In this experimental study the clarification process of the drinking water treatment plant (WTP) of Florence (Italy) has been evaluated. At present, the most common way to optimise the clariflocculation process (best type and dosage of coagulants and mixing conditions) is the jar-test procedure which can give information about the final turbidity, and consequently the process efficiency, after a settling period in a batch procedure at laboratory-scale. An alternative method with a Photometric Dispersion Analyser (PDA2000) was recently introduced at the WTP to provide quick and continuous information (flocculation index, correlated with the flocs size) about the aggregation state of particles during a modified jar-test procedure. The PDA2000 was applied to a real suspension (namely Arno river water) providing useful data for the determination of best type and optimum dosage of coagulants. Furthermore a strong correlation between the removal efficiency of the turbidity and PDA a parameter derived from the PDA 2000 data (defined as the rate of the square root of the flocs size index and the maximum slope of the growing curve) was observed.

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