Do You Really Know Your Water? - Part II
Now that we know about the possible sources for drinking water contamination, what can we do about it? Water treatment is what protects us from ingesting undesirable organisms with our drinking water. Treatment of a water supply is a safety factor, not a corrective measure. Keep this in mind in the discussion that follows:
There are several ways of purifying water. In evaluating the methods of treatment available, the following points regarding water disinfectants should be considered:
A disinfectant should be able to destroy all types of pathogens and in whatever number present in water. It should destroy the pathogens within the time available for disinfection, it should function properly regardless of any fluctuations in the composition or condition of the water, not cause the water to become toxic or unpalatable, it should function within the temperature range of the water, be safe and easy to handle, be such that it is easy to determine its concentration in the water and provide residual protection against re-contamination. Techniques such as filtration may remove infectious organisms from water, but they are not a substitute for disinfection.
Methods of disinfecting water:
Place water in a container over heat. Bring it to the boiling point. Hold it at this temperature for one minute. This will disinfect the water. Perhaps you have used this technique after a flood or when a water main has burst as an emergency aid. Boiling water is an effective method of treatment because no important waterborne diseases are caused by heat-resisting organisms.
The use of ultraviolet light is an attempt to imitate nature. As you recall, sunlight destroys some bacteria in the natural purification of water. Exposing water to ultraviolet light destroys pathogens. To assure thorough treatment, the water must be free of turbidity and color. Otherwise, some bacteria will be protected from the germ-killing ultraviolet rays. Since ultraviolet light adds nothing to the water, there is little possibility of its creating taste or odor problems. On the other hand, ultraviolet light treatment has no residual effect. Further, it must be closely checked to assure that sufficient ultraviolet energy is always reaching the point of application.
VARIOUS CHEMICAL DISINFECTANTS
The most common method of treating water for contamination is to use one of various chemical agents available. Let us review these briefly.
Bromine is an oxidizing agent that has been used quite successfully in the disinfecting of swimming pool waters. It is rated as a good germicidal agent. Bromine is easy to feed into water and is not hazardous to store. It apparently does not cause eye irritation among swimmers nor are its odors troublesome.
One of the most widely used disinfecting agents to ensure safe drinking water is chlorine. Chlorine in cylinders is used extensively by municipalities in purification work. However, in this form chlorine gas (CL2) is far too dangerous for any home purpose. For use in the home, chlorine is readily available as sodium hypochlorite (household bleach) which can be used both for laundering and disinfecting purposes. This product contains a 5.25% solution of sodium hypochlorite which is equivalent to 5% available chlorine.
Chlorine is also available as calcium hypochlorite which is sold in the form of dry granules. In this form, it is usually 70% available chlorine. When calcium hypochlorite is used, this chlorinated lime should be mixed thoroughly and allowed to settle, pumping only the clear solution.
For a variety of reasons not the least of which is convenience, chlorine in the liquid form (sodium hypochlorite) is more popular for household use. Chlorine is normally fed into water with the aid of a chemical feed pump. The first chlorine fed into the water is likely to be consumed in the oxidation of any iron, manganese or hydrogen sulfide that may be present. Some of the chlorine is also neutralized by organic matter normally present in any supply, including bacteria, if present.
When the "chlorine demand" due to these materials has been satisfied, what's left over -the chlorine that has not been consumed - remains as "chlorine residual". The rate of feed is normally adjusted with a chemical feed pump to provide a chlorine residual of 0.5 - 1.0 ppm after 20 minutes of contact time. This is enough to kill coliform bacteria but may or may not kill any viruses or cysts which may be present.
Such a chlorine residual not only serves to overcome intermittent trace contamination from coliform bacteria but also provides for minor variations in the chlorine demand of the water. The pathogens causing such diseases as typhoid fever, cholera and dysentery succumb most easily to chlorine treatment. The cyst-like protozoa causing dysentery are most resistant to chlorine. As yet, little is known about viruses, but some authorities place them at neither extreme in resistance to chlorination.
CHLORINE DEMAND - There are three basic terms used in the chlorination process: Chlorine demand, chlorine dosage and chlorine residual. Chlorine demand is the amount of chlorine which will be reduced or consumed in the process of oxidizing impurities in the water.
Chlorine dosage is the amount of chlorine fed into the water. Chlorine residual is the amount of chlorine remaining in water after oxidation takes place. For example, if a water has 2.0 ppm chlorine demand and is fed into the water in a chlorine dosage of 5.0 ppm, the chlorine residual would be 3.0 ppm.
For emergency purposes, iodine may be used for treatment of drinking water. Much work at present is being done to test the effect of iodine in destroying viruses which are now considered among the pathogens most resistant to treatment. Tests show that 20 minutes exposure to 8.0 ppm of iodine is adequate to render a potable water. As usual, the residual required varies inversely with contact time. Lower residuals require longer contact time while higher residuals require shorter contact time. While such test results are encouraging, not enough is yet known about the physiological effects of iodine-treated water on the human system. For this reason, its use must be considered only on an emergency basis.
Silver in various forms has been used to destroy pathogens. It can be added to the water as a liquid or through electrolytic decomposition of metallic silver. It has also been fed into water through an absorption process from silver-coated filters. Various household systems have been designed to yield water with a predetermined silver concentration. However, fluctuations in the flow rate often result in wide variations in the amount of silver in the water.
In minute concentrations, silver can be highly destructive in wiping out disease-bearing bacteria. While long contact time is essential, silver possesses residual effect that can last for days. Silver does not produce offensive tastes or odors when used in water treatment. Further, organic matter does not interfere with its power to kill bacteria as in the case with free chlorine. Its high cost and the need for long periods of exposure have hindered its widespread acceptance.
ODORS AND TASTES
Various odors and tastes may be present in water. They can be traced to many conditions. Unfortunately, the causes of bad taste and odor problems in water are so many, it is impossible to suggest a single treatment that would be universally effective in controlling these problems. Tastes are generally classified into four groups - sour, salt, sweet and bitter.
Odors possess many classifications. There are 20 of them commonly used, all possessing rather picturesque names. The names, in many cases, are far more pleasant than the odors themselves. To name a few of them - nasturtium, cucumber, geranium, fishy, pigpen, earthy, grassy and musty. Authorities further classify these odors in terms of their intensity from very faint, faint, distinct and decided to very strong.
In many cases, it is difficult to detect what constitutes a taste or an odor. The reason is obvious. Both the taste buds and olfactory organs work so effectively as a team, it is hard to realize where one leaves off and the other begins. To illustrate: hydrogen sulfide gives water an "awful" taste yet actually it is this gas's unpleasant odor that we detect rather than an unpleasant taste. Unfortunately, there is little in the way of standard measuring equipment for rating tastes and odors. Tastes and odors in water can be traced to at least five factors. They are:
- Decaying organic matter
- Living organism
- Iron, manganese and the metallic product or corrosion
- Industrial waste pollution from substances such as phenol
- High mineral concentrations
In general, odors can be traced to living organisms, organic matter and gases in water.
Now how can these objectionable tastes and odors be removed from water?
Some tastes and odors, especially those due to organic substances, can be removed from water simply by passing it through an activated carbon filter. Other tastes and odors may respond to oxidizing agents such as chlorine and potassium permanganate. Where these problems are due to industrial wastes and certain other substances, some of the above types of treatment may completely fail. In some cases, for example, chlorination may intensify a taste or odor problem.
TURBIDITY and suspended matter are not synonymous terms although most of us use the terms interchangeably. Correctly speaking, suspended matter is that material which can be removed from water through filtration or the coagulation process. Turbidity is a measure of the amount of light absorbed by water because of the suspended matter in the water. There is also some danger of confusion regarding turbidity and color. Turbidity is the lack of clarity or brilliance in water. Water may have a great deal of color - it may even be dark brown and still be clear without suspended matter.
Pick up a glass of water and hold it to the light. Can you see any finely divided, insoluble particles suspended in the water? Does the water seem hazy? If so, the water is turbid. When water has a large amount of such suspended particles, we lose our zest for it. While it may be safe to drink, it seems offensive.
The suspended particles clouding the water may be due to such inorganic substances as clay, rock flour, silt, calcium carbonate, silica, iron, manganese, sulphur or industrial wastes. Again, the clouding may be due to a single foreign substance in water, chances are it is probably due to a mixture of several or many substances. These particles may range in size from fine colloidal materials to coarse grains of sand that remain in suspension only if the water is agitated. Those particles which quickly sink to the bottom are usually called, "sediment".
If you take water from a swiftly flowing river or stream, you generally find that it contains a considerable amount of sediment. In contrast, you find that water taken from a lake or pond is usually much clearer. In these more quiet, non-flowing waters, there is greater opportunity for settling action. Thus, all but very fine particles sink to the bottom. Least apt to contain sediment are wells and springs. Sediment is generally strained from the water as they percolate through sand, gravel and rock formations.
Mechanical filtration will remove all forms of turbidity. Of course, the smaller the turbid particles, the finer the filter openings must be in order to strain them out. These types of filtration systems must be certified NSF/ANSI Standard 53.
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