“Doing what they need to do, with what they have.” That would be an appropriate tagline for the Attleboro (Mass.) Wastewater Treatment Facility.

Superintendent Paul Kennedy and his staff have used their ingenuity to overcome a number of challenges while operating the treatment processes cost-effectively and meet tightening discharge standards.

Plant team members have creatively used excess capacity to achieve nitrification-denitrification. In addition, they’re dealing with high ammonia andcopper in the influent stream and have implemented an effective industrial pretreatment program. In 2013, the facility won the Best Medium Plant Performance Award from the Massachusetts Water Pollution Control Association.

“That award was really big for us,” says Kennedy. “We’ve always been a good operating plant, but there are 40 or 50 plants our size in Massachusetts. “We’ve never received an award like that before. It was great.”

Cutting down nitrogen

The Attleboro plant is designed for 8.6 mgd and now handles 4.0 mgd. The plant was built in 1980 to replace an old trickling filter operation. A $30 million expansion in 2008 essentially upgraded all the process equipment.

Wastewater passes through the headworks, equipped with a bar screen, a pair of comminutors and an aerated grit chamber. Then it flows through a Parshall flume before two flash mixers and flocculation mixers where ferric chloride is added for phosphorus removal and lime for pH adjustment and copper precipitation. (Lightnin mixers were supplied by Cole-Parmer and lime slakers by Merrick Industries.)

Settling occurs in three 80-foot-diameter, 12-foot-deep clarifiers (Walker Process Equipment). Primary sludge is sent to gravity thickeners, and the overflow moves on to a second-stage wet well ahead of secondary aeration.

“We have a total of ten aeration tanks, each equipped with mechanical surface aerators [Philadelphia Mixing Solutions]. Four tanks are dedicated as anoxic zones and contain two mixers each [Flygt – a Xylem Brand]. We add MicroC from Environmental Operating Solutions as a carbon source to achieve denitrification. We have spring and summer nitrogen limits.”

The remaining six basins serve as aerobic zones for nitrification. Each basin is 10 feet deep and holds 224,000 gallons. The aeration tank effluent flows to three 12-foot-deep, 100-foot-diameter secondary clarifiers. The overflow moves on to a trio of traveling bridge sand filters (Aqua-Aerobic Systems). Imported sodium hypochlorite is used for disinfection, and sodium bisulfate for dechlorination.

Waste activated sludge is blended with primary solids, thickened, dosed with lime and ferric chloride, and dewatered on a plate-and-frame filter press (Evoqua). Kennedy says the staff likes the unit’s automatic cloth washing feature.

Onsite landfill

Biosolids cake at 30 to 35 percent solids drops through hoppers to trucks that transport it to a landfill on the plant site. Kennedy reports that the current landfill cells are nearly full and another cell is under construction. Being close by, the landfill has proven cost-effective for handling biosolids. “We’ve looked at liquid hauling, but it was double the cost of landfilling — as much as $1 million a year.”

To control odors, Attleboro operates two biofilters (Duall, a CECO Environmental Company) — at the headworks and in the solids area, which houses the blended sludge tanks and filter press. A carbon filter deals with odors in the grease and scum collection area.

Plant processes are controlled by a SCADA system with eight terminals around the plant that allow operators to switch equipment, make process changes, and adjust air rates or turn the blowers on or off. The system also displays and controls pumping and sludge wasting rates and chemical tank and wet well levels. For preventive maintenance, Attleboro uses JobCalc maintenance software (Hach), which assigns work orders and keeps track of work history.

Solving problems

In New England, an old-fashioned expression is, “Solve your own problems.” It perfectly represents the attitude with which the Attleboro staff attacks challenges. Total nitrogen removal was perhaps the biggest hurdle.

“We had just finished our $30 million upgrade, and we had inquired about a total nitrogen limit in our new permit,” explains Kennedy. “A TMDL [total maximum daily limit] had not been established for nitrogen on our river, and we believed we would not have a limit when our permit was renewed.”

As it turned out, Attleboro was indeed subject to new nitrogen limits. Given the full upgrade, Kennedy and the city council were not keen on getting funds for more equipment, so plant team members concentrated on what they had.

In the original plant design, four aerators were used in a small first-stage biological system, followed by a set of first-stage clarifiers. The idea was to reduce BOD and prepare the wastewater for the second stage. “They ran it a while, but it didn’t really work,” Kennedy says. Since then, Attleboro has achieved full nitrification in the second stage, saving on operational costs, while using the first-stage system to enhance copper precipitation.

Looking upstream

Faced with the new nitrogen limits, Attleboro convinced the U.S EPA to allow the plant to experiment with a full-scale nitrogen removal pilot plant. “We took our existing configuration of aerators and made that work,” says Kennedy. “We converted four of the 10 second-stage aerators to anoxic zones and added mixers along with a carbon-source chemical, and we are able to denitrify.”

The full-scale pilot plant has been running for about two years, and the results of nitrification-denitrification sequence have been good, especially after an outside ammonia issue was identified and removed. “A local industrial plant had been discharging ammonia to the sewers at 7,000 to 8,000 ppm total Kjeldahl nitrogen [TKN],” Kennedy says. “We could not meet our permit [8 mg/L] with what they were discharging.”

The solution involved an ammonia recovery system at the industrial plant that reduced its discharge to 200 to 300 ppm TKN. “We worked with the industry and the city,” Kennedy says. “There was no TKN limit in the city ordinance. We revised the local limits and issued a compliance order.”

Success followed. With the pretreatment order in place and the anoxic zones and carbon source fully functioning, Attleboro has done well meeting its total nitrogen permit. At times, the plant has recorded effluent values of 2 to 3 mg/L.

Smart use of clarifiers has solved other problems. Attleboro slakes lime and adds the slurry to wastewater at headworks and copper is removed in the primary sludge. The copper comes from household plumbing and from septage. “We have a copper limit of 13 micrograms per liter on a monthly average,” says Kennedy. “With pH in the 7.5 to 7.8 range, the lime successfully precipitates copper into the sludge. If we’re in the mid to low pH 6 range, the copper will go back into solution and we’ll see it in the effluent.”

While copper precipitation works well, the lime had created a maintenance issue as it built up in the clarifier. “However, once we started adding lime at headworks into our grit chamber, maintenance issues were drastically reduced because now the bulk of the lime grit settles in our grit chamber and is mechanically removed.

In addition to its industrial pretreatment requirements, which affect 17 industries, Attleboro maintains an aggressive FOG (fats, oil and grease) program covering 109 commercial businesses.

The road ahead

Not content to rest on their laurels — even with the new award plaque hanging in the lobby along with congratulations from the mayor and council — Kennedy and his staff are keenly aware of future challenges. “In the next two to three years, we’re looking at renewal of our NPDES permit,” says Kennedy. “We expect that at least we’ll have to go from 8 to 5 mg/L on total nitrogen. I think they may want us to go lower on aluminum as well.”

That could be a problem: “Right now, we add polyaluminum chloride for phosphorus removal. We use a dual feed — ferric chloride at the headworks and polyaluminum chloride in the aerators — and it works really well.” The plant can meet its current limit of 122 micrograms per liter for aluminum, but if the limit is lower, that will be difficult.

Since the plant discharges into a river that is part of the Narragansett Bay watershed, Kennedy is well aware of water-quality standards for the stream and regularly attends meetings that discuss control strategies. “We’re doing a good job of cleaning up the Ten Mile River,” he says. “Whether it’s phosphorus, metals or coliform, our numbers are good. We didn’t win that award for nothing. The biggest problem is stormwater and snow melt. It’s not regulated, and a lot of the pollution is coming from runoff from impervious surfaces.”

Whatever lies ahead, Attleboro will figure it out. “I’ve been here 33 years,” says Kennedy. “We’ve done a lot of things that you won’t find in the book.” Maybe not, but they seem to work.

More Information

Aqua-Aerobic Systems, Inc. - 877/271-9694 - www.aqua-aerobic.com

Cole-Parmer - 800/323-4340 - www.coleparmer.com

Environmental Operating Solutions, Inc. - 866/642-7621 - www.eosenvironmental.com

Evoqua Water Technologies, LLC - 815/623-2111 - www.evoqua.com

Flygt - a Xylem Brand - 855/955-4261 - www.flygtus.com

Hach Company - 800/227-4224 - www.hach.com

MERRICK Industries, Inc. - 850/265-3611 - www.merrick-inc.com

Duall, a CECO Environmental Company  - 989/725-8184 - www.dualldiv.com

Philadelphia Mixing Solutions - 800/956-4937 - www.philamixers.com

Walker Process Equipment A Div. of McNish Corp. - 800/992-5537 - www.walker-process.com