Paso Robles, California, made a plan in 2013 to go to tertiary wastewater and then recycling, but the city didn’t intend to modify the treatment plant until 2025. California’s prolonged drought changed that plan.

In 2016, the city had nearly completed a plant upgrade, including equipment to generate electricity from biogas, when the state began offering grants for recycled water projects.

“We had master-planned space so we could easily plug in tertiary treatment later,” says Matt Thompson, wastewater treatment manager. “Because of >span class="s4">on board, we decided to go ahead and build the tertiary treatment facilities.”

The work, completed in 2019, included innovations in UV disinfection and nutrient harvesting. The project received the Engineering and Research Achievement Award for 2019 from the California Water Environment Association.

Site advantages

The Paso Robles Wastewater Treatment Plant (4.9 mgd design, 1.9 mgd average flow) is on a site that slopes toward the Salinas River. The tertiary process, which includes filtration and UV disinfection, makes use of that slope.

“We can take advantage of the fall of water to run effluent through the tertiary treatment process entirely by gravity,” Thompson says. “We intercept the effluent from the secondary clarifiers and run it through some flow equalization basins on its way to filtration.”

The equalization basins are repurposed secondary clarifiers that were taken offline during the 2016 plant upgrade. They probably would have been demolished if the tertiary project had kept to the original schedule.

“I wanted to save some money so I had Black & Veatch explore repurposing the clarifiers,” Thompson says. “They happened to be in roughly the right location within both the plant space and the hydraulic profile to serve as flow equalization.” Reusing the concrete basins saved about $750,000.

The highly variable flow from the secondary clarifiers collects in the basins to be released at a steady pace to the filters. “We decided to go with cloth media filtration,” Thompson says. “The advantage is that there is very little headloss. We don’t want to have to pump the water through our filtration process. We’re able to flow entirely by gravity through our cloth media.”

The filtration system (Aqua-Aerobic Systems) enables operators to plug in a flow rate through the control system. Motor-actuated valves then adjust automatically to even out the flow. “That’s a nice thing to have because filters don’t like the normal fluctuations of flow that you see coming into a wastewater treatment plant,” Thompson says.

Dealing with hard water

The Paso Robles water supply, which comes from wells under the Salinas River, is mineral rich. That is a problem for UV disinfection because hard water tends to create a film over the lamps. The solution was a TrojanUVSigna system with ActiClean technology.

The quartz tubes that hold the UV lamps have Teflon collars on the outside that automatically move across the surface, wiping the tubes and discharging citric acid to clean the minerals off the quartz. Thompson calls the system a game changer for small plants that treat hard water: “Previously cities like ours would shy away from using UV light because they would be so concerned about fouling of the lamps.”

Lamp maintenance is also easier because the system hydraulics lift the banks of lamps out of the channel for easy access. Casey Shepherd, chief plant operator, observes, “We’re very fortunate as an operations-maintenance staff to have it be so user friendly. We can pull up the entire light banks hydraulically. We do it once a week on both channels. We add new ActiClean gel once a month.”

Before tertiary treatment, Paso Robles disinfected with chlorine. The advantage of UV is the absence of residual chemicals. “Not only are we producing tertiary-quality recycled water, but we’ve also greatly improved the quality of the water that we discharge to the river during the wet season when there is low demand for recycled water,” Thompson says.

During that season, the recycled water is used for irrigation around the treatment plant and for cleaning, but most is discharged to the river. “We consider that an interim situation until we complete the purple-pipe distribution system and sell it to the wine grape growers around Paso Robles,” Thompson says. “That part of the recycled water project is still in design.”

The distribution system is to begin operating in 2022. The largest customer is expected to be a new water district that formed just outside the city. A storage pond for recycled water is in place. “We’re essentially developing a new water supply for Paso Robles,” Thompson says.

Nutrient harvesting

The utility is also developing a new fertilizer supply. As the tertiary processes were under construction, the treatment plant suffered an onset of struvite formation in the piping, making nutrient control a priority.

The city had a long-range plan for nutrient harvesting, but Thompson had it added to the tertiary treatment project as a change order. The old process sent biosolids filtrate back to the treatment process. That allowed nutrients to build up and eventually form struvite.

The new system sends the filtrate to a cone-shaped device where it is mixed with magnesium. When caustic soda is added to raise the pH, struvite forms in a controlled reaction and falls to the bottom. From there, it is pumped into porous sacks that allow water to drain.

The nutrient harvesting equipment (Multiform Harvest) was assembled and is run by the treatment plant team. “It’s incredibly productive,” Thompson says. “Our operators are pulling more than a ton of struvite out of the process every week. Our plan is to sell it to a fertilizer distributor.”

The main motivation was to deal with the struvite problem, but there’s a sustainability benefit as well. “We consider ourselves to be recycling nutrients,” Thompson says. “Every ton of phosphorus fertilizer that we pull out of our plant is one less ton of material going out into the environment. We’re getting ahead of the nutrient regulations.”