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Within membrane processes, fouling is one of the critical issues affecting the productivity, plant operation and maintenance costs. Focusing on wastewater treatment processes, it has been reported that extracellular polymeric substances (EPS) are one of the main causes of membrane fouling. In membrane filtration research, sodium alginate often serves as a model compound for EPS. Sodium alginate is a hydrophilic unbranched binary copolymer. In the presence of divalent cations, e.g. Ca2+, alginates form complexes, which lead to a significant change in filtration mechanisms in dead-end filtration and also to a change in filtration performance during cross-flow filtration experiments. Filtration conditions (e.g. transmembrane pressure or cross flow velocity), feed composition as well as membrane material have a major influence on the fouling behavior of the system. In this study ceramic hollow fiber membranes were used due to their high chemical and thermal stability coupled with a high specific membrane surface. In addition to the evaluation of the filtration data using conventional cake filtration model, nuclear magnetic resonance imaging was used to elucidate the influence of Ca2+ on the fouling layer structure for alginate filtration with ceramic hollow fiber membranes. In order to visualize the alginate layers inside the opaque ceramic hollow fiber membranes by means of MRI, specific contrast agents were applied. Supplementary to multi slice multi echo imaging, flow velocity measurements were performed to gain more insight into the hydrodynamics in the fouled membranes. MRI reveals the structure of the alginate layers and confirms the assumption obtained from the evaluation of filtration data, that the addition of Ca2+ is leading to the formation of an alginate gel layer on the membrane, whereas in the absence of Ca2+, the structure of the alginate layer is rather of concentration polarization manner, hence more fluid and hydrodynamically better controllable.

Mass transfer in biofilms is determined by diffusion. Different mostly invasive approaches have been used to measure diffusion coefficients in biofilms, however, data on heterogeneous biomass under realistic conditions is still missing. To non-invasively elucidate fluid-structure interactions in complex multispecies biofilms pulsed field gradient-nuclear magnetic resonance (PFG-NMR) was applied to measure the water diffusion in five different types of biomass aggregates: one type of sludge flocs, two types of biofilm, and two types of granules. Data analysis is an important issue when measuring heterogeneous systems and is shown to significantly influence the interpretation and understanding of water diffusion. With respect to numerical reproducibility and physico-chemical interpretation, different data processing methods were explored: (bi)-exponential data analysis and the Γ distribution model. Furthermore, the diffusion coefficient distribution in relation to relaxation was studied by D-T 2 maps obtained by 2D inverse Laplace transform (2D ILT). The results show that the effective diffusion coefficients for all biofilm samples ranged from 0.36 to 0.96 relative to that of water. NMR diffusion was linked to biofilm structure (e.g., biomass density, organic and inorganic matter) as observed by magnetic resonance imaging and to traditional biofilm parameters: diffusion was most restricted in granules with compact structures, and fast diffusion was found in heterotrophic biofilms with fluffy structures. The effective diffusion coefficients in the biomass were found to be broadly distributed because of internal biomass heterogeneities, such as gas bubbles, precipitates, and locally changing biofilm densities. Thus, estimations based on biofilm bulk properties in multispecies systems can be overestimated and mean diffusion coefficients might not be sufficiently informative to describe mass transport in biofilms and the near bulk. Refereed/Peer-reviewed