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, 2007Tempe, Arizona

Sponsored byArizona Water Institute

U.S. Bureau of Reclamation

August, 2008

Executive CommitteeWendell Ela*, University of ArizonaChuck Graf, Arizona Water InstituteTom Poulson, U.S. Bureau of ReclamationJim Baygents, University of ArizonaJan Theron, Northern Arizona UniversityPeter Fox, Arizona State UniversityChris Scott, University of Arizona

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

Supporting SponsorsBrown and CaldwellErrol L. Montgomery & AssociatesDamon S. Williams & Associates

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

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I. Introduction

Central Arizona Project (CAP) water originating in the Colorado River is a primarysource of potable and irrigation water in Arizona. This water source, along with the SaltRiver in central Arizona, brings over 1,000,000 tons of salt each year into central andsouthern Arizona, where it accumulates as the carrier water is used. These salts go primarilyinto the region’s soils and the effluent discharged from wastewater treatment plants. Both ofthese temporary repositories ultimately feed the salt into the groundwater, which is a criticalwater supply for the region.

On August 6, 2007, a workshop was convened in Tempe, Arizona to focus on identifyingthe state-of-the art technologies for membrane removal of salt from the region’s watersources while minimizing the attendant water loss and the environmental impact of thetreatment residuals disposal. The workshop focused on identifying and prioritizing the technicalhurdles that must be overcome to improve efficiency and economic viability of treatmentprocesses, and establishing a practical roadmap forward for achieving sustainable, viabledesalination of inland, moderate salinity waters including wastewaters. These workshopobjectives were examined in the narrow context of source water salinities ranging from CAPwater with a total dissolved solids of about 700 mg/l (milligrams per liter) to brackish waterscontaining up to 10,000 mg/l of TDS. Although ocean disposal is a potential option in thedistant 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 waterdemand requires full utilization of the State’s allotment of CAP water as well as its availablesurface water sources, including the Salt and Verde River resources. The salinity of CAP andSalt River water, by far the bulk of the surface supply, exceeds the Environmental ProtectionAgency’s Secondary Standard of 500 mg/L for total dissolved solids (TDS). Demand greaterthan these surface and historical groundwater supplies can only be met by wastewaterreclamation and tapping into and treating the region’s brackish water resources. The salinityof 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 significantportion of the region’s water resources.

To illustrate, water demand in the Tucson Active Management Area (TAMA) is estimatedat 400,000 acre-feet per year. However, the rate of natural groundwater replenishment isonly about 60,000 acre-feet per year. The unavoidable shift from ground water to CAPwater as the primary regional water resource will have significant water quality implications.The average TDS concentration in delivered ground water has historically been about 260mg/l. TDS levels at the Tucson terminus of the CAP canal are greater than 700 mg/l andlikely to rise. Full utilization of CAP water allocations will bring 200,000 tons of salt intothe TAMA each year. Without some form of salt management, the majority of this salt willremain, 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 rateof about 1.1 million tons annually. This is not a reversible situation and cannot meet thepublic’s concept of water supply sustainability.

The strategy for salt management in central and southern Arizona will almost certainlyconsist of a combination of reverse osmosis (RO) treatment and benign brine disposal.

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Water recovery (the percentage of treated water that is collected as permeate) is of excep-tional importance to the community. Although it is difficult to assign a marginal benefit towater supply augmentation through recovery enhancement, Tucson residents have shown awillingness to pay $3 per 1,000 gallons or about $1,000 per acre foot for the highest incre-ment in the tiered rate structure for delivered residential water. Using this value to illustratethe benefits of brine minimization, if the entire TAMA CAP entitlement is RO treated tomanage local salt levels in ground water, each 1% increase in recovery efficiency wouldprovide a benefit to the community of greater than $2M per year. Water recovery from ROtreatment of CAP water is currently limited to about 80%. If methods could be developedto 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 inother Arizona desalination projects.

In response to this critical challenge to Arizona, the Arizona Water Institute and theBureau of Reclamation convened a technical workshop entitled “Improving SalinityManagement and Desalination Technology for Brackish Water Resources in the Arid West”on August 6, 2007, in Tempe, Arizona, at the Tempe Mission Palms Hotel. Workshopinvitees from governmental agencies, water utilities, consulting firms, academia, and otherstakeholder 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 representativesof the key stakeholder groups on ways to address the inland desalination and concentratemanagement challenges and on identification of the critical research hurdles that must beovercome for implementation of viable strategies.

The closest to a unanimous conclusion reached by the participants in the Workshop wasthat ‘there is no silver bullet’. Although there is no single, one-size-fits-all technologicalsolution to the inland salinity management problem, a range of options is available that canbe applied in various combinations to meet case-specific conditions. A sustainable, economi-cally viable and technically feasible solution will consist of a number of different mutuallyreinforcing technologies and efforts, the particular nature of which will vary according tothe size, location and circumstances of the water supplier and other factors. Within theoverall 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 needand 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, and5) 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, allissues raised and points made in the workshop do not neatly fall into only one of the areasenumerated. Consequently, a best fit for each topic was attempted, but some topics areaddressed in more than one category with some overlap.

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II. Source Control

Short of desalting surface water before it enters the region, such as by implementing ROtreatment at the mouth of the CAP canal, a significant mass of salt will enter the regionalong with the surface water supplying Arizona’s water needs. However, the water conveyedby the CAP canal is not the only source of imported salt. In the Phoenix region a significantmass of salt is contributed by the Salt and, to a lesser extent, the Verde River. In addition, alarge and rapidly increasing mass of salt is introduced because of the use of ion exchangehome water softening devices. Although the devices remove multivalent cations, the totalsalinity (in terms of equivalents per volume) of the product water is not decreased and, inaddition, a highly saline brine is generated. Because this salt addition occurs after treatmentby centralized water treatment facilities and at the point of use of the drinking water, theextra salt load is conveyed to the sewer and seen as a higher wastewater salinity than standardcalculations predict. This load has potential negative impacts on the riparian ecosystemsupported by the wastewater discharges, on the uses of the reclaimed water, on the WWTPprocesses 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-relatedhome appliances, such as water heaters, irons, swamp coolers and coffee makers, due toprecipitative scaling by hardness-causing cations. As the salinity and hardness of drinkingwater supplies increase, it is expected that watersoftener use will also increase. Two approacheswere discussed to counteract this trend: imple-ment system-wide water softening to eliminate theneed for individual point-of-use water softeners ordiscourage their use through disincentives or othermeasures. These approaches are not mutuallyexclusive and could be implemented to somedegree in tandem.

The former approach is typically accomplishedby centralized lime softening (or variations of thisprocess), although nanofiltration or reverseosmosis processes may be used if other waterquality considerations suggest they may provideadditional benefits. Alternatively, implementationof centralized capture and regeneration of spention exchange softening resins (for instance via aprovider switch-out program) would allowimproved residual management and enhancedwater recovery by incorporation of processesthat are not amenable to point-of-use homesoftening (e.g., brine recycle, rinse/backwashrecovery, precipitative softening of brines).

As to the latter approach, workshop participantssuggested several means of discouraging home

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water softener use. These include a tax/surcharge on softener salt, a surcharge for softenerinstallation in new homes, or restrictions on the use of self regenerating softeners. This latterstrategy would force regeneration of spent softener resins at centralized facilities where thehigh salinity regenerant residual could be more easily treated or managed to minimizenegative impacts. Although the practicality of implementation of these (dis)incentives towater softener use was not discussed, there was considerable interest voiced in furtherinvestigating such means to discourage use.

A final potential approach to decreasing the salt impact of home water softeners is theuse of capacitive deionization devices. This technology is still not fully developed and com-mercialized, 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 loadsince softening is driven by an input of electrical energy rather than monovalent ions. Theprocess produces an ion-reduced finished water and a brine concentrate, but with no netincrease 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 membranematerial (normally Aerogel) and the low ion site capacity of the membranes.

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III. RO Pre-Treatment

Reverse osmosis pre-treatment generally includes a variety of technologies and tech-niques to decrease both fouling and scaling of the membranes. Fouling occurs due to thebuild-up of particles (organic and inorganic, colloidal and particulate) that are in the feedwater and deposit on the membrane surface. Fouling is usually most pronounced on themembranes in the front-end stages of an RO array. In contrast, scaling occurs due to theprecipitation of supersaturated salts on the membrane surface. The degree of precipitationincreases as the concentrate stream progressively moves through the sequential RO elementsand increases in salinity. Scaling is a problem typically in the back-end stages of an ROarray where the reject stream’s salt concentration is near its maximum.

Conventional water treatmentprocesses (e.g., sedimentation,coagulation/flocculation, filtration)as well as other processes, suchas slow sand filtration, micro-and ultrafiltration, and activatedcarbon filtration, can be effectiveat removing fouling componentsof the raw water. Micro- andultrafiltration are becomingmore widely used to removefoulants ahead of RO becausethey have a relatively smallinstallation footprint, are reliableand well field tested, and theirprice is becoming competitivewith alternative processes. Formany Arizona utilities, availabilityof land is not a primary constraint, so slow sand filtration (SSF) may be an option to lessenfeed water fouling indices. The Water Quality Improvement Center at Yuma, initiated bythe US Bureau of Reclamation, the National Water Research Institute, the U.S. Army andother research institutions, has studied pre-treatment by slow sand filtration and recommendsit for consideration, particularly for rural installations. There is the possibility of improvingSSF removal of colloidal solids and dissolved organics by developing better engineered filtermedia and by sand amendment with such things as granular activated carbon and ironparticles. Improved engineering to control schmutzedecke development and various periodiccleaning methods should also be pursued to optimize SSF performance.

Pre-treatment technologies and techniques to control or delay the onset of scalingreceived considerable discussion. These RO pre-treatment processes can be classified intotwo 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 ofprocesses acts by removing the limiting salts prior to membrane application. The secondgroup includes processes that increase the solubility of these sparingly soluble salts. In eithercase, the outcome is increased water recovery prior to the onset of scaling.

Membrane scaling of supersaturated salts (BaSO4)

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The salts most commonly responsible for scaling are calcium carbonate (CaCO3), bariumsulfate (BaSO4), silica (SiO2), calcium sulfate (CaSO4), calcium fluoride (CaF2), strontiumsulfate (SrSO4) and magnesium hydroxide (Mg(OH)2). The processes discussed by work-shop participants to remove these components from the feedwater prior to RO were 1) ionexchange and 2) precipitative processes, including softening. As to the first, cationic ionexchange resins are more commonly used than anionic resins, primarily because neithercarbonate, 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) tomaintain the regenerant brine, so there is a net increase in salt in the system (although it isconfined to the waste brine stream).

More commonly, precipitative removal of the divalent cations is used to control scalingsalts 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 compo-sition and the cations targeted for removal. A number of workshop participants commentedon the need to improve the selectivity and efficiency of precipitative processes by suchthings as polymer addition (to increase floc settling rates and sludge dewatering), specificion 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 ofrecovery of reusable and potentially saleable products from the sludge residual (althoughthis discussion focused primarily on selective precipitation as a post-RO treatment processand will be covered in that section of this report).

The alternative to removing the limiting salts prior to RO is to manipulate the feedwatercomposition to increase the solubility of the limiting salts. Sulfuric acid is often added tolower the feed pH, which decreases both the carbonate concentration by converting bicar-bonate to volatile carbon dioxide and the hydroxide concentration. However, if sulfuricacid addition is not prescribed because sulfate salts are limiting (e.g., BaSO4), hydrochloricacid is often used. A disinfectant must also be added to feedwater to prevent microbialgrowth in the RO system. Polyamide (PA) membranes have largely replaced celluloseacetate (CA) membranes because of their greater chemical and physical stability, greaterwater fluxes and salt rejections and resistance to bacterial degradation. However PAmembranes are intolerant of free chlorine, so disinfection in these systems is typically bycombined chlorine (chloramines). Considerable work has been performed to improve theoxidant tolerance of polyamide membranes in particular, and other membrane materials ingeneral. There are a wide variety of proprietary antiscalant additives, which allow thesupersaturation (increase in solubility) of the limiting salts to varying degrees. Inorganicphosphate additives have been largely replaced by organic polymer additives. These addi-tives allow supersaturation factors of from 2 to several orders of magnitude depending onthe type of limiting salt, the particular additive used, and the other constituents in thefeedwater. (For instance, it has been found that water with high iron can greatly decrease theeffectiveness of many antiscalants.) Anecdotes from various workshop participants suggestthat there is a wide range in cost, ease of handling, and performance of the available antis-calants and that with the growth of this market, there is regular introduction of new productsonto the market. There are few independent, comprehensive, comparative studies of theeffectiveness of the different commercial antiscalant formulations and this was earmarkedby workshop participants as an area where research effort should be directed.

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IV. Membranes

As noted by workshop participants, the formulation and fabrication of the actual ROmembranes has been done primarily in the private sector and under proprietary restrictionsthat limit comparative research on the various products. Additionally, development of mem-brane materials is a relatively specialized technology and it was considered that only a fewcompanies have the expertise. This is not expected to change in the near future, so therewas general agreement that improvement of membrane materials should not be a primaryfocus of public or academic research and policy efforts. However, there was discussion of howevaluation 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 ofmembrane ‘additions’ might be worthwhile areas of investigation by the research community.In addition, it was suggested that a long-term collaboration between academic and industryresearchers in understanding the flow dynamics near membrane surfaces (such as influencedby spacer design, membrane surface roughness, and inlet/outlet structures) could potentiallylead to significant improvements in fouling and scaling resistance, and membrane hydraulicflux, without substantial material changes to the membranes themselves.

A large number of membrane additions that could improve RO performance werediscussed, although in nearly all cases these were identified as longer-term research andimplementation developments that were not likely to impact RO applications within theshort-term (5 year) horizon. Membrane additions of interest included electromagneticmembranes which separate ions by their charge density; nanoparticle-embedded membranesthat create reaction as well as separation of water constituents; biomimetic membranes thatmimic the lipid bilayer/protein channel functionality of natural cell membranes; and targetelectrodialysis membranes for selective ion separations.

A presentation during the morning session of the workshop, as well as discussion duringthe breakout sessions, highlighted the community’s interest in forward osmosis and questionsregarding 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 feedsolution through the membrane into the draw solution not due to hydrostatic pressure (asin RO), but due to osmotic pressure. For this to occur, the draw solution must have a higherosmotic pressure than the feed solution. However, if the dissolved species in the drawsolution can be readily separated from it after FO, then desalinated water may ultimatelybe produced. The feasibility of most present FO configurations depends on using a drawsolution 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 dioxide-ammonia mixture in the draw solution which can be volatilized at relatively low tempera-tures (~60°C) to effect final water purification and recycle of the solutes. FO has the advan-tage over RO of working at near zero hydraulic pressure, having very high rejection formost solutes, and avoiding much of the fouling associated with pressure driven systems. FOpermeation rates are much lower than for RO and, at present, there are no commerciallyavailable processes. Advancement of FO will depend largely on development of membranesspecifically engineered for FO application (rather than using RO designed membranes), FOmembrane reactor configurations that efficiently incorporate draw solute separation andrecycle processes, and improvement of draw solute formulations to increase draw osmoticpressure and solute separation.

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V. Post-treatment

The type of concentrate treatment, if any, implemented following reverse osmosis largelywill be dictated by the options for and cost of final residuals disposal. If chemical componentsof the concentrate stream are economically recoverable and useable, then not only is aproduct generated, but the concentrate salinity is decreased and the options for beneficialuse of the remaining aqueous stream increased. The ionic composition of most concentratessuggests that the possible recoverable minerals will be mainly chlorides, carbonates andsulfates of major cations such as sodium, potassium, magnesium and calcium. These mineralswill be either in pure or mixed forms. Since the removal of scale-forming calcium andmagnesium is often necessary to process concentrates, the production of calcium andmagnesium 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 followingapplications.• 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 fromselective precipitation and gypsum from crystallization. Calcite used as a whitening agentfor paper production has a market value of $300/ton. Magnesium hydroxide is feedstockfor the production of magnesium, and gypsum is used for the production of drywall. Theevaporation of concentrates may yield sodium sulfate if the calcium and magnesium arepreviously removed as mixed salts. Products may also be recovered from concentrates usingelectrolytic processes. Potential high value products include chlorine, bromine, dilute acidsand bases, and metals. The production of products from concentrates has not been the goalof 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 tosustainable salt management. Selective precipitation of commercially valuable solids byengineered chemical addition to cause supersaturation of selected solids has been success-fully used in Australia and South Africa, but so far has not been demonstrated in the U.S.

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A relatively recent technology marketed for post-treatment of RO concentrates is theproprietary vibratory shear-enhanced process (VSEP®). The VSEP technology was developedand is marketed by New Logic Research and has been used in manufacturing for severalyears on a small scale. The principle of operation is based on energy input (vibration) to ROmembranes to prevent particle attachment to the membranes and perturb and mix thechemical boundary layer near the membrane surface. Both activities minimize membranefouling in very concentrated chemical mixtures or suspensions. VSEP technology offerspotential advantages over alternative concentrate management options. These includemoderate environmental footprint (land, energy, noise), competitive capital cost, and poten-tially very high water recovery factors. However, VSEP may significantly increase operationand maintenance costs. For this reason, VSEP needs to be tested on a larger scale wherethe economics, reliability, versatility and other properties can be evaluated as they apply tomunicipal use.

DewVaporation is an evaporative-condensation process, which produces a low TDSdistillate and a high TDS concentrate from impaired water. The poor quality feed waterruns down one side of a heat transfer wall inside the DewVaporation tower, while air flowsupward, evaporating some of the water. The rest of the water with the salts flows down andout 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 theair 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 sideassisting the evaporation process. The condensing water is very low in TDS and is collectedat the bottom of the tower.

Dr. James Beckman, the Bureau of Reclamation, and the City of Phoenix operated 25DewVaporation Towers over the winter of 2006-2007 at the 23rd Ave WWTP. Data wascollected on energy use, TDS of distillate, TDS of concentrate, production rate and otherinformation. A final report of the project has been completed and can be accessed on theBureau 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, and3. 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), and2. 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 betterengineering, such as a reliable steam source (ideally a waste heat source), improved pump-ing equipment, tower base redesign to minimize catch basin leakage and energy loss, anddiagonal fluid flow paths to increase distillate production. The DewVaporation technologypotentially could be applied in a number of scenarios, including remote, low demand siteswith impaired well water; at the end of a Zero Liquid Discharge (ZLD) facility; and toreplace a crystallizer, which would be receiving low flow-high TDS water.

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VI. Local Management and Disposal of Residuals and Concentrate

Concentrate disposal alternatives are relatively well known. Because no single technologyworks for every concentrate management case, the objective is to put the right ones togetherfor any particular disposal problem considering the volume of concentrate, amount of landavailable, cost, and other considerations such as social and environmental impacts.Concentrate management could be accomplished in a large centrally located facility operatedby several entities in common or as dispersed units at the site of each individual concentrate-generating entity.

The two most common methods of concentrate disposal in Arizona are evaporationponds and sewer disposal. Both methods are acceptable on the small scale but as larger ROfacilities are built to meet the demand for potable water, these disposal methods havesevere drawbacks. The major costs associated with evaporation ponds are land and liners.Methods to enhance evaporation typically increase the contact area between the air andwater by spraying or pumping, and the efficiency of these methods may depend on localwind conditions. Although wind- and spray-aided evaporation processes reduce therequired pond size, problems have been reported with the design and long-term operationof spray nozzles and possible transport of salt particles into downwind, off-site locations. Bycoupling evaporation ponds in series, low economic value products such as sodium sulfatemay be produced. Solar gradient ponds, which are relatively deep brine-containing pondsdesigned to produce energy, are also an option. Solar gradient ponds are not evaporationponds, but the basic design is similar to evaporation ponds. Therefore, a regional saltprocessing facility could use some ponds for energy production while the remaining pondsare used for evaporation. Some workshop participants felt that solar ponds were difficult tomaintain and the energy recovery small compared to those difficulties.

Concentrate also may be disposed of by land application. This is currently done in someinstances for dust abatement, although this practice is not likely to be feasible for large-scale operations because of both the volume of concentrate requiring disposal and issuesresulting 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 IndianCommunity plans to mix the concentrate produced from their RO facility with the treatedwastewater coming out of the Lone Butte WWTP. This reclaimed water/concentrate mixwould be used to irrigate crops on Community land. Irrigation of halophytes is an analogueto evaporation ponds to reduce concentrate volume. The evapotranspiration rate for halo-phyte irrigation is seldom higher than the unaugmented evaporation rate from a pond. Forhalophyte irrigation to be economical, irrigators must produce a commercial product orreplace a beneficial, non-salt tolerant species (e.g., turf grass). Greater use of halophytes isdependent on demonstration that long-term aquifer contamination or soil fertility degradationwill 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, thentransportation must be considered. Commonly, this is by discharge to sewer. However, thispractice may cause (and has caused) problems with the ability of the treated wastewater to bereused 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 centralfacility. Smaller PVC pipe could be placed inside existing sewer lines to carry concentrate toanother location. This would be contingent on the existing sewer main having additional

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capacity to handle the added pipe. Alternatively, the existing storm drain system could beused 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 injectionwells, as classified under the U.S. EPA’s Underground Injection Control (UIC) program, areused in Florida and will be used in El Paso, Texas. Under the federal program, it is commonlyassumed that the receiving water must be at least 10,000 mg/l TDS and be isolated frompotable aquifers. It has been suggested that the Arizona concentrate be injected into thesalt dome caverns being formed in the Luke Salt Body by solution mining by Morton SaltCompany. However, the annual cavern volume increase would only handle about 66,000gallons a day of concentrate. Furthermore, concerns about potential contamination ofMorton’s food-grade product with undesirable ions in the injected concentrate almostcertainly rule out this option. Alternatively, artificial recharge into poor quality aquiferscould be used for concentrate disposal. This may be a way of saving the water associatedwith concentrate until technology catches up to inexpensively remove the salts. The potentialfor suitable locations in Arizona has not been sufficiently investigated. In Arizona, suchwells also are regulated and must be issued a permit under the State’s Aquifer ProtectionPermit (APP) program. However, under Arizona’s APP program, a number of hurdles mustbe overcome, including the requirement for the Arizona Department of EnvironmentalQuality (ADEQ) to reclassify the relevant aquifer from drinking water use before concentrateinjection into such repositories could be permitted; ADEQ has never reclassified an aquifer.Nevertheless, some workshop participants suggested that an updated study of potentialdeep 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 volumerequiring disposal, although it remains an open question as to what degree economies ofscale apply to particular disposal strategies. The workshop participants considered a numberof concentrate volume reduction options – both non-patented and proprietary. Brineconcentrators are an expensive, yet viable, technology. They are used primarily at powerplants 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 ina zero liquid discharge (ZLD) system. They can be relatively high maintenance due to thecorrosivity of very high TDS levels of the fluids. Although freeze crystallization is used insome industrial settings, it would not be applicable on a municipal water production scaledue to high energy demand and installation complexity. Lime softening or other precipitativevariants may be used as a post-treatment prior to second stage RO, analogous to pre-treat-ment applications of the technology. The softening objective is to remove the limiting saltions (e.g., Ca and Mg) which would otherwise scale the secondary RO membranes. This isconsidered a relatively ‘messy’ technology (from the perspective of reagent and wastesludge handling), but is mature and well proven in practice. One such installation, at thePalo Verde Nuclear Generating Station, is the largest lime softening operation in Arizona.Three proprietary technologies were discussed for concentrate reduction. These include theDewVaporation and VSEP technologies discussed earlier and High Efficiency ReverseOsmosis (HERO). HERO uses proprietary methods to remove the hardness, so a second ROcan process the concentrate from the first RO unit.

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VII. Regional-scale Concentrate Disposal

By the year 2020, an estimated 11 MGD of concentrate will be produced in the greaterPhoenix area. This concentrate must be managed in an environmentally friendly manner.During the workshop, a number of ideas were discussed in which several entities wouldwork together using large engineered systems to manage concentrate. Although the followingideas were discussed there was no consensus on which would be practical and who wouldpartner in any particular project.

CASI – The Central Arizona Salinity Interceptor was the recommendation found by theTucson 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 theSea of Cortez. The capital cost was estimated between $86M and $189M in 2004without considering cost overruns. The Arizona Department of Water Resources opposedthe idea because it removed large amounts of water (in the form of concentrate) fromArizona. Issues of national sovereignty, and the export of waste are also importantconsiderations. On the other hand, this option would move the salts to the sea asoccurred naturally in pre-dam times. Alternatively, it has been suggested that theconcentrate be pumped to the Salton Sea to reduce its salinity. In either case, expensescould be recouped from future concentrate producers, who would pay a premium todischarge their concentrate into the interceptor.

Palo Verde Nuclear Generating Station (PVNGS) Discharge – This strategy putsthe concentrate into the PVNGS cooling water pipeline. This is a large diameterpipeline 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 tohandle the additional salt load. Using lime softening, RO, and brine concentratorsPVNGS would implement a ZLD using excess power capacity when available. The entitiescontributing concentrate into the pipeline would pay for the additional operational costsby PVNGS. However, Arizona Public Service, the operator of PVNGS, has stated thatthey do not want to be the brine receptor of Arizona.

Gila River Salt Marsh – In this strategy the brackish waterlogged area downstream ofTres Rios Wetlands would receive concentrate from across the Phoenix greater metro-politan area. Before the concentrate is released to the Gila River it would go through aconstructed wetlands, which would remove heavy metals, selenium and other non-environmentally friendly ions. The flow in the river would mix with the fresh water flowexiting the current Tres Rios Wetlands and create a salt wetlands. The current TDS ofthe Gila River in this area ranges from 2000 mg/L to 4000 mg/L TDS. Approximately80% 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 devel-oped but would rely on cheap land available on the range for either overland flowapplication or rapid infiltration. Another possibility would be construction of evaporation

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ponds on the range. Advantages of this proposal are the availability of large areas ofcheap federal land that would not significantly impair the area’s primary use as a targetrange for military ordinance.

Discharge into Sacaton Open Pit – The idea was advanced to discharge concentratedbrines into the abandoned Asarco Sacaton Pit just north of Casa Grande. This very largeopen pit copper mine has been closed for several decades and during that period therehas been little if any groundwater influx, despite much of the pit lying below the watertable elevation that exists in the alluvium surrounding the bedrock area containing thepit. This suggests the bedrock matrix into which the pit was excavated is highly imper-meable and could naturally prevent migration of brines to an aquifer if used for disposalpurposes. The pit could be used as a combined evaporation pond/final disposal site forbrine wastes from both the Phoenix and Tucson areas.

Workshop participants generally felt that all of the above regional-scale disposal optionsshould be more thoroughly examined to better identify and analyze advantages, disadvantages,ballpark costs, and technical, institutional and regulatory considerations.

Tres Rios Wetlands

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VIII. Summary and Roadmap

It is clear there is not currently, nor likely to be in the future, a single technologicalsolution to achieving high water recovery from inland desalination. Likewise there is no singleidentifiable means of handling the very large volumes of desalination residuals that will begenerated and must be disposed of. It is expected that high efficiency desalination willrequire a hybrid treatment train consisting likely of pre- and post-membrane treatmentprocesses. 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 andin 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 systemfrom the conceptual to the implemention stage. Microfiltration and slow sand filtration tocontrol membrane fouling require additional evaluation under the specific conditions likelyto be met in Arizona or similar regions. Ion exchange, chemical precipitation and nanofil-tration individually and possibly in combination show promise for significantly delayingmembrane scaling, but have not reached the stage of understanding or field testing toachieve widespread implementation. Various post-treatment options also are promising,including VSEP, DewVaporation, and selective precipitation combined with second stageRO. However, none of these options have yet been sufficiently evaluated nor developed tobe considered for full-scale implementation without considerable project-specific testing.Finally, although a number of different large-scale residuals disposal strategies have beenproposed and evaluated to varying degrees, there are none that do not have considerableassociated drawbacks or uncertainties. The Central Arizona Salinity Study group is continuingto evaluate some of these options and endeavoring to find additional alternatives. Thiseffort must continue if a regional level residual disposal strategy is to be found.

There are many feasible combinations of unit operations for membrane treatment ofmoderately saline water. Maximizing the overall performance of the treatment train in termsof 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 inter-relationships between the different, possible, single processes (e.g., lime softening, slowsand filtration, ion exchange, VSEP, microfiltration, capacitative deionization,DewVaporation) in the context of the available energy resources, local costs, and specificproject constraints before a decision can be made as to the best approach. In the end, theoptimum strategy almost certainly will be an integrated, multi-process approach dependenton particular project conditions and the overall desalination approach selected for the solu-tion. Although not addressed in detail in this workshop, energy costs and the trajectory offuture costs in interplay with alternative energy sources, are critically important and must beincluded in the equation.

Both short- and long-term research and development needs are evident from theworkshop discussions. Perhaps foremost among these is the need for a regionally focuseddesalination R&D center that could consolidate the technical expertise, pilot-testingcapabilities, transfer of evolving technologies, and the core required research thrusts into anorganizational structure that would be accessible to the various stakeholders. The multi-unitnature of the optimum treatment scheme and the lack of systems level modeling programsindicate that pilot studies will be required for most proposed schemes. At the same time

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such studies are often too expensive and difficult to perform by many utilities, and fewentities are capable of undertaking such studies. The need for systems level integration oftechnologies in response to case specific conditions; pilot-scale assessment of likely processconfigurations; and an accessible and affordable locus for research, testing and evaluationsuggests that there is a state (and arid region) infrastructure research need for such adesalination facility.

In summary, the following projects are recommended as means to address short-termdesalination challenges and provide relatively rapid benefits. Who would/should fund,oversee and conduct these projects was only scantly discussed. Ideally, they would beadministered, coordinated and staged under a regional desalination R&D center as advo-cated above, however they could also be administered individually through the various cur-rent, 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 greaterRO water recovery before the onset of scaling. These strategies should specifically utilizewaters from or representative of the CAP canal water, Salt River water, or other Arizonasource waters with average TDS greater than 600 ppm. The pre-treatment strategies ofimmediate 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 recoveryfrom the brine residuals particular to Arizona waters that are likely to be subject to ROtreatment

• Undertake a preliminary feasibility, impact and cost study of the potential for develop-ment of a salt marsh in the Gila River channel south of Phoenix

• Evaluate the relative efficacies of the various commercially available anti-scalants andidentify a standard testing protocol by which these and future products in the marketcan be comparatively rated

• Evaluate, at the scoping level, the potential for use of the Sacaton Pit and other regional-scale options for waste brine disposal

• Evaluate the efficiency, shortcomings and energy costs associated with direct implemen-tation of VSEP™ technology for increasing water recovery from an RO process appliedin typical inland, arid west conditions.

• Investigate feasible strategies for and means of encouraging centralized ion exchangemedia regeneration (vis-à-vis in-home regenerative water softeners) to minimize waterconsumption and brine disposal impacts.

• Investigate regulatory and economic means to discourage the use of in-home regenerativewater softeners.

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• Develop the capability for case specific, systems-level evaluation of complete processschemes for desalination and brine management, including development of the necessarymodeling tools and metrics.

• Develop and/or identify halophytes with commercial value that can be grown in linedfacilities irrigated with RO brines.

The following projects are recom-mended to address long-term desali-nation challenges and provide potentiallylarge, yet longer horizon, payback oninvestment.

• Establish a program by which newmembranes, membrane treatmentrelated products, and alternativedesalination technologies are inde-pendently and comparatively testedunder the conditions and con-straints relevant to the arid westdesalination situation.

• Establish a combined basic andapplied research program toimprove the efficiency anddecrease the cost of capacitativedeionization.

• 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 house-hold-level water harvesting that will affect wastewater volumes, and thereby, wastewatersalinities.

• If the results of the short-term scoping study recommended above are positive, under-take detailed cost and feasibilities studies of using the Sacaton Pit and/or a Gila RiverSalt Marsh as brine disposal options.

• Conduct combined basic and applied research on the potential for incorporating novelmembrane functionalities, such as electromagnetic membranes, biomimetic membranes,and ion-specific electrodialysis membranes

• If appropriate circumstances arise where brine generation occurs proximate to a wasteor cheap heat source (e.g., power plant cooling water stream), refine and retest theDewVaporation technology based on the findings of the study done by the City ofPhoenix and U.S. Bureau of Reclamation.

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IX. Afterword

As part of the Arizona Water Institute grant which funded this workshop, Ashley Smithof the University of Arizona completed an annotated review of recent, key publicationspertinent to the state of the art regarding inland desalination. This literature review was notmeant to be exhaustive as there are well over 1,000 papers on desalination in peer-reviewedjournals alone and many more in conference proceedings, industry journals, agency projectreports and other publicly available resources. The literature review is included herein asAppendix 3 to give workshop attendees and other interested persons a point of entry intothe literature on a particular aspect of the subject, with the intent that additional in-depthinformation could be accessed using the citations and ideas within the point of entry literature.

Scottsdale Water Treatment Facility

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Appendix 1

Salinity Workshop Agenda

Appendix 2

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Salinity Workshop Attendees

Adams, Angela U.S. Bureau of ReclamationAlexander, Kevin Separation Processes, Inc.Androwski, James Northern Arizona UniversityArnold, Bob University of ArizonaBaygents, Jim University of ArizonaBeckman, James Arizona State UniversityBenemelis, Perry AZ Dept of Water ResourcesBiggs, Jeff Tucson WaterBoyd, Basil City of TempeBoyer, John Arizona Public ServiceBrandy, Matt Intel CorporationBrown, Kurt U.S. Bureau of ReclamationCampbell, Marc Salt River ProjectCardoza, Mark American Water CompanyChang, Yu Jung HDRChapman, Michelle U.S. Bureau of ReclamationChavez, Laura Brown & CaldwellChinn, Tim HDRChoi, Chi Chi Arizona State UniversityCullom, Chuck Central Arizona ProjectDay, Henry Arizona Public ServiceDosSantos, Placido Arizona Water InstituteDrago, Len Intel CorporationEla, Wendell University of ArizonaEngle, Tee Arizona State UniversityFox, Peter Arizona State UniversityFranks, Rich HydranauticsGoldman, Fred Kennedy/Jenks ConsultantsGraf, Chuck Arizona Water InstituteGreen, Jerry CDMGremillion, Paul Northern Arizona UniversityGulizia, Lynne Toray Membrane AmericaHaymore, Tonya Arizona Water Institute

He, Charlie Carollo EngineersHoller, Eric U.S. Bureau of ReclamationJohnson, Bruce Tucson WaterKelso, Brandy City of PhoenixKinshella, Paul City of PhoenixKottenstette, Richard Sandia National LaboratoryLaMartina, Karen Tucson WaterLant, Tim Arizona State UniversityMadole, Jim E. L. Montgomery & AssocMansfield, David City of ScottsdaleMardam, Tony CH2M HillMarra, Ralph Tucson WaterMitchell, Stuart American Water ChemicalsMoody, Chuck U.S. Bureau of ReclamationNewell, Pete HDRNorris, Mike U.S. Bureau of ReclamationPeterson, Joel E. L. Montgomery & AssocPoulson, Tom U.S. Bureau of ReclamationRayhel, John Intel CorporationRiley, Jim University of ArizonaRobertson, Michele Az Dept of Environ QualityRoth, Glen Damon S. Williams & AssocRussell, David Professional Water TechRussell, Jerry Carollo EngineersSacks, Richard City of ScottsdaleScott, Chris University of ArizonaSmith, Ashley University of ArizonaTerrey, Andy City of PhoenixTheron, Jan Northern Arizona UniversityThomas, Harold Brown & CaldwellThomure, Tim Tucson WaterWallace, Greg E. L. Montgomery & Assoc

Appendix 3

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