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    Nano filtration and reverse osmosis

Reverse osmosis (RO) and nano-filtration (NF) are applied for removal of dissolved substances and contaminants from water.

Both technologies are based on the principle of reverse osmosis. Reverse osmosis has a high removal yield for all dissolved substances and is therefore exceptionally well-suited for desalinization and demineralization for drinking water, boiler feed water and process water production. Nano-filtration has a lower removal efficiency for monovalent ions and is therefore ideally suited for softening, pesticide removal and removal of dissolved organic substances (DOC Dissolved Organic Carbon).

Application of reverse osmosis and nano-filtration technology offers the following advantages:

  • Extensive removal of dissolved substances
  • Minimal use of chemicals relative to ion exchange
  • Continuous production is possible, instead of batch-wise
  • Minimal oversight and maintenance
  • Simple, compact and modular construction, resulting in high flexibility

Operation nano filtration and reverse osmosis

Reverse osmosis is a pressure-driven process that makes use of a semi-permeable membrane. The semi-permeable membrane functions as a barrier for the dissolved particles. A semi-permeable membrane is an industrial copy of the natural cell membrane.

Osmosis is the flowing of water through a semi-permeable membrane from a less concentrated solution to a more concentrated solution. It is a natural process based on the search for a balance. The water continues to move until an identical concentration is reached on both sides of the semi-permeable membrane.

The pressure that has to be applied to the concentrated solution in order to counteract this natural water movement is called the osmotic pressure of the solution.

With reverse osmosis, a pressure greater than the osmotic pressure is imposed on the concentrated solution. This causes the water to move from the concentrated solution, through the semi-permeable membrane, to the less concentrated solution. Because the dissolved substances do not pass through the membrane, the concentrated solution becomes more strongly concentrated, and the less concentrated solution becomes diluted. The concentrated water is called the concentrate; the water pressed through the membrane is called the permeate.

In contrast to microfiltration and ultrafiltration, nano-filtration and reverse osmosis membranes are not porous. The stopping or passing of substances here is not only dependent on the particle size, but to a large degree on the affinity that the semi-permeable membrane has with the particle. The membranes are therefore not an absolute barrier.

Membranes are made of polymers, so of plastic. They are just flat, thin sheets. In order to create sufficient membrane surface, most membrane elements are ‘spiral wound’. The flat membrane is glued on three sides to a non-penetrable sheet (like an envelope) and then wound tightly around a tube. The open side of the envelope is glued to the tube lengthwise, over openings. The entire thing is placed into a tube. The water flows through the tube, along the rolled-up membrane, whereby a large part will be pressed through the membrane and will enter the permeate tube through envelope. The rest leaves the tube at the other side as concentrate. There are multiple membranes in a tube and multiple tubes in an installation.

Experience RWB

RWB has a great deal of experience with the designing and realization of reverse osmosis and nano-filtration installations. The design of an installation will be largely determined by the kind of supply water. RWB has experience with applying pre-treated surface water, drinking water, various kinds of process water and with effluent from a biological purification with MBR technology.

Next to customer-specific installations RWB also has a standardized Culligan® line.

The conducting of pilot tests is one of the options, next to process simulation, to come up with an optimal design for your application.

More information about nano filtration and reverse osmosis?

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