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 C/N ratio for denitrification. Influent of sewage directly into the anoxic zone is not useful to maintain a high C/N ratio. The adsorptive capacity of activated sludge can rapidly increase the C/N ratio. Similar to a contact-stabilization process, a contact tank can be combined with the Pasveer ditch; it provides contact time (zone) between raw sewage and return sludge before entering the ditch. In principle, insoluble organic substrate can be easily adsorbed onto the floc surfaces and enmeshed in the floc structure at a short retention time. After the contact, mixed influent is introduced into the anoxic zone. As a result, a high C/N ratio is obtained which enhances denitrification. Using this set up, the Pasveer ditch was operated. The experimental results show that the efficiency of denitrification has been enhanced from 45 to 83% for NO3-N removal. The corresponding denitrification 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 examination indicated improved settleability of the sludge. Further enhancement of denitrification needs an exact control of the dissolved oxygen level in the ditch and/or a concentration increase of denitrifying microorganisms.

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