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Dynamic variations of carbonaceous and nitrifying activities in hybrid reactors with different operating conditions

Authors Li, Chunju
Issue Date 2004
Summary Two bench-scale hybrid systems, employing tyre beads and Kaldnes elements as carriers, respectively, were used in this research. The aim was to evaluate the dynamic changes of carbonaceous and nitrifying bioactivities in suspended and attached growth under different operating conditions such as SRT, carrier concentrations and loading rates. The distinct advantage of hybrid systems in achieving nitrification at short SRT operation was demonstrated in the comparison with an activated sludge system. With the operating SRT of suspended growth varying from 0.17 d to 7.1 d, the two hybrid systems gradually changed from a biofilm-dominant to a suspended-growth-dominant system. At the fixed loading rates of 4.2 kg COD/m3-d and 0.78 kg TKN/m3-d, the relative contribution of attached growth towards the overall organic removal was reduced from 50% at 0.17-d SRT to 5% at 7.1-d SRT and its corresponding contribution to nitrification was reduced from 95% to 15%. For both growth types, with the increasing SRT, carbonaceous activities decreased due to the limiting substrate while nitrifying activities increased because of the accumulation of nitrifiers. It was found that the attached growth always showed a higher nitrifying activity but a lower carbonaceous activity as compared to the suspended growth. The long SRT of the biofilm favored the nitrifying growth at the short operating SRT, which enabled the hybrid systems to achieve a remarkable nitrification rate of 0.45-0.6 kg TKN/m3-d at a short SRT of 1 to 2 days. This rate was about 3 to 8 times as high as that of the activated sludge system. In addition, the suspended growth in the hybrid systems also showed a considerable nitrification rate of 0.2 to 0.4 kg TKN/m3d at short SRT operation because of the continuous input of nitrifiers from the biofilm detachment. Moreover, the suspended growth was found to shoulder 80 to 90% of total organic loading when the SRT was 1 to 2 d. This greatly relieved the organic burden on attached growth and exempted it from the negative influence of increased organic loading rate. With this kind of cooperation between suspended and attached growth, the hybrid system was able to achieve a higher nitrification rate at a fixed organic loading rate, as compared to an activated sludge system. As biofilm became dominant at short SRT operation, the hybrid system encountered a severe oxygen deficiency in biofilm at a high organic loading rate, which greatly limited the system nitrification rate. Both the operating DO and the surface organic loading rate were found to have profound impacts on the nitrification rate of the biofilm. At the same loading rate (4.2 kg COD/m3-d and 0.78 kg TKN/m3-d), the surface organic loading was reduced by varying the carrier concentration from 10 to 30 g/L. This drastically boosted the system nitrification rate from 0.01 kg TKN/m3-d to 0.40 kg TKN/m3-d and also made the biofilm less sensitive to the changes of operating DO concentration and applied volumetric loading rate. As comparing the two carriers used in this study, it was found that Kaldnes elements were able to maintain a relatively constant attached biomass concentration under the varying surface organic loading rate due to its relatively closed structure. This feature would be favorable for treating a seasonally fluctuating loading. However, the closed structure of Kaldnes element also incurred a significant diffusion resistance. Therefore, more energy input was needed to overcome the diffusion resistance to increase the nitrification rate. Key words: hybrid reactor, specific carbonaceous and nitrifying activities, suspended growth, biofilm, effect of DO and surface loading, simultaneous organic removal and nitrification, Kaldnes elements, shredded tyre beads
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2004
Language English
Format Thesis
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