Abstract
This paper presented the conditions of oxygen supply which enabled the simultaneous denitrification with nitrification in full-scale oxidation ditches.
Oxygenation capacity (OC) per organic load ratio was introduced as an important control parameter for the simultaneous removal of organic substances and nitrogen compounds. This OC/organic load ratio has been used as a “rule of thumb” to calculate the required oxygen supply.
Above OC/CODcr load of 0.75kgO2·kg-1CODcr, 80-95% CODcr removal was obtained in full-scale oxidation ditches. At OC/CODcr load ratio above 1.0kgO2·kg-1CODcr, the sufficient nitrification was achieved, but below this value the nitrification rate decreased rapidly. On the other hand, the denitrification efficiency was more than 90% below 1.3kgO2·kg-1CODcr of OC/CODcr load, although the denitrification rate decreased with increasing OC/CODcr load. Consequently, optimum OC/CODcr load ratio for the nitrogen removal existed from 1.0 to 1.3kgO2/ kgCODcr, which corresponded to 1.4-1.9kgO2·kg-1BOD5 in OC/BOD load.
Operation with the simultaneous removal of organic substances and nitrogen compounds in oxidation ditches reduced the amount of oxygen supply by approximately 20%, compared with operation without denitrification. Hence, the removal of organic substances by the denitrification reduced to a considerable extent, the energy demand of aeration.
