Formation and stability of aerobic granular sludge in a sequential batch reactor for the simultaneous removal of organic matter and nutrients from low-strength domestic wastewater

Simultaneous removal of organic matter, nitrogen, and phosphorus, via simultaneous nitrification and denitrification (SND) and enhanced biological phosphorus removal processes, was evaluated in a pilot-scale sequential batch reactor. The focus was on granule's morphology, stability, microbiological composition, and reactor performance while treating diluted domestic wastewater with total chemical oxygen demand (CODt) of ≈ 200 mg.L-1. The applied organic loading rate was 0.9 ± 0.3 kg CODt.m−3.d−1 in the experiment. Aerobic granular sludge developed gradually. After 87-day operation, granules (diameter ≥ 0.2 mm) were ≥ 50 % of the biomass, and after 168 days, complete granulation was obtained (≥ 80 % of biomass). In the third period (days 168–247, complete granulation), mixed liquor biomass reached a volatile suspended solids (VSS) concentration of 1.2 ± 0.3 g VSS.L−1, with the granules remaining stable until the experimental end. In this period, low effluent concentrations of COD, nitrogen (NH4+-N, NO2−-N and NO3−-N) and phosphate (PO43—P) were obtained (mg.L−1): 36 ± 11; 4 ± 5; 3 ± 3, 4 ± 5; and 0.9 ± 0.4, respectively. COD, NH4+-N, and PO43−-P removal efficiencies (%) were 80 ± 11; 83 ± 20; and 55 ± 24, respectively. Heterotrophic nitrification and SND were observed, resulting in a process efficiency of 31 % even with dissolved oxygen applied to saturation. The phosphate removal was mainly attributed to denitrifying phosphorus accumulating organisms. Pseudomonas, the dominant genus found, acted in nitrogen and phosphorus removal. Pseudoxanthomonas also assisted in phosphorus removal. Bacterial communities in the flocs (≈ 20 % of biomass) during the last period were similar to those in the granules; therefore, they constituted the basis for granule formation, directly contributed to the simultaneous good removal of organic matter and nutrients. [Display omitted] •Complete granulation was achieved in 168 days with low-strength real wastewater.•Larger diameter granules favored SND and phosphorus removal.•The removal efficiencies were 80 % (COD), 83 % (N) and 55 % (P).•Pseudomonas genus was dominant during the complete granulation period.•A highly specialized biodiversity was clearly developed after granulation.