Impacts of Operating Parameters on Stability of Sequencing Batch Reactors Inoculated with Mixed Yeasts in Treating Edible Oil Refinery Wastewater

Yeasts have distinct advantages in degrading many kinds of oil-containing wastewater produced from edible oil, olive oil or palm oil mill. Key operating parameters are crucial to set up a bioreactor with a stable and sound performance. In this study, mixed yeasts were inoculated in pilot-scale sequencing batch reactors (SBR) to treat edible oil refinery wastewater. Effects of carbon/nitrogen ratio, sludge retention time(SRT), as well as pH on the performance of SBR and yeast cell morphology were investigated. The results show that nitrogen is an important factor affecting yeast morphology and system stability. At a BOD:N:P ratio of 100:5:1, the SBR was operated successfully and steadily with SVI of 49.1 +/- 4.0 mL g(-1). However, at a BOD:N:P ratio of 100:2.5:1, yeast cell morphology transformed from yeast to long pseudohyphae which resulted in the pronounced increase of SVI and followed by extensive filamentous sludge; A mild increase of SVI and formation of short pseudohyphae were observed without perceptible change of pollutant removal efficiency when SRT was decreased from 60 d to 6 d; Increasing of operating pH from 5.5 to 7.0 led to a drastic increase of suspended solid and oil up to 612 and 2290 mg L-1, respectively, in the effluent.