POTABILITY AND PALATABILITY
Is the water potable? Is it palatable?
These basic questions must be asked of all water intended for human use. Potable water is safe to drink. Palatable water looks good and tastes good. But the two don't always go hand in hand.
Water that's cool, clear, and sparkling appeals to the user. It is palatable, but it may not be safe to drink. On the other hand, some water may have an objectionable taste, odor, and/or color-yet be perfectly safe.
Water intended for human use must, first be completely free of germs.
Water is the natural breeding ground for many types of tiny organisms. Some are harmless. Others have mild nuisance value. But still others can cause serious disease.
For the most part, you need worry only about organisms in the past category. There are many different types, such as bacteria, viruses, and protozoa. As a group, they're known as "pathogens." To make water safe to drink, these pathogens must be destroyed.
In testing water for potability, the idea is not to find high disease-bearing organisms are present. There are too many varieties to make this practical. Instead, the test of potability is whether human and/or animal wastes are present. If so, the water is not considered safe to drink.
Actually, water contaminated by wastes doesn't necessarily contain disease-bearing bacteria. But their presence indicates danger. So all such waters should be considered unsafe.
When a water supply is found to contain wastes, you must correct the situation at once. But remember this basic rule: it's more important to eliminate the source of contamination than to kill the bacteria content. When you run into a contaminated water supply, your first job should be to get rid of the source of trouble. Then take other safety measures as needed.
How to Purify Water. There are several ways of purifying water by killing the pathogens in it. Among them are the following:
Boiling. Holding water at the boiling point for 15-20 minutes will disinfect. This is a safe practice that's often used in emergencies.
Ultra-Violet Light. Irradiating water with U-V light duplicates one of nature's own purification methods. Instead of sunlight, you use a U-V source such as a germicidal lamp. To assure thorough treatment, the water must be fairly free of suspended matter. Otherwise some bacteria may be protected from the light.
Chemical Treatment. This is the most common method. Among the chemical agents that can be used are chlorine, bromine and iodine, potassium permanganate, and ozone. Silver ions (as from silver nitrate) are also very effective in killing bacteria, even in small concentrations. But they require long contact time.
Unfortunately, it's not easy to test the effectiveness of some treatment methods. You can, however, test for chlorine readily. And for this reason, it's the chemical most widely used for making water safe to drink.
For household needs, a 5.25% solution of sodium hypochlorite is the most convenient source of chlorine. You can also use calcium hypochlorite, which is sold in the form of dry granules. But the sodium solution is available more widely.
Hypochlorite solutions are normally fed into a water supply by chemical feed pumps. The first chlorine fed into the water supply will react with iron, manganese, or hydrogen sulfide if any of these are present. Another part will be neutralized by the small amount of organic matter always present in water.
What's left over-the chlorine that has not been consumed-is known as the "chlorine residual." In operating household chlorinators, the feed is usually adjusted to provide a chlorine residual of 0.5-1.0 ppm after 20 minutes contact time. This enough to kill all pathogens.
A more recent development in chlorination involves the use of higher concentrations for short contact periods. This process is known as superchlorination-dechlorination. Again, remember that chlorine treatment is only a safety measure. It can't take the place of eliminating the source of contamination.
Taste and Odor. Water may be potable, yet not "fit to drink." Tastes and odors make it offensive to many users. While natives of a given area may get used to a peculiar taste or smell in the local water supply, strangers often gag at the first sip-or sniff.
Palatability is judged by four senses: sight, taste, smell, and touch. To be palatable, water should be free fro color (sight), odor (smell), turbidity (sight), and objectionable taste.
It should be of moderate temperature (touch) the year round, and aerated (taste).
Many factors can cause bad taste and/or odor. Industrial wastes, especially phenolic compounds, impart an objectionable flavor. So do excess amounts of chlorine and other disinfectants. Similar effects can be produced by iron, manganese, decaying organic matter, algae and other micro-organisms, and the products of metallic corrosion.
Some tastes and odors, especially those from organic sources, can be removed by filtration through an activated carbon bed. Others respond to chlorination. But tastes and odors from industrial wastes and other sources may be completely unaffected by such treatments. Indeed, chlorination sometimes makes matters worse.
In any event, you may have to try various methods to get rid of bad tastes and odors. And if the simple methods don't work, it's sometimes cheaper to find a new source of water.
Turbidity. Turbidity is a cloudy condition Of the water. This problem may be due to the presence of iron or manganese. Or it may be caused by finely divided particles of clay, sand, organic matter, industrial wastes, or tiny organisms.
As used by water analysts, the terms "suspended matter" and "turbidity" are not identical. Strictly speaking, suspended matter is the amount of material that can be removed by filtration. Turbidity is a measure of how much light the water absorbs because of the presence of suspended matter.
Mechanical Filters. All turbidity can be removed by mechanical filtration. But the efficiency of any filter depends on the size of the openings through it. And to remove very fine particles, filters with extremely small pores must be used. Such filters are not always practical because of the high pressure drop as water flows through them.
Filters containing specially graded sand and gravel are effective in clearing up mildly turbid water. The only maintenance they require is periodic back washing.
In some cases, the so-called "cartridge" filters have been found helpful. But these have a fairly high pressure drop. And the cartridge must be replaced from time to time. For this reason, such filters are usually placed on individual water lines used for cooking and drinking. They're rarely used on the entire water system.
Public water systems often treat turbidity by a process known as "coagulation." This involves feeding in a coagulating agent such as alum. When mixed with the water, this creates a "floc" or voluminous, gel-like precipitate. As the floc settles, it pulls the suspended matter down to the bottom of the tank. A filter is then used to get rid of the last traces of floc and suspended matter.
You can use the same principle (on a smaller scale) in household systems where turbidity is a major problem. But the technique is tricky. And such systems are best installed by people experienced in this work. Then, too, household coagulation systems require quite a bit of regular attention.
Alkalinity. As noted in earlier articles, hydroxides, carbonates, and bicarbonates make water alkaline. All are present in natural water. But except for bicarbonates, significant concentrations are rare.
Low to moderate alkalinity isn't troublesome in household water supplies. But high concentrations give the water a soda-like taste. Moreover, too much alkalinity can cause excessive drying of the skin.
Lime softening can be used to reduce alkalinity. But this process is normally restricted to industrial and public systems. For private household water supplies, an alternative answer is ion exchange.
This method makes use of a negative exchange resin. It works just like a regular water softener with one exception: instead of pulling positive ions-calcium and magnesium-out of solution, it gets rid of the negative ions which cause alkalinity. The resin is regenerated with table salt.
There's only one trouble with this approach: in getting rid of alkalinity, the sulfates, carbonates, and bicarbonates will be replaced by chloride ions. This will result in a taste change which may be a problem.
Another method involves feeding a mineral acid, such as hydrochloric (muriatic) or sulfuric acid. This form of chemical treatment generates large amounts of carbon dioxide gas. And you must make some provisions for allowing the gas to escape.
Mineral acid treatment has serious drawbacks. The acids are somewhat dangerous to handle. And you must provide extremely close control of the feed rate in order to avoid the possibility of excess concentrations. These could be highly corrosive, as well as painfully poisonous.
In general, alkaline water is much less a problem than acid water. Sometimes other treatment methods, notably chlorination, help reduce alkalinity. In other instances, the condition is mild enough to be tolerated. But if you find some form of treatment necessary, negative ion exchange is probably the best answer.