Posts Tagged ‘in situ’

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

Electrical Resistance Heating (ERH) White Paper

June 9th, 2013

When chlorinated hydrocarbons, such as dry cleaning fluids and industrial solvents, are released into the environment, they migrate downward until they reach groundwater. Because they are heavier than water, they continue sinking through the aquifer until they find a layer of tight soil where they form pools of pure product. As these solvent pools slowly dissolve into the groundwater, they feed large contaminant plumes capable of impacting human health and the environment well down gradient of the impacted site.

These solvent pools are often found at depths of over 30-feet below ground surface and 20-feet below the water table, making them impossible to access using conventional remediation technologies. For example, the shoring and dewatering requirements for soil excavation and disposal at these depths represent insurmountable safety, engineering, and cost challenges. grs-3Phase-Diagram

Electrical Resistance Heating (ERH) is an aggressive in situ remediation technology that can simultaneously treat the solvents found in saturated and unsaturated soil, groundwater, and pools deep below the groundwater table. ERH is often used to clean sites where other technologies have had limited success. Using ERH, even large sites can be remediated quickly and completely. The technology is so robust that it can return highly contaminated groundwater to drinking water quality.

Developed for the US Department of Energy, ERH takes power from standard utility lines and applies it to electrodes placed in a grid pattern across an impacted site. As the subsurface resists this application of electricity it is heated to the boiling point of water producing steam and contaminant vapors. Installed to the maximum depth of contamination, ERH systems can heat to over 100-feet below grade.

During ERH, pools of solvents located below the water table are boiled first and this large contaminant mass is quickly removed from the subsurface. Next, steam formed in the subsurface starts driving contaminants out of soil and groundwater. As steam tries to escape from the subsurface, it sweeps contaminants to the recovery wells where they are collected and carried to the surface for treatment.
At the surface, steam and contaminant vapors are condensed into water, liquid contaminants, and cool contaminate vapors. Liquid contaminants are collected for recycling while condensate water and contaminant vapors are treated prior to release to the local sewer system and the atmosphere.

ERH can be applied in flowing gravel aquifers, heterogeneous glacial tills, and tight clays. It can treat fuel hydrocarbons, chlorinated solvents (dry cleaners), and polycyclic aromatic hydrocarbons (PAHs) such as creosote and coal tar. It can be safely used under roads, parking lots, and occupied buildings without disruption of traffic or occupancy. GRS would be happy to review any site for the use of ERH, and an easy to use site evaluation tool can be found on our web site.