Stage 1: Conditioning Fouling
The initial conditioning stage arises where strong interactions take place between the membrane surface and the EPS/SMP present in the mixed liquor. Ognier et al. [107] described rapid irreversible fouling in this initial stage and passive adsorption of colloids and organics have been observed even for zero-flux operation and prior to particle deposition [103]. Another detailed study based on passive adsorption revealed the hydraulic resistance from this process to be almost independent of tangential shear and the initial adsorption to account for 20%–2000% of the clean membrane resistance depending on the membrane pore size [108]. In a more recent study, the contribution of conditioning fouling to overall resistance was found to become negligible once filtration takes place [109]. By applying a vacuum pump (rather than suction) coupled with air backflushing, Ma et al. [110] were able to reduce colloidal adsorption onto the membrane. These studies suggest that colloid adsorption onto new or cleaned membranes coupled with initial pore blocking may be expected in MBRs [12]. The intensity of this effect depends on membrane pore size distribution, surface chemistry and especially hydrophobicity [107]. In a test cell equipped with direct observation through a membrane operating with crossflow and zero flux, flocculant material was visually observed to deposit temporarily on the membrane [103]. This was defined as a random interaction process rather than a conventional cake formation phenomenon. While some flocs were seen to roll and slide across the membrane, biological aggregates typically detached and left a residual footprint of smaller flocs or EPS material. Biomass approaching the membrane surface was then able to attach more easily to the membrane surface to colonize it and contribute to Stage 2 [10].