Mechanisms Contributing to MBR Performance

FIGURE 28.2 Schematic interrelationship between factors that determine process performance in MBRs. Wastewater characteristics are implicit in this outline. The term biological reactions inclndes exogenous and endogenous metabolisms, extracellular enzymatic activity, 

The aggregation rate of colloidal and particulate materials is dependent on the permeation rate, hydrodynamics, and surface forces between the membrane and colloid material therefore, fouling rates are system dependent. However, certain broad trends are evident with respect to fouling. 

Below a certain permeation rate, the TMP varies linearly with flux, and above this transition flux, a sharp increase in TMP is observed concomitant with a permeate flux decline. A time-dependent flux decline is also observed. The critical flux hypothesis is that upon start-up, there exists a flux below which a time-dependent flux decline does not occur, whereas fouling takes place above this critical value [44]. It is known that the critical flux in MBRs depends on hydrodynamics, particle size, and membrane surface characteristics. Typically, MBRs are operated below the critical flux in order to minimize fouling. However, the validity of the critical flux concept in MBR operation has been questioned since after prolonged operation, irreversible fouling has been observed at subcritical fluxes [45]. 

The importance of biopolymer deposition on the membrane has been highlighted by Chu and Li [46] who noted that such material may accelerate bacterial attachment to the membrane in addition to its contribution to fouling resistance. The formation of a slime layer on the membrane, comprising an EPS matrix with embedded bacteria, may be analogous to a biofilm, the significance of which is the difficulty of its removal by nonchemical methods. The question of whether biofilm formation plays a significant role in MBRs is yet to be addressed. Indeed, a comprehensive model of fouling in MBRs is not yet available. 

Handbook of Membrane Separations Chemical, Pharmaceutical, Food and Biotechnological Applications 

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