Nano filtration and Reverse Osmosis

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

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.

1 2 3

1. Water flows from a column with a low dissolved solids content to a column with a high dissolved solids content
2. Osmotic pressure is the pressure that is used to stop the water from flowing through the membrane, in order to create balance
3. By pursuing pressure that exceeds the osmotic pressure, the water flow will be reversed; water flows from the column with a high dissolved solids content to the column with a low dissolved solids content

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).

Summarized, the applications of Reverse Osmosis application are:
· Water softening
· Drinking water production
· Process water production
· Ultra pure water production (electronic industries)
· Concentration of molecular solvents for food and dairy industries

The pre-treatment of feed water for nano filtration or Reverse Osmosis installations greatly influences the performance of the installation. The required form of pre-treatment depends on the feed water quality. The purpose of pre-treatment is reducing the organic matter content and the amount of bacteria, as well as lowering the MFI.

The organic matter content and the amounts of bacteria should be as low as possible to prevent the so-called biofouling of membranes. The application of a pre-treatment has several benefits:
· Membranes have a longer life-span when pre-treatment is performed
· The production time of the installation is extended
· The management tasks become simpler
· The employment costs are lower

Next to pre-treatment one can perform a chemical dosage (acid, anti-scalent), to prevent scaling and precipitation of insoluble solids, such as calcium carbonate and barium sulphate on the membrane surface.
The applied acids are hydrochloric acid (HCl) and sulphuric acid (H₂SO₄).
Sulphuric acid is the most widely used chemical for this purpose. However, hydrochloric acid is applied more and more because sulphuric acid can negatively influence the fouling speed of a membrane. When the feed water contains high amounts of sulphate ions, hydrochloric acid replaces sulphuric acid. The dosage of sulphuric acid would enhance the chances of scaling by sulphate ions on the membranes in this case. (Baker, 2000)

This is an overview of the main pre-treatment techniques and the substances, which are reduced during these processes.

Besides the above-mentioned methods there are also other ways to purify the feed water, such as disinfection and active-carbon filtration.

Microfiltration and ultrafilration

Reverse Osmosis membranes

Reverse Osmosis chemicals

Membrane technology