چكيده به لاتين
In wastewater treatment plants (WTP), the COD/N ratio depends on wastewater characteristics and its control is costly. For this reason, COD/N is usually considered as one of the input variables for design of WTPs. Conversely, sludge age is a design variable and can be selected depend upon the treatment aims. Therefore, when encountering different COD/N ratios, choosing appropriate sludge ageS can help to achieve higher nitrogen removal efficiencies. In this study, the effect of COD/N ratio and sludge age on simultaneous nitrification and denitrification in a moving bed sequential batch reactor (MBSBR) was studied. For this purpose, three reactors were operated in two phases under the sludge ages of 5, 10 and 15 days. The applied COD/N ratios for phases 1 and 2 were 10 and 20, respectively. The reactors had a working volume of 12.9 L. About 40% of each reactor was filled with Kaldnes-3 media. According to the obtained results, high COD removal efficiencies were obtaind for all reactors under the COD/N of 10, while, with increasing the COD/N ratio to 20 in phase 2, the effluent COD increased, particularly for the sludge ages of 5 and 10 days. In both phases, increasing the sludge age had no significant effect on biofilm mass, but with increasing the COD/N ratio in phase 2, biofilm mass increased significantly in all three reactors. In Phase 1, as the sludge age increased from 5 to 10 days, the nitrification increased as well. However, higher sludge age of 15 days leaded to decrease of nitrification. In Phase 2, increasing of the sludge age from 5 to 15 days steadily increased the nitrification. In phase 2, with the two fold increasing of COD/N ratio, the nitrification decreased significantly in all reactors. Increasing the COD content multiplied the population of aerobic heterotrophic bacteria. Competition and dominance of such a high population of heterotrophs over nitrifier bacteria for consumption of the dissolved oxygen significantly reduced the nitrification in phase 2. In both phases, increasing the sludge age improved the denitrification, but this increase was much more obvious in phase 2. In addition, with increasing the COD/N in phase 2, the denitrification decreased significantly in all 3 reactors. This was despite more favorable conditions for denitrification in this phase, i.e. increasing the biofilm mass and its anoxic sites as well as remaining part of COD until the end of the cycle. Actually, the main reason for reduction of the denitrification in phase 2 was the lower nitrate availability for denitrifier bacteria as a result of the weaker nitrification in this phase. In other words, under the higher COD/N ratio, nitrification (nitrate concentration) became the controlling factor of the denitrification and considerably affected the total nitrogen removal efficiency. Overall, it can be said that when using a MBSBR system for treatment of wastewater with COD/N ratio of about 10 (common COD/N ratio for municipal wastewaters), applying sludge ages of about 10 days will result in higher nitrogen removal efficiencies, while for the COD/N ratio of about 20 days, sludge ages over 15 days are recommended.
