Biofilm image observation and thickness
Considering the biological treatment of OSPW without VX-11e pretreatment, SBRs (HRT 11.5 days) seeded with activated sludge and with MFT showed a much higher AEF removal, i.e. 19.4% and 38.7% respectively. Results show that when the HRT was increased from 1 day to 11.5 days, significant increase of AEF removal was found. However, the relatively long HRT makes it not realistic for the bioreactors to be widely applied in the field. For bioreactors with HRT of 1–2 days, the MBBR applied in this study performed the highest AEF removal of 18.2%. Considering the biological treatment of OSPW with ozone pretreatment, the continuous flow biofilm reactor of Hwang et al. (2013) and biofilm reactors operated in batch or continuous modes (Choi and Liu, 2014) showed very high AEF removal of 85–89%, while the MBBR removed 41% of the total AEF. However, the higher AEF treatment was due to the high levels of ozone dose (80 mg/L). Deducting the part of ozone degradation, 14.5% of total OSPW AEF was removed by the biological treatment of MBBR, which was the highest among all the studies above (9.6%, 8.2% and 11.4% respectively). One conclusion could be drawn that after the ozone pretreatment, MBBR in this study showed excellent biodegradation performance of OSPW. Therefore, combining the ozone pretreatment cost and biodegradation efficiency, ozonation combined MBBR in this study may be a good choice for OSPW treatment.