USE OF CONTACT TANK TO ENHANCE DENITRIFICATION IN OXIDATION DITCHES

Poor denitrification in a Pasveeer oxidation ditch is attributed to a lack of carbon sources available in the anoxic zone as it is essential to maintain a high CM ratio for denitrification. Influent of sewage d irectly into the anoxic zone is not useful to maintain a high C/N rati o. The adsorptive capacity of activated sludge can rapidly increase th e C/N ratio. Similar to a contact-stabilization process, a contact ran k can be combined with the Pasveer ditch; it provides contact time (zo ne) between raw sewage and return sludge before entering the ditch. In principle, insoluble organic substrate can be easily adsorbed onto th e floc surfaces and enmeshed in the flee structure at a short retentio n time. After the contact, mixed influent is introduced into the anoxi c zone. As a result, a high CM ratio is obtained which enhances denitr ification. Using this set up, the Pasveer ditch was operated. The expe rimental results show that the efficiency of denitrification has been enhanced from 45 to 83% for NO3--N removal. The corresponding denitrif ication capacity of the sludge is increased by 240%. The contact tank has also the same principle as a 'selector' to control bulking sludge caused by filamentous bacteria. The SVI data and microscopic examinati on indicated improved settleability of the sludge. Further enhancement of denitrification needs an exact control of the dissolved oxygen lev el in the ditch and/or a concentration increase of denitrifying microo rganisms. Copyright (C) 1996 IAWQ.