DENITRIFICATION OF DRINKING-WATER - A BIOENERGETIC EVALUATION

An investigation into denitrification of groundwater, using fluidised bed reactors with ethanol as carbon source and electron donor included , inter alia, determination of the bioyield and the ethanol to NO3- ra tio required to effect complete denitrification. The values measured f or these parameters are in close agreement with other published data, Y(met) equals 0. 15g cells/g NO3- metabolised or 0.3 g.cells/g ethanol metabolised, and the g ethanol utilised/g NO3- removed is 0.5. Howeve r, theoretical investigation into the denitrification process indicate s that the observed yield data are very much lower than those expected from theoretical considerations, i.e. an approximate expected theoret ical anoxic yield of 0.69 g cells/g ethanol metabolised. To investigat e possible reasons for the discrepancy, a bioenergetic model was formu lated. Application of the model to anoxic processes using reasonable a ssumptions based on expected anabolic and catabolic efficiencies also gives theoretical yield data which are much higher than those obtained experimently here and in other published data. To obtain agreement be tween theory and practice the catabolic efficiency has to be reduced t o approximately one third of its expected value. Possible reasons for this discrepancy are proposed.