Membranes with a pore size of 0.1 – 10 µm perform micro filtration. Microfiltration membranes remove all bacteria. Only part of the viral contamination is caught up in the process, even though viruses are smaller than the pores of a micro filtration membrane. This is because viruses can attach themselves to bacterial biofilm.
Micro filtration can be implemented in many different water treatment processes when particles with a diameter greater than 0.1 mm need to be removed from a liquid.
Examples of micro filtration applications are:
· Cold sterilisation of beverages and pharmaceuticals
· Clearing of fruit juice, wines and beer
· Separation of bacteria from water (biological wastewater treatment)
· Effluent treatment
· Separation of oil/ water emulsions
· Pre-treatment of water for nano filtration or Reverse Osmosis
· Solid-liquid separation for pharmacies or food industries
For complete removal of viruses, ultra filtration is required. The pores of ultra filtration membranes can remove particles of 0.001 – 0.1 µm from fluids.
Examples of fields where ultra filtration is applied are:
· The dairy industry (milk, cheese)
· The food industry (proteins)
· The metal industry (oil/ water emulsions separation, paint treatment)
· The textile industry
Ultra filtration can also be applied for pre-treatment of water for nano filtration or Reverse Osmosis.
Pre-treatment of water is very important when these filtration techniques are applied, because membrane fouling can easily disturb the purification process. Pre-treatment is not only important for nano filtration and Reverse Osmosis processes, but also for the above-mentioned microfiltration and ultra filtration processes. A pre-treatment needs to be determined as soon as the composition of the wastewater is known.
To prevent plugging or damaging of membranes by hard and sharp particles from the feed water, water needs to be pre-filtered before micro filtration or ultra filtration processes take place. The pores of the pre-filtration unit need to be between 0.5 and 1.0 mm, depending on the composition of the wastewater. Further pre-treatment will not be necessary when micro filtration or ultra filtration is performed.
Nano filtration
Nano filtration is a technique that has prospered over the past few years. Today, nano filtration is mainly applied in drinking water purification process steps, such as water softening, decolouring and micro pollutant removal.
During industrial processes nano filtration is applied for the removal of specific components, such as colouring agents.
Nano filtration is a pressure related process, during which separation takes place, based on molecule size. Membranes bring about the separation. The technique is mainly applied for the removal of organic substances, such as micro pollutants and multivalent ions. Nano filtration membranes have a moderate retention for univalent salts.
Other applications of nano filtration are:
· The removal of pesticides from groundwater
· The removal of heavy metals from wastewater
· Wastewater recycling in laundries
· Water softening
· Nitrates removal