Aerobic granulation with petrochemical wastewater in a sequencing batch reactor under different operating conditions

The aim of this study was to define the best operating conditions for the successful start-up of a granular sludge sequencing batch reactor (GSBR) treating petrochemical wastewater (PW) produced by the integrated gasification combined cycle and characterized by high NH4+-N concentration (up to 630 mg/L). Two different start-up strategies were applied during Phases A and B, using different shear forces, inoculum source and pH. The GSBR was fed with a mixture of synthetic medium and real wastewater, and a readily degradable growth substrate (i.e. sodium acetate) was supplied in order to maintain the volumetric organic loading rate at 3 g COD/L d: the share of PW in the influent was gradually increased, depending on the observed reactor performance. In Phase A, aerobic granules irreversibly deteriorated when the share of PW in the influent was increased to 30%. Conversely, a faster and more stable granulation was observed during Phase B, likely due to the higher superficial gas velocity applied (i.e. harsher conditions) and the different sludge used as inoculum (already acclimated to toxic substances): under such conditions, the synthetic medium was completely replaced by the PW. The observed total organic carbon removal efficiencies were similar in Phases A and B (92 ± 6–94 ± 2%, respectively), while NH4+-N removal efficiency was higher during Phase B (up to 78% when PW% was 100%). The inoculum with acclimated sludge and the harsher selective pressure (Phase B) were proved to speed up the formation of compact and well settling granules, more resistant to the increase of PW share in the influent.