Summary and conclusions, Miljøstyrelsen

Chemical-biological filter for degradation of chlorinated solvents in groundwater

Summary and conclusions

The chemical and biotic degradation of primarily trichloroethylene (TCE) but also perchloroethylene (PCE) has been studied in various filter matrix combinations. The main component for chemical degradation was iron filings (0.01-0.05 mm) and for biotic degradation the use of sphagnum, poplar chips, plant material and green pills was studied.

The tests included studies of the degradation potential of 13 different combinations of filter matrices. The filter matrices consisted of iron / sand matrices, biomass / sand matrices and combinations of these.

The filter matrices were tested in 8 reactors made of polypropylene pipes with an inner diameter of 15 cm and a height of 120 cm. The water led to the reactors came from the water supply and mainly TCE - but also PCE - was added. The oxygen content in the water was in the interval 8-11.5 mg/l and the pH value was approx. 8.

The test period ran for approx. one year and included the determination of inlet and outlet concentrations of chlorinated ethenes, mainly by means of GC/MS. The inlet concentration of TCE was on the scale of 200-3,400 µg/l. In addition measurements of pH, E_H and oxygen were included in the sampling.

The iron matrices showed half-lives for degradation of TCE on the scale of 40-175 minutes. Generally, no degradation products containing chlorinated ethenes were registered in the outlet from these reactors.

The results indicate that an iron / sand mixture at the ratio of 14/86 W/W% will be suitable for further tests in a combined filter, as this mixture only resulted in a moderate rise in pH. A doubling of the iron content led to serious problems with the matrix permeability and no enhanced effect on the half-lives was registered compared to degradation of TCE.

For some of the iron matrices, clogging of the matrix was observed. By means of XRD it was not possible to determine clearly the reason for the clogging, but we assume that the clogging was caused by the generations of amorphous ferrihydrate.

For the biotic matrices, the use of poplar chips did not have a degradation effect and only a moderate degradation using sphagnum. However, the green pills and plant material showed very fine half-lives for degradation of TCE. These were on the scale of 20-45 minutes, but at the same time the biomass matrices generated cis-DCE.

The combination of iron / sphagnum and iron / poplar chips had no enhanced effect on the degradation of TCE compared to the iron matrices. On the contrary, they had a poorer degradation capacity than the iron / sand reactors.

The combination of green pills and iron in a series had a good degradation effect with half-lives of about 1 hour for degradation of TCE.

Nevertheless, the indications were that in the course of time a certain quantity of cis-DCE will be generated in the outlet. However, current tests indicate that iron / green pills mixed in an overall matrix only result in insignificant quantities of cis-DCE in the outlet water.

The test showed that it is possible to degrade TCE in an iron matrix, in a biomass matrix, and in a combined chemical / biotic matrix. We assess that in certain situations it may be possible to use a combined iron / biomass filter for purification of e.g. TCE-contaminated groundwater. However, before such potential can be finally determined, several studies will have to be conducted in order to optimise the filter considering both the design of the matrix and the operation, the long-term effect and the robustness e.g. towards variable water types.