Comparison of a MBR system start-up with and without inoculum: experimental and mathematical model application

The performance of a membrane bioreactor (MBR) system and the extension and mechanisms that lead to the formation of the fouling may differ in relation to the start-up. As a consequence, the MBR start-up may constitute a crucial point that drastically influence the MBR performance in its lifespan. Indeed, the start-up influences the mechanisms of membrane fouling whose effects, in the case of irreversible cake deposition, can be modified only by a chemical membrane washing. In order to gain insights about the effect of the start-up on a MBR system, a MBR pilot plant was build-up and a mathematical model developed. The MBR system was constituted by a hollow fibre membrane module installed in submerged configuration in a pilot aerobic reactor. Particularly, two different start-up procedures have been considered: inocolum of biomass and no inoculum of biomass. The whole experimental campaign was divided in two periods, lasting 65 days each: during the first period, the pilot plant was started-up without inoculum of activated sludge and no sludge withdrawal was performed; on the contrary during the second period, the MBR pilot plant was started-up with sludge inoculum and the sludge concentration was kept constant. A mathematical model was applied to the two periods in order to integrate the insights coming the experimental analysis about the advantages/disadvantages of carrying out a MBR start-up with or without inoculum. The model takes into account both biological and physical phenomena that jointly play important rules in the assessment of MBR system performance. The evolution of fouling and TMP in both periods were analyzed. Both experimental and model results demonstrated that the best choice in terms of start-up of MBR systems comes from a compromise between minimization of the fouling issues and, on the other hand, high system performance.