Particle size distribution and biomass growth in a submerged membrane bioreactor

Abstract

Submerged membrane bioreactors (MBR)associate in a single treatment unit a process ofbiological treatment and a phase of solid/liquidseparation by membrane filtration. Due to reduced membrane porosity, suspendedsolids and large amount of bacteria and virusesmay be retained in the reactor; consequently, highbiomass concentration is generally established inthe system. Indeed, this latter circumstance leadsto obtain a system able to operate with a low F/Mand high SRT and hence, as a result, a low sludgeproduction. In technical literature, the definition of fou-lants in MBR systems is a controversial task.More specifically, the influence of TSS onmixed liquor filterability has not been yet fullyunderstood [1]. On the other hand, morphologyand flocs size distribution have been identified asimportant factors affecting mixed liquor filter-ability [2]. Therefore, theoretically, in MBR sys-tem flocculation should not play a direct role inbiomass separation since there is not any furthersettling phase, indeed it is strictly connected withcake permeability and filtration resistance. Inparticular, the deflocculation process, that can beregarded as a consequence of the hydrodynamicand physical–chemical conditions, and of thephysiological state of the mixed liquor, causes anincrement in the filtration resistance due to botha decreased sludge floc size and an EPS produc-tion caused from the deflocculation itself [3]. Bearing in mind the consideration discussedabove, the study aimed to analyse the start-up of asubmerged membrane bioreactor with completesludge retention; furthermore, particular carewas addressed towards the problem regardingthe best operating conditions which drive metab-olism towards the bacterial maintenance ratherthan new cells production, in order to minimizesludge production. Granulometric characteristicsof sludge flocs and the control of mixed liquordeflocculation have been investigated.