Many water treatment plants are currently facing the challenge of removing both turbidity and organic matter at a reasonable cost. This double objective often requires high coagulant doses, resulting in high operating costs due to the increase in both reagents use and sludge production. An attractive approach to reducing operating costs while maintaining the finished water quality is to add the coagulant in a double step sequence, which is referred to as double step coagulation. In this article, bench-scale experiments were conducted to assess the impact of a double step addition of coagulant on both turbidity and organic matter removal in clarification processes. The results showed that coagulant dose required for a given turbidity target can be significantly reduced. For example, for a final turbidity of 1 NTU, the coagulant dose can be reduced by 20% with respect to the Coagulant Reference Dose (CRD), which represents 200,000 Euros per year without any additional investment on infrastructure. Following these bench scale demonstrative tests, the double step coagulation strategy was adopted at full scale by the Sidi Said Maachou Water Treatment Plant in Morocco. Operated by the Société des Eaux de l'Oum Er-rbia, this plant treats 200,000 m3/day to supply the city of Casablanca. The double step coagulation was implemented to achieve a double objective: turbidity and organic matter removal at the lowest chemical cost.

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