A REVIEW OF ARSENIC REMOVAL TECHNOLOGIES AND TECHNOLOGY SELECTION APPROACH ZAID CHOWDHURY, PHD, PE, BCEE SOUTHWEST AWWA CONFERENCE OCTOBER 16, 2017
Arsenic Current MCL 10 ug/l EPA is revisiting health effects data Next 6-yr Review? MCL may be lowered
History of regulatory development and research to assist affected utilities USEPA s Arsenic Technology and Cost document AWWA s national compliance cost estimate WRF projects: Interactive Decision and Costing Tool Innovative technology research Technology Demonstration Secondary Impacts of As Rule National Drinking Water Advisory Council s (NDWAC) committee to evaluate regulatory costs
Some basic facts about arsenic Arsenic naturally occurs in two forms: Organic (seafood) Inorganic (water) Inorganic arsenic is more toxic than organic arsenic Inorganic arsenic occurs in two forms Arsenite, As(III) Arsenate, As(V) As (III) is more toxic than As(V)
Health effects of arsenic Arsenic causes cancer of Skin Bladder Lung and Prostrate Arsenic is also implicated in cardiovascular disease, diabetes and reproductive disorders Health effects research points to very low safe level
Arsenic occurrence in the US
Arsenic Rule impact Arsenic Standard Number of Systems Impacted National Compliance Costs EPA AWWA 3 µg/l 14,500 $7 Billion $28 Billion 5 µg/l 9,000 $4 Billion $14 Billion 10 µg/l 4,000 $2 Billion $5.5 Billion 20 µg/l 1,600 $0.6 Billion $1.5 Billion ~97% of impacted systems serve less than 10,000 people Small systems cost: $3-$30 per month per household Large system cost: $2-$4 per month per household
Technologies for arsenic removal Adsorption; GFH, GFO, ArsenX, AA, etc. Coagulation/Filtration or Coagulation/MF (softening) Ion Exchange Membranes (NF, RO, or EDR) Most of these technologies can achieve >90 % removal when optimized
Adsorption Adsorption capacity varies with media type Media performance is site specific and varies with: ph Concentrations Competing ions Raw Water Spent media must be removed and replaced W A T E R Media As(V) Adsor bent Spent Backwash Treated Water Backwash
Coagulation/Filtration Coagulant + Water = Flocs As(V) Arsenic is removed by incorporation into flocs or adsorption on to flocs Flocs are removed by filtration or micro/ultra filtration Solids contain majority of arsenic and disposed as non-hazardous waste
Ion Exchange Lower arsenic capacity compared to adsorption media Requires frequent regeneration cycles Sulfate interferes Spent regenerant may be hazardous Brine recycle increases the utilization of regenerant Treatment for regenerant is possible leading to the formation of a solid waste containing arsenic
Membranes RO/NF achieves high degree of removal EDR is partially effective Generates concentrated brine
Treatment interferences Adsorption: Phosphate interferes with adsorption Coagulation: Silica interferes with coagulation Ion Exchange: High sulfate limits the use of ion exchange Nitrate and selenium peaking may occur Membranes: High TDS may be problematic for membrane applications
Technology applicability for arsenic removal Technology GW System Applicability SW System Applicability Prone to Interferences Need Sewer Connection Adsorption? Low No Anion Exchange? High Yes Coagulation Moderate No Lime Softening Moderate No Membranes Moderate Yes
Capital cost for arsenic removal technologies $14,000 Capital Cost (in $1,000s) $12,000 $10,000 $8,000 $6,000 $4,000 IX, at 20 mg/l influent SO4 IX, at 50 mg/l influent SO4 GFH, ambient ph = 7.5, 75,000 BVs AA, ambient ph = 7-8, 10,000 BVs AA, ambient ph = 8-8.3, 5,200 BVs GFH, adjusted ph = 6.5, 110,000 BVs AA, adjusted ph = 6.5, 23,100 BVs AA, adjusted ph = 6.5, 15,400 BVs CMF (w/o Sedimentation Basin) $2,000 $- 0.27 1.2 7 Design Flow (mgd)
Total cost for arsenic removal technologies Total Cost (in $/kgal) $10.00 $9.00 $8.00 $7.00 $6.00 $5.00 $4.00 $3.00 IX, at 20 mg/l influent SO4 IX, at 50 mg/l influent SO4 GFH, ambient ph = 7.5, 75,000 BVs AA, ambient ph = 7-8, 10,000 BVs AA, ambient ph = 8-8.3, 5,200 BVs GFH, adjusted ph = 6.5, 110,000 BVs AA, adjusted ph = 6.5, 23,100 BVs AA, adjusted ph = 6.5, 15,400 BVs CMF (w/o Sedimentation Basin) $2.00 $1.00 $- 0.27 1.2 7 Design Flow (mgd)
Some actual capital cost numbers from utilities in Arizona Capital Cost Treatment unit Flow Total Cost Unit Cost Arizona Installations GPM $/gpd granular iron media 900 $ 959,000 $ 0.74 granular iron media 1,100 $ 1,346,879 $ 0.85 granular ferric oxide 1,900 $ 2,202,383 $ 0.80 iron-enhanced media 725 $ 1,600,000 $ 1.53 granular ferric hydroxide at 3 facilities 3,900 $ 4,000,000 $ 0.71 granular ferric hydroxide at 2 facilities 3,100 $ 3,900,000 $ 0.87 granular ferric hydroxide 3,685 $ 2,500,000 $ 0.47 granular ferric hydroxide 4,097 $ 1,000,000 $ 0.17 granular ferric hydroxide 7,777 $ 1,500,000 $ 0.13 Average $ 0.70
TECHNOLOGY SELECTION APPROACH
Technology selection guidance available from published literature reports
Technology selection decision tree
Red flags for technology selection Sulfate > 50 mg/l or Nitrate > 5 mg/l Ion Exchange Discharge of liquid residual to sewer or evaporation pond not available High ph, well-buffered source-water Regenerable Adsorbents Ion Exchange Membranes Adsorbents Coagulation/Filtration Membrane
Technology selection drivers $ Water quality and interfering ions Operational Complexity
Pilot testing leads the way to the most effective treatment system design Figure 1. Adsorption Test SkidFigure 2. Adsorption Test Skid Operate small-scale column to determine arsenic break-through profile 0.020 GFH Effluent Raw Water or Influent Arsenic Concentration (mg/l) 0.015 0.010 0.005 24,000 BVs MCL At 96,000 BVs As is 9.4 µg/l 0.000 0 20,000 40,000 60,000 80,000 100,000 Bed Volumes Processed
Design considerations
Take-away points Water quality and operational constraints driver for technology No arsenic level is safe get to the lowest feasible level MCL may be lowered be ready Pro-active planning saves money