Simple Solutions for Process Improvement

Low-cost modifications to the aeration process at a small Massachusetts plant bring substantial effluent nutrient reductions and other money-saving improvements.
Simple Solutions for Process Improvement
Grant Weaver, left, president of The Water Planet Company, and Bob Gabry, chief operator at the Sunderland Wastewater Treatment Plant, at the MCC panel housing the sc200 controller (Hach).

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Sometimes it doesn’t take a major plant upgrade to drive down the nutrient content of effluent and improve overall plant performance.

Bob Gabry, chief operator at the Sunderland (Massachusetts) Wastewater Treatment Plant, will gladly so attest. A few inexpensive changes to the aeration process at his 0.5 mgd (design) activated sludge plant yielded consistently low effluent total nitrogen, plus elimination of filament problems, better sludge settling, reduced biosolids hauling costs, and substantial savings on chlorine.

It all resulted from a few months’ work and experimentation in 2014 with Grant Weaver, P.E., president of The Water Planet Company, a wastewater treatment consulting firm.

Flying solo

Gabry, employed by private contract operator Warner Brothers, runs the Sunderland plant himself except for fill-ins during vacations and weekends. The 40-year-old extended aeration plant averages 175,000 to 200,000 gpd; flow is somewhat seasonal because the community of 2,300 hosts a number of apartment-dwelling college students from the University of Massachusetts in nearby Amherst.

Nitrogen reduction is an issue among plants in the area because of an initiative to protect Long Island Sound. The Sunderland plant’s permit allows discharge of 35 pounds of total nitrogen per day to the Connecticut River.

Gabry, who has been at the plant for 30 years, had taken steps to make the process more energy-efficient by placing the mechanical aerator on a timer and operating it at intervals instead of continuously, looking to keep dissolved oxygen (DO) in the basin at 1 mg/L. Still, he faced frequent explosions of filamentous bacteria, which led to bad solids settling in the final clarifier, effluent turbidity, and sludge bulking, all of which he sought to control by chlorinating the return activated sludge (RAS) line. “It was a battle for years,” Gabry says. “I was putting thousands of dollars’ worth of chlorine per year into the return line.”

Weaver stopped at the plant during his travels, introduced himself to Gabry, and offered to demonstrate a process change that could improve nitrogen removal and address the other issues Gabry was facing. Weaver observes, “There are two basic kinds of operators. Some want to learn how they can do the most with what they have and buy what their community needs, but really no more. Others see it as their job to invest in the latest and greatest — the most up-to-date, sophisticated equipment and technology. Bob is in that first category.”

At first, Gabry was skeptical about what Weaver presented. Then he talked to operators at a few other plants who had adopted the suggestions. “They were almost doing cartwheels it was so good,” he recalls.

Taking action

Weaver says, “In Bob I saw an operator who on his own initiative had the idea to turn the aeration equipment on and off to keep down filaments and save electricity. He had a good grasp of that. It showed me that he was willing to change the status quo.

“Therefore, it was easy to enter a discussion of extending the air-off time to allow the plant to go anoxic and remove the nitrate that is created when you remove ammonia. If you get rid of the nitrate, that should help the clarifier settle a little better, and it should act as somewhat of a selector for filaments. You create two environments: air-on time where the basin is more aerobic, and air-off time when it’s more anoxic. Bacteria that thrive in either condition get banged by cycling through the opposite condition. Bob’s head wrapped around that quickly.”

The process adjustments included:

  • Replacing existing aeration timers with a 10-hour timer (Eagle Signal)
  • Installing a Hach sc200 controller, LDO oxygen sensor and Digital ORP sensor
  • Acquiring a hand-held ORP probe (YSI, a Xylem brand)

The DO probe operates in a feedback loop with the aerator to maintain the aeration basin DO at 1 mg/L at times when the blower is running.

The next step was to test-drive different blower on and off intervals. After a period of discussion and experimenting, Gabry arrived at a “sweet spot” of 3.5 hours on (for nitrification) and 1.5 hours off (for denitrification), around the clock. Along the way, he acquired equipment to perform nitrate, nitrite and ammonia testing, notably a colorimeter (Hach). Data from the DO probe is fed back to the plant control room where a seven-day chart recorder (Honeywell) tracks the DO levels and on-off cycles.

Seeing results

“It didn’t take that much time to get the process locked in,” Gabry says. “Within about a month-and-a-half I had identified the on times and off times that worked best. The plant really responded quickly. I had a grin on my face 24/7.”

Plant influent ammonia averages 40 mg/L, and effluent ammonia now averages 0.1 to 2 mg/L, versus 15-20 mg/L previously. Effluent total nitrogen averages 8 mg/L, versus 25 mg/L before the process changes. The plant used to discharge 25 and 30 pounds of nitrogen per day into the river; it now releases 8 to 12 pounds.

In addition, the filament issues cleared up quickly. “My sludge volume index (SVI) levels before were in the upper 200s to 400s, and sometimes even 500 or 600,” says Gabry. “So you can imagine the stress on the clarifier. Within a couple of months after starting the new process, my SVIs were going down, down, down. The effluent was getting cleaner, and the filaments went away. SVIs have gone down to under 100.

“The stability under the microscope is phenomenal. I can look at it today and look at it in a week and it hasn’t changed a bit. I’m running a higher mixed liquor suspended solids (MLSS) — about 3,500 mg/L versus 1,500 mg/L before. The growth curve goes higher, so the free swimmers go away. We have stalked ciliates and rotifers now, and they’re very happy.”

In addition, the process change has greatly improved sludge settling and sharply reduced the volume of biosolids hauled offsite. The plant had been sending up to 10 9,000-gallon loads per month at less than 1 percent solids. That has been reduced to three or four loads at 2 to 3 percent, saving up to $2,000 per month. Because chlorination of the RAS line is no longer needed, chlorine usage has dropped from as much as 1,600 gallons per year to 800 gallons, saving up to $2,000.

Avoiding upgrades

Weaver says the lesson from Sunderland is that operators willing to modify their day-to-day operations can significantly reduce effluent nitrogen in most facilities, in some cases replacing plant upgrades that could cost in the millions of dollars.

“It does take more operator attention,” he observes. “Bob now spends time checking things he didn’t used to check. My role is to help operators move from one comfort zone to a new comfort zone. Every person is different.”

Says Gabry, “If we had not done this, we would be building tanks, adding anoxic zones, putting in selectors. It would be hundreds of thousands of dollars.”


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