BIOAUGMENTATION CONSIDERATIONS - Bioaugmentation for Groundwater Remediation - page 140

5.2.2 Temperature and pH

Temperature and pH are two other factors that affect the growth and activity of Dhc.
Temperature will affect the rates of Dhc growth and solvent biodegradation. Complete dechlo-
rination from trichloroethene (TCE) to ethene has been observed between 10 C and 30 C for
commercial dechlorinating cultures (with the exception of a site located in Alaska where the
complete dechlorination of TCE to ethene was observed at groundwater temperatures between
6-8 C). Dechlorination stalled at cis -1,2-dichloroethene ( cis -DCE) at temperatures less than
4-10 C (depending on the electron donor added) and above 40 C (Friis et al., 2007 ). Maximum
growth rates ( m ) and zero order degradation rates were highest for TCE dechlorination at 30 C
with lactate as a substrate ( m TCE of 7.00 0.14 days 1 ). In general, maximum growth rates and

TCE dechlorination rates were up to an order of magnitude higher than rates for utilization of
cis -DCE and vinyl chloride (VC). Temperature dependence of maximum growth rates and
degradation rates of cis -DCE and VC were similar and highest at 15-30 C (Friis et al., 2007 ).
Therefore, Dhc growth and dechlorination rates will be slower in regions where groundwater
temperatures are lower and faster in regions where groundwater temperatures are higher. To
mitigate for colder temperatures, additional Dhc culture can be added.
Like most microbial processes, dechlorination activity is affected by pH and is highest near
neutral. The optimal pH for the growth of the KB-1 ® bacterial culture is between pH 6.0 and 8.3.
Complete degradation to ethene occurs within this pH range, while partial degradation of TCE
to cis -DCE and VC occurs between the 5-6 and 8.6-10 pH ranges. Dechlorination was not
observed to occur below pH 5 and above pH 10 (Rowlands, 2004 ). At pH values less than 5,
dehalogenation of perchloroethene (PCE; also termed tetrachloroethene) was found to be
completely inhibited in Dhc -containing cultures (Vainberg et al . , 2009 ). Others have found VC
dehalogenation to be more sensitive to pH than TCE dechlorination, with strong inhibition
occurring at a pH less than 6 (Eaddy, 2008 ). Buffers can be used to adjust the native groundwa-
ter pH to near neutral conditions to improve rates, as discussed in Section 5.3.2.2 .

5.2.3 Competition for Electron Donor/Geochemical Conditions

Successful bioaugmentation and TCE dechlorination to ethene requires sufficient electron
donor to drive the process. When other electron acceptors, such as nitrate, dissolved iron,
dissolved manganese and sulfate are present, bacteria utilizing these alternate electron accep-
tors will require electron donor to drive their reduction, resulting in competition for electron
donor. Generally this competition can be overcome by adding sufficient electron donor to meet
the demand of these other processes.
The presence of high background sulfate concentrations appears to adversely impact bioaug-
mentation and dehalorespiration in some cases but not in others. For example, complete dechlori-
nation of TCE to ethene was not observed in bioaugmented microcosms (Pinellas culture)
containing high concentrations of sulfate (3,000-6,000 mg/L), despite stimulating active sulfate
reduction to remove sulfate, repeated reamendments with the Pinellas culture, and the application
of different electron donors (Battelle, 2004 ).
However, groundwater at the site of origin for KB-1 ® contained more than 1,000 mg/L of
sulfate, and as such, the KB-1 ® culture appears to have adapted to high concentrations
of sulfate. As part of the Source Area Bioremediation (SABRE) project, the effect of high
sulfate concentrations (e.g., 1,250 mg/L) on reductive dechlorination using the KB-1 ® culture
was studied. At these concentrations, sulfate reduction was concurrent with cis -DCE reduction.
The transformation of VC to ethene occurred once sulfate concentrations were reduced below