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Membrane filtration

 
Membrane filtration is a physical separation process in which water flows through a semi-permeable membrane due to a pressure difference and is thus purified of dissolved substances.

For porous membranes, the following relationship applies:

Retention of a substance if: substance size > pore diameter

 

Filtration process

The process is essentially based on the filtration of water through a membrane of a certain pore size. This enables constant filtration values even with changing feed conditions. Diffusion and electrostatic repulsion forces, especially with small pore diameters, also contribute to the filtration result. In this case, water is able to pass through the membrane, but substances are retained depending on their size and composition. The effect of concentration polarisation, which is often referred to in science, must also be taken into account in water treatment with the help of membranes.

Diagram of filtration process:

Filtrationsprozess in der Membranenfiltration

Effect of concentration polarisation

Due to the accumulation of substances, a laminar boundary layer forms at the surface of the membrane, whereby the particle concentration at this point is significantly higher than in the turbulent part of the inflow. As shown in the diagram, the concentration of particles Cp of the boundary layer increases from Cp,0 of the turbulent part of the inflow to a maximum value Cp,max at the membrane surface. Over time, this creates an ever-increasing resistance to the passage of further particles. An increase in pressure on the inflow side increases the water passage through the membrane (yield), but this also promotes and accelerates the further formation of a boundary layer and the associated resistance.

To counteract this effect, Chriwa Wasseraufbereitungstechnik GmbH offers a suitable cleaning and anti-scaling system for each membrane filtration.

Diagram of concentration polarisation:

Filtrationsprozess in der Membranenfiltration

A comparison of the Chriwa membrane processes

Depending on the pore sizes or the retention of certain substances, a distinction is made between micro-, ultra- and nanofiltration, as well as reverse osmosis. Which process is specifically required for your process can be quickly determined by us in a consultation and by analysing the raw water parameters.

Overview of membrane processes:

Mikrofiltration

Microfiltration

Microfiltration is mostly used in the food and pharmaceutical industries. Other possible applications include the treatment of waste water and use as a purification stage upstream of a reverse osmosis system.

Among others, the following substances can be removed from the water:

  • Zooplankton
  • Algae
  • Bacteria
  • Suspended particles

The filtration process is essentially based on the permeability of a porous membrane. Larger particles form a filter cake on the surface (surface filtration), while smaller particles penetrate into the pores of the membrane and are retained there by adsorption or electrostatic attraction (depth filtration).

By using a Chriwa microfiltration system, the water is not demineralised. This means that salts remain in the water and are not retained by the membrane.

Ultrafiltration

Ultrafiltration

Ultrafiltration is mostly used in the treatment of drinking water and in the beverage industry.

Among others, the following substances can be removed from the water:

  • Substances that can be removed from the water by microfiltration are
  • Macromolecules
  • Viruses
  • Colloidally dissolved solids

The filtration process is essentially based on the permeability of a porous membrane. Larger particles form a filter cake on the surface (surface filtration), while smaller particles penetrate into the pores of the membrane and are retained there by adsorption or electrostatic attraction (depth filtration). In this way, the best filtration results can be achieved with minimal pressure loss, which also means that the total pressure in the system can be kept low.

By using a Chriwa ultrafiltration system, the water is not demineralised. This means that salts remain in the water and are not retained by the membrane.

Nanofiltration

Nanofiltration

Nanofiltration is mostly used in the chemical, pharmaceutical, food or textile industries. Another field of application is the treatment of waste water.

Among others, the following substances can be removed from the water:

    Substances that can be removed from the water by microfiltration and ultrafiltration
    Organic substances
    Multi-charged larger ions (bicarbonate reduction etc.)

The filtration process is mainly based on electrostatic attraction forces and the diffusion of particles through the non-porous membrane.

Umkehrosmose

Reverse osmosis

Reverse osmosis is mostly used in the production of low-salt product water and boiler water treatment, brackish and seawater treatment, as well as in water treatment for the beverage industry.

Among others, the following substances can be removed from the water:

  • Substances that can be removed from water by micro-, ultra- and nanofiltration
  • Multi-charged ions
  • Single-charged ions

The filtration process is essentially based on the diffusion of the particles through the non-porous membrane. Remineralisation is possible with specially selected salts, or through raw water blending.