September 2007, Vol. 19, No.9
Water Reuse: What's on the Horizon?
Recent advances in water reuse technology have reduced operating costs, improved efficiencies, and significantly enhanced the quality of treated effluent — enabling municipalities to consider reuse projects that previously would have been economically and technically infeasible.
These improvements, coupled with growing concerns about maintaining adequate water supplies in drier regions of the world, have grown the market for membranes and other water reuse technologies.
According to Jean Debroux, a research scientist and water quality resource specialist at Kennedy/Jenks Consultants (San Francisco), wastewater reuse has become an integral part of many communities’ water resource planning, especially in California, Florida, and the arid Southwest. “The majority of reuse is for nonpotable purposes, although there are several significant indirect potable reuse projects in the U.S.,” he said.
Val Frenkel, membrane technologies group leader at Kennedy/Jenks, agreed. “Water treated from wastewater is increasingly becoming a substitution for major water use applications, such as agricultural irrigation, use in golf courses, and for groundwater supplementation,” he said.
While most of the technologies suitable for water reuse have existed for some time, in recent years they have enjoyed a higher profile. Water quality professionals cited lower cost and demonstrated success as two reasons why these technologies are capturing newfound attention.
Membrane bioreactors. Membrane bioreactors (MBRs), which combine biological treatment with membrane filtration, have become an increasingly cost-effective option for wastewater treatment plants to meet more stringent nutrient limits. Their ability to produce highly polished effluent also makes MBRs a popular choice for water reuse applications.
“MBRs can treat wastewater effluent to near drinking water standards in removing particles and other constituents,” Frenkel said. He added there is a significant demand for MBRs on the U.S. West Coast, where the majority of projects incorporate the technology.
MBR costs have declined dramatically in the past several years. In addition, industry acquisitions “may effectively lower the prices” of MBRs, since large companies that have purchased smaller membrane manufacturers could take advantage of large-scale production, Frenkel said. Compared to conventional processes that require more concrete and steel — which are becoming more expensive — “the economics of MBR are improving year to year,” he said.
Reverse osmosis. According to the Brackish Groundwater National Desalination Research Facility (Alamogordo, N.M.), saline and brackish waters constitute more than 97% of the water in the world. Now that reverse osmosis technology has become more efficient and less expensive, water professionals said, desalination of this plentiful water supply has become a viable reuse option.
“Traditionally, energy demand has been the biggest threat to desalination,” Frenkel said. “However, there have been advances in desalination devices and technologies which have significantly reduced this demand.”
In California alone, 20 new desalination plants are planned, with the largest in Carlsbad and Huntington Beach, according to Frenkel.
Debroux said reverse osmosis has become a reliable desalination technology for use in generating a droughtproof water supply. “The cost and energy requirements of reverse osmosis are dropping due to advances in membranes and energy recovery systems,” he said. “However, disposal of brine continues to be a major issue, especially in inland areas.”
Desalination technologies also are gaining prominence because of the degree of water purity that can be produced at lower costs than in the past, according to Rebecca Bright, a senior research analyst in the Technical Insights group of Frost & Sullivan (San Antonio).
“Distillation and membrane technology have been around for quite some time,” Bright said. “Traditionally, these technologies were expensive. However, as a result of concentrated research in membrane technology, there have been significant advances that have effectively dropped operating costs by as much as 80%.”
Bright added that aside from the residual stream, desalination is an environmentally friendly treatment technology. “With dedicated research in brine disposal, this challenge could be overcome in the next 5 to 10 years,” she said.
Recently, researchers at the University of California–Los Angeles Henry Samueli School of Engineering and Applied Science announced the development of a new nanoscale reverse osmosis membrane with a cross-linked matrix of polymers and engineered nanoparticles. The highly porous technology attracts water while repelling dissolved salts, organics, bacteria, and other impurities. Civil and environmental engineering assistant professor Eric Hoek said the new reverse osmosis membranes are as effective as current membranes but less expensive, more productive, and become soiled less quickly. According to initial tests, these membranes could reduce the cost of desalinating water by as much as 25%, he said.
Another technology being developed is forward (or direct) osmosis desalination, which uses an osmotic pressure gradient as its driving force for separation. While the technology is not currently available commercially, proponents expect it will have lower energy requirements than reverse osmosis.
Microfiltration and ultrafiltration. Widely used for industrial, drinking water, and wastewater treatment, microfiltration (MF) and ultrafiltration (UF) membranes have a bright future for water reuse applications, water professionals said.
Anthony Wachinski, senior vice president and technical director for the Water Processing Division of Pall Corp. (East Hills, N.Y.), said hollow-fiber MF is the “kingpin” of all membrane treatment technologies.
“Many performance advances have been related to membrane MF, because it can handle dirtier water at higher flows with a lower pressure drop. This combination of capabilities translates into superior treatment at a higher throughput with lower costs,” he said. “MF provides an effective barrier for particles, bacteria, and protozoan cysts.”
Wachinski said the market for hollow-fiber UF also is on the rise. “Currently, there is considerable momentum with using UF treatment systems in France and other European countries,” he said. “In the United States, the largest pressurized UF plant in North America is being designed for Minneapolis Water Works.”
Wachinski suggested that MF and UF membranes could be used to pretreat seawater and enhance the protection of high-pressure reverse osmosis systems. “We believe they are the future of desalination,” he said.
Bright agreed that MF and UF technology can be effective as a pretreatment stage prior to the adoption of reverse osmosis. “The scientific community has been focusing on pretreatment since the Tampa Bay (Fla.) Desalination Project, in which the pretreatment systems failed,” Bright said. “Pretreatment is very essential as it helps in increasing the life of the membrane and also improving productivity.”
Ultraviolet light. “Ultraviolet [UV] light is being used more for disinfecting pathogens and for the photolysis and oxidation of trace levels of organic compounds,” Debroux said. “UV light has the advantage of disinfecting without the typical formation of regulated chlorination byproducts but has the disadvantage of not producing a residual disinfectant that travels with the water in the distribution system.” He added that UV treatment is used in indirect potable reuse projects and can be effective for the removal of certain photosensitive trace organic compounds or other microconstituents. However, the process is also energy-intensive.
Meeting Energy Demands
Despite recent improvements in their energy requirements, most water reuse technologies still require more energy than conventional treatment. “As such, life-cycle analyses must be considered when evaluating more energy-intensive processes,” Debroux said.
Given that energy prices show no sign of dropping, the increased cost of operations could be a primary decision factor for municipalities considering a water reuse project.
Bright said fossil fuel prices will determine the cost of desalination as long as the process relies on oil-based products.
“That could change if industry starts to adopt nuclear or renewable energy for desalination technologies,” she said. Renewable energies, such as wind, solar thermal, and photovoltaics, could help municipalities bring down the energy costs of water reuse technologies, she said.
Bright cited a study being conducted by Texas Tech University (Lubbock) and GE Water & Process Technologies (Trevose, Pa.) on the feasibility of using wind energy in combination with desalination. “This could come around at large-scale plants by the end of 2015,” she said.
— Jeff Gunderson, WE&T
About This Series
This is the second part of a two-part series on water reuse. Part 1 (August) explored innovative water reuse solutions.
Stemming the Flow of Nitrate to the Gulf
Innovations in agricultural conservation and cropping practices could significantly reduce the hypoxic zone in the Gulf of Mexico, according to a number of leading scientists. Like water quality groups, they’re calling on the U.S. Congress to increase funding for conservation programs in reauthorizing the Farm Bill, which is set to expire this month, in order to get the job done.
“It’s not rocket science,” said Don Scavia, a professor at the University of Michigan (Ann Arbor) School of Natural Resources and the Environment. “These are things we know how to do; what we need are the funds to actually make it happen.” Scavia, together with several colleagues, lays out these practices in a new book, From the Corn Belt to the Gulf, published by Resources for the Future (RFF), a Washington, D.C., think tank.
Scavia and his colleagues recommend improvements in the precision of fertilizer applications, targeted upland habitat and wetland restoration, and the production of perennial crops, such as switch grass. Adopting traditional and innovative conservation practices on a widespread basis could scale back the amount of nitrogen reaching the gulf by 40% and significantly improve water quality nationwide, they say. Nearly half of the nation’s rivers, lakes, and bays are impaired, they note, adding that agriculture is the most commonly cited source of impairment.
In a May 16 letter to Congress, the Water Environment Federation (Alexandria, Va.), along with 11 other water sector groups, also urged legislators to not only promote more funding for Farm Bill conservation programs but also to ensure that the Farm Bill makes water quality a top priority for program funding.
Such a shift in agricultural policy could be achieved without costing taxpayers more money simply by redirecting some of the money currently going to farmers in commodity payments into conservation subsidies, said Joan Nassauer, a professor of landscape architecture at the University of Michigan School of Natural Resources and the Environment and co-author of the RFF book. Farmer participation “would still be voluntary, but this shift in expenditures could provide incentives for farmers to do it right,” she explained.
Agricultural Inputs Are Key
In an integrated assessment published in 2000, scientists pinpointed fertilizer-laden runoff from Midwestern farm fields as a leading source of excess nutrients, especially nitrogen, feeding the growth of a so-called dead zone in coastal waters off Louisiana and Texas. Every summer, oxygen levels in bottom waters fall to less than 2 ppm of dissolved oxygen across an area spanning 12,700 km2, on average, according to measurements by Nancy Rabalais, director of the Louisiana Universities Marine Consortium (Chauvin).
Overall, fertilizer runoff is responsible for more than half of the nitrogen loading to the gulf, atmospheric deposition for nearly 20%, and animal waste and municipal wastewater for around 10%, respectively, according to various modeling studies.
An interagency federal and state task force led by the U.S. Environmental Protection Agency (EPA) used the results of the integrated assessment to develop an action plan for reducing the dead zone’s size to less than 5000 km2 by 2015. To meet that goal, the task force recommended a 30% reduction in nitrogen loading, mainly through voluntary measures implemented by the agricultural community.
Some 7 years later, however, that goal is nowhere close to being met. The gulf’s dead zone swelled last year to more than 17,000 km2, Rabalais’ measurements show. With the current ramp-up in ethanol production from corn, an especially fertilizer-intensive crop, scientists said they fear nutrient loading from agriculture will only grow.
Meanwhile, as Congress debates reauthorization of the Farm Bill, EPA’s Science Advisory Board (SAB) is putting the finishing touches on a reassessment of the state of the science surrounding the gulf’s hypoxic zone. Essentially, the SAB panel is finding that as the system deteriorates, it’s becoming less resilient to recovery, said Donald Boesch, president of the University of Maryland Center for Environmental Science (Cambridge) and a reviewer of the SAB report. As a result, SAB is upping the ante, now recommending that nitrogen loads be reduced by as much as 45%. The scientific panel also is recommending reductions in phosphorus loads this time around, according to the draft assessment.
Changing the Rural Landscape
To achieve such significant reductions, it’s clear that farmers need to change their agricultural practices across wide swaths of the Mississippi River basin, said Scavia, who was heavily involved with the hypoxia task force’s initial integrated assessment.
Scavia and his colleagues set out to determine what types of conservation practices it would take to solve the problem. They honed in on the Corn Belt — especially in Iowa and Illinois — which is “where most of the nitrogen is coming from and where most of the attention ought to be focused,” Scavia said.
“We were looking at ways to make it happen on the ground,” Nassauer explained, “asking how agriculture could be changed in a way that would maintain the agricultural enterprise but still allow for environmental benefits.” As part of that study, they analyzed the kinds of practices individual farmers could take under three different scenarios in two small Iowa watersheds — one in a flat landscape and the other in a hilly one.
The goals of the three strategies they considered included maximizing commodity production, improving water quality and reducing downstream flooding, and enhancing biodiversity within agricultural landscapes.
In the first scenario, which focused on getting the most out of the land, more acres would be planted in corn and soybeans, not unlike the current situation. The U.S. Department of Agriculture (USDA) predicts a record 36.6 million ha (90.5 million ac) of corn will be planted this year, a 15% jump from last year.
Under this strategy, farmers would use fertilizer more efficiently and practice no-till agriculture. Still, water quality modeling showed that nitrate loading would increase by 8% in the flat watershed and 19% in the rolling watershed.
In the second and third scenarios, by contrast, the researchers found nitrate loading in both watersheds could be reduced by upwards of 50%. These strategies involved such practices as more pasture land near streams, crop rotations, widening stream buffers, wetland restoration, and increasing the amount of acreage in perennial crops, such as native grasses or switchgrass, in addition to practicing precision agriculture and no-till farming.
“The take-home message is that just expanding the use of better nutrient management, such as precision agriculture, while it’s part of the solution, it won’t be enough,” Nassauer said. Additionally, she said, “given the explosion of corn production, even if we use off-the-shelf technologies available to apply fertilizer with great precision, we’ll still be going backwards with regard to the amount of nitrogen that’s getting into the gulf and other rivers, lakes, and bays.”
What’s more is that with scenario three, which focused on incorporating more perennial crops into the landscape, economists found farmers could achieve these significant water quality benefits with no impact on their bottom line, Nassauer said.
Nassauer, Scavia, and others are pinning their hopes on the new Farm Bill to shift U.S. agricultural policy toward a stronger emphasis on conservation. “There’s so much money going to our farmers in the form of crop subsidies anyway,” Scavia said. “If we could just put them in the form of conservation subsidies, we could get this done.”
Ethanol Production Fueling Concerns
The growing demand for ethanol, however, is pushing up corn prices, as ethanol producers work to meet renewable fuel targets set under the federal Energy Policy Act of 2005. Currently, 114 ethanol refineries are on-line to produce more than 21 million m3 (5.6 billion gal) annually, according to the Renewable Fuels Association (Washington, D.C.). Another 80 refineries are under construction, and seven are expanding to add more than 22.7 million m3 (6 billion gal) of capacity.
This quick expansion could cause even lands currently enrolled in conservation programs to be pulled back into production, said Mary Santelmann, an ecologist at Oregon State University (Corvallis) and co-author of the RFF book. Indeed, USDA estimates that enrollments for 1.9 ha (4.6 million ac) in the Conservation Reserve Program won’t be renewed or extended, and of those, 566,570 ha (1.4 million ac) are located in corn-producing states.
These developments don’t bode well for the gulf. “It looks like it’ll make the problem that much more difficult,” Boesch said. “Many people are concerned about what the push towards ethanol means for water quality and environmental quality in general.”
— Kris Christen, WE&T