Successful growth of dechlorinating microbial culture
The growth of the dechlorinating culture in an optimized medium was successfully realized on 10 L scale in a lab fermentor and on 500 L scale in an industrial reactor. The 10 L batch, as well as the 500 L batch had showed a very good dechlorinating capacity. Stable values of the physico-chemical parameters were obtained (pH and redox potential). Furthermore, molecular analyses showed high concentrations of dechlorinating bacteria.
The results of the chemical analyses during growth are given in Figure 1.
Figure 1 Theoretical additions and evolution of the measured concentrations of the different chlorinated compounds and ethene in the liquid phase of the bioreactor. TOC was dosed on day 0 and 259. 1,2-DCA and PCE were added on day 0, 110 and 166.
Three doses of CAH were added to the industrial reactor during the 11.5 months of follow-up. The chemical analyses indicated that the chlorinated compounds were effectively degraded in the bioreactor. After each dose, as expected, a significant decrease of the CAH concentrations was observed. Moreover, after 11.5 months, an increase of the concentration of the Dehalococcoides, as monitored by the 16S-rRNA gene density, was observed. During the growth of the 500 L batch, Avecom performed regularly activity tests to investigate the dechlorinating capacity of the culture towards CAH. The sample of Multidechlorobac taken after 11.5 months (end of production) showed no lag phase and a good degradation of 1,2-DCA and TCE (Figure 2). Moreover, no accumulation of VC was observed.
These results confirmed that the culture had sufficiently grown in the industrial reactor and contained high concentrations of active dechlorinating bacteria after 11,5 months. These tests clearly demonstrated that the bacteria were sufficiently grown at the time the production was finished. It was the first time that the growth of Multidechlorobac was realized at industrial scale.
Figure 2 Results of the standard activity tests of the Multidechlorobac sample after 11.5 months of growth (l: liquid phase; g: gas phase)
Monitoring HGB cell biostimulation
The monitoring results of the HGB cell for biostimulation have shown that during the start-up phases not only favourable biodegradation conditions were created within the HGB cell, but also that 12DCA biodegradation along the expected degradation pathway was taking place.
During the monitoring of the HGB cell in 2012 and 2013 the concentration levels for 12DCA in the monitoring wells as well as the concentration of other CAHs, dropped almost below detection level. Moreover, this drop was accompanied by a rise in ethylene concentration, the aimed degradation product. As a result, it can be concluded that the operation of the first activated HGB cell for biostimulation can be regarded as successful! In figure 3 the monitoring results of the CAH concentration in monitoring well LV102 are shown.
In July 2013 the HGB cell halted due to pressure built-up in filter and injection wells. Analyses of sludge and observations during cleaning of the piping and wells of the HGB cell show that this last pressure built-up was only due to the accumulation of FeS. The formation of FeS is an integral part of the chemical reactions taking place in the aquifer within the radius of influence of the HGB cell. The sulphates present in the groundwater are reduced to sulphides which subsequently precipitate with Fe2+. This was the first time that the formation accumulation of FeS in the filters was observed. Before the start-up of the HGB cell in November 2013 the installation, pumping and injection wells were cleaned thoroughly. The HGB cell operates now at a flow rate of 18 m³/h.
Figure 3 Monitoring CAH concentration in monitoring well LV102 (logarithmic scale)
As a first step in the modeling of the performance of an HGB cell, a detailed model has been constructed in 2013. Simulation runs have been done with HYDPARIDEN and MOCBAC3D software. The HYDPARIDEN software allows a simulation of the evolution of the hydraulic heads in the aquifer during the operation of a HGB cell (see figure 4). The MOCBAC3D software simulates a working HGB cell once an equilibrium situation in the aquifer has been reached and allows for the simulation of transport of a marker in the groundwater (see figure 5).
Figure 4 Hydraulic heads along the well screen
Figure 5 Marker concentration along the well screen from low (blue) to high (red) marker concentration
The most important observation of the first model simulations is that the HGB cell has a vertical influence reaching almost the base of the aquifer. This leads to the conclusion that a vertical upscaling of the HGB cell for biostimulation within the project will not be needed.
Update results groundwater reactor tests
An update can be found on the results of the successful lab tests for growing dechlorinating microorganisms with site polluted groundwater. These tests are an important step in evaluating the feasibility of an on-site growth reactor for the dechlorinating culture at the site.
Growth of dechlorinating microbial culture with site-polluted groundwater
The growth of the dechlorinating bacteria in contaminated groundwater was stepwise investigated on labscale by Avecom.
The tests consisted of the following phases:
- Investigation of the feasibility of an on-site groundwater reactor for the growth of a dechlorinating culture. This was done by evaluating 2 types of carrier material.
- Upscaling to a semi-continous reactor to evaluate the practical applicability for an on-site reactor. For the production of sufficient inoculum for bioaugmentation it was important to test whether a lower hydraulic residence time and higher volumetric loading rates were feasible.
- Determination of nutrient requirements for the growth of the dechlorinating microbial culture in site-polluted groundwater.
- Further optimization of the process parameters to stimulate the degradation of vinylchloride.
In summary of the results the following conclusions can be made:
- From the two carrier materials tested (S and P), carrier material S gave the best results in terms of 1,2-DCA and VC degradation. Also important concentrations of micro-organisms were found in the water phase of the reactor which makes it a suitable inoculum for the bioremediation. Moreover, the carrier material retains enough micro-organisms in the reactor to maintain continuous growth.
- Good 1,2-DCA degradation was obtained with a hydraulic residence time of 10 days and a 1,2-DCA loading rate of 2,4-3,1 mg 1,2-DCA/L.day.
- Further optimization was necessary for the degradation of VC. Increasing the hydraulic residence time to 20 days resulted in a good removal of VC with degradation rates around 100 μg VC/(L.day).
- Based on the test results it can be concluded that the groundwater probably contains limited amounts of N, P and trace elements that can support the growth of the dechlorinating bacteria. Therefore it is advised that nutrients can be dosed at the start of the on site reactor and afterwards monthly or two monthly.
- COD dosage will need to be adjusted based in function of the contamination present in the groundwater (higher concentrations needed for the degradation of VC than for 1,2-DCA)
- Filtering the effluent to remove FeS (potentially clogging injection wells) did not reduce the amount of dechlorinating bacteria.
Fig. 6 – Evolution of the concentration of 1,2-DCA in the liquid phase of the bioreactors