Aerobic granular sludge treating ammonium-rich petrochemical wastewater: influence of different growth substrate concentrations

Among industrial effluents, petrochemical wastewater is characterized by a highly variable composition, depending on the processes applied in different facilities worldwide. In this study, the possibility to apply aerobic granular sludge to the treatment of ammonium-rich petrochemical wastewater produced by the integrated gasification combined cycle (IGCC) was investigated, in view of replacing or integrating the existing full scale treatment system based on steam stripping and trickling filtering units. A lab-scale granular sludge sequencing batch reactor (GSBR) was fed with IGCC wastewater, with the addition of an organic growth substrate (sodium acetate, NaAc) in order to sustain granulation. Considering a full-scale application perspective, NaAc supply was progressively reduced and finally suspended. As the resulting volumetric organic loading rate (vOLR) decreased, overall process performance and granules physical properties were thoroughly evaluated. At high vOLR, granular sludge showed good NH4+-N and TOC removal efficiencies (75 and 90%, respectively), as well as excellent sludge settleability (SVI8 was 35±6 mL/gTSS). As expected, the reduction and final suspension of NaAc supply caused the decrease of volatile suspended solids (VSS) concentration in the GSBR (from 5.3 to 2.6 gVSS/L), although no negative effect on overall process performance and granules physical properties was observed. The results showed that: (i) the aerobic granular sludge technology can be considered as a suitable alternative for the treatment of ammonium-rich IGCC wastewater; (ii) the supply of an external carbon source can be progressively reduced and suspended if successful GSBR start-up has been previously achieved; (iii) the consequent reduction of the operating costs would represent an important pre-requisite in view of process scale-up. Moreover, the results achieved in this study provide useful information for the successful long-term operation of aerobic granular sludge reactors treating similar industrial wastewaters, characterized by high ammonium concentration.