Color characterization of anammox granular sludge: Chromogenic substance, microbial succession and state indication

Vision is a direct and convenient way to get information, and color characteristics are important visual information to identify objects. In this study, the L*a*b* color space was introduced for the first time to digitize the surface color of anammox granular sludge (AnGS). Three AnGSs under typical biological loading rates were chosen and the color value a*/b* was found to have a positive correlation with the specific anammox activity and the biological loading rate of AnGS. Cytochrome c was detected to be the key chromogenic substance determining the red hue of AnGS. The concentration of cytochrome c was highly consistent with the abundance of anammox bacteria. Nitrospira was observed to compete with Candidatus Kuenenia for nitrite under survival state, resulting in the obvious decrease of a* value; while the growth of sulfur-related Limnobacter and Thiobacillus was enhanced under starvation state, leading to the production of Fe-S compounds covering over the surface and decrease of a* and b* value simultaneously. The unique color characteristics of AnGS were evaluated as a visual indicator to serve the on-line control and better judgement of anammox process.