Cross Effects of Organic Loading Rates and Substrate COD/N/P Ratios on Aerobic Granulation

We have operated sequencing batch reactors at different organic loading rates (OLRs) of 0.5, 1.5 and 4.5 kg COD m^-3 d^-1 or substrate COD/N/P ratios of 100/5/1 and 100/5/0, respectively. Results showed that granules failed to be cultivated under the OLR of 0.5 kg COD m^-3 d^-1 for the absence of filamentous bacteria during the full cultivation period. granules remained not detected before Day 38 under the OLR of 1.5 kg COD m^-3 d^-1 and substrate COD/N/P ratio of 100/5/1. Granules were initially observed on Day 25 under the OLR of 4.5 kg COD m^-3 d^-1 and COD/N/P ratio of 100/5/1, however, The settleability of these granules began to deteriorate within 10 days after their maturation time. It demonstrated that a certain quantity filamentous bacteria act as the framework of granules, which is the most crucial step to achieve a quick and stable granulation. The higher the OLR and lack of phosphor, the more likely the growth of filamentous bacteria is favored with promoting the granulation by strongly hydraulic shear force. Serious sludge bulking would be more likely to occur resulting in biomass washout and the decrease of COD removal efficiency.