Turnkey Automated In Situ Delivery Systems
A primary limitation of in-situ remediation processes (i.e. chemical oxidation, bioremediation) is the lack of contact between delivered amendments and the contaminants. Unfortunately, many pilot-scale and full-scale in situ processes are implemented using slug injections, which are often ineffective because they have minimal radius-of-influence, so they fail to maintain appropriate subsurface conditions or deliver the required mass of amendments. In order to overcome these limitations, ETEC has developed automated recirculation systems that can deliver liquid and slurry-phase substrates, nutrients, or chemical oxidants to the subsurface on a consistent, scheduled basis. The In Situ Delivery (ISD™) systems are fully automated and programmable, allowing you to optimize delivery of subsurface amendments based on your site-specific remediation needs.
Reductive Dechlorination of Chlorinated Compounds
Reductive dechlorination remains a popular and effective method for remediation of chlorinated solvents like PCE, TCE, TCA and pentachlorophenol. Major advantages of anaerobic biological processes include the ability to achieve required cleanup levels (i.e. 1 ppb for PCE and TCE). Typical reductive dechlorination projects generally require only the addition of a carbon substrate (lactate, edible oil, etc.) to produce reductive groundwater conditions. However, many solvent plumes are very large and the main challenge is being able to deliver the correct mass of substrate throughout an entire impacted area. The ISD™ systems are specifically designed for plume-wide delivery of soluble or viscous substrates. The ISD™ systems accomplish delivery by performing consistent groundwater recirculation (extraction, amendment and re-injection) to optimize substrate delivery throughout the treatment zone, thus stimulating large-scale reductive dechlorination of dissolved constituents and DNAPL.
Substrates for Reductive Dechlorination
Many anaerobic substrates being used today are thick, viscous products designed to slowly dissolve in groundwater, thereby providing long-term reductive conditions. The primary drawback with this approach is that these substrates have a very limited radius-of-influence, so they can’t provide treatment of an entire plume efficiently. ETEC’s soluble, highly-degradable, nutrient-amended CarBstrate™ substrate makes it possible to treat an entire plume efficiently. By combining continuous groundwater recirculation with pulsed substrate injection, the ISD™ systems produce targeted, site-wide reductive conditions and explosive growth of biomass in the soil pore space quickly and efficiently. As this biomass begins to “rot”, it acts as an ideal substrate for long-term dechlorination. Current site data indicates that with distribution of CarBstrate™ active dechlorination can be maintained for years after treatment.
Chemical oxidation processes (Fenton’s, permanganate, persulfate) are used to oxidize a broad range of organic compounds. Since chemical oxidation relies on contact between the oxidant and the target contaminant, in situ delivery is critical. Specifically, oxidant loading rates and soil oxidant demand significantly affects the oxidant mass required to complete remediation. Because oxidant cost is very high (and often cost-prohibitive), minimizing these costs via efficient subsurface injection is imperative. Recent studies show that slug injections of high-concentration oxidant solution are ineffective, unsafe, and can waste a significant mass of oxidant (thus increasing costs). Smaller injection volumes on a consistent basis using lower oxidant concentrations are more cost-effective and more successful. The ISD™ systems provide precise injection control of the volumes and concentrations of oxidants and catalysts, which saves you money, increases contaminant destruction and protects your field personnel from dangerous chemical reactions.
ETEC TURNKEY EQUIPMENT LINE UP
Anaerobic Bioremediation | In Situ Delivery (ISD™) System
Our In Situ Delivery (ISD™) equipment and anaerobic bioremediation products address the critical governing principles that control the success or failure of any in situ anaerobic bioremediation effort. These principles are discussed below in greater detail.
All remediation processes come down to mass balance. How many pounds or kilograms of a contaminant exists in the subsurface, and how much of this mass can be removed by the proposed remediation system? With reductive dechlorination, the mass balance is a function of the stoichiometry of the oxidation-reduction reactions that govern the biological utilization of a particular compound. For example, oxidation-reduction reactions for the reductive dechlorination of PCE are as follows:
Oxidation Reaction: C6H12O6 + 6 H2O → 6 CO2 + 24 H+ + 24 e-
Reduction Reaction: 3 C2Cl4 + 12 H+ + 24 e- → 3 C2H4 + 12 Cl-
Overall Reaction: C6H12O6 + 3 C2Cl4 + 6 H2O → 3 C2H4 + 12 Cl- + 12 H+ + 6 CO2
The C6H12O6 requirement for this reaction is that 1 mole of C6H12O6 can degrade 3 moles of C2Cl4. Converting to the appropriate mass ratio results in the following:
1 unit C6H12O6 : 3 x (180/166) units of C2Cl4, or 1 unit C6H12O6 can degrade more than 3 units of C2Cl4.
Theoretically, anaerobic bioremediation using 1 pound of carbon substrate can degrade 3 pounds of PCE in the subsurface. However, our practical experience has proven that the amount of carbon substrate necessary for complete dechlorination of PCE is a significantly greater mass. Our soluble, nutrient-amended electron donor substrate, Carbstrate™, fully addresses the problems with supplying a large mass of substrate on a site-wide basis.
By utilizing our ISD™ equipment in combination with localized injection and extraction wells, artificial groundwater gradients can be produced within the plume area to induce circulation of a soluble carbon substrate and essential nutrients through the groundwater and smear-zone soil. By placing injection and extraction wells at specific locations, individual “circulation cells” can be created throughout a plume zone (i.e. source area, sidegradient, downgradient, etc.), resulting in:
Optimized electron-donor substrate delivery
Hydraulic plume control
Induced groundwater gradients via GW mounding/drawdown
The importance of hydraulic influence as part of a groundwater remediation process cannot be overstated…groundwater management results in long-term flexibility, accelerated cleanup, and shorter remediation timeframes.
Consistent groundwater recirculation is also the best available technology for approximating complete-mix CSTR conditions in the subsurface. This is an important factor for reductive dechlorination, since research has shown that stable concentrations of hydrogen in groundwater favor dehalogenating bacteria (as opposed to methanogens and acetogens).
Dissolution & Desorption
Contaminants in the subsurface partition into various phases depending upon their specific physical/chemical characteristics. However, contaminants in the adsorbed phase (i.e. the constituents that are bound to the organic soil fraction) usually represents over 70% of the total subsurface mass. Successful in situ reductive dechlorination must address the adsorbed-phase mass in conjunction with the dissolved-phase constituents to achieve site closure. By constantly replacing and re-circulating groundwater within the soil matrix, the our reductive dechlorination process accelerates dissolution of sorbed constituents via:
Maximized and continuous smear-zone soil contact
Enhanced delivery of biological products
Stimulation of a large microbial population
By forcing sorbed constituents into solution, they are rendered bio-available and susceptible to subsequent biological degradation. More importantly, desorption can be enhanced 300% to 600% by stimulating a highly-active microbial community! The combination of these dissolution processes ensures treatment of both sorbed- and dissolved-phase contaminant mass.
Complete Dechlorination without Stall
In the past, many reductive dechlorination projects experienced a “stall” effect that resulted in a buildup of vinyl chloride (VC). Currently, there is significant debate in the industry regarding the need for bioaugmentation (addition of dechlorinating bacterial species such as Dehalococcoides) in order to ensure complete dechlorination. ETEC has completed many full-scale in situ dechlorination projects, and all these projects have shown complete dechlorination to ethene. Like some industry professionals, we believe that complete dechlorination is achieved by providing and maintaining proper subsurface conditions. We believe our success results from the following:
Supply of an easily degradable carbon substrate
Consistent delivery at controlled concentrations throughout the solvent plume area
Availability of specific nutrients (N, P, K, micro-nutrients) to stimulate growth of indigenous dechlorinating bacteria
By providing these components, our reductive dechlorination process has resulted in complete dechlorination and no build-up of hazardous daughter products like VC. An added benefit of our process is the production of significant biomass within the saturated soil matrix. Research and our own project experience has shown that the slow decomposition of this accumulated biomass serves to continue the dechlorination process, reducing or eliminating contaminant rebound following active treatment.