The remediation of VOX- contaminated groundwater by means of molasses injection on a former industrial site of Philips Company in Rambouillet, France

Transcription

1 LIFE99 ENV/NL/ The remediation of VOX- contaminated groundwater by means of molasses injection on a former industrial site of Philips Company in Rambouillet, France Layman s report, March 12 th, 2004

2 Introduction This comprehensive summary contains the results of the project: LIFE99 ENV / NL / The remediation of VOXcontaminated groundwater by means of molasses injection on a former industrial site of Philips Company in Rambouillet, France. The project started in November of 1999 and continued till October Due to the innovative aspects of the applied remediation technique the European Community granted a LIFE programme subsidy for this project. The contamination on this location consists of the following constituents: trichlororoethene (TCE) and cis-dichloroethene (DCE). These constituents are part of the group of volatile chlorinated organic hydrocarbons (VOX). TCE is a constituent that has been used on a large scale as degreasing agent in metal industries and chemical laundries for many years. Inventories in the United States and in the Netherlands show that about 30% of known groundwater contamination cases are related to VOX. Pump & treat versus enhanced natural attenuation The currently most common remediation technique for the VOX based contaminations is to extract the polluted groundwater and treat it ex-situ (pump & treat). The physical and chemical behaviour VOX requires that the pump & treat needs to be continued for a long time. In this project an alternative technique is applied. In this method the naturally occurring biological degradation of VOX is enhanced in situ by optimising the necessary physical and chemical soil conditions for the decomposing processes. The optimisation occurs by adding special substances to the groundwater. The remediation period of this alternative technique is assumed to be significantly shorter than with conventional techniques. Costs of applying this new technique are assumed to be significantly lower than those related to conventional techniques. Moreover this alternative technique is meant to remove the complete contamination. Objective of the Project The objective of the project is to demonstrate the feasibility of VOX contaminated groundwater remediation by means of enhanced biodegradation with molasses injection. The project goals are: - To determine the essential site and soil information to estimate the success of enhanced biodegradation remediation of VOX by means of molasses injection; - To study the dosing, injection and distribution strategies of the molasses solution in order to optimise the technique. Stimulating the biodegradation of VOX. Chlorinated hydrocarbons like TCE are degraded into other compounds by soil bacteria metabolism processes under naturally occurring conditions. These metabolism processes are complicated chemical reactions where electrons are transferred from one compound to the other (redox reactions). The degradation of VOX takes place in a number of steps. In each step a chlorine atom is removed from the molecules. TCE is decomposed into DCE, DCE into vinylchloride (VC) and VC into ethene and ethane. The bacteria that degrade VOX compounds are most active under specific soil conditions. For example, the acidity of the soil and groundwater should be within certain limits. The most controlling soil characteristic determining the biodegradation is the so-called

3 redox potential. Naturally occurring redox conditions are at best only sub-optimal for fast and complete biodegradation of VOX. Especially, the final steps in the biodegradation process are slow or not taking place at all resulting in the accumulation of the derivatives DCE and VC. By adding extra food for the soil bacteria, redox conditions can be manipulated such that they become favourable for the VOXdecomposing bacteria. This bacteria food should contain an organic carbon source such as molasses. Molasse is a waste product of sugar production processes. Project phases The project is carried out in a number of phases: - Phase 1 comprises the geohydrological and chemical characterisation of the pollution and the determination of site-specific factors. - The goal of phase 2 to study groundwater flow and transport processes on this site by means of a groundwater model. Based on the model results phase 3 can be executed. - The goal of phase 3 is to design and engineer the remediation system (e.g. amount of molasses needed, injection rate and frequency, number of injection and extraction wells). - The goal of phase 4 is to remediate the VOX contamination on the site by means of enhanced biodegradation with molasses injection and determine the economic and technical applicability of the technique. - Phase 5 comprises the dissemination of the results. These phases are partly performed simultaneously. Usually the main part of phase 1 is conducted at the beginning of the project. However, the characterization task turned out to be a continuous and cyclic process. In the final stages of the project specific information was needed to answer specific questions that rose during the project execution. Besides, part of the characterisation process was repeated since more advanced methods became available. Phase 1: Characterisation With the characterisation, insight was obtained in site-specific conditions and in the physical and chemical behaviour of the soil and groundwater. Moreover, it gave information on the extent and dynamics of the groundwater contamination. The studied site is located on an undulating terrain and is almost totally covered with buildings, roads and pavements. Groundwater flow is to the south and southwest and the groundwater table is at about 9 metres below surface. From the field investigations it was found that the soil is consisting of very fine sands with local marl and clay layers. The vertical spreading of the VOX contamination is to a depth of meters below surface. The in-situ redox conditions are not optimal for biodegradation of the VOX. The level of organic matter in the soil is very low. A preliminary study showed that the presence of the necessary anaerobic bacteria group was poor. A more sophisticated detection method performed in a later stage of the project showed that the essential bacteria Dehalococcoides ethenogenes only occurred in a small population on this site. These field results gave the initial impression that there was a poor chance of success for the enhanced biodegradation of VOX by molasses injection. To improve the feasibility estimation of the new technique some laboratory tests were performed. In these tests, soil and groundwater of the site were put in batches and brought under the conditions similar to those at the site (Biological Activity Test or BAT). Subsequently molasses were injected into one of the batches and the changes in the redox potential and VOX concentrations were monitored. The first test showed that the conclusions derived from the field investigations were valid. The biodegradation was taking place at a slow rate only after adding the molasses. During the fieldwork it was observed that there was a lack of relevant soil bacteria. Hence in a second batch, groundwater with a viable bacteria population was injected next to the molasses. It was observed that the biodegrading process was stimulated sufficiently. Bases on these tests it was Injection

4 concluded that remediation by means of enhancing VOX biodegradation was feasible. additives to this groundwater it is spread over 11 clusters of injection wells (see figure 3). The system is equipped with a computerized on-line monitoring and management system enabling the process to be controlled from an office in the Netherlands (figure 4). Figure 1: Groundwater flow pattern and path lines based on model calculations in case of injection of water additives in injection filter I7 and extraction of groundwater in well I20 Phase 2: Groundwater modelling The results of the first task were used as input for the groundwater model. First of all, the model has been essential in studying the groundwater system: in what direction and how fast is the groundwater flowing? Secondly, the groundwater model has been used to study the effects on the flow pattern and on the spreading of injected molasses and bacteria. Thirdly, it was used to study the geohydrological effects of measures like extracting and injecting of water (see figure 1). Furthermore, the groundwater model gave the opportunity to study the containment of contaminated groundwater through geohydrological isolation. Figure 2: Mixing tank where molasses and other additives such as bacteria and micronutrients are added to the groundwater Phase 4: Remediation In an ideal situation the enhancement of biodegradation starts at a single injection well. Subsequently, the biodegradation is scaled up to the entire area by expanding the number of injection wells. It is obvious that the groundwater model has been an essential tool for the design and engineering of the remediation system. Phase 3: Design and engineering of remediation system Based on the results of the former two phases the remediation system has been designed and engineered. The system consists of a large number of injection and extraction wells distributed over the site in such a way that the remediation strategy can be adjusted flexibly. In this system, groundwater is extracted in some wells on the site and transported to the central dosing unit. The adding of the molasses and additional bacteria takes place in a mixing tank of 3,5 m 3 (see figure 2). After adding the Figure 2: Infiltration pipes in central dosing unit The start up needed two steps. In the first try, the molasses concentration in the groundwater was too low. With this low concentration (4 mg/litre) bacteria living directly nearby the injection well started to grow explosively resulting in clogging of the filter. After regeneration of this clogged well, another volume of molasses was injected at a much higher concentration (250 mg/litre). Only after

5 some time, the molasses solution is diluted to such degree that soil bacteria are able to consume it and bacteria growth is inhibited. In this way clogging of the well is prevented. Observations Following observations were made in this project: - Injection of molasses created strongly reduced soil conditions. In the area where the start up of the remediation was performed active reduction of nitrates and sulphates occurred. It was found that reducing conditions could easily be manipulated in the soil by adding molasses. - More than one year after injection of the molasses in well I7, organic carbon was found at 40 meters downstream of the injection location. It appears that the injection and spreading of the molasses into the soil on this site is feasible - The active soil bacteria that have been added to the groundwater at the site in Rambouillet were derived from a remediation site in the Netherlands. During storage (more than a month) the acidity in this Dutch groundwater decreased and

6 Figure 4: Online monitoring and control system Consequently only part of the active biomass survived. - By lowering the redox conditions the biodegradation of TCE has been stimulated. The biodegradation of DCE to VC is however not taking place completely. In figure 5 it can be observed that the redox potential in well I9 (at 20 meters from the injection location) dropped after injection of molasses in September and October Only after optimising the soil conditions the percentage of converted TCE increased from 0 to 30%. - The incomplete biodegradation of the VOX compounds is possibly due to the fact that the sulphate concentration in the start up zone exceeded our estimates. The amount of injected molasses was therefore not sufficient for reduction of this high amount of sulphates. - The geohydrological manipulation in the start up zone has not been satisfactory. Part of the injected molasses has spread in a southwest direction resulting in too much dilution. Hence the amount of molasses available per cubic meter of polluted soil was too low. Recommendations for feasibility determination To determine the feasibility of remediation by enhanced biodegradation of VOX following aspects should be studied: - The existence of the various VOX compounds and their biodegraded derivatives. - The redox conditions i.e., the concentration of various electron acceptors have to be determined. To define the right amount of necessary molasses, the total mass of these acceptors should be known. - The bacteria relevant for the VOX biodegradation should exist in a viable population at the site. In case of VOCdegradation also the existence of nitrate and sulphate reducing bacteria is important. - Micronutrients determine the activity of the soil bacteria and thus it should be studied whether additional micronutrients need to be injected into the soil. - From our study it is concluded that the most important soil characteristic is the organic matter content in the soil matrix. At the Rambouillet site this organic matter content is extremely low.

7 Recommendations for implementing and running in-situ remediation by means of molasses injection Based on our research we consider the organic matter content as most controlling soil factor for the success of enhanced biodegradation since it is nearly impossible to manipulate this particular soil characteristic. Obviously, the redox conditions in the soil are important as well since these are dictating the rate of biodegradation. The redox conditions in the soil can be manipulated relatively easily and therefore they are not controlling the success of enhancement. Lack of organic matter results in insufficient adsorption capacity for the bacteria and micronutrients. Without this attachment the injected bacteria are not able to develop a viable population. Secondly, the reduced adsorption of the matrix results in smaller retardation of the injected molasses. The spreading of the molasses is too fast. Bio-augmentation of groundwater with water containing active bacteria from other contaminated sites may cause procedural problems. In our case, the Dutch authorities only agreed with transporting a groundwater volume with active bacteria from one contaminated site to another after lengthy negotiations. To avoid delays one should involve the relevant authorities as soon as possible. The conservation of active bacteria in groundwater in a tank and especially growing a viable bacteria population in it appears to be most efficient when also soil particles are put in the tank. The soil particles deliver the necessary surface for bacteria and micronutrient attachment as mentioned above. Before injecting the groundwater with the bacteria, the soil particles should be removed to prevent the well from clogging. It is obvious that the groundwater with active biomass has to be stored and conserved under the right external conditions. From out study it appears that the bulk of the injected molasses on this site was used to develop the favourable reduced conditions. The bacteria using the VOX compounds in their metabolism use only a minor part of the injected molasses. When injecting the molasses one should take into account the viscosity of the solution. Obviously, when the solution is too viscous it will be hard to inject and distribute it through the soil. A dose of 1 volume of molasses with 2 volumes of groundwater is sufficient to avoid these viscosity problems. When the injected molasses solution is too diluted problems with explosive bacterial growth may lead to clogging of the injection filters. By raising the initial molasses concentration in the injected volume of groundwater it acts as a preserving substance. Only after some dilution due to diffusion and transport processes soil bacteria are able to consume the added organic carbon. This takes place at some distance from the injection well. Results of this remediation are compared with the result of application of the similar technique at two other sites. At these sites the biodegradation runs faster and is more complete. This is probably related to high organic matter content in the soil matrix and the existence of a relevant population of soil bacteria. Environmental benefits The environmental benefits of the enhanced biodegradation of VOX by means of additives injection are: - The technique is based on naturally occurring processes. It only enhances these processes resulting in faster and more complete biodegradation. - The technique does not use synthetic substances and uses only limited amounts of energy. - When the technique is optimised, after remediation only minor contamination is left. - The technique uses a closed groundwater balance. Extracted groundwater at the site is mixed with the molasses and is subsequently injected. There is no loss of groundwater from the site. - Generally the duration of remediation is short. - Hardware and substances needed for running this technique are readily available. - High adsorption capacities of soil materials like clay and peat mostly limit the effectiveness of conventional remediation techniques. These high adsorption capacities are however favourable when

8 Figure 5: Decrease of redox potential and the increase in percentage of biodegraded TCE after molasses injection applying enhanced biodegradation with molasses injection. In case of an average organic matter content and some initial biological degradation of VOX it is found that remediation at this site is more cost effective by means of molasses injection than pump & treat, However in the real situation of low organic matter content and poor initial biological activity pump & treat is most cost-effective. Generally, costs for applying this technique are low. Intensive monitoring of the progress contributes significantly to total costs. More information Results of this study are described in following articles: - Brummeler, Dr. ir. E ten & Ing. G. Wijn, Remedie voor een verzande in-situ sanering, Bodem, jaargang 12, nummer 5, 2002 (in Dutch) - European subsidy for innovative redevelopment techniques, Environmental Info (PHILIPS), Edition 5, March 2001; - Midden, R. van, Voormalig Philipsterrein wordt gesaneerd met stroop, Locatie (ARCADIS), nr. 1, maart 2001 (in Dutch). For more information please contact: - Philips Environmental Services Mr. J. Schreurs P.O. box MD, Eindhoven, The Netherlands phone: +31 (0) fax: +31 (0) jack.schreurs@philips.com - A&G Milieutechniek Mr. G. Wijn P.O. Box AR Waalwijk, The Netherlands phone: + 31 (0) fax: + 31 (0) More information can also be found on following websites: