Numerous industries (metal processing companies, dry cleaner’s, …) in Europe face groundwater contamination with CAHs. These are volatile organo-chlorine compounds (C1 and C2) that are toxic and carcinogenic. The group of CAHs can be divided into several subgroups: chloro-ethenes, chloro-ethanes and chloro-methanes. Pure CAHs are liquids heavier than water that can migrate to large depths. Because they are slowly degraded, they tend to form large groundwater plumes that are very difficult to remediate. ’Traditional’ remediation techniques such as pump and treat are often inadequate, time-consuming and expensive. Due to the chemical characteristics and the large number and extent of sites with CAH contamination, they are considered by Flemish and European authorities as priority sites for remediation.
Anaerobic dechlorination by soil microorganisms is a promising remediation approach for CAH plumes. The technology requires that, site conditions are favourable or can be engineered to become favourable.
There are several types of unfavourable conditions which may hamper anaerobic bioremediation:
- Insufficiant presence of suitable micro-organisms in the contaminated subsoil. The different contaminants of the CAHs and their metabolites are degraded by different bacterial species. Because of this, the bacterial population that is introduced into the groundwater should be a mixture of different types of bacteria that can degrade the entire mix of CAH.. By using bioaugmentation technology, the 50% of cases in which ENA does not work can be overcome. In the current demonstration we intend to show that this strategy can be implemented on a full scale for a CAH contamination where mainly chloro-ethenes are present
Recent pilot-scale research indicates that it is possible to introduce such micro-organisms:
a) by simple groundwater transfer, improving bioremediation.
b) by .introducing bacteria which have been grown in a laboratory. The purchase of bacteria every time a new inoculation of injection wells must be done is extremely expensive for large sites. Therefore, it is necessary to develop a methodology to transfer bacteria in the field from already inoculated injection wells to new injection wells. Such a work methodology did not exist before the start of this project.
- Unfavourable groundwater conditions such as low pH. From a microbial point of view, almost every injection well at this site had an unfavourable pH (<6) and a redox-potential that was too high for microbial dechlorination.
- Operational issues with implementation of bioremediation: Halorespiring bacteria are strictly anaerobic. Their activity and growth are impeded by the presence of oxygen in the groundwater. When injecting organic substrate and pH modification agents in the groundwater oxygen may be introduced in the subsurface. This may be the case when groundwater is mixed above ground with glycerin and bicarbonate. When there are only a few infection wells than this can be solved by flushing the mixing container with nitrogen gas. For the Full Scale demonstration , this is technicallyharder to achieve. Moreover, Flemish regualtions require the use of ‘clean’ water to be injected in the subsurface. That has typically less favorable conditions (i.e. more oxidized that the receiving groundwater) whih contributes to this issue.
References
On anaerobic bioremediation of chlorinated solvents:
Engineered Approaches to In Situ Bioremediation of Chlorinated Solvents: Fundamentals and Field Applications
On bio-augmentation for chlorinated solvent sites:
BIOAUGMENTATION FOR REMEDIATION OF CHLORINATED SOLVENTS: TECHNOLOGY DEVELOPMENT, STATUS, AND RESEARCH NEEDS (2005)