Download - Arsenic/Iron Co-Precipitation and High Rate Filtration in the City of Portage

Arsenic/Iron Co-Precipitation and High Rate Filtration in the City of Portage

Christopher Barnes, P.E., City of Portage

Kendra Gwin, P.E., City of Portage

Daniel Starkey, P.E., AECOM

City of Portage

• Population of 45,000

• Groundwater source of supply

• Water supply and distribution system operated under contract by United Water

City of Portage, Garden Lane Water Treatment Plant

Background

• Portage operates 14 well fields located throughout the City

• Limited treatment includes chlorination, fluoridation and phosphate for iron sequestration

• Average day pumping: 5.5 MGD

• Peak day: 17.6 MG


Garden Lane Well Field

Garden Lane Well Field Site


Well GL-3

Well GL-2

Well GL-1

Well GL-4 (New)

Treatment Plant Location

Garden Lane Well Field• Three existing wells

• Wells contain elevated levels of iron, manganese and arsenic

• Two wells consistently above the 10 ug/L arsenic MCL

• All wells above the 0.3 mg/L iron SMCL

• Well field relegated to “stand-by” service

• This well field is in the center of the City and is a critical component of the overall water supply and distribution system

• Shift base load capacity to a more reliable and protected aquifer


Feasibility Study Prepared in 2006

• Evaluated various treatment methodologies• Coagulation/filtration• Ion exchange• Chemical oxidation/filtration• Adsorption

• Treatment iron removal alternatives were considered• Pressure filtration• Aeration + Detention + Pressure Filtration• Aeration + Detention + Gravity Filtration• Partial Flow Treatment + Blending

• Pressure filtration was identified as the most cost-effective option


Co-Precipitation of Arsenic and Iron using Pressure Filtration

• Proven technology

• Soluble iron (Fe 2+) and arsenic (As 3+) are oxidized to insoluble iron (Fe 3+) and arsenic (AS 5+)

• Arsenic is adsorbed and enmeshed in the resulting iron floc

• Filtration removes insoluble iron/arsenic precipitate

• Iron must be present in sufficient quantity


Arsenic Treatment Process Selection Guide


• Iron must be present in sufficient quantity • Ratio of iron to arsenic of 20:1 (minimum) is “rule of thumb”• In Portage this ratio is 50+:1 with naturally occurring iron

Preliminary Investigation

• City investigated various pressure filtration systems

• Three systems were selected for pilot testing in 2007

• Visits made to other cities to observe and seek input from operators

• Filtronics (Anaheim, CA) was selected• Use of proprietary media allows for higher than normal filtration rate• Short (4-min) backwash and at reduced interval (8-hour)• Backwash reclamation system was a standard offering• Significant experience in the western US where arsenic is a major problem


Pilot Testing

• Well GL-2 with highest levels of arsenic was tested

• Chlorine (sodium hypochlorite) used as oxidant

• 3-inch pilot unit


Pilot Testing Results

Raw Water Finished Water• Arsenic: 20 - 27 ug/L (range) 1.5 - 3.0 ug/L (range)

• Iron: 1.0 - 1.1 mg/L (range) ND- 0.002 mg/L (range)

• Manganese: 0.084 - 0.086 mg/L (range) 0.018 – 0.027 mg/L (range)

• Chlorine breakpoint concentration of 19 ppm

• Excellent removals observed at chlorine feed rates of 6 to 8 ppm


System DesignSupply

• 3,000 GPM (4.32 MGD) firm capacity

• 4,000 GPM (5.76 MGD) total system capacity

• New 1,200 GPM supply well, GL-4


System Design


Filtronics Treatment and Filtration System

System Design

Filtration

• Reaction vessels (2)– 3.5 minutes CT at design flow rate

• Filtration vessels (4)– Filtration rate 10 gpm/ft2

– Backwash rate 20 gpm/ft2

– Filter area 100 ft2 per filter

• Provisions for future addition of a 5th filter


Raw Water Quality


Finished Water Quality


Waste Backwash Water Management Options

• On-site management using ponds or infiltration beds• High groundwater table• Heavy, “muck”-type soils• Limited soil permeability would result in large ponds • Well field located in a park setting

• Discharge to municipal sewer system• Arsenic limits would likely be exceeded without pretreatment• Prohibitively expensive

• Reclamation and re-use • Separate solids and reuse backwash water• Dewater and dispose iron/arsenic solids in a Type II landfill


Filter Backwash


Filter Backwash

• Backwash interval: 8 hours

• Backwash duration: 4 minutes

• Rinse duration: 1 minute

• Backwash rate: 20 gpm/ft2

• Rinse rate: 10 gpm/ft2


Filter Backwash

• Waste backwash volume: 9,000 gallons per backwash

• Tank sized to accommodate 16 filter backwashes

• 150,000 gallon total capacity

• Iron/arsenic precipitate settles in the conical tank bottom


Backwash Reclamation• Decanter collects supernatant, which

is pumped to the head of the plant for filtration and reuse

• Backwash recycle rate is limited to <10% of the forward flow


Backwash Reclamation

• Solids accumulate in the waste backwash tank

• Estimated that solids will be removed 2x year

• Plate and frame filter press is provided to dewater precipitate

• Solids were tested during pilot study and are non-hazardous


New Well House, GL-4


New Well, GL-4


Sustainability Features

• Natural Lighting

• Pervious Pavement (Parking Area)

• Natural Prairie Site Restoration

• Stormwater “Rain Garden”


Current Status• System was started in June 2010

• Arsenic and iron removals have been consistent with results of pilot work

• Finished water quality:• Arsenic 2-3 ug/L• Iron <0.01 mg/L

• Waste backwash sludge dewatering not performed to date

• Resolving communication issues between the local filter control panel and the central SCADA system

• Developing a level of comfort with operating the City of Portage’s first full-scale treatment system


Ribbon Cutting Ceremony August 2, 2010
