THE USE OF VEGETABLE OIL SUBSTRATE S IN THE TREATMENT OF CONTAMINATED GROUNDWATER

Transcription

1 THE USE OF VEGETABLE OIL SUBSTRATE S IN THE TREATMENT OF CONTAMINATED GROUNDWATER M. Pienaar 1, S. Labuschagne and D. Duthe 2 1 SRK Consulting, IBM House, 54 Norfolk Terrace, Westville, Durban, KwaZulu Natal, South Africa; mpienaar@srk.co.za 2 SRK Consulting, Johannesburg, Gauteng, South Africa; dduthe@srk.co.za Edible vegetable oil (EVO) substrates have been successfully used to stimulate the in situ anaerobic biodegradation of groundwater contaminated with chlorinated solvents as well as numerous other anaerobically biodegradable contaminants like nitrates and perchlorates at many commercial, industrial and military sites throughout the world. EVO substrates are classified as a slow release fluid substrate, and comprise of food grade vegetable oil such as canola or soya bean oil. The EVO substrate serves as an easily biodegradable source of carbon (energy) used to create a geochemically favourable environment for the anaerobic microbial communities to degrade specific contaminants of concern. EVO substrate s can either be introduced into the subsurface environment as pure oil, in the form of light non aqueous phase or as an oil/water emulsion. The emulsified vegetable oil substrates has several benefits over non-emulsified vegetable oil as the fine oil droplet size of the commercially manufactured emulsified oils can more easily penetrate the heterogeneous pore and fracture spaces of the aquifer matrix. The use of this technology to stimulate in situ biodegradation of groundwater contaminants is still relatively unknown in South Africa. This paper gives an overview of the EVO technology and its application, specifically looking at the advantages of using this relatively inexpensive, environmentally friendly based technology to remediate contaminated groundwater within fractured rock environments commonly encountered in South Africa. 1. INTRODUCTION Remediation of contaminated land forms part of South Africa s comprehensive environmental strategy for the sustainable management of the environmental resources. One of our most important resources is groundwater. Negligence and mismanagement due to historical environmental management practices that historically was not as rigorous as present, has had significant negative environmental impacts with respect to soil, surface water and groundwater and resulted in the necessity for remediation of contaminated sites in South Africa. One of the most cost effective and least disruptive remedial methods is the In situ treatment of groundwater using bioremediation. In situ Bioremediation relies on the microbial populations present within subsurface environments to treat contaminated soil and groundwater. One of the most recognised techniques is to stimulate the naturally occurring in situ microbial populations to degrade specific contaminants. The degradation is stimulated either through Biostimulation, where a carbon rich substrate is added to enhance the naturally occurring microbiological population to degrade the specific contaminant or, alternatively through Bioaugmentation, where a commercially available microbiological culture is introduced into the groundwater bearing unit to enhance the viable numbers of the key microbial species. Edible Vegetable Oil (EVO) substrates have been successfully used to stimulate in situ anaerobic biodegradation of chlorinated solvents and other groundwater contaminants, including nitrates, perchlorates, explosive compounds (e.g TNT), dissolved metals (e.g. hexavalent chromium), at commercial, industrial and military sites throughout the world.

2 This paper focusses on the background to anaerobic bioremediation using EVO substrates focussing and its use in the treatment of chlorinated solvent contaminated groundwater, its function and advantages, as well as how this technique is favourable to treating contaminated groundwater sites in South Africa. 2 BACKGROUND What is Anaerobic Biodegradation? The subsurface environment is home to a broad and diverse array of microbial communities that result in complex ecosystems. The composition and activities of these populations are dependent on the aquifer geochemistry and they are affected by any changes to these environments. By altering the geochemical environment and by adding nutrients through substrates such as EVO, these microbial ecosystems can be altered to preferentially degrade specific contaminants of concern (ESTCP, 2006). What is Biostimulation? Biostimulation with various types of substrates has been used to stimulate anaerobic biodegradation. The main groups of substrates substrate types are soluble and slow-release substrates. Soluble substrates are easily fermentable substrates such as Lactate, Methanol or Molases, these compounds can easily distribute in the subsurface whether in sandy material or a fractured rock environment as is commonly found throughout most of South Africa. However, the disadvantage to using soluble substrates is that it is utilised quickly and therefore required frequent re-injection to maintain the microbial substrate requirement. Another group of substrates are slow-release substrates such as EVO or Hydrogen Release Compound (HRC), these compounds are slowly fermented by the microorganisms to release Hydrogen into the environment. This allows for longer periods between re-injection as the substrate requirement can be maintained for longer (ESTCP, 2006).. What is EVO? Edible Vegetable Oil is relatively inexpensive, innocuous, food-grade substrates (ESTCP, 2006). When properly prepared and injected, EVO are slowly biodegraded in most aquifers. A single, low-cost injection can provide sufficient carbon to drive reductive dechlorination for several years. This is expected to significantly lower operational and maintenance (O&M) costs compared to aqueous-phase injection of soluble carbon sources (e.g., lactate and carbohydrates) and will allow addition of slowrelease substrates at locations where placement of solid-phase carbon in trenches is not feasible (e.g., at depths, fractured rock) (ESTCP, 2006 and AFCEE 2004b). How does EVO enhance anaerobic biodegradation? All triglycerides (edible fats and oils) can be anaerobically fermented to hydrogen and organic acids like acetate. The anaerobic fermentation is believed to occur through a two-step process where the ester linkages between the glycerol and the fatty acids are hydrolyzed releasing free fatty acids and glycerol to solution. The glycerol is then degraded to 1,3-propanediol and subsequently to acetate. The saturated fatty acids undergo further breakdown by beta-oxidation resulting in the formation of two molecules of hydrogen (H 2 ), one molecule of acetate (C 2 H 3 O 2 ) (ESTCP, 2006). Through the successive oxidation at the beta carbon atom, the long-chain fatty acids are progressively shortened into fatty acids and acetic acid. This progressive oxidation generates four hydrogen atoms for every acetic acid produced from saturated fatty and two atoms of hydrogen are released for every acetic acid unit from unsaturated fatty acids (ESTCP, 2006). The acetic acid and hydrogen generated through this fermentation process is then used in a variety of reactions during biodegradation. If oxygen and nitrate is present in the subsurface then the hydrogen and acetic acid is rapidly oxidized to carbon dioxide and water. Once the oxygen and nitrate is depleted then the, excess hydrogen and acetate can be utilised for degradation of contaminants e.g chlorinated solvents through reductive dechlorination, or it can also be used to reduce sulfate and

3 oxidized forms of manganese and iron in the sediments. If methanogenic bacteria are present at the site the hydrogen and acetic acid can also be fermented to methane (ESTCP, 2006). What type of subsurface treatment can be done using EVO? EVO can be injected directly into the subsurface using conventional wells or using temporary direct push injection wells (that can be installed where the geology allows). The most common treatment configurations include direct source area treatment, where EVO is injected directly into the characterised source area, or plume treatment options such as a permeable reactive barrier also referred to as a biobarrier. The type or combination of treatment configuration applied at a site is dependent on the overall objectives of the project and on the site specific conditions. For example if the objective is to control or reduce the contaminant concentrations in the groundwater emanating from the source, to reduce the risks of exposure to specific receptors, then a permeable reactive barrier configuration is applicable as it will attenuate the plume, as it moves through the biobarrier, thereby reducing the risks and limit plume migration. The permeable reactive barrier is created by injecting the EVO through a series of temporary or permanent wells installed perpendicular to groundwater flow, downgradient of the source area. This will allow the key microbiological communities to establish within the bio-reactive zone created through the injection of the emulsified EVO. These microorganism populations will then treat the groundwater moving through this bio-reactive zone degrading the contaminants and lowering the potential risk to off-site receptors. Source area treatment is more difficult due to the toxicity associated with high concentrations of chemicals within source areas. However, anaerobic biodegradation has been proven to be successfully stimulated by the addition of EVO. The EVO will first stimulate rapid biodegradation of dissolved contaminants, then, as contaminants are slowly released from the aquifer matrix or residual the edible oil will still be present to support biodegradation. Types of EVO Substrates Two forms of EVO is used: Non Aqueous Phase Liquid (NAPL) also referred to as Pure EVO or Emulsified Vegetable Oil. Pure EVO can be used to treat contaminated sites, but results in high oil saturations and large reductions in the permeability within the injections zone. Typically, oil injected in this form will occupy between 40% and 90% of the aquifer pore space immediately adjoining the injection point (ESTCP, 2006). Therefore to distribute the oil over a greater distance, larger volumes of oil must be injected. This could result in a large permeability loss that could negatively impact your chosen treatment system e.g the permeability of a reactive barrier system, but would be potentially beneficial in limiting groundwater flow from source areas. Therefore the use of pure EVO is restricted to treatment of source areas, or where the groundwater velocities are too high and the groundwater flow needs to be retarded to create a bio-reactive zone. NAPL EVO can be sourced locally from any food oil supplier and can be delivered in drums or in larger quantities depending on the need and the injection system design. The other form of EVO is an oil-in water emulsions. Oil emulsions are prepared by mixing edible oils with emulsifying agents and water, yielding a smooth blend of small oil droplets suspended in a water matrix (i.e., an oil-in-water emulsion). The ideal emulsion should be stable, have small, uniform droplets to allow transport in most aquifers; and have a negative surface charge to reduce droplet capture by the solid surfaces (ESTCP, 2006). The emulsion is then injected into the aquifer with chase water to distribute and immobilize the oil droplets. As oil droplets migrate through the aquifer pore spaces, they collide with sediment surfaces and stick. The sediment surfaces gradually become coated with a thin layer of oil droplets that provides a carbon source for long-term biodegradation. One of the main advantages of emulsified EVO is the small droplets can be transported substantial distances (up to 15m~ dependant on the geological conditions). Field and laboratory studies (Borden et al., 2004 and Coulibaly and Borden, 2004)) have shown that emulsified oils can be transported over substantial distances with low to moderate oil retention and little permeability loss. The emulsified EVO

4 can be sourced from commercial suppliers that produce stable emulsions for the purpose of stimulating in situ bioremediation. These suppliers provide the relevant support to design the injection system and can help calculate the substrate requirement using site specific parameters. The geochemical conditions at the site, the geology as well as the injection borehole design and the groundwater plume contaminant concentrations will influence the amount of substrate required to address the site specific substrate (hydrogen) demand. If the geochemical conditions are not optimal for microbial growth (e.g ph very low) then specific additives can be added to the emulsified EVO to optimise the system. The addition of other soluble substrates and nutrients (e.g., lactate, yeast extract, vitamins) can enhance the geochemical conditions to stimulate the establishment of a robust microbial community in the groundwater plume, capable of degrading the contaminants of concern. 3. APPLICATION IN SOUTH AFRICA The use of EVO as organic substrate alternative to the more readily known and used soluble substrates holds several benefits for use in South Africa. EVO is an environmentally friendly product that has been used and researched extensively across the world to enhance in situ bioremediation. The benefit to us in South Africa is that years of research conducted at sites in America and Europe, has resulted in a variety of tried and tested commercially available products that can sourced from oversees. The technical support and product quality is also valuable when trying to negotiate with clients and convince authorities that the product and method selected for the site can successfully enhance bioremediation and result in effective contaminant biodegradation. Geologically, South Africa has a diverse environment, with many sites having very heterogeneous geological conditions making in situ remediation very complex. Emulsified EVO has the benefit of small droplet sizes that can easily penetrate a variety of aquifer and geological conditions including fractured rock environments. This has several benefits over alternative technologies and can even be adapted to suit aquifer conditions in instances where larger droplets are required to slow flow in these same fractured rock environments. 4. CONCLUSION The use of EVO to stimulate in situ bioremediation to treat contaminated groundwater has several advantages over other technologies available. These include: Relatively low cost, NAPL EVO is readily available in South Africa, however commercially available emulsions are imported and can be subject to currency fluctuation and import costs Non-Toxic food grade materials are used that are biodegradable and are not harmful to the environment Has been proven internationally to be extremely effective in treating groundwater contaminated with chlorinated solvents EVO slowly biodegrades in comparison to other soluble substrates that requires frequent reinjection and the residual organic amendment can remain effective in the aquifer for an extended period of time (e.g., 5 to 10 years). The emulsified vegetable oils have been successfully injected into fractured rock environments and can easily penetrate small pore spaces depending on the size of the emulsification. Other soluble substrates and amendments can be added to the EVO to enhance and/or optimise the geochemical environment (e.g compounds to adjust ph and essential vitamins).

5 5. REFERENCING Air Force Center for Engineering and the Environmental (2004) Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents (AFCEE 2004b). Air Force Center for Engineering and the Environmental. (2007). Protocol for in situ Bioremediation of Chlorinated Solvents using edible oil. (AFCEE 2007) Borden, R.C(2005). Effective Distribution of Emulsified Edible Oil for Enhanced Anaerobic Bioremediation, Journal of Contaminant Hydrology Borden, R. C. and B. X. Rodriguez. (2005). Evaluation of Slow Release Substrates for Anaerobic Bioremediation. Bioremediation Journal. Coulibaly, K.M. and R.C. Borden. (2004). Impact of edible oil injection on the permeability of aquifer sands. Journal of Contaminant Hydrology, 71(1-4): Coulibaly, K.M. (2003). Permeability Reduction and Emulsified Soybean Oil Distribution in Aquifer Sediments: Experimental and Modeling Results. Doctor of Philosophy in Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC. Environmental Security Technology Certification Program (ESTCP)(2006) Protocol for Enhanced In situ Bioremediation Using Emulsified Edible Oil. United States Environmental Protection Agency (1998) Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water EPA/600/R-98/128.Office of Research and Development Washington DC