Struvite had become a serious problem for the Tres Rios Water Reclamation Facility in Arizona’s Pima County.

The buildup of struvite occurred downstream of the anaerobic digesters and especially in the centrifuge facility where biosolids are dewatered from 2.5% to 18-20% solids. “We’d see it form especially in the centrate lines because the centrate was so phosphorus-laden,” says Steven Richey, plant manager.

“Struvite is kind of like kidney stones. It’s made up of the same material. With the amount of phosphorus we had in the system, we were almost totally occluding pipes. It was almost like the formation of concrete on the pipe walls. We started seeing high pressure on the discharge side of pumps. Eventually you have a 10-inch pipe with only a 3-inch hole down the middle, if that.”

Jason Hartman, assistant plant manager for operations, adds, “The main centrate header coming off the centrifuges is an 18-inch line, and it was almost completely occluded when we got in there and cleaned it out.”

To solve the problem, the Tres Rios team worked with the HDR engineering firm and Schwing Bioset to implement North America’s first installation of the NuReSys nutrient recovery technology. The $6 million system precipitates the struvite within the digestate stream so that it can be removed with the biosolids.

The system converts more than 85% of the orthophosphate in the centrate to struvite crystals, resolving the buildup. It also greatly reduces the phosphorus recycled back to the treatment plant headworks, leaving no more than 50 mg/L of orthophosphate in the return stream.

An essential facility

The Tres Rios plant (50 mgd design, 30 mgd average) serves the Tucson metropolitan area and is the central biosolids treatment site for all eight of Pima County’s clean-water facilities. It is permitted for A+ reclaimed water through the Arizona Department of Environmental Quality. Some of the reclaimed water is used on site and for irrigation at a neighboring park, but most of the effluent discharges to the Santa Cruz River.

Biosolids cake is hauled from the site, land-applied and incorporated into the soil by Synagro. The Tres Rios facility produces 180 wet tons per day of dewatered biosolids from its own operations and the other Pima County plants.

The plant was built in the early 1970s with a high-purity oxygen activated sludge process. In 2006 a mixed liquor extended aeration process came online with the high-purity oxygen still in service. In 2014, as part of a Regional Optimization Master Plan, the five-stage Bardenpho process for ammonia and total nitrogen removal was brought online.

“We started having the struvite problem when the mixed liquor extended aeration process was added, but we really saw it when we put in the five-stage Bardenpho,” Richey recalls. To slow struvite buildup, the plant team added ferric chloride to the process stream to change the pH of the biosolids, and added dilution water in the centrate return lines.

“But then we still had all that phosphorus going back to the plant in the centrate,” Richey says. “Now our NuReSys facility is helping us get the phosphorus out in the biosolids, so it can go into the field for land application.”

Choosing a remedy

In exploring potential remedies for the struvite buildup, HDR worked with the county to establish a set of performance specifications, including at least 85% reduction in orthophosphate and an electrical demand less than 10 kWh per kilogram of phosphorus removed. The specifications were then shared with two technology vendors for cost proposals.

One technology was pilot-tested. The NuReSys process, although not pilot-tested at Tres Rios, had been proven in numerous installations in Europe. It was ultimately chosen in 2018 based on a financial evaluation. In addition, the Tres Rios plant had experience with Schwing Bioset, having used the company’s positive displacement piston pumps to lift dewatered biosolids into the elevated silo from which the material is dispensed into trucks.

“We’ve had a good relationship with Schwing Bioset,” says Richey. “We had confidence in them, and they were willing to work with us on design modifications going forward.” 

Creating crystals

Struvite control starts with managing the pH of the feed streams to influence the reaction of magnesium with the phosphorus in solution. This happens in an air stripper. Once an ideal pH condition is created, magnesium precipitates the soluble orthophosphate ions as struvite.

The addition of magnesium chloride enables continual precipitation of struvite from the waste stream. For every one pound of orthophosphate removed, an additional half-pound of ammonia nitrogen is also captured.

In the process at Tres Rios, the three Westfalia centrifuges (GEA Group) are downstream from the NuReSys process. Digestate is pumped into the NuReSys CO2 release tanks (air strippers). There, carbon dioxide is released via coarse-bubble aeration, raising the pH to the desired condition. Overflow from the air stripper proceeds to a crystallizer, where magnesium chloride is added in proportion with the orthophosphate concentration and stream’s solids content.

In the crystallizer, a continuously stirred tank reactor, the magnesium reacts with the soluble orthophosphate ions and nitrogen, allowing the struvite crystals to form and grow. The process yields 1-3 mm struvite crystals. “You can definitely see them in the cake biosolids,” Hartman says. “They almost look like rhinestones in the sunlight.”

Fine-tuning

The NuReSys process was commissioned in November 2020. It has functioned well, although some midstream adjustments have been made. A significant challenge was in constructing and integrating the technology while keeping the Tres Rios biosolids process operating without interruption. “We are an end-of-the-line facility,” Hartman observes. “We don’t have the ability to close the gates and stop the flow.

“We’re processing all of Pima County’s biosolids, so our centrifuge building is in operation 24 hours a day, seven days a week. The integration and the ability to bring this new process online, while still producing biosolids and sending it offsite, required awesome coordination by our staff and our contractors.”

Richey adds, “We’ve had to do some workarounds and some modifications. Schwing Bioset has been very accommodating, especially during the first year, in allowing us to make those modifications without voiding our warranties.”

One issue involved the process overflow from the air stripper tanks to the crystallization reactor, as struvite constricted the lines, and cleaning them was labor-intensive. That was corrected by a plumbing change that made it possible to waterjet the lines.

Another adjustment involved the addition of polymer ahead of the centrifuges. “We expected to use an amount of polymer comparable to what we used before,” says Richey. “But we found the dry polymer we were batching and mixing on site didn’t have a high enough molecular weight to process the biosolids without the ferric chloride and the same amount of phosphorus in the system. We had changed the consistency of our material, so we had to switch to an emulsion polymer that is considerably more expensive.”

Keeping it running

Tres Rios operation and maintenance technicians operate the system 24/7 and collect the process control samples required for troubleshooting and process decision-making. During startup and commissioning, daily orthophosphate testing was performed on site to fine tune the magnesium chloride feed rate.

Central maintenance heavy-duty mechanics remove and clean struvite-laden piping during process upsets. Especially before piping modifications for CO2 tank overflow pipe cleaning, maintenance also incorporated weekly flushing of the piping with plant process water to help keep the formation of struvite from starting.

Instrumentation technicians clean and calibrate the system’s pH and level control instruments. Quarterly tank cleanings are ongoing. A contracted combination truck service (Vactor) clears debris from the tanks being cleaned and jets the struvite formation from overflow piping between the CO2 stripper tanks and crystallization reactor. Buildup of rags and hair in the reactor after startup has decreased with a prescreening size change from 5 mm to 2 mm.

Close collaboration

A residual benefit of the NuReSys process is substantial savings on ferric chloride. “We’ve reduced ferric chloride addition by about 75%,” Hartman says. “Now we just add a small amount to the digesters, mainly for H2S control. We were pumping more than 360,000 gallons a year, and now we’re down to less than 100,000 gallons a year.” A chemical that once cost nearly $1 million per year now costs about $300,000.

Hartman cites close relationships among the plant team, HDR and Schwing Bioset as important to the project’s success. The work was complicated by the onset of the COVID pandemic, as a slowdown in supply chains caused delays in receiving some equipment. That aside, “Between HDR and Schwing Bioset we always had good response. People were professional in helping the work move forward.”