocs). “We were back to the level at which wastewater plants must operate.”

Nasty characters

The 19 mgd (design) activated sludge plant averages 10.5 to 11 mgd. Microthrix blooms began after a 2011 upgrade to biological nutrient removal. Temperatures just below 60 degrees F triggered the bacteria’s proliferation, usually in late October or early November.

Outbreaks began with nuisance foam but accelerated to the “very common” and then “abundant” stage at 57 degrees F. As Microthrix entered the mixed liquor, it slowed the settling rate. “Our remedy was to increase wasting and do borderline nitrification because we have a higher ammonia limit,” says Millard. “However, the plant isn’t very stable with partial nitrification.”

The blooms made sludge light and fluffy, dropping its return wasted volume from 5,300 mg/L to 3,900 mg/L. To compensate, operators increased wasting from 150 to 200 gpm. Of even greater concern was the Microthrix entering the four anaerobic digesters. “In severe outbreaks, foam enters and plugs the gas lines and can even blow off the domes,” says Millard.

In summer, operators dewater biosolids in drying beds, but they use two Humboldt centrifuges in winter. During the Microthrix upset, they increased feed to the units from 170 to 250 gpm.

Silver bullet

Lindstrom discussed the plant’s condition with Tafadzwa “Tee” Mariga, Kemira applications manager. Mariga did numerous jar tests because operators were also concerned about phosphorus levels. “By the time Tee entered the picture, the demand for Kemira tanks and pumps had outstripped the supply,” says Millard. “Tee rented similar equipment, and the Kemira field services team set up the feed pumps to ensure accurate dosing.”

Kemira set the chemical storage tank on a concrete pad alongside the pre-anoxic, anoxic and anaerobic complex, then ran the feed line sleeved in a second hose to the return activated sludge channel in the pre-anoxic basin. The storage tank holds 47,000 pounds of chemical. The trial used 148,144 pounds, or 13,158 gallons.

Operators began feeding FEX-208 at 12 gph on Dec. 20, 2014. They reduced the amount to 9.6 gph on Dec. 31 and lowered it to a maintenance dose of 5.6 gph on Jan. 15. “Watching an SVI pushing 300 drop under 200, then down to 100 within two and a half months was exciting,” says Millard. “We also saw phosphorus levels drop from 1.4 mg/L to 0.104 mg/L.”

Offensive strategy

Staying alert to changing plant conditions is the operators’ strongest game. Every shift checks the basins for signs of foam and does a settleometer test. If the settling rate increases significantly, they check the SVI and do a microscopic examination to count and identify the filaments.

At the first sign of trouble, operators are ready to administer a maintenance dose. Last summer, a contractor converted the chlorine feed room to hold the PAC storage tank with lines to the return activated sludge pumps. A tank of acetic acid will introduce carbon to the anoxic zone to increase denitrification.

“My best advice to operators is if you had trouble last winter, call Kemira,” says Millard. “Get your name on the list early to test the system using their tanks and pumps. If you must rent equipment, let Tee help find the proper vendor. He is hugely responsible for our success.”

The rental equipment from a local company cost $10,000. The chemical for the trial cost $45,920. “These expenditures make a lot more sense than being fined by the EPA for permit violations,” says Gene Michael, wastewater director.

According to Mariga, if operators had been able to feed a maintenance dose a little sooner last year, they may not have had to hammer the outbreak as hard, and the cost could have been lower.

Millard isn’t bothered by what might have been. He’s happy, relieved and proud of his staff: “They did a wonderful job with the drawdowns and remain vigilant on their rounds.”


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