Miljøprojekt, 1148; Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Pilotprojekt med stimuleret in situ reduktiv deklorering - Hovedrapport

Pilotprojekt med stimuleret in situ reduktiv deklorering - Hovedrapport

Summary and conclusions

Chlorinated solvents constitute a significant problem for groundwater in Denmark. Remediation of such sites is a challenge in source zones of contaminated sites as well as in contaminant plumes. Recently, in situ methods for remediation of groundwater pollution have emerged from international studies as cost-effective technologies. This has generated a strong interest in applying and testing promising remediation methods under Danish conditions.

Stimulated reductive dechlorination has been applied with promising results in a pilot scale system with recirculation at Rugårdsvej 234-238 in Odense. The pilot scale has been established in a secondary sandy aquifer overlaid by clay till. The clay till is heavily contaminated by chlorinated solvents.

Addition of donor (carbon source) and bacteria has stimulated anaerobic dechlorination in the sandy aquifer as the composition of chlorinated solvents has changed significantly. Initially, cis-DCE dominated in all boreholes in the source area, and was converted to lower chlorinated ethenes. Vinylchloride and ethene dominated due to anaerobic dechlorination 201 days after initiation of the recirculation loop with lactate.

Monitoring results down-gradient the source area in August 2006 showed a DCE content of 21 µg/l and VC content of 21 µg/l. Prior to initiation of the pilot scale the content of DCE was 2,420 µg and the content of VC was 12 µg VC/l. The concentrations in the source area have been reduced to a lesser degree. In this part cis-DCE from the overlying clay is transferred into the sand layer. The significance of this is not quantified, but this will happen regardless of the remediation technology chosen as long as the contamination in the clay till lasts.

Stimulated reductive dechlorination as a remedy takes advantage of a sequential transformation of chlorinated ethenes under anaerobic conditions:

Trichloroethene -> Dichloroethene -> Vinylchloride -> Ethene/Ethane

The process can be stimulated by adding an electron donor creating anaerobic conditions and generating hydrogen. Hydrogen is used as the electron donor for dechlorination, while the chlorinated ethenes are used as electron acceptors. Complete dechlorination is needed in order to form harmless ethene/ethane instead of toxic intermediates like vinylchloride. A prerequisite for anaerobic dechlorination is the presence of dechlorinating bacteria, which can either be present as indigenous bacteria or have to be added (bioaugmentation).

In this case lactate has been applied for biostimulation of anaerobic dechlorination. Lactate has been converted to hydrogen and acetate and thereby the appropriate redox conditions were created in the aquifer (sulphate reducing) within three months. Bioaugmentation with dechlorinating enrichment (KB-1^TM) was done, but it is not yet clear whether dechlorination has been enhanced due to this. On the other hand, a significant growth of dechlorinating bacteria has been achieved.

The pilot scale system was an active (forced gradient system) with recirculation, which was run for 200 days. The treatment area is 40 m in flow direction and has a width of 20 m. Groundwater was extracted continuously (ca. 3,500 l/day) and injected into three injection wells. The groundwater pore flow velocity was depicted by a tracer to ca. 0.5 m/day. The residence time was approximately 60 days, corresponding to recirculation of three aquifer pore volumes.

The project has shown that it is possible to design, engineer and run an active system with extraction and recirculation of groundwater added with a donor. In the design of the system a detailed site characterization was important. The most significant operational problems were due to build-up of pressure caused by clogging. In future design of active remediation schemes, selection of pumps, filtration of extracted groundwater and regeneration of boreholes (screens) need careful consideration.

Risk assessment of the pilot scale system shows that addition of lactate does not constitute a problem as such, however, the conversion of lactate by sulphate reduction generates bad odour (sulphide) in the treatment area. Methane production has not caused problems. The extraction – hydraulic control -has been so efficient that the recirculation system has not enhanced spreading of chlorinated solvents down-gradient. An enhanced spreading of donor in a south-easterly direction was observed, but this is considered acceptable.

The project costs for the pilot scale system have been DKK 1.9 mill.. The cost is about DKK 2,000 /m³ soil assuming a treatment area of 900 m³. The high costs are caused by the fact that it is a technology demonstration project, where detailed documentation of hydraulic and biogeochemical processes was mandatory. A corresponding commercial project with an active system will probably cost about DKK 1,000 /m³ soil. The costs are comparable to other in situ methods such as chemical oxidation.

A large potential for remediation of chlorinated ethene by stimulated anaerobic dechlorination has been shown based on the experience from the pilot scale experiment at Rugårdsvej 234-238. A contamination dominated by TCE/cis-DCE has been transformed. The key process is biological which has the advantage that the process can continue even after the injection of donor is terminated. The main disadvantage is the generation of intermediates which demand complete dechlorination for a successful clean up. It has been estimated that complete dechlorination to ethene would have been likely within 12 months, without constant input of cis-DCE from the overlying clay till.

Active and passive systems have advantages and disadvantages. Operation of an active remediation system requires a permeable setting. This was the case at Rugårdsvej and this will be the case at many other contaminated sites in Denmark. On the other hand many sites will not be suitable for active systems and this gives rise to a need to develop both active and passive systems. After six months to a year, some systems will change mode of operation from active to passive. Since day 250, the system at Rugårdsvej has been operated as a passive system.

Stimulated anaerobic dechlorination will be applicable at many Danish sites. Naturally occurring anaerobic conditions will be optimal. The remediation technology under these conditions will be cost-effective compared to other in situ technologies. The choice between anaerobic dechlorination and other in situ methods should be based on an evaluation of all advantages and disadvantages for clean up at the actual site.