Sea Water Filtration

40-70 bar

http://en.wikipedia.org/wiki/Reverse_osmosis

Correct for Reverse Osmosis on seawater.

Dissolved materials cannot be removed from water by techniques intended to take out particulate solids. Ions can only be removed by techniques such as ion exchange, membrane processes, evaporation/boiling/distillation or electrodialysis. Catalysts, like activated carbon, can be used to remove organics and free halides responsible for causing taste and smell.

It won´t work because a sand filter is not a substitute for RO membrane filter. Different principles apply to each. Sand filters (usually referred to as multmedia filters)will only remove suspended solids whereas the right RO filter will remove dissolved salts and virtually everything else that is found in waters.

...up to a point. Membranes are usually rated in general terms of their "percentage rejection". Some ions always get through.

Reverse Osmosis followed by remineralisation and either ultraviolet light or free chlorine sterilisation is usually sufficient for potable water, and is widely used upon ships. It is an energy-intensive process, which is why it is only used on land in certain circumstances.

Low-tech approaches in developing countries usually dictate something different. In the Himalayas for example, hill fog is condensed from the atmosphere using fine nylon mesh, and the droplets collected for drinking!

Check out www.wateraid.org.uk

I was looking at replicating what was nature was doing,i.e.: well at x metres for coast would produce fresh water, the salt having been absorbed by land mass in between.

nano-filters can be expensive, especially where its required the most!! plus it's difficult to regenerate - you can't backflush.

In magnesium extraction from sea water, the hydroxide is extracted using lime, now this would make the remaining sea water 'thinner'/more brackish, making it easier to RO.

are there other similar sub-processes whereby more dissolved salt could be extracted economy? subsequent processes may be irrelevant, for now.

Rmg21,

"I was looking at replicating what was nature was doing,i.e.: well at x metres for coast would produce fresh water, the salt having been absorbed by land mass in between."

Unfortunately, your thinking and suppositions are flawed. You cannot remove salt from seawater by filtering or absorbing it with pebbles, sand etc. As mentioned in the earlier post, a reverse osmosis membrane is completely different than filtering out particulates.

The well water is fresh at a distance from the coast because it is ground water (originally from rain or melting snow) flowing in the aquifer out towards the ocean, rather than sea water that the sand has removed the salt from. On Long Island, where I live, many wells near the coast have been closed due to over pumping and sea water flowing back through the aquifer, resulting in what is commonly called salt water intrusion. When you pump water out of a well, it results in a depression cone in the local water table. The faster you pump, the steeper and larger the cone, and if the edge of the cone gets below sea level in sandy soils near the coast, the sea water starts flowing in through the sand mixing with and contaminating the fresh water.

If it were really possible to remove the salt from sea water by filtering it through sand even if it took a great distance of travel through sand, don't you think it would have been done centuries ago?

Trust me, it will not work.

A link explaining RO:

http://science.howstuffworks.com/question29.htm

Regards, Greg

Well, the water doesn't necessarily come in from the sea to x metres inland. Some of it comes from precipitation over the land mass and its subsequent drainage.

Most water treatment processes adapt what Nature can do and work with it. Replicating/substituting can be done, though it will produce water at a higher cost-per-unit-volume than a Nature-assist string of processes.

Potable water is delivered to nearly every home in the UK 24h/7d/52w per year, ready for consumption at £0.50 Sterling per m³. Beat that!

Do that and you put GE out of business. Modern man has only been trying for 100 years or so. When i was in Florida last year there were islands of trees in the salt flats whos name escapes me, but the guide expalined their root system uses a natrual reverse osmosis to desalinate. I always thought their might be a way to combine that process with producing water for human consumption. Probably no way to accomplish economies.

What you are proposing violates several thermodynamic 'laws'. It is basically a 'perpetual motion' device, and not achievable so far as we know.

Thanks for the insight, Greg, and Keith, could you explain a little on the perpetual motion bit- I can't see the link.

How about the second part of my last post, removal of some constituent salts, that should help. With less density, system pressure should lower, and membrane life gets longer.

I was also thinking capillary action - this should improve salt separation as sea water moves towards dry zone. Produced vapour would require less vacuum, say, under tropical conditions.

Membrane life is more a function of the antiscalant regime than of the density of the fluid being treated: prevention of precipitation of the calcium and magnesium carbonates, sulphates, hydroxides and halides, barium sulphate and carbonate, aluminium hydroxide and phosphate and a host of other things is the primary concern. Autopsies of failed membranes often indicate inattention to the above and a lack of regular chemical cleaning.

Prevention and cleaning regimes are determined by the ion content in the fluid and by operational experience.

1. So what do you gain? Pumping the membrane desalinated water back to sea level will cost as much as pumping to pressurize a membrane filter chamber.

2. Sand won't work as a desalinator.

The best solution it to use low pressure distillation. This requires a chamber where water can be heated by the sun and that is at bellow atmospheric pressure.

The boiling point of water going down with a decrease in pressure. It is a type of solar still often used by people crossing desert regions, where a small hollow is made and people urinate during the day and then place a plastic film over this at night so the water vapour condenses and can be run into a container yielding fresh water to drink. On a larger scale rather than make a single unit, several smaller units would work better because the amount of solar radiation (heat) required to start the process will be less. A large surface area is required corrugated sheet would help here. The surface painted or coated black and lenses plased so as to concentrate the suns energy. Concave reflectors can be used to further enhance the effect.

I leave the final design to you but I suggest you crack on with this because the Australians already have my data.

well, I was hoping salt separation thru' soil media wouldn't require the kind of pressure required for typical RO, (should have googled/researched more prior posting my discussion/question - sorry guys!!).

...guess its back to square one, perhaps with better membrane maintenance program.

Also, vacuum distillation on solar heat looks promising too(vacuum distillation is widely used on board ships for distilled/fresh water production, with normal water purity of 1.5 ppm salt content. you need this for the boiler!).

Although this company does not have anything "low power" for RO, it does offer an interesting twist to the typical spiral would or hollow-fiber membrane technologies. It uses vibration to keep the membrane clear of plugging. Although they do not clearly explain how they prevent fatigue failure in the systems, it appears to be a possible solution for slurries and heaily contaminated feeds.

http://www.vsep.com/technology/applications.html

thanks guys, ...looks like I need to redesign my commercial plan!!