Microstructure and granulation cycle mechanisms of anammox-HAP coupled granule in the anammox EGSB reactor.

The formation of anammox-hydroxyapatite (HAP) coupled granules has been shown to be an approach to efficient nitrogen removal and phosphorus recovery in the anammox EGSB reactor. However, the granulation cycle mechanism of anammox-HAP coupled granules for sustainable regeneration and growth is still not well understood. In this study, the microstructure, chemical composition and microbial structure of a total of six different-sized granules, from 0.25 mm to 2.8 mm, was determined. An SEM-EDS analysis indicated that the small granules (<0.5 mm) were composed of poly-pellet clusters with anammox biofilms attached to the HAP cores, and the large granules (>0.5 mm) consisted of a three-layer structure: a surface anammox biofilm layer, a middle connection layer, and a HAP mineral inner core. The analysis of elemental composition and microbial structure suggested homogenous granular characteristics regardless of granule size. The dominant microorganisms were anammox bacteria of Candidatus Kuenenia stuttgartiensis and heterotrophic denitrifying bacteria. Based on these results, a granulation cycle mechanism for anammox-HAP coupled granules was proposed for the first time. The growth of the small granules with the simultaneous enlargement of anammox biofilms and HAP cores results in the formation of large granules. Large granules regenerate new small granules in a two-step procedure. The first step is the separation of embryo HAP crystals from the mother core via heterogeneous growth, and the second step is the separation of the biofilms due to biodegradation and shear stress.