Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4b. Landfill Assessment and Remediation

Formatted Title:

ISTR Pilot Testing for Improvement of Full-Scale Design: How to Deal with Surprises and Improve Design

Background/Objectives:

A mixed waste landfill near Rome, Italy owned by Tre Monti srl. is scheduled for remediation using thermal technologies. The waste layers contain volatile and semi-volatile compounds such as TCE, PCE, hexachloroethane and hexachlorobenzene. Before initiating full-scale design, a pilot test of thermal conduction heating (TCH) was conducted on approximately 10% of the landfill area. The pilot area was isolated by the installation of a 14 meter deep sheet pile cell. The test objective was to heat the subsurface material to near 100^oC, capture and treat the generated vapors, and to sample soils before and after to illustrate achievable reductions in VOC and SVOC concentrations. Lessons learned during the pilot test were to be used to improve the full-scale design.

Approach/Activities:

Sheet piles were installed to form a square pilot test area of 15 by 15 m.The target treatment interval was from 9 to 13 m below grade. Thirty-nine heater borings with heaters from 7.5 to 14.5 m depth were installed. Seven extraction wells and four temperature monitoring borings were placed between the heaters. A TCH power delivery system capable of delivering 250 kW of power to the heaters was used. Vapors and liquids were conveyed to an on-site treatment system with cooling, condensation, phase separation and treatment of vapors and liquids using granular activated charcoal.

Results/Lessons Learned:

The presentation will focus on several unexpected observations, including:

After installation of the sheet-pile cell, heaters and extraction wells, the water level was found 3-4 m below grade. The site conceptual model and the TCH pilot design was based on a water level deeper than 8 m.
With shallow groundwater, vapor extraction screens were submerged. Groundwater was pumped from seven wells to create drawdown and to make it possible to apply a vacuum to the formation.
Heating was slower than expected. The ability to inject energy using TCH was limited by the upward flow of steam around the heaters. Where unsaturated soil was expected, groundwater was present and made the steam more buoyant.
Water kept coming into the pilot zone from below and/or from the sides. A water balance showed that more than one pore volume of water was removed, yet the water level never dropped to the desired depth near the bottom of the treatment interval.
Due to the steam migration issues and incoming water, target temperatures could not be reached in the whole test volume.

Despite these challenges, soil concentrations after the heating period showed substantial reductions from starting levels. TCE was reduced by over 99% to concentrations below 0.5 mg/kg, PCE was reduced by 98.5%, hexachloroethane was completely removed, and hexachlorobenzene concentrations were reduced by 31%. The full-scale design will be derived and enhanced by these observations. Due to the presence and migration of water, the thermal technology of choice will be evaluated. Electrical resistance heating (ERH) can cope with more water than TCH and will be considered. Also, water usage during installation of the thermal system will be minimized and monitored. A detailed characterization effort is being undertaken to ensure that the full-scale design is based on realistic data on geology, water levels and contaminants present.

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Matteo Donati