BIOAUGMENTATION CONSIDERATIONS - Bioaugmentation for Groundwater Remediation - page 141

~50 mg/L (Dworatzek et al., 2006 ). Treatability studies may be warranted to assess the effects
of high sulfate concentration given that it enhances reductive dechlorination at some sites while
inhibiting it at others.
One possible problem with high levels of sulfate is the potential for the production of
excessive sulfide concentrations as a result of sulfate reduction, which may inhibit anaerobic
dechlorination as well as some fermentation reactions (Maillacheruvu and Parkin, 1996 ).
The sulfide levels that may potentially inhibit dechlorinating microorganisms are not well
documented. One study that investigated sulfide inhibition indicated that 5.0 millimolar
(mM) (161 mg/L) sulfide stopped all dechlorination activity, but no inhibition was observed at
1 mM (32.1 mg/L) (He et al., 2005 ). In general, dissolved sulfide and hydrogen sulfide are
rapidly co-precipitated with ferrous iron (a byproduct of ferric iron reduction), providing
sufficient iron is present to react with the sulfides.
The role of iron- and manganese-reduction in inhibiting bioaugmentation performance is
largely uninvestigated. At some sites, high dissolved iron or manganese concentrations are
thought to adversely affect dechlorination in a manner similar to other competing electron
acceptors. The concentrations of dissolved iron and manganese that may inhibit anaerobic
dechlorination have not been well documented or defined (Parsons Corporation, 2004 ).

5.2.4 Volatile Organic Compound (VOC) Concentration

It was once thought that bioremediation processes were ineffective for treating high concen-
trations of chlorinated ethenes such as those found in dense nonaqueous phase liquid (DNAPL)
source areas, which historically limited the application of the technology to dissolved plume
treatment or containment. However, data collected over the last several years demonstrate that
dechlorinating microorganisms are active over a wide range of chloroethene concentrations.
Duhamel et al . ( 2002 ) reported dechlorination of PCE, TCE, cis- DCE and VC at initial concentra-
tions of 132, 197, 77 and 87 mg/L, respectively, in microcosm studies. Inmicrocosms conducted for
the SABRE project, complete dechlorination of TCE to ethene occurred at 400 mg/L and, in some
cases, as high as 800 mg/L (Harkness et al., 2006 ). Similar results were reported by Yang and
McCarty ( 2000 ), who observed PCE dechlorination in the presence of cis- DCE and ethene at
concentrations of 0.66 and 1.05 mM (64 and 29 mg/L, respectively). The presence of such high
concentrations of PCE, cis- DCE and ethene can be inhibitory to methanogenesis (Yang and
McCarty, 2000 ), improving electron donor availability for dehalorespiration. The occurrence of
dechlorinating activity at very high chlorinated solvent concentrations has permitted bioremedia-
tion/bioaugmentation to be used for DNAPL source remediation (Schaefer et al., 2010b ).
It is also important to recognize that there is a lower volatile organic compound (VOC)
concentration limit for sustaining reductive dechlorination. This level has not been well-defined
in the field. However, in laboratory studies, Cupples et al. ( 2004 ) indicate that concentrations of
cis -DCE and VC must be above 0.7 micromolar ( m M) (44-68 m g/L) so that growth of Dhc
outpaces its decay.

5.2.5 Inhibitory Constituents

While chloroethenes appear to be inhibitory only at extremely high aqueous concentrations,
several VOCs have been demonstrated or are suspected to exert inhibitory effects at much
lower concentrations. Both chloroform (CF) and 1,1,1-trichloroethane (TCA) slowed rates of VC
dechlorination to ethene by Dhc , with complete inhibition at concentrations of 450 micrograms
per liter ( m g/L) (3.8 m M) and 700 m g/L (5.2 m M), respectively (Duhamel et al., 2002 ). Compara-
ble results have been reported for other chloroethenes,

including inhibition of cis- DCE