COMPARISON OF BUTYRIC KID, ETHANOL, LACTIC-ACID, AND PROPIONIC-ACID AS HYDROGEN DONORS FOR THE REDUCTIVE DECHLORINATION OF TETRACHLOROETHENE

Previous studies indicated that dechlorinators can utilize H-2 at lowe r concentrations than can methanogens. This suggests a strategy for se lective enhancement of dechlorination-managing Hz delivery so as to im part a competitive advantage to dechlorinators. Four H-2 donors-butyri c and propionic acids, which can only he fermented when the Hp partial pressure is lower than 10(-3.5) or 10(-4.4) arm, respectively, and et hanol and lactic acid, which are readily fermented at H-2 partial pres sures 2-3 orders of magnitude higher - were administered to anaerobic mixed cultures. Comparison of the resulting enrichment cultures during time-intensive, short-term tests showed significant differences in pa tterns of donor degradation, H-2 production and use, and distribution of reduction equivalents between dechlorination and competing methanog enesis. Amendment with butyric and propionic acids resulted in less me thanogenesis than did amendment with ethanol or lactic acid, which gen erated much higher H-2 levels. Ethanol did not support complete dechlo rination during shortterm tests, but it was a viable donor over long-t erm testing because a portion was converted to a pool of slowly degrad ed propionic acid and because during long-term tests, cultures were ro utinely co-amended with pre-fermented yeast extract, a source of slowl y fermented volatile fatty acids. Understanding the fate of electron d onors and their fermentation products is an important component in und erstanding dechlorinating communities.