Treatment Process Selection

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Treatment Process Selection

• Baiyang Chen
• January, 2007

2
Background

• Occurrence
• Arsenic (As) 0.036 mg/L
• Fluoride (F) 3.2 mg/L
• Total dissolved solids (430 mg/L)
• Maximum contaminant levels (MCLs)
  of City of Surprise, AZ and USEPA
• As 0.007 mg/L
• F 2.0 mg/L
• TDS 500 mg/L

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Arsenic (As) Removal

• Best Available Technologies (USEPA)
• Activated Alumina
• Ion Exchange
• Reverse Osmosis
• Electro-Dialysis Reversal
• Oxidation/Filtration
• Enhanced Lime Softening
• Enhanced Coagulation/Filtration
• (---gt 7 is favored! System to be provided by
  Layne-Christensen)

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Fluoride (F) Removal

• Comparison of Processes
• Activated Alumina
• Ion Exchange
• Reverse Osmosis
• Electro-Dialysis Reversal
• Coagulation
• Others researched processes

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Activated Alumina (AA)

• Advantages
• Best available technology (BAT) by USEPA
• High efficiency (up to 90 removal)
• Removal of arsenic at the same time
• Treatment is cost-effective
• Low water loss (typically 35)
• Low energy consumption

• Limitations
• Works well in narrow pH range (56).
• High TDS and organics can result in fouling
• Presence of other anions can compete
• Regeneration is required and effectiveness of
  adsorbent reduces after regeneration
• Users do not like the taste of treated water

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Ion Exchange (IX)

• Advantages
• Also efficient to Arsenic removal
• Easy to operate

• Limitations
• Not a BAT of USEPA
• Efficiency depends on other anions like sulfate,
  carbonate, bicarbonate, and phosphate.
• Regeneration of resin forms F-concentrated
  wastewater
• Need adjustment of pH before and after treatment
• Treated water contains high level of chloride
• Cause odorous and DBP problems

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Reverse Osmosis (RO)

• Advantages
• BAT of USEPA
• Very effective for fluoride (gt90)
• Treat possible contaminants of future concern
• Life of membrane is longer, less regeneration is
  needed
• High integrity to ensure constant water quality
• No interference by other ions
• Automated, simple operation with little manpower

• Limitations
• More expensive than adsorption methods
• Treated water becomes acidic and needs pH
  adjustment
• Significant water loss (20)
• Brine is hard to deal with at inland plant
• Impacted by silica content (optimal at lt30mg/L,
  silica concentration of 75mg/L may enlarge RO
  water loss to 40)

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Electro-Dialysis Reversal (EDR)

• Advantages
• High removal of all ions
• Not interfered by silica
• Less fouling than RO
• Fully automated
• Chemical addition not required

• Limitations
• Not a BAT of USEPA
• Water loss is high (20)
• More expensive than RO, not economical for TDS lt
  3,000 mg/L
• Discharge of concentrated brine is a problem
• Side-products like hydrogen gas is dangerous

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Coagulation-Precipitation

• Limitations
• Low removal efficiency (lt33)
• Optimal condition is not well recognized yet
• High alum dosage needed, 80mg/L Al2(SO4)3 per
  1mg/L fluoride
• Less Al(OH)3 is formed, leaving more aluminum
  occurrence in water
• Removal of organic matter is lowered too
• Silica, temperature, and pH can impact the
  fluoride removal

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Other Processes

• Powdered Activated Carbon (PAC)
• Lime Softening (LS)
• Ultrafiltration (UF)
• Nanofiltration (NF)
• Coagulation-Microfiltration (C/MF)
• Electro-coagulation (EC)
• Membrane Distillation (MD)
• Costly. Inefficient. Lack Maturity. etc

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Recommendation RO System

• Key Reasons
• High efficiency in removing most ions
• Suitable for middle level budget
• Gain credits by treating pollutants of future
  concern
• Side effects/products can be properly handled
• Concerns can be overcome and not vital for this
  project
• Manufacturers
• Zenon, Ionics, Severn Trent, GE-Osmonics,
  Layne-Christensen, etc

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Plant Planning

• Procedures
• Treat water by coagulation-filtration process, to
  meet arsenic MCL requirement
• A portion (40) of water undergo a primary RO
  system (with 99 F rejection 20 water loss),
  rejected water pass through lime softening,
  followed by secondary RO system
• Blending RO-treated and non-RO treated water to
  meet fluoride MCL standards
• Disposal of RO brine in sewage

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Schematic of RO System for F Removal
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Expected Results

• Arsenic lt 0.007 mg/L
• Fluoride lt 2.0 mg/L
• Overall water loss 2
• TDS in drinking water lt 263 mg/L
• Brine with TDS of 15,000 mg/L

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Disposal of RO Brine

• Method
• Discharge to sewage
• TDS 437 mg/L in sewage system
• Pose insignificant effects on WWTP operation.

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Summary of Plant Design

• Coagulation-filtration is favored for arsenic
  removal
• Reverse Osmosis (RO) is recommended for fluoride
  removal
• 40 of water need RO treatment, followed by
  blending with non-RO treated water
• Minimized water loss (2), As F below MCLs
• RO rejected water can be properly disposed
• Details to be provided upon approval

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Comments?