Corey Scales

Optimizing Bioremediation with Low pH Tolerant Dhc 

Corey Scales, Bioaugmentation Coordinator, SiREM

Tetrachloroethene (PCE) and trichloroethene (TCE) can be reductively dechlorinated to cis-dichloroethene (cDCE) under anaerobic conditions by a variety of microbes. Continued dechlorination of cDCE to VC and subsequently to ethene is conducted uniquely by Dehalococcoides (Dhc) bacteria containing the appropriate vinyl chloride reductive dehalogenase gene. For sites that are lacking native Dhc with the appropriate gene, commercial Dhc cultures are often injected into the groundwater to complete the cDCE and VC dechlorination to ethene step.   Bioremediation of chlorinated ethenes and many other chlorinated compounds is optimal at neutral pH (i.e., 6.8-7.5). pH values below 6.0 is problematic for bioremediation of chlorinated ethenes with the cDCE and VC to ethene step being the most affected with either slow dechlorination rates or even complete stalling. Given that both reductive dechlorination and fermentation of commonly used electron donors are acid generating processes, enhanced bioremediation has the potential to decrease pH into the inhibitory range, even if prior to bioremediation amendment addition the pH was acceptable.      Modifying aquifer pH using buffering agents such as sodium bicarbonate and various commercial formulations has become increasingly common. Aquifer pH modification has shown  varying degrees of success depending on application method, site geology and geochemistry but is generally considered challenging.  In certain cases, especially where pH is near or slightly below 6.0, the use of bioaugmentation cultures acclimated to lower pH has the potential to reduce the need for aquifer neutralization. Increasing evidence indicates that complete dechlorination to ethene is possible below pH 6.0 with pH tolerant bioaugmentation cultures.  These cultures can be used to minimize or avoid the need for aquifer neutralization.     The development and field use of low pH acclimated cultures will be discussed and case studies presented.  At one Site in Florida, with the pH ranging between 5.5 and 6.0, PCE, TCE and cDCE were completely dechlorinated to VC and ethene within 6 months of bioaugmentation with a low pH tolerant bioaugmentation culture.  At another Site in Florida, where the pH was as low as 5.5, TCE (up to 100,000 ug/L) was dechlorinated to ethene   after bioaugmentation with a low pH tolerant bioaugmentation culture.  The benefits of reducing or eliminating the need for aquifer neutralization will be discussed along with lessons learned.     

Corey has a B Sc. in Biocehmistry and has worked at SiREM for more than 7 years as the Bioaugmentation Coordinator and a Service Area Manager. He has technical experience in the growth, scale-up, and field implementation of several anaerobic microbial bioaugmentation cultures to degrade contaminants including chlorinated solvents, petroleum hydrocarbons, and other recalcitrant compounds. Throughout his career, he has provided technical support throughout the planning, injection, post-injection monitoring and data analysis stages for hundreds of field bioaugmentation events. He is focused on providing insights for optimal bioremediation success.