| Aerobic granules are granule-shaped microbial aggregates, which were formed by selected microbial flocs under certain operational conditions. Compared with traditional activated sludge, aerobic granules have granular shape, compact structure and good settling ability, which may retain high concentration biomass in the system, obtaining high resistance to temperature, shock load and toxic substances. Therefore, it has greatly improved loading rate, removal efficiency and stability, giving a small footprint of the system. Nowadays, aerobic granular sludge technology has been widely accepted as one of the most advanced technologies for high efficient wastewater treatment and reuse.Aerobic granules have high mass-transfer resistances in the inner part due to its granular shape and compact structure. Therefore, there may be an anoxic or anaerobic zone inside granules with big mean sizes. This is of great importance for the operation of biological nitrogen removal process, which demands aerobic conditions for nitrification and anoxic conditions for denitrification. Many studies have proved that single aerobic granules can realize nitrogen removal through nitrification and denitrification at the same time. However, nitrogen removal by aerobic granules with anoxic area may be affected by many factors, such as-concentration of dissolved oxygen in the system, mean size, density and microbial species distribution of aerobic granules, etc.. Among them, size of aerobic granules has great influence on nitrogen removal efficiency of the system by affecting the rate of nitrification and denitrification for its different size of aerobic and anoxic zone. However, it is very difficult to control the size of aerobic granules in practice, which has caused the instability of granular nitrogen removal system.This research was based on the cultivation of denitrifying granular sludge in inorganic synthetic wastewater and explored the mechanism of formation and tability. In the meantime, characteristics of the aerobic granules were thoroughly investigated, which may give some basics for the study of the aerobic granules with different sizes. The reseach shows that:1. Organic load concentration has a certain influence on stability of aerobic granules system. High organic load is beneficial to the formation of aerobic granules. But a high organic load condition cause a rapidly growth of microbial which may reduce the mechanical strength of the three dimensional structure,And, it also caused a floc-booming in the systems easily which is fatal for granules as floc will over-compete granules and finally lead to dieing-out of granules.However. a lower organic loading could cause a lack of substrate in the system. especially in the center of the large aerobic granules, it is prone to cause the perforation and microbial decomposition of aerobic granules. It is easy to collapse and broken under the action of the hydraulic shear force due to the the decrease of the strength of the aerobic granules. There were a big nitrite and nitrate accumulation for the low organic substrates in the influent which are important electron donors for denitrification. There was a big decrease in the effluent nitrite and nitrate concentration when increased the COD concentration in the influent, which indicated that denitrification was very easy to conduct for the system as long as organic substrates should meet the conversion needs. A moderate range of organic load concentration need to be controlled to ensure the stable operation of the system.2. The change of ammonia nitrogen removal rate embodies the dissolved oxygen penetration depth changing with the development of growth of aerobic granules. At the early stage of formation, the dissolved oxygen inside the aerobic granules will be reduced because of the increase of the mass transfer resistance and only a small amount of nutrients can reach to the inner layers of the granules because of the increase of mass transfer resistance along with the formation of the granules and increase of mean size. So that cause a reduce of the nitrification rate. There may be an anoxic or anaerobic zone inside granules with big mean sizes as the increasing of mean size of the aerobic granules and the continuous improvement of the gaps and channels. Denitrification react in these two areas and the denitrification ability increase rapidly.3. Compared with biomass concentration of the aerobic granules in the system, mean size of the aerobic granules increased rapidly at a low biomass concentration. It increased slowly or even stopped increasing when the biomass concentration reached to the maximum. Contrast phenomenon indicate that biomass concentration has a certain relevance with the growth of mean size of the aerobic granules. Aerobic granules with different sizes were successfully cultivated in the two sequencing batch reactors (SBR), which were initially seeded with activated sludge of different biomass concentrations. The experiments show that:(1) Mean size of the aerobic granules in Rl and R2 reached 1300μm and 900μm, respectively, during 100 days’ cultivation. At the same time, SVI30 maintained 40-80mL/g in R1 and 30-70mL/g in R2, respectively.(2) During the formation period, aerobic granules in R1 had a bigger size due to the lower biomass concentration, which led to a lower shear force in the reactor; while aerobic granules in R2 had a smaller size due to the higher biomass concentration, which led to a higher share force in the reactor.(3) FISH results show that nitrifying bacteria in the smaller aerobic granules are located in the outer layers of the granules, where there are higher concentrations of nutrients and dissolved oxygen. Therefore, smaller aerobic granules are more active and stable than the bigger ones. |
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