It has been well over two decades that contaminated groundwater has been successfully treated by stimulating natural biological processes within polluted aquifers. This process, coined “in-situ bioremediation,” caught on in the 1990s and has since become a commonly employed strategy for cleaning pollution within groundwater systems and soils onsite.

The particular process of employing bioremediation to chlorinated solvent contamination is rather complex, often involving injection of organic chemical substrates into the contaminated subsurface. In turn, these substrates stimulate various biochemical processes that ultimately result in contaminant degradation or destruction.

The cost effectiveness of applying in-situ bioremediation to chlorinated solvent contamination hinges directly on the chemical and physical characteristics of the substrate applied and the ability to successfully distribute the material in the subsurface. Recent developments in substrate chemistry now allow for remediation engineers to distribute advanced substrates over significant subsurface volumes with lower capital costs.

Anaerobic bioremediation of chlorinated solvents

Enlarge this picture Figure 1: The reductive dechlorination process.

The most commonly encountered chlorinated solvent pollutants in groundwater and soil are chlorinated ethenes, commonly used as dry cleaning solvents and degreasers. These include perchloroethene (PCE), trichloroethene (TCE) and trichloroethane (TCA).

Bioremediation of these compounds takes place in the subsurface through a biological process known as “reductive dechlorination.” To stimulate this process, the environmental engineer must apply a substrate directly to the contaminated subsurface zone, usually though borings or wells. The substrate is then attacked by naturally occurring soil microbes in what is technically a fermentation reaction to produce small amounts of dissolved hydrogen. In turn the hydrogen is used as an energy source by another set of subsurface microbes known as dechlorinating microbes.

In the final step of the process, the dechlorinating microbe donates an electron to the chlorinated solvent pollutant removing a chlorine atom in the process. This dechlorination proceeds stepwise, removing multiple chlorine atoms until achieving complete dechlorination. Once the chlorine atoms are removed from a contaminant like PCE, the remaining portion is rapidly degraded in the subsurface by a number of processes (Figure 1).

Soluble substrates

Much has changed since the first reductive dechlorination projects were undertaken. Initially, experiments were performed in the field with the direct injection of gaseous hydrogen. This approach, while successful, was not readily adopted commercially due to the logistics of applying the hydrogen gas itself into the subsurface. Research conducted in a variety of university laboratories around the world then focused on the potential of common organic substrates and their ability to ferment in the subsurface thus releasing hydrogen in place.

Fermentation through the use of many low-cost, common, soluble substrates has been shown to result in adequate hydrogen generation within the subsurface environment and to stimulate reductive dechlorination. Such soluble substrates include: sugar solutions, organic acids such as lactate, alcohol and others. Each of these compounds are easily pumped into the subsurface and rapidly dissolved in groundwater.

It was soon realized, however, that while these soluble substrates were low-cost on a per-pound basis, the cost of application was often high, leading to very high life cycle project costs.^[1] The unexpected costs of using soluble substrates are usually associated with:

Rapid fermentation, resulting in much of the hydrogen generating methane rather than stimulating dechlorination. This requires either a continuous addition of the substrate or frequent reapplication. It is not uncommon to require monthly reapplications of soluble substrates such as lactate solutions.

Fouling of injection wells due to rapid biological growth during continuous injection requiring injection well maintenance

“Washout” as fast-moving groundwater systems can carry the soluble substrate down gradient out of the treatment zone.

Controlled-release substrates

A variety of controlled-release substrates have been developed that offer the remediation industry a lower-cost approach to injecting soluble organic compounds. These alternatives include compounds such as specialized polylactate esters that slowly release lactate^[2], to varying formulations of emulsified vegetable oils that are applied in high volumes through multiple wells onsite. The common element of all of these products is that upon application to the subsurface, they produce a controlled-release of the hydrogen needed for chlorinated solvent contaminant reduction without the need for costly reapplication. The use of these products, while higher on a unit costs basis (dollars per pound of substrate), usually result in much lower overall project life-cycle costs.^[1]

When employing a controlled-release substrate for chlorinated solvent bioremediation, the initial subsurface distribution of the product is key to the success of the project. Without necessary hydrogen within the target treatment zone, the desired contaminant degradation will not be achieved. The question put before the remediation design engineer is how to most cost-effectively distribute the product within the aquifer matrix? This requires maximizing the subsurface volume impacted by the controlled-release product from each point of subsurface injection (boring or well).

One of the most important factors influencing the distribution of bioremediation substrates is hydrophile/lipophile balance (HLB). The HLB is an index used to describe the tendency for a chemical substance to dissolve in water (hydrophilic) or oil (lipophilic).^[3] The more hydrophilic a compound (high HLB) the more readily it dissolves and distributes in the subsurface. Conversely, the more lipophilic (low HLB) the less likely it will dissolve in water and the more likely it will bind to the aquifer or soil matrix and distribute poorly. A substrate with very high HLB such as lactate (HLB 30) will distribute rapidly in the subsurface through diffusion and flow with groundwater movement, but as mentioned previously, the high solubility may cause it to ferment too rapidly and may wash out of the treatment area.

Conversely, emulsified oil substrates have a very low HLB (-6), which significantly limits its distribution. Emulsified oil substrates, when injected into the subsurface, tend to rapidly bind to the aquifer-mineral surface. This generally occurs within the first 1 to 2 meters from the injection point.^[4] Laboratory studies have shown, and field data has corroborated, that the distribution of emulsified oil substrates is independent of concentration of the oil emulsion injected and of the speed with which the emulsion is applied. Unfortunately, injecting amounts of additional “chase water” does not push these emulsified oil substrate droplets further out as once it is bound to the aquifer the oil is immobile.

Due to its very low HLB and insolubility, these substrates will not re-dissolve and distribute by diffusion or groundwater flow. Instead, emulsified oil substrates remain sorbed to the aquifer directly in the area injected. The result is that the proper use of emulsified oil substrates requires many injection points or wells packed closely together in order to achieve adequate coverage of the aquifer volume to be treated.

Controlled-release substrates with balanced HLB

Enlarge this picture Figure 2: Treatment design and cost summary comparison.

In an effort to lower the cost of applying bioremediation substrates, researchers at Regenesis, San Clemente, Calif., have developed a substrate with a balanced HLB having tendencies to adsorb, yet also to dissolve. This product (known as 3-D Micremulsion) is injected into the subsurface as a suspension mixed with water and has a tendency to sorb onto the aquifer matrix, similar to emulsified oil substrates.

However, after sorbing to the aquifer surface, the majority of the substrate droplets re-dissolve into groundwater, leaving behind only a thin coating of the substrate rather than a heavy layer of adhered oil droplets. The dissolved substrate then moves with the groundwater flow and diffusion until it re-adsorbs nearby onto the aquifer surface. Research has shown that if the dissolved concentration of these balanced HLB substrates exceeds what is referred to as the critical micelle concentration, or CMC, the dissolved substrate can actually form molecular spheres containing very small droplets (0.01 to 0.05 microns) of the substrate called “micelles.” These micelles move outward in the subsurface until adhering nearby to the aquifer matrix.^[5]

Once attached, this cycle of partial dissolution/transport/adsorption repeats until a thin layer of substrate coats the aquifer matrix target area, where it then stimulates the desired bioremediation. The self-distributing property of these substrates allows for better penetration of the contaminated aquifer than has been available in a controlled-release biological substrate.

The benefit of gaining increased distribution in the subsurface upon injecting a substrate is very apparent: much lower capital cost. The self-distributing property of advanced substrates allows the remediation engineer to achieve treatment of the target area with fewer injection wells and application of less substrate material. A typical aquifer treatment design and cost summary is presented in Figure 2.

The technical approaches used in the bioremediation of chlorinated solvent contamination in groundwater and soils have evolved substantially over the past two decades. This is very apparent when selecting a substrate for injection into contaminated zones. While a range of options are available, controlled-release substrates have been shown to present lower overall project costs by saving time and money on operations and reapplication.

Recent advances in substrate chemistry now offer the remediation engineer the opportunity to utilize a controlled-release substrate that self-distributes over the target volume of the subsurface. This allows lower up-front capital life cycle costs in projects treating chlorinated solvents in-situ. PE

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* Per pound pricing is for “standard emulsion.” HRC Advanced standard emulsion is prepared on-site as a 1:10 microemulsion avoiding the excess shipping costs associated with transporting pre-mixed oil in water emulsions.