Complex processes hold abundant challenges and satisfaction for staff members at the Millard H. Robbins Jr. Water Reclamation Plant.
An operator joining the team at the Millard H. Robbins Jr. Water Reclamation Plant will find enough processes, learning opportunities and challenges to last a long time.
“We’re a wastewater treatment plant with a water plant bolted onto the back,” says Brian Owsenek, P.E., deputy executive director for process and maintenance for the Upper Occoquan Service Authority, the facility’s owner. From end to end, the plant’s multiple treatment steps have been tested, automated, monitored and fine-tuned. Hundreds of standard operating procedures guide operators through tasks from routine maintenance to unplanned repairs.
The operations team faces the challenge of producing water for indirect potable reuse: The facility discharges to Virginia’s Occoquan Reservoir, source of drinking water for Northern Virginia.
The wastewater treatment side uses the modified Ludzack-Ettinger (MLE) secondary process. The water treatment side is built around a challenging but extremely effective high lime process, followed by multimedia filtration and carbon adsorption. Final disinfected effluent consistently meets a phosphorus permit limit of 0.1 mg/L and a COD limit of 10 mg/L.
It’s all overseen by a meticulously trained and highly qualified staff. “For a process guy, working here is like being a kid in a toy store,” observes Bob Canham, director of the Treatment Process Division. “For operators, there isn’t a lot of boredom here because there’s so much to learn. We keep our folks engaged by moving them around.”
Guarding the reservoir
The Upper Occoquan Service Authority was formed in the late 1970s as development around Washington, D.C., began to overload 11 small package wastewater treatment plants, whose discharges were degrading the Occoquan Reservoir. The Millard H. Robbins plant was commissioned in 1978 and expanded from an initial design capacity of 15 mgd to 54 mgd.
Effluent from the plant travels about 20 miles in the reservoir before reaching the water treatment plant at Occoquan that treats and distributes water to a significant portion of northern Virginia. “During normal weather, we account for a small fraction of the flow into the reservoir,” says Owsenek. “During drought, we can account for up to 90 percent, and we become the reason why northern Virginia is not exposed to water shortages. We have allowed the safe yield of that reservoir to grow because of the water we recycle into it.
“We also recognize that the majority of water we discharge ultimately flows into Chesapeake Bay, which is a national treasure. The bay has been recovering over the years, but the major problems are nitrogen and phosphorus. From day one, our permit requirements for those nutrients have been on the bleeding edge.”
Owsenek and Canham bring strong credentials to their roles, each backed by 30-plus years of industry experience. Owsenek holds bachelor’s and master’s degrees in mechanical engineering, while Canham holds a bachelor’s in civil and a master’s in environmental engineering. With John Connelly, training manager, and the shift managers and assistants, they work diligently to make sure the 38 plant operators are proficient in the plant’s multiple processes and cross-trained for flexibility.
Driving down nitrogen
The UOSA is a wholesaler, receiving flow from Fairfax County, the Prince William County Service Authority, and the cities of Manassas and Manassas Park.
Influent enters the headworks and passes through 1/2-inch climber bar screens (SUEZ) and a four-chamber PISTA Grit system (Smith & Loveless). A chemical scrubber at the headworks uses bleach and caustic soda to remove hydrogen sulfide and other odorous compounds. “We’re proud that our plant is essentially odor-free and has essentially no impact on our neighbors,” says Owsenek.
The flow then passes to six 125-foot-diameter primary clarifiers with aluminum covers and an odor scrubber using activated carbon (Calgon Carbon). Next, Internalift screw pumps (Evoqua Water Technologies) deliver the flow to a biological selector process where return activated sludge is added to the primary effluent for a short detention time in a zone with a high food-to-microorganism ratio, a step designed to limit bulking.
That’s followed by seven separate trains of 15-foot-deep aeration basins using the MLE process to remove ammonia, TKN and COD, while lowering nitrate to about 8 mg/L. Fine-bubble diffusers (Sanitaire, a Xylem brand) supply oxygen delivered mainly by a pair of 600 hp high-efficiency APG-Neuros turbo blowers. Several older blowers are operated alternately during summer peak flow periods. The last step on the wastewater side consists of 10 125-foot-diameter secondary clarifiers.
Complex — and effective
Secondary effluent enters the high lime chemical treatment process. “The high lime process was in vogue in the 1970s,” says Owsenek. “Other entities that tried to use it gave up on it.
Given our unique mission, and because we’re stubborn cusses, we kept at it. It is the most robust process for broad-spectrum treatment of water. Our staff takes pride that we are the only ones using this expensive, difficult process.”
The main purpose of the high lime process is to meet the plant’s extremely low phosphorus limit, but in addition it provides a barrier against pathogens. Canham states, “We add lime slurry to the secondary effluent and rapidly raise the pH to about 11 in six rapid-mix chambers. Then we go into a slow, quiescent mixing step where we flocculate it to promote attraction between the calcium and the phosphorus.”
In an automated control loop, lime slurry is added based on flow to maintain the pH setpoint.
Seven 125-foot-diameter clarifiers then allow the calcium phosphate to settle by gravity. The plant team discovered through testing that reseeding the incoming flow with the calcium phosphate sludge enhances clarifier performance.
Clarification is followed by first-stage recarbonation by injection of carbon dioxide, produced as exhaust from on-site combustion devices including a biogas-fueled engine-generator. “We drop the pH to about 10 to promote the precipitation of hardness — the calcium and magnesium carbonates,” says Canham. “Downstream from that is another set of seven clarifiers where we settle out the carbonate sludge.”
Next comes second stage recarbonation, where more carbon dioxide addition lowers the pH to about 7.5. Ten multimedia gravity-flow filters (Leopold Type S underdrains) and then 12 pressure filters use anthracite, sand and garnet media to remove remaining suspended solids. Alum is also added to precipitate more phosphorus.
The flow then passes through one-time-reactivated granular activated carbon (Calgon Carbon) for adsorption of refractory organic compounds (to reduce COD) and to remove color, taste and odor. On-site furnaces regenerate the carbon as required. The water is disinfected with sodium hypochlorite and dechlorinated with sodium bisulfite before discharge with turbidity of 0.2 NTU.
Chemical sludge is dewatered in gravity thickeners (Walker Process and EIMCO/Ovivo USA) to 5 to 15 percent solids and then to 50 percent solids in recessed-chamber filter presses before burial in an on-site landfill. The recessed-chamber presses replaced older hydraulic filter presses that yielded solids percentages in the 40s, according to Kevin Gately, solids process manager. The drier material will help extend landfill life: “If you produce wetter cake, then you either have to let it dry or add soil to it.” That added soil would consume landfill space.
Wastewater treatment sludges are delivered to three 1-million-gallon anaerobic digesters that produce about 250 cfm of biogas. After treatment for hydrogen sulfide and siloxane removal, the gas feeds an 850 kW Jenbacher engine-generator (GE Energy) that in 2016 produced 6.72 million kWh, about 25 percent of the plant’s electricity usage. Heat captured from the engine exhaust and jacket water warms the digesters. Cogeneration system availability exceeds 95 percent.
Biosolids from the digesters are dewatered to 22 percent solids in four centrifuges (Alfa Laval and Westfalia/GEA Group). Two natural-gas-fueled pelletizers (Andritz) yield pellets at 95 percent solids, which contractor Synagro markets and sells to farms and commercial fertilizer blenders.
The addition of the Andritz pelletizer about three years ago ended production and hauling of lime-stabilized material. “Pelletization saves us a ton of money in hauling fees,” Gately says.
“When we did a combination of lime stabilization and pelletizing, we were spending upwards of $30,000 a month in hauling fees. Now our hauling fees in a given month are below $10,000 and are usually much lower.”
“Generally, we only run one pelletizer train, but especially in spring when we’re trying to get our solids inventory down for the summer, we have to run both periodically. The Andritz unit is a newer technology and is more automated. The second unit is an older one and we have to make the adjustments manually.”
At each step of the process, the UOSA team takes pride in striving for continuous improvement based on data. “We’re like Major League Baseball here,” says Canham. “We keep statistics on everything. We do that for process control and economic reasons.”
Essential to plant performance are managers accountable for specific process areas. Juergen Roessler serves as process manager responsible for the technical side and C.G. Goldizen as operations manager in charge of operations. Other key managers and their duties in addition to Gately are:
- Gary Cooper, evening shift manager and digester/cogeneration process
- Frank Hogan, assistant shift manager and advanced treatment (filters, carbon system, carbon regeneration furnaces)
- Kevin Shrewsbury, assistant shift manager, solids thickening and transfer
- Mike Clark, night shift manager, preliminary and primary treatment
- Ben Caoili Jr., day shift manager, chemical treatment and disinfection
- Nas Fahmy, assistant shift manager, recarbonation
Illustrating the strategies employed to keep the process in tight control, Owsenek describes the phosphorus removal feedback loops. “We have two processes that control phosphorus: lime addition and alum addition. Ben Caoili controls the lime setpoint to maintain the pH necessary to remove the large bulk of the phosphorus. Frank Hogan controls alum addition to regulate the phosphorus actually leaving the plant.
“They work together with extensive rules we’ve developed over the years to control the two setpoints in what we think is the most efficient way. Every day, Ben and Frank pull up our lab data sheet, check the phosphorus numbers, coordinate with each other, and set those pH and alum targets.” Control on the alum side is aided by inline phosphorus analyzers (Hach).
Strict discipline applies to process control and purchasing decisions. For the activated carbon used to scrub the air from the primary clarifiers, plant leaders specified carbon not just on the basis of price per pound of material but on pounds of hydrogen sulfide removed per dollar. They explored on-site regeneration of that carbon but found it wasn’t feasible.
The team also tested polymer addition as a way to improve settling in the first set of chemical treatment clarifiers; it helped, but not enough to justify the added expense. “Sometimes we step up to bat and strike out, but we always step up to bat,” says Canham.
The high lime process itself is a challenge for operators. “Calcium carbonate scale forms all over the process,” Owsenek says. “It’s a very labor-intensive process, but it is extremely effective, and in our role as a reclaimer of water it is an essential part of our strategy. We basically have to make it work.” Several team members spend most of their time year-round descaling clarifiers, floc basins, rapid mix basins, mixers, pumps and other equipment.
Trained for the tasks
Given the challenges, it’s easy to understand UOSA’s emphasis on training. “As most people in the industry know, it’s very hard to find and bring in experienced operators,” Owsenek says. “Because we have so many processes, even when we are able to find people from the outside, they come in here at close to the ground level.
“John Connelly, who used to be a shift manager, moved into the training manager role and administers our career ladder process. When we hire people, even those with a lot of experience, they go through a multiyear development program where they have to pass tests in each section to demonstrate mastery and knowledge.
“There are 14 tests available, and each one is associated with a pay adjustment — basically a promotion. That allows us to bring in people at a salary commensurate with their limited knowledge of the plant and over the course of two to five years promote them as they gain knowledge, skill and confidence. It’s a very effective way of motivating people to learn the nitty gritty of the plant.”
The curriculum encompasses theoretical training by way of the California State University, Sacramento courses, and on-the-job training side by side with experienced operators, followed by sign-off upon mastery of specific tasks. “Then we have a capstone where we encourage our people to take the Commonwealth of Virginia Class 2 Wastewater Operator certification test,” says Owsenek. “That’s one of the final steps of the career ladder.”
Electronic tools support the operators and management. Software called eLogger lets operators from each plant section enter observations, problems, work orders and other items. These are available to the entire operations and management staff. “It helps us stay up to date on what’s going on around the plant,” says Owsenek. “We have a permanent archive.”
Meanwhile, PolicyTech software (NAVEX Global) serves as a repository for standard operating procedures: “All our SOPs are legible, direct and short. We capture the knowledge as best we can. Several hundred standard procedures and policies are in the system.”
The UOSA team is by no means content with the status quo. One future project: Finding a way to recycle the 3 to 5 percent of biosolids pellets that don’t pass quality control and at present are landfilled. “There are always things to learn,” Owsenek says. “There’s always research to be done, always a higher level of quality we can provide.”
A twist on nitrate reduction
The Upper Occoquan Service Authority is in an unusual position related to nitrate discharges: It’s required to discharge more nitrate than its neighbors to help protect the health of its receiving water, the Occoquan Reservoir.
“In northern Virginia, we’re very concerned about nutrient discharges into Chesapeake Bay,” observes Brian Owsenek, deputy executive director. “Most neighboring facilities have much lower effluent limits for nitrate than we do.”
The Occoquan Reservoir thermally stratifies in summer — warmer, oxygen-rich water in the surface layer and colder, oxygen-poor water in the depths. As a result, anaerobic processes in the deep water release ammonia, phosphorus, manganese, hydrogen sulfide and other problematic compounds. “We are compelled to discharge more nitrate than is normal in this area in order to control that anaerobic activity,” Owsenek says. “As part of a partnership involving Fairfax Water and the Occoquan Watershed Monitoring Laboratory, we actively control the nitrate we discharge to regulate the health of the reservoir and prevent those secondary releases.”
In the absence of oxygen, nitrate substitutes as a terminal electron acceptor, preventing the release of ammonia and keeping anaerobic fermentation processes from occurring in the deep water of the reservoir.
“We seasonally control the amount of nitrate we discharge,” says Owsenek. “In summer we turn off our modified Ludzack-Ettinger process, and even then we sometimes struggle to get our nitrate high enough to satisfy the reservoir’s needs. In winter, we run the MLE process, optimize it, and remove nitrate. Overall, we have an annual cap of 1.3 million pounds of nitrate that we’re allowed to discharge, and that we have to discharge.”