text: Electrical resistance heating is used by the environmental restoration industry for remediation of contaminated soil and groundwater.  ERH consists of constructing electrodes in the ground, applying alternating current (AC) electricity to the electrodes and heating the subsurface to temperatures that promote the evaporation of contaminants.  Volatilized contaminants are captured by a subsurface vapor recovery system and conveyed to the surface along with recovered air and steam.  Similar to Soil vapor extraction, the air, steam and volatilized contaminants are then treated at the surface to separate water, air and the contaminants.  Treatment of the various streams depends on local regulations and the amount of contaminant.
  Some low volatility organic contaminants have a short hydrolysis half-life  For contaminants like these, i.e. 1,1,2,2-Tetrachloroethane and 1,1,1-trichloroethane, hydrolysis can be the primary form of remediation.  As the subsurface is heated the hydrolysis half-life of the contaminant will decrease as described by the Arrhenius equation.  This results in a rapid degradation of the contaminant.  The hydrolysis by-product may be remediated by conventional ERH, however the majority of the mass of the primary contaminant will not be recovered but rather will degrade to a by-product.
  There are predominantly two electrical load arrangements for ERH: three-phase and six-phase.  Three-phase heating consists of electrodes in a repeating triangular or delta pattern.  Adjacent electrodes are of a different electrical phase so electricity conducts between them as shown in Figure 1.  The contaminated area is depicted by the green shape while the electrodes are depicted by the numbered circles.
  Six-phase heating consists of six electrodes in a hexagonal pattern with a neutral electrode in the center of the array.  The six-phase arrays are outlined in blue in Figure 2 below.  Once again the contaminated area is depicted by the green shape while the electrodes are depicted by the numbered circles.  In a six-phase heating pattern there can be hot spots and cold spots depending on the phases that are next to each other.  For this reason, six-phase heating typically works best on small circular areas that are less than 65 feet in diameter. 
  ERH is typically most effective on volatile organic compounds (VOCs).  The chlorinated compounds perchloroethylene, trichloroethylene, and cis- or trans- 1,2-dichloroethylene are contaminants that are easily remediated with ERH.  The table shows contaminants that can be remediated with ERH along with their respective boiling points.  Less volatile contaminants like xylene or diesel can also be remediated with ERH but energy requirements increase as the volatility decreases.
  Electrode spacing and operating time can be adjusted to balance the overall remediation cost with the desired cleanup time.  A typical remediation may consist of electrodes spaced 15 to 20 feet apart with operating times usually less than a year.  The design and cost of an ERH remediation system depends on a number of factors, primarily the volume of soil/groundwater to be treated, the type of contamination, and the treatment goals. The physical and chemical properties of the target compounds are governed by laws that make heated remediations advantageous over most conventional methods. The electrical energy usage required for heating the subsurface and volatilizing the contaminants can account for 5 to 40% of the overall remediation cost.
  There are several laws that govern an ERH remediation.  Dalton’s law governs the boiling point of a relatively insoluble contaminant.  Raoult’s law governs the boiling point of mutually soluble co-contaminants and Henry’s law governs the ratio of the contaminant in the vapor phase to the contaminant in the liquid phase.