A deliberate and patient approach to operating changes helps the Oakdale Wastewater Treatment Plant maintain consistent effluent quality.


To Cody Bridgewater, an unexpected change in effluent quality is no cause for hasty action — it’s a time for deliberate steps and patient observation.

“One thing I really stress to our team is that when we make changes to the system, we’re going to make them one at a time, so we can determine whether that change has had an effect, negative or positive,” says Bridgewater, plant supervisor at the Oakdale (California) Wastewater Treatment Plant.

“We have to be patient enough to allow changes to take effect. Usually, after two to three weeks at the earliest, we’ll re-evaluate and see what kind of impact we’ve made. So far we’ve had a lot of success with that.”

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The approach has served the plant well since Bridgewater came on board two years ago. Effluent typically shows BOD and ammonia below detection limits and total nitrogen below 3 mg/L (permit level 15 mg/L). The latest plant upgrade, to tertiary treatment in 2011, earned a Wastewater Treatment Project of the Year award from the American Society of Civil Engineers Sacramento Section.

Long history

Oakdale, known as the Cowboy Capital of the World, lies at the base of the Sierra Nevada foothills in central California. Its treatment plant, built in the 1940s, has seen several upgrades. A 2001 project added four cement-lined biosolids drying beds and a new headworks structure.

The 2011 project included:

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  • Lining of the plant’s two aeration ponds
  • Upgrade of the existing Biolac aeration system (Parkson) to include the Wave-Ox process
  • A 120-foot-diameter, 16-foot-deep secondary clarifier (Envirodyne) with return activated sludge pump station
  • Four AquaDisk cloth-media tertiary filter cells (Aqua-Aerobic Systems)
  • AquaRay 40 vertical-lamp UV disinfection system (Ozonia)
  • Two biosolids dewatering screw presses (Huber Technology)

The plant (4 mgd design flow) serves a community of 20,700 that includes a candy company as its only industrial discharger, contributing about 20 percent of influent BOD. “Their discharge is consistently about 40,000 to 50,000 gpd, but the strength is pretty inconsistent,” Bridgewater says. “We’ve had samples come in as low as 200 mg/L BOD, and as high as 9,600 mg/L.”

Process stability

Influent averaging 1.59 mgd comes to the plant by way of an aerial pipe that crosses the Stanislaus River. It passes through a CleanFlo Monoscreen 3 mm fine screen (WesTech), followed by a Schloss vortex grit collector (Smith & Loveless).

From there the influent mixes with return activated sludge and the flow is split between the east and west lined aeration basins, each 380 feet long, 176 feet wide and 16 feet deep, and with 9.12 million gallons total capacity.

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The Biolac process uses a long hydraulic retention time with more biomass in the system, providing stability to handle load and flow fluctuations. The process uses fine-bubble aeration, and at Oakdale the air is supplied by eight positive displacement rotary-lobe blowers: two 100 hp Robuschi units, along with two 100 hp, two 75 hp and two 40 hp Sutorbilt units (Gardner Denver).   

In each basin, 20 6-inch air lines (aeration chains) float on the surface, each with 15 air manifolds suspended on 1-inch air lines half a foot above the basin bottom. Each manifold has five diffusers. When air is activated, the manifolds oscillate in the water in response to the rising air bubbles. This enables highly energy-efficient aeration and mixing.

Removing nutrients

The Biolac Wave-Ox technology simplifies total nitrogen removal by enabling automatic aeration control among groups of aeration chains to create multiple oxic (nitrification) and anoxic (denitrification) zones.

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“We can dictate if a given chain is always open or always closed, or if it is in Group A or Group B,” says Bridgewater. “So, for example, while Group A is aerating, Group B is becoming anoxic and causing denitrification to happen. Then after a certain time, which right now is set for 20 minutes, those zones switch. We can change that time based on what kind of response we’re getting out of the basins. We can pick which chains we want to be in each grouping, and we can work each basin independently.”

From the aeration basins, the flow travels to the 120-foot secondary clarifier. An older 90-foot clarifier, now being rehabilitated with a new drive mechanism (WesTech), will be available for redundancy. After clarification, a low-lift pump station feeds the cloth-media filters, operated in pairs in alternating months. UV disinfection is the final step before discharge to rapid infiltration ponds.

Biosolids are dewatered to 14 to 15 percent solids on two screw presses (Huber Technology). The cake solids are sent to the local landfill for use as an alternative daily cover. During summer, waste activated sludge can be pumped to drying beds to enable additional wasting.

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Attentive operators

Plant team members take pride in keeping a close eye on the process. “When I started, we did our in-house labs three to four days a week to track what was going on in the system and where we needed to be operating,” Bridgewater says. “At one time the system was a little bit solids-bound. Since I started, we’ve been wasting five days a week for six to seven hours a day. We’re wasting consistently to keep our mean cell residence time at a good level.

“We do our morning reads every day at the same time. We check the DO levels and the equipment runtime hours. We clean the floatables out of the clarifier scum box twice a day. We check the return activated sludge pumps, the low-lift pump station and the filters to make sure they are operating properly with no alarms.

“I think the biggest item has been staying on top of our UV system cleaning and maintenance. We pull the UV modules out of the channels and put them in the acid dip tank every Tuesday and Friday, making sure the intensity of those lamps is as high as it can be. That really helps us maintain an effective disinfection process. We also have very clear water coming to the UV system. Average turbidity at that point is under 1 NTU.”

Operators monitor the process of a SCADA system installed by Tesco Controls and using Wonderware software (Schneider Electric - Invensys). “We have a lot of ability to trend data, whether it be influent flow or DO,” Bridgewater says. The blowers operate on a feedback loop using a DO sensor (Hach) at the effluent end of each basin and a Hach SC200 controller. Changing blower operation from manual to automatic last year has saved about 10 percent on electric bills.

Being methodical

Upsets in the process have been rare at Oakdale, but when some parameter starts to stray, the team responds with care and discipline. For example, when effluent nitrates began to climb, Bridgewater saw a need to make adjustments to the basins. He decided to try reconfiguring the aeration zones in one basin and shared the plan with the operators.

“I wanted to change the way the zones operated and create a bigger area for the bugs to move around and come in contact with the nitrates,” he says. “The biggest thing I could stress was just to watch it — leave it alone. We kept monitoring it, keeping to our normal routine and normal tests. And sure enough, over the course of a few weeks, the nitrates started to drop.”

On a daily basis, Bridgewater takes pride in general housekeeping: “I truly believe the appearance of your plant is a representation of how you feel about your job.”

Technology helps the team keep tabs on maintenance, analyze data and prepare reports. They use the cloud-based MobileMMS computerized maintenance management system, which generates work orders for emailing to team members every morning.

“We went from paper work orders and paper checklists to everything being done on iPads,” Bridgewater says. “Our morning rounds are all done electronically. We hand-enter the data as we go through, and it’s automatically uploaded to a secure server. Everything is held there so that I can filter the data and create reports.

“If I want blower runtimes for December through April, I can generate that. I can set a couple of filters and the system brings me all the DO readings for the past month. I don’t have to thumb through file drawers to find something — it’s there at the click of a mouse. The ability to integrate technology with hands-on experience has been valuable. We’re a lot more organized now. The wealth of data we have at our fingertips helps a lot with troubleshooting.”

IPads and wireless technology enable the team to operate and monitor the SCADA system from home, which allows them to work minimal two-hour shifts on weekends and holidays. The plant is normally staffed weekdays from 7 a.m. to 3:30 p.m.

Still learning

The Oakdale plant marks a step in Bridgewater’s progression as an operator. His grandfather is retired as a Grade III operator with the city of Modesto. “Shortly after I graduated high school, he told me, ‘You need to get into this field.’ Being the typical teenager, I knew better.”

He went into land surveying instead, but after a half-dozen years found himself out of work within a weak economy. “I started applying to wastewater jobs throughout California,” he recalls. “It took me a year and a half to get in as an operator in training.”

He worked two years at a small trickling filter plant in Jamestown, 40 minutes east of Oakdale. Two years later, as a Grade II operator, he moved to the Salida Sanitary District to help operate a 1.1 mgd activated sludge plant. After two more years, certified Grade IV, he came to Oakdale, where he received his Grade V certification in November 2015.

Now he’s pursuing an online four-year degree in water resource management through Florida Gateway College and enjoying another step up in the profession. “At Oakdale I’ve been able to get to a whole new level with tertiary treatment and UV disinfection,” he says. “It has really opened my eyes to how much opportunity there is in the industry.”


Across generations

Team members at the Oakdale Wastewater Treatment Plant cross generational lines, but work well together regardless.

Senior operator Lovanna Brown and maintenance mechanic Todd Cordoni are in their 50s, plant supervisor Cody Bridgewater is 32, and operator Sam Soth is 29. All pitch in to help keep the plant running efficiently and in compliance.

“Lovanna’s biggest asset is her 25-year history with the plant,” Bridgewater says. “She knows all the ins and outs, the idiosyncrasies that someone can only know from being around,” she holds Grade III Wastewater Treatment and Grade I Lab Analyst certifications.

Soth started in early 2015 as a part-time lab technician and later that year became a full-time operator in training. He completed his 1,800 hours for certification earlier this year and became a full-fledged operator with Grade III Wastewater Treatment certification. He also holds Grade II Lab Analyst and Grade II Environmental Compliance Inspector certifications.

Cordoni came aboard in 2007. “During construction for the 2011 upgrade he was out there daily just learning,” Bridgewater says. “He knows where every pipe is in this plant.” An expert fabricator, he built the pontoon used for replacing diffusers in the aeration basins. It has an electric winch and solar panels to keep the batteries charged: “It’s amazing what he can do with a welder.”

Bridgewater adds, “Being able to communicate and having commonality of experience in the industry has allowed us to work through our generation gap.”


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