The influence of short values of hydraulic and sludge retention time on performance of a membrane bioreactor treating sunflower oil refinery wastewater

In this work, the performance of organic pollutant removal, membrane fouling and sludge morphology in a submerged membrane bioreactor (MBR) treating sunflower oil refinery wastewater (SORW) containing high oleic content was studied during 52 days' operation at short values of 18 h and 10 days for hydraulic retention time and sludge retention time, respectively. The removal efficiencies of chemical oxygen demand (COD), oil and grease (O&G) and turbidity were found to be 73.2±6.6%, 75.1±2.5%, and 99.7±0.1%, respectively and the need for membrane cleaning never rose. The results showed a statistically significant linear correlation between the mixed liquor O&G with soluble microbial products and extracellular polymeric substances (EPS) (rP=0.792; p-value=0.034 and rP=0.920; p-value=0.003, respectively). Additionally, increase in MLVSS concentration which was due to an increase in cell concentration and was not related to accumulation of O&G and biopolymers inside the bioreactor, increased specific oxygen uptake rate. The trend of changes in sludge volume index (SVI) and supernatant turbidity (ST) with EPStotal was also found to be statistically significant (rP=0.736; p-value=0.037 and rP=0.773; p-value=0.024, respectively). The results of SVI, ST, EPS, particle size distribution and microscopic observation showed change in sludge morphology to flocs of smaller size (unimodal, with mode of 20 μm) with high compressibility (SVI=44.0 mL g–1 MLSS) and bioflocculating ability (ST=20.4±3.3 NTU). The results of the present study were indicative of a very good potential of the MBR for treatment of SORWs.