Terra Systems :: Rationale for Biological Approach

Rationale for Biological Approach

TSI’s experience indicates that site remediation costs for various technologies are generally consistent with the cost metrics which are discussed in Quinton et al's 1997 paper entitled “A method to compare groundwater cleanup technologies”. In that paper, the authors predict costs (on a net present cost basis) for Natural Attenuation to be 9% and Accelerated Anaerobic Bioremediation to be 13% of a Pump-and-Treat System, i.e. a savings of 91% and 87% respectively.

There are basically three different techniques for in situ biological transformation of chlorinated contaminants: Natural Attenuation; Passive Accelerated Anaerobic Bioremediation; and Accelerated Anaerobic Bioremediation. Depending on site conditions, economics, and the site owner's goals, one or more of these techniques can be used at the site. We define these techniques in the following manner:

Natural Attenuation: Involves the use of unenhanced natural processes as part of a site remediation strategy. These processes which can transform contaminants to less harmful forms or immobilize them to reduce risks result from biological, chemical, and physical reactions that take place in the subsurface. The dominant attenuation process for chlorinated organics is biotransformation.

Passive Accelerated Anaerobic Bioremediation: Using a direct push method, a slow release substrate such as an edible oil or edible oil emulsion is injected into the formation to generate reducing conditions and provide the native micro flora with sufficient carbon to conduct reductive dechlorination. Although a number of factors impact the speed of the biological process, the technique most likely will require less than half the time required for Natural Attenuation.

Accelerated Anaerobic Bioremediation: Using a recirculating groundwater system as the transport mechanism, a quick release substrate such as sodium lactate is continuously added and moved throughout the formation. Depending on site conditions, this technique can be expected to remediate a site in a time frame of six months to 5 years.

The accelerated anaerobic bioremediation process uses either native or introduced microorganisms to degrade chlorinated solvents such as PCE to innocuous end products including ethene and ethane. An organic substrate must be added to the groundwater to generate reducing conditions and provide the necessary carbon to support biodegradation of the chlorinated solvents. The organic substrate can be soluble compounds such as benzoate, lactate, molasses, acetate, or methanol. These soluble substrates must be added periodically at either high batch dosages or continuously at low dosages to provide the necessary carbon. Groundwater recirculation is generally necessary to distribute the dissolved organics throughout the contaminated site. Another option for the substrate would be to use a relatively insoluble compound such as edible oil (i.e. soybean oil) which would be added to the groundwater periodically (probably every two to ten years) and usually does not require the groundwater recirculation system. Additional nutrients such as nitrogen, phosphorus, trace elements, and vitamins are also needed.

TSI has used edible oils as substrates to support reductive dechlorination. We have found that soybean oil, corn oil, corn oil margarine, coconut oil, and beef tallow can support reductive dechlorination in both batch microcosm tests and in column studies for over two years (Lee et al. 2000). The oils are an inexpensive and long-lasting substrate. We have also developed a patented system for injecting an emulsion of edible oil that can be distributed to a wide area of the aquifer (Borden and Lee, 2002).

SRSä Approach - The SRSä approach for stimulating in situ reductive dehalogenation dramatically reduces both the initial capital and long-term O&M costs of treating chlorinated solvent impacted formations. An injection of a low solubility, slowly biodegradable food-grade edible oil (SRS™) is made through a direct push technology or temporary injection wells. Depending on site-specific conditions, a mixture of several different oils and soluble substrates may be required. The soluble substrates are used to induce rapid, initial biodegradation of the chlorinated solvents in the more bioavailable regions (sand layers). Over time, the oils slowly dissolve and are degraded providing a steady release of dissolved organic carbon to degrade any contaminants that slowly bleed out of low permeability zones.

Our approach eliminates the need for groundwater recirculation systems, and their associated high capital and operating costs. The SRS^TM technology is especially useful in low permeability formations where it is difficult and expensive to circulate treatment fluids. The SRS™ process is superior to soluble substrate systems, in which the substrate can be rapidly biodegraded before it reaches the target contaminants. Our low solubility oil technology overcomes this problem by placing the slowly biodegradable oil in close contact with the contaminant through conventional wells or using direct push technology.

The key to successful implementation of an in situ bioremediation project is cost-effective distribution of electron donors. Shown below is an evaluation of the following substrates: molasses, corn syrup, lactate, polylactate ester, and SRS^TM. Molasses, corn syrup, lactate and other soluble substrates can be dissolved in water and flushed through the treatment zone to very effectively treat the high permeability zones. However these substrates are very rapidly degraded so they must be frequently replenished. Molasses and corn syrup are available quickly and may lead to inhibitory, low pH conditions as organic acids are rapidly released. The polylactate esterhas some advantages since it can last six months or more. However, the polylactate ester is extremely viscous so it is difficult to move the material more than a few feet away from the injection well. The polylactate ester that is hydrolyzed to release lactic acid and groundwater flow will carry some dissolved lactic acid out away from the injection point. However this is a slow and inefficient process. As a consequence, polylactate ester injection points must be very closely spaced in lower permeability formations. However the greatest problem with polylactate ester is the very high operation and maintenance cost. Polylactate esteris a relatively expensive material ($6.00/Lb for 50% organic substrate – 50% water mixture) and must be replaced after approximately six months. This results in a very high maintenance costs that continues as long as solvents are diffusing out of the lower permeability zones.

We believe the SRS^TM approach has several very significant advantages over other approaches. First, SRS^TM can be distributed and immobilized over substantial distances in a range of aquifer materials. At a recent project in Long Island, NY, edible oils were distributed 35 ft away from the injection point. However at most sites, we find that it is more economical to use closer injection point intervals to reduce the time required for injection. Second, the edible oil substrate (SRS^TM) can be designed to be very long lasting, tremendously reducing long-term operation and maintenance costs.

Substrate	Distance¹	Longevity	Readdition Frequency	Cost²
Molasses & Corn Syrup	limited < 10 ft.	< 1 mo.	~ monthly	$0.20 to 0.40/lb.
Lactate	limited < 10 ft.	< 1 mo.	monthly	$2.00 to 2.50
Polylactate ester	limited < 5 ft.	< 9 months	every 9 months	$12.00/lb.³
SRS^TM	20 ft. or more	typically 2 - 5 yrs.	2 yrs. or longer	$2.00 to 3.50/lb

1 Distance indicates expected radial distribution away from injection point.

2 Prices reflect those obtainable for bulk chemicals in 2001. Price per pound is based upon weight of actual electron donor. Where present, weight of water or viscosity reducing agents is factored out of cost calculation.

3 Harkness, M. 2000. “Economic Considerations In Enhanced Anaerobic Bioremediation” In The Second International Conference on Remediation of Chlorinated and Recalcitrant Compounds: Vol. 4. Battelle Press, Columbus, OH. pp. 9-14.

Injection of pure, nonaqueous phase edible oil was not included in our review because of the inherent delivery problems associated with this substrate. Because of the viscosity and interfacial characteristics, it is difficult to distribute pure liquid oil more than a few feet away from the injection well. Over time the oil then flows back into the injection point, further reducing treatment system effectiveness. Terra Systems and others normally experience a significant amount of the injected oil being recovered at the injection point over a short time frame. Terra Systems has developed an edible oil substrate (SRS^TM) that not only travels much further from the injection point and also does not generally become free floating recoverable product.