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A multi-process groundwater treatment plant was recently started up at a southern New England groundwater treatment plant to reduce concentrations of aromatic and chlorinated aromatic solvents present in the site's groundwater. VOC and SVOC impacted groundwater is treated to meet strict State Surface Water Discharge Criteria (SWDC) concentrations using metals precipitation, sedimentation and filtration, followed by organics treatment process using UV and hydrogen peroxide. Following UV/peroxide treatment, groundwater is treated using a shallow tray air stripper for polishing, followed by two catalytic activated carbon units for excess hydrogen peroxide removal. The treated groundwater is discharged to a nearby surface water body following pH neutralization. In addition, there is a soil vapor extraction (SVE) offgas treatment system consisting of a catalytic oxidation unit and acid gas scrubber in operation to treat impacted soil gas from the subsurface residual source areas.

The UV oxidation system operates with a residual hydrogen peroxide concentration of up to 50 mg/L, and this concentration must be reduced to less than 5 mg/L prior to discharge. Operational difficulties occurred with the full-scale catalytic carbon units. Excessive headloss through the units developed over short intervals and carbon fines were periodically observed in the treated effluent. In addition, the acid gas scrubber uses a slipstream of the treated process water effluent to quench the offgas from the catalytic oxidizer prior to discharge into the atmosphere. Carbon fines were entrained in this slipstream causing increased maintenance related issues with the acid gas scrubber.

As a result of these problems, two alternative chemical methods for removal of the excess hydrogen peroxide were evaluated. The purpose of these tests was to identify, test and prove-out a possible replacement for the catalytic carbon in the event that the operational difficulties became excessive and/or performance was adversely affected to a point where the effluent could not reliably meet the SWDC. The alternative methods evaluated were catalase addition, which breaks down hydrogen peroxide into water and oxygen through biochemical enzymatic action, and sulfur dioxide, which reacts with hydrogen peroxide to form sulfuric acid. Field tests carried out at the site included both bench and full-scale evaluations of catalase and sulfur dioxide. Results obtained are presented and compared for process efficacy and operability, as well as operating costs. Sulfur dioxide was the recommended option due to its high process efficiency, instantaneous reaction time, and lower capital and operational costs, provided that personnel safety measures are enhanced due to concerns over sulfur dioxide's hazardous and toxic characteristics. Catalase is an easily implemented option and safer to use, but is more critically dependent on reaction time and mixing limitations in conventional stirred tanks and more expensive to use. Field tests demonstrated that additional catalase reaction time through inclusion of the effluent discharge line could compensate for these limitations.

Ultimately, full-scale alternate hydrogen peroxide technologies were never deemed necessary because the operational difficulties were effectively managed by routine equipment cleaning and maintenance. Moreover, after approximately three years of continual operation of the groundwater pump and treat system, influent target SVOC concentrations decreased to a point where the operation of the UV/Oxidation and catalytic carbon units could be suspended. This paper presents bench-scale and full-scale testing objectives, methods and results and provides a conceptual design for a full-scale alternate hydrogen peroxide removal system for this particular site.
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Document Type: Research Article

Publication date: 2004-01-01

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