From: Public Services and Procurement Canada
Biodegradation is a process by which organic contaminants in solid or liquid matrices are transformed by microorganisms to produce cell material, energy, organic compounds (generally less toxic than the parent compounds), CO2, and water. Under favourable conditions, microorganisms are capable of degrading a wide variety of organic compounds. The biodegradation of oil and polycyclic aromatic hydrocarbons (PAHs) (Ukiwe et al. 2013) in sediments has been clearly demonstrated, but biodegradation of polychlorinated biphenyls (PCBs) and chlorinated aliphatic hydrocarbons (CAHs) remains (for now) an emerging technology. The main advantage of biodegradation is the persistence of the biological reactions that can lead to complete transformation or mineralization of organic contaminants to a less-toxic or non-toxic form. This is also a method where the cost per unit of volume treated compares favourably with ex situ technologies and other in situ technologies.
Biodegradation may occur under aerobic or anaerobic conditions. Biodegradation occurs relatively easily for most petroleum hydrocarbons, PAHs (particularly low molecular-weight PAHs), PCBs and a small number of CAHs (dichloroethane [DCE], vinyl chloride) under aerobic conditions. If oxygen is not easily renewed, as is usually the case in sediments, the rapid reduction in dissolved oxygen concentrations due to microbial respiration creates anaerobic conditions. In such cases, anaerobic microorganisms can use electron acceptors other than oxygen, such as nitrate, sulfate, manganese (Mn) (IV), iron (Fe) (III), or CO2, to continue contaminant mineralization. Complex chlorinated compounds are easier to biodegrade under anaerobic conditions. Petroleum hydrocarbons, light PAHs and PCBs, and CAHs can be degraded under anaerobic conditions when electron acceptors other than oxygen are available for microorganisms.
Generally, two methods are used to promote the biological treatment of contaminated sediments: biostimulation and bioaugmentation.
Biostimulation is the introduction of additives (amendments) into sediments to stimulate the indigenous microorganisms and accelerate the biodegradation of specific contaminants (for instance, through oxidation or nutrient addition). Biostimulation can occur in both aerobic and anaerobic environments, provided the necessities are available for microbial life. It is often advantageous to focus on biostimulation under either aerobic or anaerobic conditions. The preferred environmental condition depends on the biodegradation potential of contaminants, access to biostimulants, and existing site conditions.
Bioaugmentation involves the introduction, directly on or into the contaminated sediment, of cultured microorganisms with specific catabolic abilities. This procedure enables or accelerates the biodegradation of target contaminants. The key factor in the success of bioaugmentation is selecting the appropriate bacterial strain. This selection must be made taking into account not only the ability of the strain to degrade the contaminants of concern, but also the ability of the strain to survive within the environmental conditions of the new habitat (e.g., pH, salinity, redox potential). It should be noted that, under real-world conditions, the effectiveness of bioaugmentation has not been conclusively demonstrated. This technology is still experimental, and there are significant challenges in applying it to underwater environments on a large-scale basis. In Canada, the injection of microorganisms into the environment is regulated by a federal agency, and must follow the 1999 Canadian Environmental Protection Act (CEPA 1999). The microorganisms must be registered on the Domestic Substance List (DSL) in order to be considered for the bioaugmentation remediation technique.
U.S. EPA, 1993. Guide for Conducting Treatability Studies under CERCLA: Biodegradation Remedy Selection. https://semspub.epa.gov/work/HQ/175669.pdf
Biodegradation techniques are highly site-specific, and selecting an approach requires consideration of contaminant type, sediment grain size distribution, natural environmental conditions (e.g., anaerobic/aerobic conditions), site access, and location. The main issue implementing biodegradation remediation is the application and distribution of the treatment media (amendments) to the contaminated zone. Implementation includes achieving adequate surface coverage, reducing loss of amendments to suspension, and treating contaminants at depth. The process for selecting and implementing a biodegradation technique at a site may include the following:
Amendments used in biodegradation vary greatly, depending on individual site conditions and needs. When biostimulation is used, amendments may include oxidizers, such as hydrogen peroxide, calcium nitrate, and sodium nitrate; nutrient fertilizers, such as nitrogen, phosphorus, and organic fertilizers; and products affecting the ionic balance, such as sodium chloride. Bioaugmentation materials include biodegraders and bacterial culture, such as KB-1, whereas inhibitors may consist of chemical or biological surfactants or dispersants, such as TritonX-100, bile salts, surfactin, sodium taurocholate, rhamolipid, and sophorolipid.
Biodegradation in sediments risks the loss of contaminants and/or amendments to suspension into the water column. Sediment contamination may be released when the water body is agitated, for example, by extreme weather events (e.g., flooding) or through human activities (e.g., construction). Loss of amendments may be mitigated through application techniques, such as application during periods of low energy (e.g., no current and wave action) and injection into sediment. Amendment application plans should be evaluated for their suspension potential, and preference should be given to products and/or techniques with little potential for loss into the water column. Sediment traps may be installed when contaminant or amendment loss is expected, to capture the fraction lost to suspension.
Materials and amendments should be safely stored to prevent accidental spills from contacting the water body or seeping into the groundwater. As a precautionary measure, sites should be equipped with spill kits and sorbent pads. Excess amendments may be returned to the supplier or disposed of in a manner acceptable to the local aut
Notes:
On-site treatment trials will establish the efficiency of the technology and the parameters that influence the treatment time and cost (e.g. residence time, pump flow rate, requirements for pre-treatment, etc.).
Contaminants must be accessible at the surface of the sediment for amendment application, and be located at a reasonable distance from shore, to make application feasible and cost-effective. If contaminants are below the sediment surface, the depth to the contamination should be shallow enough to allow mechanical mixing-in of the amendments from the surface, or within the bioturbation zone (mixing of the sediments by benthic organisms at the sediment-water interface)
While biodegradation generally occurs more rapidly under warmer conditions, it has also been observed in Arctic environments. Biodegradation techniques that have been applied in northern environments include:
Biodegradation techniques remain in the developmental phase. They are heavily studied and discussed in the scientific research literature. However, technical reviews and case studies are seldom available.
Treatment time can vary from one to many years, dependent on the contaminant type, quantity, site conditions, and amendment/technique used. Following the initial treatment, regular monitoring is required to demonstrate that contaminant levels are trending toward remediation objectives over time. Additional amendment applications may be required.
In theory, biodegradation treatments are permanent if they result in the complete destruction of contaminants. However, there have been no long-term (30–50 years) monitoring studies completed to date.
In most cases, biodegradation by-products are water, carbon dioxide and, in the case of chlorinated compounds, inorganic chlorine. However, in cases where bacterial catabolic activities are not present, incomplete degradation of the target molecule may occur. For example, the incomplete biodegradation of some PCBs (dihydrodiols and dihydroxybiphenyls) may lead to breakdown products that are significantly higher in toxicity to bacteria, even in short incubation periods. Trichloroethene (TCE) and perchloroethene (PCE) lead to the production of dichloroethene (DCE), and the incomplete biodegradation of DCE can lead to vinyl chloride (VC) production, which is more toxic than the parent compounds.
Addition of amendments and increased biological activity may lead to a change in the geochemical conditions within the sediment, for example a change in pH. These changes may result in increased bioavailability and mobilization for certain metals and other contaminants.
Surfactants may increase contaminant solubility, allowing them to be more available for microbial uptake. Care must be taken when using surfactants to ensure that they do not inhibit biodegradation, which can occur if the surfactant is toxic to the native bacteria or if there is competition for substrate utilization (Liu et al., 2001).
No secondary treatment is required if the target contaminant concentrations are reached.
University of Alaska Fairbanks, 2015. Biodegradation and Transport of Crude Oil in Sand and Gravel Beaches of Arctic Alaska. http://www.boem.gov/BOEM-2015-041/
Environment Canada, 1995. in situ Treatment of Hamilton Harbour Sediment. Injection of Calcium Nitrate and Nutrients to Degrade Organic Compounds (PAHs) within Freshwater Sediments. http://link.springer.com/article/10.1007%2FBF00116654
The performance of biodegradation systems in sediments, reported in the literature during pilot scale testing or remediation projects, can achieve up to a 90% reduction in sulphide concentrations and a 50% to 80% reduction in concentrations of PAHs, petroleum hydrocarbons, and BTEX. The dechlorination of PCBs or halogenated hydrocarbons in the laboratory has demonstrated that it is possible to eliminate all contamination. It is important to note that achieving this level of biodegradation in situ is highly unlikely due to the intrinsic heterogeneity of sediment and the difficulty in controlling all parameters, as can be done in the laboratory. The efficacy of biodegradation may be compromised by groundwater advection and tidal action, which may carry sediment and amendments out of the treatment zone. However, the laboratory results demonstrate the potential of the technology, which can be exploited in the future due to improved understanding and control of the processes.
Biodegradation is one of the most sustainable in situ technologies and has fewer environmental impacts than more invasive remedial options, such as dredging or excavation. Sustainability and overall success may be improved by developing a site-specific plan that reduces consumption of energy, material, and water, minimizes harmful air emissions and waste generation, and protects human health and ecosystems during monitoring. For example, to the extent possible,
Composed by : Bruno Vallée M.Sc, LVM Inc.
Latest update provided by : Bruno Vallée, M.SC. (1), Sharilyn Hoobin, M.Sc. (2) and Ashley Hosier, Ing. (2), LVM Inc. (1) and Royal military college royal (2), respectively
Updated Date : November 24, 2016
