|MadSci Network: Earth Sciences|
First, let’s recall that a water molecule is just an oxygen atom with a couple of hydrogen atoms stuck onto it. This simple little atom provides the medium for all life on earth, yet most of the water on earth is in the oceans, with so much salt in it that people and other land animals can’t use it. We need “fresh” water, which starts out as rain from clouds in the sky. The clouds get their water by evaporation of water from the oceans, but also lakes, rivers, and anywhere there is water on the surface of the earth. Most water is simply recycled in this way, and has been for millions of years. It is possible to make “new” water. Early in the US space program, NASA generated electricity on spacecraft with hydrogen-oxygen fuel cells. The by-product was ultrapure water, which the astronauts drank. The trouble with this is that water that pure is unnatural—it dissolves a little of almost anything it touches, and then it’s not ultrapure anymore. When we want very pure water in the laboratory, we distill it. This starts with boiling water to evaporate it, then chilling the steam in a “condenser” to turn it back into liquid. Distillation gets rid of a lot of impurities, but further steps are often needed. This is expensive, and storing the purified water without contaminating it is tricky. Drinking water comes either from a well (“groundwater”) or from a lake or river (“surface water”). Most homes out in the countryside, and some cities, use groundwater. Often, the groundwater is so purified by filtration down through the soil that it doesn’t need much treatment to make it safe to drink, but water that has passed through a lot of soil tends to be “hard.” It has dissolved minerals that make it turn soap to scum, but the minerals are usually good for our bodies when we drink hard water. Some places, though, the dissolved minerals are harmful, so testing is needed. Communities that use surface water have a harder time with purification because water in lakes and rivers is water that has run off of surfaces after rain falls, and it may carry many contaminants. Also, the water may already have been used by an upstream community and discharged into the waterway as treated, disinfected (we hope!) wastewater (sewage). Purification is needed, but distillation (or making new water out of hydrogen and oxygen) is too expensive in most cases; the energy cost to boil the water would be huge. Instead, water at the “intake” is coarse-filtered to take out big objects, including fish. The water may also receive some chlorine at this stage. Then, a “coagulant” such as alum is added, to make smaller particles stick together, and these coagulated particles are settled out in a step called “sedimentation.” The water is usually passed through of filter of sand or other fine material before being disinfected with chlorine, or sometimes ozone or ultraviolet; note that “disinfected” means viruses and bacteria that might cause infections are killed—some bacteria are still alive. At this point, it is called “finished water,” but it’s still at the treatment plant rather than at your house, where you can use it. Sending the finished water out through “mains” to homes, schools, and other users is not easy. There are always things dissolved in the water and bacteria that can use them, so water testing is done mainly from samples in the distribution system, rather than in the treatment plant, to see how things are going. The main problems are build-up of slime from bacterial growth and of slow accumulation of material that settles out of the water when it’s standing still in the pipes. Sometimes, though, water has to be used in large quantities to fight a fire, and the pressure in the pipes drops enough that water outside the pipes gets slurped in. Of course, the water that may be in the soil around the outside of the water main is not pure, so when it gets slurped in, the “drinking water” gets contaminated. When things go really wrong, people are told to boil their water before drinking it; this seldom lasts long. To prevent the development of coliform bacteria (these are called “indicators”—most are harmless, but they show us that the water hasn’t been properly taken care of), water suppliers try keep a low level of active chlorine all the way to the user. The nitrate you ask about is usually present because the source water had been used before. It can be removed by a difficult process called “denitrification”; but the levels of nitrate are usually harmless, so they are just left in the water. Now, it’s being found that used water may also contain people’s medications (aspirin, for example), vitamins, and other things that got into sewage. So, why not completely purify drinking water? The main reason is that most drinking water serves purposes other than drinking. An average grown-up’s daily water intake is about a half gallon, much of it as drinks other than water (milk, soft drinks, coffee, etc.). Kids need less. The average amount of “drinking water” an American uses, though, is from 50 to 100 gallons. Much of the water that is delivered to your home is used for washing dishes, clothes, bodies, and even cars. Most homes in the U.S. have flush toilets; these use 3 to 5 gallons of water each time they are flushed (some are made to use less). Of course, this water, as well as the water that was used for washing things, now goes through the sewers to be treated, disinfected, and discharged for someone else to use again. Rivers that join to make up the Mississippi drain half the U.S., including a lot of cities, by the time they get to what used to be New Orleans. This is very “experienced” water, and drinking water treatment is a very demanding and expensive task in downstream cities. They may use special chemical processes and even activated carbon, even if most of the drinking water is used for other things than drinking. Those who don’t trust what comes out of their taps may choose to drink bottled water, but there is no guarantee that the water in the bottle is really purer. In the U.S., the Environmental Protection Agency writes and enforces the rules for tap water, and the Food and Drug Administration regulates bottled water; they aren’t always “on the same page.” Some people also buy purification devices. These can work if they are designed to take care of the specific impurities that come with your tapwater (no two tapwaters are quite the same), but they have to be maintained. They can easily make the water worse, rather than better, if they aren’t taken care of. I get to travel to other countries (Mexico, Ukraine, and Egypt this year, with plane changes in Germany and Austria). I have my own ideas of where I can safely drink the tapwater, but I don’t worry about drinking water when I visit places in the U.S. We are very lucky to have enough water, and pure enough water to know that what we drink from our taps won’t make us sick. Much of the rest of the world’s population is not so lucky. Dean O. Cliver
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