Brackish Ground Water Desalination: Challenges to Inland Desalination Technologies (It sure ain t seawater desalination)

Transcription

1 Brackish Ground Water Desalination: Challenges to Inland Desalination Technologies (It sure ain t seawater desalination) Bruce Thomson Dept. of Civil Engineering University of New Mexico (bthomson@unm.edu) 1

2 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 ) Convergence of these leads utilities to consider low quality water as potential source: Wastewater for reuse Brackish & saline water Treatm ent Cost Unit Cost Willingness to Pay Tim e 2

3 Definition of Brackish & Saline Water (US BuRec, 2003) Mildly brackish Moderately brackish Heavily brackish Seawater and Brine 1,000-5,000 mg/l 5,000-15,000 mg/l 15,000-35,000 mg/l > 35,000 mg/l 3

4 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 4

5 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 5

6 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 6

7 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 7

8 Thermal vs Membrane Desalination (NAS, 2007) 8

9 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 µm - 2 nm (MWCO KDaltons) Nanofiltration - < 1 nm (MWCO 10-1 KDaltons) RO - ~0.2 nm Diameter of H 2 O molecule ~0.2 nm 9

10 Reverse Osmosis Use pressure to force water through semi-permeable membrane - considered diffusion of H 2 O molecules through membrane, not filtration Osmotic pressure - depends on ionic concentration & nature of ions P P = Pressure (bar) C = Conc. of dissolved ions (mol/l) T = Temperature (K) φ = Osmotic coefficient (depends on solute) R = Gas constant = CφRT 10

11 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 Feed Water P Pump Concentrate Energy Recovery or PRV Permeate 11

12 RO Process Spiral wound membrane cartridges 12

13 Osmotic Pressure Determined by thermodynamics - not membrane characteristics 13

14 3 Major Differences Between Seawater & Inland Desalination Feed water recovery Membrane fouling Brine disposal 14

15 Feed Water Recovery 15

16 Feed Water Recovery Recovery (r) = Fraction of feed water recovered as permeate If rejection ~100%: C = C 1 1 r High recovery = High conc. of solutes in concentrate c f 25 Objective of inland desalination is high recovery Cconcentrate /Cfeed Recovery (%) 16

17 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 17

18 Membrane Fouling 18

19 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 19

20 Inorganic Fouling Minerals Precipitate log K so Equil. Cation Conc. (mg/l) Equil. Anion Conc. (mg/l) CaCO CaSO. 4 2H 2 O SiO CaHPO Concentration of HCO 3 - in units of mg CaCO 3 /L 2 Other phosphate phases such as apatite (Ca 5 (PO 4 ) 3 OH) are several orders of magnitude less soluble than CaHPO 4. 3 Ignoring ionic strength and complexation effects. 20

21 Seawater vs. Ground Water K + 0.9% Mg % Ca Ca 2+ 31% Cl - 9% HCO 3-11% Na % Seawater TDS = 35,400 mg/l SO 4 2-3% HCO 3-0.2% Cl Mg 2+ 9% K + Tularosa Basin, NM TDS = 2,860 mg/l 4% Na + 10% SO % Sandoval Co. water TDS = 12,500 mg/l Cl - = 3,100 mg/l, SO 4 2- = 4,400 mg/l As = 600 ug/l 21

22 Inorganic Fouling - UNM Tap Water Module 1 Specific Flux, L/m2-h-bar Module 2 Module Run Time, hr 22

23 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) 23

24 Brine Disposal 24

25 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 25

26 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 26

27 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) 27

28 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 CO 2 /d 28

29 Energy Comparison to Conventional Treatment ABC WUA Treatment Plant uses.17 Kwh/m 3 for treatment NAS cites Kwh/m 3 for seawater desal Desal energy costs principally depend on TDS. New technologies will not significantly reduce energy costs. 29

30 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 30

31 Sustainability 31

32 NOI to Appropriate ABQ Journal 2/13/9 176 filed as of 1/15/9 32

33 NOI to Appropriate in Bernalillo & Sandoval Cos. 33

34 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 34

35 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 35

36 My Opinions-1 I do not categorically oppose development of brackish/saline water resources 36

37 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 37

38 Acknowledgements Kerry Howe - UNM Civil Engineering John Hawley - Hawley Geomatters Guy Bralley - Sandoval County Nabil Shafike - NM ISC 38