Environmental Engineering Reference
In-Depth Information
methanogens, excessive dosing of biogenic organic compounds in situ may result in
groundwater contamination by the fermentation products. To overcome this drawback,
hydrogen may be added directly as an electron donor source to accelerate PCE and TCE
reduction. The efficiency concern of hydrogen utilization remains to be further
investigated as methanogens may consume large parts of hydrogen and out-compete the
dechlorinators. Readers are referred to a recent study (Aulenta 2005) for dechlorination
efficiency comparison of using different electron donors such as methanol, butyrate, and
hydrogen for long-term PCE degradation.
Briefly, under the normal feeding condition of PCE, the methanol-fed bioreactor
and butyrate-fed bioreactor experienced much lower PCE reduction rates than that of
hydrogen bioreactor. The dechlorination performance of bioreactors with methanol and
butyrate as electron donors depended on the concentrations of hydrogen produced from
the fermentation of these organic compounds. It was found that the amount of hydrogen
produced from methanol or butyrate fermentation was much lower than the theoretical
prediction and not sufficient for the dechlorination under usual PCE feeding conditions.
During the long-term performance tests, however, the hydrogen-fed bioreactor appeared
to have a distinct decline of the dechlorination rate after 90 days as opposed to around
230 days in methanol or butyrate-fed bioreactors. These results suggest that direct
hydrogen addition is effective in rapid PCE dechlorination but may be less attractive for
long-term degradation of chlorinated aliphatic compounds in contaminated groundwater.
In a feasibility experiment of PCE dehalogenation using acetate as an electron
donor, the observed results showed that acetate can be used as a electron donor by many
organisms to convert PCE to TCE and further to cDCE (Lee 2007), although acetate
utilizing organisms preferred to use hydrogen in the process of dehalogenation of PCE to
cDCE. If fermentable biogenic compounds are added into a single stage dehalogenation
process, both acetate and hydrogen will be produced from fermentation, and hydrogen
will be quickly consumed as a preferred electron donor by organisms. This causes
inefficient usage of acetate and high organic accumulation in groundwater. Recently,
two-stage processes was proposed, in which acetate from organic substrate fermentation
was used in the first stage for reduction of PCE to cDCE, and hydrogen was used in the
second stage for reduction of cDCE to ethane (Lee 2007). In this two-stage system, the
organic substrate was added down-gradient to convert cDCE to VC and further to ethane
by hydrogen produced from the fermentation of the organic substrate. Through the
recycle system, the fermentation products including acetate were added up-gradient to
convert PCE and TCE to cDCE. The beneficial factors of the two-stage system
compared to a single stage system include reduced organic substrate dose requirement
(50 to 75% reduction of excess acetic acid production), reduction of residual chemical
oxygen demand (COD) in groundwater and minimal methane production.
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