Membrane technology has become aimportant utility in separation technology over the past few decades. The main force of membrane technology is the fact that it works without the addition of chemicals, with a relatively low energy use and easy and well-arranged process conductions. Membrane technology is a generic term for a number of different, very characteristic separation processes. These processes are of the same kind, because in each of them a membrane is used.
Membranes are used more and more often for the creation of process water from groundwater, surface water or wastewater. Membranes are now competitive for conventional techniques. The membraneseparation process is based on the presence of semi permeable membranes.The principle is quite simple: the membrane acts as a very specific filter that will let water flow through, while it catches suspended solids and other substances.There are various methods to enable substances to penetrate a membrane. Examples of these methods are the applications of high pressure, the maintenance of a concentration gradient on both sides of the membrane and the introduction of an electric potential.Membrane filtration can be divided up between micro and ultra filtration on the one hand and nano filtration and Reverse Osmosis (RO or hyper filtration) on the other hand.Membrane filtration has a number of benefits over the existing water purification techniques.
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).
- he textile industry.
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.
Reverse Osmosis (RO)
Reverse Osmosis is based upon the fundamental pursuit for balance. Two fluids containing different concentrations of dissolved solids that come in contact with each other will mix until the concentration is uniform. When these two fluids are separated by a semi permeable membrane (which lets the fluid flow through, while dissolved solids stay behind), a fluid containing a lower concentration will move through the membrane into the fluids containing a higher concentration of dissolved solids. (Binnie e.a., 2002)
After a while the water level will be higher on one side of the membrane. The difference in height is called the osmotic pressure.
By pursuing pressure upon the fluid column, which exceeds the osmotic pressure, one will get a reversed effect. Fluids are pressed back through the membrane, while dissolved solids stay behind in the column.
Using this technique, a larger part the salt content of the water can be removed.
Reverse Osmosis is a technique that is mainly applied during drinking water preparation. The process of drinking water preparation from salty seawater is commonly known. Besides that, Reverse Osmosis is applied for the production of ultra pure water and boiler feed water. It is also applied in the food sector (concentration of fruit juice, sugar and coffee), in the galvanic industry (concentration of wastewater) and in the dairy industry (concentration of milk for cheese production).
Sea Water Desalination
Spiral Wound Seawater Reverse Osmosis modules The most common RO membrane used in desalination are spiral wound Thin Film Composite. They consist in a flat sheet sealed like an envelope and wound in a spiral.
There are 3 typical membrane diameters : 2.5″, 4″ and 8″. Seawater RO membranes have a maximum permeate flowrate ranging from 1.4 to 37.9 m3/d, therefore many membranes are often required to meet the plant permeate production requirements.
The membranes are enclosed in series in pressure vessels and the number of membrane element per pressure vessels can vary from 1 to 8.
Pressure vessels are then arranged in parallel to satisfy the membranes flow and pressure specifications as well as the plant production requirements.
The total number of membranes and pressure vessels required and their arrangement (the array) depends on permeate flow and applied pressure, i.e., salinity and temperature.
TRL offers turn key membrane system along with pre-treatment and post treatment along with required dosing system.