Indirect potable reuse is assuming a life of its own in the American West. Are we heading in the right direction?
Toilet to Tap—with all that’s gone
on in the West in the last half-dozen years, from drought to reallocation of
Colorado River water, and restrictions coming out of California’s Sacramento
Delta, the once-maligned, supply-side strategy seems to be an idea whose time
has finally come.
Supposedly attributed to a clever
copy editor at the Los Angeles Daily
News, “Toilet to Tap” brought down a 33,000-acre-foot groundwater recharge
project slated for Los Angeles’ San Fernando Valley, as well as projects in San
Diego and Dublin, CA. But the continuing issue for water professionals is that
the negative and potentially divisive phase suggests that developing new sources
of potable reuse is a simple and capacious undertaking.
In traditional water systems, raw
water is diverted from its source in a lake, stream, or aquifer; treated; and
distributed, with little more to do. Wastewater is subsequently collected,
treated, and discharged to a receiving body. The fact that, in many places in
the US, this results in unplanned potable reuse (as the Southern Nevada Water
Authority puts it, “borrowing water”) does not in any way diminish the
well-developed planned reuse projects emerging in this country.
Planned potable reuse in the US is
largely indirect, wherein treated effluent is subject to multiple
contaminant-removing barriers, from extensive chemical and physical treatment to
dilution and natural cleansing in soil or a body of water. In a 1998 report, the
Water Science and Technology Board of the National Research Council’s Commission
on Geosciences, Environment, and Resources concluded that, while analytical and
toxicological testing, as well as epidemiological studies, have identified no
significant health risks in communities using reclaimed water, indirect potable
reuse projects should exceed the requirements for conventional water treatment
and should employ strong chemical disinfection processes in addition to physical
treatment systems. Also, barriers for microbiological contaminants should be
more robust than in conventional water treatment.
So, what does it look like out
there? Is jumping on the reverse osmosis (RO) bandwagon the way to go? Or is
nature perhaps a resource we’ve bypassed in our regulatory zeal? Is it more
effective to pull out all the stops before the effluent goes into the ground or
treat it as it’s drawn out?
Southern California is served by a
complicated mix of city and county utilities, which are in turn regulated by a
Byzantine web of agencies, so it might be surprising to learn that Los Angeles
has been practicing potable reuse since the 1960s. While Orange County has made
a splash with its huge 70-million-gallon-per-day Groundwater Replenishment
Project, the Water Replenishment District of Southern California (WRD) has been
quietly recharging groundwater with tertiary-treated wastewater, in part with
effluent supplied by West Basin Municipal Water District. The WRD’s original
rationale was similar to Orange County’s emphasis in its groundbreaking public
outreach campaign—protection of natural groundwater by maintaining the barrier
that keeps saltwater from contaminating the region’s aquifers. And if some of
this water also makes it into raw supplies used for drinking water, well, so be
it.
Over the years, WRD has used a mix
of treated effluent, stormwater running off the San Gabriel Mountains, and
potable water supplied by the Metropolitan Water District of Southern California
to recharge the Central and West Basin aquifers, two of the most heavily used
groundwater basins in California, serving four million Los Angeles County
residents.
Recurrent drought convinced
various powers-that-be that neither nature nor the Metropolitan Water District
was reliable enough to keep the aquifers and the sea barriers supplied, and, in
1995, West Basin christened its own advanced water treatment plant. Today, it
produces what it describes as five distinctive grades of “designer” recycled
water: tertiary, nitrified tertiary
(with the ammonia removed for use in industrial cooling towers), softened RO (secondary treated
wastewater pretreated by either lime clarification or ultrafiltration, then
followed by RO and disinfection—the water that’s now used for groundwater
recharge), pure RO (secondary treated
wastewater that had undergone microfiltration, RO, and disinfection for
low-pressure boiler feed water), and ultrapure RO (microfiltration, RO, disinfection, and second-pass RO for
high-pressure boiler feed).
Both West Basin and WRD have
committed to increasing use of recycled water as a means of diversifying their
water supply portfolios. To this, West Basin has added more efficient water
conservation and ocean desal. The target shared by both agencies, to increase
the amount of recycled water used in Los Angeles’ seawater barriers from 75% to
100%, is also a goal in Orange County.
At least one industry observer
suggests that West Basin’s effluent treatment chain, which mimics what Orange
County established at its original Waterworks 21 and is currently using in its
Groundwater Recharge Project, has set a standard that has caused state
regulators to be preoccupied with RO. An extensive groundwater basin underlies
northern Orange County, although, as in Los Angeles County, the aquifers are
subject to seawater intrusion. And, although the Groundwater Replenishment
project was largely sold to the public as a way to shore up the saltwater
barrier and manage wastewater effluent, the project’s spreading grounds are only
six month’s travel time from the groundwater supplies that local utilities
depend on for drinking water.
These factors considered, the
Orange County Advanced Purification Facility first subjects secondarily treated
effluent to microfiltration, which—as Ron Wildermuth, former public information
point person for Orange County and now West Basin, suggests—can be thought of as
the last step in tertiary treatment, or the first step in RO. Out come suspended
particles, protozoa, bacteria, and some viruses freeing up the RO to concentrate
on smaller microscopic salts and organic constituents. Ultraviolet (UV) and
hydrogen peroxide then eliminate any remaining organic compounds. According to
Shivaji Deshmukh, program manager of the Orange County’s Groundwater
Replenishment System, the advantage of disinfecting with UV instead of chlorine
is that it avoids creating any additional trihalomethanes.
The highly purified effluent is
either injected into wells at the saltwater barrier or sent to the Santa Ana
spreading grounds where it is blended with other water sources. The California
Department of Health Services considers a stay underground to be an additional
barrier to viruses, and blending as a means to control unregulated chemicals.
Extensive monitoring at multitudinous critical stages is slated to cost the
district an estimated $1 million annually.
Orange County’s Groundwater
Recharge Project is now the largest of its type in the world, but Scottsdale,
AZ, has a similar history of using effluent to recharge its groundwater. Lacking
ample surface supplies, for years Scottsdale used its groundwater as an
exclusive source of supply. As late as 1996 with a population of just under
200,000, the city was using some 23 billion gallons per day. At that rate, with
the population expected to jump to 285,000 by 2012, the city would need twice
that supply.
This unsustainable level of
groundwater pumping came to a halt with Arizona’s 1980 Groundwater Management
Act, which established safe yield as the goal statewide. Through a combination
of strategies that included using effluent for aquifer recharge, the city hit
its safe yield milestone in 2006, when as much water was recharged into the
aquifer as was pumped out from its wells. In addition to effluent, which is
treated to drinking water standards before it’s injected, the Scottsdale Water
Campus also injects surface water from the Central Arizona Project into shallow
dry wells, as well as treated drinking water directly in the aquifer.
Scottsdale’s effluent treatment
chain includes: 400-micrometer strainers, followed by ammonia to eliminate free
chlorine, which is followed by microfiltration and an antiscalant. Next comes pH
adjustment using sulfuric acid, then 20 micrometer cartridge filters, a thin
film composite polyamide RO in a three-stage configuration of 24:10:5 with a
recovery rate of 85%, degasifier towers for reduction of carbon dioxide, and,
finally, lime feed for RO permeate stabilization. The injected water percolates
through several hundred feet of soil, where it commingles with local groundwater
and is pulled out by down-gradient production wells.
Emergency wells are designed to
recharge tertiary effluent diverted from the water treatment plant when
necessary, to prevent hydraulic overloading during Scottsdale’s short wet
season. These are monitored and controlled collectively and discharged into a
three-fourth-inch gravel pack roughly 20 feet below the ground surface. While
Central Arizona Project water is used for recharge primarily during the summer
months when irrigation demand is high, reclaimed water is used for recharge
primarily during the winter months. To achieve the goal of 450 milligrams per
liter total dissolved solids (TDS) per liter prior to recharge, some reclaimed
water receives RO treatment year-round to blend with water from the Central
Arizona project, which has a TDS of about 700 milligrams per liter.
 |
| Photo: Tom Stewart |
| Highly purified RO is one step in the process to turn wastewater into a purified product. |
To do all of this, the Arizona
Department of Environmental Quality requires a wastewater reuse permit and an
Aquifer Protection Permit. The Arizona Department of Water Resources requires an
underground storage facility permit. All aquifers in Arizona are currently
classified for drinking water protected use, and the state has adopted national
primary drinking water maximum contaminant levels as its aquifer water quality
standards. The initial construction costs for the first two phases of the
Scottsdale Water Campus for tertiary and advanced water treatment facilities
totaled $75 million (compared to the multi-millions required today), and
Scottsdale estimates its cost to produce potable quality water via this method
is less than $1.30 per 1,000 gallons.
“Although Scottsdale has been
reclaiming water since 1984, it wasn’t until 1998 that we started reusing it,”
says Water and Wastewater Treatment Director Art Nuñez. “Until then, we just
poured usable water down the drain and paid to dispose of it.”
Aside from its groundwater
recharge program, Scottsdale also markets its reclaimed water for irrigation to
the city’s numerous golf courses.
Close to the border in El Paso,
TX, the El Paso Water Utilities once had similar ideas about ensuring potable
supply sustainability. Circumstances changed, however, such as the utility
finding it more cost effective to sell effluent than put it in the ground.
“In 1979, we undertook an
assessment that suggested we would be in serious trouble by the year 2030, with
respect to our groundwater pumping,” says Water Resources Manager Bill
Hutchison. “Actions were taken including expanding our surface water use,
implementing a pretty stringent conservation program, and increasing our
reclaimed water use. All of this has helped make the Heco Bolson essentially
sustainable.”
In addition, the utility
constructed the essentials for a groundwater recharge project, including a water
reclamation facility, injection wells, and monitoring systems. “Then the golf
course opened and the power plant started sniffing around, and we built lines to
supply them both with recycled water, which meant less and less water was going
in the ground,” says Hutchison. “But it also decreased potable water use. In
addition, the injection wells were presenting their own set of problems with
clogging, collapsing, and having to be re-drilled. At one time, we were putting
as much as 20,000 acre-feet a year in the ground. Now it’s down to about 1,500
acre-feet a year.”
To hedge its bets, however, El
Paso also instituted studies which determined that spreading basins were a
better alternative to wells, so that the water that goes into the ground these
days goes through spreading basins.
And, while El Paso is feeling
comfortable with less is more, the 700 resident community of Cloudcroft is among
a number of New Mexico communities committing to technological innovation.
Cloudcroft relies on snow melt to recharge the small pockets of groundwater that
provide the town’s drinking water supply, and, with less precipitation than
normal over recent years, the community literally found itself running out—to
the point that the National Guard had to bring in truckloads to sooth the dry
throats of summer tourists. Stuck in a considerable bind, the residents of the
small community were saved by the Governor, who, concerned about drought
conditions throughout the state, established a water innovation fund to finance
the development of additional water supplies and help conserve what supplies
were available.
Thus, Cloudcroft was able to
secure the $3.5 million it needed to build the system that its designer, Eddie
Livingston, of Livingston and Associates in Alamogordo, NM, likes to point out
is the first of its kind in the country. ITT/Advanced Water Treatment supplied
the equipment for the wastewater treatment/reuse project.
As Livingston describes it, the
elaborately redundant system will reclaim 100% of the town’s wastewater to
drinking water quality, blend this with existing well and spring water, and then
retreat everything before the water is introduced into the town’s drinking water
supply. On average, 100,000 gallons will be added to the Cloudcroft system
annually. This elaborate treatment chain was necessary, in part because the town
has no opportunities for groundwater storage, is not on top of a mountain, and
has no natural surface resources.
Cloudcroft’s water treatment
begins with a membrane bioreactor wastewater treatment plant, which replaces the
town’s existing trickling filter plant. The effluent is filtered through
microfiltration membranes, disinfected with chloramines, and pumped to a storage
tank, from which it gravity feeds three miles to the town’s potable water
facilities. Here it receives its first run through RO.
“Because the facility is downhill,
we have enough pressure that we don’t need a pump on the RO system, which is
very energy efficient,” says Livingston. “We end up with about 175 psi
pressure.”
Again, local conditions help
define the treatment process. “The spring water is moderately hard, and it gets
higher in dissolved solids by the time it goes through the wastewater plant, so
we’re using the same RO membranes Orange County used in Water Factory 21,” he
adds.
For redundancy and public health
concerns, the system also mimics Orange County’s use of advanced oxidation,
using hydrogen peroxide and UV light.
The highly purified RO permeate is
then stored in a million-gallon, lined and covered reservoir. From there, it’s
blended approximately half with spring water and the other half with well water
at another reservoir, and the blended water is subjected to ultrafiltration to
remove not only particulates and large pathogens like giardia and cryptosporidium, but also bacteria and
viruses. The blended water is then disinfected again with UV.
“We use UV after ultrafiltration
for a couple of reasons,” says Livingston. “Number one, it’s a very good
disinfection method, but also the state required us to have at least 5.5 LOG
[inactivation versus contact time plotted on a Logarithmic (LOG) scale] removal
of cryptosporidium. The regulators gave us 4.5 LOG for the ultrafiltration
membranes and two LOG for the UV. After the UV disinfection, we polish
everything off with activated carbon to remove emerging contaminants, then
disinfect one last time with chlorine.”
Too much of a good thing? It
depends. Given the restrictions of Cloudcroft’s geology and the relatively small
amount of water that will be processed, the town will not be facing the same
challenges with RO brine disposal that inland facilities treating large amounts
of water are exposed to, and will, in fact, use the brine to keep the dust down
on its roads and to make snow at the local ski area.
Anything extra will be injected
into one of the dry wells the consultants dug when they were looking for
additional sources of water. And as Livingston points out, not only will
residents now have a reliable source of water, the quality of that water will
improve, meaning their hot water heaters will last longer than three years.
An entirely different set of
circumstances prevailed outside metropolitan Dallas that convinced the North
Texas Municipal Water District (NTMWD) to take a more natural approach to
indirect potable reuse. Taking advantage of treated effluent that flows down the
East Fork of The Trinity River from facilities the district either owns or
manages, it will use constructed wetlands to treat the river water, which will
then be blended with raw water to help sustain the region’s potable supply.
As a state agency, NTMWD provides
water, wastewater, and solid waste services to 61 municipalities on the north
and east side of Dallas, one of the fastest growing areas of the nation. Surface
water storage is provided by four manmade lakes, which are fed by an annual
rainfall of 40 inches a year, but which recede considerably under drought
conditions. An additional consideration is that NTMWD’s service area is
currently growing at the rate of 4–5% annually, and it expects to serve 700,000
additional residents by 2020.
According to Assistant General
Manager Mike Rickman, the district had developed all easily developable local
resources. “There are no additional reservoirs that can be constructed in or
adjacent to our area,” he says. “We were having to go further out, at
considerably more expense, so we started looking at what options we had
locally.”
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| Photo: Southern California Water Replenishment District |
| Both West Basin and WRD have committed to increasing use of recycled water as a means of diversifying their water supply portfolios. |
The utility took its cue from
neighboring Tarrant Regional Water District, which will construct a similar
project to serve the Fort Worth area. NTMWD will draw water out of the river,
run it through a 2,000-acre constructed wetlands to reduce phosphorous and
nitrogen, and reintroduce it to 22,000-acre Lake Lavon, one of its four
reservoirs, where it will remain for over a year before it’s drawn out and
treated as raw water for potable use. The lake serves as a blending basin for
fresh water from the three other lakes, so the river water will also be
diluted.
The project required a deal with
the state that NTMWD would only capture 70% of the flow its upstream facilities
contribute to the river, leaving 30% for the environment and downstream uses.
Estimates are that the $300-million project, which was financed by selling
bonds, will produce 102,000 acre-feet of water in the next 10 years.
“We’re making very efficient use
of the land to produce water,” says Rickman. “Using current technology, we can’t
put any more reuse water into Lake Lavon, because it will have reached its
assimilative capacity once this project is fully operational. But that doesn’t
mean we can’t take additional reuse water that has gone through wetlands to
another supply source and do the same thing. What we’re doing with this project
is allowing nature to help us.”
A similar river source water
project is underway in Aurora, CO. When completed, the $750-million Prairie
Waters Project is expected to move as much as 50 million gallons of water a day,
boosting the community’s water supply by approximately 3.3 billion gallons per
year and effectively doubling the value of its $300-million water rights. The
project will draw water from the South Platte River, use it, treat it, and then
discharge it back into the river. The water then flows downstream, where it is
recaptured in wells the city has constructed, filtered through the riverbank,
and pumped back to Aurora for additional treatment. Travel time is seven to 10
days, and this riverbank natural filtration method—which is in regular use in
Europe—will remove most of the nitrates, phosphorous, and other organic
compounds.
From there, the recovered water
will be pumped to an artificial aquifer, where it will remain for approximately
30 days to provide enhanced biological and organic treatment. Next comes a water
purification facility where the water is softened and treated with advanced
ultraviolet oxidation, then flocculation, sedimentation, and filtration. After
this, it will be subjected to an activated-carbon gravity filter and, finally,
disinfected with chloramines before it enters the regular distribution
system.
The two river projects warm the
heart of Peter Fox, who is a Professor of Civil and Environmental Engineering at
Arizona State University and a long-time advocate of using natural systems to
treat reclaimed wastewater, in particular soil aquifer treatment (SAT).
“In my viewpoint, soil aquifer
treatment has the potential of using biological processes to remove the majority
of the organics that are present in a lot of different waters,” says Fox. “Given
sufficient time—a year or so in the subsurface—you can expect that the amount of
organic carbon might be reduced to one milligram per liter or less, which is
similar to a lot of natural groundwater. So if you’re thinking of RO, maybe you
should also look at soil aquifer treatment.
“In the Aurora project,” he
continues, “after soil aquifer treatment, they’re going to treat that water with
activated carbon and advanced oxidation to destroy or remove residual compounds.
That way they don’t have to use reverse osmosis to remove everything, because
the matrix is so much cleaner that the oxidation technologies should be much
more effective. To my mind, this is a much more sustainable type of
operation.”
Fox further gives his opinion: “My
thought is we should be looking a lot closer for other types of indirect potable
reuse, instead of doing all of this reverse osmosis,” he says. “The Montebello
Forebay in Los Angeles County has been doing basically soil aquifer treatment
since 1962, and they’ve done epidemiological studies to show there’s been no
health effects. Scottsdale has seriously considered getting rid of their
system—which they modeled after Water Factory 21—
because they’re having such
problems with salt disposal. They’re saying maybe they should look to just doing
groundwater recharge and treat the water when they recover it. With Hydrosystems
Inc., in Phoenix [AZ], they’ve pioneered Beta zone injection wells, which can be
used where you don’t have enough land for SAT.”
“I think we need to think ‘big
picture,’” says Hoover Ing, of the WRD. “RO is a very energy-intensive process,
and you’ve got the salts. The studies I’ve seen have shown that soil does a
tremendously effective job of removing a lot of contaminants. A few more
pharmaceuticals tend to go through the soil than persist with RO, but a lot of
these are removed with organic carbon. And, the water begins to look like what
it was before it became wastewater.
“In Los Angeles, we are trying to
get the regulators to allow us to use 100% recycled water in our seawater
barriers,” Ing goes on to say. “Right now, this is kind of uncharted territory.
One concern is the RO water may leach out chemicals in the ground—that it’s so
pure it hasn’t been quite stabilized. Which means, all things considered,
percolation may not be as rudimentary as we’ve been thinking. Have enough
barriers, have enough blending, and have enough travel time—these are at the
heart of any kind of requirements. Monitor it carefully, and, if something isn’t
going right, shut it off.”
From Los Angeles comes news that
the city has revised its 1990 “Toilet to Tap” project. Ing remembers that, at
the time it was first conceived and then abandoned, $60 million in combined
federal, state, and local funds had gone into constructing a 10-mile, 60-inch
pipeline to take disinfected tertiary effluent from the Donald C. Tillman
Wastewater Treatment Plant in the eastern San Fernando Valley, to spreading
grounds at the far northern end of the San Fernando Valley Aquifer. Today, under
Mayor Antonio Villaraigosa, indirect potable reuse has been given a new lease on
life, although, at 15,000 acre-feet per year, the project will be approximately
half the size of what was originally planned.
According to Jim McDaniel,
Assistant General Manager at the Los Angeles Department of Water and Power
(LADWP), a significant determining factor has been restoration of the Los
Angeles River, in that effluent from the Tillman plant is needed to keep the
river running in the summer. The city’s recently completed comprehensive water
supply plan emphasizes increased water conservation and expanded use of recycled
water to generate an additional 100,000 acre-feet of new water a year, with
35,000 acre-feet of the recycled total coming from purple pipe uses and the
remainder from the groundwater replenishment project.
The city is currently in the
process of developing a Recycled Water Master Plan, which it hopes to have
completed by 2011, and then the effluent flowing by 2019. Although the treatment
chain is yet to be developed, McDaniel says the project will include a
$500-million upgrade of the Tillman plant.
In the meantime, LADWP is tackling
the nemesis that brought down the 1990 East Valley Recycling Project. Taking a
page from Orange County’s book, it is already in the process of developing its
public outreach campaign.