Dispersion in water and wastewater treatment refers to the distribution of fine particles in a liquid in which the particles remain evenly suspended in the medium without combining or separating. Dispersion processes are essential for many applications in industrial water technology and wastewater treatment, as they improve the stability and homogeneity of particle distributions and prevent the accumulation of solids. Stable dispersion is required to prevent deposits and fouling in pipes, reactors and on membrane surfaces.

In industrial practice, the formation and stabilization of a dispersion is particularly important in cooling circuits, boiler water treatmentsas well as in dye works and the paper industry, where various particles such as minerals, pigments or fibers have to be dispersed. Dispersing agents support the homogeneity of the dispersion and extend the service life of the systems.

Technical background to dispersion

A dispersion occurs when solids, liquids or gases are distributed in another medium and form a suspended phase there. The fine particles form the dispersed phase, while the water or another liquid forms the continuous dispersion medium.

The stability of a dispersion is influenced by several factors:

• Particle size: Small particles with a large surface area remain in suspension more easily, as the buoyancy forces of the liquid are sufficient to separate the particles.
• Surface charge of the particles: Electrically charged particles are less inclined to join together, as particles with the same charge repel each other.
• Viscosity of the medium: A higher viscosity of the medium helps the particles to remain in suspension for longer and to distribute themselves evenly.
• Dispersing agents: Chemical dispersing agents are added to reduce the surface tension of the particles and create stable, long-term dispersion systems.

Types of dispersions in industrial water technology

1. Colloidal dispersions:

   • Colloids are extremely fine particles that generally do not sediment but remain in suspension due to Brownian motion. These particles can be both hydrophilic and hydrophobic and pose a challenge in water treatment, as they often cannot be removed by simple sedimentation. They are usually stabilized by coagulation and flocculation processes.

2. Suspensions:

   • These are coarser particles that are kept in suspension by agitation or other mechanical action. Suspensions are often found in the treatment of sludge, for example in sludge conditioning and solids removal in wastewater treatment.

3. Emulsions:

   • Emulsions are dispersions in which liquids (often oil and water) are finely dispersed. Emulsions are generally undesirable in water treatment and are removed by special emulsion splitting and precipitation.

Use of dispersion processes in practice

In water and wastewater treatment, dispersion processes are often used to prevent deposits and to optimize the treatment steps:

1. Cooling water treatment:

   • Dispersed solids such as salts and mineral deposits in the cooling water can reduce the efficiency of the system. Dispersants that keep the deposits stable and in suspension prevent the formation of limescale and other solid deposits on the heat exchanger surfaces. This stable dispersion reduces fouling and ensures optimum heat transfer.

2. Boiler water treatment:

   • High temperatures in boilers and heating systems cause calcium and magnesium salts to form, which can lead to boiler scale. Dispersing these deposits and stabilizing them with dispersants prevents sedimentation and extends the service life of the heating surfaces.

3. Membrane systems and reverse osmosis:

   • In membrane systems, fine particles and colloidal substances often lead to fouling, which impairs the operation of the membrane. By dispersing the solids, the membranes can be protected from deposits. This increases the service life and reduces the need for chemical cleaning.

4. paper industry:

   • In paper production, solids and fillers are often kept in a stable dispersion. Dispersing agents prevent these substances from depositing on machine components, which improves production efficiency and the quality of the end product.

5. Textile and dyeing industry:

   • Here, colors and pigments are kept in a stable dispersion to ensure a uniform coloring result. Dispersing agents ensure that the color particles remain evenly distributed and are not deposited on machine surfaces or in pipes.