The physical properties of suspended particles and the relationship between particle size and structure were investigated. In situ properties of the aggregates in a coagulation–flocculation process were obtained using a non-intrusive image analysis technique. Derived properties, including density, porosity and the number of primary particles in a floc, were estimated from aggregate structure using a fractal approach, which better represents the distribution of mass in an aggregate, compared with a conventional Euclidean approach which considers uniform mass distribution in an assumed spherical shape. A spherical particle assumption overlooks the highly porous nature of real aggregates and underestimates volume, which subsequently influences coagulation and settling estimates in solid–liquid separation processes. The present results illustrate a strong inverse relationship between the fractal dimension and aggregate length, consistent with the idea that larger aggregates in general are more porous. In addition, correlations between the solids content, floc density and the number of primary particles that constitute a floc of a given size were established. It is suggested that the aggregation process produces flocs of constantly changing morphology and related physical properties. Overall, these findings can provide additional information for understanding and modeling suspended particle characteristics.

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