The water loss in packed beds was studied spatially and time‐resolved via magnetic resonance imaging on the pore scale. The packed beds were measured under water‐saturated and unsaturated conditions, while the magnetic resonance method allowed a quantitative differentiation between air, liquid, and solid phases exploring the native contrast of the named phases without additional contrast agents. Beside a qualitative image comparison, subsequent quantitative image processing allowed for a detailed spatially resolved determination of water distribution, the differentiation between water transport processes, and the quantification of liquid clusters in 3‐D. Results are presented for two packed beds that show significant differences in their evaporation and drainage dynamics, which are mainly determined by the physical properties of the packed beds. The water loss of the packed bed of 2–4 mm quartz particles reached a level below interpretability after 18.2 hr; meanwhile, a successive decrease of the largest liquid cluster volume from 82.5 to 0.7 mm³was observed. The water content of the packed bed of 2 mm glass spheres was still observable after 70.9 hr. During the experiment, no significant changes in the structure of the liquid clusters were measured. The current work displays the applicability of magnetic resonance imaging for pore‐scale investigations without the addition of contrast agents.

The removal or degradation of particulate organic matter is a crucial part in biological wastewater treatment. This is even more valid with respect to aerobic granular sludge and the impact of particulate organic matter on the formation and stability of the entire granulation process. Before the organic part of the particulate matter can be hydrolyzed and finally degraded by the microorganism, the particles have to be transported towards and retained within the granulated biomass. The understanding of these processes is currently very limited. Thus, the present study aimed at visualizing the transport of particulate organic matter into and through an aerobic granular sludge bed. Magnetic Resonance Imaging (MRI) was successfully applied to resolve the different fractions of a granular sludge bed over time and space. Quantification and merging of 3D data sets allowed for a clear determination of the particle distribution within the granular sludge bed. Dextran coated super paramagnetic iron oxide nanoparticles (SPIONs, d p = 38 ± 10 nm) served as model particles for colloidal particles. Microcrystalline cellulose particles ( d p = 1-20 μm) tagged with paramagnetic iron oxide were applied as a reference for toilet paper, which is a major fraction of particulate matter in domestic wastewater. The results were supplemented by the use of real wastewater particles with a size fraction between 28 and 100 μm. Colloidal SPIONs distributed evenly over the granular sludge bed penetrating the granules up to 300 μm. Rinsing the granular sludge bed proved their immobilization. Microcrystalline cellulose and real wastewater particles in the micrometer range accumulated in the void space between settled granules. An almost full retention of the wastewater particles was observed within the first 20 mm of the granular sludge bed. Moreover, the formation of particle layers indicates that most of the micrometer-sized particles are not attached to the biomass and remain mobile. Consequently, these particles are released into the bulk phase when the granulated sludge bed is resuspended.

Microorganisms settle in diverse partially saturated porous media in the form of biofilms. The alteration of hydraulic properties and diffusive transport processes occurs simultaneously with biofilm growth in porous media. Imaging methods offer the ability to directly visualize and quantify alterations on the pore scale. However, imaging methods have mainly observed biofilm growth in completely saturated porous media. The current study used magnetic resonance imaging (MRI) to dynamically visualize biofilm growth within a porous medium under alternating drainage and flushing events. Prior to the MRI experiments, the sample was cultivated for 6 days within a porous medium consisting of 2 mm glass spheres. Starting from day 6, growth was monitored using MRI over a period of 7 days. The approach allowed for a visualization of all fractions (biofilm, water, air, and porous material) after drainage as well as flushing events. Biofilm was found to preferentially grow within permanently wetted areas situated next to pore throats. Furthermore, an increase in the water retention and connectivity of the liquid phase was found. The largest liquid cluster covered 11% (day 6) and 91% (day 12) of the total retained water, suggesting that biofilm growth might improve diffusive transport processes within partially saturated porous media.

Aerobic granular sludge is one of the most promising technologies in wastewater treatment of the past decades. By now, around 50 full scale plant have been set up at full scale under the trademark Nereda©. With approx. 50 % of the chemical oxygen demand (COD) particulate organic matter is a crucial fraction of municipal wastewater. Theories about its degradation in granular sludge reactors typically start with the assumption that particulate organic matter is adsorbed at the granule surface after the feeding phase (De Kreuk et al. 2010, Pronk et al. 2015). Despite of the ideal case, unattached particles after the feeding phase would be available for degradation under aerobic conditions or could be washed out with the effluent. To extent the knowledge about the degradation process, the present study aimed at visualizing the transport and fate of particulate organic matter into and through a bed of granular sludge. The main perspectives are to directly show their distribution and retention mechanism inside a granular sludge bed. Magnetic Resonance Imaging (MRI) was successfully applied to visualize the different fractions of a granular sludge bed resolved in time and space (x, y, z). According to particle size, three particle consortia have been chosen to represent municipal wastewater: Dextran coated super paramagnetic iron oxide nanoparticles (SPIONs, mean diameter dmean = 20 nm) served as model particles for colloids. As a reference for toilet paper, paramagnetically tagged microcrystalline cellulose with a size fraction between 1 and 20 µm was used. The results are supplemented by the use of real wastewater particles with a size fraction between 28 and 100 µm. No paramagnetic tagging was applied in the latter case. The retention mechanism is found to be size dependent. Colloidal particles are able to attach and penetrate the granules. Therefore, they will constantly release substrate during their degradation inside the granule. In contrast, larger particles accumulate within the void space between the granules. Moreover, the formation of particle layers indicates that most of the particles are not attached to the biomass and remain mobile after an initial feeding phase. Thus, they remain available under aerobic conditions and might be partially washed out with the effluent if no attachment is taking place in the aerobic mixing phase (Ranzinger et al. 2020). References: De Kreuk, M., Kishida, N., Tsuneda, S. and Van Loosdrecht, M. (2010) Behavior of polymeric substrates in an aerobic granular sludge system. Water research 44(20), 5929-5938. Pronk, M., Abbas, B., Al-Zuhairy, S., Kraan, R., Kleerebezem, R. and Van Loosdrecht, M. (2015) Effect and behaviour of different substrates in relation to the formation of aerobic granular sludge. Applied microbiology and biotechnology 99(12), 5257-5268. Ranzinger, F., Matern, M., Layer, M., Guthausen, G., Wagner, M., Derlon, N. and Horn, H. (2020) Transport and retention of artificial and real wastewater particles inside a bed of settled aerobic granular sludge assessed applying magnetic resonance imaging. Water Research X, 100050.