Texture of anammox sludge bed: Composition feature, visual characterization and formation mechanism

Anammox granular sludge biotechnology has been successfully applied in the environmental engineering for nitrogen removal from wastewater. However, the startup and control of anammox process was difficult and lacks an easy-recognized indicator. In this work, the texture of anammox sludge bed in a long-term (>15 months) stable-operated and high-rate anammox reactor was investigated for the first time. The results showed that the anammox sludge bed took on the unique texture which was closely related to the biological loading rates of sludge bed at different heights of the reactor. The texture was ascribed to the pattern created by granule gradation (granules size distribution) and granule number density (spaces between granules). The sludge bed texture displayed the distinctive vision and could be characterized by the reflected light of sludge bed. The lightness indicator was found to have an excellent linear relationship with the height of anammox sludge bed. The texture of anammox sludge bed was revealed to originate from the periodic cycle of gas-driven segregation and density-based aggregation of granules. The results hint at a convenient and efficient strategy for operators to assess and control the working state of anammox system.