Low pH impacts on enhanced in situ bioremediation

Full text: Introduction - Enhanced In-Situ Bioremediation (EISB) is an innovative and cost-effective treatment technology for chlorinated solvents and other recalcitrant chemicals in groundwater. Through appropriate selection of electro donors (biostimulation) and the injection of bacterial cultures into groundwater (bioaugmentation), the degradation of compounds like trichloroethene (TCE) to the non-toxic end-product ethene is achieved. However, low pH is a common condition encountered in bioremediation. The in-situ biological degradation of insitu chlorinated compounds can be impacted by reduced pH environments as many biological cultures have a preference for neutral pH (6 to 8.5). Sites with low pH, or sites that have reduced pH as a result of electron donor addition are more likely to accumulate degradation daughter products cis-dichloroethene (cDCE) or vinyl chloride (VC), which is more toxic compound than the parent TCE. Methods - The presentation will provide the technical reasons for decreases in pH as a result of biostimulation by electron donors via the production of volatile fatty acids in the subsurface and hydrochloric acid in the dechlorination process of TCE to ethene. Also, for existing sites, the impact of low pH will be discussed and data provided from bench scale testing and field sites on what levels of low pH cause impacts to the bioremediation process and solutions to overcome these impacts. Results and discussion - The presentation will provide several sets of data to highlight the low pH problems at EISB sites and provide solutions to preventing and overcoming reduced biodegradation as a result of low pH, including: A discussion on the impact of pH and how small changes in pH can result in large reductions in microbial activity and the rate of biodegradation and potential accumulation of cDCE and VC will be presented including microcosm testing results. Options and challenges with aquifer neutralisation using products such as sodium bicarbonate, potassium hydroxide and sodium hydroxide will be discussed. The impact of different electron donors on the generation of hydrochloric acid will be discussed. The use of low pH tolerant bioaugmentation cultures will be presented as an innovative solution to bioremediation in low pH conditions. The success of bioaugmentation and the rates of biodegradation at various pH levels will be presented. Finally, case studies including a site in Western Australia, will be presented showing bioremediation at low pH sites. Conclusions - Low pH is a common condition at sites requiring remediation. There are several aspects to consider when implementing bioremediation at low pH site such as electron donor selection, buffering options, and selection of bioaugmentation cultures. However, these challenges can be overcome and successful bioremediation at low pH sites is achievable. (author)