Resistance and resilience of nitrifying bacteria in aerobic granules to pH shock

To investigate the response difference between flocculent sludge and granular sludge in unfavourable environmental conditions, 1-h pH shock was exerted to both granular sludge and flocculent sludge with the same biomass concentration and the same population sizes of nitrifying bacteria. It was found that nitrite-oxidizing bacteria (NOB) were more sensitive to pH than ammonia-oxidizing bacteria (AOB). In addition, the deviation of 3 pH units to alkaline from the optimal pH was much more detrimental to AOB and NOB in both flocculent sludge and granular sludge compared with the deviation of 3 pH units to acidic. However, respirometric activities of both AOB and NOB in granular sludge after pH shock at 5 and 11 were much higher than those in the flocculent sludge. The recovery of nitrifying activities by only one batch culture indicated that nitrifying bacteria in granular sludge experienced a much less irreversible loss compared with flocculent sludge. The results in this study showed that compact structure and the big size of granules could play a buffering role to unfavourable conditions which resulted in a much higher resistance and resilience to shock.

This study aims to provide some guidance to the application of aerobic granular sludge to real wastewater which might experience fluctuated pH. The findings in this study indicate that resistance and resilience of nitrifying bacteria in mature granules to pH shock at steady state is much better than flocculent sludge, suggesting more robust and flexible operation of granular sludge compared with traditional flocculent sludge. In addition, the results in this study underlined a more serious inhibition from free ammonia at high pH than free nitrous acid at low pH as well as the inaccuracy of reported bell-shaped modelling of pH effect on nitrification.