Posts Tagged ‘contaminants’

How does Electrical Resistance Heating Work?

April 1st, 2014

ELECTRICAL RESISTANCE HEATING

Electrical resistance heating is accomplished by passing an electrical current through the volume of subsurface soil requiring treatment. The process is equally effective in vadose and saturated zones. As the subsurface resists this flow of electricity, it is heated to the boiling point of the water/contaminant mixture present, regardless if the water source is soil moisture or a flowing aquifer. Following Dalton’s and Raoult’s laws, the temperature at which subsurface boiling occurs is dependent upon depth below the groundwater table, the type of contaminants present, and the ratio of contaminant concentrations. Increasing depth below the groundwater table and the presence of less volatile contaminants produces higher boiling points requiring increasing levels of energy consumption to reach remedial goals.

grs-3Phase-Diagram

3-Phase ERH Diagram

The temperatures created by Electrical Resistance Heating-ERH is sufficient to evaporate targeted contaminants and produce an in situ steam source by boiling soil moisture and groundwater. This steam than strips contaminates from the soil matrix and carries them to vapor recovery (VR) wells that may be co- located with the electrodes.

ERH brings the subsurface to boiling in a smooth and controlled manner. Because Electrical Resistance Heating-ERH  needs soil moisture to remain above 5% to be successful, the technology cannot desiccate soil or cause subsidence. ERH electrodes do not become significantly hotter than surrounding soil and no excess energy is stored in the subsurface in the form of temperatures above boiling. If ERH application ceases for any reason, such as a site-wide power failure, steam generation in the subsurface stops quickly. For this reason, backup generators for vapor recovery and treatment systems are not a requirement for  Electrical Resistance Heating-ERH remediation projects.

Once heating at an ERH site starts, pure contaminants boil first, then groundwater with high levels of dissolved phase contaminants, and finally clean groundwater. Thus, ERH specifically targets the most impacted sections of the subsurface, including non-aqueous phase liquids (NAPLs) and contaminants adhering to the soil matrix. Additionally, ERH cleans saturated soil without having to dewater it, eliminating the need for groundwater extraction, treatment, and disposal systems.

Electrical Resistance Heating Contaminants

The type of contaminants and desired cleanup goals affect the energy, time, and cost to clean a given treatment volume. The two most important factors in evaluating the performance of ERH at a site are the level of total organic carbon (TOC) in soil and the presence of SVOCs such as heavy fuel hydrocarbons. Both of these substances hold CVOCs in the subsurface, making them more time consuming and expensive to remove regardless of the remediation technology deployed.

EPA-In Situ Technologies for Contaminated Soils