Desal World II

In the first Desal World activity, you used your imagination to drum up new ideas for alternative sources of drinking water. This time around, you will use your hands to build a solar-powered desal plant of your own - and turn your classroom into Desal World!

Learning goal

To learn about an alternative source of drinking water through a hands-on activity

Subject

Science

Materials

• one egg
• two 2-liter plastic soda bottles
• black paint
• one foot of clear plastic tubing, 1/2" or 1 cm diameter or larger (as for an aquarium)
• duct tape
• salt
• water
• bowl

Sunshine State Standards

Science: Force and Motion, SC.C.2.3; How Living Things Interact in the Environment, SC.G.2.3; Nature of Science, SC.H.1.3, SC.H.3.3

Activity

1. With safety in mind, have your teacher or parent spray one of the 2-liter bottles black.
2. In a clear pitcher, mix 1/2 cup of salt in a quart of water.
3. Pour the salt water into the black 2-liter bottle.

4. Attach the clear tubing to both 2-liter bottles and secure with duct tape. See diagram on reverse.
5. Set both bottles in a sunny window. Place the black bottle higher than the clear bottle.
6. After several days, water will have moved from the black bottle to the clear bottle.
7. In order to show the change in the saltiness of the water without tasting it, try the low-tech egg test. In a bowl of salt water, a raw egg will float; in fresh water, it will sink. Use the egg test before and after the activity to note the change in the water's salinity. Another optional instrument that will determine precisely the change in the water's salt content is a hydrometer.

Background

A gulp or two while swimming in the ocean or Gulf isn't so bad, but could you imagine drinking nothing but salt water? Salt water doesn't refresh a human body - it actually makes a person more thirsty! Eventually a person on a saltwater diet would become ill from dehydration.

Humans cannot survive on salt water alone - unless, of course, the salt is removed.

Over the last 50 years in seaside population centers where drinking water is scarce, desalination plants have been built that turn salt water into fresh water. Currently in Florida, roughly 180 desal plants purify brackish water (a mixture of salt water and fresh water). At present, no desal plants purifying sea water are operating in west central Florida. The Southwest Florida Water Management District and other agencies and local governments intend to have seawater desalination plants operating in the future.

One way to purify salt water is through a process called distillation. Distillation means heating the water until it boils and turns into steam. The steam is then collected in a separate container. When the steam cools and returns to liquid form, it is pure enough to drink.

The problem with this method is that it requires a vast amount of energy to produce small amounts of purified water. The expense of the energy has caused most places to seek alternatives other than saltwater distillation.

Another new technology used to desalinate water is called reverse osmosis. Though reverse osmosis may sound difficult, the process itself is actually simple. High pressure pushes salt water through a membrane. A membrane is made from material that allows liquid but not solids (like salt) to pass through it. After the water has passed through the membrane, what is left is purified water and a brine by-product (salt). Reverse osmosis is expensive - though cheaper than distillation.

Two traditional drawbacks to desalination include the high cost of the energy needed to operate the plants and the safe disposal of the brine. These factors have made researchers find new ways to desalinate water with greater energy efficiency and to dilute the brine and return it safely to the Gulf so that it doesn't harm marine life.

Water that currently comes out of the tap costs less than half a penny to produce per gallon. While desalinated water will cost more to produce, it will still cost less than one-half penny per gallon to produce.

Seawater desalination can potentially ease the strain on traditional water sources, especially the groundwater aquifers from where most of west central Florida's water is drawn. Desalination, however, will meet only a small percentage of our water needs. With or without seawater desalination, Floridians need to conserve water!

Discussion

• Why can't people drink salt water?
• Which desal process does your model most closely resemble, distillation or reverse osmosis?
• Do you believe the benefits of desalination are worth the extra cost?