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Water, Water Not Everywhere

Employing conservation and reuse, the Denver Water Recycling Plant in Colorado treats water to get the most out of it in a water-scarce region

"We set this plant up for tours because educating our public on water reuse is our mission," explains Russell Plakke, manager of the new Denver Water Recycling Plant. "We live up to our slogan of The Right Water for the Right Use." As a utility, Denver Water supplies water for the city and county of Denver, Colorado and 70 suburban water districts.

As the largest water recycling facility in Colorado, the plant, which came online last year, treats up to 30 million gallons of effluent a day coming from the neighboring Metro Wastewater Reclamation District Plant to standards that allow for use in irrigation and industrial applications. Then pumps push the water through 18 miles of purple pipes to Xcel Energy, the Denver Zoo, Denver parks, golf courses, school systems, and other users. This helps stretch the use of water in the arid prairie region just east of the Rocky Mountains that clamors for more water as its population burgeons and diverse interests compete for riparian rights.

Plakke averages two tours a week during the school year, with groups ranging from elementary students to professional engineers. For today's tour, I've joined a Colorado School of Mines class on water and wastewater treatment. The previous week, Plakke and CH2M HILL environmental engineer Larry Schimmoller, senior project manager from the design team, showed off the state-of-the-art plant to four tour groups from the WateReuse Conference meeting in Denver. The groups also toured the neighboring Xcel Energy Cherokee Power Plant, the plant's main customer. Cherokee uses up to 10 million gallons per day (MGD) of the non-potable water for its cooling towers.

The history of the Denver Water Recycling Plant goes back several decades. To supply drinking water, Denver Water treats water taken from the South Platte River and Colorado's western slope of the Rockies. The Blue River Decree of 1955 mandates that the utility maximize its use of trans-basin water to minimize or defer new imports of water from the western slope. With this in mind, Denver Water has been investigating methods of reusing treated wastewater since that time. Denver Water teamed with the University of Colorado Environmental Engineering Department to develop a five-gallon-per-minute pilot as the new plant's predecessor known as the Successive Use Project. After completion in 1970, they operated it for ten years.

Then, in the 1980s and 1990s, the Potable Reuse Demonstration Project, a larger scale study, sought to establish the relative safety of reclaimed wastewater as a potable, or drinking water, supply. However, Denver Water chose not to pursue direct potable reuse because of high costs, customer perceptions, and regulatory constraints. They conducted an Integrated Resource Planning study from 1994 to 1997 that deemed recycling trans-basin effluent and conserving water preferable in the short-term. These measures formed the basis for the current recycling project.

In simple terms, the water recycling plant diverts effluent from the wastewater treatment plant, which treats wastewater from municipal sewage and industrial sources to a level that it could be safely discharged to streams or rivers. The recycling plant then treats the water to a higher level than the wastewater effluent but technically not as clean as drinking water, for use in industrial processes and irrigating crops, golf courses, parks, and school grounds. Previously, such processes would use potable water treated to drinking water standards. As a result, the plant reduces unnecessary treatment and transport of water by treating it only for its intended use.

High-Level Treatment
The first stop on the tour is the testing laboratory, where Plakke draws two beakers of water, one from each process train, or separate flow of water being treated. Here, the water is tested for pH balance, turbidity, organics, and chlorine content. "We treat to a higher standard than the state of Colorado requires," he explains. "In fact, 20 years ago, the process we use may have met potable standards."

When the engineering team consisting of consulting firms Boyle Engineering and CH2M HILL designed the plant, Colorado had not yet established regulations for reclaimed water quality, so Denver Water and the engineers decided to design the recycling plant to meet California Title 22 standards for reclaimed water quality and to the quality required by its major customer, Xcel Energy.

The next stop is the computer control center, where monitors provide 150 windows into the treatment processes and the purple-pipe distribution system. This high degree of automation means fewer staff members are required to operate the system, resulting in significant savings.

Because it takes as many workers to run the plant as a potable plant that produces ten times the amount of water, the recycling process is expensive. Also, because this plant sits at its low-elevation effluent source, three fourths of the delivery pipeline is uphill, doubling the cost of electricity for pumping. However, recycling proves cost effective when compared to finding new water and developing the resources, according to Plakke.

Next comes the Biological Aerated Filter (BAF) Building, where ammonia-eating bacteria in basins catch a ride on polystyrene beads, filtering the water as they go. In the rapid mix room, a coagulant is added to attract the particles to one another; slippery polymers aid in the process. Then it's on to the flocculation paddles. Three separate sets of paddles turning in opposite directions agitate the water, forcing collisions of the particles to further bind them together. The snowflake-like particles grow bigger and heavier.

Then the water flows into the sedimentation basin, where it moves up a series of closely-spaced stainless steel plates, causing sediment to fall to the bottom. From there, the flow goes to basins where the water filters through anthracite coal, and remaining sediment gets trapped. Then the water flows into the contact basin, passing through a series of baffled walls, where chlorine reacts with the water for at least 30 minutes. The water terminates in the finished water reservoir, a 300-foot-wide, 23-foot-deep, concrete-covered reservoir that can hold 11 million gallons. From here, 1,000-horsepower distribution pumps send the water to its municipal and industrial customers.

An Exciting Opportunity
Back in his office, Russell Plakke discusses his tenure at the plant: "We're all excited to be working here. We were able to handpick our staff, some from within Denver Water who asked for transfers." Twenty staff members consisting of two supervisors, process control folks, plant technicians, and maintenance workers operate the plant around-the-clock. Plakke joined Denver Water eight years ago as a technician at the Foothills Treatment Plant. Then he transferred to the recycling plant in the planning stages, and he became an assistant to Brian Good, the plant supervisor. The project's success contributed to promotions for Good to operation and maintenance director and for Plakke to plant supervisor.

You might question why two different engineering firms with strong water experience partnered on this project when either firm could have designed the entire project. "We were both pursuing the project individually, when Denver Water suggested we combine forces to make a stronger team," explains Tom Roode, Boyle Engineering project manager. "So we did, and Denver Water awarded us the project."

Boyle Engineering took the lead as primary consultant, providing overall project management and consultant team management. Also, Boyle served as the lead consultant for the final design of the source water pump station, site civil, yard piping, chlorine contact basins, distribution pump station, site electrical, and the solids handling facilities associated with the BAFs. With 500 employees, the firm has offices in Colorado, California, Utah, New Mexico, and Florida.

CH2M HILL served as the lead consultant for pilot testing and schematic design of all water, solids, and chemical treatment processes. They also led the final design effort for the BAFs, coagulant rapid mixers, flocculation basins, inclined plate clarifiers, and high-rate deep bed anthracite media filters. Headquartered in Denver, CH2M HILL provides engineering, construction, operations, and related technical services in the areas of water, transportation, energy, environment, communications, construction, and industrial facilities with more than 15,000 employees in 200 offices worldwide.

Multiple Challenges
Many design challenges confronted the design team. For starters, sedimentation basins 250 feet long normally require ample room. But with space at a premium on the recycling plant site, the engineers designed a basin with a series of 1,200 closely spaced stainless steel plates. As water moves up the series, particles hit the plates and drop out of the flow. The adoption of this parallel-plate system saved an estimated $4 million.

Water used for industrial purposes can have requirements of zero ammonia and low phosphorus, and this demanded complex solutions. The original plan called for a joint ammonia removal facility constructed by Denver Water and Denver Metro Wastewater Plant to meet upcoming wastewater discharge regulations. But when the regulations didn't change as anticipated, CH2M HILL had to scramble to help Denver Water select a process to remove the ammonia and suggested the BAFs.

"The BAFs were a $10-million change order after construction had started," Larry Schimmoller of CH2M Hill reveals. Although BAF systems had operated for 10 years in Europe, only a handful existed in the U.S., according to Schimmoller. Good, Schimmoller, and Jane Fisher, the Denver Water project manager, toured three BAF facilities around the country. Then CH2M HILL assisted Denver Water in selecting vendors, ordering equipment, and designing the facility. The system works so well, Plakke and Schimmoller presented a paper on it at this year's Water ReUse Conference. Since the plant is the only one in the world using upflow BAFs for recycled water, professionals from as far away as Paris, France have expressed interest in the technology.

Starting in 1997 with the planning and pilots, Schimmoller spent eight years of his professional life working on the Denver Water Recycling Plant. A project engineer in CH2M HILL's water and wastewater engineering group, he has been with the firm 15 years. He earned a bachelor's degree in civil engineering from Clarkson University and a master's in environmental engineering from the University of Illinois.

Tom Roode, who has worked at Boyle Engineering four years, has a mechanical engineering degree from Colorado State University and an MBA from the University of Colorado at Denver. "This was a good experience with so many different aspects," he says of the project. "Having the contractor (Western Summit/Lillard joint venture) involved throughout the design process was great, as we could integrate constructibility feedback into the design on a continual basis while keeping the design one step ahead of the construction pace."

Meanwhile, on the tour, Plakke explains that Denver Water has a second-phase project on the drawing board, which will increase plant capacity to 45 mgd. The expansion may take eight to 10 years, depending on economic conditions allowing for distribution system expansion. At full capacity, the recycling plant will then save 17,000 acre-feet of water a year, equivalent to what a small dam and reservoir would hold, or enough water to serve 35,000 households annually. This clearly shows that while Denver Water has come a long way in its efforts to conserve and recycle water, they can get even better.


For more information on the Denver Water Recycling Plant, visit www.denverwater.org/recycle/project_overview.html


Progressive Engineer
Editor: Tom Gibson
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