# Presentation: 461276

## Arsenic Removal by Coagulation & Precipitation Processes

- Presentation Prepared by:
- Joe Chwirka - Camp Dresser & McKee (chwirkajd@cdm.com)
- Bruce Thomson - University of New Mexico (bthomson@unm.edu)
- Presented by: YuJung Chang – HDR Engineering, Inc.

## Introduction

- Arsenic removal by coagulation & filtration is effective for many applications
- Presentation will discuss:
- Chemistry of the process
- Variables affecting process performance (especially pH & coagulant dose)
- Process variations
- Coagulation & granular media filtration
- Coagulation & membrane filtration
- Design considerations

## Acid-Base Chemistry of As(V)

## Acid-Base Chemistry of As(III)

## Redox Chemistry of As

## Solubility of As(III) Compounds

## Solubility of As(V) Species

## Important Points

- As has two oxidation states - As(III) & As(V)
- As(III)
- Non-ionic (H3AsO3) at neutral pH
- High solubility
- More toxic to many organisms
- As(V)
- Ionic (H2AsO4-/HAsO42-) at neutral pH
- Some phases are less soluble
- More reactive in solution:
- Membranes
- IX
- Adsorption

## Coprecipitation

- Coprecipitation involves removal of two or more constituents by a precipitation reaction.  Coprecipitation of As with Fe(OH)3 is an effective treatment process:
- 		FeCl3 + 3H2O  =  Fe(OH)3(s) + 3H+ + 3Cl-
- Points:
- Produces HCl which will lower pH
- Typical Fe dose ~10-4 M, whereas As conc. ~10-7 M, hence As is minor component within precipitate
- As likely removed by adsorption onto Fe(OH)3 surface with subsequent enmeshment as floc particle grows
- Al(OH)3 also effective

## Solubility of Fe(OH)3

## Effect of pH on Surface Charge of Fe(OH)3

## Covalent Bond Formation(Grossl et al., 1997)

## pH of Zero Point of Charge

- Electrostatic attraction is important first step in adsorption
- pHzpc = pH at which net surface charge = 0
- Surface is positive at pH < pHzpc
- Most clay minerals have pHzpc < 6
- Hence poor adsorption
- Clays dominate surface chemistry of soils
- Fe(OH)3 and Al2O3 have relatively high pHzpc
- Good adsorbents of As(V)

## As Removal by Conventional Treatment (McNeill & Edwards 1997)

- Survey of conventional coagulation-flocculation water treatment plants
- Correlate As removal to removal of Fe, Mn, & Al
- [Fe] = Iron Precip. Formed (mM)

## As Removal by Conventional Trt. - 2

## As Removal by Conventional Trt. - 3

- Strong correlation to removal of Fe & use of FeCl3 as coagulant
- Weaker correlation to removal of Al & use of Alum
- Possible sorption onto colloidal Al(OH)3 which passes through granular media filters
- Improved As removal achieved by minimizing effluent total Al concentration
- Note the importance of particulate As

## Arsenic Removal vs FeCl3 Dose, Albuquerque NM

- Arsenic Removal vs FeCl3 Dose, Albuquerque NM

## Ambient pH: FeCl3 vs As Leakage, NAS Fallon

## El Paso Jar Testing

## pH Adjustment with CO2, NAS Fallon, NV

## Silica Impacts Arsenic Removal at pH  7.0 and Above

- pH 8.5

- pH 7.5

- pH 6.5

- FeCl3 Dose: 4 mg/L

- After Clifford and Ghurye

## Silica Speciation with pH

## Silica in US Water Supplies (NAOS)

- After Davis and Edwards

## Polymeric Silica

- After Davis and Edwards

- 50 mg/L

- 17 mg/L

- 5 mg/L

## Coagulation/ Filtration

- Use pressure filters
- Direct Filtration frequently used for iron and manganese removal.
- Limited to low ferric dose applications.
- High coagulant dose will result in frequent backwash requirements
- Increased residuals production & handling costs
- Increased production of wastewater

## Schematic of Coagulation/ Filtration

- Aeration

- FeCl3

- Rapid Mix

- Solids to Dewatering

- Treated Water

- Pressure Filter

- Solids to Landfill

- Raw Water

- CO2

- CO2

## Calculation of Filter Loading Limitation

- Rule of Thumb, no more than 10 mg/L of FeCl3
- Limit Solids Loading to 0.1 lbs/SF
- May need to add sedimentation

## Vertical Pressure Filter

## Direct Filtration Performance(Based on 0.1 lbs Solids/sf)

## Backwash Water as Percent of Production(Based on 250 gal/sf)

## C/F O&M Issues

- Large backwash volume (20 gpm/sf for 10 minutes)
- Tanks may need internal painting, 10 yr intervals.  Use 316 SST.
- Standby filters, typically provided, but need to evaluate.
- Pneumatic or electric valve operators.

## Coagulation/ Pressure Filtration

- Particle size
- Particle breakthrough
- backwash requirements
- filter ripening
- Backup Filters

## Coagulation/ Microfiltration

- Pilot tested in Albuquerque, 1998
- Pilot tested in NAS Fallon, NV, 2001
- Pilot tested in El Paso, TX, 2001/2002
- Fallon Paiute Shoshone Tribe: 0.5 mgd
- City of Albuquerque: 2.3 mgd

## Microfiltration General Concepts

- Low Operating Pressure, 5 - 30 psi
- 0.1 to 0.2 micron pore size
- Water flow from Outside to the Inside
- Air-Water Backwash
- Backwash Every 25 to 30 minutes (95% recovery)
- Flux rate defined as Gallons/SF/Day (GFD)
- Chemical Cleaning Frequency > 30 days

## What is C/MF?

## Pressure Driven Membranes

## MF Process Operates in Direct Filtration Mode

## Solids are Removed from Module by an Air-Water Backwash

## Coagulation/ Microfiltration

- Pilot tested in Albuquerque, 1998
- Pilot tested in NAS Fallon, NV, 2001
- Pilot tested in El Paso, TX, 2001/2002
- Fallon Paiute Shoshone Tribe: 0.5 mgd
- City of Albuquerque: 2.3 mgd

## Microfiltration General Concepts

- Low Operating Pressure, 5 - 30 psi
- 0.1 to 0.2 micron pore size
- Water flow from Outside to the Inside
- Air-Water Backwash
- Backwash Every 25 to 30 minutes (95% recovery)
- Flux rate defined as Gallons/SF/Day (GFD)
- Chemical Cleaning Frequency > 30 days

## What is C/MF?

## Pressure Driven Membranes

## MF Process Operates in Direct Filtration Mode

## Solids are Removed from Module by an Air-Water Backwash

## Pall Microfilter

## Memcor PerformanceNAS Fallon, 15 mg/L FeCl3

- 25 GFD (No FeCl3)

- 27 GFD

- 38 GFD
- (No FeCl3)

- 32 GFD

- 27 GFD

## Memcor Cleaning EfficiencyNAS Fallon, Citric Acid

## Pall PerformanceNAS Fallon, FeCl3 45 mg/L

## El Paso C/MF Pilot Studies

## El Paso Pilot Studies

- Only Pall MF tested
- Ferric dose 10 mg/L
- pH lowered to 6.8 with CO2

## El Paso Pall Performance

## Fallon Paiute Shoshone Tribe C/MF PFD

## Fallon Paiute Shoshone Tribe C/MF

## Fallon Paiute Shoshone Tribe As Treatment Faciliy

## Fallon Paiute Shoshone Tribe Start-upDecember 2004

## C/MF Summary

- Emerging Technology for Arsenic Treatment
- Can be designed for high flux rates with Low TOC groundwater
- Optimize solids loading by pH pre-treatment
- Cost competitive with other technologies

## Recent Studies on Particle Size Filtration and Arsenic Removal

## C/MF O&M Issues

- Membrane Replacement: Pall warrantees membranes for 10 years, prorated.
- Chemical cleaning with citric acid, can not be recycled, must be disposed of.
- Provide sufficient replacement parts, not system redundancy.

## Comparison of C/MF to Pressure Filters at the Fallon Paiute Shoshone Tribe 

## Residuals Characteristics for C/MF and C/F

- C/MF around 4% to 5 % Backwash.
- C/F  around 5% to 10% Backwash.
- Recycle the backwash water to minimize wastewater.
- Ferric residuals will pass TCLP, however, may not pass the Cal WET.

## Residuals Handling

- Mechanical dewatering will be complicated: Ferric sludge is difficult to dewater
- Need body additives, Diatomaceous Earth
- Filter Bottom Dumpsters and polymer for small applications.
- Solids drying ponds:
- Ponds need to be lined.
- Anaerobic conditions may release the As.
- Provide access for sludge removal equipment.

## Concurrent Iron, Manganese, and ArsenicDrinking Water Standards

- Fe: Secondary Standard of 0.30 mg/L
- Mn: Secondary Standard of 0.05 mg/L

## Iron and Arsenic Removal

- Oxidize Fe with Cl2 or O3
- Adsorb As onto Fe(OH)3 precipitate
- pH needs to be around 7.3

## Manganese and Arsenic Removal

- As requires low pH for adsorption
- Mn requires high pH (>10) for oxidation with Cl2
- Mn oxidation by ClO2 is rapid & appears to be independent of pH
- ClO2 reported to be ineffective for As(III) oxidation.
- May need to add Cl2 in addition