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Forsøg med biofiltre til rensning af poreluft forurenet med chlorerede
opløsningsmidler
The use of biofilters for removal of tetrachloroethylene (PCE) and trichloroethylene
(TCE) and vinylchloride(VC) is expected to be extremely competitive to conventional
technologies both regarding cost efficiency and conversion of the contaminants to
chloride, water and carbondioxide. Biological conversion of gaseous trichloroethylene
(TCE) and tetrachloroethylene (PCE) was investigated in compost and sphagnum based
biofilters under both aerobic and anaerobic conditions. The filter matrices were nutrient
enriched and mixed with three different sources of micro organisms including contaminated
soil, activated sludge and manure. The matrices were packed in reactors with a length of
105 cm, an inner diameter of 10 cm and a working volume of 8.2 liters. For each
biologically active reactor an identical autoclaved matrix was used as reference reactor.
The gas flow through the reactors was maintained at 0.5 liters/min resulting in an empty
bed residence time of 16 min. PCE and TCE were supplied with a carrier gas in
concentrations of 77-212 mg/m3 and 68-155 mg/m3 respectively. To the
aerobic biofilters, moisturised air was used as carrier gas and propane-butane gas, 9:1
(w/w) was supplied as primary C-source in a concentration of 1% (v/v). Moisturised N2
served as carrier gas in the anaerobic filters. The biofilters were run for a period of
117 days. Analysis of all supplied gases and vinyl chloride was performed by gas
chromatography on gas samples taken from gas tight inlet and outlet sample ports on the
reactors. At the end of the trial all matrices were analysed for pH, chloride and moisture
content.
From day 57 to day 102 biological activity in the aerobic biofilters were observed with
an average degree of conversion of propane and butane of 72% and 50%, respectively in the
compost based matrix and 46% and 27%, respectively in the sphagnum based matrix. For both
aerobic reference reactors the degree of conversion of propane and butane was close to 0
%. During the same period a significant conversion of TCE and PCE was observed in both
biologically active aerobic biofilters with the highest removal efficiency in the compost
based matrix. At the end of the trial removal efficiencies of TCE and PCE increased to 39
% and 17% respectively as an effect of decreasing the concentration of the primary
C-source to 0.25 % (v/v). It is concluded that the conditions for co-metabolistic
degradation of TCE were present in the aerobic biofilters. Further it is suggested that
the conversion of PCE is a result of anaerobic zones in these biofilters. In the anaerobic
biofilters no TCE and PCE conversion was observed in the sphagnum based matrix , but after
78 days removal efficiencies between 7% and 20 % were observed in the compost matrix. For
all biologically active biofilters except the anaerobic compost matrix an increase in the
chloride content compared to the respective reference reactors was observed. The increase
was explained by a total degradation of TCE and PCE during the last 45 days of the trial.
This was consistent with gas-analyses of in- and outlet samples from all reactors which
showed no content of vinyl chloride and 1,2-cis-dichloroethylene. The increase of chloride
content suggests that total degradation may have taken place already 20-40 days after the
beginning of the trial. It is concluded that the reactor system is suitable as use for
biological conversion of TCE and PCE. Due to the short time span of the investigation of 4
months the process and design parameters of the reactor system was tested under one set of
conditions. Therefore it is expected that the system can be optimised to achieve higher
removal efficiencies.
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