Cometabolic Bioremediation

Aerobic Cometabolic Bioremediation

In aerobic cometabolic bioremediation indigenous bacteria or bioaugmented strains are stimulated by adding oxygen and a cometabolic growth substrate to trigger the production of enzymes that can oxidize or degrade the target pollutant via cometabolism. An iSOC® system is used to deliver oxygen as a terminal electron acceptor along with a gaseous growth substrate, typically an alkane gas, in a process known as Co-infusion to stimulate complete in situ contaminant destruction. The method is most useful for bioremediation of pollutants that are not themselves good aerobic growth substrates for bacteria. Cometabolic treatment has been found to be particularly effective for the chlorinated solvent trichloroethylene (TCE) and other lower or less oxidized chlorinated aliphatichydrocarbons (DCE, VC, TCA, DCA, CF, and MC). At some sites with mixed groundwater contaminants (i.e. petroleum hydrocarbons with chlorinated solvents), the necessary substrate for inducing aerobic cometabolism of the target pollutant may already be present and all that is needed is the infusion of oxygen. Various aliphatic and aromatic hydrocarbon compounds have been found to function as cometabolic treatment substrates and significant research and development has focusedon the use of alkane gases (i.e. methane and propane) for the purpose.

Delivery of Oxygen and Gaseous Cometabolic Substrates with the iSOC^® System for Bioremediation

Implementation of the aerobic cometabolic bioremediation processes utilizes the iSOC^® gas delivery system based on inVenture's patented Gas inFusion technology - a unique method of infusing supersaturated levels of dissolved gas into liquids. At the heart of iSOC® is a proprietary structured polymer mass transfer device that is filled with microporous hollow fiber material and provides a large surface area for mass transfer - in excess of 7000 m2/m3. The fiber is hydrophobic and therefore excludes water. The system efficiently delivers gas to liquid by mass transfer without sparging.

The Co-inFusion process involves the configuration of iSOC^® treatment wells to deliver oxygen and a dilute (2 to 4.5%) non-flammable mixture of alkane gas (i.e. propane) or alkene gas (i.e. ethene) in inert nitrogen gas to the groundwater treatment area. The iSOC units are placed in treatment wells and provide an inherently large surface area that allows for oxygen and alkane gas mass transfer to groundwater without sparging. The treatment area established by Co-inFusion wells can be designed as a grid to treat source areas or as a treatment curtain or fence of treatment wells to cut off plume migration.

In an aerobic cometabolic bioremediation application, the iSOC^® saturates the treatment well with dissolved oxygen (DO), typically 40-200 PPM depending on the immersion depth of the iSOC^® in groundwater. A natural convection current and a designed release bubble from the top of the iSOC® mixes the water column in the well resulting in a relatively uniform DO concentration. The alkane or alkene gas is delivered to the iSOCs intermittently, either to dedicated cometabolic gas delivery wells or alternately switching gases at the oxygen delivery wells. A curtain of dissolved gas rich water around and downgradient of the treatment well is formed by diffusion, advection and dispersion, forming a treatment zone where enhanced bioremediation removes target contaminants. The cometabolic gas/nitrogen mixture is added intermittently by manually switching with a gas manifold system or by a mechanical system controlled by a timer as shown in the following schematic.

Using a dilute mixture cometabolic gas supply limits the dissolved concentration of the cometabolic gas to target concentrations. The recommended gas mixtures are also nonflammable and are stored in high-pressure cylinders, ideal for gas delivery to iSOC® treatment wells. Where necessary the treatment well can be actively vented via an airflow exhaust blower operated on a 12-volt rechargeable battery system that can also serve to operate the solenoid valves on the timer controlled gas management system.

Placement of injection wells depends on site-specific conditions and treatment objectives as described further below. Treatment well screens typically span the full thickness of the contaminated groundwater zone with the iSOC® unit placed near the bottom for maximum hydraulic head. The iSOC® system is installed in a few hours to days depending on the number of treatment wells. iSOC® units are easily moved from well to well to optimize performance and remediation strategies.

The iSOC® unit is constructed of high quality SS316 stainless steel and a proprietary structured polymer mass transfer device. iSOC® is 1.62" (41 mm) in diameter and 12.65" (321 mm) long with a barb connector for 0.167" ( or 4 mm) ID polyurethane tubing. The housings for the pressure and flow control unit and the drain plug are made from nylon. iSOC® has a lifting ring for connecting to a suspension line for insertion in 2" (50 mm) or larger treatment wells. High flow well screen is recommended for treatment wells. The units are connected to regulated supplies of industrial-grade compressed oxygen and the cometabolic substrate specialty gas.

Experience in the field has shown that in each treatment well where an iSOC® is installed, high levels dissolved oxygen levels of can easily be achieved with concentration depending on the head of the water in the well and groundwater flow velocity. Each atmosphere of pressure allows for an approximately 40 ppm of dissolved oxygen. The solubility of an example alkane cometabolic substrate, 4.5% propane gas mixture, is approximately 2.7 mg/L propane at one atmosphere of pressure. Oxygen and the alkane gas mixture are intermittently infused into the aquifer at a rate of 15 to 20 cubic centimeters/minute.