Discussions thus far have focused on pathogens and methods of destroying them in the process of making water potable - safe to drink. This is highly important but it's not the whole story; for water must be palatable as well as potable. The obvious question to ask is - What makes a water palatable?

To be palatable, a water should be free of detectable tastes and odors. Immediately, we come to a stumbling block. What constitutes a detectable taste or odor? Undoubtedly when you have traveled around the country, you have tasted waters which must have had unpleasant tastes or odors. Natives in the area may be surprised to note you reaction, for after drinking the water for many years, they find nothing peculiar to either the taste or odor of the water. Then, there are those waters which have tastes and odors so obnoxious (hydrogen sulphide water, for example), even the long time inhabitant can't stomach them. Turbidity, sediment and color play important roles in determining whether a water is a delight to drink. 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 in four groups - sour, salt, sweet and bitter.

Odors possess many classifications. There are 20 of them commonly used, all possessing rather picturesque names. In fact 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. Now your taste buds and olfactory organs are not necessarily of the same acuteness as your neighbors. So there may be some disagreement on the subject. Generally you or your neighbor should not be made aware of any tastes or odors in water if there is to be pleasure in drinking it. If you are conscious of a distinct odor, without specifically seeking for such, the water is in need of treatment. 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 sulphide 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:

1) decaying organic matter

2) living organism

3) iron, manganese and the metallic product or corrosion

4) industrial waste pollution from substances such as phenol

5) chlorination

6) high mineral concentrations

In general, odors can be traced to living organisms, organic matter and gases in water. Likewise, tastes can be traced generally to the high total minerals in water. There are some tastes due to various algae and industrial wastes. 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 actually intensify a taste or odor problem. Potassium permanganate has been found to be extremely effective in removing many musty, fishy, grassy and moldy odors. Two factors make this compound valuable - it is a strong oxidizing agent and it does not form obnoxious compounds with organic matter. However, a filter must be used to remove manganese dioxide formed when permanganate is reduced.

Turbidity and suspended matter are not synonymous terms although most of us use the terms more or less interchangeably. Correctly speaking, suspended mat- ter 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 a water. Water may have a great deal of color - it may even be dark brown and still be clear without suspended matter. The current method of choice for turbidity measurement in Canada is the nephelometric method; the unit of turbidity measured using this method is the nephelometric unit (NTU). Turbidity in excess of 5 NTU becomes apparent and may be objected to by a majority of consumers. Therefore an Aesthetic Objective (AO) of < = 5 NTU has been set for water at the point of consumption. 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 course grains of sand that remain in suspension only as long as the water is agitated. Those particles which quickly sink to the bottom are usually called, "sediment". There are no hard and fast rules for classifying such impurities. 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 these water as they percolate through sand, gravel and rock formations. Turbidity varies tremendously even within these various groupings. Some rivers and streams have water that appears crystal clear with just trace amounts of turbidity in them especially at points near their sources. These same moving waters may contain upwards of 30,000 ppm of turbidity at other points in their course to the oceans. In fact, turbidity in amounts well over 60,000 ppm have been registered. Again there are significant fluctuations in the amount of turbidity in a river at different times in a year. Heavy rainfalls, strong winds and convection currents can greatly increase the turbid state of both lakes and rivers. Warm weather and increases in the temperature can also add to the problem. For with warm weather, micro-organisms and aquatic plants renew their activity in the water. As they grow and later decay, these plant and animal forms substantially add to the turbid state of a water. Also, they frequently cause a heightening of taste, odor and color problems.

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. Under some circumstances, the openings may have to be so small that they cause an excessive pressure drop as the water creeps through the filter and the unit may be impractical. In many cases, filters containing specially graded and sized gravel and sand are effective in screening out turbid particles. With such units, a periodic backwashing to remove the filtered material is all the maintenance necessary. As discussed in later chapters, the use of filter aids is necessary in treating many water sources. A filter aid is a chemical that is added onto the top of the filter bed immediately after backwashing. The filter aid traps fine dirt particles producing a more a sparkling clear water and keeps dirt from penetrating the filter bed, insuring better bed cleansing during backwashing. In some cases, cartridge filters are effective.

Municipal and industrial systems frequently make use of the coagulation process to aid in the removal of turbidity. In this economical process, a coagulating agent such as aluminum sulphate is fed into the water. After rapid mixing, the coagulating agent forms a "floe" generally in the form of a gelatinous precipitate. This floe gives the appearance of a soft, gentle snowfall. A settling period is then needed to allow the floe to fall gently through the water. As the floe forms and settles, it tends to collect or entrap the turbid particles and form them into larger particles which sink to the bottom. On large installations, huge settling basins provide the necessary time and space for the process. After the settling period, the water flows through a filter to remove the last traces of the coagulant and any remaining turbid particles. An additional water quality parameter of importance is color. Ordinarily we think of water as being blue in color. When artists paint bodies of water, they generally color them blue or blue-green. While water does reflect blue-green light, noticeable in great depths, it should appear colorless as used in the home. Ideally, water from the tap is not blue or blue-green. If such is the case, there are certain foreign substances in the water. Among these substances: Infinitely small microscopic particles add color to water. Colloidal suspensions and non-colloidal organic acids as well as neutral salts also affect the color of water; The color in water is primarily of vegetable origin and is extracted from leaves and aquatic plants; Naturally, water draining from swamps has the most intense coloring. The bleaching action of sunlight plus the aging of water gradually dissipates this color, however. All surface waters possess some degree of color. Like some shallow wells, springs and an occasional deep well can contain noticeable coloring. In general, water from deep wells is practically colorless. An arbitrary standard scale has been developed for measuring color intensity in water samples. When a water is rated as having a color of five units, it means: The color of this water is equal in intensity to the color of distilled water containing 5 milligrams of platinum as potassium chloroplatinate per liter. Highly colored water is objectionable for most process work in the industrial field because excessive color causes stains. While color is not a factor of great concern in relation to household applications, excessive color lacks appeal from an aesthetic standpoint in a potable water. Further, it can cause staining. The Aesthetic Objective (AO) for color in drinking water is < = 15 true color units. The provision of treated water at or below the AO will encourage rapid notification by consumers should problems leading to the formation of color arise in the distribution system. In general, color is reduced or removed from water through the use of coagulation, settling and filtration techniques. Aluminum sulphate is the most widely used coagulant for this purpose. Superchlorination, activated carbon filters and potassium permanganate have been used with varying degrees of success in removing color. Table 11 summarizes water treatment methods currently used.

Table 11. Wastewater Treatment Methods

Objective of Treatment

Method or Technology

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