There's a "death clock" on the Internet that -- for absolutely no cost -- revealed to me the day, month, and year when I will take my last breath. Other clocks keep track of who's currently shuffling off (and onto) this mortal coil. Suffice to say, the world population is something more than 6.6 billion and ticking upward every second.

I hate to blame newborns for anything, but the babies who push the population clock higher are worsening a critical problem: Water scarcity already affects 40% of people. New people eat and drink. They bathe and create waste. They buy manufactured products and go to water parks.

Let's mine some Foolish lessons from the above information. First, if the death clock is accurate, I'm not going to need a whole lot of money for retirement. Second, there are numerous ways for investors to take advantage of the swelling business of purifying water.

A quick look at a few companies in the sector shows that investors need to think big and small, and stretch their research across the globe.

The small of it
Keep an eye on smaller players whose fortunes hinge on bringing something new to the sector. A private company called Aqua Sciences, for instance, wasn't on the list of government-funded researchers when it developed what the Pentagon wanted: a machine that pulls potable water out of the air in even the driest regions.

Aqua Sciences' patented technology uses salts to attract moisture that's in the air, then separates the water and salt. This Florida company's process goes beyond traditional filtering, opening up a whole new angle for purification.

There's also money in companies working to improve existing methods of sweeping impurities out of water. In Los Angeles, private NanoH20 picked up $5 million a year ago from Khosla Ventures -- the venture capital firm of Vinod Khosla, co-founder of Sun Microsystems (Nasdaq: JAVA) -- for further development of its nanotechnology-enhanced membrane materials that will fit into existing desalination plants.

The big of it
There's no overlooking the presence of General Electric's (NYSE: GE) big fingers in seemingly all pieces of the water-purification pie. It is No. 2 in the water treatment market, behind only Nalco (NYSE: NLC), and GE recently cut the ribbon on the largest seawater desalination plant in Africa. But as a testament to GE's girth, its water unit accounts for less than 2% of overall revenues.

Israeli-based IDE Technologies is slowly working its way toward going public. The company already has a reputation and a reach, and is perhaps best known for partnering with Veolia (NYSE: VE) and Elran D.D. to design, build, and operate the world's largest seawater desalination plant, in Ashkelon, Israel.

For its part, water giant Veolia says it will invest 15 billion to 20 billion euro between 2007 and 2009 on internal projects and acquisitions. That includes 1 billion euro on acquisitions this year alone.

What the Ancient Mariner tells us
It's not true that there's water, water everywhere, and not a drop to drink. But there are water problems everywhere, and people working to solve them. The segment of the industry working on making water fit for consumption is home to fish of every size and color. Study the waterscape before deciding where to drop your line. Just don't hook an albatross.

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Fool online editor Kris Eddy does not own shares in any company mentioned in this article. The Fool has a disclosure policy.