| The principle of micro filtration and ultra filtration is physical separation. The extent to which dissolved solids, turbidity and microrganisms are removed is determined by the size of the pores in the membranes. Substances that are larger than the pores in the membranes are fully removed. Substances that are smaller than the pores of the membranes are partially removed, depending on the construction of a refuse layer on the membrane. |
Micro filtration and ultra filtration are pressure-dependent processes, which remove dissolved solids and other substances from water to a lesser extend than nano filtration and Reverse Osmosis.
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
Protection of membranes
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.
When wastewater is treated it needs to answer certain demands. It depends on these demands whether or not permeates need additional treatment after microfiltration or ultrafiltration.
Purification techniques that are eligible for additional treatment are:
· Disinfection techniques
· Quick filtration
· Active carbon filtration
Nano filtration and Reverse Osmosis (RO)