The International Information Center for Geotechnical Engineers

Soil Remediation Techniques: Examination of In Situ Chemical Oxidation

 

7. Implementation Design

To successfully reduce contaminants, the oxidant must come into contact with the contaminant molecules. As generally preferred, the distribution technique chosen should ensure that the oxidant is uniformly circulated throughout treated area (Interstate Technology and Regulatory Council, 2005).

In order to achieve adequate contact between oxidant and contaminate, the injected volume of the oxidant should represent an adequate fraction of the void space in the subsurface. If there is groundwater near, the volume used needs to be carefully determined (conservative) in order to not spread the contaminated groundwater beyond your site. The injection volume is largely dependent on the site-conditions and the type of oxidant being utilized. ISCO is typically used where contaminant concentration is high and it is usually introduced to the contaminant by pumping the oxidant into wells in the contaminated area. Common options include injecting into preexisting wells, specially installed wells, temporary direct push points, and permanant direct push wells (ITRC, 2001). These wells are installed at different depths in order to reach as much dissolved and undissolved contaminants as possible (USEPA 2012). Efficiency can be promoted by recirculating oxidants between wells so that the groundwater mixed with an oxidant is pumped out while more of the oxidant is pumped in through another well. Recirculation is especially useful for remediating large areas. This discussion will limit to describing application of the commonly used oxidants: permanganate, ozone, persulfate, and hydrogen peroxide (Interstate Technology and Regulatory Council, 2005). Implementation of these oxidants into the site requiring remediation is done in various stages which include a conceptual site model, bench-scale treatability testing, screening and selection of oxidant and delivery approach, pilot-scale testing, design development and full-scale implementation. The different methods for initiating this process are described as follows (City Chlor 2013).

7.1 Injection via Filters

As shown in Figures 5 and 6, the oxidant is injected into the soil with the use of pressure which requires vertical filters. It is rather simple to inject multiple rounds of injection but the cost of the material used in the injection filters can be high since the filter material should be resistant to the oxidants used. 

Figure 5: Schematic View of Injection (City Chlor, 2013)

Figure 6: Schematic Side View of Injection (City Chlor, 2013)

7.2 Injection via Direct Push Methods

In this process the oxidant is injected into the soil via direct push methods. Direct Push Technology refers to the driving, pushing, or vibrating of small-diameter hollow steel rods into the ground. The oxidant can be inserted into this steel rod to be distributed as needed. The maximum depth, however, is limited to 20 meters. The shape and dimensions of the oxidant plume of one injection point depends on the injection rate, pressure, heterogeneity, permeability, and speed at which the oxidant is consumed. It is overall best to use pumps that can handle a rate of 20 liters/minute and a pressure of at least 1500 kPa for sandy soils to 5500 kPa for loam soils. The most important soil characteristic that determines the success of this method is permeability. The lower the permeability, the more difficult the injection will be. 

This is relative simple to use to a certain depth but if a second round is needed, the cost can increase further because a new rod is needed. The radius of influence is slightly smaller than injection filters. This can be an advantage if you are treating according to the presence of contaminants. Direct push methods can be expensive if several injection rods are required (City Chlor 2013). 

Figure 7

Figure 7: Example of protected-screen Direct Push well installation (Interstate Technology and Regulatory Council, 2006)

7.3 Recirculation

This method combines direct injection or infiltration and groundwater abstraction. As seen in Figure 3, the oxidant is injected into one well and the groundwater with the oxidant is pumped out another well at a specific distance away which is a function of its time to oxidize the contaminant. Through the process of infiltration and extraction, a larger hydraulic gradient is created within the contaminated area. This increase ensures that the remediation can take place faster than with only an injection system. This larger hydraulic gradient increases the area of influence (CityChlor 2013). This can be at a high cost but can minimize the amount of oxidant needed and for shorter time duration. This process can be very efficient if there is good hydrological control. Figure 8 shows a simpler schematic of the process.

Figure 8

Figure 8: Diagram of recirculation method used in ISCO treatments (Interstate Technology and Regulatory Council, 2005)

7.4 Infiltration

This method is implemented by infiltrating the oxidant passively through horizontal or vertical filters. This is only possible in highly permeable soils but can be adapted the specific conditions of the site with vertical filter configuration. Horizontal filters are less recommended if there are heterogeneities in the soil. In a passive system, the infiltration capacity of the soil, the groundwater level, groundwater flow rate, and life of oxidant must be accounted for. Figure 9 shows this processs.

Figure 9:Injection through passive Infiltration (City Chlor, 2013)

The infiltration capacity of the soil is heavily influenced by the soil type. The coarser the soil, the higher success of infiltration via filters. Infiltration tests should be performed before this. For indirect application, the groundwater flow should be greater than 0.05 m/day and if it is lower than passive treatment should be sufficient. Furthermore, the following conditions must be satisfied for infiltration via filters to be a reliable method (City Chlor 2013):

  • The oxidant must remain reactive in the soil long enough to oxidize the contaminants
  • The oxidant must remain stable long enough to create a sufficient large radius of influence
  • The injection of an oxidant must not have harmful products to people or nature

7.5 Soil Mixing

In this technique, the contaminated soil is mixed with the oxidant with the use of auger drills which have a diameter of 1 - 3.5 meters. This creates excellent contact between the oxidant and contaminants but is limited to shallow soil (less than 2 meters in depth) and there is a loss of soil structure. As you increase in depth, the costs increase dramatically. The oxidant is applied by a dosing system in the auger. 

 

Figure 9

Figure 10: Soil Mixing using In Situ Oxidation (Regenesis, 2015)

7.6 Sparging

This is the injection of ozone into the saturated zone. It is only applicable for ozone. These methods have been derived from conventional compressed air injection techniques and have more stringent requirements and require expensive material due to the corrosive nature of ozone. During this process, ozone is produced continuously. Pilot tests can be performed to determine the radius of influence.

The three application possibilities are as follow (City Chlor, 2013):

Injection Filters

  • Microporous injection filter
  • The air/ozone is injected into the soil in very small bubbles
  • Able to spread a relatively great distance in the soil

Recirculation Well

  • Two injection filters and an underwater pump
  • Consists of 2 injection filters and an underwater pump
  • The injection causes an upward flow of the injected gas which in correspondence with the pump, creates a circulation effect
  • This results in a larger radius of influence

Perozone

  • Combination of gas/liquid injection
  • The injection filter is coated with glass beads where H2O2 is passed
  • This results in a mixture of ozone and hydrogen peroxide which is injected in the soil

 

 

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