The difference between night and day is for real at the Butler Water Reclamation Facility in Peoria, Ariz. Proud owners of a brand-new membrane bioreactor (MBR) system, one of the largest in the world, plant supervisor Ray Trahan and his staff have had to adjust their processes in response to dramatic changes in diurnal flows. The hot desert weather is a factor, as well.

“Depending on the time of year, we may have almost no flow between 4 and 6 a.m.,” says Trahan. “We’ve had to make changes in order to provide a steady flow to our membranes. And our hot weather and warm water temperatures reduce the slack in our membranes, altering our maintenance plans.”

The crew at the Butler WRF has resolved these issues. Effluent from the new plant is pristine, as it needs to be for the community’s groundwater recharge system. “We’ve steadily improved operations as we’ve gone along,” says Trahan. “We’re meeting our nitrogen removal requirements, turbidity is less than 0.01 NTU, and our fecals are non-detectable.” The pure effluent is also netting the city valuable credits for recycled water (see sidebar).

Brand-new facility

The plant was built from scratch and started up in June 2008 to serve the rapidly growing population of Peoria, a Phoenix suburb. “In 2001, the city began working toward two goals: increasing the community’s capacity to treat wastewater, and producing a high-quality effluent for aquifer recharge,” says Brad Hemken, project director in the Phoenix office of Black & Veatch, which designed the plant. Before the new facility was built, Peoria sent its wastewater to a neighboring community for treatment.

“Not only was the city paying the cost of the transfer and treatment, but we weren’t realizing any credits for recharging the groundwater,” Trahan says. Flygt (ITT Water & Wastewater) influent pumps bring an average of 7 mgd to the head of the treatment train. Off-site bar racks remove large solids and debris, and a vortex grit removal system is located on site. The wastewater then passes through 2 mm fine screens to protect the membrane process against remaining solids.

Ahead of the 10 membrane tanks (GE Water & Process Technologies – Zenon), the MBR system has four parallel trains, each containing three anoxic basins operating in series and one oxic basin. Contents in the anoxic areas are mixed mechanically, and diffused aerators mix the oxic zones. The LAMSON blowers are from Gardner Denver. The system achieves complete nitrification and partial denitrification before the flow passes through the membranes.

High-quality product

“The return activated sludge is high in dissolved oxygen, so it is returned to the oxic basins rather than the anoxic basins, where it would hamper the denitrification activity,” Trahan says. After membrane filtration, the flow passes through a UV disinfection system supplied by Trojan Technologies.

The effluent, well within an 8 mg/l alert level and a 10 mg/l effluent limit for total nitrogen, is gravity-fed to underground storage and percolation beds a little more than five miles away. As a standby, the plant maintains an NPDES-permitted outfall into the New River, which is a dry channel most of the year.

The plant has a design capacity of 13 mgd, but flow is now capped at 10 mgd, and just six of eight membrane cassettes are installed and operational in each tank. “We can build out to 13 mgd in the future if necessary,” he says.

During normal operation, an intermittent air-scour process prevents solids from building up on the surface of the hollow-fiber membranes. In addition, the membranes “relax” for a minute every 15 minutes as permeate production stops and aeration continues. The membranes can also be back-pulsed if needed to remove accumulated material. During regular maintenance, the membranes soak in a cleaning solution of citric acid or sodium hypochlorite.

Biosolids removed from the treatment process are conditioned with polymers and dewatered in GEA Westfalia centrifuges, which achieve 18-20 percent cake with no pre-thickening. Trucks take the cake to an area landfill.

Knowing the process

The Butler WRF staff and management took part in extensive training on the new system, including visits and conversations with other MBR plant operators in Arizona, and weeklong in-house training sessions with the manufacturers. It was helpful that a Zenon membrane users group met in Phoenix last year.

“We’ve met other MBR operators and have stayed in contact with them through e-mail,” Trahan says. “And Zenon has helped us optimize the plant.” Still, nothing is better than hands-on experience in getting to know the ins and outs of new wastewater treatment technology. “We knew going in that this was a complex process, but it turned out to be a little more complex than we thought,” Trahan says.

The diurnal flow issues forced several process adjustments. “It was a challenge for our instrumentation and control people, as well as our operators,” Trahan recalls. His crew tweaked the control program and the pumping volumes and worked with the manufacturer to re-program the set points that control volumes in the aeration basins ahead of the membranes — all to smooth out flow to the membranes.

The plant sees wide seasonal swings because the San Diego Padres and Seattle Mariners baseball teams hold their spring training camps nearby. March is an especially high-flow month.

Beyond the membranes, the in-vessel UV controls were affected by flow rates. “The UV looks at flow rates and will shut down if the flow is too low to avoid overheating,” says Trahan. “If the UV shuts down, the membranes also shut down to avoid discharging untreated water.”

That situation led to repeated re-starts of the system, wasting energy and increasing wear and tear of the equipment. But the process has steadily improved through trial and error. “We’ve achieved success as we’ve gained more experience,” he says.

Beating the heat

The desert climate presented more challenges, especially with the membranes. The fibers are designed with a certain amount of slack so that they can be shaken periodically to dislodge debris accumulating on the outside surfaces.

“The high heat and water temperature here initially caused the membranes to shrink and lose the required slack,” says Trahan. “We didn’t anticipate that, and we’ve made adjustments to the amount of slack in all 60 cassettes to account for the shrinkage. The slack must be restored in order to clean the membranes properly.”

The membrane cleaning procedures have gone as prescribed. For normal cleaning, the staff uses sodium hypochlorite or citric acid, soaking the membranes for about an hour once a week. For more thorough recovery cleaning, they soak the membranes overnight at intervals of about a year. The trains are cleaned individually, so that flow continues uninterrupted.

At first, the plant experienced some issues with the internal wiping mechanism on its UV units. “The company was very responsive and made changes to the drive system hydraulics,” says Trahan. “They changed out a carriage and sleeves. The units are working very well now.”

Trahan’s team has also adjusted the controls on the blowers, fine-tuning blower ranges to accommodate surges in the flows.

Lessons learned

What’s been learned in starting up an MBR operation of this size and scope? “Always expect the unexpected, and plan a year ahead of time if you can,” Trahan says. “Our staff has been outstanding. Our team is small and there have been serious demands on them — the anticipation of startup, and not knowing what to expect.”

Trahan says having lead operator Roger Carr on staff was invaluable, because of his experience as a lead operator at a membrane facility. “Only two of our people had any experience with membranes, and that was on the clean-water side,” Trahan says. “We shared our experiences.” That and the training made the project a success. “There’s always room for improvement, but we’ve been able to handle it,” Trahan says.

With startup behind them, the Butler WRF staff can look forward to the next phase of the project: development of a park around the facility. The location of the plant was one of the issues thoroughly aired in public meetings before construction. There have been no complaints, even though some residences are only a half-mile away, and the public should be even more pleased in the next few years as the city develops the park.

Trahan says the effluent water will then be used to irrigate vegetation and fill small recharge lakes around the property, or be injected into the groundwater table. That was one of the reasons for choosing MBR technology in the first place. “It has a small footprint,” says Trahan, “and it’s compatible with the surroundings.”