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Brackish Ground Water Desalination: Challenges to Inland Desalination

Technologies(It sure ain’t seawater desalination)

Bruce ThomsonDept. of Civil EngineeringUniversity of New Mexico

(bthomson@unm.edu)

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Introduction

• Demand leads to increased willingness to pay for water• Improvements in treatment technologies, including

desalination, lead to reduced costs• Water rights laws that don’t govern saline water sources

(NMSA 72-12-26)• Convergence of these leads utilities to consider low quality

water as potential source:• Wastewater for reuse• Brackish & saline water

Time

Unit

Cos

tWillingness to Pay

Treatment Cost

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Definition of Brackish & Saline Water(US BuRec, 2003)

Mildly brackish 1,000 - 5,000 mg/l Moderately brackish 5,000 - 15,000 mg/l Heavily brackish 15,000 - 35,000 mg/l Seawater and Brine > 35,000 mg/l

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Desalination

• Desalination• Traditional application - Removal of salts from sea

water.• Current interests - Remove all dissolved constituents

from source water (including organics)• Interest in desalination technologies for advanced

wastewater treatment for indirect (& possibly direct) potable reuse

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Seawater vs. Inland Desalination

• Much experience & familiarity with seawater desalination• Limited understanding & appreciation of differences of

inland desalination• Important differences include:

• Feed water recovery objectives• Water chemistry• Brine disposal options

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Objectives

• Provide brief review of desalination technologies• Focus on RO

• Discuss differences between seawater & inland desalination

• Identify limitations of inland desalination on development of brackish & saline ground water resources

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Desalination Technologies(NAS, 2007)

• Membrane technologies - RO, EDR• Phase transfer technologies - Variations of distillation

including thermal distillation, multistage flash distillation, multiple effect distillation, vapor compression

• Ion exchange

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Thermal vs Membrane Desalination(NAS, 2007)

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Membrane Technologies

• Increasing use in water & wastewater treatment due to improved performance & reduced cost

• Classified according to size of particles or solute that’s rejected• Microfiltration - 10 µm - .03 µm• Ultrafiltration - 0.1 µm - 2 nm (MWCO 100 -10 KDaltons)• Nanofiltration - < 1 nm (MWCO 10 - 1 KDaltons)• RO - ~0.2 nm• Diameter of H2O molecule ~0.2 nm

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Reverse Osmosis

• Use pressure to force water through semi-permeable membrane - considered diffusion of H2O molecules through membrane, not filtration

• Osmotic pressure - depends on ionic concentration & nature of ions

• P = Pressure (bar)• C = Conc. of dissolved ions (mol/L)• T = Temperature (K)• φ = Osmotic coefficient (depends on solute)• R = Gas constant

RTCP φ=

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RO Terminology

• Permeate - Water that passes through membrane• Concentrate (Brine) - Solution containing retained solutes• Recovery - Fraction of feed water recovered as permeate• Rejection - Fraction of solutes not passing through

membrane

Energy Recoveryor PRV

Concentrate

PFeedWater

Pump

Permeate

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RO Process

• Spiral wound membrane cartridges

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Osmotic Pressure

• Determined by thermodynamics - not membrane characteristics

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3 Major Differences Between Seawater& Inland Desalination

• Feed water recovery• Membrane fouling• Brine disposal

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Feed Water Recovery

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Feed Water Recovery

• Recovery (r) = Fraction of feed water recovered as permeate• If rejection ~100%:• High recovery = High conc. of solutes in concentrate

−=

r11CC fc

0

5

10

15

20

25

0 20 40 60 80 100

Recovery (%)

Cco

ncen

trat

e/C

feed

• Objective of inland desalination is high recovery

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Recovery - Fraction of Feed Water Recovered

• Consequences of high recovery:• High osmotic pressure• Reduced quality of permeate - leakage through RO

membrane is proportional to feed water quality• Concentrations of dissolved salts may exceed solubility

limits = fouling due to precipitation• Sea water:

• Unlimited supply hence recovery is less important. Chemistry is principally Na+ & Cl- - Low inorganic fouling potential

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Membrane Fouling

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Membrane Fouling

• One of biggest challenges to O&M of RO plant is fouling.• Four types:

• Colloidal fouling• Inorganic fouling• Organic fouling• Biological fouling

• Low quality ground water may be cause all four types. • Most challenging is inorganic fouling

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Inorganic Fouling Minerals

Precipitate log Kso

Equil. Cation Conc. (mg/L)

Equil. Anion Conc. (mg/L)

CaCO3 -8.48 60 751

CaSO4.2H2O -4.58 205. 492

SiO2 -2.71 116 - CaHPO4 -6.6 20.0 15.62

1 – Concentration of HCO3

- in units of mg CaCO3/L 2 –Other phosphate phases such as apatite (Ca5(PO4)3OH) are several orders of magnitude less soluble than CaHPO4. 3 – Ignoring ionic strength and complexation effects.

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Seawater vs. Ground Water

Mg2+

9%K+

4% Na+

10%

SO42-

26%

HCO3-

11%

Cl-

9%

Ca2+

31%

Tularosa Basin, NMTDS = 2,860 mg/L

K+

0.9%

Na+

41.6%

Mg2+

5.1%

Ca2+

0.0

SO42-

3%

HCO3-

0.2%

Cl-

0.5

SeawaterTDS = 35,400 mg/L

Sandoval Co. water TDS = 12,500 mg/LCl- = 3,100 mg/L, SO4

2- = 4,400 mg/LAs = 600 ug/L

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Inorganic Fouling - UNM Tap Water

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 50 100 150 200 250 300 350 400Run Time, hr

Spec

ific

Flux

, L/m

2-h-

bar

Module 1

Module 3

Module 2

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Fouling Control & Membrane Cleaning

• Fouling control• Reduce recovery• pH adjustment• Antiscalants• Antimicrobial agents (chloramines)• Pretreatment - Softening

• Membrane cleaning• Acidic & alkaline solutions• Complexing agents• Surfactants• Oxidants (depends on membrane)

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Brine Disposal

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Concentrate Disposal Options

• Seawater desalination disposal options• Return to sea

• Inland desalination disposal options (NAS, 2007)• Discharge to surface water• Evaporation ponds• Land application• Deep well injection• Landfill of solid wastes

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Concentrate Disposal Considerations

• Very high TDS• Concentrated by 4x at 75% recovery

• High concentrations of toxic constituents present in feed water (As, Se, U, etc.)

• Reduced evaporation of salt saturated solutions• High TDS solutions are corrosive• Impacts on deep well injection process

• Corrosion of equipment & well screens• Precipitation & cementing of subsurface formations

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Brine Disposal Case Study - Phoenix

• Study of disposal options for 20 Mgd concentrate stream• Evaporation ponds

• 10 square miles in area• Total capital cost = $410,000,000

• Pipeline to Gulf of California• 184 miles of 30- to 60-inch pipeline, additional distance

through existing canal• Would require approval from Mexico• Total capital cost = $456,000,000

(includes Tucson)

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Energy & Environmental Considerations

• 40 Mgal/d plant (44 KAF/yr), TDS = 12,500 mg/L, 75% recovery• Will pump 50 Mgal/d of brackish water

• 10 Mgal/d concentrate• Pressure ~1,000 psi• Energy reqt. ~12 Mwatts (16,000 horsepower)

• With energy recovery• Will produce 370,000 lbs CO2/d

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Energy Comparison to Conventional Treatment

• ABC WUA Treatment Plant uses .17 Kwh/m3 for treatment• NAS cites 2.5 - 7.0 Kwh/m3 for seawater desal

• Desal energy costs principally depend on TDS. New technologies will not significantly reduce energy costs.

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Cost

• Cost - Design study for 5 Mgd system in NM (TDS ~12,000 mg/L)• Capital cost =$143M• Total cost of water treatment = $8.50/1,000 gal

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Sustainability

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NOI to Appropriate…

• ABQ Journal 2/13/9• 176 filed as of 1/15/9

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NOI to Appropriate in Bernalillo & Sandoval Cos.

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Conclusions-1

• 3 important differences between seawater & inland desalination:• Recovery - Inland desalination will only recover 50 -

75% of feed water.• Fouling - Ground water has high fouling potential for

fouling by Ca and Si minerals• May limit recovery• Adds complexity & cost

• Brine disposal - Adds complexity & cost to operation

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Conclusions-2

• Also:• Expensive• Energy intensive• Complicated - Requires highly trained operators

• My take home message - Inland desalination is much more challenging than sea water desalination• More uncertainty in design, cost & operation

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My Opinions-1

• I do not categorically oppose development of brackish/saline water resources

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My Opinions-2

• Brackish water in central NM is not a sustainable source of supply

• We should not allow residential development that is dependent on non-sustainable water supplies

• Possible uses of brackish water formations:• Industry, mining, & agriculture• Drought reserve

• Must recognize that the formation has intrinsic value for use as secure water storage.• Should include its management in deliberations

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Acknowledgements

• Kerry Howe - UNM Civil Engineering• John Hawley - Hawley Geomatters• Guy Bralley - Sandoval County• Nabil Shafike - NM ISC