Achieving mainstream partial nitritation with aerobic granular sludge treating high-rate activated sludge effluent

New configurations for wastewater treatment plants (WWTPs) are under development to increase their self sufficiency and sustainability. One is the A -B process, in which the A -stage is designed to maximize the redirection of influent chemical oxygen demand (COD) to the production of biogas. A promising A -stage alternative is the high -rate activated sludge (HRAS). B -stage, on the other hand, can consist of the combined partial nitritation (PN)-anaerobic ammonium oxidation (anammox). In this study, PN with aerobic granular sludge (AGS) was attempted under mainstream conditions at pilot scale. The effluent of a HRAS system - ideally operating at 2 g/L of total suspended solids, 0.5 mg/L of dissolved oxygen and 1 h of hydraulic retention time (52 +/- 13 % COD removal) - was fed to the PN-AGS reactor initially inoculated with floccular sludge. Granulation and PN were studied with daily/seasonal fluctuations and without adding external reagents. Selective wasting from the top of the settled sludge bed was found a successful strategy to trigger granular sludge formation. The maintenance of a low sludge volume index (SVI < 100 mL/g) was helpful enough for improved operation of the system (i.e., COD removal and PN). Stable PN was achieved due to the suppression of the nitrite oxidizing bacteria activity by inhibitory levels of free nitrous acid. Finally, working with fast settling biomass at long settling times reduced the influent-biomass-nitrite contact and, thus, denitritation.