Samples of rag layer were collected using multiple bench-top, batch centrifugations with optical microslides inserted to a centrifuge cell in the last spinning. Image analysis of the samples provided data for approximate sedimentation rate analysis, based on fractal floc dimension and Smoluchowski coagulation kinetic equation. The analysis shows that, if a flocculating chemical is used as a demulsifier, overdosing will necessarily result in deterioration of the process performance. Optimization of the chemical dosage and accurate process control are thus necessary conditions to attain satisfactory dewatering process performance