Granular activated carbon promoting re-granulation of anammox-hydroxyapatite granules for stable nitrogen removal at low phosphate concentration.

Anaerobic ammonium oxidation (anammox) coupled with hydroxyapatite (HAP) crystallization not only achieves simultaneous nitrogen removal and phosphorus recovery, but also cultivates excellent anammox granules. However, a floatation and wash-out of anammox-HAP granules was occurred at low phosphate concentrations. In this study, a reactor inoculated with mature anammox-HAP granules and fed with low phosphate (5 mg P/L) was added with granular activated carbon (GAC) to maintain sludge granulation and nitrogen-removing stability. At influent total nitrogen >800 mg/L and nitrogen loading rate ~ 9.8 kg/m/d, a satisfactory nitrogen removal of around 88% was maintained during 140 days of operation. Insufficient phosphate supplement resulted in a sludge bulking, with suspended solid and sludge density decreased whereas sludge water content and expansion ratio increased due to HAP loss. Nevertheless, the sludge re-granulation was found at the later stage as the proportion of granules in 2.8- 3.35 mm went up to 37.4% after large granules disintegrated into small pieces at the initial stage. The settling velocity was finally ranging from 129.8 to 182.2 m/h. In addition, Candidatus Brocadia was increased from 2.1% to 20.1% and dominated in the microbial community. These findings suggest GAC was able to promote re-granulation of anammox-HAP granules at low phosphate concentration, which avoids sludge flotation and widens their application as an inoculum.