Magnetic resonance imaging (MRI) as non-invasive approach for quantifying the transport of particulate organic matter within a bed of settled aerobic granules

Abstract

<p>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©.</p> <p>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.</p> <p>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:</p> <p>Dextran coated super paramagnetic iron oxide nanoparticles (SPIONs, mean diameter d<sub>mean</sub> = 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.</p> <p>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).</p> <p><strong>