Membrane Processes

Photo: Membrane Processes

Factors such as a growing population, decreasing supplies of surface water, and the price of potable water consumption, represent the reasons for the increasing need to optimize existing water preparation procedures with regard to their economy. For the past 20 years, membrane processes have experienced rapid development and today they are increasingly used in the water preparation sphere, where they are replacing the technologies that have been used thus far.

Principle

The purpose of using membranes in the preparation process is to remove pollutants from raw water and to obtain pure water.

The membrane must be permeable for the solvent, but not for the dissolved substances. The osmotic pressure of the system corresponds to the stabilized difference of the levels on both sides of the membrane. If the pressure on the side of the dissolved substance is higher than the osmotic pressure, the osmotic phenomenon will be reversed and certain components of the solvent will pass through the membrane while others will be concentrated before it.

This means that after passing through the membrane module, the incoming flow provides two flows - pure water, i.e., permeate, and wastewater containing substances that cannot get through the membrane, i.e., concentrate.

The driving force of the entire process is generally pressure (reverse osmosis, nano-filtration), but it can also be the difference of concentrations (dialysis), vapour pressure (pervaporation, membrane distillation), or electric potential (electro-dialysis). Based on the membrane pore size, the processes can be classified into:

Method

Pore size

Retains

Replaced technology

Ultra-filtration

3 – 50 nm

Macromolecules, organic substances (viruses, bacteria), colloid substances

Sand filtration, coagulation, sedimentation

Nano-filtration

1 – 3 nm

Multivalent ions, dissolved organic substances (pesticides, secondary products of disinfection)

Standard water softening methods (ion exchangers, lime-soda)

Reverse osmosis

0,1 – 1 nm

Univalent ions, all dissolved substances

Distillation

The field of prospective applications for membrane technologies is continuously expanding with the increasing requirements for the availability of potable water for an ever growing population. All over the world, the impairment of the quality of surface and ground water is registered due to the influence of industrialization, farming production, and human activities in general. It is true that desalination technologies of sea water have been available, but their costliness has limited their use in practice. With the onset of membrane technologies and their affordability, it has become economically advantageous to prepare potable water from sea water with the use of membranes.

Application
- Mobile water preparation plants
- Sea water desalination
- Potable water desalination (softening, removal of colloid substances and bacteria…)
- Elimination of heavy metals and dangerous substances from water

Advantages - Lower demands for the built-up area than with conventional technologies
- Minimum operational requirements
- Low consumption of chemicals