Flagstaff Upgrades Treatment Plant
Professional golfers have a quote that neatly summarizes their game: "Drive for show — putt for dough." Like other sports quotes, it reflects a larger reality. It's human nature to focus our attention on spectacular feats like the impossibly long drives down the middle of the fairway. But, it's the 6-foot putts that determine the champions.
Wastewater treatment plants are the 6-foot putts of the construction world. They're not like the soaring glass and steel wonders that define our skylines and our cities. They're not the gracefully spectacular bridges that span our picturesque canyons, rivers and bays. In fact, they're usually hidden away from view in locations that are well away from populated areas.
WWTPs aren't glamorous, but they are absolutely essential elements of civilized life. Take them off-line for a few days, and things get ugly fast.
The Wildcat Hill Wastewater Treatment Plant in Flagstaff, AZ, is typical. It's only about a mile from the Flagstaff Mall, but it's a safe bet that only about 5 percent of the people in Flagstaff know it's there. The plant opened nearly 30 years ago and has been quietly processing as much as 6 million gallons of wastewater per day since then.
The Wildcat Hill plant, Flagstaff's largest, began a major expansion and upgrade in May 2007. PCL Construction Inc. of Tempe, AZ, won the contract and went to work with a two-year schedule to completion.
"This job initially went to bid in 2005," said PCL Superintendent Dave Weesner. "We were awarded the project in the design/preconstruction phase with a $28 million estimate and drawings that were 60 percent complete. The city felt $28 million was too expensive. They dropped us as the PM at Risk and went to a hard bid. That came in at $34 million. The city came back to us and asked us to do quite a bit of value engineering with them and Black & Veatch, the design engineer. Through this value engineering we were able to get the contract back down to $28 million.
"Since notice to proceed, we have worked with Black & Veatch and with the city's representative, Construction Management Associates. So far, we're at about $500,000 in value engineering. That's money we give back to the city of Flagstaff."
Modern treatment plants are a mix of cutting-edge technology and science that would have made perfect sense to Leonardo da Vinci. A mountain stream cleans water by churning it and injecting oxygen to burn off bacteria and other organic particles. Sand and dirt scrub organic contaminants from surface water as the water percolates down to the water table.
Oxygen, sand, heat, and gravity do most of the work at Wildcat Hill. A sewer main runs from the city of Flagstaff to the Rio de Flag, a dry wash next to the plant. Sewage enters the plant at a headwork that contains the primary screen, a system that separates solids from liquids. The organic solids are routed to the digester building, and the liquids go to the primary clarifiers.
The primary clarifiers are concrete tanks that serve as settling ponds to allow sediment to drop out of suspension and sink to the bottom. Water from the top of the primary clarifiers continues to the next stage. This step is where the plant is seeing its most significant change in the upgrade.
Currently, the water from the primary clarifiers is delivered to a pair of bio-towers. The bio-towers are the plant's equivalent to a mountain stream. Their job is to expose organic material in the water to oxygen and micro-organisms. Pumps at the base of the towers constantly inject outside air into the water. The aerated water is pumped to the top of the tower, and it trickles down through screens. The screens are magnets for colonies of micro-organisms that eat the organic material in the water as it trickles by.
A Major Upgrade
The two bio-towers will be taken out of commission when the upgrades are in place. Both towers will be replaced by an Integrated Fixed-Film Activated Sludge process. IFAS is a relatively new process, and the Flagstaff plant may be Arizona's first installation.
IFAS systems are housed in concrete vaults that are similar to very deep swimming pools. Air is delivered to the water through narrow piping systems on the bottom of the tank. Cylindrical, stainless steel retention sieves filled with honeycombed slices of plastic (media) sit in racks above the piping. The plastic media creates an ideal home for micro-organisms that provide the same function as they did in the old bio-towers. The air bubbles up through the media, adding oxygen to the process and circulating organic particles to be removed from the water.
"The media is something for micro-organisms to bind to," said PCL's Weesner. "The IFAS process requires far less space, both vertically and horizontally, than the bio-towers. That's one benefit, but the big gain comes in water quality. Currently, the reclaimed water this plant creates is rated B quality. The IFAS process will get the plant's water to A-plus quality."
That's a significant improvement, and it will allow the city to use reclaimed water for a wider range of applications. That would include making snow for the Snow Bowl ski area if the objections to that application can be overcome.
The old bio-towers will not be torn down when the IFAS process comes on line. The sludge collection equipment inside the towers will come out, and the tanks could eventually be used to store reclaimed water. PCL crews will remove the dome cover from one tower and place it on a nearby tank that is currently serving as a storage vault.
Water leaving the IFAS process goes to the secondary clarifiers. PCL has added a fifth train-of-process to the secondary clarifier system, and the contract calls for replacing all the mechanical equipment in the four existing trains.
From the secondary clarifiers, treated water moves to a chlorine contact basin and then indoors to a final filtering building. Currently, final filtering is done with sand beds in long troughs. Water comes in over the sand and percolates down to catch basins below. PCL has replaced one sand bed with a pair of Kruger disk filter units that work much like smaller reverse-osmosis filters. (PCL also installed three smaller disk filter units at the nearby Rio de Flag WWTP.) Water is forced through progressively finer membranes to capture any fine sediment that made it past the other processes.
Wildcat Hill Plant Specialist Charlie Hernandez said that water exiting the current final filter building typically contains 5 ppm of sediment. That's good. Regulations call for the plant to produce water with less than 50 ppm. When all the improvements are complete, the plant's number should drop below 1 ppm.
The turbidity improvement will come from the IFAS — not the disk filters — said PCL Field Engineer Lane Mullenaux.
"Although they don't match the filtration of the sand filtering system, the Kruger disk filter units allow significantly greater flow, approximately four times the flow of the sand method," Mullenaux explained. Maintaining the disk filter units is easier as well. Each membrane can be changed out quickly.
Before it leaves the plant, the water goes through a dechlorinating process. After that, much of the water goes directly into the city's non-potable water system where it is used to irrigate golf courses, lawns, school fields, parks, and other property. Unneeded water can be stored in the storage tank. Otherwise, the excess is discharged into the dry bed of Rio de Flag and soaks into the ground.
What about Solids?
A significant amount of the work being done by PCL at Wildcat Hill involves upgrades to the digestion building, which takes solid material separated out at the beginning of the process. PCL is installing two new centrifuges in the building to separate as much liquid as possible from the solid waste.
From the centrifuge, solid waste goes into a tank that functions much like the digestive system. It sits for 15 days or more at a constant temperature of 98.6 degrees F. Predictably, the process produces a copious volume of methane gas. For years, Wildcat Hill has used that gas to help power the plant.
"We had an old Waukesha generator that runs on the gas," Hernandez said.
The upgrades and improvements to the plant will deliver much more methane than the plant requires. Arizona Public Services offered to install a 280-kilowatt cogeneration system at the plant to tap the excess methane and use it to produce electricity to feed into the APS grid. Heat from the generator is captured by the system and used to heat water for the plant's needs. The new cogeneration system installed by APS and Kinetics includes a Jenbacher engine that replaces the old Waukesha.
Challenges and Accomplishments
Most of the work PCL has done to date has been fairly routine, Weesner and Mullenaux agreed.
"We had a difficult RAS (Reactivated Sludge) pipe installation, due to existing electrical and plumbing congestion," Mullenaux recalled. "It was heavy 20-inch pipe that required an overhead installation in an area where our access with lift trucks was limited. We had to be careful not to damage existing lines."
That caution carried through to every aspect of the project.
Weesner noted that, "Working at an active wastewater plant is always a challenge. You have to make sure that all the processes that are currently running continue to run as you add new equipment and take away old equipment. We had to take tanks off line, and we tried to hit the digester and the primaries at low-flow."
PCL managed to get three months ahead of schedule last year, Mullenaux said.
"We had a significant amount of concrete work on this project. The digestive control building and the secondary sedimentation trough were scheduled for pouring the first summer. We decided to pour the IFAS structure, too. We did our final pour just before Thanksgiving. We were lucky the snow didn't start early."
If PCL can maintain that three-month cushion, its crew should complete the project in late February or early March 2009. That will give Flagstaff a modern and efficient treatment plant producing millions of gallons of high-quality reclaimed water every day. It's just a coincidence that the biggest customer of the "putt-for-dough" treatment plant is a golf course.