Miljøprojekt Nr. 1175 2007 Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Afprøvning af elektrokemisk reaktor til rensning af grundvand indeholdende klorerede opløsningsmidler

Afprøvning af elektrokemisk reaktor til rensning af grundvand indeholdende klorerede opløsningsmidler

Summary and conclusions

This report describes the development and pilot-scale testing of an electrochemical reactor for cleaning groundwater contaminated with chlorinated solvents. The project was carried out in a cooporation between DHI – Water and Environment and Rambøll Denmark A/S.

In a previous project, a lab-scale electrochemical reactor was developed and tested for degradation of organic contaminants in water. Degradation of the organic contaminants in the electrochemical reactor is achieved by direct transfer of electrons between the organic contaminants and the electrodes when an electrical current is applied to the electrodes. The electrochemical reactor has shown promising results at lab-scale but it has not previously been tested at larger scale under field conditions.

During the project period, a pilot-scale electrochemical reactor and power supply was constructed. This reactor was tested at a site designated by the County of Roskilde, which had previously housed a dry-cleaning facility. The groundwater on this site, was contaminated with chlorinated solvents and a full-scale remediation project for cleaning the groundwater using active coal is ongoing.

The purpose of the pilot-scale testing was to test the reactor under realistic field conditions with natural groundwater as opposed to the spiked tap water that had been used in previous lab tests. The groundwater at the site was influenced by saltwater intrusion and had relatively high electrical conductivity.

The electrochemical reactor consisted of a buffer tank and a reactor unit with a total volume of approximately 140 litres and an electrode surface of 1.9 m². The reactor unit was constructed to ensure turbulent flow across the electrodes to minimise the diffusion layer surrounding the electrodes, thereby optimising the degradation rate. To minimise precipitates on the electrodes caused by the alkaline conditions on the cathode, the power supply was equipped with a facility to change the direction of the direct current supplied. The polarity of the electrodes was changed every 7 minutes, which was sufficient to prevent significant precipitation of carbonates from taking place.

The first tests performed in the lab showed that the reactor achieved degradation rates that were comparable to the rates achieved in previous lab tests. Also, these tests showed that the degradation was complete, since no unwanted intermediate degradation products were detected. Following these first tests, the reactor was optimised with respect to the groundwater at the chosen site. These optimisation tests showed that for both PCE and TCE, degradation followed a first-order process and that 99.8 % of TCE and 98 % of PCE was degraded over one day.

Subsequently, the reactor was moved to the field site in Greve. Here, batch tests of ½-1 day’s duration and longer term tests with continuous operation for one week were performed. These tests showed that the reactor, the power supply, the control unit, and all other components could function for such periods without need for adjustment and without technical problems. Batch tests performed before and after the long-term test showed that the efficiency of the reactor was significantly less after the long-term test than before, especially for PCE, where the degradation rate decreased by a factor of almost 3. For TCE, the decrease in efficiency was less significant. The decrease in degradation rate for PCE is primarily thought to be caused by a much smaller initial concentration in the batch test performed after the long-term test than in the batch test performed before the long-term test (140 µg/l after as opposed to 910 µg/l before) but also general differences in water quality and precipitates on the electrodes could play a role. Altogether, the pilot-scale tests showed that long-term flow-through operation periods can lead to precipitates on the electrodes causing decrease in reactor efficiency. It is thought that these precipitates could be avoided/minimised by reducing the time between polarity changes for the electrodes and/or by introducing a procedure for cleaning the electrodes.

Apart from the pilot-scale tests, the efficiency of the electrochemical process with respect to other compounds than chlorinated solvents was also tested in the laboratory. Lab-scale tests with tap water contaminated with BAM and MTBE showed that electrochemistry is a very effective method to degrade BAM in water but not a very effective method with respect to MTBE in water. It was also found that the electrochemical process is disinfectious.

Tests generally showed that satisfactory degrees of cleaning could be achieved but that the time to reach this was too high. The possibility of increasing the degradation rate with respect to the chlorinated solvents by combining electrochemistry with ultrasound was investigated in preliminary experiments. These experiments showed that the inclusion of ultrasound in the setup could increase degradation rates by up to a factor of 3. Also, experiments with variation of the flow velocity in the reactor showed that increased flow and thereby increased turbulence resulted in increased degradation rates. These results indicate that further optimisation of current and flow conditions and inclusion of ultrasound could increase the efficiency of the electrochemical reactor towards chlorinated solvents significantly. In light of the results achieved, it is thought that an increase in degradation rates of at least a factor of 10 is possible.