COMPARISON OF A2O AND UCT OPERATION MODE AT FULL-SCALE EBPR PROCESSES – PERFORMANCE, KINETICS AND PAO POPULATION

Comparison of the P removal performance with UCT mode versus A2O was conducted at a full-scale enhanced biological phosphorus (P) removal (EBPR) plant. The plant has four treatment trains in parallel that allowed for simultaneous operation of different modes. The UCT mode produced lower effluent ortho-phosphate concentration and had better P removal performance than the A2O mode. Investigation of ortho-P and TON (NO2+NO3) profiles across different anaerobic, anoxic and aerobic zones in the two processes indicated that UCT mode had higher and quicker P release peak than A2O mode and there were much more and faster anoxic P-uptake took place in the UCT mode than A2O mode. This led to better denitrification and lower effluent nitrate concentration in UCT process than A2O process. Batch tests with sludge samples from the UCT process mode showed higher specific phosphorus removal rates and VFA utilization efficiency than that from the A2O process, indicating higher PAO activity level in the UCT system. Quantification of poly-phosphate accumulating organisms (PAOs) in the two processes using DAPI stain and fluorescence in situ hybridization (FISH) showed similar level of total PAOs in the two processes and, Rhodocyclus-like PAOs in UCT process was found to be slightly higher than A2O (11.6% and 9.9%, respectively). These results showed consistent evidence for better P removal performance in UCT mode than A2O mode, although, there was no apparent quantitative correlation between population abundance, P release and uptake rates and effluent ortho-P concentration. This study suggests that there is advantage to apply UCT mode, especially when the effluent P limit becomes very low and the small extent of difference in effluent P concentration starts to become more significant.