Imaging of particle deposition and resulting flow field during flocculation filtration within a granulated activated carbon filter

Abstract

Flocculation filtration onto granulated activated carbon (GAC) filters is an advanced treatment method to achieve lower phosphorous and micropollutant discharge in wastewater treatment plants. However, the suitability is put into question as the deposition of suspended solids (SS) onto a GAC filter is aligned with more frequent backwashing ongoing with abrasion of GAC and downtimes. To evaluate the filtration process of SS onto GAC filters on the pore scale, we resolved the particle deposition in a timely and spatially resolved image series by means of magnetic resonance imaging (MRI). Besides structural images, flow measurements were acquired to evaluate the impact of SS accumulation onto the local flow field. The column had an inner surface area of 1.54 cm2 and was filled with GAC up to 10 cm in height. In total, 6.4 L of wastewater originating from a secondary clarifier were added to the GAC filter in steps of 0.2 L. The wastewater contained approximately 8.4 mg/L TSS after flocculation and precipitation. Results of one experimental run are presented. Particle accumulations within the GAC filter as well as a formation of a deposit layer was found. During the filtration process preferential inflow points into the GAC filter were recognizable. The transition of a preferential inflow point to another was often aligned with the redistribution of upper parts of the GAC filter and a compaction of accumulated SS. Analysis of flow distributions revealed that the equivalent filter velocity of 28.8 m/h is 2.3 times higher compared to the average filter velocity (12.5 m/h including porosity). Further, the equivalent filter velocity increased up to 100.8 m/h at the end of the experiment. As particles do not distribute equally over the filter height, those velocities are not representative for the whole height of GAC filters. The current study provides insights into a real flocculation filtration process without using artificial materials for imaging purposes.