Figure 1:  Steam Injection to Clean Up Semi-Volatile DNAPLs

     Figure 1 is a drawing of a Steam Injection project.  Steam, and air, is injected in widely spaced wells.  As steam condenses, water sinks and flushes NAPL towards the producer, while uncondensed steam rises, vaporizing NAPL, and moves over the top of the treated zone.  Since the producing wells are at reduced pressure, they are probably cooler than 100^o C while the injectors are very hot. Vaporization is the main producing mechanism, and clay layers are cleaned as the layers heat by conduction.

     Steam has been used to increase production of petroleum for over 40 years, and millions of barrels of steam are injected daily.  Steam reduces the viscosity of liquids, introduces a gas phase (thereby raising NAPL mobility), and increases the NAPL vapor pressure so that extraction is enhanced.  In addition, the higher temperature promotes chemical reactions, while at lower temperature microbial activity increases.  Steam temperature increases with pressure and depth.  At 125 feet, steam injected at 100 psi has a temperature of 328^o F.  This can vaporize semi-volatiles like pyrene.

      If more is needed, High Temperature Thermal Conduction can heat everything to 700 ^oC.

Figure 2: Thermal Conduction to Clean Up Low-Volatility DNAPLs

     This technology, shown in Figure 2, will vaporize less volatile contaminants like PCB’s, pesticides, and even mercury. The soil is heated by electrical heating elements in closely spaced wells.  The heaters can be as hot as 900^o C and the soil will reach 700 ^o C in a few weeks.  Wells are under vacuum, so air drawn in from the edge of the pattern oxidizes and vaporizes organic material.  Anything that survives the well is destroyed by thermal oxidizers on the surface.

     The technology works best in the vadose zone, since it is costly to boil an aquifer.  To achieve a high temperature the wells are closely spaced, and this much heat is expensive, so the cost can be higher than other technologies.  However, this is the only way to completely clean a site.   If less heat is needed, the cost is much lower, and other heat sources are possible.

Figure 3: Electrokinetics to Remediate Water Soluble Metals

     Electrical Current - Electrical resistance heating and electrokinetics can move metals, mobilize NAPLs, or heat the soil as high as 250 ^oF to vaporize NAPLs.  Both technologies work because moist soil conducts electric current.  Thus, it can either be heated, and vaporized NAPLs can be collected at wells or electrodes, or ions can move with the current and be collected at other types of electrodes.

     The resistance of the soil and mobility of fluids are controlled by the soil moisture so this must be controlled, sometimes by injecting water.  The wells must be closely spaced so capital cost for a project is higher.   While heating is a fast process, moving ions with a current is much slower since the current must change the charge on the soil in order to mobilize ions.  However, electrokinetics is one of the few technologies for recovering metals, and resistance heating works well in clay layers.
 

Figure 4: Surfactant Injection to Clean Up Low Viscosity DNAPLs

     Flushing with surfactants and chemicals can recover NAPLs, and some oxidized metals, or immobilize other metals in the soil.  The technology works best in saturated soil, since water is being injected, but can also work in the vadose zone.  Surfactants clean up NAPLs by reducing the capillary pressure that traps them.  A surfactant (soap) is used with a polymer, foam, or microbes to almost completely remove NAPLs and some metals.

      Reducing agents are used to precipitate other metals.   Oxidizing agents can decompose NAPLs. The cost of many treatments is not high and comparable to that of steam flushing.

     MK Tech Solutions believes that each of the technologies has unique advantages and that none can be used at all sites.   We also believe that restoring value to property is a powerful driving force for cleanup.