Image analysis of sludge aggregates

Abstract This study investigates the hypothesis that the mechanisms of chemical coagulation–flocculation and electrocoagulation produce specific imprints in nature, and the structure of sludge aggregates-floccules (flocs). Scanning electron microscopy (SEM) methods were used to produce 480 images of 32 types of wastewater sludge. The analyzed sludge was obtained by chemical coagulation and electrocoagulation of four types of synthetic effluents containing four red dyes. Perimeter P and area A of 120–170 differently sized objects were determined in 256 selected and contrast-enhanced images with the use of Image Analysis software. lg A  ∼  lg P plots revealed that the analyzed sludge samples were made of self-similar aggregates-flocs with fractal characteristics. The slope s of log plots was used to determine surface fractal dimension D a which was extrapolated to volumetric fractal dimension D v . Dimension D v was applied in a quantitative description of sludge aggregates-flocs. Aggregates-flocs of Al sludge group were characterized by higher values of D v in comparison with Fe sludge group, which quantitatively confirmed the compactness of Al sludges and the jagged character of Fe sludges observed in SEM analyses. No flocculation of dye alone was observed, but the results of the experiment showed that phosphates were required for the destabilization of the colloidal system containing dye. In view of the above, a simple model of P-PO 4 and dye sorption on a colloidal sorbent made of {Al(OH) 3 } or {Fe(OH) 3 } was proposed. The structure of {Al(OH) 3 } and {Fe(OH) 3 } aggregate-flocs was graphically simulated to determine the effect of volumetric fractal dimension D v on sweep flocculation and sludge separation and dehydration. The modeled processes of P-PO 4 sorption and sweep flocculation of dyes and the simulated images of aggregate-flocs confirmed that the analyzed wastewater sludges were mainly formed in the process of diffusion limited cluster–cluster aggregation DLCA, even the particle-cluster type aggregation with pre-polymerized coagulants (PAC) could be involved.

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