Size characterization of red cores in partial nitritation/anammox granular sludge: Nitrogen removal activity and bacterial structure indication

In this study, the size characterization of partial nitritation/anammox (PN/A) granules was conducted using image analysis technique to develop a simple and effective method for monitoring the operating of the PN/A process. The statistical analysis of the projected area of the brick-red and compact cores of granules (A(Red)) was conducted for numerous samples. Both batch testing and high-throughput pyrosequencing were employed to evaluate the bacterial activity and community responses to variations in the mean ARed values of the granules. A positive linear correlation (R-2 > 0.93) was identified between ARed and specific nitrogen removal activity; both increased with increasing total inorganic nitrogen removal rate (from 0.07 to 0.42 g N.g(-1)VSS.d(-1)) in a continuous-flow bioreactor. ARed shared a similar tendency with the accumulation of extracellular polymeric substances, indicating larger anoxic cores existed in mature granules. This was also confirmed by the observed monotonic 3.7-fold increase in the relative abundance of the anammox populations (genus Candidatus Kuenenia), accompanied by the wash-out of ammonium- and nitrite-oxidizing bacteria. As no special testing instrument is required to quantify ARed, it constitutes an easy-to-visualize microbe-related indicator to evaluate the working states of the PN/A granules, exhibiting a clear potential in engineering applications.