A Phoenix wastewater treatment plant focuses on optimization to operate more effectively, reduce costs and protect the environment
The 91st Avenue Wastewater Treatment Plant in Phoenix, Ariz., has a simple mission: meet permits; reduce chemical, energy, and biosolids costs; automate where practical; and innovate. Despite the lack of any large capital projects, the plant has achieved big savings, says assistant plant superintendent Jim Coughenour.
“Electricity has increased by 40 percent, chemical costs are up, and even the price for biosolids land applicators has increased. Still, our division has reduced overall cost for those three from $18.2 million to $15.2 million in five years,” Coughenour says. “We expect to reduce it by another $1 million this year. We’re trying to craft the entire plant to optimize each step.”
The 230 mgd regional facility serves three million people in Phoenix, Glendale, Scottsdale, Tempe and Mesa. It uses single-stage nitrogen removal, single-stage anaerobic digestion, chloramination for disinfection, and a revamped solar drying biosolids operation. “We’re looking at this in a little different way,” Coughenour says.
Process enhancements begin at the headworks. Centrate ammonia from the digested sludge dewatering centrifuges is nitrified to nitrate or nitrite at the Centrate Treatment Facility and recycled to the headworks. The nitrate/nitrite is used up prior to reaching the aeration basin, reducing soluble COD and cutting total nitrogen loading on the aerators by 15 percent. It also serves as an odor control chemical at no cost in the headworks and primary clarifiers. That step alone saves the plant $40,000 a year.
A small dose of ferric chloride in the headworks helps meet sulfide-related air-quality permit limits and causes small particles to come together and settle out in the primary clarifiers.
The primary clarifiers were updated several years ago to remove much of the plant’s BOD load before the aeration phase to reduce the use of blowers. Spiral scrapers and Stamford baffles (NEFCO) were added, along with energy dissipating center wells. Intermittent high-rate pumping was replaced by continuous slow-rate pumping. Density meters and VFDs maintain primary sludge density at about 3 percent.
“The only sludge blankets we have are in the cones, and that is just enough to thicken it to 3 percent,” says Coughenour. “We are able to stop acid hydrolysis from occurring in the clarifiers, preventing soluble COD from increasing. With centrate treatment and ferric chloride, we’re getting about 80 percent solids removal, almost 60 percent COD removal and 15 percent soluble COD removal.”
In the last five years, while the amount of suspended solids and COD coming into the plant has increased sharply, the amount going into the aerators has decreased. “That efficiency has probably saved $600,000 a year,” he notes.
A sophisticated aerator DO control strategy relies on a number of blowers. “The most efficient are the Atlas Copco blowers that have inlet and discharge guide veins,” Coughenour notes.
The plant also uses first-pass and fourth-pass anoxic zones in the aerators, allowing for proper treatment with less oxygen and less blower use. Oxygen is added in the center of the aerator, where ammonia is nitrified and BOD is oxidized, to maintain dissolved oxygen at 2.0 mg/l. About 7 percent of the primary effluent is added to the fourth-pass anoxic zone.
“The fourth-pass feed allows us to pass a small amount of ammonia for chloramination,” he says. “Operating the fourth-pass of the aerator, the mixed liquor channel, and the secondary clarifier as anoxic, causes endogenous denitrification, lowering our total nitrogen and freeing up significantly more oxygen.”
He says the plant started using the denitrification-nitrification-denitrification strategy in the mid-1990s and has been refining it ever since. “We are using roughly 600 kWh per million gallons here for blowers, as opposed to 900 kWh at our 23rd Avenue plant,” Coughenour says. Phoenix Water Services has transferred about 10 mgd of flow from 23rd Avenue to take advantage of the savings at 91st Avenue.
Unusual water reclamation
About half of the plant’s effluent is piped 50 miles to the Palo Verde Nuclear Generating Station, the largest nuclear plant in the country and the only one not on a lake or river. It uses the effluent for condensing steam into water in its cooling towers. At peak times, about 80 mgd of effluent goes to Palo Verde.
Because it doesn’t have to be chlorinated, wastewater destined for Palo Verde is treated in an older part of the plant. It flows by gravity to the power plant. “We saved $50,000 the last two months because we didn’t have to pump the water, and we reduced our chlorine cost by $50,000,” says Coughenour.
Help from wetlands
The plant recently finished a $34 million U.S. EPA-funded Tres Rios Constructed Wetlands Demonstration Project on 750 acres along the Salt River. That cost compares to an estimated $625 million for a plant upgrade.
The first 500 acres are treatment cells called Flow Regulating Wetlands. Water leaving the cells must meet all discharge permit requirements. Currently receiving 45 mgd of treated secondary effluent, the wetlands are designed to accept up to 400 mgd.
“91st Avenue reduces primary effluent total nitrogen from about 50 mg/l to about 6 mg/l,” says Coughenour. “The wetlands further reduce it to 3 mg/l.”
91st Avenue uses sodium bisulfite to dechlorinate effluent prior to discharge, but chlorine is removed naturally in the wetlands. “Total chlorine residual of 2 mg/l enters the wetlands, and it’s all gone before it gets halfway through.”
Chlorine analyzers are gathering data and calculating decay rates to determine if peak flows will still be fully dechlorinated. Coughenour believes that as the wetlands mature, they will still remove all chlorine even if the wetlands accept 100 percent of the plant’s effluent.
The last 250 acres are called the Overbank Wetlands where a city park will be built. “It will be a great place for people to come and enjoy the environment,” says Coughenour.
Plant efficiencies extend to the management of biosolids. Development has taken over much of the property where the plant’s biosolids were land-applied. “We decided to see if we could resurrect a solar drying program we had used many years ago.”
Being in the Sonoran Desert presents some advantages in drying biosolids. No greenhouses are needed, the biosolids are just placed on asphalt pads.
“We are drying 20 percent solids cake and getting about 75 percent of the water out using Brown Bear tractors for aeration,” he says. “We’ve reduced what we pay for land application to $1.9 million a year. Five years ago we were spending more than $5 million. We believe we’ll be able to reduce it to less than $1 million.”
Solar drying creates a product that is essentially Class A. Eliminating odors and vector attraction problems allows land application closer to homes and businesses. “Instead of land-applying 60 to 80 miles away, we are doing it 40 to 50 miles from the plant,” Coughenour says. “We’re taking trucks off the highway and producing a product that is more attractive to farmers. Demand is growing.”
Incremental change has been an effective approach at 91st Avenue. “We’re trying to manage lots of different chemical and energy applications,” says Coughenour. “We just improved our power monitoring, and the operators have been able to use that to limit when equipment is turned on to reduce our energy bill. That is producing substantial savings.”
The staff is using that new ability to find other opportunities to limit equipment and energy use during times of low flow. “Innovation is part of our mission,” he adds. “Not just to stagnate and accept what’s happened before.”