Biodegradation of chlorinated aromatic hydrocarbons in slow sand filters simulating conditions in contaminated soil—Pilot study for in situ cleaning of an industrial site

Biodegradation of a mixture of chlorinated aromatic hydrocarbons (CAH) by a natural biocoenose of microorganisms was demonstrated in four semi-scale plants which served as models for a contaminated industrial site (concept of slow sand filters). The plants consisted of columns filled with contaminated soil taken from the ground of the spilled site and water storage containers that contained the contaminated water. The gas saturated water was continuously recycled through the fixed bed columns. The gases supplied were either pure oxygen, air or nitrogen, with the nitrogen-supplied plant serving as a control simulating the microaerobic conditions in the natural habitat. A fourth plant was filled with sand instead of contaminated soil and supplied with air. It served only as a fixed bed and not as a reservoir for organic substrate. After 110 days, in all aerated plants more than 99% of the chlorobenzenes (CB), 96% of the chlorophenols (CP) and 94% of the adsorbable organic halogens (AOX) were removed. In comparison, only 82, 78 and 49% of these compounds were removed in the nitrogen-supplied plant. Chloride ion concentrations in the aerated plants, measured as mineralization product of the AOX-concentrations, increased from 130 mg/l to 240 and 250 mg/l and coincided with the decrease of AOX-concentrations. Total cell counts of water and soil were approximately ten times higher in the aerated plants compared to the anaerobic or sandfilled plants. Microbial activity measured as respiration in the fixed beds showed the same profile.