Air & Water - Continued

When we last left our itinerant energists we and they were concerned about ensuring clean air and water for succeeding generations. We had reviewed some scenarios through which we might generate electricity and desalinate water at the same time or clean up some dirty water as another by-product of power generation.

Let’s look a little closer.

Scene 1 was the ideal where you had a usable waterfall near ocean or salt (brackish) water. A mile or so is not far away but there are not very many good sized waterfalls that close.

Scene 2 was where a supply of natural gas was fairly close to the shore. This is more likely than Scene 1 but the message for both of these scenes – is desalinization of the ocean and sea water.
Ocean water contains more than just salt and the chemistry of the category known as “seawater” will be examined later. Our purpose today is to determine the feasibility of providing generation of electricity coincident with the removal of salts and other matter from seawater or polluted land water to provide that most critical and desirable commodity – clean drinking water.

There are three basic methods for desalination; Vaporization (or distilling), Electrolytic Processing and Reverse Osmosis. We will look briefly at these and come to a surprising and pleasing recommendation.

There are several time honored means of distilling water to its purest state; boiling and flashing in order to create water vapor which is then condensed to usable liquid.

In electrodialysis, water is placed in a container with negative and positive electrodes to which the salt and other unwanted ions are attracted and collected for removal. This method was tried on a large scale in Saudi Arabia in 1958 but the extreme electrical demands made it impractical except where abundant energy was available.

The third and most promising system is the Reverse Osmosis Process (ROP). In this procedure water is subjected to great pressure which forces it through a special membrane which passes only pure water leaving behind more concentrated salt water.

The proven economics of ROP are what put the “icing on the cake.” While the flash distillation system costs about $4.00 per 1,000 gallons of product, ROP costs about $2.00 per 1,000. ($0.002).

An ROP motor 10 HP at 30 GPM costs $0.16 per hour to run.

30 GPM = 1800 Gallons per hour Power cost = $0.0000888 / gallon – not bad.

(For Comparison, tap water in NY and CT runs under $0.0007 per gallon – but that’s from reservoirs (rainwater). Super market bottled water runs $1.20 per gallon (90% for bottle, label & cap) = $0.12/gal water).

So, we can see that when taken together with energy from the cheap hydro power – or waste heat from gas fired generator – the generation/desalination process makes a lot of sense- especially in areas where fresh potable water is not available.

The major problem is – how do we get the processes to where they are most desperately needed? Please stay tuned.