Miljøprojekt, 983 Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Stimuleret in situ reduktiv deklorering. Vidensopsamling og screening af lokaliteter - Hovedrapport

Stimuleret in situ reduktiv deklorering. Vidensopsamling og screening af lokaliteter

Summary and Conclusions

This report highlights the possibility of applying stimulated in situ reductive dechlorination as a remediation technology on Danish sites polluted by chlorinated solvents. The project was carried out under the Danish Environmental Protection Agency's Technology programme of soil and groundwater pollution in cooperation with the County of Funen.

The project was carried out in cooperation between the Technical University of Denmark - Environment & Resources DTU, GeoSyntec Consultants and COWI A/S.

The project includes a collection of the knowledge of the subject available nationally and internationally. Additionally a screening model is developed for an initial assessment of remediation by means of stimulated in situ reductive dechlorination on a certain location. Initially the model will be applied for screening of 13 sites on Funen, all polluted by chlorinated solvents.

The project is confined to including the degradation of chlorinated ethenes and 1,1,1-TCA.

Literature review

Reductive anaerobic dechlorination
Stimulated in situ reductive dechlorination is a remediation technology applied for sites polluted by chlorinated ethenes. The natural degradation processes in the groundwater system are stimulated by addition of electron donor and/or bacteria. At anaerobic dechlorination there will be a stepwise removal (substitution by hydrogen) of the chlorine atoms, so that from PCE there will be formation of TCE, cis-DCE, VC and finally ethene. Anaerobic dechlorination is a redox process, in which certain bacteria can use the chlorinated ethenes as electron acceptor for generation of energy in a respiration process often called dehalorespiration. Most dehalO^-respiring bacteria use hydrogen as the primary electron donor at the dechlorination of chlorinated ethenes. The process may proceed naturally in polluted groundwater systems under reduced redox conditions, but will often be limited because of lack of electron donors.

A number of dehalO^-respiring bacteria are able to dechlorinate PCE or TCE to cis-DCE, but today the only isolated bacterium known is, Dehalococcoides ethenogens 195 that can reductively dechlorinate PCE or TCE completely to ethene. The last dechlorination step from vinyl chloride to ethene takes probably place by cometabolic transformation and is not connected to dehalO^-respiration. Dehalococcoides ethenogens 195 belongs to the bacterial strain Dehalococcoides, which has the characteristics that it can carry out reductive dechlorination from cis-DCE to VC. In practice this is extremely important, as it means that the anaerobic dechlorination from PCE til cis-DCE can only be regarded as the first part of the process. The very presence of cis-DCE does consequently not prove that a complete degradation to vinyl cloride and ethene can take place on a certain location. Absence of Dehalococcoides means probably that anaerobic dechlorination from cis-DCE to VC does not take place.

Application of stimulated in situ reductive dechlorination as a remediation technology
At application of stimulated in situ reductive dechlorination as a remediation technology, electron donors - often in the form of organic matter - and nutrients are added to the saturated zone. At fermentation of the electron donor there is a generation of hydrogen, which is used in the reductive dechlorination. Addition of electron donors alone (without addition of bacteria) is called "biostimulation". If a bacterial culture is added simultaneously, it is called "bioaugmentation".

The majority of experience with the method in pilot and full-scale has been achieved in North America. There are however some activities in Holland, but in the international literature there are but few publications from there. In Denmark there is only one example of field experience in terms of a pilot test.

The method can be used on both chlorinated ethenes and 1,1,1-TCA. Removal of the free phase is always recommended prior to remediation by stimulated in situ reductive dechlorination. On the basis of laboratory tests it seems there is a potential for stimulation of degradation of free phases or on the contact surface between free phases and the pollution plume. This is a significant research field and is at present investigated in pilot tests in the field in North America.

As stimulated in situ reductive dechlorination involves addition of electron donors and fermentation substrates to groundwater, this technology is so far only applicable for treatments in the saturated zone.

Remediation principles
Field techniques used for stimulated in situ reductive dechlorination involve both active and passive systems.

Active systems use constant or pulsating addition of electron donors - often combined with pumping and recirculation of groundwater. Compared to passive systems, active systems require more equipment on the surface, such as pumps and pipings, injection equipment and automatic control and monitoring equipment. Active systems are most often used in deposits with good hydraulic conductivity, such as sand/gravel deposits, but they are also used in fissured deposits.

The advantage of active systems is a better distribution of addition agents in the aquifer compared to the passive systems. This means a shorter and a more efficient treatment time. The largest disadvantages of active systems are that they are not suitable for low-permeable deposits. Additionally the operation of remediation systems is typically more difficult than that of the passive systems.

Passive systems include one or more injections of donor in the treatment area or of biobarriers. Passive systems are generally simple and easy to implement. The risk of operational problems and clogging of bore holes is less significant than in connection with active systems. In comparison to active systems, passive systems are godless efficient at achieving the same contact between electron donor, bacteria and the chlorinated ethenes. The treatment effect is therefore inferior to that of the active systems. When assessing the experience with active and passive injection systems, it should be considered that the data of passive systems collected are of inferior quality.

Hydrogeology
In situ anaerobic dechlorination as a remediation technology has mainly been used in sandy aquifers, and lately there has been a rapid development in the application in fractured rock types (lime, granite). The knowledge basis regarding treatment of low-permeable aquifers is poor. The gained experience indicates that the same problems as with other techniques are to be expected on such sites, i.e. problems with addition and mixing (creating contact with the pollution), difficult monitoring and distant time horizons. As pollution with chlorinated solvents in Denmark is often related to low-permeable deposits, it is necessary to clarify - by means of pilot tests - the possibility of remediation of low-permeable deposits.

Redox conditions
At the review of literature it was found that the method has been applied in both anaerobic and aerobic aquifers. A significant accumulation of the degradation products cis-DCE, VC and ethene together with considerably reduced conditions are true indications that the groundwater chemistry on the location is favourable for in situ reductive dechlorination. On sites with aerobic conditions, where the concentration of degradation products is insignificant, stimulated in situ anaerobic dechlorination might still be a suitable method. The adaptation period, before the anaerobic dechlorination is effective, will however be longer than that on sites with already anaerobic conditions. On aerobic sites the need for addition of bacteria (bioaugmentation) will probably also be higher. Prior to bioaugmentation it is important that reducing conditions have been obtained as exposure to oxygen might harm the bacteria.

Electron donors
Substances normally used as electron donors are lactate-based polymers (HRC), oil-based slow-releasing compounds, soluble compounds such as molasses, foodstuff-based acids (acetate or lactate), alcohols (methanol or ethanol) and other naturally occurring materials (chitin or composted wood/bark debris). Under anaerobic conditions all these materials are subjected to fermentation, at which hydrogen (H₂) is generated. It is distinguished between dissolved, easy-reacting electron donors and slow-releasing electron donors.

Sand and gravel aquifers and fissured structures such as lime aquifers are often suitable for active addition of dissolved, easy-reacting electron donors, such as sodium lactate, methanol, ethanol and acetate. In low-permeable deposits are typically used slow-releasing electron doners, such as HRC, vegetable oils or emulsified soya bean oil.

Generally there are no electron donors that can in advance be pointed out as being better than others, as many electron donors have turned out to be suitable. In order to ensure an optimal donor selection as regards the process, it is suggested - on the present experience base - to carry out laboratory tests with donors that are potential candidates on the location in question. These tests would also highlight the donor consumption on the location in question. The most significant uncertainty as to the need for electron donor is connected to the volume of biologically accessible iron. In literature there is little focus on this problem, but at the same time high concentrations of dissolved iron has been reported at many field tests. Irrespective of the fact that iron has significant importance as electron acceptor, it is worth noticing that the total electron donor requirements are typically lower than the quantities of chemical oxidants added at e.g. chemical oxidation.

Bacterial addition (bioaugmentation)
Many of the pilot and full-scale remediation projects indicate that it is necessary to add bacterial culture to obtain a complete dechlorination to ethene, as the dechlorination often stop at cis-DCE without addition of bacteria.

Another significant discussion is whether it would be advantageous to add more bacteria to a system, where anaerobic dechlorination to ethene takes place. Today there is no answer to this question, as too little knowledge is available on the ability of the bacteria to grow under field conditions compared to existing bacteria. The literature review indicates that there is an advantage in the form of a faster degradation. This would be a very important argument, as it might be possible to reduce the time horizon of the treatment.

Treatment effect and time frame
The best remediation effects were found at active systems under application of bioaugmentation. Here, a remidiation effect below 10 μg/l of the sum of chlorinated solvents was achieved.

In literature there are few reports on completed cases that include the necessary time horizons of treatment. The fastest treatment were found at active systems under application of bioaugmentation, whereas the slowest treatment were found using passive systems in low-permeable deposits. It is however probable that the time at the present stage of stimulated in situ anaerobic dechlorination will be several years, so there has been no revolution in the efficiency of the remediation of chlorinated solvents compared to other in situ methods. The method is assessed to have a considerable development potential, if the applied development and the application of the method in Denmark and abroad continue.

Spreading and survival of bacteria
The risk at injection of bacteria is considered insignificant, if the procedures developed in North America are followed. Immediately the greatest problem is the risk of injection of pathogenic microorganisms, which is to be ensured by means of certificates stating that the bacterial cultures have been tested and found free of pathogens. It is stressed that the known bioaugmentation techniques use bacteria, which are enriched on the basis of naturally occurring bacteria. That is to say they are not genetically manipulated bacteria (GMO).

A frequently asked question concerns the risk of spreading and survival of the injected bacteria. The bacteria can be conveyed in aquifers - actually a condition for the use in connection with bioaugmentation. The speed of the spreading is probably lower than the speed of the natural groundwater flow. Dehalococcoides' survival in aquifers has not been thoroughly investigated. A heavy growth of the bacteria outside areas polluted by chlorinated solvents is not expected. The culture will probably disintegrate or die under aerobic conditions, but the level of knowledge is very sparse.

Proposal for a investigation scheme
Compared to the present Danish pollution investigations there will in future be a need for measuring ethene and ethane in the groundwater as a standard. It is also recommended to include a determination of the organic matter in the sediment as a standard. Additionally there is a need for determination of the redox conditions. The supplementary costs for these investigations will be relatively low - under the precondition that they are carried out simultaneously with the other investigations.

Economy
Compared to other remediation methods, such as air sparging, chemical oxidation and preventive pumping, stimulated reductive dechlorination is considered competitive. It would however depend on a specific assessment of the individual case.

Legislation
Addition of bacteria and substrate to the groundwater and a possible recirculation of bacteria-containing groundwater will in Denmark normally require an authorization according to the Environmental Protection Act's § 19, 1. The county is the authorizing authority according to § 19, 4. Requirements of a possible monitoring will be determined by the county in connection with the authorization and be included in the authorization as a condition. It is however stipulated in § 63 of the Land Pollution Act that in cases of public investigations or preventive efforts, these can be carried out without authorization according to e.g. the Environmental Protection Act and the Water Supply Act. It is a precondition that it concerns an actual preventive or survey project. The County Council decides whether a case is acceptable as public survey and preventive efforts.

Development needs
Compared to many other remediation methods stimulated reductive dechlorination is relatively complicated, as it - in addition to the traditional fields of activity ( hydrogeology, pollution chemistry, dimensioning and implementation) involves a significant microbial and geochemical element, especially after that bioaugmentation (addition of bacteria) has become part of the technology. This means that there is a need for cooperation between authorities, researchers and consultants.

The below topics show where there is a need for development of anaerobic dechlorination as preventive technology on field scale in Denmark:

Well-documented pilot tests and full-scale implementations, including an assessment of choice and consumption of donor
Remediation of low-permeable sites or in unsaturated zone
Remediation of free phases or on the contact surface between free phases and the pollution plume
Handling and injection of bacteria
Assessment of need for bioaugmentation
Laboratory methods for assessment of anaerobic dechlorination potential and rates of degradation
Authorities' approval of injection of donor and bioaugmentation

In connection with the solution of the above topics it would be relevant to involve both Danish and foreign consultants/researchers and Danish authorities (the Danish EPA).

Screenings of sites

A screening model has been prepared for an initial assessment of remediation by means of stimulated in situ reductive dechlorination on a certain location. The model presents a list of criteria for an assessment of the applicability, and it adds a score for each criterion. The criteria are organised in four general categories: Preliminary investigations of the location/hydrogeological profile, pollution profile, geochemical profile and logistic factors. The criteria are weighted according to their relevance. Criteria rendering probable the use of stimulated in situ reductive dechlorination are weighted positively, and criteria reducing the possibility are weighted negatively. Additionally, criteria minimizing the costs of the implementation are weighted positively. The most important factors in the model are the hydraulic conductivity and presence of degradation products. A high hydraulic conductivity and presence of vinyl chloride and ethene/ethane result in a high score. The aim was a model that is relatively simple, and that it must be applicable on the basis of the data typically found at Danish extensive/successive investigations.

The results generated by the model serve solely as an indication of the possibility of stimulation of the reductive dechlorination - and are consequently not to be used instead of investigations on the applicability in the field. The model presupposes that the mother product is only PCE or TCE.

The model has initially been used for screening of 13 sites on Funen that are all polluted by chlorinated solvents. The screenings show a considerable variance among the individual sites. Thus it has been relatively easy to select the most suitable sites.

When a location has achieved a low score, it does not mean that stimulated in situ reductive dechlorination cannot be attractive as a remediation method on the concrete location compared to other techniques. It must be expected however that it would be more difficult to treat the pollution by means of the method in question.