Theme: 08. Metals

Topic Area: 8c. Precipitation and Stabilization of Metals

Formatted Title:

Evaluation of a New Treatment Amendment to Remove Mercury from Recovered Groundwater/Leachate at a Landfill

Background/Objectives:

Groundwater contaminated from a closed, unlined landfill is contained with a series of recovery wells and treated with air strippers to remove volatile organic compounds. Treated water is released to receiving streams via an NPDES permit. A change in a regulatory guidance document required low levels of mercury in the outfall to be treated to an even lower concentration. To this end a low cost and easy to maintain Hg pre-treatment system is being assessed. The Hg pre-treatment system has been developed through a series of increasing size and capacity field-scale pilot systems. A new Hg treatment amendment has been evaluated and has demonstrated the ability to very efficiently removed Hg from the water stream using simple equipment and low amendment dosage rates.

Approach/Activities:

The work that has been completed to date has progressed through several phases. The first phase of testing consisted of an initial demonstration of the Hg amendment removal efficiency through a bench-scale batch testing process. Following the success of the bench-scale testing, a field-scale pilot using large diameter (4-in PVC) flow-through columns was completed to evaluate various amendment loading rates, water/amendment contact times, Hg removal efficiency, and amendment longevity. Additional testing was also completed to evaluate the potential for the organics constituents to impact the Hg removal efficiency. The last step of the testing process involved the scale-up of the flow-through columns to a single 55-gal drum. Water from a recovery well having the highest load of Hg was used to evaluate the removal efficiency under the sites most extreme conditions. The single drum testing was designed to evaluate the selected amendment dosage at higher flow rates in order to determine the optimal throughput and other operational parameters that will provide the basis for the design and implementation of a full-scale Hg pre-treatment system.

Results/Lessons Learned:

Data from the completed bench-scale testing, flow-through column testing and the single drum testing have been collected and evaluated. The results of the bench-scale study indicated that low dosages of the new Hg amendment had the potential to remove significant concentrations of Hg with removal efficiencies of up to 99%. Additional data from the bench-scale testing will also be presented. The bench-scale study result supported the continued evaluation of the amendment. In the next phase of testing, large diameter flow-through column testing was completed to evaluated three amendment loading rates (1%, 3% and 5% by wt.) with flow rates of up to 0,5 gallons per minute (gpm) over a total testing period of more than 30 days. The results of the large diameter flow-through column testing indicated that a 3% loading rate of the Hg treatment amendment was an optimal dosage. Additionally, the flow-through column results demonstrated that with more than 60,000 pore volumes passing though the columns the Hg removal efficiency was observed to remain consistent between 90-99%, with effluent concentrations in the range of 20-50 ng/L. Additional data from the flow-through column study will also be presented. The flow-through column study results supported the continued evaluation of a larger scale pilot system using the Hg treatment amendment. In the last phase of testing, using a single 55-gal drum, has yielded lessons learned related to modifications to the design and packing of the drum with the treatment media. Modifications to the internal water distribution system and internal filtering media were completed over a series of short-term single drum tests in order to optimize the flow rate through the single drum without significantly impacting the Hg removal efficiency. The current single drum configuration maintains approximately 2 gpm throughput while reducing inlet Hg concentrations from ~795 ng/L to <20 ng/L at the outlet. The single drum testing was subjected to a long-term test of approximately 3 months to provide detailed data for the proof-of-concept and design details for a full-scale treatment system that will treat up to 50 gpm. The results of the long-term testing from the single-drum test will be available for presentation in 2024. Lessons learned throughout the testing process will also be presented.

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David Vollero