(225f) Engineering Challenges of Implementing the Peroxone Process for in-Situ Oxidation of Ground Water Contaminated By Acetone and Petroleum Hydrocarbons At a Former Industrial Facility

Authors: 
Qadir, R. M., Enpro Solutions, Inc.



In-situ treatment of dissolved chemicals of concern (COC) presents unique challenges due to variability in the subsurface hydrogeology, pH, and chemistry of the soil and ground water matrix. At this former acetylene manufacturing facility, very high concentrations – currently up to 15 x 10 6 micrograms per liter (µg/L) of acetone are present in the ground water! Other COCs include gasoline range petroleum hydrocarbons and benzene, toluene, ethylbenzene and xylenes (BTEX).  The Peroxone process – simultaneous ozone (O3) and hydrogen peroxide (H2O2) injection - was selected for in-situ chemical oxidation of dissolved species in the ground water. This advanced oxidation process produces highly reactive hydroxyl (OH-) free radicals during the spontaneous decomposition in the ground water of O3 to oxygen (O2). Adding H2O2 increases the OH-concentration and the overall oxidation rate. In addition, both O3 and H2O2 break down into O2 which leads to increased bio-stimulation.

In 2010 six dual-completion (two well casings in the same well bore) injection wells were installed to deliver the oxidants to the subsurface. Six ground water monitoring wells were used to track water quality parameters and ground water COC concentrations within the treatment zone during remediation. In addition, one well is placed up-gradient to the treatment zone and another off-site sentinel well is placed down-gradient to the site. Baseline ground water samples were collected and analyzed to estimate the mass-in-place of COCs dissolved in the ground water and adsorbed onto subsurface soils. The natural chemical oxygen demand (COD) based on naturally occurring organic materials and minerals present in the soil and ground water was estimated.  The total COD is many times greater than that required to react with the COCs alone.  The optimal oxidant dosage and flow rates required to overcome natural COD and consume the COCs over a period of several years were estimated.  A packaged remediation system of appropriate capacity was installed to produce ozone from compressed air and deliver a mix of ozone, enriched O2 and H2O2 to the subsurface through injection wells. The Peroxone process control system software allowed individually varying process variables such that injection flow rate, injection time and other parameters could be customized for each individual well. A complete injection cycle consisted of delivering oxidant to all six wells in turn for the injection times determined by the individual set point at each well. On site monitoring well data show that source area COC concentrations are being reduced.  The off-site sentinel well data show that COC concentrations down-gradient to the source area have been reduced by as much as 90% since the Peroxone system was installed.

Challenges encountered during system installation included the presence of subsurface obstructions such as buried utilities and concrete slabs; and logistical and safety concerns due to limited site access and restricted work hours. A major operating challenge was process alarms and shutdowns caused by apparent leaks that turned out to be migration of the injected ozone from the sub-surface through secondary containment piping back to the process equipment trailer. Additional challenges include monitoring remediation progress and optimizing process variables and injection cycles over time.

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