This study is accomplished within the framework of SILPHES financed by ADEME, the French Environment and Energy Management Agency (AMI 2013 program). SILPHES is a "technology demonstrator" project which aims at developing innovative solutions for in situ remediation of a mixture of recalcitrant chlorinated solvent, mainly composed of hexachlorobutadiene, hexachloroethane, PCE, TCE and hexachlorobenzene. SILPHES is organized around two fundamental and complementary tasks:
- The remediation of chlorinated solvents point source pollution. This part is devoted to the optimization of the treatment of the point source, which includes physical, chemical and thermal treatments, associated with diagnostic and monitoring;
- The remediation of chlorinated solvents plume. This part is devoted to the improvement of environmental diagnosis and the design and monitoring of natural attenuation, bioremediation or chemical treatment.
This paper only covers a part of the first point stated above, i.e. the remediation of the residual phase of chlorinated solvents remaining after pumping.
Since the 1990s, in situ chemical remediation is frequently considered because of good treatment efficiency without carrying out an excavation or without additional processing step. Among the in situ remediation techniques, those for the treatment of chlorinated solvents are highly widespread, essentially for the treatment of PCE and TCE [1-4] by using strong oxidizing or reducing agents. The particularity of this study is the diversity of the chlorinated solvents mixture encountered in situ: both aliphatic and aromatic compounds. As a result, molecules have different chemical affinities with reactants, so both oxidation and reduction have been studied.
The chemical degradation of the mixture was investigated in vials to define the relative efficiency of various reagents. Three oxidizing agents (potassium permanganate, Fenton's reagent and sodium persulphate) and four reducing agents (a suspension of nanoscale zero-valent iron, with or without surfactant, sodium dithionite and sodium sulfide) have been used.
All experiments were performed in 100 mL vials filled with 50 mL of deionized water and 1 mL of the mixture of the chlorinated compounds. Different concentrations of reactants were added to the batch system after removing dissolved oxygen. Six replicates of each experimental condition were performed, and all sealed vials were stirred at 100 rpm in a thermostatic chamber at 12°C, the average groundwater temperature. Samples were analyzed at specified times in order to monitor the evolution of the amounts of chlorinated compounds. Ion chromatography was used to determine the production of chloride ions. A percentage of dechlorination was calculated from the measured concentrations of chloride ions by considering a complete dechlorination of all chlorinated compounds contained in the mixture. Gas chromatography –injecting aqueous and headspace samples– was used to determine the amounts of initial compounds and intermediate products of their degradation.
Experimental results have shown that, in these operating conditions, reductants are more efficient than oxidants. The highest percentage is about 12.5 % after 80 days of dechlorination, and is obtained with a ratio of sodium sulfide equal to 5 times the theoretical stoichiometry. This low value can be explained by worse operating conditions, with amount of chlorinated solvents well beyond saturation. The maximum percentage obtained for both nanoscale zero-valent iron and dithionite is about 9%, but at different stoichiometry: at stoichiometry ratio for dithionite and at 0.39 the stoichiometry ratio for iron. Increasing the amount of iron should enhance the degradation of chlorinated solvents. Regarding oxidants, only KMnO4 have shown relevant results, but lower than those obtained with reductants.
Results obtained by gas chromatography have illustrated the evolution of concentrations of chlorinated solvents. Hexachloroethane and PCE seems to be preferentially dechlorinated in the mixture: the most important variations in concentrations were observed with them or intermediate products of their degradation. Complex patterns of degradation have been observed, so individual chemicals mechanism could not be explicitly illustrated, except for the reduction of hexachloroethane and PCE. The products of this pathway were mainly PCE (β-elimination) and pentachloroethane (hydrogenolysis), but no quantitation could be obtained.
Further studies are currently in progress. The next phase consists in studying the degradation of the mixture with amounts of reagents widely in excess to define the limits of treatment efficiency. Others chemicals will also be tested, in particular potassium ferrate (oxidant) and palladium-doped microscale zero-valent iron (reductant). Once the best reagent compositions are known, several studies will be done with three chlorinated solvents – hexachlorobutadiene, hexachloroethane and hexachlorobenzene – taken individually to establish the reaction kinetics and mechanism at different experimental conditions (pH, temperature, concentration…).
References:
[1] Amir A, Lee W (2011). Chemical Engineering Journal 170, 492-497.
[2] Amonette JE, Templeton J, Speed R, Zipperer J (1992). In: SSSA Meetings, Soil Science Society of America: Minneapolis, pp 1-16.
[3] Amonette JE, Szecsody JE, Schaef HT, Templeton JC, Gorby YA, Fruchter JS (1994). In: In-situ Remediation: Scientific Basis for Current and Future Technologies. Part 2, Gee GW, Wing NR (Eds). Battelle Press: Columbus, pp 851-881.
[4] Xie Y, Cwiertny DM (2010). Environmental Science and Technology 44, 8649-8655.
Chlorinated alkanes, including 1,2-dichloropropane (1,2-DCP), 1,2,3-trichloropropane (1,2,3-TCP), and 1,1,2-trichloroethane (1,1,2-TCA), have been extensively used as fumigants, intermediates in chemical syntheses or solvents. Due to their toxicity and recalcitrant nature, some of these chemicals are no longer manufactured but remain present at historically contaminated sites. Information describing the transformation of chlorinated alkanes under anaerobic conditions is scarce and it is limited to a few bacterial genera including Dehalococcoides, Desulfitobacterium, Dehalobacter, and Dehalogenimonas. These organohalide-respiring bacteria (ORB) can couple the reductive dechlorination of chlorinated alkanes to energy conservation and growth; therefore they are of greater interest for in situ bioremediation. To date, there are only four isolates belonging to the genus Dehalogenimonas, all of them isolated in the United States. Here, we show a stable sediment-free enrichment culture derived from river estuary sediments in Barcelona (Spain) that exclusively dechlorinates vicinally chlorinated alkanes via dichloroelimination. This reaction involves the simultaneous removal of two chlorines from adjacent carbon atoms with the formation of a carbon-carbon double bound. PCR with genus-specific primers revealed the presence Dehalogenimonas in our culture. To gain insights into the identities of dominant bacterial populations present in the enrichment culture, prominent DGGE bands were excised and sequenced. Compound specific stable isotope analysis (CSIA) was used to determine the carbon isotope fractionation during reductive dechlorination of polychlorinated alkanes. Our results show that dechlorination was accompanied by significant isotope fractionation that differs from those values described previously by other ORBs. Advances in the fundamental understanding of carbon isotope fractionation during reductive dechlorination can contribute to confirm and quantify in situ bioremediation of these chemicals in contaminated sites containing Dehalogenimonas.
KEYWORDS: Fully automated, Anaerobic bioreactor, Chlorinated ethenes, TCE concept, Large plume treatment
ABSTRACT:
Overview
A special category of soil remediation covers sites that have been contaminated by the use of solvents, especially chlorinated ethenes. These contaminants can be removed by enhanced natural attenuation using the so-called TCE concept. Artificially cultivated bacteria (Dehalococcoides ethenogenes, DHC) with the required carbon source and nutrients are introduced into the soil in order to stimulate anaerobic degradation. The process can be controlled by constantly monitoring the influent, infiltrate and bioreactor streams for pH, oxygen and ORP. The strength of this concept lies in its relatively short active stimulation phase with complete degradation of the chlorinated hydrocarbons to harmless end products.
The general remedial approach involves extraction, amendment and re-circulation of groundwater in the targeted aquifer zone. Extracted groundwater is passed through an anaerobic bioreactor to enrich the water with DHC. After filtration, the feed is infiltrated through injection screens or wells located up gradient of the treated zone. This anaerobic bioremediation approach can be applied in urbanized areas with limited site access to prevent site disruption and reduce impacts to the environment. This semi-passive method can expedite the time of remediation, compared to passive approaches. The fully automated remotely controlled bioreactor system provides clients with a complete biodegradation of chlorinated ethenes.
Specifications
The system consists of a control unit, a bioreactor and a filtration and charging system. Once the DHC inoculum is added to the bioreactor, their growth is enhanced by feeding the system, until a cell count of 105 cells/ml is achieved. Bioreactor operating conditions include ORP levels of less than -300 mV, while oxygen levels are reduced to zero at a temperature of about 20 ºC. The bioreactor operates at an extraction/infiltration flow rate of about 10 m3/hr. Depending on the size of a pore volume for the targeted zone, the system may be able to treat a site within a period of several months. Based on previous experiences, amendment of one pore volume of the targeted aquifer zone is sufficient for complete chlorinated ethenes treatment at most sites.
Process and performance monitoring
Monitoring our process and performance is crucial to the success of the enhanced bioremediation projects. Therefore, defined parameters such as oxygen concentration, pH, water temperature (ºC) and oxidation-reduction potential (ORP) are monitored continuously. When the parameters exceed threshold levels, the bioreactor is automatically switched off. Monitoring wells in the field are used to assess the efficacy of enhanced bioremediation. The design and operation of this in-situ system of extraction and infiltration can be adjusted depending on the volumes of groundwater to be treated, site specific circumstances and remedial targets to be met.
Technology application
Leakages that had occurred, because of a former dry cleaner in the center of The Hague in The Netherlands, left the shallow and deeper groundwater up to 15 m –gl contaminated with chlorinated solvents (PCE and TCE).
Soil properties, in the area of The Hague, are characterized by sediments of medium fine sand and heterogeneous intermediate layers of clay and/or peat. Groundwater contamination plumes had spread to a volume of 400,000 m3 underneath private properties and form a thread for the deeper aquifers and the abstraction of drinking water.
The design by Bioclear opted for a phased approach, with the infiltration and extraction filters all in the public road. The plume was divided into six successive steps which were provided with a carbon source and micro-organisms. After completion of the first phase, the extraction wells of phase 1 were used as infiltration wells for phase 2. This function shift is also applied in subsequent phases, for which a total of 55 wells are placed.
The contaminants are biologically degraded in the subsurface within the course of a few months to a few years at most. After this time the risks will have been removed and we are left with clean groundwater. The location in The Hague is situated in an urban area; even so, the entire remediation system could be placed without problems. All required wells and pipes were placed underground. The remediation unit was on-site for approximately sixteen months. This is very short when compared to more traditional remediation techniques. This also meant that the disturbance to the community was kept to a minimum. Furthermore, this technique is sustainable and environmentally friendly, as it requires very little energy and only natural nutrients are used. In a period of sixteen months, the bacteria and nutrients were introduced into the soil. A large portion of the contamination had already been entirely degraded in that time. The remediation project was therefore a great success.
Conclusions
The TCE treatment results in a complete degradation to ethane/ethane in all the projects.
Using the fully automated telemetric system, it is possible to continuously control the pH, redox and oxygen in the groundwater.
Logging data indicated the working of the TCE unit with minimum human supervision and maximum efficiency.
TCE is a sustainable solution for large plume remediations in comparison to conventional techniques.
The project was completed in time, with no rebound observed and with no cost overruns.
Bioremediation of chlorinated solvents including chlorinated ethenes, chlorinated ethanes and chlorinated methanes in groundwater is a proven remedial approach and has been used widely in Europe and North America for over a decade. In many Northern locations in Europe, the U.S., and Canada low groundwater temperatures are ubiquitous and low permeability matrices are widespread. Technologies that can mitigate injection issues in low permeability matrices and growing experience with low groundwater temperature bioremediation have led to a better understanding of remediation strategies and timelines under these conditions.
Understanding the feasibility of bioremediation and the practical limits of bioremediation of chlorinated solvents under cold conditions is important in remedy selection and expectation management for colder climate bioremediation projects. Groundwater temperatures defined as cold for bioremediation applications (i.e., below 10 ºC) are commonly found in Europe north of approximately 55 degrees latitude including much of Scandinavia, the Baltic Countries and Scotland. In North America cold groundwater is generally found north of 45 degrees latitude in the Northern contiguous US, Alaska and much of Canada. Specific strategies for approaching cold water bioremediation will be discussed including bioaugmentation, electron donor selection and application. Examples of successful bioremediation at cold climate sites in Denmark, United States (Alaska) and Canada, will be presented with a focus on degradation half-lives, concentrations of dechlorinating bacteria (Dehalococcoides) and remediation outcomes.
Low permeability strata are common in some of the most highly industrialized areas of Europe and North America. Low permeability materials pose a challenge for in situ remedial technologies as delivery of the amendments and contact with the compounds of interest can be challenging. Closely spaced injection points using multiple injection intervals per point can aid distribution, but can also be time consuming and costly. Novel technologies, such as Electro Kinetic bioremediation (EK-Bio), can be used to deliver electron donor in low permeability matrices as well as transporting dechlorinating bacteria through the low permeability materials. Hydraulic fracturing, a technique originally conceived as an oil and gas extraction technology, can also be used to improve distribution of bioremediation amendments, thereby improving bioremediation outcomes. Examples of successful implementation of EK-BIO and hydraulic fracturing for bioremediation in clay strata will be discussed.
A metal processing site in Flanders, Belgium is characterized by a groundwater contamination with chloro-ethenes that has migrated 1 km off-site. The groundwater has a high seepage velocity and is acidic. Laboratory tests have shown that enhanced natural attenuation by addition of organic substrate induces partial dechlorination of PCE, which stalls at cis-DCE. The objective of this EU-LIFE+ sponsored project BACAd is to demonstrate that bioaugmentation can be achieved on full-scale in a cost efficient way by optimizing the in-situ propagation of injected cultures.
In a first stage, five microbial cultures which were derived from different sites and two electron donors were screened with laboratory microcosm tests. The two best cultures were used for execution of two push-pull pilot tests. Each test was done with a specific culture and electron donor. Laboratory column tests were performed with these cultures and site materials to evaluate and optimize their migration in the soil. The culture that performed the best in the push-pull and column tests was injected in a pilot test with 4 injection wells. At the same time, a similar field test has been performed in an adjacent area with injection of groundwater from another site where complete dechlorination of PCE has occurred. Afterwards, the remediation has been scaled up to a reactive zone with 40 + 20 injection wells that covers the entire plume width. Full-scale bioaugmentation with transfers of the microbial population from the initial pilot test remediation areas to the reactive zone has started in spring 2014. By doing this, the costs for the production and injection of the microbial culture may be decreased, improving remediation efficiency. The in-situ propagation of microbial cultures is monitored with QPCR and DGGE-analyses.
Laboratory microcosms have demonstrated complete dechlorination with bio-augmentation in the presence of the substrates Nutrolase (a residue from potatoe processing) and glycerol. The column tests confirmed the need for bio-augmentation and the dechlorination capabilities of the two cultures that were used in the field. They have demonstrated the mobility of the cultures in aquifer materials of the site.
The two push-pull test with cultures grown on Nutrolase and glycerol induced complete dechlorination in the field. Acidic groundwater conditions have slowed the process and required neutralization. Glycerol was a better substrate than Nutrolase. The culture grown on Nutrolase was contaminated by pathogenic bacteria, which was caused by the Nutrolase. The in-situ evolution of the pathogens (faecal Streptococci) has been monitored. Results of microbial and molecular analyses by QPCR will be presented.
The first small scale pilot test with injection of glycerol and a microbial culture has achieved complete dechlorination in the injection wells following bio-augmentation. QPCR analyses have demonstrated that DNA of Dehalococcoides and of dehalogenating enzymes has increased consistently over time. Full dechlorination has not been achieved yet in the injection wells of the second small-scale test in which groundwater from another site was injected, although removal of PCE, TCE and DCE has been achieved to a large extent.
The establishment of suitable environmental conditions in the full-scale test area by regular injections of glycerol and bicarbonate has required more time than expected. This was the result of high groundwater velocity, oxidizing redox conditions and an acidic pH of 4. The transfers of groundwater from bioaugmented pilot test areas into the full-scale reactive zone were initiated as soon as favourable environmental conditions were established in the injection wells. They have been ongoing at regular time intervals since spring 2014. The monitoring results will be presented.