Assessing inorganic components of cake layer in A/O membrane bioreactor for physical-chemical treated tannery effluent

Abstract

In this study, an anoxic-oxic membrane bioreactor (A/O-MBR), was used to treat effluent tannery wastewater pretreated by physicochemical processes. The A/O-MBR performed well during the experimental period and was able to produce a high-quality effluent containing 90 ± 10 mg-CODcr/L and 0.5 ± 0.1 mg-NH4+-N/L. However, it was observed that at rates of approximately 1.02 kPa/day and 1.2 μm/day, both transmembrane pressure (TMP) and thickness of cake layer increased during wastewater treatment. The eventual thickness of the cake layer was between 47.8 and 51.5 μm. Furthermore, an Inductively Coupled Plasma-Optical Emission Spectrometer, used to analyze inorganic components of the cake layer, revealed that four inorganic elements, Cr, Ca, Mg and Al were predominant (weight percentage rate 4:13:10:72). Due to low solubility (Cr(OH)3: Ksp 6.3 × 10−31; Al(OH)3: Ksp 6.3 × 10−19), the elements of Cr and Al mainly existed in the forms of Cr(OH)3 and Al(OH)3, respectively. Other minerals in the cake layer included Al2O3, CaCO3, and MgCO3. Additionally, using an SEM-EDX (Scanning electron microscopy-energy dispersive X-ray analyzer), we discovered that inorganic particles that formed within the activated sludge of the A/O-MBR steadily accumulating on the membrane surface, resulted in an evenly distributed inorganic layer which could be observed along the cross-sections of the cake layer.