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

City of Portage, Garden Lane Water Treatment Plant

Garden Lane Well Field

Garden Lane Well Field Site

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

Arsenic Treatment Process Selection Guide

City of Portage, Garden Lane Water Treatment Plant

• 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

City of Portage, Garden Lane Water Treatment Plant

Pilot Testing

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

• Chlorine (sodium hypochlorite) used as oxidant

• 3-inch pilot unit

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

System Design

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

Raw Water Quality

City of Portage, Garden Lane Water Treatment Plant

Finished Water Quality

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

Filter Backwash

City of Portage, Garden Lane Water Treatment Plant

Filter Backwash

• Backwash interval: 8 hours

• Backwash duration: 4 minutes

• Rinse duration: 1 minute

• Backwash rate: 20 gpm/ft2

• Rinse rate: 10 gpm/ft2

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

City of Portage, Garden Lane Water Treatment Plant

City of Portage, Garden Lane Water Treatment Plant

City of Portage, Garden Lane Water Treatment Plant

City of Portage, Garden Lane Water Treatment Plant

New Well House, GL-4

City of Portage, Garden Lane Water Treatment Plant

New Well, GL-4

City of Portage, Garden Lane Water Treatment Plant

Sustainability Features

• Natural Lighting

• Pervious Pavement (Parking Area)

• Natural Prairie Site Restoration

• Stormwater “Rain Garden”

City of Portage, Garden Lane Water Treatment Plant

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

City of Portage, Garden Lane Water Treatment Plant

Ribbon Cutting Ceremony August 2, 2010

City of Portage, Garden Lane Water Treatment Plant