Foaming in membrane bioreactors: Identification of the causes

Abstract

a b s t r a c tMembrane bioreactors (MBRs) represent by now a well established alternative for wastewater treatment. Their increasing development is undoubtedly related to the several advantages that such technology is able to guarantee. Nevertheless, this technology is not exempt from operational problems; among them the foaming still represents an "open challenge" of the MBR field, due to the high complexity of phenomenon. Unfortunately, very little work has been done on the foaming in MBRs and further studies are required. Actually, there is not a distinct difference between conventional activated system and MBR: the main difference is that the MBR plants can retain most Extracellular Polymeric Substances (EPSs) in the bioreactor. For these reason, unlike conventional activated sludge systems, MBRs have experienced foaming in the absence of foam-forming micro-organisms. Nevertheless, the actual mechanisms of EPS production and the role of bacteria in producing foam in activated sludge in MBRs are still unclear. In this paper, the authors investigated the roles of EPS and foam-forming filamentous bacteria by analyzing samples from different pilot plants using MBRs. In particular, in order to define the macroscopic features and the role of EPS and filamentous bacteria, a Modified Scum Index (MSI) test was applied and proposed. Based on the MSI and the foam power test, the causes of biological foaming were identified in terms of the potential for foaming, the quality and the quantity of the foam. The results indicated that the MBR foaming was influenced significantly by the concentration of bound EPSs in the sludge. In addition, the quantity and stability of MBR scum increased when both bound EPSs and foam-forming filamentous bacteria were present in the activated sludge.

The role of EPS in the foaming and fouling for a MBR operated in intermittent aeration conditions

“…In period III, the MSI 0 decreased to about 20%. Thus, in agreement with previous studies [9,12], also in an IA-MBR the general trend of MSI 0 is correlated with the variation of EPS S in the bioreactor. Contrarily, after the complete process of "purification" the effect of EPS S was "masked".…”

“…5d). On the other hand, the absence of significant correlations between MSI 2 and EPS S confirms that the purification step reduces the effect of EPSs on foaming, in favor of the contribution of filamentous bacteria [9,13]. In particular, when the mixed liquor was purified from scum during the foaming tests, the major amount of EPS was removed with the scum and so the correlation between the extracellular polymeric substances and the subsequent scum indexes became gradually less important.…”

“…By contrast, FP is scientifically rigorous, and essential for analysis of foaming potential and prediction of foaming risk. Furthermore, FP has been closely correlated with EPS concentration [9], and so can be indirectly associated to the physiology of the activated sludge and with other phenomena dependent on them (such as membrane fouling rate). However, both tests are simple and can be conducted directly in situ with similar simple equipment.…”

“…Furthermore, other studies outlined also the presence of other bacteria species during foaming occurrence. For this reason, bacterial strains such as the Eikelboom Type 0092 have been defined "not famous foam-forming bacteria" [9,11].…”

“…However, both EPSs and filamentous bacteria influence the sludge hydrophobicity and therefore impact on foaming occurrence. Although the foaming occurrence can seriously affect the correct working of a MBR, studies dealing with foaming in MBRs are scarce [9,12,13]. Furthermore, foaming occurrence can be encouraged by the implementation of new management strategies such as the Intermittent Aeration (IA).…”