Temporal microbial community studies have broadened our knowledge of the dynamics and correlations among microbes in both natural and artificial engineering systems. Using activated sludge as a model system, we utilized the intensive longitudinal sampling method to identify overlooked diversity and the hidden dynamics of microbes, detect cross-associations among microbes after detrending, and reveal the central microbial dynamics during sludge bulking and foaming. We discovered that the accumulative alpha diversity in activated sludge sampled daily over 392 days could be as high as 14 000 OTUs, and that the bacterial community dynamics followed a gradual succession, drifting away from the initial observed day and displaying a significant time-dependent trend. Cross-associations among bacteria were modulated after removing potential spurious correlations based on autocorrelation in microbial time series. Moreover, clusters of bacteria displaying rapid turnover were discovered during the beginning, ongoing, and fading of sludge bulking and foaming, and their physicochemical parameters are resolved. These identified groups of bacteria and their related environmental factors could potentially supply clues to form hypotheses for treating operational problems, such as sludge bulking and foaming.