Salinity Management
and
Desalination Technology
for Brackish Water Resources
in the Arid West
Summary Report of a Workshop
held on
August 6, 2007
Tempe, Arizona
Sponsored by
Arizona Water Institute
U.S. Bureau of Reclamation
August, 2008
Salinity Management and Desalination Technology for
Brackish Water Resources in the Arid West
Summary Report of a Workshop
held on
August 6, 2007
Tempe, Arizona
Sponsored by
Arizona Water Institute
U.S. Bureau of Reclamation
August, 2008
Executive Committee
Supporting Sponsors
Wendell Ela*, University of Arizona
Chuck Graf, Arizona Water Institute
Tom Poulson, U.S. Bureau of Reclamation
Jim Baygents, University of Arizona
Jan Theron, Northern Arizona University
Peter Fox, Arizona State University
Chris Scott, University of Arizona
Brown and Caldwell
Errol L. Montgomery & Associates
Damon S. Williams & Associates
Workshop report prepared for publication by
*contact and corresponding author. Chemical and Environmental Engineering, University of Arizona,
Tucson, AZ 85721. E-mail: wela@engr.arizona.edu
Table of Contents
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
II. Source Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
III. RO Pre-Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
IV. Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
V. Post-Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
VI. Local Management and Disposal of Residuals and Concentrate . . . . . . . . . . . . . . .10
VII. Regional-scale Concentrate Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
VIII. Summary and Roadmap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
IX. Afterword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
Appendices
Appendix 1- Salinity Workshop Agenda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Appendix 2 - Salinity Workshop Attendees . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Appendix 3 - Annotated Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
I. Introduction
Central Arizona Project (CAP) water originating in the Colorado River is a primary
source of potable and irrigation water in Arizona. This water source, along with the Salt
River in central Arizona, brings over 1,000,000 tons of salt each year into central and
southern Arizona, where it accumulates as the carrier water is used. These salts go primarily
into the region’s soils and the effluent discharged from wastewater treatment plants. Both of
these temporary repositories ultimately feed the salt into the groundwater, which is a critical
water supply for the region.
On August 6, 2007, a workshop was convened in Tempe, Arizona to focus on identifying
the state-of-the art technologies for membrane removal of salt from the region’s water
sources while minimizing the attendant water loss and the environmental impact of the
treatment residuals disposal. The workshop focused on identifying and prioritizing the technical
hurdles that must be overcome to improve efficiency and economic viability of treatment
processes, and establishing a practical roadmap forward for achieving sustainable, viable
desalination of inland, moderate salinity waters including wastewaters. These workshop
objectives were examined in the narrow context of source water salinities ranging from CAP
water with a total dissolved solids of about 700 mg/l (milligrams per liter) to brackish waters
containing up to 10,000 mg/l of TDS. Although ocean disposal is a potential option in the
distant future, nearer-term inland disposal options were emphasized for evaluation.
The historical, high quality groundwater resources are not sufficient to sustain the current,
much less, projected municipal, industrial, and agricultural demands in Arizona. The water
demand requires full utilization of the State’s allotment of CAP water as well as its available
surface water sources, including the Salt and Verde River resources. The salinity of CAP and
Salt River water, by far the bulk of the surface supply, exceeds the Environmental Protection
Agency’s Secondary Standard of 500 mg/L for total dissolved solids (TDS). Demand greater
than these surface and historical groundwater supplies can only be met by wastewater
reclamation and tapping into and treating the region’s brackish water resources. The salinity
of both of these latter water resources is greater than current surface water source salinities.
The inevitable conclusion is that future water needs will require desalination of a significant
portion of the region’s water resources.
To illustrate, water demand in the Tucson Active Management Area (TAMA) is estimated
at 400,000 acre-feet per year. However, the rate of natural groundwater replenishment is
only about 60,000 acre-feet per year. The unavoidable shift from ground water to CAP
water as the primary regional water resource will have significant water quality implications.
The average TDS concentration in delivered ground water has historically been about 260
mg/l. TDS levels at the Tucson terminus of the CAP canal are greater than 700 mg/l and
likely to rise. Full utilization of CAP water allocations will bring 200,000 tons of salt into
the TAMA each year. Without some form of salt management, the majority of this salt will
remain, contributing an average of 5 mg/l to the regional aquifer each year.
An analogous situation exists in the Phoenix area, where salts are accumulating at a rate
of about 1.1 million tons annually. This is not a reversible situation and cannot meet the
public’s concept of water supply sustainability.
The strategy for salt management in central and southern Arizona will almost certainly
consist of a combination of reverse osmosis (RO) treatment and benign brine disposal.
1
Water recovery (the percentage of treated water that is collected as permeate) is of exceptional importance to the community. Although it is difficult to assign a marginal benefit to
water supply augmentation through recovery enhancement, Tucson residents have shown a
willingness to pay $3 per 1,000 gallons or about $1,000 per acre foot for the highest increment in the tiered rate structure for delivered residential water. Using this value to illustrate
the benefits of brine minimization, if the entire TAMA CAP entitlement is RO treated to
manage local salt levels in ground water, each 1% increase in recovery efficiency would
provide a benefit to the community of greater than $2M per year. Water recovery from RO
treatment of CAP water is currently limited to about 80%. If methods could be developed
to even halve the water loss (increase water recovery from 80 to 90%), it would produce a
$20M per year benefit within the TAMA alone. The technology would be equally useful in
other Arizona desalination projects.
In response to this critical challenge to Arizona, the Arizona Water Institute and the
Bureau of Reclamation convened a technical workshop entitled “Improving Salinity
Management and Desalination Technology for Brackish Water Resources in the Arid West”
on August 6, 2007, in Tempe, Arizona, at the Tempe Mission Palms Hotel. Workshop
invitees from governmental agencies, water utilities, consulting firms, academia, and other
stakeholder groups were selected for their technical expertise and involvement in the subject.
(A workshop agenda and a list of participants are included as appendices to this report.)
The workshop was specifically structured to encourage discussion between representatives
of the key stakeholder groups on ways to address the inland desalination and concentrate
management challenges and on identification of the critical research hurdles that must be
overcome for implementation of viable strategies.
The closest to a unanimous conclusion reached by the participants in the Workshop was
that ‘there is no silver bullet’. Although there is no single, one-size-fits-all technological
solution to the inland salinity management problem, a range of options is available that can
be applied in various combinations to meet case-specific conditions. A sustainable, economically viable and technically feasible solution will consist of a number of different mutually
reinforcing technologies and efforts, the particular nature of which will vary according to
the size, location and circumstances of the water supplier and other factors. Within the
overall Arizona region the solution will include technologies and strategies for:
1) minimization of the transport of salt into the region (e.g., by minimizing the need
and motivation to use home water softeners),
2) pre-treatment of RO feed water to increase water recovery and membrane performance,
3) improving the performance of membranes and the membrane separation process itself,
4) post-RO treatment of the concentrate stream to increase the recovered water, extract
economically viable concentrate components, and reduce the concentrate volume, and
5) management and disposal of the remaining concentrate and other residuals.
The following five sections of the report address discussions that occurred regarding
each of these five components of a holistic approach to salinity management. Obviously, all
issues raised and points made in the workshop do not neatly fall into only one of the areas
enumerated. Consequently, a best fit for each topic was attempted, but some topics are
addressed in more than one category with some overlap.
2
II. Source Control
Short of desalting surface water before it enters the region, such as by implementing RO
treatment at the mouth of the CAP canal, a significant mass of salt will enter the region
along with the surface water supplying Arizona’s water needs. However, the water conveyed
by the CAP canal is not the only source of imported salt. In the Phoenix region a significant
mass of salt is contributed by the Salt and, to a lesser extent, the Verde River. In addition, a
large and rapidly increasing mass of salt is introduced because of the use of ion exchange
home water softening devices. Although the devices remove multivalent cations, the total
salinity (in terms of equivalents per volume) of the product water is not decreased and, in
addition, a highly saline brine is generated. Because this salt addition occurs after treatment
by centralized water treatment facilities and at the point of use of the drinking water, the
extra salt load is conveyed to the sewer and seen as a higher wastewater salinity than standard
calculations predict. This load has potential negative impacts on the riparian ecosystem
supported by the wastewater discharges, on the uses of the reclaimed water, on the WWTP
processes themselves, and on the overall rate of salt accumulation in the region.
Proliferation of home and small commercial water softener use is, at least partially,
motivated by a desire to avoid premature failure or frequent maintenance of water-related
home appliances, such as water heaters, irons, swamp coolers and coffee makers, due to
precipitative scaling by hardness-causing cations. As the salinity and hardness of drinking
water supplies increase, it is expected that water
softener use will also increase. Two approaches
were discussed to counteract this trend: implement system-wide water softening to eliminate the
need for individual point-of-use water softeners or
discourage their use through disincentives or other
measures. These approaches are not mutually
exclusive and could be implemented to some
degree in tandem.
The former approach is typically accomplished
by centralized lime softening (or variations of this
process), although nanofiltration or reverse
osmosis processes may be used if other water
quality considerations suggest they may provide
additional benefits. Alternatively, implementation
of centralized capture and regeneration of spent
ion exchange softening resins (for instance via a
provider switch-out program) would allow
improved residual management and enhanced
water recovery by incorporation of processes
that are not amenable to point-of-use home
softening (e.g., brine recycle, rinse/backwash
recovery, precipitative softening of brines).
As to the latter approach, workshop participants
suggested several means of discouraging home
3
water softener use. These include a tax/surcharge on softener salt, a surcharge for softener
installation in new homes, or restrictions on the use of self regenerating softeners. This latter
strategy would force regeneration of spent softener resins at centralized facilities where the
high salinity regenerant residual could be more easily treated or managed to minimize
negative impacts. Although the practicality of implementation of these (dis)incentives to
water softener use was not discussed, there was considerable interest voiced in further
investigating such means to discourage use.
A final potential approach to decreasing the salt impact of home water softeners is the
use of capacitive deionization devices. This technology is still not fully developed and commercialized, but could potentially compete with ion exchange for home softening purposes.
The advantage of capacitive deionization is that it does not increase the overall salt load
since softening is driven by an input of electrical energy rather than monovalent ions. The
process produces an ion-reduced finished water and a brine concentrate, but with no net
increase in overall ion content. This technology (at least, at its current stage of refinement)
is not economically competitive with current technologies due to the high cost of membrane
material (normally Aerogel) and the low ion site capacity of the membranes.
4
III. RO Pre-Treatment
Reverse osmosis pre-treatment generally includes a variety of technologies and techniques to decrease both fouling and scaling of the membranes. Fouling occurs due to the
build-up of particles (organic and inorganic, colloidal and particulate) that are in the feed
water and deposit on the membrane surface. Fouling is usually most pronounced on the
membranes in the front-end stages of an RO array. In contrast, scaling occurs due to the
precipitation of supersaturated salts on the membrane surface. The degree of precipitation
increases as the concentrate stream progressively moves through the sequential RO elements
and increases in salinity. Scaling is a problem typically in the back-end stages of an RO
array where the reject stream’s salt concentration is near its maximum.
Conventional water treatment
processes (e.g., sedimentation,
coagulation/flocculation, filtration)
as well as other processes, such
as slow sand filtration, microand ultrafiltration, and activated
carbon filtration, can be effective
at removing fouling components
of the raw water. Micro- and
ultrafiltration are becoming
more widely used to remove
foulants ahead of RO because
they have a relatively small
installation footprint, are reliable
and well field tested, and their
price is becoming competitive
with alternative processes. For
Membrane scaling of supersaturated salts (BaSO4)
many Arizona utilities, availability
of land is not a primary constraint, so slow sand filtration (SSF) may be an option to lessen
feed water fouling indices. The Water Quality Improvement Center at Yuma, initiated by
the US Bureau of Reclamation, the National Water Research Institute, the U.S. Army and
other research institutions, has studied pre-treatment by slow sand filtration and recommends
it for consideration, particularly for rural installations. There is the possibility of improving
SSF removal of colloidal solids and dissolved organics by developing better engineered filter
media and by sand amendment with such things as granular activated carbon and iron
particles. Improved engineering to control schmutzedecke development and various periodic
cleaning methods should also be pursued to optimize SSF performance.
Pre-treatment technologies and techniques to control or delay the onset of scaling
received considerable discussion. These RO pre-treatment processes can be classified into
two groups based on their mode of action in dealing with the limiting (least soluble) salts,
which are responsible for initiating scaling on the final stage membranes. One group of
processes acts by removing the limiting salts prior to membrane application. The second
group includes processes that increase the solubility of these sparingly soluble salts. In either
case, the outcome is increased water recovery prior to the onset of scaling.
5
The salts most commonly responsible for scaling are calcium carbonate (CaCO3), barium
sulfate (BaSO4), silica (SiO2), calcium sulfate (CaSO4), calcium fluoride (CaF2), strontium
sulfate (SrSO4) and magnesium hydroxide (Mg(OH)2). The processes discussed by workshop participants to remove these components from the feedwater prior to RO were 1) ion
exchange and 2) precipitative processes, including softening. As to the first, cationic ion
exchange resins are more commonly used than anionic resins, primarily because neither
carbonate, fluoride, nor silicate are readily removed by anion exchange, whereas calcium,
magnesium, strontium and barium have relatively high affinities for common cationic resins.
However as discussed earlier, ion exchange requires addition of salt (typically NaCl) to
maintain the regenerant brine, so there is a net increase in salt in the system (although it is
confined to the waste brine stream).
More commonly, precipitative removal of the divalent cations is used to control scaling
salts in central treatment facilities. Traditionally, this is done by either addition of caustic
(NaOH), lime (CaO), or lime and soda ash (Na2CO3); depending on the raw water composition and the cations targeted for removal. A number of workshop participants commented
on the need to improve the selectivity and efficiency of precipitative processes by such
things as polymer addition (to increase floc settling rates and sludge dewatering), specific
ion addition (to target early precipitation of certain cations) and designer particle addition
(to provide preferential nucleation sites and increase settling rates). There was also discussion of
recovery of reusable and potentially saleable products from the sludge residual (although
this discussion focused primarily on selective precipitation as a post-RO treatment process
and will be covered in that section of this report).
The alternative to removing the limiting salts prior to RO is to manipulate the feedwater
composition to increase the solubility of the limiting salts. Sulfuric acid is often added to
lower the feed pH, which decreases both the carbonate concentration by converting bicarbonate to volatile carbon dioxide and the hydroxide concentration. However, if sulfuric
acid addition is not prescribed because sulfate salts are limiting (e.g., BaSO4), hydrochloric
acid is often used. A disinfectant must also be added to feedwater to prevent microbial
growth in the RO system. Polyamide (PA) membranes have largely replaced cellulose
acetate (CA) membranes because of their greater chemical and physical stability, greater
water fluxes and salt rejections and resistance to bacterial degradation. However PA
membranes are intolerant of free chlorine, so disinfection in these systems is typically by
combined chlorine (chloramines). Considerable work has been performed to improve the
oxidant tolerance of polyamide membranes in particular, and other membrane materials in
general. There are a wide variety of proprietary antiscalant additives, which allow the
supersaturation (increase in solubility) of the limiting salts to varying degrees. Inorganic
phosphate additives have been largely replaced by organic polymer additives. These additives allow supersaturation factors of from 2 to several orders of magnitude depending on
the type of limiting salt, the particular additive used, and the other constituents in the
feedwater. (For instance, it has been found that water with high iron can greatly decrease the
effectiveness of many antiscalants.) Anecdotes from various workshop participants suggest
that there is a wide range in cost, ease of handling, and performance of the available antiscalants and that with the growth of this market, there is regular introduction of new products
onto the market. There are few independent, comprehensive, comparative studies of the
effectiveness of the different commercial antiscalant formulations and this was earmarked
by workshop participants as an area where research effort should be directed.
6
IV. Membranes
As noted by workshop participants, the formulation and fabrication of the actual RO
membranes has been done primarily in the private sector and under proprietary restrictions
that limit comparative research on the various products. Additionally, development of membrane materials is a relatively specialized technology and it was considered that only a few
companies have the expertise. This is not expected to change in the near future, so there
was general agreement that improvement of membrane materials should not be a primary
focus of public or academic research and policy efforts. However, there was discussion of how
evaluation and modeling of the performance of the whole RO system train (pre-treatment,
membrane filtration, post-treatment, and residuals disposal), and the long-term development of
membrane ‘additions’ might be worthwhile areas of investigation by the research community.
In addition, it was suggested that a long-term collaboration between academic and industry
researchers in understanding the flow dynamics near membrane surfaces (such as influenced
by spacer design, membrane surface roughness, and inlet/outlet structures) could potentially
lead to significant improvements in fouling and scaling resistance, and membrane hydraulic
flux, without substantial material changes to the membranes themselves.
A large number of membrane additions that could improve RO performance were
discussed, although in nearly all cases these were identified as longer-term research and
implementation developments that were not likely to impact RO applications within the
short-term (5 year) horizon. Membrane additions of interest included electromagnetic
membranes which separate ions by their charge density; nanoparticle-embedded membranes
that create reaction as well as separation of water constituents; biomimetic membranes that
mimic the lipid bilayer/protein channel functionality of natural cell membranes; and target
electrodialysis membranes for selective ion separations.
A presentation during the morning session of the workshop, as well as discussion during
the breakout sessions, highlighted the community’s interest in forward osmosis and questions
regarding the impact it is expected to have on the desalination field. In forward osmosis
(FO, also variously referred to as osmosis or direct osmosis), water moves from the feed
solution through the membrane into the draw solution not due to hydrostatic pressure (as
in RO), but due to osmotic pressure. For this to occur, the draw solution must have a higher
osmotic pressure than the feed solution. However, if the dissolved species in the draw
solution can be readily separated from it after FO, then desalinated water may ultimately
be produced. The feasibility of most present FO configurations depends on using a draw
solution species that can be separated and then recycled for reuse in new draw solution.
The most promising of the FO processes for large volume desalination uses a carbon dioxideammonia mixture in the draw solution which can be volatilized at relatively low temperatures (~60°C) to effect final water purification and recycle of the solutes. FO has the advantage over RO of working at near zero hydraulic pressure, having very high rejection for
most solutes, and avoiding much of the fouling associated with pressure driven systems. FO
permeation rates are much lower than for RO and, at present, there are no commercially
available processes. Advancement of FO will depend largely on development of membranes
specifically engineered for FO application (rather than using RO designed membranes), FO
membrane reactor configurations that efficiently incorporate draw solute separation and
recycle processes, and improvement of draw solute formulations to increase draw osmotic
pressure and solute separation.
7
V. Post-treatment
The type of concentrate treatment, if any, implemented following reverse osmosis largely
will be dictated by the options for and cost of final residuals disposal. If chemical components
of the concentrate stream are economically recoverable and useable, then not only is a
product generated, but the concentrate salinity is decreased and the options for beneficial
use of the remaining aqueous stream increased. The ionic composition of most concentrates
suggests that the possible recoverable minerals will be mainly chlorides, carbonates and
sulfates of major cations such as sodium, potassium, magnesium and calcium. These minerals
will be either in pure or mixed forms. Since the removal of scale-forming calcium and
magnesium is often necessary to process concentrates, the production of calcium and
magnesium by-products is logical and potentially economical, if local markets are available.
In any case, concentrates and products from concentrates have been used for the following
applications.
• Irrigation of salt tolerant crops
• Supplements for animal dietary needs
• Fertilizers (mainly potassium salts)
• Soil conditioners for remediation of sodic and acidic soils
• Sealants for reduction of seepage from water channels, ponds and other effluent
holding basins
• Fire retarding and proofing chemicals
• Manufacture of magnesium oxide and magnesium metal
• Manufacture of light-weight and fireproof building products
• Manufacture of plastics, paint, ink, and sealant products
• Dust suppression
• Stabilizers for road base construction and salt for deicing roads
• Flocculating agents for water/wastewater treatment
• Various applications in food and chlor-alkali industries
Products may be recovered from concentrates by selective precipitation, crystallization,
and evaporation. Common products would include calcite and magnesium hydroxide from
selective precipitation and gypsum from crystallization. Calcite used as a whitening agent
for paper production has a market value of $300/ton. Magnesium hydroxide is feedstock
for the production of magnesium, and gypsum is used for the production of drywall. The
evaporation of concentrates may yield sodium sulfate if the calcium and magnesium are
previously removed as mixed salts. Products may also be recovered from concentrates using
electrolytic processes. Potential high value products include chlorine, bromine, dilute acids
and bases, and metals. The production of products from concentrates has not been the goal
of public agencies and will add a new level of complexity to concentrate treatment.
However, salt by-products will help to eliminate salts from the water cycle and help lead to
sustainable salt management. Selective precipitation of commercially valuable solids by
engineered chemical addition to cause supersaturation of selected solids has been successfully used in Australia and South Africa, but so far has not been demonstrated in the U.S.
8
A relatively recent technology marketed for post-treatment of RO concentrates is the
proprietary vibratory shear-enhanced process (VSEP®). The VSEP technology was developed
and is marketed by New Logic Research and has been used in manufacturing for several
years on a small scale. The principle of operation is based on energy input (vibration) to RO
membranes to prevent particle attachment to the membranes and perturb and mix the
chemical boundary layer near the membrane surface. Both activities minimize membrane
fouling in very concentrated chemical mixtures or suspensions. VSEP technology offers
potential advantages over alternative concentrate management options. These include
moderate environmental footprint (land, energy, noise), competitive capital cost, and potentially very high water recovery factors. However, VSEP may significantly increase operation
and maintenance costs. For this reason, VSEP needs to be tested on a larger scale where
the economics, reliability, versatility and other properties can be evaluated as they apply to
municipal use.
DewVaporation is an evaporative-condensation process, which produces a low TDS
distillate and a high TDS concentrate from impaired water. The poor quality feed water
runs down one side of a heat transfer wall inside the DewVaporation tower, while air flows
upward, evaporating some of the water. The rest of the water with the salts flows down and
out of the tower where it is disposed or routed to a second tower for further water recovery.
The air, now higher in humidity, continues to the top of the tower.
Energy in the form of steam is added to the tower at this point, saturating the air. As the
air travels down the other side of the heat transfer wall it cools and begins to condense.
The heat of condensation travels through the heat transfer wall to the evaporation side
assisting the evaporation process. The condensing water is very low in TDS and is collected
at the bottom of the tower.
Dr. James Beckman, the Bureau of Reclamation, and the City of Phoenix operated 25
DewVaporation Towers over the winter of 2006-2007 at the 23rd Ave WWTP. Data was
collected on energy use, TDS of distillate, TDS of concentrate, production rate and other
information. A final report of the project has been completed and can be accessed on the
Bureau of Reclamation’s Science and Technology Office web site. On the positive side:
1. The DewVaporation towers were reliable and ran constantly for extended
periods of time,
2. The distillate was low in TDS - approximately 10 mg/l, and
3. The towers could process high TDS feed.
Whereas on the negative side:
1. Small amounts of distillate were produced from each tower (5-8 gallons/hour), and
2. the energy multiplication factor was approximately 2.5 (while theory predicted a
energy multiplication factor of 5).
The evaluation trials suggest the DewVaporation towers work but the process needs better
engineering, such as a reliable steam source (ideally a waste heat source), improved pumping equipment, tower base redesign to minimize catch basin leakage and energy loss, and
diagonal fluid flow paths to increase distillate production. The DewVaporation technology
potentially could be applied in a number of scenarios, including remote, low demand sites
with impaired well water; at the end of a Zero Liquid Discharge (ZLD) facility; and to
replace a crystallizer, which would be receiving low flow-high TDS water.
9
VI. Local Management and Disposal of Residuals and Concentrate
Concentrate disposal alternatives are relatively well known. Because no single technology
works for every concentrate management case, the objective is to put the right ones together
for any particular disposal problem considering the volume of concentrate, amount of land
available, cost, and other considerations such as social and environmental impacts.
Concentrate management could be accomplished in a large centrally located facility operated
by several entities in common or as dispersed units at the site of each individual concentrategenerating entity.
The two most common methods of concentrate disposal in Arizona are evaporation
ponds and sewer disposal. Both methods are acceptable on the small scale but as larger RO
facilities are built to meet the demand for potable water, these disposal methods have
severe drawbacks. The major costs associated with evaporation ponds are land and liners.
Methods to enhance evaporation typically increase the contact area between the air and
water by spraying or pumping, and the efficiency of these methods may depend on local
wind conditions. Although wind- and spray-aided evaporation processes reduce the
required pond size, problems have been reported with the design and long-term operation
of spray nozzles and possible transport of salt particles into downwind, off-site locations. By
coupling evaporation ponds in series, low economic value products such as sodium sulfate
may be produced. Solar gradient ponds, which are relatively deep brine-containing ponds
designed to produce energy, are also an option. Solar gradient ponds are not evaporation
ponds, but the basic design is similar to evaporation ponds. Therefore, a regional salt
processing facility could use some ponds for energy production while the remaining ponds
are used for evaporation. Some workshop participants felt that solar ponds were difficult to
maintain and the energy recovery small compared to those difficulties.
Concentrate also may be disposed of by land application. This is currently done in some
instances for dust abatement, although this practice is not likely to be feasible for largescale operations because of both the volume of concentrate requiring disposal and issues
resulting from salt build-up in the upper soil layer in the application site.
Alternatively, land application may take the form of irrigation. The Gila River Indian
Community plans to mix the concentrate produced from their RO facility with the treated
wastewater coming out of the Lone Butte WWTP. This reclaimed water/concentrate mix
would be used to irrigate crops on Community land. Irrigation of halophytes is an analogue
to evaporation ponds to reduce concentrate volume. The evapotranspiration rate for halophyte irrigation is seldom higher than the unaugmented evaporation rate from a pond. For
halophyte irrigation to be economical, irrigators must produce a commercial product or
replace a beneficial, non-salt tolerant species (e.g., turf grass). Greater use of halophytes is
dependent on demonstration that long-term aquifer contamination or soil fertility degradation
will not occur due to salt build-up in the soil.
If the concentrate is not used or disposed of on the site where it is generated, then
transportation must be considered. Commonly, this is by discharge to sewer. However, this
practice may cause (and has caused) problems with the ability of the treated wastewater to be
reused and may cause sewer capacity issues as additional desalination facilities go on-line.
Two ideas were discussed at the workshop for non-sewer concentrate transport to a central
facility. Smaller PVC pipe could be placed inside existing sewer lines to carry concentrate to
another location. This would be contingent on the existing sewer main having additional
10
capacity to handle the added pipe. Alternatively, the existing storm drain system could be
used to move concentrate, as long as the storm drain also has sufficient capacity.
Injection wells are a current and historical option for concentrate disposal. Class 1 injection
wells, as classified under the U.S. EPA’s Underground Injection Control (UIC) program, are
used in Florida and will be used in El Paso, Texas. Under the federal program, it is commonly
assumed that the receiving water must be at least 10,000 mg/l TDS and be isolated from
potable aquifers. It has been suggested that the Arizona concentrate be injected into the
salt dome caverns being formed in the Luke Salt Body by solution mining by Morton Salt
Company. However, the annual cavern volume increase would only handle about 66,000
gallons a day of concentrate. Furthermore, concerns about potential contamination of
Morton’s food-grade product with undesirable ions in the injected concentrate almost
certainly rule out this option. Alternatively, artificial recharge into poor quality aquifers
could be used for concentrate disposal. This may be a way of saving the water associated
with concentrate until technology catches up to inexpensively remove the salts. The potential
for suitable locations in Arizona has not been sufficiently investigated. In Arizona, such
wells also are regulated and must be issued a permit under the State’s Aquifer Protection
Permit (APP) program. However, under Arizona’s APP program, a number of hurdles must
be overcome, including the requirement for the Arizona Department of Environmental
Quality (ADEQ) to reclassify the relevant aquifer from drinking water use before concentrate
injection into such repositories could be permitted; ADEQ has never reclassified an aquifer.
Nevertheless, some workshop participants suggested that an updated study of potential
deep brackish and saline repository formations in south-central Arizona be conducted,
incorporating geophysical and deep drilling data collected in the last few decades.
The economics of concentrate/residual disposal are directly linked with the volume
requiring disposal, although it remains an open question as to what degree economies of
scale apply to particular disposal strategies. The workshop participants considered a number
of concentrate volume reduction options – both non-patented and proprietary. Brine
concentrators are an expensive, yet viable, technology. They are used primarily at power
plants where electricity is cheap, but tend to be large, high maintenance structures.
Crystallizers are a very expensive technology, which are primarily utilized as the last stage in
a zero liquid discharge (ZLD) system. They can be relatively high maintenance due to the
corrosivity of very high TDS levels of the fluids. Although freeze crystallization is used in
some industrial settings, it would not be applicable on a municipal water production scale
due to high energy demand and installation complexity. Lime softening or other precipitative
variants may be used as a post-treatment prior to second stage RO, analogous to pre-treatment applications of the technology. The softening objective is to remove the limiting salt
ions (e.g., Ca and Mg) which would otherwise scale the secondary RO membranes. This is
considered a relatively ‘messy’ technology (from the perspective of reagent and waste
sludge handling), but is mature and well proven in practice. One such installation, at the
Palo Verde Nuclear Generating Station, is the largest lime softening operation in Arizona.
Three proprietary technologies were discussed for concentrate reduction. These include the
DewVaporation and VSEP technologies discussed earlier and High Efficiency Reverse
Osmosis (HERO). HERO uses proprietary methods to remove the hardness, so a second RO
can process the concentrate from the first RO unit.
11
VII. Regional-scale Concentrate Disposal
By the year 2020, an estimated 11 MGD of concentrate will be produced in the greater
Phoenix area. This concentrate must be managed in an environmentally friendly manner.
During the workshop, a number of ideas were discussed in which several entities would
work together using large engineered systems to manage concentrate. Although the following
ideas were discussed there was no consensus on which would be practical and who would
partner in any particular project.
CASI – The Central Arizona Salinity Interceptor was the recommendation found by the
Tucson RO study as the best alternative for concentrate disposal for a large RO facility
(Reverse Osmosis Treatment of Central Arizona Project Water for the City of Tucson:
Appraisal Evaluation, Bureau of Reclamation, Desal R&D Report No. 36, January 2004,
Revision 1). It consists of a pipeline or canal from Tucson and possibly Phoenix to the
Sea of Cortez. The capital cost was estimated between $86M and $189M in 2004
without considering cost overruns. The Arizona Department of Water Resources opposed
the idea because it removed large amounts of water (in the form of concentrate) from
Arizona. Issues of national sovereignty, and the export of waste are also important
considerations. On the other hand, this option would move the salts to the sea as
occurred naturally in pre-dam times. Alternatively, it has been suggested that the
concentrate be pumped to the Salton Sea to reduce its salinity. In either case, expenses
could be recouped from future concentrate producers, who would pay a premium to
discharge their concentrate into the interceptor.
Palo Verde Nuclear Generating Station (PVNGS) Discharge – This strategy puts
the concentrate into the PVNGS cooling water pipeline. This is a large diameter
pipeline supplying effluent from the Phoenix 91st Avenue WWTP to the PVNGS.
Advocates of this idea stated their belief that PVNGS has the expertise and capacity to
handle the additional salt load. Using lime softening, RO, and brine concentrators
PVNGS would implement a ZLD using excess power capacity when available. The entities
contributing concentrate into the pipeline would pay for the additional operational costs
by PVNGS. However, Arizona Public Service, the operator of PVNGS, has stated that
they do not want to be the brine receptor of Arizona.
Gila River Salt Marsh – In this strategy the brackish waterlogged area downstream of
Tres Rios Wetlands would receive concentrate from across the Phoenix greater metropolitan area. Before the concentrate is released to the Gila River it would go through a
constructed wetlands, which would remove heavy metals, selenium and other nonenvironmentally friendly ions. The flow in the river would mix with the fresh water flow
exiting the current Tres Rios Wetlands and create a salt wetlands. The current TDS of
the Gila River in this area ranges from 2000 mg/L to 4000 mg/L TDS. Approximately
80% of the existing vegetation is tamarisk, and additional halophytes could be planted.
Periodic floods down the Gila and Salt Rivers would flush the salt build-up to the ocean,
preventing the marsh from salting itself out.
Discharge onto the Barry Goldwater Air Force Range – This idea was not fully developed but would rely on cheap land available on the range for either overland flow
application or rapid infiltration. Another possibility would be construction of evaporation
12
ponds on the range. Advantages of this proposal are the availability of large areas of
cheap federal land that would not significantly impair the area’s primary use as a target
range for military ordinance.
Discharge into Sacaton Open Pit – The idea was advanced to discharge concentrated
brines into the abandoned Asarco Sacaton Pit just north of Casa Grande. This very large
open pit copper mine has been closed for several decades and during that period there
has been little if any groundwater influx, despite much of the pit lying below the water
table elevation that exists in the alluvium surrounding the bedrock area containing the
pit. This suggests the bedrock matrix into which the pit was excavated is highly impermeable and could naturally prevent migration of brines to an aquifer if used for disposal
purposes. The pit could be used as a combined evaporation pond/final disposal site for
brine wastes from both the Phoenix and Tucson areas.
Workshop participants generally felt that all of the above regional-scale disposal options
should be more thoroughly examined to better identify and analyze advantages, disadvantages,
ballpark costs, and technical, institutional and regulatory considerations.
Tres Rios Wetlands
13
VIII. Summary and Roadmap
It is clear there is not currently, nor likely to be in the future, a single technological
solution to achieving high water recovery from inland desalination. Likewise there is no single
identifiable means of handling the very large volumes of desalination residuals that will be
generated and must be disposed of. It is expected that high efficiency desalination will
require a hybrid treatment train consisting likely of pre- and post-membrane treatment
processes. This treatment train will operate in an environment in which physical, regulatory,
and economic measures are in place which minimize the total salt flux into the region and
in which a moderate to large-scale strategy is operated for final residuals disposal.
Considerable research and evaluation work is still needed to bring such a holistic system
from the conceptual to the implemention stage. Microfiltration and slow sand filtration to
control membrane fouling require additional evaluation under the specific conditions likely
to be met in Arizona or similar regions. Ion exchange, chemical precipitation and nanofiltration individually and possibly in combination show promise for significantly delaying
membrane scaling, but have not reached the stage of understanding or field testing to
achieve widespread implementation. Various post-treatment options also are promising,
including VSEP, DewVaporation, and selective precipitation combined with second stage
RO. However, none of these options have yet been sufficiently evaluated nor developed to
be considered for full-scale implementation without considerable project-specific testing.
Finally, although a number of different large-scale residuals disposal strategies have been
proposed and evaluated to varying degrees, there are none that do not have considerable
associated drawbacks or uncertainties. The Central Arizona Salinity Study group is continuing
to evaluate some of these options and endeavoring to find additional alternatives. This
effort must continue if a regional level residual disposal strategy is to be found.
There are many feasible combinations of unit operations for membrane treatment of
moderately saline water. Maximizing the overall performance of the treatment train in terms
of multiple, simultaneous objectives such as water recovery, waste minimization, economics,
resilience, and public acceptance is the real goal of any desalination project. Consequently,
each application must be viewed from a systems-level perspective that addresses the interrelationships between the different, possible, single processes (e.g., lime softening, slow
sand filtration, ion exchange, VSEP, microfiltration, capacitative deionization,
DewVaporation) in the context of the available energy resources, local costs, and specific
project constraints before a decision can be made as to the best approach. In the end, the
optimum strategy almost certainly will be an integrated, multi-process approach dependent
on particular project conditions and the overall desalination approach selected for the solution. Although not addressed in detail in this workshop, energy costs and the trajectory of
future costs in interplay with alternative energy sources, are critically important and must be
included in the equation.
Both short- and long-term research and development needs are evident from the
workshop discussions. Perhaps foremost among these is the need for a regionally focused
desalination R&D center that could consolidate the technical expertise, pilot-testing
capabilities, transfer of evolving technologies, and the core required research thrusts into an
organizational structure that would be accessible to the various stakeholders. The multi-unit
nature of the optimum treatment scheme and the lack of systems level modeling programs
indicate that pilot studies will be required for most proposed schemes. At the same time
14
such studies are often too expensive and difficult to perform by many utilities, and few
entities are capable of undertaking such studies. The need for systems level integration of
technologies in response to case specific conditions; pilot-scale assessment of likely process
configurations; and an accessible and affordable locus for research, testing and evaluation
suggests that there is a state (and arid region) infrastructure research need for such a
desalination facility.
In summary, the following projects are recommended as means to address short-term
desalination challenges and provide relatively rapid benefits. Who would/should fund,
oversee and conduct these projects was only scantly discussed. Ideally, they would be
administered, coordinated and staged under a regional desalination R&D center as advocated above, however they could also be administered individually through the various current, dispersed funding and research oversight groups in the region (e.g., AWI, BOR, TRIF,
individual utilities, AwwaRF).
• Evaluate implementation and cost of various pre-treatment strategies to allow greater
RO water recovery before the onset of scaling. These strategies should specifically utilize
waters from or representative of the CAP canal water, Salt River water, or other Arizona
source waters with average TDS greater than 600 ppm. The pre-treatment strategies of
immediate interest are:
• Ion exchange
• Nanofiltration softening (with and without microfiltration particle removal)
• Chemical softening
• Combined and hybrid versions of these approaches
• Pursue development of selective precipitation as a means for beneficial product recovery
from the brine residuals particular to Arizona waters that are likely to be subject to RO
treatment
• Undertake a preliminary feasibility, impact and cost study of the potential for development of a salt marsh in the Gila River channel south of Phoenix
• Evaluate the relative efficacies of the various commercially available anti-scalants and
identify a standard testing protocol by which these and future products in the market
can be comparatively rated
• Evaluate, at the scoping level, the potential for use of the Sacaton Pit and other regionalscale options for waste brine disposal
• Evaluate the efficiency, shortcomings and energy costs associated with direct implementation of VSEP™ technology for increasing water recovery from an RO process applied
in typical inland, arid west conditions.
• Investigate feasible strategies for and means of encouraging centralized ion exchange
media regeneration (vis-à-vis in-home regenerative water softeners) to minimize water
consumption and brine disposal impacts.
• Investigate regulatory and economic means to discourage the use of in-home regenerative
water softeners.
15
• Develop the capability for case specific, systems-level evaluation of complete process
schemes for desalination and brine management, including development of the necessary
modeling tools and metrics.
• Develop and/or identify halophytes with commercial value that can be grown in lined
facilities irrigated with RO brines.
The following projects are recommended to address long-term desalination challenges and provide potentially
large, yet longer horizon, payback on
investment.
• Establish a program by which new
membranes, membrane treatment
related products, and alternative
desalination technologies are independently and comparatively tested
under the conditions and constraints relevant to the arid west
desalination situation.
• Establish a combined basic and
applied research program to
improve the efficiency and
decrease the cost of capacitative
deionization.
• Assess future wasteloads and waste stream through analysis of long-term growth patterns,
degree of incorporation of reuse in new development and sub-divisions, and household-level water harvesting that will affect wastewater volumes, and thereby, wastewater
salinities.
• If the results of the short-term scoping study recommended above are positive, undertake detailed cost and feasibilities studies of using the Sacaton Pit and/or a Gila River
Salt Marsh as brine disposal options.
• Conduct combined basic and applied research on the potential for incorporating novel
membrane functionalities, such as electromagnetic membranes, biomimetic membranes,
and ion-specific electrodialysis membranes
• If appropriate circumstances arise where brine generation occurs proximate to a waste
or cheap heat source (e.g., power plant cooling water stream), refine and retest the
DewVaporation technology based on the findings of the study done by the City of
Phoenix and U.S. Bureau of Reclamation.
16
IX. Afterword
As part of the Arizona Water Institute grant which funded this workshop, Ashley Smith
of the University of Arizona completed an annotated review of recent, key publications
pertinent to the state of the art regarding inland desalination. This literature review was not
meant to be exhaustive as there are well over 1,000 papers on desalination in peer-reviewed
journals alone and many more in conference proceedings, industry journals, agency project
reports and other publicly available resources. The literature review is included herein as
Appendix 3 to give workshop attendees and other interested persons a point of entry into
the literature on a particular aspect of the subject, with the intent that additional in-depth
information could be accessed using the citations and ideas within the point of entry literature.
Scottsdale Water Treatment Facility
17
Appendix 1
Salinity Workshop Agenda
18
Appendix 2
Salinity Workshop Attendees
Adams, Angela
Alexander, Kevin
Androwski, James
Arnold, Bob
Baygents, Jim
Beckman, James
Benemelis, Perry
Biggs, Jeff
Boyd, Basil
Boyer, John
Brandy, Matt
Brown, Kurt
Campbell, Marc
Cardoza, Mark
Chang, Yu Jung
Chapman, Michelle
Chavez, Laura
Chinn, Tim
Choi, Chi Chi
Cullom, Chuck
Day, Henry
DosSantos, Placido
Drago, Len
Ela, Wendell
Engle, Tee
Fox, Peter
Franks, Rich
Goldman, Fred
Graf, Chuck
Green, Jerry
Gremillion, Paul
Gulizia, Lynne
Haymore, Tonya
He, Charlie
Holler, Eric
Johnson, Bruce
Kelso, Brandy
Kinshella, Paul
Kottenstette, Richard
LaMartina, Karen
Lant, Tim
Madole, Jim
Mansfield, David
Mardam, Tony
Marra, Ralph
Mitchell, Stuart
Moody, Chuck
Newell, Pete
Norris, Mike
Peterson, Joel
Poulson, Tom
Rayhel, John
Riley, Jim
Robertson, Michele
Roth, Glen
Russell, David
Russell, Jerry
Sacks, Richard
Scott, Chris
Smith, Ashley
Terrey, Andy
Theron, Jan
Thomas, Harold
Thomure, Tim
Wallace, Greg
U.S. Bureau of Reclamation
Separation Processes, Inc.
Northern Arizona University
University of Arizona
University of Arizona
Arizona State University
AZ Dept of Water Resources
Tucson Water
City of Tempe
Arizona Public Service
Intel Corporation
U.S. Bureau of Reclamation
Salt River Project
American Water Company
HDR
U.S. Bureau of Reclamation
Brown & Caldwell
HDR
Arizona State University
Central Arizona Project
Arizona Public Service
Arizona Water Institute
Intel Corporation
University of Arizona
Arizona State University
Arizona State University
Hydranautics
Kennedy/Jenks Consultants
Arizona Water Institute
CDM
Northern Arizona University
Toray Membrane America
Arizona Water Institute
19
Carollo Engineers
U.S. Bureau of Reclamation
Tucson Water
City of Phoenix
City of Phoenix
Sandia National Laboratory
Tucson Water
Arizona State University
E. L. Montgomery & Assoc
City of Scottsdale
CH2M Hill
Tucson Water
American Water Chemicals
U.S. Bureau of Reclamation
HDR
U.S. Bureau of Reclamation
E. L. Montgomery & Assoc
U.S. Bureau of Reclamation
Intel Corporation
University of Arizona
Az Dept of Environ Quality
Damon S. Williams & Assoc
Professional Water Tech
Carollo Engineers
City of Scottsdale
University of Arizona
University of Arizona
City of Phoenix
Northern Arizona University
Brown & Caldwell
Tucson Water
E. L. Montgomery & Assoc
Appendix 3
20
21
22
23
24
25
26
27