- What is ISCO?
ISCO is a remediation technology that uses strong chemical oxidizers to react with organic contaminants resulting in harmless residual compounds. ISCO is typically used for source zone treatment of moderate to high contaminant concentrations. Oxidation is the process of destroying molecular bonds of Contaminants of Concern (COC) resulting in removal of electrons. The reaction chemistries vary based on the specific oxidants, site conditions, and contaminants treated. I will spare the reader the detailed discussions about direct and indirect oxidation pathways, reaction kinetics, and radical formation. The important thing to know is that ISCO treatment is considered a “contact sport” where oxidizing chemicals must be placed directly in contact with the contaminant to affect treatment. For this reason, oxidant delivery and distribution are of key importance. Proper dosing is also important since natural organics in the treatment area will also react with the oxidant. Oxidant selection is also paramount for success since each product has its advantages, disadvantages and contaminant selectivity.
- What are the primary ISCO reagents used by industry?
There are several oxidant products available for ISCO remediation – the selection of which depends on your target contaminants, concentrations, site geochemistry, and COC mass distribution. The following table summarizes key ISCO reagents and their general properties (ozone gas technologies are omitted).
|Potassium Permanganate (KMnO4)||RemOx-S®|
|Purple solid crystals delivered in 55-lb pails or 330-lb drums at 100% concentration, typically applied at 1-3% using potable mix water. Temperature dependent and less soluble than NaMnO4. Activation is not required. Appropriate for treating low range COC concentrations. Can typically persist in the formation up to 6 months.|
|Sodium Permanganate (NaMnO4)||RemOx-L®|
|Purple liquid delivered in 5-gal pails, 55-gal drums, 275-gal totes at 40% concentration, typically applied at 3-20% using potable mix water. Very soluble and not temperature dependent. Activation is not required. Appropriate for treating high range COC concentrations. Can persist in the formation 12 months or longer.|
|Sodium Persulfate (Na2S2O8) + Activator||Klozur®|
|A white fine powder often delivered in 55-lb bags or pails. Klozur persulfate requires activation using liquid caustic (NaOH), iron, peroxide, or heat. PersulfOx contains a built-in activation catalyst (Sodium Silicate). Persulfate is highly soluble and is commonly mixed at concentrations of 10 to 30%. Activated persulfate stays reactive in the formation for about 2 to 4 weeks.|
|Sodium Percarbonate (Na2CO3) + Activator||RegenOx®||Sodium Percarbonate (soda ash) is combined with a sodium silicate-ferrous sulfate catalyst (activator) in a 2-part commercial oxidant called “RegenOx”. Percarbonate is a white powder that comes in a 5-gallon bucket (Part A) and for activation is mixed with a catalyst gel that also comes in a 5 gallon bucket (Part B). Typically injected at a 4-10% concentration, it stays reactive in the formation for up to 30 days.|
|Catalyzed Hydrogen Peroxide (H2O2)||Various||Hydrogen peroxide is a colorless liquid that is thermodynamically unstable at high concentrations. Usually catalyzed using transition metals (natural iron is often sufficient) forming hydroxyl radical. Typically injected at 4-10% concentration. Fast reaction (not persistent). Distribution challenges due to gas evolution limits it application. Stays persistent in the formation for a few days at best. Less popular oxidant these days.|
Combined Remedies – There is a growing trend for “combined remedies” where multiple technologies are used to attack challenging plumes. There are several modified ISCO products that include additives for extended release oxygen to support enhanced aerobic bioremediation. These hybrid ISCO products are not addressed in this article.
3. What contaminants are treated by ISCO?
|Potassium Permanganate (KMnO4)||Chlorinated Ethenes (primarily PCE and TCE). Can degrade some PAHs, Phenolics and pesticides.|
|Sodium Permanganate (NaMnO4)||Chlorinated Ethenes (primarily PCE and TCE). Can degrade some PAHs, Phenolics and pesticides.|
|Sodium Persulfate (Na2S2O8) + Activator||Most Chlorinated Solvents (PCE, TCE, DCE, VC, etc.), BTEX, GRO, DRO, MTBE, TBA, Chlorobenzenes, Phenols, PAHs, Pesticides, others.|
|Sodium Percarbonate (Na2CO3) + Activator||Most Chlorinated Solvents (PCE, TCE, DCE, VC, etc.), BTEX, GRO, DRO, MTBE, TBA, and others.|
|Catalyzed Hydrogen Peroxide (H2O2)||Most organic COCs|
4. What are the primary advantages of using ISCO?
ISCO is most ideal for source zone treatment but is also used for grid treatment of plumes where COC concentrations are moderate to high. ISCO has also been used for recirculation and barrier treatment of groundwater plumes using oxidants with a high persistence (ie. Permanganate) or where injection accessibility is limited. It has also been used for aquifer “polishing” but is considered expensive for most low-concentration treatment efforts. ISCO can be effective for vadose zone remediation using soil mixing since contact time can be managed. Vadose zone ISCO treatment using injection requires tightly spaced injection grids which can make it cost prohibitive. Similarly, sites with highly organic soils may eliminate chemical oxidation as a practical option highlighting the importance of testing for Natural Oxidant Demand (NOD). ISCO typically requires a minimum of 2 to 3 treatment events. Good site assessment and bench testing data is essential for ISCO success.
Permanganate has advantages due to its persistence and density. It reacts comparatively slower than persulfate and so can migrate further into the formation before reacting. Its higher density (as NaMnO4) allows it to travel vertically through density-driven diffusion. Also, permanganate’s persistence can support treatment of heterogeneous conditions where matrix back diffusion is an issue. Permanganate is effective over a wide pH range (~pH 3-12). Its strong color makes it easy to trace in the subsurface.
Persulfate reacts rather fast (once activated) but has the advantage of effectively oxidizing a large range of target contaminants. Despite its fast-acting and aggressive reaction, persulfate treatment results in little heat or gas generation. Most effective in high pH conditions (~pH 10.5-12). Its high conductivity can be used as a tracer in the subsurface.
Percarbonate reacts rather fast (once activated) and can target a large range of contaminants. Activated Percarbonate results in short-lived oxidation with some heat generation but it is non-corrosive allowing it to be injected near utilities. Groundwater temperature increases have been known to be 5-10 deg F for about 10+ days.
Peroxide has the advantage of being inexpensive, very reactive, and capable of degrading most types of contamination. However, exothermic heat generation is often problematic with peroxide and so application concentrations should be kept below 10% if possible. Subsurface distribution of peroxide can be problematic due to reactivity and reagent surfacing is a common problem.
5. What parameters are needed to support a successful ISCO design?
ISCO planning and pricing requires several common parameters, some of which can be estimated (assumed) and others supported with testing/measurements. Bench testing can be performed to determine Natural Oxidant Demand (NOD), treatability, and other soil properties that might limit ISCO success. Pilot testing is usually recommended to confirm planning assumptions such as soil properties, tooling needs, flow rates, injection pressures, and ROI. ISCO dosing calculations and implementation costs are driven by some minimum parameters as outlined below. Safety factors are applied to those calculations based on the level of data certainty.
|Permanganate||Permanganate Oxidant Demand (PNOD), treatment area & thickness, soil type, soil density, effective & mobile porosity, hydraulic conductivity, foc, gradient, design COC concentrations (soil & gw), assumed ROI.|
|Persulfate||Natural Oxidant Demand (NOD), treatment area & thickness, soil type, soil density, effective & mobile, hydraulic conductivity, foc, gradient, design COC concentrations (soil & gw), assumed ROI, buffering capacity (alkalinity) if activating with NaOH.|
|Percarbonate||Natural Oxidant Demand (NOD), treatment area & thickness, soil type, soil density, effective & mobile, hydraulic conductivity, foc, gradient, design COC concentrations (soil & gw), assumed ROI.|
|Catalyzed Hydrogen Peroxide (H2O2)||Acid buffering and natural carbonates, treatment area & thickness, soil type, soil density, effective & mobile porosity, hydraulic conductivity, foc, gradient, design COC concentrations (soil & gw), assumed ROI.|
6. How are the oxidants delivered?
There are several optional approaches for delivering oxidants for subsurface treatment. The most common approaches are summarized as follows:
|DPT Injection||Most common delivery method for ISCO since it provides more control over distribution. Injection pressures can vary from low to high but usually less than formation fracture pressure. Typically uses 1.5 to 2.25-inch diameter rods with radial injection pattern delivered in 3-5+ foot thick intervals depending on lithology and treatment interval. More common delivery method for heterogeneous sands, silts, and clays.|
|Well Injection (vertical, horizontal & recirc.)||Oxidants are pumped down injection wells using a well adapter and drop pipe using low pressures. More common delivery method for homogeneous sands, gravels, and bedrock. Nested injection wells may be required if heterogeneous or massive lithologic units are targeted. Stainless steel construction is recommended for peroxide. Wells can be manifolded for simultaneous injection and strategies can include groundwater recirculation to maximize distribution.|
|Augering or Sonic Drilling w/Packer Injection||For difficult geologic conditions, injection holes are advanced using small diameter auger or sonic methods to support packer injection at target intervals in a bottom-up approach. This approach may include proppants or sand-reagent mixtures (RemOx® SB).|
|Pneumatic Fracturing w/Packer Injection||For bedrock or clayey sites, the formation can be pneumatically fractured to support ISCO using packers.|
|Soil Mixing||For shallow sites with accessibility, oxidants can be mixed into saturated soils in treatment cells and lifts.|
|Specialty Methods||Oxidants have both been manufactured into paraffin wax cylinder products for slow-release, passive/barrier treatment approaches.|
Oxidant Delivery Comments: For well injection methods, Potassium Permanganate >2.5% can plug well screens, pre-filtering is recommended. Sodium Permanganate is easy to handle and ideal for well injection. For Sodium Persulfate, the reagent and activator can be injected separately or together if pre-mixed (in small batches) and immediately injected. For Percarbonate, the reagent and activator are typically pre-mixed (in small batches) and immediately injected. For peroxide, catalysts are typically injected separately from the reagent due to fast reaction and gas evolution (surfacing is a common problem).
Tooling issues: Persulfate is corrosive to galvanized tooling such as injection rods and fittings. Inner-hoses are often used to protect tooling. Permanganate is not corrosive to tooling but is incompatible with strong acids, reductants, and organic compounds. Excess permanganate can be neutralized effectively using a mixture of vinegar, peroxide, and water.
7. What do chemical oxidants cost?
Like most material purchases, oxidant costs will vary based on order quantities. The price ranges presented below reflect my experience with close to 100 typical ISCO projects. Sales tax is regionally variable. In California, sales taxes range from 7-10%. Freight costs depend on distance, weight, and unloading criteria but I have found that it generally ranges from 8-15% of the product costs. Carrying costs are any mark-ups by the consultant or contractor needed to coordinate the order and float the cashflow to support the purchase.
|Potassium Permanganate (100% KMnO4)||$2-$3/lb + sales tax + carrying costs + freight (quantity dependent)|
|Sodium Permanganate (40% NaMnO4)||$1.75-$2.50/lb + sales tax + carrying costs + freight (quantity dependent)|
|Sodium Persulfate (Na2S2O8)||$1.50-$2.25/lb + sales tax + carrying costs + freight (quantity dependent)|
|Sodium Percarbonate (Na2CO3) + Activator||$1.50-$2.25/lb + sales tax + carrying costs + freight (quantity dependent)|
|Catalyzed Hydrogen Peroxide (H2O2)||$0.75-$1.50/lb + sales tax + carrying costs + freight (quantity dependent)|
8. What are the common drivers and typical costs for ISCO implementation?
There are a multitude of site remediation scenarios for ISCO making cost estimation a subjective endeavor that is dependent on many variables. The following are some rules-of-thumb guidance to assist in ISCO field planning (contractor & supplier costs).
|Percent Effective Pore Volume Targeted||25% to 50% (25% is common)|
|Typical Oxidant Concentrations||Potassium Permanganate: 1-3%
Sodium Permanganate: 3-20%
Sodium Persulfate: 5-30%
Sodium Percarbonate: 4-10%
Hydrogen Peroxide: 4-10%
|Activator Costs for Persulfate or Peroxide||Typically, 25% of oxidant cost|
|# Injection Events to Achieve Remediation Goals||Typically, 3 events separated by 2-3 months (depends on oxidant persistence & seepage velocity)|
|Average Per-Location Flow Rate||1-3 gpm (Silt – Silty Sand), 2.5-5 gpm (Sands)|
|Average # of manifolded locations (DPT)||Commonly 3-5 depending on lithology, pressures, volumes/interval & flow rates but can be up to 10 or more for large efforts.|
|Average # of manifolded locations (Wells)||Commonly 5-10+ depending on allowable flow rates, volumes/screen and pressures|
|Actual Injection hours per day |
(8 hours onsite)
|DPT: 6 hours (higher setup, breakdown, and tooling advance time). Wells: 8 hours (minimal setup & breakdown time)|
|Key Drivers for Project Duration||Total injection volume, number of locations, injection depth (DPT)|
|Average length of target interval||15 feet|
|Well Injection Contractor||Mixing/Injection (pump) rig, 2-3 man crew………………………$3K-$5K/day|
|DPT Injection (easy-mod drilling)||DPT rig, mixing/Injection rig, 3-4 man crew………………………$4K-$6K/day|
|DPT Injection (deep/hard drilling)||Large DPT rig, mixing/Injection rig, 3-4 man crew……………$5K-$8K/day|
|DPT Injection – High Volume ISCO||Rigs + equip. for high-vol. production, 4-5 man crew……..$6K-$10K/day|
|Pneumatic Fracturing w/ISCO||Sonic rig, fracture module, mix system, packer system….$15K-$20K/day|
|Other Potential Injection Contractor Costs||Mobilization, per diem, reagents, fence rentals, fork lift, water meter, bulk storage tank rental, anaerobic water generation.|
|Soil Mixing ISCO (5-ft lifts)||Typically 150-250 cy/day, excavator w/mixing head, crew, reagent slurry mixing/pumping system, loader, water truck……$10K/day + setup|
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