Fouling analysis and mitigation in a tertiary MBR operated under restricted aeration

Abstract

Operation of tertiary membrane bioreactors at low dissolved oxygen concentration (DO) reduces energy consumption associated with biological aeration. However, low oxygen concentrations affect biological activity, sludge bioflocculation and fouling propensity. This study assesses the effectiveness of an alternative backwash initiation strategy for fouling control in a tertiary membrane bioreactor operated under restricted oxygenation. Long-term filtration tests were carried out at five DOs (0.09–1.23 mg/L) in a pilot-scale unit operated in dead- end filtration mode (i.e. without air-scouring) under a broad-range of permeate fluxes (25–65 L/h m2 ). Bioflocculation was negatively affected at low DO ([O2]≤0.25 mg/L), resulting in relatively high values of biopolymeric clusters concentration (BPC ≥4.3 mg DOC/L). This produced small floc sizes (D (v,0.5)≤49 μm), poor filterability (TTF≥573 s) and low critical fluxes (Jc≤20 L/h m2 ). However, above moderate oxygen concentrations ([O2 ]≥0.38 mg/L), sludge bioflocculation significantly increased, showing good filterability (TTF≤173 s) and relatively high critical fluxes (Jc≥45 L/h m 2 ). The filtration tests showed that the controlled backwashing initiation was effective in limiting the residual fouling even at a very low oxygen concentration, since it automatically increased backwashing frequency as sludge bioflocculation decreased. The reversible fouling, it can be described by the compressible cake model, with a specific cake resistance at zero pressure governed by the bioflocculation state, while the compressibility is determined by the filtration flux. Optimal sustainable conditions were established at moderate DO ([O2]=0.38 mg/L), which involves a low specific energy demand for the filtration process and biological aeration (SED=0.25 kWh/m 3 ), and a high net permeate flux (J =42.2 L/h m 2 ).