Role of extracellular polymeric substances and metals in fouling of membranes in the wastewater treatment

Abstract

The use of membrane bioreactors (MBRs) for wastewater treatment has been increasing since the 1990s and is gaining more importance due to rising demands for water in general. There is yet one main limitation in full-scale applications of MBRs: the membrane fouling, leading to a substantial loss of membrane performance and subsequently to high costs because of cleaning and replacing the membranes. Since no consensus on the exact fouling phenomenon or a single fouling parameter in MBRs has been reached yet the present study was aiming at characterizing membrane foulants in the full-scale MBR Nordkanal (Düsseldorf, Germany), in pilot-scale MBRs as well as in fermenter-/lab-scale studies. The focus of the present study was on two classes of substances that are known to cause fouling in membrane applications: extracellular polymeric substances (EPS) and metals. The long-term observation of activated sludge, permeate and autopsied membranes from the MBR Nordkanal showed a significant contribution of EPS and iron to membrane fouling. The membrane fouling layers where shown to have the following composition (in weight %): humic substances (39%), carbohydrates (30%), iron (22%) and fatty acids (8%). Contrary to other, mostly pilot-scale studies, biofilm development was not dominant in this full-scale MBR due to vigorous aeration and frequent backwashing of the fibres. Interestingly, precipitations of iron hydroxides were detected on the membranes, even on the permeate side, whereas Ca2+ and Mg2+ were not involved in fouling in this MBR. The retention by the ultrafiltration membranes in the full-scale MBR was shown to be 39 ± 27% for humic acids and 79 ± 30% for carbohydrates (average of 2.5 years of monthly samples) although the molecular weight of the humics fraction was demonstrated to be below the molecular weight cut-off of the membrane. Fatty acids (FA) of microbial origin (C16:0, C18:0) were the dominating FA in EPS of activated sludge flocs, sludge supernatant, permeate and membrane fouling layers in the MBR Nordkanal. A remarkable finding was that a shorter FA of a probable synthetic origin (C9:0) was preferentially deposited into the membranes where it accounted for 10% of the FA in total EPS whereas in sludge floc EPS it was only 1% of total FA concentration. The results from the full-scale study were used to propose a fouling mechanism that is based on complexation and adsorption processes. Iron from the coagulant FeCl3 in form of Fe2+ and Fe3+ is playing a central role in this mechanism as it is able to form complexes with humic acids and with uronic acids that were detected in the microbial polysaccharides of samples of the MBR Nordkanal. For such complexes as well as for fatty acids, reduced electrostatic repulsion, narrowing of the membrane pores and hydrophobic interactions with the membrane material could explain the observed fouling phenomenon. In lab-scale surveys regarding membrane cleaning strategies, it was further verified that citric acid is especially suited to remove iron and EPS from fouled membranes. Moreover, it was demonstrated through lab-scale sludge digestion how soluble Fe2+ can be formed and released together with EPS from the sludge floc matrix under anaerobic conditions. Such conditions also deteriorated sludge properties in terms of filterability and settling behaviour. Finally, in the pilot-scale study it was shown that low sludge retention time (SRT) leads to an excess production of EPS in correlation with deteriorating sludge properties. The formation of fouling layers on the membranes was also more distinct at low SRT, with 40-fold higher amounts of deposited proteins and 5-fold higher concentrations of carbohydrates compared with high SRT. However, membrane permeability was similar at both SRTs. The conclusion for operators of full-scale MBRs in general is that high SRT is beneficial for activated sludge properties. For the specific case of the MBR Nordkanal which is already operated at high SRT (28 d), it can be concluded that the presence of iron in conjunction with anaerobic conditions in the denitrification tank, prior to membrane filtration, is more decisive for the membrane fouling. As shown by results of the present study, the applied cleaning strategy with citric acid in Nordkanal is appropriate for removing the major membrane foulants. For future studies aiming at clarifying distinct fouling parameters and fouling mechanisms, a focus on metal complexation with specific EPS or with wastewater components, as well as the interaction of such complexes with different membrane materials looks promising