Plant loads triple with ski season

Located in the heart of the Colorado Rockies at an elevation of 8'574 feet, the Fraser treatment plant is permitted as an activated sludge facility with a hydraulic design capacity of 2.0 Million Gallons Per Day (MGD).

The plant, which was built in 2003, is located near Winter Park Resort. The resort ranks as one of the most popular ski destinations in Colorado, especially during weekends since it’s located approximately 90 miles from Denver.

When ski season isn’t in full swing, the plant normally treats approximately 1.2 MGD of wastewater when measured over a 30-day period. However, things change considerably for the plant during the ski season when the local population climbs from approximately 2'000 people to as many as 20'000 people. The increase in population calls for the plant to operate at high capacity without fail. The plant typically sees a major increase in loads on weekends during the ski season, said Fuqua.

“Our flows on Thursdays are usually around one-half million gallons, but by Friday night during the peak season we’re at 1.5 million gallons. That puts a premium on the reliability and efficiency of the plant, including machinery like aeration blowers,” Fuqua said.

Dual wastewater treatment processes

To treat wastewater, the Fraser plant uses pre-treatment, nitrification in dual activated sludge basins with clariflocculators followed by ultraviolet disinfection. The plant is one of only a few in the United States that uses a combination of re-air, anoxic and standard aeration zones for nitrogen and ammonia removal. 

The plant is designed with two identical wastewater treatment trains, each of which is used to process approximately one MGD. The plant normally operates one train from mid-July until early November when it then activates the second train to handle the increased loads as the population begins to swell in advance of the peak ski season. 

Primary systems at the plant include the pretreatment operation with a grit removal system, which then feeds into separate anoxic basins for denitrification. From there, sewage flows to a Sequencing Batch Reactor and onward to two separate aeration basins with fine-bubble diffusers and secondary clarifiers before effluent is discharged to the Fraser River. Waste Activated Sludge (WAS) is pumped from the clarifiers to two separate aerobic digesters. The WAS is then pumped to the plant’s drying and solids handling operation, which leverages a centrifuge for during Class B solids for use as fertilizer.

Originally, the plant was designed to use two, 150 horsepower (hp) centrifugal aeration blowers to feed air to the aeration basins on each wastewater processing train. Each centrifugal blower is rated to provide to 2'500 scfm at 7 psi. The plant was also originally equipped with three 125 hp Positive Displacement (PD) blowers to supply air to the digester with one blower serving each process train and the third unit serving as a standby machine. Each PD blower is rated to provide 1'200 scfm at 11 psi.

After being named plant manager Fuqua placed aeration and blowers on his list of priorities, knowing the plant could save energy and improve a number of maintenance-related issues with upgrades.

Tackling multiple aeration blower issues

Early on, Fuqua decided to focus on the centrifugal blowers used on the aeration basins. The blowers were a relatively easy target for energy savings, he said.

“The two original centrifugal blowers were oversized for the loads and flow for most of the year,” Fuqua said. “I basically had a setup that provided more air than I needed.”

To better match blowers to the aeration basins – and save energy costs –  the plant installed a 60 hp centrifugal blower rated to deliver up to 800 scfm at 7 psi. The blower supplied ample air for aeration during the off-peak season when the plant experienced relatively low flows. When the ski season kicked in, the plant’s SCADA system would shut down the 60 hp blower and activate the two 150 hp blowers to meet the need for more air due to  increased flows. The strategy allowed the plant to save energy costs while promoting the growth of microorganisms to breakdown waste in addition to maintaining targeted Dissolved Oxygen (DO) levels.

Fuqua said operating the 150 hp blowers for short periods during peak load conditions, however, still proved costly.

“When we had peak loads coming in, the 150 hp blowers would cycle on quite a number of times,” Fuqua said, adding that the larger centrifugal blowers would operate as much as 30% of the time during the ski season. As such, electric utility demand charges began to add up.

“We’re billed on a demand charge so every time one of those 150 hp centrifugal units would cycle on it bumped up our costs for electrical power, which is one main reason why we began to look at other options for aeration blowers.”

Another reason to evaluate blowers was the need to address noise levels created by the PD blowers used for the digesters, which would waste as much as 20'000 gallons of water a day during peak season. Given the noise issue, Fuqua opted to only run the blowers from 6 pm until 7 am when no plant operators were normally at the facility. Hearing protection was essential for operators anytime they needed to be at the plant during those same hours.

Fuqua, a trained mechanic, also wanted to replace older model blowers with newer technology to bolster the reliability of the aeration blower system.

“During peak loads, the centrifugal blowers would cycle on and off so much the system couldn’t keep up and they’d throw a fault, or we’d get an alarm at 2 am because our ammonia levels were starting to get high,” he said. “Those types of issues made it imperative to look at newer blowers.”

A main concern related to reliability with any new blowers, he said, was the ability to withstand the dry, dusty environment.

“We have very low humidity here and the air is super dry. That means we get a lot of fine dust that can be hard on blowers and other equipment,” Fuqua said. “That’s something I wanted to ensure we dealt with as far as new blowers.”

Upgrade features two high-speed turbo compressors

After evaluating blower technologies, the Fraser wastewater treatment plant chose to upgrade its blower system with two Sulzer HST™ 20 turbocompressors to provide aeration for the plant at low pressure. The plant also reconfigured the aeration piping to allow the new blowers to provide air to both the aeration basins and digesters as needed – meaning the plant can meet its aeration goals with far fewer blowers.

Installed in fall 2018, each 200 hp turbocompressor delivers 2'100 to 2'550 scfm at pressures ranging from 7.2 to 10.7 psi. Each blower also offers a turndown ratio of 50%. The new blowers, which were installed in the same blower room as the original blowers, replaced one 150 hp centrifugal blower and one 125 hp PD blower.

Today, the plant typically operates only one turbocompressor throughout the year, regardless of load fluctuations and the first full peak winter ski season. The remaining blowers serve as standby machines.

“Now, I can run the one turbocompressor and supply air to the aeration basins or the digesters, depending on water depths and the DO level,” Fuqua said. “If the digesters are full, I run air to the digesters and shave off air to the aeration basins. If the digesters are somewhat empty, I supply more air to the aeration basins. I’ll alternate the two turbocompressors based on current conditions, but I’m only running one blower at a time to supply air to everything. I’ve haven’t had to turn the other blowers on in a year.”

The HST turbocompressors use magnetic bearings instead of air bearings. When the machine is powered, a single shaft that functions as the motor’s rotor is lifted and levitated by balanced magnetic forces. There is no contact between rotating and static components of the machine, which appeals to Fuqua.

“With the fine dust we have, I was more inclined to choose machines with magnetic bearings since they never touch,” he said, adding how a single blower has allowed the plant to keep pace with the plant’s demand for air and greatly diminished the need for operators to address warning alarms.

“During the peak season before the installation of the new blowers, we were probably getting three alarms a week. I don’t think we’ve had three alarms in the last nine months,” Fuqua said.

The HST turbocompressor also operate quietly since each machine rated to keep noise to 70 dBA when fully operational. It’s a marked difference from the old machines, allowing the team to forget worries about working in a noisy environment Fuqua said.  

“I never envisioned that things could be as quiet at this plant as they are now,” he said.

Project nets annual energy savings of USD 30'000

While improved reliability and less noise were top priorities for the aeration blower upgrade project, Fuqua said energy savings drove the decision to upgrade to new blowers. Toward that end, he said the project was a success.

“We’re always looking at way to save energy costs and protect the environment, whether it’s new blowers or a project we have underway for an even higher level of treatment for metals and phosphorous,” he said.

Since the upgrade, the treatment plant has reduced energy costs by approximately 10%. With the new blowers in place the plant saves approximately USD 2'500 per month, which equates to an energy savings of USD 30'000 per year.

“The main reason we went with the project was because of the potential energy savings and the hope that we would save that much,” Fuqua said. “It’s the green thing to do.”

Looking back, Fuqua said the biggest challenge to the project was to talk his two plant operators into taking on the blower upgrade installation with little outside help – which was especially important since the plant is in a remote location.

“I had no doubt in my mind we could do it. It was just a matter of convincing my team we could do it,” he said, adding the project achieved every goal.

“I’m tickled to death,” Fuqua said. “It worked out better than I expected.”

By Mike Grennier, article courtesy of Blower & Vacuum Best Practices® Magazine