What is flocculation?

How do aggregates form by bridging flocculation?

Bridging flocculants are usually synthetic water-soluble polymers of high molecular weight that strongly adsorb onto particles and span between them. They adsorb onto many particles to form a 3D matrix, as represented right. In general, the higher the molecular weight, the better the flocculant, although many factors affect this.

Fractal aggregate structures

		The aggregates formed by flocculation are highly porous, with this porosity having a critical impact upon aggregate growth, sediment dewatering and rheology. It is usually described through fractal geometry, thereby accounting for aggregates becoming increasingly porous as size increases.
Df 2.05	Df 2.40

The total enclosed volume of an aggregate (solid particles plus void volume) is treated as a sphere and taken to increase with the cube of a characteristic length (L) either diameter or radius. The mass (m) increases with a lower power, using the relationship:

Where D_f is the mass-length fractal dimension and can take a value between 1 and 3, the latter representing a solid sphere. Coagulated aggregates will typically give lower values of D_f (1.5 to 2), while flocculated aggregates will be greater than 1.8. The images above are computer simulations for aggregates of different D_f, and highlight how:

• At low fractal dimensions, the structures are very branched, and hence the fragility of the structures under shear is readily apparent.
• As size increases, the effective aggregate volume becomes even more porous.

Flocculant types

Natural product flocculants find applications in wastewater treatment where there is a demand for the use of environmentally friendly materials. They are biodegradable, largely shear-insensitive, give no secondary pollution, and are reproducibly obtained from agricultural sources. They include polysaccharides, tannins, chitosan, cellulose and sodium alginate. Dosages are high due to their moderate efficiency at inducing aggregation, and their shelf lives are short.

Most synthetic flocculants are based on polyacrylamide and its derivatives. Polyacrylamide (see right) itself is nonionic, although commercial products will contain up to 1% of anionic character. Higher degrees of anionic character are achieved by either partial hydrolysis of the nonionic homopolymer or copolymerisation of acrylamide with acrylic acid. The fully anionic homopolymer (sodium polyacrylate is shown right,) are used as a flocculant at very high pH, most notably in bauxite residue thickening.