Key differences exist between activated carbon based in situ remediation technologies such as BOS 200® and Regenesis Plumestop. These differences include but are not limited to remedial efficiency, applicable groundwater, and soil concentration ranges application technique, particle size, whether or not bacteria and nutrients are co-injected, and terminal electron acceptor selection.
This Technical Bulletin details these differences so that environmental practitioners can be informed in order to select the appropriate technology to meet project objectives and client expectations.
APPLICABLE CONCENTRATION RANGES
Significant differences exist between Regenesis Plumestop and BOS 200® technologies with respect to the contaminant concentrations that can be treated. It has been reported that total volatile petroleum hydrocarbon concentrations over 10 mg/L cannot be treated with Regenesis Plumestop. This constraint is likely due to the limited amount of activated carbon contained in Regenesis Plumestop technologies and restricts their use to polishing and/or, low concentration plume treatment.
In contrast, BOS 200® is not bound to this limitation and has been successfully applied over a broad range of groundwater and soil concentrations from polishing applications through source area (including NAPL) treatment (Figure 1). Case studies for BOS 200® source area applications can be found here.
Regenesis Plumestop and BOS 200® are applied to the subsurface in significantly different ways. Regenesis Plumestop is applied in only low-flow rate and low-pressure conditions. Given this limitation, this technology can only be applied in aquifers with high hydraulic conductivity (e.g. sands, gravels). The reagent cannot penetrate the lower permeability zones where the majority of the remaining contaminant mass typically remains. Relying strictly on diffusion from the lower to the higher permeability zones introduces inefficiency into the remedial process.
In contrast, BOS 200® is installed using high-energy emplacement methods that vary depending on the lithology. Both coarse- and fine-grained applications employ high-flow rate delivery. The injection pressure magnitude and signature will vary based on site conditions, injection depth, and lithology and is discussed in more detail below.
In coarse-grained aquifers, high-emplacement velocities are used to overcome the natural filtering effects of the lithology and create radial mixing within the treatment zone. The injection pressure signature is typically linear.
In fine-grained aquifers, enough pressure needs to be applied to cause the soil structure to fail. Once this occurs, the BOS 200® is applied at a lower sustained injection pressure, commonly called the propagation or maintenance pressure. To account for the variable nature of this technique, BOS 200® is applied using a tight grid (typically 5 to 10 ft) with adjacent injection locations having vertically offset injection intervals (e.g. 6 ft & 8 ft for Point A and 7 ft & 9 ft for Point B).
IMPACTS TO MONITORING WELLS
Unlike Regenesis Plumestop, BOS 200® will not irreversibly coat the sand particles of a monitoring well filter pack (Figure 2), which may impact the representativeness of groundwater data. To review the RPI Group monitoring well position please review the October 2016 newsletter.
Regenesis Plumestop technologies do not co-inject bacteria. BOS 200® is applied as a complete remediation system and includes a blend of facultative bacteria. The bacteria are added to the mix tank prior to application to inoculate the activated carbon. Adding a blend of facultative bacteria on the front end of the project will allow the remediation to proceed efficiently from day zero; waiting for indigenous bacteria to inhabit the carbon will delay the contaminant degradation and may not proceed at all if the proper bacteria are not present naturally.
In addition, without co-injecting a degradation mechanism, there is the possibility that the sorbed contaminants can be desorbed by another preferentially attracted contaminant or organic carbon. This is referred to as “roll over” in the activated carbon community.
ACTIVATED CARBON PARTICLE SIZE
It is accurate that BOS 200® particles are larger than Regenesis Plumestop particles, that are represented as 1 to 2 um. Over 93% of BOS 200® particles pass through a 325 mesh screen which suggests the majority of particles will have diameters of less than 45 µm although some (7%) will be larger. There is some confusion over the size of microorganisms and whether they can inhabit the pore structure of activated carbon. Depending on your reference, sizes reported include less than 1 µm, 0.2 to 2 µm, 0.5 to 20 µm, and 3 to 5 µm. The fact is that bacteria come in different geometries: spherical, oblong, chain-like, and threadlike. The organisms typically responsible for degradation of hydrocarbons fall into the spherical and oblong categories, with sizes ranging from 0.5 to 3 µm. It is apparent that these organisms are simply too big to inhabit the pore structure of LAC, however roughly 20% of the pore volume of BOS 200® is larger than 1 µm, and approximately 10% of the pore volume is larger than 5 µm. Clearly, a large percentage of the total pore volume of BOS 200® is large enough to allow bacterial invasion, enabling an efficient platform for biological assimilation.
While Regenesis Plumestop particles are marketed to move in the subsurface in permeable zones with high hydraulic conductivity (Figure 1), there are many instances where this is not desired. For example, in a permeable reactive barrier(PRB) installation, the activated carbon needs to remain emplaced in the desired zone to decrease the mass flux moving through the cross section.
ELECTRON ACCEPTOR AND NUTRIENT ADDITION
Efficient remediation is all about contact. For Regenesis Plumestop applications, electron acceptors, such as calcium oxyhydroxide, are injected in separate locations. This introduces variability and inefficiency into the remedial process. One must rely on the natural dispersive forces to bring the carbon platform and electron acceptor together. The injection of necessary electron acceptors and nutrients together with BOS 200® provides assurance for optimal biological treatment since all elements needed for sustained biological degradation are in the immediate proximity of the hydrocarbon sorbed to the carbon platform.
Another variable to consider for Regenesis Plumestop applications is longevity. For example, most oxygen releasing compounds stay active for as little as six months in high-seepage-velocity aquifers. If the project objectives have not been met, additional injections may be warranted. Once the electron acceptor pool has been depleted, contaminant rollover (detailed above) may occur. There is a fundamental difference between the above-described strategy and that of BOS 200®. An extensive effort is completed to define the contaminant mass distribution and dosing of electron acceptors is crafted to address the mass of contaminant present regardless of the time required to achieve targeted goals.
At most petroleum contaminated sites the prevailing redox conditions are anaerobic, where sulfate or carbon dioxide are being used as terminal electron acceptors. Attempting to “flip’ the aquifer back to an aerobic state will require time, again introducing an element of inefficiency.
BOS 200®, the most widely applied activated carbon-based remediation chemistry, is a complete in situ remediation system that incorporates powder-activated carbon, electron acceptors, micro and macro nutrients, and a blend of facultative bacteria to treat a wide range of contaminant concentrations in varying lithologies using proven application methods.
Regenesis Plumestop is an incomplete remedial system since the key components necessary for efficient remediation (e.g. electron acceptors, bacteria) are not added in the same injection location. In addition, this technology, due to the limitations regarding contaminant concentrations and compatible lithologies, has a finite range of applicability and should only be considered for sites with permeable lithologies and low contaminant concentrations.