Particle size distribution and biomass growth in a submerged membrane bioreactor

Submerged membrane bioreactors (MBR) associate in a single treatment unit a process of biological treatment and a phase of solid/liquid separation by membrane filtration. Due to reduced membrane porosity, suspended solids and large amount of bacteria and viruses may be retained in the reactor; consequently, high biomass concentration is generally established in the system. Indeed, this latter circumstance leads to obtain a system able to operate with a low F/M and high SRT and hence, as a result, a low sludge production. In technical literature, the definition of foulants in MBR systems is a controversial task. More specifically, the influence of TSS on mixed liquor filterability has not been yet fully understood [1]. On the other hand, morphology and flocs size distribution have been identified as important factors affecting mixed liquor filterability [2]. Therefore, theoretically, in MBR system flocculation should not play a direct role in biomass separation since there is not any further settling phase, indeed it is strictly connected with cake permeability and filtration resistance. In particular, the deflocculation process, that can be regarded as a consequence of the hydrodynamic and physical–chemical conditions, and of the physiological state of the mixed liquor, causes an increment in the filtration resistance due to both a decreased sludge floc size and an EPS production caused from the deflocculation itself [3]. Bearing in mind the consideration discussed above, the study aimed to analyse the start-up of a submerged membrane bioreactor with complete sludge retention; furthermore, particular care was addressed towards the problem regarding the best operating conditions which drive metabolism towards the bacterial maintenance rather than new cells production, in order to minimize sludge production. Granulometric characteristics of sludge flocs and the control of mixed liquor deflocculation have been investigated.