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Remediation efforts are ongoing at the Avtex Fibers Superfund Site in Front Royal, Virginia. As part of the future remediation of several surface impoundments used for viscose waste disposal during plant operation, it may be necessary to extract contaminated leachate and groundwater for ex-situ treatment and discharge to the Shenandoah River. This waste stream contains varying concentrations of sulfide (HS), carbon disulfide (CS2), metals, COD and other contaminants as illustrated in Table 1. This work describes a field pilot study designed to support preliminary remedial design efforts for treatment of this waste.

Previous bench-scale treatability testing focused on the development of a cost-effective biological treatment process to oxidize the relatively high levels of reduced sulfur compounds in the viscose basin leachate. This testing demonstrated that direct biological treatment of the concentrated leachate streams would be impossible due to the high concentration and loading of HS and CS2. Treatment by un-catalyzed alkaline pH hydrogen peroxide oxidation followed by biological treatment was identified as the most feasible and cost effective approach to meet the future permit limits for direct discharge. (Camper and Bott, 2006).

The objectives of the field pilot test conducted at the Avtex site during July-August 2006 were to develop process design criteria and to prove the effectiveness of un-catalyzed alkaline pH H2O2oxidation of HS, free CS2, CS2 associated with cellulose xanthate, and other reduced sulfur compounds present in the leachate and groundwater with minimal air emissions. For cost effectiveness, the chemical oxidation process was intended only to reduce the high concentrations of S2− and CS2, while minimizing reactions with organic carbon compounds (i.e. TOC or carbonaceous COD) in the leachate/groundwater stream to a level that would be successfully and consistently treated by a biological treatment system. Effluent H2O2 residual and temperature were also of concern because of the potentially detrimental effect on the downstream biological processes.

The pilot-scale oxidation reactor consisted of a 75-gallon continuous flow well mixed tank that was operated at a range of influent flow rates to test a hydraulic residence time (HRT) range from 3 hours to 30 minutes and H2O2 dosing from 75 to 150% of the stoichiometric dose based on the measured HS and CS2 concentrations (Figure 1). The pilot reactor was normally operated at a minimum pH of 10.5 (pH above and below 10.5 was evaluated), with NaOH addition via automatic pH control to maintain these conditions. Oxidation was achieved by dosing uncatalyzed H2O2, added as 35% product. To obtain the“design blend”(Table 2) of leachate during pilot testing, one 3,000-gallon tank containing dilute“sump”leachate, which is similar in characteristics to green leachate shown in Table 1, and one 13-gallon carboy with orange leachate were used to feed the reactor. To prevent hydrolysis of cellulose xanthate as a result of a pH decrease in the orange leachate, it was deemed critical to add the two waste streams separately to the reactor without premixing. A nitrogen gas headspace purge was provided with online H2S and O2 analysis. This purge gas was analysis for gas phase reduced sulfur compounds using a gas chromatograph with a sulfur chemiluminescence detector so that gas phase emissions of H2S and CS2 could be evaluated and a mass balance performed on the reactor.

Rapid turnaround analysis was provided for COD, total sulfide and total CS2 to support pilot operation, to set feed and H2O2 dose rates, to evaluate test variables. Reactor influent, effluent and purge gas samples were fully characterized under a wide range of operating conditions including shock loads. Reactor ORP, pH, temperature, and gas phase O2 and H2S emissions were automatically logged.

This pilot testing demonstrated that chemical oxidation with H2O2 is a viable method for pretreating the concentrated leachate streams for subsequent biological treatment and discharge. The testing proved that near stoichiometric dosing of H2O2 and an HRT of 1.0 hour provides effective oxidation of sulfide and carbon disulfide, and H2O2 dosing can be tailored to meet the needs of the downstream biological treatment process. Minimal organic carbon oxidation occurred at this optimal design condition, and total CS2 removal ranged from 92% to 95%, increasing with the decreasing influent concentration. Free CS2 removal was greater than 99% for all influent CS2 concentrations tested. Total sulfide removal ranged from 95% to 97%, again increasing with decreasing influent concentration.

Influent and effluent concentrations and representative mass balance calculations for the optimal design condition are shown in Figures 2 and Table 3, respectively. The mass balance demonstrated that over 99.5% of the HS and CS2 removal was due to oxidation, while less than 0.5% was volatilized to the gas phase, proving that the system can be operated with minimal concern for gas phase emissions. Results for a shock loading event are shown in Figure 3 and Table 3 and suggest that reactor operation was very stable even during an upset condition. The optimal reaction pH range was found to be 10.5 – 11.0, which maximized CS2 and total sulfide oxidation while minimizing contaminant loss due to volatilization. ORP measurements, ranging from −60 to −200 mV, were found to correlate well with influent loading of HS and CS2. The higher the ratio of orange to sump leachate, the more negative the effluent ORP. ORP was also observed to respond quickly to changes in H2O2 dose and changes in the leachate feed rates. Pilot testing demonstrated that ORP can be used to control H2O2 dose rate with changing influent loading to achieve a desired target effluent concentration.
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Document Type: Research Article

Publication date: 2007-01-01

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