Water Filtration: De-Ionization vs. Distillation

Define "Better".

Incomplete question.

And, provide more information concerning your application. What kind of volumes are you considering? And, what kind of production rates? If this application is something you want to do in a laboratory for a small-scale, one-time application, it may be simplest and quickest to go with distillation. A more robust commercial application might be better served by Reverse Osmosis but the quality of water needed (for semiconductor manufacture? biopharma processing? etc.) will also affect your choice. Also, do not rule out a deionizing system that does not involve reverse osmosis.

The answer is dependent on the power source available and economics.

If one has shed-loads of low-grade heat available, like the cooling circuit on a rather large engine as might be found on a ship for example, then low-pressure distillation, sometimes also called evaporation, might prove an attractive option. The pretreatment requirements for evaporation are minimal, and producing a water of, say 5ppm total dissolved solids [TDS] is relatively easy. Evaporation equipment is often constructed with significant quantities of exotic materials, like duplex stainless steel, copper-nickel and nickel-aluminium alloys. It is very tolerant of changing feed composition and temperature, and is easy on maintenance; a blanked-off tube in an evaporator tube bundle may be all that is needed to return the equipment to working order at a very high percentage of its original performance requirement. Many navies have specified evaporation equipment for drinking water production.

If one has relatively cheap electricity available, then reverse osmosis [RO] might prove more attractive for drinking water production. The performance of a RO plant is highly dependent on salinity and temperature, and requires careful pre-treatment to ensure the longevity of the RO membranes. The TDS of the permeate will vary somewhat. RO is relatively insensitive to the pitching and rolling of a ship on the high seas. Often, to cater for maintenance activities, a 2 x 100% duty/standby, 3 x 50% duty/duty/standby, or some other redundancy arrangement might be employed. RO plants can be supplied fitted, and existing ones retro-fitted, with energy recovery devices whereby the reduction in pressure of the concentrate leaving the plant can be carried out in some form of pressure-exchanger or turbine to 'help' the main pump produce the required feed pressure at a fraction of the power demand of a plant not so fitted.

Municipal RO plants are encountered on islands around the Mediterranean Sea for the production of drinking water for the local population; the one in Larnaca in Cyprus is a good example, and there are photographs of it on the internet.

Most RO permeate and evaporator condensate intended for drinking may have to go through a 're-hardening' process, where CaCO₃ is leached into the water afterwards, partly to protect the downstream pipework from corrosion and partly to improve palatability; there is no contest between the two approaches at this point.

RO is commonly employed as part of the process train in the production of technical grades, medical grades and ultrapure grades of water. In ultrapure production there comes a point in the purity where adding on successive stages of RO starts to become uneconomic in comparison with mixed bed ion-exchange polishing for instance, some units of which can be installed on a 'fit-and-forget' basis at negligible power cost; two plants producing water for the washing of semiconductor wafers in Scotland and another in Singapore doing it this way come to mind.

RO requires high consumption of electricity to drive the high pressure pumps. It also requires pre-softening to reduce wear and tear on the membranes. It is usually used as a first stage in preparation of ultrapure water. It is also used to reprocess treated domestic effluent to form potable water in California and Singapore. It's main advantage is that it can process a large volume of water in a relatively short time and is relatively compact.

Distillation requires a great deal of heat energy due to the very high SHC of water. It also requires a fairly large amount of space for the boiler and condensor. It is also quite slow. However, it can make use of waste heat, including using the heat energy being released by the condensing steam, to pre-heat the water, something RO cannot. Time can also be saved by boiling the water at low pressure and condensing it at high pressure. Unlike RO, it also doesn't require much pre-treatment and requires less maintenance overall.

Which process is better depends upon weighing all these different factors and deciding which factors are more important.

Hello,

I thank you all for your answers. I was expecting to know about the economical aspects, amount of water used/wasted during filtration, ease of construction, maintenability, etc. the various applications could be large scale of drinkable water. Its suitability for use in the cosmetic industry,etc..

I would welcome your further comments.

Regards

Wendell

Now that you have provided some additional data, some more "to-the-point" (and varied) responses are likely to occur. The "various applications" to which you refer will each be BEST served by a DIFFERENT quality of water.

For starters, you would NEVER want to provide anyone with deionized water for drinking purposes. In a relatively short time, the deionized water (which is somewhat aggressive, as water does not "like" to exist in such a pure form), would de-mineralize a person's body, causing tremendous illness, if not death. Search for "Standards" related to the specific needs that you wish to fulfill, e.g.:

http://www.lenntech.com/drinking-water-standards.htm

Best of luck in your endeavors ~