Less Is More

When it comes to operating an innovative activated sludge treatment plant, the small crew at RAE Water & Sewer in Montana does quite nicely

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At the RAE Water & Sewer District near Bozeman, Mont., just two operators do all the work at a 70,000 gpd activated sludge treatment plant. And they do an exemplary job with an innovative process.

An advanced biological nutrient removal (BNR) plant using a sequencing batch reactor (SBR) treats the water to exceptionally high standards before infiltration to the groundwater. The reactor tanks are housed in a greenhouse that prevents freeze-ups during long, cold winters. The solids are wasted to an aerobic digester and then stored and treated in reed beds. The plant is operated via PLC and monitored with a SCADA system.

 

Plant manager/operator David King runs the plant with part-time operator Dean Ridgway. They do everything from interfacing with customers to lab testing to maintenance, while running the water treatment plant, too. A part-time secretary, Mitzi Gillam, manages the office.

 

“The plant runs itself, actually, since it’s designed really well,” says King. “There’s not much we have to do on a daily basis, and that’s why we chose this design.”

 

A great team

King and Ridgway appreciate working in a smaller environment because they get to do it all — and without a manager looking over their shoulders. A district board of directors meets once a month, and King attends those meetings. “The board is there to assign the big contracts, but I recommend policy and interface with the district and grant agencies,” he says.

The biggest challenge for King and Ridgway is keeping up with maintenance, and their philosophy is to address problems as they arise rather than put things off. They make sure they keep critical spare parts, such as pumps, in stock.

 

“We spend about six hours a week on maintenance and process control, and periodically replace things like belts, air filters and UV bulbs,” King says. “We spend another six hours on monitoring and lab work. And, although we watch trends and make adjustments, we really only have to adjust the mixed liquor concentration, which we do once a week.”

 

Three times a week, they run process control tests in the lab including MLSS testing, settleometer and microscopic evaluation to classify microorganism populations, keep track of filamentous species, and evaluate floc quality.

 

King and Ridgway enjoy interacting with customers. “We have a good rapport with them,” says King. “If they have a complaint or problem, we always listen and assume we could be wrong, and they’re right.” Says Ridgway, “The downside is when you have to raise the rates, or when you have to turn off their water when they don’t pay the bill. Door-to-door collection is not my idea of a fun time!”

 

A major upgrade

Originally built in 1977, the RAE wastewater treatment system was completely upgraded in 2004. The old system used aerated lagoons, and the treated effluent was sprayed on a farm field. In 1992, the lagoons began leaking at greater than 15 inches per year; the state standard was 6 inches.

 

The town’s board of directors considered lining the lagoons or hooking into the Bozeman sewer system four miles away. But lining the lagoons would have resulted in too much water for winter storage, and while building a fourth lagoon cell would have solved the problem, the board did not want to buy more land.

 

At the same time, the farmer decided not to renew the 20-year spray irrigation contract. The board considered many options, but in the end decided to install an SBR for its ability to treat the wastewater to levels that would allow infiltration to groundwater. Their research, however, showed that some SBR plants had winter freeze-up issues, messy sludge handling and noisy blowers.

 

Minimal impact

Water quality in the area is extremely high, with many blue-ribbon trout streams. RAE needed a way to treat and discharge wastewater with minimal impact to the environment. The district ultimately chose an SBR from Aqua-Aerobic Systems consisting of two reactor basins, a post-equalization basin, and two aerobic digesters.

 

“The solution we came up with was ideal,” says King. “We decided to house the SBR reactor tanks in a greenhouse, which would eliminate freeze-up issues and create a warmer and friendlier environment for microorganisms. We use reed beds to dewater the sludge and create compost, and we moved the blowers to a separate building away from where the operators are working. This technology was a whole new concept for the state of Montana, and it has set the standard for subsequent plant designs.”

 

The greenhouse is used as an inexpensive cover for the basins. The temperature is kept just above freezing in the winter. “It is very bright inside and low-maintenance,” says King, “with nothing to paint and nothing to get wet, as greenhouses are meant to handle humidity. It also keeps anything from blowing in the basins.”

 

Wastewater enters the plant from two lift stations. A 24-inch-wide Lakeside Equipment Corporation Raptor automatic headworks screen with auto-bagger attachment allows more of the non-organic material to be filtered out, with minimal odor.

 

The material is screened, washed, compacted and bagged automatically. The operators roll the trash bin out for pick up, and clean the screen once a week. The screened wastewater enters one of the reactor basins and is treated by microorganisms via mixing, aerating, settling and decanting.

 

By creating both an aerobic and anoxic environment for the microorganisms, the plant both nitrifies and de-nitrifies the wastewater. The decanted (clear) supernatant discharges to a post-equalization basin, is pumped through a Sunlight Systems (Siemens Water Technologies) ultraviolet disinfection unit and out to one of three subsurface infiltration galleries.

 

At the end of each decant cycle, sludge is wasted to one of two aerobic digesters, where it is treated for an additional 30 days. The blowers are controlled by timers with 30 minutes of aeration followed by a 40-minute off time. The digester fluid level is kept full to maximize the solid’s residence time in the digester basins.

 

Digested sludge is pumped to one of two reed beds, planted with Phragmites, for treatment and storage. “The reed beds have been a wonderful design for this small plant,” says King, “as they dewater the sludge and provide additional treatment and storage. We have about 10 years of storage space in our reed beds, at which time we will compost it on site.”

 

There is no operator maintenance required for the solids handling. During the summer, the excess water is absorbed by the reeds through evapotranspiration, and periodically during the winter, the operators drain the filtered underdrain water back to the head of the plant.

 

Funding for the $2.5 million project came from a variety of state and federal loans and grants, as well as RAE district funds. Not long after the new plant went online, it won System of the Year in 2005 from Montana Rural Water Systems. The state’s main water trade association, Montana Rural Water Systems, provides technical support for the water and sewer industry, and is funded through federal grant programs.

 

Efficient operation

Today, the plant serves 790 people, including a large trailer court, three residential subdivisions and a large auto dealership. There is capacity for another 500 homes. New limits for total nitrogen (7 mg/l) and total phosphorus (3 lbs/day) went into effect in September 2009, and the plant meets or exceeds them easily.

 

“In addition to sampling the plant effluent, we have four downgradient monitor wells that we sample quarterly for total nitrogen, E. coli, specific conductance and water level,” says King. “Our main goal is to produce the highest-quality treated water we can and in the most cost-effective, environmentally friendly way.”

 

King and Ridgway continually look for ways to do the job more efficiently, with an eye toward sustainability. “Whenever we build something new, we build it with low maintenance and high efficiency in mind,” King says. “Over the long term, we have realized substantial savings in both labor and maintenance costs.”

 

The plant uses an Allen-Bradley (Rockwell Automation) PLC, Hach Evita in-basin dissolved oxygen sensors, HydroRanger (Siemens Water Technologies) ultrasonic in-basin level sensors, and Royce Technologies (ITT Water & Wastewater) in-basin TSS sensors. A SCADA system using Rockwell Automation RS view32 software helps them monitor and control plant operations from an office computer.

 

“I log in regularly from home or remotely when I am on trips, and we also have an auto-dialer that calls out any time-sensitive alarms,” King says. “But most everything in the plant has a backup, so we rarely have to respond to an emergency.”

 

King and Ridgway are working with Aqua-Aerobic Systems to optimize blower run times, since the plant has separate blowers for reactor basins and digesters, and the blowers turn on and off 30 or more times a day.

 

They would like to modify the control program so they can stop using the digester blowers and keep one of the aeration basin blowers running continuously. By using valve controls with set points from the oxygen sensors to scrub the excess air from the aeration basin to the digesters, they would save electricity and stabilize the dissolved oxygen levels in their basins.

 

New challenges

Besides meeting the more stringent discharge regulations, King and Ridgway had to train themselves to operate the new equipment. Ridgway, who started in 1985, was used to the lagoon system and had no experience with biological treatment.

 

“At first the new plant seemed like it would be really complicated, but I got to see the construction from the ground up and watched the equipment being installed,” he says. “Had I walked into the job with the new plant already online, it would have been difficult for me to understand how it all fits together.”

 

Ridgway received a few days of training on the system, and the SBR manufacturer provided a good manual and phone support. “Lagoons are complacent — they do their own thing,” he says. “And then this system was installed, so it was a little bit spooky to me at first. But the more I worked with it, the less mysterious it became.”

 

His background is in custodial work, and he entered the wastewater treatment field by accident. “I was standing in my driveway one day and my former manager came by and said ‘I’m going on vacation; how would you like to watch the lift station for me?’” Ridgway recalls. “I went to a one-week crash course in water treatment at Montana State University, which RAE paid for. I also had two years of electronics in college, and that helped.”

 

That investment has paid off for RAE, as Ridgway won the Operator of the Year award from Montana Rural Water Systems in 2003. Contenders for the award are nominated by their peers.

 

Trial and error

Even though King and Ridgway received training through the state’s operator certification program, they learned a lot by trial and error. Some challenges they faced included issues with filament control, digester reed bed operations, and operating the plant in cold weather.

 

During the first year, they had no effective way to control filaments. There was a layer of foam on the reactor basins, and they were adding calcium hypochlorite by hand, a time-consuming and unreliable process. So, they devised a chemical spray system with equipment ordered through USABlueBook. It was installed by a local plumber and electrician with some code changes programmed by Aqua-Aerobic Systems.

 

“It all works really slick,” says King, “and we can control the amount and duration of chlorine and polymer that we add to each batch. Most of the time we just spray water from the post-equalization basin onto the reactor basins for 50 minutes a cycle, and that keeps the foam and filaments from building up.”

 

Initially, the digesters were designed with float-level sensors, and the operators had to pump the digested sludge manually to the reed bed or decant the supernatant back to the head of the plant. In winter, they had to run the digesters at a low fluid level so they could drain excess water from the reed bed riser pipes to prevent freezing. That was not effective, since they wanted to keep their digesters full in the winter to maximize residence time and treatment performance.

 

So, they re-ran their discharge lines from the digesters to the reed beds and laid them below the frost layer, while insulating and adding heat tape to the riser pipes. Now, they can run the digesters full, and the sludge is automatically wasted to the reed beds, based on level controls.

 

Communication is key

“When you run a small operation like we do, you are really always on call,” says King. “Although each of us works independently, we take pride in providing excellent service and a high-quality product for our customers while protecting the environment.

 

“The trick is open and effective communication between our small team, since our time at work does not always overlap. This is a livelihood and not just a job that anyone can do. We have people working here who make this job their life. Fortunately, our board of directors has been very supportive when it comes to training or funding.”



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