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) characterized by high TOC and NH4+-N concentrations (up to 490 and 630 mg/L, respectively). A GSBR was used to carry out the experiments: two different start-up strategies (Phases A and B) were applied by varying shear forces, inoculum source and pH. The GSBR was fed with a mixture of synthetic and real PW, which was gradually increased depending on the observed reactor performance. In phase A, aerobic granules irreversibly deteriorated when the percentage of PW in the influent was raised to 30%. A faster and more stable granulation was observed during phase B, likely due to the higher superficial gas velocity applied (harsher conditions) and to the different sludge used as inoculum (already acclimated to toxic substances). The observed TOC removal efficiencies were similar in phases A and B (92 and 95%, respectively), while NH4-N removal efficiency was higher during phase B (28 vs 32%) . Phase B is still in progress (the rate of PW in the influent is currently set to 50%). The inoculum with acclimated sludge and the harsher selective pressure (phase B) were proved to speed-up the formation of small, dense and well settling granules, more resistant to the increase of PW rate in the influent. Further investigation is required in order to achieve stable granules treating 100% real PW, with high NH4+-N and TOC removal.