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NIBW Superfund Site: Remediation Technology

NIBW Superfund Site: Remediation Technology

 

We use three primary cleanup technologies at the North Indian Bend Wash Superfund Site:

Method #1: Air stripping water treatment technology

Groundwater treatment by air stripping, also known as packed tower aeration, is the most reliable and effective technology to remove low to moderate concentrations of volatile organic compounds (VOCs), such as TCE, dissolved in water. The process is very simple and effective as shown in the animation below. (Note: this schematic representation does not show scale accurately. In reality, TCE in groundwater is typically found at depths of up to 600 feet or greater.)

Step 1: Groundwater containing TCE or other volatile organic compounds (VOCs) is pumped from the aquifer to the top of a treatment tower that is filled with plastic packing material. The packing material consists of plastic balls, much like wiffle balls.

Step 2: The water trickles down through the packing while clean air enters the tower at the base and is blown upward by a high-speed fan. As the air comes in contact with the water trickling down the packing, the VOCs contained in the water transfer to the passing air flow.

Step 3: VOCs that are "stripped" from the incoming water move upward with the air flow, exit the tower and pass through air filters, if required. The filters, typically filled with granular activated carbon, remove the VOCs from air so that the exiting air meets federal, state, and local standards. The carbon is periodically replaced as necessary.

Step 4: The treated water is collected at the base of the tower where it is available as a clean water supply. If the treated water is intended for potable use it is disinfected at the water provider's facility before it enters the municipal drinking water system.

Method #2: Soil vapor extraction technology

Soil Vapor Extraction (SVE) systems work like a vacuum system to remove volatile organic compounds (VOCs), such as TCE, that exist in porous soils above the water table. The process is illustrated in the animation shown below. (Note: this schematic representation does not show scale accurately. In reality, TCE and soil vapors are typically extracted over about the upper 150 feet of the soil column.)

Step 1: An air vacuum blower operates to pull suction from specially constructed soil vapor extraction wells. The SVE wells consist of plastic casing that is perforated in intervals where there are known VOCs in soil. Air from the surrounding soil pores is drawn into the SVE wells thereby removing the VOCs contained in the air and removing VOCs adhered to the soils. In very porous soils, SVE will effectively remove VOCs in soil gas from a distance of over 300 feet from the extraction well.

Step 2: Air containing the VOC vapors is passed through one or more granular activated carbon filters as necessary to meet requirements. The VOCs are adsorbed by the carbon and removed from the air. The carbon is periodically removed from the canisters as it loses removal efficiency and replaced with fresh carbon. The spent carbon is returned to the supplier for regeneration.

Step 3: The treated air is discharged to the atmosphere.

Method #3: Granular Activated Carbon (GAC)

Liquid phase Granular Activated Carbon (GAC) is commonly used to remove volatile organic compounds from water.  Adsorption is both a physical and chemical process of accumulating substances at the interface between liquid and solid. Because GAC is a highly porous material, it provides a large surface area to which volatile organic compounds will adsorb. The GAC used at the NIBW GAC Treatment Facility (NGTF) is made from coconut shells.

Step 1: Groundwater containing primarily TCE and trace amounts of other VOCs is pumped from the aquifer and typically run through a pretreatment filter to remove solids.

Step 2: The groundwater flows through the GAC treatment system.  As the water comes into contact with the GAC, the TCE adsorbs to the surface on the carbon and into the porous GAC interfaces.

Step 3: Water then flows through a second GAC treatment vessel to ensure removal any trace amounts of VOCs in the water.

Step 4: Treated water is then conveyed to either the Arizona Canal for SRP distribution or to the City of Scottsdale Chaparral Water Treatment Facility for blending with treated surface water and use in Scottsdale’s potable distribution system.

Lead-lag, liquid phase GAC treatment system is considered one of the most fail-safe treatment processes available to remove VOCs from water and may also provide the added benefit of removing other impurities from groundwater, thereby reducing municipal treatment costs.