| Aerobic granules, as the self-organised granular sludge, has the advantages of good setting property, high biomass content, high flewith respecte to loading rates, removing nitrogen and phosphate in one operation unite and so on. IN this study, the formation and the impact factors of aerobic granules with low strength wastewater under low hydrodynamic was investigated.The formation of aerobic granules with low organic loading wastewater (150-200 mg/L of influent COD) at low aeration rate (0.6 cm/s of SGV) had been investigated in the anaerobic/oxic/anoxic sequence batch reactor (SBR). Aerobic granules with smooth surface and compact structure were successfully obtained after 50 days. However, these aerobic granules were unstable when the d(0.9) of granules increased to more than 1 mm. The results suggested that aerobic granules with small diameter (smaller than 1000μm) were more favorable for treating the low substrate loading wastewater at the low aeration rate. The effluent concentrations of N-NH4+ and P-PO43- were lower than 1 mg/L, and the effluent concentration of N-NO3- gradually decreased with the granulation. The cycle test revealed that most of the influent COD was removed at the anaerobic stage. Phosphate accumulating organisms (PAOs) were found to utilize O2 or NOx- as electron acceptor for phosphorus removal in the study. Simultaneous nitrogen and phosphorus removal occurred inside the granules. The PAOs mainly located on the out layer of the granules. Denitrifying phosphorous removal also occurred in the inner part of the aerobic granules. The aerobic P uptake rate was 26.2 mgP-PO43-/(gVSS·h) during the initial 60 minutes of aeration, and the anoxic P removal rate was 8.9 mg mgP-PO43-(gVSS-h).A one-dimension aerobic granule mathematical model was established, basing on mathematical biofilm model and activated sludge model. The model was used to simulate simple aerobic granule process such as nutrients removal, granule diameter evolution, cycle performance as well as depth profiles of dissolved oxygen (DO) and biomass. The effluent N-NH4+concentration decreased as the modeling processed. The simulation effluent N-NO3-concentration decreased to 3 mg/L as the granules grew. While the granule diameter increased from 1.1 mm on day 30 to 2.5 mm on day 100, and the TN removal efficiency increased from less than 60% to 91%. The denitrification capacity was believed to enhance because the anoxic zone would be enlarged with the increasing granule diameter. Simultaneous nitrification and denitrification occurred inside the big aerobic granules. The oxygen permeating depth increased with the consumption of substrate. It was about 100-200μm at the beginning of the aeration phase, and it turned to near 800μm at the end of reaction. The autotrophs were mostly located at the out layer where the DO concentration was high. The heterotrophic bacteria were distributed through the whole granule. The aerobic granule growth profiles were similar under different hydrodynamic shear force, organic loading rate, DO and setting time adjustments, which could be described into three steps:initial growth phase, steady growth phase and stable phase. The granule size under the dynamic steady-state decreased with the enhanced detachment shear force. High DO concentration and withdrawing poor-setting flocs were beneficial for granule growth because of more available oxygen and nutrients.
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