Back to Winter 2010 Newsletter
The growing public acceptance of reclaimed water is overcoming one hurdle to maximizing the use of this valuable and, at times, maligned resource. Another hurdle to be vaulted is providing the means for delivering reclaimed or recycled water from water treatment facilities to various users, for irrigation, landscape and domestic purposes, even for possible in-home use.
Many say that decentralized water treatment facilities are the best means for getting over hurdle number two. They view the design and operation of such plants as a key strategy for efficient and economic delivery of reclaimed water to users.
Decentralization has lately been getting a lot of press. It is a key concept in the ongoing quest to increase supplies of clean energy and water. It is a strategy to downsize infrastructure, thus reducing the cost of maintaining a grid, whether an electric power grid or the subsurface pipes delivering water and removing sewage. Decentralization offers a counter argument to the bigger-is-better idea.
A remedy to ineffectual concentration, decentralization occurs at different scales. Some systems are located onsite, treating relatively small volumes of water and serving individual or groups of dwellings and businesses located relatively close to each other. At a much different scale, decentralization also can serve relatively large communities and subdivisions.
Wastewater is not waste
Ever watchful for ways to increase the state’s water supplies, the Arizona Department of Water Resources is committed to increasing the state’s use of reclaimed water, which is now at about four percent of total water supplies. Karen Smith, ADWR deputy director, says “One way we can do that is to make sure when we are siting our wastewater treatment plants, we do it strategically, thinking about how we are going to use that wastewater productively.”
A strategically sited wastewater treatment plant gains the advantages of decentralization. Advantages include a significant reduction of overall capital costs, phased construction and the potential for local water reuse in dual plumbing, thus ensuring a supply of treated water for irrigation and other non-potable uses.
Designing and operating a decentralized water treatment system, whether for a large city or a local development, means changing the way business has been done. The first order of new business then is to get over what Smith calls “the legacy of wastewater as waste.”
She says, “It is a movement away from the legacy of wastewater as waste when all that was looked at was economies of scale — the building of a plant of sufficient scale and with enough partners to treat wastewater and dispose of it as inexpensively as possible — to realizing its value as a resource.”
AZ and decentralized water treatment
A system built with economies of scale foremost in mind might be a large wastewater treatment plant fed by an extensive and costly regional collection infrastructure and ideally located down-gradient of its sources of wastewater, not a plant favorable to using reclaimed water.
For example, such a system does not serve the reclaimed water needs of Vail, Arizona, a community located about 25 miles from Tucson’s water treatment plants. The town’s wastewater is moved via pump stations at a significant energy cost to the Tucson plants for treatment. If Vail wants to conserve water supplies by using recycled water, treated water would then have to be pumped back to Vail from the treatment plant at further cost. This is a disincentive to reclaimed water use.
In effect, water would be making a long round-trip journey, arriving at the plant as wastewater, then treated and pumped back to Vail for reuse. Decentralized plants would shorten the round-trip journey, from water users to treatment plant and back to reclaimed water users.
Smith says efforts are underway to encourage small-scale water treatment plants in the state. In response to a directive from Governor Jan Brewer ADWR, the Arizona Department of Environmental Quality and the Arizona Corporation Commission are forming a blue ribbon committee to address water sustainability.
She says, “The group will look at impediments to the increased use of recycled water ... We are looking at a goal of getting to 30 to 40 percent of reclaimed water use by 2030. Decentralized plants would be a big part of it, to look at how we could maximize the use of smaller scale water treatment plants in an area as vast as the Salt River Valley and metropolitan Phoenix.”
Decentralization strategies
The topic of decentralized wastewater treatment raises some important questions: How can the strategy be adapted to areas already served by large centralized treatment facilities? Can such areas be retrofitted for decentralized wastewater treatment and to what extent? What is involved in adapting and installing such systems in areas being planned and developed?
Some cities in the Phoenix metropolitan area with large centralized wastewater treatment systems have achieved to some degree decentralized operations by “water scalping.”
The most efficient and economic way of disposing of wastewater was to cooperate with the City of Phoenix and pump all wastewater to the Phoenix’s 91st Avenue Wastewater Treatment Plant. Second thoughts occurred when the value of wastewater was recognized. The cities then began operating small-scale plants, called scalping plants, located in strategic locations in the community to better distribute and use reclaimed water. The plants scalp water from the treatment process and send the solids to the 91st Avenue plant.
The City of Tempe for example built the Kyrene Reclamation Plant as a water scalping facility. The plant treats water scalped from the city’s wastewater to class A+ effluent quality for use on turf, with the solid waste then going to the 91st Avenue plant.
Small-scale decentralization
A possibility for a large city to ponder, decentralization is more adaptable at a smaller scale, when a small or modest-sized area is planned and developed, an area as small as 36 square miles. It is not retrofitting to be done, but ground-level labor. At this level, a better view of the workings and potential of decentralized treatment is evident.
Graham Symmonds is the chief technical officer of Global Water, a utility at the forefront in Arizona in promoting the use of small-scale decentralized wastewater treatment facilities. He says the best time to install a decentralized system is before development occurs.
For example, Global Water is involved with the city of Maricopa and new development around Eloy; both communities are expecting new growth and are located on the fringe of municipalities. Symmonds says the concern is not to build the 120-million gallon treatment facility Maricopa will need 100 years down the road but the one-million gallon facility that serves current needs and that could be expanded incrementally. The key to establishing such a system is installing suitable, appropriately-sized infrastructure rather than taking a one-size-fits-all approach.
He says, “The daily flow is now about 2 million gallons and getting that down a monster pipe designed for a 150 million gallon flow presents a tremendous number of operational problems from a sewer design perspective.
“We put in the infrastructure as the community is growing ... a kind of a just-in-time infrastructure solution. This saves us from having to put in monster infrastructure; we get the scale of infrastructure that is quite efficient.”
Global Water’s regional planning is based on townships of 36 square miles, with each planning area having a water reclamation facility, located on about 35 or 40 acres of land, with the capacity to grow to treat 10 to 12 million gallons per day. Water distribution sites throughout the area distribute the treated water.
Symmonds says, “You don’t have to worry about transporting recycled water back from a huge facility to where you are going to use it. We save money on the redistribution of the recycled water because it is closer to the end users.”
He says there is not much difference in the technologies used by small-scale and large-scale plants. “There was a time when some of these technologies did not scale all that well, but they do now. You can have these small facilities that produce the right water quality and do it efficiently.”
Progress of decentralization
Symmonds believes a business, not a technological breakthrough spurred interest in decentralized plants. He says, “The economics of the regionalization have become a real decision factor. Utilities are no longer willing to accept — or should not be willing to accept — inefficient infrastructure.”
He says the next step is to deliver the treated water to a homeowner’s property. “Initially it will be used outside for irrigation. But there is no reason it could not eventually be used in the house for toilet flushing.” Using a nonpotable water source that way will save 50 to 55 percent of actual demand. A further advantage is that since not all water supplies will need to be treated to meet drinking water standards a considerable reduction in treatment costs will result.
The rebuilding of the country’s water/wastewater infrastructure, an identified national priority, presents an opportunity to decentralize operations. Symmonds says this would be the time to think about laying a third pipe for recycling and installing small recycling plants to scalp water from sewer systems to provide recycled water to communities.
He says, “We will have an opportunity as infrastructure changes over the next 25 to 50 year to rethink it and redo it right.”