Support of Source Zone Bioremediation through Endogenous Biomass Decay and Electron Donor Recycling

Abstract

Enhanced bioremediation strategies employ intensive electron donor amendments that can be successful in generating high biomass concentrations within the targeted area, and this technology is increasingly being applied within source zones to address non-aqueous phase contaminants. An unintended consequence is potential electron donor recycling via the slow endogenous decay of these newly grown cells, which may persist in the source zone even after the enhanced bioremediation project is completed and the introduced electron donor is exhausted. This paper presents a conceptual model that outlines the endogenous decay process within source zones and identifies several key scenarios where it is an important contributor to long-term attenuation. A key concept is that this reservoir represented by decaying biomass is both potentially large and capable of several rounds of turnover before becoming exhausted. Thus, the slow decay of biomass and the recycling of these decay products within the source zone extended the duration of the treatment period. A recent survey on the performance of source depletion technologies strongly suggests that this process is being observed at sites where enhanced bioremediation has been implemented (McGuire et al., 2006, Ground Water Monitor. Remediat. 26:73-84). Concentrations continued to decline several years after treatment, providing a strong indication that there is an endogenous electron donor supply that is contributing to continued contaminant reduction over time. Little evidence of concentration rebound was observed relative to sites where other technologies were used, suggesting long-term benefits associated with enhanced bioremediation that appear to partially offset processes that can contribute to rebound (e.g., matrix diffusion). Because initial colonization can occur near the non-aqueous phase liquid (NAPL)-water interface when exogenous electron donor is readily available the endogenous cell decay occurs in an optimal location to continue to support reductive dechlorination. This electron donor recycling process is ideally suited to favor growth of dechlorinating organisms relative to competing populations because of the slow release rates associated with decay, and it should preferentially stimulate polychloroethylene (PCE) and trichloroethylene (TCE) source removal over

Evaluation of a laboratory-scale bioreactive in situ sediment cap for the treatment of organic contaminants

“…Regardless of this isolated analytical discrepancy, the presence of VC and ethene indicates that dechlorinating species within the OW consortium, specifically Dehalococcoides , remained active for at least one dechlorination cycle when provided only sediment effluent and a dissolved phase electron acceptor (PCE). Thus, the sources of carbon, electron donor, and micronutrients were provided by the sediment effluent or from microbial biomass (Adamson and Newell, 2009). Methane concentrations rose steadily during the dechlorination of PCE (Figure 2), indicating that methanogenic populations were also able to remain active when provided only sediment effluent.…”

Sustained treatment: Implications for treatment timescales associated with source‐depletion technologies

“…A portion of the reducing power represented by this biomass is continually recycled back into the treatment zone through the endogenous decay process that is an inherent part of the dynamic microbial growth cycle (Adamson & Newell, 2009;Sleep et al, 2005). Utilization of biomass-associated soluble microbial products and other cell components can lead to growth of new microbes (cryptic growth) and thus can continue to support desired biological reactions, such as reductive dechlorination.…”

“…For example, typical enhanced bioremediation projects provide enough substrate (several hundred mg/L for standard soluble substrates is cited in AFCEE, 2004) to generate in excess of 100 mg/L of biomass. At typical anaerobic decay rates (0.004 to 0.1 day −1 ) (Adamson & Newell, 2009), this means that up to 23 percent of the original biomass would still be present after one year. This beneficial effect is extended by subsequent cycles of cryptic growth and decay that result in multiple turnovers of reducing equivalents over the course of several years.…”

“…• Introduction of biodegradable substrate results in growth of high biomass concentrations (100s to 1,000s of mg/L), which are subject to slow decay to support further activity over years (Adamson & Newell, 2009;Sleep et al, 2005) • Certain substrates are designed to dissolve and/or degrade slowly, such that sustained substrate is present long after injection activities have been completed • Commercial slow-release and/or emulsified oil products reported to persist one to five years (AFCEE, 2004(AFCEE, , 2007Borden, 2005 • Potential decrease in pH due to formation of organic acids during fermentation of substrate (Adamson et al, 2003) continued "sustained" treatment. Specifically, the reduction in parent chlorinated volatile organic compounds (CVOC) concentration increased from a median value of 77 percent at the end of active treatment to a median value of 95 percent at the end of the (available) monitoring period.…”

“…The lifetime of these "slow-release" substrates is reported to range from one to five years (AFCEE, 2004;, meaning that sustained biological reductive dechlorination could be expected over a similar timescale. Endogenous biomass decay is also an inherent component of this technology, such that several additional years of lower-level activity should be expected for some, if not all, projects based on typical substrate concentrations, biomass yields, and decay kinetics (Adamson & Newell, 2009). Note that the contribution from endogenous decay should be significant regardless of the substrate type selected; for example, more soluble substrates tend to be rapidly but inefficiently utilized, generating high biomass concentrations that release electrons over a much longer period of time.…”