| Aerobic granular sludge is a hot research topic which is undergoing world widely recent years. However, many attentions of the work were only paid to the cultivation of granular sludge and dynamics of reaction. Less was known about the key factors to form the granular sludge and their metabolic activity. The granulation mechanism of sludge was not clearly elucidated as well. In this experimental research, the key strategy to develop granular particles and its denitrification were investigated in SBR system using four stages operational control, and the properties of aerobic particles and granulation mechanism were explored too. The main experimental results were summarized as follows:Through 120 days continuous operation, aerobic granular sludge was successfully cultivated in reactor, whose particle size was approximately 1mm. The SVI of sludge was 20—40 ml/g, settle ability 60m/h, Zeta potential–10—–12mV. COD removal efficiency reached about 95% when feeding with the influent COD 500-1500mg/L, and effluent COD was only maintained to 20-50 mg/L. Based on electronic microscope observation and elements analyses, the organic compounds aerobic granular sludge were found to be the main component, and the active mi-crobes in outer surface was higher than inner layer.A four stages operational control strategy could be used to cultivate the aerobic granular sludge, composed of initial operation stage, feed load adjustment stage, load reducing stage and stable stage. This method was effective in developing aerobic granular particles. A granulation mechanism was proposed according to the microbiological observation: first cocci and rod bac-teria with good settle ability became dominant in sludge after the initial operation stage and feed load adjustment stage. Then it converted to a'core'which was essential to promote the growth of the aerobic granular sludge after load reducing stage, together with decreasing of bacteria's surface Gibbs energy. In the stable stage, reducing selection pressure to stimulate the bacterial growth and granular development, rod and filamentous bacteria amplified greatly. Finally, a ma-tured and good granular sludge composed by rod and filamentous bacteria was formed in reactor.The aerobic granular sludge with enhanced nitrogen removal was formed by the methods of adding inorganic carbon, reducing dissolved oxygen, controlling pH and four stages operational strategy. The load reducing operation was also helpful to cultivate aerobic granular sludge with good enhanced nitrogen removal ability when the granular sludge was just appeared in reactor, but their size was comparatively smaller. In the system, it was observed that the nitrogen remov-al efficiency was 85%, 90% and 80%, respectively when influent ammonia nitrogen was 40, 60 and 100 mg/L. With the increase of influent nitrogen in operation, nitrate-N was reduced gradu-ally but nitrite-N was increased simultaneously in effluent. Nitrite was accumulated in the reac-tor and reached to 15 mg/L when the influent ammonia was higher than 80mg/L.The aerobic granular sludge's bacterial density was higher than general sludge. Fluores-cence in situ hybridization (FISH) observation and analysis revealed ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) grew in outer surface and inner surface layer of granular sludge respectively. Both AOB and NOB were aerobic bacteria. However, the growth of NOB needed not only nitrate-N but also dissolved oxygen. Consequently, spatial distribution of NOB was affected by AOB growth, and favorably retained in inner layer.Since AOB mainly grew in outer surface layer of particles, their distribution and activity were easily influenced by operational parameters such as the shock of organic load, consequently caused the effluent quality variation. The denitrification bacteria mainly distributed in the inner surface layer of granular sludge where it was anoxic or anaerobic condition. They would grow and move to outer layer of particles in high influent feeding load, which made it possible for de-nitrification reaction to occur under aerobic operational condition.
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