A Lot of Ingenuity

When a treatment plant upgrade failed to meet the nitrogen-removal specification, the town of Suffield, Conn., made it happen by using the equipment in a new way

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For years, operators at the Suffield Water Pollution Control AUTHORITY wastewater treatment plant tried in vain to meet increasingly stringent total nitrogen discharge limits. It took the help of an innovative engineering consultant and the perseverance and hard work of the operations team to solve the problem.

 

“We spent $5.3 million to upgrade our plant so we could bring it into compliance with the new Long Island Sound nitrogen limits, which will become stricter every year until 2014, when our plant will be limited to 45 pounds per day,” says chief operator Bernie Gooch. “We also wanted to accommodate a high industrial organic loading and replace chlorination with UV disinfection.”

 

In 2003, the plant’s total nitrogen limit was 89 pounds per day. The plant met its limit from then until 2005. The upgrade began in September 2005 and was completed in 2006. By 2007, the nitrogen limit was 55 pounds per day, but the effluent was averaging 75 pounds. It wasn’t until August 2009 that the plant again met its nitrogen requirement.

 

That was after the plant team made a series of process adjustments and diligently monitored progress over several weeks. Today, the plant effluent averages just 12 pounds per day of nitrogen.

 

Difficult situation

Built in 1989, the 2.03 mgd (design) dual-train, two-step oxidation ditch facility serves about 2,900 residential and 100 commercial, industrial and institutional customers, including an ice cream plant and MacDougall-Walker Correctional Institution. The plant discharges to the Connecticut River, which empties into Long Island Sound.

 

The original plant was undersized to treat the high BOD loading from the ice cream plant. Suffield conducted an engineering study in 2004 to address that issue, as well as concerns about the nitrogen limits.

 

The subsequent upgrade included a pre-anoxic tank, two internal recycle pumps (ABS), a new final clarifier (WesTech), and UV disinfection (Trojan Technologies). To improve sludge settling, the anoxic tank was built as a series of small tanks — a “selector.”

 

After the new clarifier was added, effluent BOD and TSS were consistently below 5 mg/l, easily meeting the 30 mg/l permit limit. But nitrogen was unaffected, even though the plant staff followed the anoxic equipment settings established by the design engineer.

 

“What made the situation even more difficult was that the engineering company that did the upgrade eliminated all the meters from the project plan due to budget constraints,” says Gooch. “They said we didn’t need to monitor the process, as it would work fine.”

 

Meters added

In the end, Suffield added the meters three years after the pumps were installed, using operating funds. The equipment included:

• ITT – Royce Technologies dissolved oxygen (DO) meters installed in the two oxidation ditches and wired to the computerized control system. Staff programmed the SCADA system to control aeration mixer speeds to maintain established DO settings.

• Hach oxidation-reduction potential (ORP) meters in the pre-anoxic tank to monitor biochemical activity in the anoxic tanks. The meters made it possible to operate the anoxic (low-oxygen) tank in a zero-oxygen (anaerobic) mode.

• Variable-frequency drives (VFDs) on the new internal recycle pumps to provide operator control of the pumping. As installed, the equipment could be operated in a fully on or fully off mode only.

• A Hach nitrate-nitrogen analyzer, connected the plant control system to provide continuous monitoring of effluent quality.

In trying to improve nitrogen removal, plant staff experimented with conventional process adjustments, such as changing the mixed liquor concentration, changing DO settings, and changing the return activated sludge pumping rate. Nothing worked.

 

As word of the difficulties spread, engineering firms started offering help. Suffield eventually connected with Grant Weaver, president of The Water Planet Company of New London, Conn. A Professional Engineer and ABC licensed Class 4 wastewater operator, Weaver has 30 years’ experience as a municipal operator/administrator in treatment plants from less than 0.5 mgd to 100 mgd.

 

Weaver knew right away what to do. “When I came in, my initial reaction was that the internal recycle rate was too high,” he says. “Contrary to what you read, too high a nitrate recycle rate adversely affects treatment. So we turned that down and then changed the aeration rate.”

 

Solving the problem

In summer 2009, Weaver and the plant’s staff made a variety of process changes. They turned off one of the two fixed-speed internal recycling pumps (2,000 gpm) and operated the second 18 hours a day.

 

A few weeks later, they changed the mechanical aeration mixer settings to provide less aeration in the first pass and more in the second — the opposite of the design engineer’s settings.

Again a few weeks later, they adjusted the sludge holding tank blower to operate only when the sludge press is operated — about 3.5 hours a day — instead of continuously. The staff also ran daily testing of ammonia, nitrite, nitrate and alkalinity to monitor results — an important component of making the improvements. Weekly lab testing of nitrogen parameters continued.

Some relatively minor changes in the equipment settings created new bacterial habitats and significantly altered treatment:

• The reduced internal recycle rate doubled the hydraulic retention time in the pre-anoxic zone, making the tank anaerobic. As a result, the tank now provides additional organic-nitrogen removal — an improvement of 0.7 mg/l.

• The short initial aeration zone solubilizes the high-particulate BOD waste from the ice cream plant and provides a ready supply of BOD to promote nitrate removal in the subsequent anoxic zone — a 4.6 mg/l improvement.

• The longer retention time in the second aeration zone that results from the reduced internal recycle rate provides for more consistent, complete ammonia removal — a 0.1 mg/l improvement.

 

Positive results

Although things got worse during the first six weeks after these changes, the staff spent considerable time and effort tweaking the process. It paid off: Today’s total nitrogen loading of 12 pounds per day is less than one-sixth of the 79 pounds before the changes were made, and one-third of the 2014 discharge limit of 45 pounds.

 

Effluent ammonia has improved slightly and is now consistently 0.1 mg/l or less. Effluent TKN dropped to less than 1.0 mg/l — a 0.7 mg/l organic-nitrogen reduction. Effluent nitrite plus nitrate has been reduced to less than 0.5 mg/l — a 4.6 mg/l decline in nitrate.

 

The new nitrogen results were so good that even the state Department of Environmental Protection at first did not believe the results. “They sent an inspector out to the plant to pick up a sample because they wanted to see for themselves if the numbers were correct,” says Gooch.

 

Weaver has presented papers about the Suffield plant in hopes of helping other operators with similar issues. In January 2010, he and Gooch presented at the New England Water Environment Association conference in Boston. They are scheduled to present at the National Nutrient Council meeting in Miami in January 2011.

 

Operators are key

Although Weaver’s methods have been successful at many other facilities, the success depends on plant operators. “Whenever I go into a plant, it’s up to the operators to make it work and to prove me right or prove me wrong, and I like it that way,” he says.

 

That was certainly true in Suffield, where Gooch and his team worked hard to implement the process changes and then measure the results. Measurements included daily field tests with inline instruments, test strips and colorimeters, and weekly review of lab data. The staff was aware of conditions required for optimal nitrogen removal (especially ammonia and nitrate) and maintained ongoing communication with Weaver.

 

It wasn’t an easy time for the plant’s six operators, who had already dealt with the high BOD loadings from the industrial waste, the long upgrade process, and the nitrogen specification disappointment that followed.

 

“Every July, we had gotten a check from the DEP for nitrogen credits, and everyone liked that,” says Gooch. “But then we had to start paying the DEP. Plant personnel never knew what the cost of not meeting the nitrogen specs was going to be, and it was very difficult to budget around that. The plant ended up paying $25,000 over those three years, whereas in the past four years, we had gotten back $20,000.”

 

Weaver recalls, “When I entered the picture, the operators weren’t really angry at anyone, just frustrated. Once I proposed a solution, everyone wanted to make the new process work, and they got enthused.”

 

The tedious daily testing proved to be just one more challenge. “Nobody had ever made them do this before, and they wondered if I wasn’t having them do my busy work,” Weaver recalls.

Gooch adds, “At first the operators said ‘Why am I doing all this?’ But then it started to click.” Gooch set an example for everyone with his determination and can-do attitude. “Bernie is one tough guy who doesn’t back down,” says Weaver. “That comes from his eight years in the army.”

 

Renewed success

The plant supervisor and foreman have now retired, leaving fewer people to do the work. On top of that, the plant now has even more customers. Still, the staff is satisfied with the way things are going.

 

“What makes the plant successful is the competency of the staff and the fact that the operators have been with us for a while,” says Gooch. “It’s a small community where everyone knows everyone. Four of the eight employees and the commission members live in town. The operators are getting pats on the back and being told by the commission and the public that they’re doing a great job. There is a renewed sense of pride.”

 

In the near future, the plant operators will experiment with further process modifications to maintain the high nitrogen removal while using less energy. These will include using even less recycle, lowering the aeration, and possibly operating one or more of the mechanical aerators on timers so that there are periods of off time. This will reduce the facility’s carbon footprint, and may save $50,000 a year in electricity costs.

 

Gooch says his goal is to operate within his budget and have the best-running plant possible. He offers this advice to operators everywhere: “Never back down from a challenge.”



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