December 2006, Vol. 18, No.12

Going Deep

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In places that face seasonal water shortages, it’s an idea whose time appears to have come: Storing treated wastewater deep underground during rainy months, then pumping it back up to the surface for irrigation purposes during dry periods.

Such underground water banks are one of the hottest new applications of aquifer storage and recovery (ASR) technology. In some quarters, they’re also the source of some controversy.

 ASR technology — which involves injecting treated water into an aquifer through a deep well — is not new. At least 72 fully permitted ASR well fields in 17 states house more than 300 fully permitted and operating ASR wells, according to Pyne, who recently completed an inventory of these sites for the American Water Works Association Research Foundation (Denver).

The vast majority of ASR sites, however, are used to store treated drinking water. It’s only been in the last decade that cities have begun to explore their potential for storing treated wastewater that later can be used to irrigate golf courses, public parks, and other areas, reducing the demand on a city’s potable water supply.

That’s where the controversy comes in.

“It’s one thing to take reclaimed water and use it on lawns and golf courses,” said Mark McNeal, chief executive officer of ASRus LLC (Tampa, Fla.). “It’s another to put it into what is perceived to be a pristine aquifer.” Critics worry that the water could mix with and contaminate groundwater that could be tapped for public use, he said.

Such critics were particularly vocal when officials in Sarasota County, Fla., announced plans to drill three ASR wells capable of storing up to 757,000 m3 (200 million gal) of treated wastewater at a county utility complex. A legal battle ensued for more than 2 years. The county only recently was cleared to resume work on the project.

Economics Driving Interest
Arizona was among the first states to allow ASR facilities for reclaimed water and today operates about 16 such wells, according to Pyne. There is also one in New Jersey and a handful in Texas that are fully permitted.

In other states, such as Florida, interest is increasing now that the Florida Department of Environmental Protection (DEP) has adopted rules regulating their construction. Sarasota is one of five Florida cities that are developing and, in some cases, testing ASR projects.

Why the growing interest in ASR technology for reclaimed water?

Ecomomics is one of many reasons. Aquifer storage of treated wastewater is dramatically less expensive — both to create and to maintain — than surface storage.

Consider land costs. “It can take a full city block to house a surface reservoir,” McNeal explained. Building an ASR well, on the other hand, requires a plot of land “no bigger than a person’s driveway,” he said, dramatically reducing land acquisition costs.

The wells are relatively inexpensive to construct, especially when compared to the cost of excavating a reservoir or installing large numbers of aboveground storage tanks. An ASR well with a 378,500-m3 (100-million-gal) capacity, for example, costs roughly the same as a single 19,000-m3 (5-million-gal) storage tank, according to McNeal. “ASR wells are always less than half the cost of aboveground storage — and are usually just 10[%] to 20% of the cost,” he said.

There also are environmental costs to consider. “There’s much discussion about the potential impacts of storing treated wastewater underground,” said Christianne Ferraro, program administrator for Water Facilities and Watershed Management in DEP’s Central District. “But keep in mind, you also can’t store treated wastewater in a surface reservoir without impacting the surrounding wildlife and wetlands.”

Water loss is another issue in ASR’s favor. “In reservoirs, considerable water is lost to evaporation,” McNeal noted. “With an ASR [well], you may not get 100% of your water back, but it’s close.”
Aesthetics are also a consideration. “No one wants to see a field of storage tanks,” McNeal said. “A well is more invisible; most people wouldn’t even know it was there.”

Complementing, Not Replacing, Other Systems
In virtually all cases, the ASR wells are designed to complement, rather than replace, aboveground storage systems for reclaimed water.

“On a typical wastewater reuse system, treated wastewater is transported from the treatment plant to storage tanks, each one of which can hold up to 5 million gallons [19,000 m3],” said McNeal. A utility may have 20 or more such tanks.

Such systems can be adequate if the demand for reclaimed water is relatively stable year-round. Cities in drier climates, however, often find that the demand rises dramatically during their “dry seasons,” exceeding their supply.

This is often the case in Florida. “Here, our wet season runs from August through November,” Ferraro explained. “During those months, an ASR [well] can enable a community to stockpile treated wastewater that isn’t needed immediately for irrigation. During the dry season, which runs from March to May, that water can be pumped to the surface to meet the increased demand.”

The City of Englewood, Fla., which has been testing an ASR well since 2001, realized the full benefit of its new system this year, when the region experienced a particularly dry spring. “Had it not been for the ASR well, reuse customers would have had to undergo water rationing,” McNeal said. “It provided a great supplemental supply to the city’s reclaimed water system.”

Most communities, however, are unable to depend on ASR systems exclusively for reclaimed water storage, McNeal added.

“One of the limitations of ASR is that you can’t pull the water out of the ground very quickly,” McNeal said. While a utility can drain a 5-million-gal [19,000-m3] tank in just a couple of hours to meet peak demand, it can only retrieve about 1 million gal [3785 m3] a day from an ASR well. “It’s a relatively small straw that you’re sticking in there,” McNeal said.

By combining ASR and surface storage together, however, a utility can balance its daily flows and reduce the amount of surface storage it requires. “A utility can maintain a small number of storage tanks that it can empty quickly, and then fill up with water from the treatment plant and ASR well overnight,” McNeal said.

ASR wells also can enable a utility to expand its water reuse program. Consider, for example, St. Petersburg, Fla., which operates one of the world’s largest water reuse programs, distributing 151,400 m3/d (40 mgd) of reclaimed water.

“If St. Petersburg adds an ASR well with a capacity of 100 million gallons [3785 m3], that will provide an additional 1 million gallons [3785 m3] a day to its supply for 100 days,” McNeal explained. “That means they could add the equivalent of two to three golf courses to their customer base.”

But Is It Safe?
While public concern is focused on the impact of treated wastewater on an aquifer, a bigger concern among some experts is the impact of the aquifer on the treated wastewater.

“You can’t drill an ASR well for treated wastewater just anywhere,” Ferraro said. “There are stringent guidelines, for example, regarding the distance an ASR [well] must be from our drinking water supplies.”
These wells are typically constructed along the coastlines, where a saltwater interface is moving inland, McNeal explained. Freshwater aquifers located inland are reserved for developing potable water supplies.

Utilities must also meet high treatment standards for the water that goes into an aquifer.

“The higher the quality of water in the existing aquifer, the higher level of treatment that’s required for the water going in,” Ferraro said. “Where the water is very salty, for example, less treatment is needed on the reclaimed water, because a good deal of treatment is needed to make that water a potable water source.”

“Because this ASR application is still relatively new, there’s substantial testing done on water quality, both coming in and going out of the well,” McNeal added.

One of the more surprising findings has involved the water coming out.

“The water chemistry coming out of some wells has been a little different from what went in,” said Pyne, whose firm is studying 99 microcontaminants found in water drawn from ASR wells.

Take arsenic, for example, which isn’t found in the native groundwater in Florida, but during testing, it sometimes has been found in the reclaimed water, according to McNeal.

“The groundwater in these aquifers is very stable and has been there forever,” McNeal said. “But when you introduce a different kind of water, it appears that there are some geochemical responses that result in impurities leaching out of the host rock, causing localized reactions.”

The impact of these reactions, McNeal said, lessens with each cycle testing. “We expect that they’ll flush out eventually,” he noted.

Leaching hasn’t been a universal problem, according to John Pinkston,wastewater facilities superintendent for the City of Chandler, Ariz.

Chandler operates 11 fully permitted wells and has experienced no problems with leaching since the first well began operating in 2001, according to Pinkston.

“We’re now making plans to build two more,” Pinkston said.

Chandler is not alone.

“There are many ASR sites ... in various stages of development and testing right now, both in the U.S. and in other parts of the world,” Pyne said. “And there are many more cities now looking at [ASR technology].”

“But most cities don’t want to be pioneers,” McNeal added. “Many are watching and waiting till more [ASR wells] become fully permitted before moving forward with their own.”

Mary Bufe, WE&T

 


 

Wastewater Gone Green

Wastewater treatment plants take environmental practices to new level

In addition to enhancing wetlands, restoring prairies, and turning biosolids into fertilizer, the Shakopee Mdewakanton Sioux Community (SMSC; Prior Lake, Minn.) Wastewater Reclamation Facility staff has taken up gardening — on the roof.

While it may seem a bit odd to create gardens where few can actually enjoy them, let alone see them, these special rooftop plots make sense: They are good for the environment and good for the Shakopee plant.

While Shakopee is planting, other wastewater utilities across the United States are implementing a wide variety of “green” projects, ranging from constructing more energy-efficient buildings to working toward Leadership in Energy and Environmental Design (LEED®) certification by the U.S. Green Building Council (USGBC; Washington, D.C.). It’s all part of a growing trend of turning to greener practices in the name of sustainability.

What It Means To Be Green
The term green — used to describe actions or products related to environmental welfare — is tossed around frequently as the global warming debate takes center stage. There are green cars, green homes, green cleaning products, and even green vacation packages. But what does green mean?
According to USGBC, a green building is one that is environmentally friendly, economically friendly, and people-friendly.

“LEED was created to establish a common standard of measurement for what constitutes a green building,” explained Taryn M. Holowka, a USGBC spokeswoman. “[It] is a voluntary building certification program that defines high-performance green buildings, which are more environmentally responsible, healthier, and more profitable structures.”

LEED certification encourages a holistic approach to green building. It recognizes performance in sustainable site development, water savings, energy efficiency, materials selection, and indoor environmental quality. Projects qualify for certification based on the number of points they receive in these five key areas. Benefits for certified projects include healthier buildings, lower operating costs, and potential tax rebates and other incentives.

The routes to LEED certification run the gamut, encompassing both indoor and outdoor design elements. For instance, enhancing the indoor environment and comfort level for occupants through something as simple as skylights is considered an overall building improvement.

LEED certification is new but catching on. “There are currently 613 projects that are LEED-certified and over 4500 that are registered, which just means they are in the construction process and hope to certify upon completion,” Holowka said.

There also are numerous projects, such as Shakopee’s roof garden, which fall short of LEED certification but demonstrate a significant move toward embracing sustainable building practices.

Green by Design
The green roof at the SMSC water reclamation facility was part of the plant’s original design, stemming from the tribe’s desire to leave a small environmental footprint.

Planted in July by Aloha Landscaping (Mendota Heights, Minn.), the green roof project includes thousands of sedum — a groundcover plant — and thousands of native plants covering six roofs totaling 2790 m2 (30,000 ft2). Water treated at the plant is used to irrigate the roofs, and an SMSC horticulturist tends to the plants daily.

A roof covered in plants reduces total stormwater runoff, decreases erosion, and improves water quality. Each building covered by a green roof also enjoys lower heating and cooling bills because the vegetated roof keeps the indoor temperature more moderate. Finally, the life span of the roof is extended, as ultraviolet radiation that would normally hit the roof’s surface directly is buffered by the plants.

“Because of the green roof, heating and cooling needs will see about a 25% reduction than if it was a conventional roof,” said SMSC Tribal Administrator Bill Rudnicki.

In addition to rooftop gardening, SMSC has found other ways to do good by the environment.
“One thing we’ve implemented [is] bioretention ponds created for a new 57-lot subdivision,” Rudnicki explained.

Former farmland on the tribal land has been restored as native prairies by the SMSC Land Department, a practice that helps preserve an area’s natural flora and fauna, and the plant is planning to use its treated water to irrigate commercial properties, including its 60-ha (150-ac) golf course.

Taking the LEED
At the far end of the green spectrum is the Triangle Wastewater Treatment Plant (Durham, N.C.), which completed a $28.3 million expansion in April 2005. In addition to replacing the 22,700-m3/d (6-mgd) extended aeration secondary treatment facility with a 45,400-m3/d (12-mgd) full biological nutrient removal process, Durham County incorporated a green administration building — the first wastewater treatment plant administration building in the United States to be LEED-certified.

Constructing a green, energy-efficient building was “one of the priorities” for the county, according to Chuck Hill, Durham County utilities manager.

The administration building, which houses approximately 15 employees, uses its own treated wastewater for the heating, ventilation, and air-conditioning system and low-flow toilets, “reducing our needs for potable water,” Hill explained. In fact, potable water use is down 32% within the administration building.

The building features an automatic lighting system. As one walks into a room, the lights — sensitive to motion — turn on. When the system detects no movement, it concludes that the room is no longer occupied and turns off the lights.

Additionally, the builders used recycled materials, such as carpeting, where possible. They chose locally manufactured items for interior and exterior construction.

Durham County is not alone in getting certified. The Inland Empire Utility Agency (Chino Hills, Calif.) attained platinum-level LEED certification for its headquarters building in 2002. According to Eliza Jane Whitman, deputy manager of engineering, since Inland Empire has benefited from reduced operations and maintenance costs at its new headquarters, it has begun to require many of the same green design elements — such as recycled materials, skylights, and drought-tolerant plants — in its wastewater treatment plants.

At What Cost?
The environmental benefits are clear, but does green building cost more? Not over the long term, according to USGBC.

Holowka explained that according to a 2003 report, The Costs and Financial Benefits of Green Buildings: A Report to California’s Sustainable Building Task Force, “an upfront investment of 2% in green building design, on average, results in life-cycle savings of 20% of the total construction costs — more than 10 times the initial investment.”

And the predicted 25% reduction in heating and cooling costs Rudnicki noted is consistent with cost estimates made by the U.S. Environmental Protection Agency. For example, Holowka said, “tenants [in green buildings] can save about 50 cents per square foot each year through strategies that cut energy use by 30%.” Based on agency information referenced by Holowka, “this can represent a savings of $50,000 or more in a 5-year lease on 20,000 ft2 [1860 m2],”she said.

In addition, project managers, building owners, and architects are learning that LEED-certified buildings offer other benefits beyond financial savings, such as employee retention and increased sales. For instance, according to the California Board for Energy Efficiency Third Party Program Sales, sales in stores with skylights were up to 40% higher compared to similar stores without skylights, suggesting that green benefits are far-reaching.

While similar cost–benefit studies are not available to gauge the financial impact on wastewater treatment plants, those undertaking green building projects are optimistic about the future.

Rudnicki was frank in saying that the monetary cost of planting and maintaining the green roof project is higher than initial savings from lower heating and cooling costs. Yet when the tribe considered the long-term impact of a sustainable or green practice, the benefits outweighed the costs.

“We understand that you have to have a sustainable environment and a sustainable economy,” Rudnicki said, and this belief is underscored by the community’s commitment to sustainable practices.

Meghan H. Oliver, WE&T