Cool Ways to Save
Of all the potential opportunities to save water using technology, cooling towers rank near the top.
On hot summer days, demand for water surges, and large cooling towers guzzle gallons by the tens of thousands. Flushing (“blowdown”) becomes necessary, in volumes that could fill several backyard pools. Such disposal may become necessary several times a day. A key to recycling this water to its fullest is to multiply the “cycles of concentration,” or the number of times it can be circulated until discharge becomes necessary. Each cycle thickens the contaminant minerals successively, to the point that the solution must be expelled to avoid system corrosion and damage.
The standard antidote is chemical treatment: Water softeners and assorted neutralizes are added to extend the cycles several-fold.
But chemicals alone can only go so far; there are “definite limits,” says Bill Willersdorf of Siemens Water Technologies (known until very recently as USFilter), “on levels of calcium, silica, chlorides, and other constituents that can remain in a tower” before problems erupt. To extend water reusability even further, he says, it may make sense to invest in special filtration and in reverse osmosis (RO). The latter is fairly costly; it’s usually reserved for high-quality drinking water. But in sites where water is in short supply—as is increasingly the case in some regions—or where demineralization needs are critical, added measures may be called for.
A recent showcase application illustrates: Constellation Energy, a Baltimore-based Fortune 200 firm, sought to build a new power plant in the California high desert but found that the local water supply, needed for cooling, was inadequate. The only solution: recycling, at a near-100% rate of reuse. Micro- and ultra-filtering, and advanced RO treatment systems, were installed in 2003, so that now, says Willersdorf, “virtually all of the plant’s water, including cooling tower blowdown, undergoes high-quality treatment.” Better still, after performing various cooling work, 2,000 gallons per minute are treated and pumped back into the local groundwater aquifer. Not a drop is wasted: Residual RO concentrate becomes crystallized (with the solids being sent to a landfill); the distillate is used as makeup water; and the balance goes to the cooling tower. Final liquid discharge is zero.
In this and several other cutting-edge cases, a combination of tight water restrictions and special needs truly compel innovative reuse. Says Willersdorf, who is Siemens Water Technologies’ director of corporate projects, “You physically can’t place a power plant with a wet cooling system unless you can reuse water from a municipal sewage authority or design a plant with a high number of cycles in the cooling tower.”
“For Drought Relief, Take RO”
It sounds like a cold medicine, but a few years ago, Paul Levy, an engineering consultant in Greensboro, NC, persuaded a local engineering firm to explore using RO at a major college campus, as a solution to severe local drought. More likely, he says, you’ll find RO specified for demineralizing water for boilers and other expensive or sensitive systems, and at bottling and brewing companies. RO “isn’t often thought of” right away for use in cooling towers, Levy notes. Systems must be expensively custom-engineered for cooling application. Initial costs tend to be high. Payback will be “iffy,” and the economics vary, literally, “all over the place,” he says, depending on where a project is sited in relation to local water and sewer rates. For example, in a few cities having pricey fees and charging for cooling tower blowdown—such as Boston, San Francisco, Houston, and Austin—you may well find that an RO installation comes out economically justifiable; even so, the technology will still be resisted or never explored, simply because no one is trained to consider it. Even more inhibitively, he adds, local rates and policies “don’t yet encourage water-conservation investments.”
However, RO and microfiltration systems are proving more and more viable, says Willersdorf, thanks to another “definite trend” in progress, favoring the use of tertiary reclaimed wastewater for cooling purposes. In fact, cooling service makes for an ideal reuse: The water isn’t for human consumption, and yet there’s a relatively high need for demineralization and clarity. Large-scale reuse of treated wastewater in coolers could potentially save millions of gallons of a community’s potable supply.
Then there’s the problem that, sometimes, there’s simply not enough water. As the following case examples show, arid, rapidly growing towns and cities will find that growth screeches to a halt, if the water dries up. RO literally becomes “the only way to go.” As a broad rule, notes Levy, using RO for cooling towers will likely double the cycles of concentration, or better. Front-end costs may easily be justified.
A Major Lab’s Budding Water Business
Los Alamos National Laboratory (LANL)—a world-class research center—straddles arid, drought-prone mountains in New Mexico at 7,000 feet. Even though it’s a blue-chip federal installation, well-water rights are tight there, and usage limits were capped about a decade ago, notes LANL Process Engineer and Operations Project Leader Steve Hanson.
Very soon, those limits proved inadequate. By the late 1990s LANL was envisioning erecting a massive, 300,000-square-foot Strategic Computing Complex; an acre-size room would need, says Hanson, “rather exacting temperature controls,” requiring an estimated 50,000 gallons of water daily for cooling alone. This was far more than what was readily available, as the lab was already “right up against a ceiling” as it was, he recalls.
 |
| Sanitary Effluent Reclamation Facility at Los Alamos National Laboratory |
Not to worry too much, though: LANL’s wastewater plant offered potentially lots of reusable outflow, assuming Hanson and other engineers could figure out how best to condition it.
Here, an extraordinary challenge they faced was the high silica, coming in at about 105 parts per million; when parts per million reach 160, silica “begins to plate-out,” he notes, meaning that not even two cycles of concentration could be obtained. Once scaling beings to form, silica removal is also very tough. Silica can easily damage RO membranes, too, and so manufacturers typically limit their warranties and assurances accordingly. Unbudgeted costs for repair begin to soar.
Thus, an array of hazardous chemicals must be applied, including hydrochloric acid, sodium hydroxide, magnesium chloride, ferric chloride, sodium hypochlorite, and sodium bisulphite. The typical maintenance staff’s expertise and experience with these substances will probably be limited. Hence, uncharted handling and storage issues arise. In this instance, to avoid inadvertent mixing of the wrong ingredients (as actually occurred at LANL early on and might easily happen in any storage area) the chemical tank receiving pipes were eventually retrofitted to make them operable only with the right source nozzles attached.
Next came the question of actual chemical quantities required—i.e., proportions, sequences, and application procedures. During an initial trial and testing phase, LANL allowed two membrane vendors, USFilter and Aquatex, and two chemical companies, West and Nalco, to run side-by-side tests on two small cooling towers. This would determine what combinations of chemistry, filter systems, and technology would extend the water furthest.
 |
| Reverse osmosis may be called for on sites where water is in short supply. |
All four wound up producing good results, Hanson recalls. The membrane-based systems raised the cycles of concentration up to four, and the chemical companies achieved nearly three. Even more were theoretically doable, he adds, but four cycles became the target number: This rate—an almost fourfold improvement on untreated water alone—would make construction of a big RO-based plant economically feasible.
Winning this competition and contract was USFilter, who then integrated, first, the chemical conditioners needed to induce silica precipitation; second, the company’s proprietary Memtek microfilter; and lastly, an RO system that uses seawater membranes. Construction took nearly a year, and the plant was launched in the spring of 2005. Since then, Hanson happily reports, resulting performance has “met or exceeded” all expectations.
In the first treatment stage, hydrochloric acid is applied to drop the pH to about 4.5; then, magnesium chloride and ferric chloride are added to react with the silica; pH is raised to 10.5; adding caustic soda causes a precipitate.
Next, a microfilter collects the slurry. Water—stripped of the high silicate but not of the high total dissolved solids (TDS)—comes out at about 5 or 6 parts per million silica and then travels to the RO; there, seawater membranes strain the TDS and nearly all the remaining silica.
At the end of the line, outflow is virtually silica-free. Other unwanted particles amount to barely more than a trace: Only about 20 milligrams per liter of total solids, compared to the 800 milligrams per liter of TDS and 105 milligrams per liter of silica, were present at the outset. Residue from the silica precipitation is then filter-pressed and sent to a landfill.
Water recovery comes to a remarkable 97%, and liquid discharge amounts to zero, as the 3% concentrate shunts to evaporation basins for drying.
Again, operationally, says Hanson, the plant “runs fantastic.” Two technicians manage it on a single shift. Chemistry conditions, flow, flux rates, salt passage, and every other imaginable reading are automated. (Note: Running a wastewater plant at a world-class science lab means you have the benefit of top-of-the-line metering systems already being “standard equipment.”)
Too, the tricky chemical balancing that had been anticipated turned out to be a bit easier in practice: Magnesium chloride levels have been adjusted downward, and the ferric chloride levels raised slightly, for modest overall cost savings.
“And it’s been easy to maintain,” Hanson continues, although the plant’s operation has been only intermittent. Surprisingly, the original microfilter membranes are still in place. They’re working “extremely well,” he reports, producing “better than 99% salt rejection.” Flux rate hasn’t declined more than a percent or two, “and there has been no membrane degradation,” he adds.
Treated output since 2005 has totaled about 10 million gallons (curtailed somewhat by an interruption in mid-2006). Hanson says that, once the plant gets into full operation, regular output should reach 20-plus million gallons yearly. A planned expansion in the near future will raise this to over 100 million gallons by the addition of a second RO unit and another microfilter.
Too, now that virtually all of the facility’s wastewater is being, or eventually will be, “hyper-treated” to this very high level of quality, not only is there plenty of purified feedwater for use in the Computing Center’s 1,200-ton, 3-cell cooling tower, but, all told, there’s potentially an additional 400 acre-feet per year available: That’s sevenfold what was originally needed for the cooling project.
So, further options on how to use this bounty are already being discussed. At this point, besides serving the original tower as planned, the next water loop will go to cooling the laboratory’s power plant and then a linear accelerator beam.
All of these reuses together will comprise more than 30% of all the water used at LANL. This means that tens of millions of gallons of potable water will be freed and available to serve the larger community.
Already, says Hanson, several important lessons have emerged. Number one: “Be creative” in evaluating all possible reuses. “Even though [the RO plant] was targeted for a specific facility,” he says, “we find that the potential applications for the water are myriad. There are lots of places to look” for other users of this water—including, perhaps, even winter snowmaking. Once the treatment plant investment can be justified by one key application—in this case, cooling tower makeup water—then achieving larger economies of scale with many others makes all the more sense.
Another lesson: In water reuse planning, he says, remember that reclaimed water comes out “with variable quality at different stages in the process.” Thus, you have the option of deciding whether every user needs the same high quality, or whether water of lesser purity can be used in some areas, “being diverted at a different point in the plant,” he says, “amounting to less treatment cost for that application.”
A third insight gleaned: If multiple reuse applications are envisioned, some advance thought should be given to the question of plant siting, as in, “Where’s the best locale for the water distribution system, to allow as many end users as possible to get it?” he says. LANL’s RO plant—although intended mainly for the supercomputing center—was actually positioned more than a mile away, in order to be closer to the power plant and another facility that will use the RO output for cooling.
A final issue you may be wondering about: How do the economics stack up?
LANL’s up-front costs were admittedly quite high, Hanson says (particularly as a federal installation with unusually stringent demands); his costs were not at all comparable, he says, to what a municipality might pay, which would undoubtedly be “much more affordable.”
At any rate, his operational savings on water and sewer bills now come to over $100,000 annually. In time—when the full complement of uses can be found for all of the output—total savings will multiply several times higher, being expected to come out nearly offsetting plant operating costs.
RO Design Factors, Limitations, Costs
Engineering Consultant Levy (doing business as Mechanical Construction Solutions of Greensboro, NC) offers an outline of several of the key considerations when evaluating a project for RO and/or the installation of high-quality filtration:
- If fouled by silicates, RO filters are often tough to repair.
- “They’re sensitive to chlorine,” he says; exposure to heavily chlorinated water—which can easily occur with treated water—should be minimized.
- Sand particles may be damaging; “a dispersant may be needed to prevent clogging,” he says. Upstream filters can also help by removing 5-micron particles.
- If tertiary water with live organisms is being piped through for reuse, ultra-filtration may be needed.
- Anti-algae and anti-scaling chemicals will still be required, as in any typical chemistry mix for cooling tower water.
All in all, though, if the native water you’re using isn’t too problematic, RO membranes can be, he says, “a terrific option” for extending the life of reclaimed wastewater in cooling towers, for power plant cooling, and for boiler water makeup. Water recovery in each tower cycle typically ranges from 70% to 80%, depending on condition. (Performance improves with frequent filter cleaning.) And lastly, he notes, there’s “both an art and a science” to finding the “sweet spot” or optimal water pressure, which is usually between 100 and 300 pounds per square inch. Higher flows and pressures boost output but may cause ruptures.
And how about payback?
High-quality treatment begins to make sense economically, he says, only if you’re processing 10 million gallons of blowdown a year or more. Of course, local water and sewer rates will be determinative in every case.
Willersdorf suggests too that in some water-starved areas, two RO systems might conceivably be justified, to extend reuse and multiply the cycles of concentration up to a dozen times or more: “It all depends,” he says, “on how many cycles … you need to get to, and how much you want to spend up-front or down the road.”
Conversely, going without the costly RO, but using microfiltration by itself, might also be a smart solution to consider, he adds; for example, if flow rates are below 1,000 gallons per minute, a microfilter would allow a system to operate in a smaller footprint and remain indoors at only modest additional expense.
General Dynamics C4 Systems, Scottsdale, AZ
On a final note, a visit to the very first commercial RO skid for cooling tower reclamation, developed by WRI (maker of the Aquatex line), finds that it is still processing blowdown after nearly a dozen years.
Performance over that time, says C4 Systems Director of Facilities and Real Estate George Adams, has been remarkable, with “no failures or breakdowns that I know of, for years.”
WRI performs servicing and remotely monitors the pH, conductivity, and chlorine levels, he says.
Spurring C4 Systems’ adoption in the mid-1990s wasn’t a water crisis but simply the philosophical appeal of doing water conservation for the community. Also, due in part to this successful reclamation effort, General Dynamics received a coveted LEED designation, making this site one of the first anywhere to achieve “green” status for an existing building.
Operationally, says Adams, the system must cope with high silica too; as at Los Alamos, it’s the main hurdle to extending the cycles. Processing begins with blowdown from the 4,500-ton tower going to a holding tank. From this, it’s pulled into the reclamator and pressed through a thin film membrane doing nanofiltration and extracting particles—then fed back into the cooling tower supply.
About 75% to 80% of the water is reclaimed, with balance heading to sewers.
Using RO and filtration has reduced chemical treatment needs by about 40% and has yielded a modest reduction in sewer fees. Before doing the installation, cycles of concentration were around 3 or 3.5; afterwards, this jumped to about 5, notes Adams.
Best of all, though, in the decade-plus of its operation, the company estimates that about 25 million gallons of water have been reclaimed. As far as the desert neighbors are concerned, says Adams, “It all counts.”
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WRI President Randall Jones—an early developer of quality treatment processes for reclaiming retort cooling water in the canning industry—concedes that adoption of RO and ultra-filtration, purely for conservation needs, is still rather rare. Low water/sewer rates make it easy to forego conservation, unless other extremities dictate. Even in the few cities where rates do make it quite viable—where paybacks could come in less than two years—there’s not been keen interest, because water bills remain such a fraction of expenses: Why bother?
Cooling remains a promising reuse application, though, and greater demand could materialize, particularly in some western states: Populations have surged, “but our water resources have dwindled,” Jones says. “So we are now at a point where things are really very different … this is a new era in the West.” As industrial and agribusiness water users seek to adapt, he suggests, “It’s going to get interesting.”
Author's Bio: Writer David Engle specializes in construction-related topics.
November-December 2006
Cool Ways to Save
Of all the potential opportunities to save water using technology, cooling towers rank near the top.
On hot summer days, demand for water surges, and large cooling towers guzzle gallons by the tens of thousands. Flushing (“blowdown”) becomes necessary, in volumes that could fill several backyard pools. Such disposal may become necessary several times a day. A key to recycling this water to its fullest is to multiply the “cycles of concentration,” or the number of times it can be circulated until discharge becomes necessary. Each cycle thickens the contaminant minerals successively, to the point that the solution must be expelled to avoid system corrosion and damage.
The standard antidote is chemical treatment: Water softeners and assorted neutralizes are added to extend the cycles several-fold.
But chemicals alone can only go so far; there are “definite limits,” says Bill Willersdorf of Siemens Water Technologies (known until very recently as USFilter), “on levels of calcium, silica, chlorides, and other constituents that can remain in a tower” before problems erupt. To extend water reusability even further, he says, it may make sense to invest in special filtration and in reverse osmosis (RO). The latter is fairly costly; it’s usually reserved for high-quality drinking water. But in sites where water is in short supply—as is increasingly the case in some regions—or where demineralization needs are critical, added measures may be called for.
A recent showcase application illustrates: Constellation Energy, a Baltimore-based Fortune 200 firm, sought to build a new power plant in the California high desert but found that the local water supply, needed for cooling, was inadequate. The only solution: recycling, at a near-100% rate of reuse. Micro- and ultra-filtering, and advanced RO treatment systems, were installed in 2003, so that now, says Willersdorf, “virtually all of the plant’s water, including cooling tower blowdown, undergoes high-quality treatment.” Better still, after performing various cooling work, 2,000 gallons per minute are treated and pumped back into the local groundwater aquifer. Not a drop is wasted: Residual RO concentrate becomes crystallized (with the solids being sent to a landfill); the distillate is used as makeup water; and the balance goes to the cooling tower. Final liquid discharge is zero.
In this and several other cutting-edge cases, a combination of tight water restrictions and special needs truly compel innovative reuse. Says Willersdorf, who is Siemens Water Technologies’ director of corporate projects, “You physically can’t place a power plant with a wet cooling system unless you can reuse water from a municipal sewage authority or design a plant with a high number of cycles in the cooling tower.”
“For Drought Relief, Take RO”
It sounds like a cold medicine, but a few years ago, Paul Levy, an engineering consultant in Greensboro, NC, persuaded a local engineering firm to explore using RO at a major college campus, as a solution to severe local drought. More likely, he says, you’ll find RO specified for demineralizing water for boilers and other expensive or sensitive systems, and at bottling and brewing companies. RO “isn’t often thought of” right away for use in cooling towers, Levy notes. Systems must be expensively custom-engineered for cooling application. Initial costs tend to be high. Payback will be “iffy,” and the economics vary, literally, “all over the place,” he says, depending on where a project is sited in relation to local water and sewer rates. For example, in a few cities having pricey fees and charging for cooling tower blowdown—such as Boston, San Francisco, Houston, and Austin—you may well find that an RO installation comes out economically justifiable; even so, the technology will still be resisted or never explored, simply because no one is trained to consider it. Even more inhibitively, he adds, local rates and policies “don’t yet encourage water-conservation investments.”
However, RO and microfiltration systems are proving more and more viable, says Willersdorf, thanks to another “definite trend” in progress, favoring the use of tertiary reclaimed wastewater for cooling purposes. In fact, cooling service makes for an ideal reuse: The water isn’t for human consumption, and yet there’s a relatively high need for demineralization and clarity. Large-scale reuse of treated wastewater in coolers could potentially save millions of gallons of a community’s potable supply.
Then there’s the problem that, sometimes, there’s simply not enough water. As the following case examples show, arid, rapidly growing towns and cities will find that growth screeches to a halt, if the water dries up. RO literally becomes “the only way to go.” As a broad rule, notes Levy, using RO for cooling towers will likely double the cycles of concentration, or better. Front-end costs may easily be justified.
A Major Lab’s Budding Water Business
Los Alamos National Laboratory (LANL)—a world-class research center—straddles arid, drought-prone mountains in New Mexico at 7,000 feet. Even though it’s a blue-chip federal installation, well-water rights are tight there, and usage limits were capped about a decade ago, notes LANL Process Engineer and Operations Project Leader Steve Hanson.
Very soon, those limits proved inadequate. By the late 1990s LANL was envisioning erecting a massive, 300,000-square-foot Strategic Computing Complex; an acre-size room would need, says Hanson, “rather exacting temperature controls,” requiring an estimated 50,000 gallons of water daily for cooling alone. This was far more than what was readily available, as the lab was already “right up against a ceiling” as it was, he recalls.
|
| Sanitary Effluent Reclamation Facility at Los Alamos National Laboratory |
Not to worry too much, though: LANL’s wastewater plant offered potentially lots of reusable outflow, assuming Hanson and other engineers could figure out how best to condition it.
Here, an extraordinary challenge they faced was the high silica, coming in at about 105 parts per million; when parts per million reach 160, silica “begins to plate-out,” he notes, meaning that not even two cycles of concentration could be obtained. Once scaling beings to form, silica removal is also very tough. Silica can easily damage RO membranes, too, and so manufacturers typically limit their warranties and assurances accordingly. Unbudgeted costs for repair begin to soar.
Thus, an array of hazardous chemicals must be applied, including hydrochloric acid, sodium hydroxide, magnesium chloride, ferric chloride, sodium hypochlorite, and sodium bisulphite. The typical maintenance staff’s expertise and experience with these substances will probably be limited. Hence, uncharted handling and storage issues arise. In this instance, to avoid inadvertent mixing of the wrong ingredients (as actually occurred at LANL early on and might easily happen in any storage area) the chemical tank receiving pipes were eventually retrofitted to make them operable only with the right source nozzles attached.
Next came the question of actual chemical quantities required—i.e., proportions, sequences, and application procedures. During an initial trial and testing phase, LANL allowed two membrane vendors, USFilter and Aquatex, and two chemical companies, West and Nalco, to run side-by-side tests on two small cooling towers. This would determine what combinations of chemistry, filter systems, and technology would extend the water furthest.
|
| Reverse osmosis may be called for on sites where water is in short supply. |
All four wound up producing good results, Hanson recalls. The membrane-based systems raised the cycles of concentration up to four, and the chemical companies achieved nearly three. Even more were theoretically doable, he adds, but four cycles became the target number: This rate—an almost fourfold improvement on untreated water alone—would make construction of a big RO-based plant economically feasible.
Winning this competition and contract was USFilter, who then integrated, first, the chemical conditioners needed to induce silica precipitation; second, the company’s proprietary Memtek microfilter; and lastly, an RO system that uses seawater membranes. Construction took nearly a year, and the plant was launched in the spring of 2005. Since then, Hanson happily reports, resulting performance has “met or exceeded” all expectations.
In the first treatment stage, hydrochloric acid is applied to drop the pH to about 4.5; then, magnesium chloride and ferric chloride are added to react with the silica; pH is raised to 10.5; adding caustic soda causes a precipitate.
Next, a microfilter collects the slurry. Water—stripped of the high silicate but not of the high total dissolved solids (TDS)—comes out at about 5 or 6 parts per million silica and then travels to the RO; there, seawater membranes strain the TDS and nearly all the remaining silica.
At the end of the line, outflow is virtually silica-free. Other unwanted particles amount to barely more than a trace: Only about 20 milligrams per liter of total solids, compared to the 800 milligrams per liter of TDS and 105 milligrams per liter of silica, were present at the outset. Residue from the silica precipitation is then filter-pressed and sent to a landfill.
Water recovery comes to a remarkable 97%, and liquid discharge amounts to zero, as the 3% concentrate shunts to evaporation basins for drying.
Again, operationally, says Hanson, the plant “runs fantastic.” Two technicians manage it on a single shift. Chemistry conditions, flow, flux rates, salt passage, and every other imaginable reading are automated. (Note: Running a wastewater plant at a world-class science lab means you have the benefit of top-of-the-line metering systems already being “standard equipment.”)
Too, the tricky chemical balancing that had been anticipated turned out to be a bit easier in practice: Magnesium chloride levels have been adjusted downward, and the ferric chloride levels raised slightly, for modest overall cost savings.
“And it’s been easy to maintain,” Hanson continues, although the plant’s operation has been only intermittent. Surprisingly, the original microfilter membranes are still in place. They’re working “extremely well,” he reports, producing “better than 99% salt rejection.” Flux rate hasn’t declined more than a percent or two, “and there has been no membrane degradation,” he adds.
Treated output since 2005 has totaled about 10 million gallons (curtailed somewhat by an interruption in mid-2006). Hanson says that, once the plant gets into full operation, regular output should reach 20-plus million gallons yearly. A planned expansion in the near future will raise this to over 100 million gallons by the addition of a second RO unit and another microfilter.
Too, now that virtually all of the facility’s wastewater is being, or eventually will be, “hyper-treated” to this very high level of quality, not only is there plenty of purified feedwater for use in the Computing Center’s 1,200-ton, 3-cell cooling tower, but, all told, there’s potentially an additional 400 acre-feet per year available: That’s sevenfold what was originally needed for the cooling project.
So, further options on how to use this bounty are already being discussed. At this point, besides serving the original tower as planned, the next water loop will go to cooling the laboratory’s power plant and then a linear accelerator beam.
All of these reuses together will comprise more than 30% of all the water used at LANL. This means that tens of millions of gallons of potable water will be freed and available to serve the larger community.
Already, says Hanson, several important lessons have emerged. Number one: “Be creative” in evaluating all possible reuses. “Even though [the RO plant] was targeted for a specific facility,” he says, “we find that the potential applications for the water are myriad. There are lots of places to look” for other users of this water—including, perhaps, even winter snowmaking. Once the treatment plant investment can be justified by one key application—in this case, cooling tower makeup water—then achieving larger economies of scale with many others makes all the more sense.
Another lesson: In water reuse planning, he says, remember that reclaimed water comes out “with variable quality at different stages in the process.” Thus, you have the option of deciding whether every user needs the same high quality, or whether water of lesser purity can be used in some areas, “being diverted at a different point in the plant,” he says, “amounting to less treatment cost for that application.”
A third insight gleaned: If multiple reuse applications are envisioned, some advance thought should be given to the question of plant siting, as in, “Where’s the best locale for the water distribution system, to allow as many end users as possible to get it?” he says. LANL’s RO plant—although intended mainly for the supercomputing center—was actually positioned more than a mile away, in order to be closer to the power plant and another facility that will use the RO output for cooling.
A final issue you may be wondering about: How do the economics stack up?
LANL’s up-front costs were admittedly quite high, Hanson says (particularly as a federal installation with unusually stringent demands); his costs were not at all comparable, he says, to what a municipality might pay, which would undoubtedly be “much more affordable.”
At any rate, his operational savings on water and sewer bills now come to over $100,000 annually. In time—when the full complement of uses can be found for all of the output—total savings will multiply several times higher, being expected to come out nearly offsetting plant operating costs.
RO Design Factors, Limitations, Costs
Engineering Consultant Levy (doing business as Mechanical Construction Solutions of Greensboro, NC) offers an outline of several of the key considerations when evaluating a project for RO and/or the installation of high-quality filtration:
- If fouled by silicates, RO filters are often tough to repair.
- “They’re sensitive to chlorine,” he says; exposure to heavily chlorinated water—which can easily occur with treated water—should be minimized.
- Sand particles may be damaging; “a dispersant may be needed to prevent clogging,” he says. Upstream filters can also help by removing 5-micron particles.
- If tertiary water with live organisms is being piped through for reuse, ultra-filtration may be needed.
- Anti-algae and anti-scaling chemicals will still be required, as in any typical chemistry mix for cooling tower water.
All in all, though, if the native water you’re using isn’t too problematic, RO membranes can be, he says, “a terrific option” for extending the life of reclaimed wastewater in cooling towers, for power plant cooling, and for boiler water makeup. Water recovery in each tower cycle typically ranges from 70% to 80%, depending on condition. (Performance improves with frequent filter cleaning.) And lastly, he notes, there’s “both an art and a science” to finding the “sweet spot” or optimal water pressure, which is usually between 100 and 300 pounds per square inch. Higher flows and pressures boost output but may cause ruptures.
And how about payback?
High-quality treatment begins to make sense economically, he says, only if you’re processing 10 million gallons of blowdown a year or more. Of course, local water and sewer rates will be determinative in every case.
Willersdorf suggests too that in some water-starved areas, two RO systems might conceivably be justified, to extend reuse and multiply the cycles of concentration up to a dozen times or more: “It all depends,” he says, “on how many cycles … you need to get to, and how much you want to spend up-front or down the road.”
Conversely, going without the costly RO, but using microfiltration by itself, might also be a smart solution to consider, he adds; for example, if flow rates are below 1,000 gallons per minute, a microfilter would allow a system to operate in a smaller footprint and remain indoors at only modest additional expense.
General Dynamics C4 Systems, Scottsdale, AZ
On a final note, a visit to the very first commercial RO skid for cooling tower reclamation, developed by WRI (maker of the Aquatex line), finds that it is still processing blowdown after nearly a dozen years.
Performance over that time, says C4 Systems Director of Facilities and Real Estate George Adams, has been remarkable, with “no failures or breakdowns that I know of, for years.”
WRI performs servicing and remotely monitors the pH, conductivity, and chlorine levels, he says.
Spurring C4 Systems’ adoption in the mid-1990s wasn’t a water crisis but simply the philosophical appeal of doing water conservation for the community. Also, due in part to this successful reclamation effort, General Dynamics received a coveted LEED designation, making this site one of the first anywhere to achieve “green” status for an existing building.
Operationally, says Adams, the system must cope with high silica too; as at Los Alamos, it’s the main hurdle to extending the cycles. Processing begins with blowdown from the 4,500-ton tower going to a holding tank. From this, it’s pulled into the reclamator and pressed through a thin film membrane doing nanofiltration and extracting particles—then fed back into the cooling tower supply.
About 75% to 80% of the water is reclaimed, with balance heading to sewers.
Using RO and filtration has reduced chemical treatment needs by about 40% and has yielded a modest reduction in sewer fees. Before doing the installation, cycles of concentration were around 3 or 3.5; afterwards, this jumped to about 5, notes Adams.
Best of all, though, in the decade-plus of its operation, the company estimates that about 25 million gallons of water have been reclaimed. As far as the desert neighbors are concerned, says Adams, “It all counts.”
WRI President Randall Jones—an early developer of quality treatment processes for reclaiming retort cooling water in the canning industry—concedes that adoption of RO and ultra-filtration, purely for conservation needs, is still rather rare. Low water/sewer rates make it easy to forego conservation, unless other extremities dictate. Even in the few cities where rates do make it quite viable—where paybacks could come in less than two years—there’s not been keen interest, because water bills remain such a fraction of expenses: Why bother?
Cooling remains a promising reuse application, though, and greater demand could materialize, particularly in some western states: Populations have surged, “but our water resources have dwindled,” Jones says. “So we are now at a point where things are really very different … this is a new era in the West.” As industrial and agribusiness water users seek to adapt, he suggests, “It’s going to get interesting.”