The study of physicochemical and morphological characteristics of activated sludge mixed liquor in a submerged membrane bioreactor treating vegetable oil wastewater

In this work the physicochemical and morphological characteristics of activated sludge mixed liquor in a submerged membrane bioreactor (MBR) treating real vegetable oil wastewater (VOW) was studied during the 30 days MBR operation at hydraulic retention time and solid retention time of 48 h and 20 days, respectively. With decrease in aerobic metabolic activity and hence the activated sludge growth rate during the MBR operation, MLSS and MLVSS decrease resulting in a decrease in viscosity and sludge volume index (SVI). Univariate correlation analysis revealed the presence of a statistically significant linear correlation between average concentration of MLSS and MLVSS and also food to microorganism (F/M) with values of SVI throughout the operation of the MBR. During MBR operation bioflocculating ability of the bacterial population has not changed. The trend of change in SVI data with extracellular polymeric substances (EPS) (mainly EPSP) and EPSP/EPSC was found to be statistically significant. This direct effect has been partly explained through the corresponding effect of EPS on surface charge of flocs (due to stronger electrostatic repulsion between negatively charged floc components as described by the DLVO theory) and partly by steric forces that the EPS may create when the EPS molecules extend out from cell surfaces and physically prevent the cells from forming close contact. The results of SVI, EPS (mainly EPSP), EPSP/EPSC, particle size distribution (PSD) and microscopic observation showed change in sludge morphology from filamentous bulking to smaller flocs size with high compressibility. Also during MBR operation the rheology of the mixed liquor was Newtonian.

Keywords:

membrane bioreactor , vegetable oil wastewater , morphology , sludge volume index , viscosity , filamentous bulking , extracellular polymeric substances (EPS)