INTERMITTENT ADDITION OF EXTERNAL CARBON TO ENHANCE DENITRIFICATION IN ACTIVATED-SLUDGE

Efficient nitrogen removal in a pre-denitrifying activated sludge proc ess strongly depends on the amount and nature of the carbon source in the wastewater. If denitrification has to be supported at periods, the response to a supplemental carbon source should be immediate and subs tantial. The aim of this work was to study how intermittent addition o f ethanol affects process performance and the denitrifying capacity of the biomass in a pre-denitrifying activated sludge process. The inves tigated intermissions; simulated a possible operational situation with a short stop of ethanol addition or a strategy with addition only at weekends. The long-term effect of intermittent addition was also studi ed. Experiments were performed in a pilot-scale activated sludge plant . Effluent nitrate was measured on-line and potential denitrification rates were determined with the acetylene inhibition technique. Once th e bacteria were adapted to the external carbon source, the studied int ermission of 24 hours as well as an intermission of six days showed no severe effect on denitrification capacity of the sludge. In order to maintain process stability with intermittent addition, the denitrifyin g bacteria had to sustain a high capacity at each stop. Five weeks of intermittent addition of ethanol did not change the level of nitrate r eduction in the system during periods of ethanol addition. Denitrifica tion potential was, however, negatively affected. The results suggest that it is possible to use an intermittent strategy when adding ethano l as an external carbon source in a pre-denitrifying system and for in stance only support the denitrifying bacteria during weekends. (C) 199 8 IAWQ. Published by Elsevier Science Ltd