Optimisation of the coagulation–flocculation process of raw water by optimal design method

In the present paper, a systematic study of the influence of raw water quality and operating conditions on the effectiveness of the coagulation–flocculation process using aluminium sulphate (AS) is presented. The influence of raw water composition is studied at two levels, fractional factorial designs and a Doehlert uniform shell design to determine the influence of the operating conditions. The analysis of the experimental responses (residual turbidity and minimally AS dose are denoted as (Tum) and (ASm), respectively) shows that the parameters which have the greatest influence change depending on the response. It is not always possible, for a given water composition, to obtain simultaneously low residual turbidity and minimally AS dose. However, under the conditions of this study, the search for a criterion of optimisation must bear the minimisation of residual turbidity, because it remains the principal criterion of effectiveness of the process, even, if in a few cases, it results in an increase in the cost of treatment.