Flocculation is a common operation for urban and industrial wastewaters and sludge treatment. Analysis and models are mainly devoted to elementary particles whereas the flocs, usually poorly known and little characterized, are the physical objects treated in separation devices. Operators of sewage treatment plants have to adapt daily the conditions to water/sludge variability but owing to a lack of knowledge on chemical conditioning mechanisms and flocs behavior, the adaptation remains usually crude and poorly mastered.

A better knowledge of their generation, properties and behavior under constraints in separation processes is required for works design and performances optimization. The presentation will review existing practices to form and measure intrinsic and behavioral flocs properties with standardized methods or original techniques, will describe particle agglomeration mechanisms and point out the importance of mixing conditions and their environment, will point out the necessity for models to take into considerations particles agglomeration and flocs break-up under constraints and the water release in the spaces between end inside flocs and the required multiscale approach.

The challenges for scientists and industrials are a better identification of key parameters at micro and macroscale ruling flocs formation and separation processes, a better quantification of constraints undergone by flocs in industrial machines, the adaptation of existing separation models to flocs properties, and the possibility to replace, partially or totally, chemical products by bio-sourced products to reduce the environmental impact of coagulation / flocculation.