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Q: De-salinating water ( Answered,   1 Comment )
Subject: De-salinating water
Category: Science
Asked by: theseekers-ga
List Price: $17.50
Posted: 10 Jul 2003 20:17 PDT
Expires: 09 Aug 2003 20:17 PDT
Question ID: 227685
What are the costs, issues, and complications on a small scale?

Request for Question Clarification by alanna-ga on 10 Jul 2003 21:53 PDT
Hi - Could you tell me what kind of water is to be desalinated
(brackish or sea); the location of the operation (US, Africa, Tuscon,
Arizona, etc,); and how small a scale (village, town, household). 
This info would help target the response.  Thanks a lot.  Alanna-ga

Clarification of Question by theseekers-ga on 11 Jul 2003 06:32 PDT
Seawater, Pacific Coast, village scale.
Subject: Re: De-salinating water
Answered By: alanna-ga on 11 Jul 2003 15:09 PDT
Hi theseekers-ga,

Thanks for narrowing down your question.  Here's some information on
desalination.  I hope it is of help to you.

Fresh water may be obtained from sea water in a number of ways.  The
underlying process for all desalination is extracting or separating
water molecules from the salts, ions, metals, and other ingredients
that make up salt water.  Nature performs this feat continuously.  Sea
water evaporates, rises into the atmosphere, cools, falls as rain and
enters fresh water rivers, lakes and reservoirs.

Artificial desalination can occur by mimicking nature, that is by
using evaporated water (heat-bases processes),  or by forcing the
issue and chemically separating water from sea salts and other
minerals (membrane-based processes).

The evaporation technique used in desalination is distillation.  There
are a number of distillation processes, of which the most common are:

-. Multiple Effect Evaporation (ME) and Multistage Flash Evaporation
(MSF) which involve boiling the salt water and  use heat-exchange
tubes to effect the releaser of water;

-Vapor Compression (VC) in which water vapor is collected and

-solar distillation in which water is collected in a greenhouse-type
structure, condenses on the cooler inside surface of the glass, and is
collected s it runs down the glass.

MSF distillation is most commonly used, but multiple-effect distillers
are becoming increasingly competitive for small and medium-size

Membrane techniques include Reverse Osmosis, Electrodialysis, or Ion

Reverse Osmosis (RO) puts salty water to one side of a semi-permeable
membrane where it is subjected to pressures of 800 to 1,200 LB/sq. in.
for seawater. “Pure” water will diffuse through the membrane leaving
behind a salty concentrate containing most of the dissolved organic
and inorganic contaminants.

With Electrodialysis salt water is pumped between many flat, parallel,
 membranes in a stack.  Some membranes let pass positive charged ions,
some negative.  A charge is applied leaving partially desalted water
between each set of membranes

In Ion Exchange salt and mineral ions  in salty water are "traded" for
desirable ions of fresh water through granular chemicals called ion
-exchange resins.

Reverse Osmosis is far and away the most common separation technique
used today for desalination.

There are many ways to break down costs for building, operating and
maintaining a desalination facility.  Total costs is one way, costs
per unit of product is another.

Using a total figure, the Monterrey Water Management District has
broken down its costs for three facilities as follows:

A 3 million gallon/day plant:  facility: $29.2 million; operation and
maintenance $2.1 million for seasonal operation

A 6mpd plant: capital costs 53.2 million; operation and management
$3.9 million seasonal

A 7-14 mpd plant: Desalination facility: $72.0-171.3 million; op and
maintenance $4.6-9.3 for seasonal operation

A breakdown per unit of water obtained is offered by the Encyclopedia
of Desalination and Water Resources.  They show the average cost of
desalination, among all types of facilities throughout the world, to
be  $1.50 to $3.00 per cubic meter (equivalent to about $5.70-11.30
per 1000 gallons). An installation (again among all types) would add
an additional $1.00 to the cost.

The above costs compare most unfavorably to the costs of water at, for
example, the Tampa Bay, Florida facility.  This is a huge desalination
effort in which the costs have been estimated at only $2.02/1000

Tampa Bay is mentioned here to show economies of scale.  The smaller
scale will always be very expensive.

A comparison by the UN Environmental Program of desalination costs in
island states around the globe found a wide range of costs ranging
from cents per cubic meter to $5 - $6/cubic meters. Most of these were
reverse osmosis types of process.


To desalinate or to pipe in fresh water is a question of need, costs,
and the environment

Need, of course, must be determinedf locally.  Costs may be balanced
against environment.  The process that uses solar power to heat and
distill sea water is also the most expensive in capital outlay and
maintenance costs. These range from $50 to $80 per 1,000 gallons of
final water product.(Compare Reverse Osmosis or Multiflash
Distillation at about $9.80/1000 gallons)

I hope this has beedn helpful to you.

All the best.

Subject: Re: De-salinating water
From: theo44-ga on 12 Oct 2003 14:24 PDT
An insulated pipe from a depth in the ocean to obtain a water temp.
below condensation temp. eg; 45 degrees, and run through any thinwall
potable water pipe in loops or a condenser will produce a continuos
"rain" of pure drinking water. the cold water is fed back to the
ocean, so only a circulating pump is needed and this is non polluting,
low maintenance, low tech etc, also you can use it for air
conditioning before returning it to the ocean. Hawaii, Big Island has
this going on now and one small operation gets 400 inches a year.
Initial cost and expense but low operating & maintenance costs there

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