A Settling Process Using Magnetite Ballast Helps A Massachusetts Plant Meet Strict Phosphorus Limits

A phosphorous reduction process using magnetite ballast helps a Massachusetts treatment plant consistently meet strict effluent permit limits.
A Settling Process Using Magnetite Ballast Helps A Massachusetts Plant Meet Strict Phosphorus Limits
Dave Garabedian, operations and maintenance specialist, checks the shear mixer after the chemical bond between the floc and magnetite has been broken.

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From April through October (in-season), effluent from the Billerica (Mass.) Wastewater Treatment Plant averaged 0.80 mg/L total phosphorus. From November through March (off-season), it averaged 1.55 mg/L. In 2008, the U.S. EPA lowered the plant’s total phosphorus limit to 0.20 mg/L in-season and 0.75 mg/L off-season.

To comply, operators added sodium aluminate at the head of the treatment train and polyaluminum chloride solution (USALCO) at the end. “That lowered our in-season total phosphorus to 0.50 mg/L,” says Jeff Kalmes, plant supervisor. “We couldn’t use sodium aluminate in winter because it froze, so our off-season total phosphorus averaged 1.29 mg/L.”

As a remedy, the Woodard & Curran Engineering firm recommended the CoMag phosphorus reduction system (Evoqua Water Technologies) for its economical operation and small footprint to accommodate a retrofit. The plant was the nation’s first to receive a full-size CoMag system installation.

Preparing the plant

With 80 percent of the town’s 40,000 residents connected to the sewer system, the 5.4 mgd (design) activated sludge plant treats 3.83 mgd on average. Operators wasted secondary sludge to one of two diffused air flotation thickeners (DAFT), then stored it in a 70,000-gallon tank. A second identical tank held primary sludge, and a third mixed the contents of the first two before a Fournier rotary press dewatered the solids. Effluent was chlorinated, dechlorinated and discharged to the Concord River.

A two-month pilot study familiarized Evoqua Water Technologies engineers with the plant’s secondary effluent as it ran through the phosphorus removal system in a portable trailer. “Whenever they took a sample to test for phosphorus, we split it,” says Kalmes. “I ran my own test and sent a sample to a third-party laboratory. Our results always came in lower, which convinced us the system worked. However, we all used different EPA-approved tests, and that could account for the variances.”

In late 2008 the DAFT units were replaced with gravity thickeners (settling tanks), five 3 mgd tertiary pumps in the basement, and a storage area for the alum, caustic and polymer used to condition the secondary effluent. Workers also installed two magnetic drums to collect and recycle magnetite (Fe3O4), the mineral in the CoMag system that helps remove particulate containing phosphorus.

With the gravity thickeners online, operators routed sludges to them without stopping operations. As the holding tanks emptied, workers converted them to two 24-foot-diameter tertiary settling tanks and a pump room for the CoMag return activated sludge pumps. They installed the phosphorus removal system — four 11-foot-square by 11-foot-deep reaction tanks — alongside the former holding tanks. The workhorse of the system is magnetite, a mineral resembling black talcum powder but five times heavier than sand.

Mineral with a mission

Operators condition secondary effluent (CoMag influent) in the first two tanks to remove small amounts of TSS. The liquid then flows to the third (reaction) tank, where it mixes with fresh and recirculated magnetite.

Operators add 50 pounds of magnetite per day to a slurry tank; a 30 gpm on-demand pump then sends the mixture to reaction tank 3. Recirculated magnetite comes from the tertiary tanks. Operators add a small dose of Superfloc polymer (Kemira) per day to reaction tank 4 to create floc and enhance the capture rate of fine particulates.

The mixture then flows to the tertiary settling tanks. The high specific gravity of the ballasted floc produces rapid, reliable settling. The increase in solids density improves the capture of contaminants, enhances plant stability and enables the system to withstand high fluctuations in loads and flows.

Sludge wasted from the tertiary tanks flows through a shear mixer that breaks the chemical bond between the floc and magnetite. As the freed liquid rises, it hits a spinning magnetic drum that captures the mineral and returns it to the slurry tank for recirculation. The separated sludge flows back to the gravity thickener.

Surpassing expectations

“Initially, Evoqua Water Technologies said we’d probably lose 30 pounds of magnetite per day,” says Kalmes. “During the four years we’ve run the system, we’ve learned that conditioning effluent to 6.3 pH maximizes settling of phosphorus, aluminum and magnetite. The result is a loss of 1 to 6 pounds of mineral per day.”

Daily, two operators manually test the pH in the reaction tanks, and Kalmes collects a sample of CoMag waste activated sludge to ensure that it has a 6.3 pH. “Operators must be diligent and at the top of their game,” he says. “For example, by looking at our chemical usage every morning, they can identify a problem within five minutes.”

And therein lay the rub. The retrofit switched the plant from manual operation to automation. After Evoqua Water Technologies trained personnel on SCADA operation, the biggest challenge was getting them to use it. Kalmes made daily checklists to help operators familiarize themselves with the SCADA screens by cycling through them.

“Automation was a big move on many levels,” says Kalmes. “For example, we’re still running three shifts, but we’re going to one shortly. We’ll keep the off-shift operators, enabling us to tackle more projects and have backup personnel to cover vacations or illness.”

Tinkering with treatment

Billerica is unique in that the CoMag system is at the end of the treatment train, enabling operators to split the flow or bypass the reaction tanks and go directly to chlorination. The arrangement allows Kalmes and Dave Garabedian, operations and maintenance specialist, to experiment. During the off-season, they run 1 mgd through the CoMag system, testing ways to optimize performance by varying alum, polymer or magnesium volumes.

“Making small changes to the tertiary sludge wasting schedule taught us that we’ll have the best result if we waste for nine minutes on and 45 minutes off,” says Kalmes. “If we adjust the time a minute either way, we notice differences in the phosphorus and aluminum levels.”

The numbers speak for themselves. In-season effluent total phosphorus dropped to between 0.07 and 0.14 mg/L, and off-season went to 0.50 mg/L. “Our in-season influent has 196 mg/L BOD and the final effluent has 2 mg/L BOD and TSS,” says Kalmes. “In winter when we co-blend effluents from straight treatment and CoMag, we have 12 mg/L BOD and 9 mg/L TSS.” The permit is 30 mg/L for both.

A meter downstream of the tertiary settling tank but before chlorination measures turbidity. “It averages 0.51 NTU, but I’ve seen it as low as 0.20 NTU,” says Kalmes. “We can see a coin on the bottom of our 8-foot-deep chlorine tanks.” Nevertheless, he believes the team can work the system even harder to achieve still better results.

“As far as chemical and mineral usage, we’re exceeding anything projected by the technology providers,” says Kalmes. “They’re here often to see what we do and they use what they learn in future installations. Operators and engineers who have heard of our success come from China, Russia, Poland, Cyprus and across the U.S. At the end of the day, we know what we’ve done equals quality going out and we’re mighty proud of it.”   



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