Monitoring settling of anaerobic digestates using low-field MRI profiling and NMR relaxometry measurements

Dewatering of anaerobic digestate from red meat processing was assessed using low field MRI profiling and NMR relaxometry. Samples were flocculated using a cationic flocculant (EM640CT) at dosing range (0 to 1.6% v/v) and monitored during the initial 30 min of settling via MRI profiling to assess changes in water fraction, settling time and initial settling velocity. The profiles showed decreasing settling time and increasing initial settling velocity with increased dosing, while sample porosity was observed to increase up to the optimal dosing point (0.8% v/v). Significant increases in sample variability were observed past this point due to flocculant overdosing. The samples were then analysed in terms of turbidity and NMR relaxometry. Increasing flocculant concentration caused turbidity to decrease from 210 to 13 NTU. The relaxation rate of free water showed a strong positive correlation with turbidity. T2 peaks observed before overdosing could be assigned to different water structures (free, interstitial, vicinal and hydration). An additional T2 population emerged in the T2 distributions at the optimal dosing point. Multivariate exploratory data analysis (MEDA) showed that this T2 population from the solids layer was strongly correlated with the total solids layer height and turbidity of the watery layer. This T2 peak formation may therefore be used to study opaque flocculated solids to monitor for water structures associated with flocculant overdosing. Further studies using this technique will aim to assess the potential of low field T2 relaxometry monitoring inline before mechanical dewatering, to monitor optimal flocculant dosing during continuous operations on systems with high solids concentration.