Water Disinfection One More Time

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Treatment of a water supply is a safety factor, not a corrective measure. There are a number of ways of purifying water. In evaluating the methods of treatment available, the following points regarding water disinfectants should be considered:

a. A disinfectant should be able to destroy all types of pathogens and in whatever number present in water.

b. A disinfectant should destroy the pathogens within the time available for disinfection.

c. A disinfectant should function properly regardless of any fluctuations in the composition or condition of the water.

d. A disinfectant should not cause the water to become toxic or unpalatable.

e. A disinfectant should function within the temperature range of the water.

f. A disinfectant should be safe and easy to handle.

g. A disinfectant should be such that it is easy to determine its concentration in the water.

h. A disinfectant should provide residual protection against recontamination.

Techniques such as filtration may remove infectious organisms from water. They are, however, no substitute for disinfection. The following are the general methods used for disinfecting water:

Boiling - This involves bringing the water to its boiling point in a container over heat. The water must be maintained at this temperature 15 to 20 minutes. This will disinfect the water. Boiling water is an effective method of treatment because no important waterborne diseases are caused by heat-resisting organisms. Ultraviolet Light - 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 reaching the point of application at all times.

Use of Chemical Disinfectants - The most common method of treating water for contamination is to use one of various chemical agents available. Among these are chlorine, bromine, iodine, potassium permanganate, copper and silver ions, alkalis, acids and ozone. 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 or 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 sulphide 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.

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 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 still 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.

Copper ions are used quite frequently to destroy algae in surface waters but these ions are relatively ineffective in killing bacteria.

Disease-bearing organisms are strongly affected by the pH of a water. They will not survive when water is either highly acidic or highly alkaline. Thus, treatment which sharply reduces or increases pH in relation to the normal range of 6.5 to 7.5 can be an effective means of destroying organisms.

There are numerous other agents which have proved to be successful in destroying pathogens. Many of these must still be subjected to prolonged testing with regard to their physiological effect on man. Among these are certain surfactants and chlorine dioxide. There are several types of surfactants which aid in destroying pathogens. The cationic detergents readily kill pathogens. Anionic detergents are only weakly effective in destroying pathogens. Surfactants have not been seriously considered for treating drinking water because of their objectionable flavor and possible toxic effects. Chlorine dioxide has unusually good germ killing power. Up to the present time, no valid tests for its use have been developed because of the lack of means for determining low residual concentrations of this agent. It's such a strong oxidizing agent, a larger residual of chlorine dioxide would probably be needed than is the case with chlorine. At present, chlorination in one form or another is regarded as the most effective disinfectant available for all general purposes. It has full acceptance of health authorities. Still there are certain factors which affect its ability to disinfect waters. These should always be kept in mind. They are:

a. "Free" chlorine residuals are more effective than "combined" or "chloramine" residuals. Disinfection regardless of the type of chlorine becomes more effective with increased residuals. Chloramine is the compound formed by feeding both chlorine and ammonia to the water. This treatment has been used for controlling bacteria growth in long pipelines and in other appliances where its slower oxidizing action is of particular benefit.

b. A pH of 6.0 to 7.0 makes water a far more effective medium for chlorine as a disinfecting agent than do higher pH values of around 9.0 to 10.0.

c. The effectiveness of chlorine residuals increase with higher temperatures within the normal water temperature range.

d. The effectiveness of disinfection increases with the amount of contact time available.

e. All types of organisms do not react in the same way under various conditions to chlorination.

f. An increase in the chlorine demand of a water increases the amount of chlorine necessary to provide a satisfactory chlorine residual.

In order to ensure the destruction of pathogens, the process of chlorination must achieve certain control of at least one factor and, preferably two, to compensate for fluctuations that occur. For this reason, some authorities on the subject stress the fact that the type and concentration of the chlorine residual must be controlled to ensure adequate disinfection. Only this way, they claim, can chlorination adequately take into account variations in temperature, pH, chlorine demand and types of organisms in the water. While possible to increase minimum contact times, it is difficult to do so. Five to ten minutes is normally all the time available with the type of pressure systems normally used for small water supplies. Many experts feel that satisfactory chlorine residual alone can provide adequate control for disinfection. In their opinion, superchlorination-dechlorination does the best job. Briefly, what is this technique and how does it operate?

The success of superchlorination-dechlorination system depends on putting enough chlorine in the water to provide a residual of 3.0 to 5.0 ppm. This is considerably greater than chlorine residual of 0.1 to 0.5 ppm usually found in municipal water supplies when drawn from the tap. A superchlorination-dechlorination systems consists of two basic units. A chlorinator feeds chlorine into raw water. This chlorine feed is stepped up to provide the needed residual. A dechlorinator unit then removes the excess chlorine from the water before it reaches the household taps. The chlorinator should be installed so that it feeds the chlorine into the water before it reaches the pressure tank. A general purpose chemical feed pump will do the job. The size and the placement of the dechlorinator unit depends on the type of treatment necessary. This will usually be an activated carbon filter. If pathogen kill is all that is required, a small dechlorinator can be installed at the kitchen sink. This unit then serves to remove chlorine from water used for drinking and cooking. The advantage in dechlorinating only a part of the water is obvious. A smaller filter unit does the job and since only a small portion of the total water is filtered under such conditions, the unit lasts longer before either servicing or replacement is necessary. Essentially dechlorination is not needed to ensure a safe drinking water. Once the water is chlorinated, the health hazard is gone. The chlorine residual is removed merely to make the water palatable. If the problem is compounded due to the presence or iron and/or manganese, all the water should be filtered. Under such conditions, a larger central filter is necessary and should be placed on the main line after the pressure tank. The prime advantage of the superchlorination-dechlorination process is that it saturates water with enough chlorine to kill bacteria. Simple chlorination sometimes fails of its objective because homeowners may set the chlorine feed rate too low in order to avoid giving their water a chlorine taste. Sodium Dichloroisocyanurate - Sodium Dichloroisocyanurate can sterilize drinking water, swimming pool, tableware and air, or be used for fighting against infectious diseases as routine disinfection, preventive tableware and environmental sterilization in different places, or act as disinfectant in raising silkworm, livestock, poultry and fish. It can also be used to prevent wool from shrinkage, bleach the textile and clean the industrial circulating water, The product has high efficiency and constant performance with no harm to human beings. It enjoys goods reputation both at home and abroad. Table 8 summarizes some of this chemical's properties.

Table 8. Properties of Sodium Dichloroisocyanurate

UN No.






Physicochemical Properties

White crystalline powder, granular, or tablets


Powder or Granular

Available Chlorine

56% min.

60% min.

pH Value



25 or 50 kg plastic drums

Qty/20' FCL (MT)


Trichloroisocyanuric Acid - With strong bleaching and disinfection effects, Trichloroisocyanuric Acid is widely used as high effective disinfectant for civil sanitation, animal husbandry and plant protection as bleaching agent of cotton, gunny, chemical fabrics, or as shrink-proof agent for woolens, battery materials, organic synthesis industry and dry-bleaching agent of clothes. Effervescent Tablets (250, 500, 650 or 1000 mg) of TCCA are available for household use. Table 9 provides some general properties.

Table 9. Properties of Trichloroisocyanuric Acid,

UN No.






Physicochemical Properties

White crystal powder, granular or tablets, with stimulant smell of Hypochloric Acid, slightly soluble in water, easily soluble in Acetone.




Tablet :20g)

Available Chlorine

90% min.


0.5% max.

pH Value (1% W. S.)



25 or 50 kg plastic drums

Qty/20' FCL (MT)


Isocyanuric Acid - Cyanuric Acid is widely used for the stabilization of available chlorine swimming pool water treatment. CYA is also the starting compound for the synthesis of many organic derivatives. Table 10 provides some general properties.

Table 10. Properties of Isocyanuric Acid,





Physicochemical Properties

White crystalline solid powder or granular, ion- toxic and odorless




Cyanuric Acid

98.5% min.

98% min.


0.4% max.

0.5% max.


0.3 mm max. 90% hrough

0.6-2 mm 90% hrough

pH Value (1% Water Solution)



Melting Point (Centigrade)

330 min.

330 min.


25 PPM max.

25 PPM max.


woven bags

fiber drums

Qty/20' FCL (MT)



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


Rotten Egg Smell

a) Manganese green-sand filter up to 6 ppm H2S with pH not lower than 6.7

b) Over 6 ppm H2S constant chlorination by filtration / dechlorination

c) Open aeration followed by oxidizing-catalyst filter

The water should be tested at the source for H2S determination as the gas escapes rapidly.


Locate and eliminate seepage. Activated carbon will adsorb oil and gasoline (most hydrocarbons) on a short term basis. Airstrip with (40:1 air/water ratio) followed by 2 ft3 carbon units in series

Aromatic, Fishy, Earthy, or Woody Smell

a) Activated carbon type filter, or

b) Cartridge-activated carbon filter for drinking and cooking

Sharp Metallic Smell

a) Water softener can remove 0.5 ppm or iron (Fe) for every grain/gal. of hardness up to 10 ppm at minimum pH of 6.7 (unaerated water)

b) Over 10 ppm Fe: chlorination with sufficient retention tank time for full oxidation followed by filtration and dechlorination

c) pressure aeration plus filtration for up to 20 ppm Fe



a) up to 10 ppm iron removed by manganese greensand filter if pH is 6.7 or higher; or

b) Manganese-treated pumicite catalyst filter if pH is 6.8 or higher and oxygen is 15% of total iron content

c) Downflow water softened with good backwash, up to 10 ppm, use calcite filter followed be downflow water softener

Black Staining

a) Manganese greensand or manganese zeolite-type catalyst-filter to limit of 6 ppm or 15 ppm, respectively (combined Fe and Mn), with pH not lower than 6.7 value

b) Process used for iron removal usually will handle manganese

c) Manganese punicite* medium catalyst-filter with ultrafiltration-type membrane element

Objective of Treatment

Method or Technology

b) For whole-house system, remove by absorption via special macroporous Type 1 anion exchange resin regenerated with NaCl. up to 3 ppm

c) Above 3 ppm, constant chlorination with full retention time, followed by filtration and/or dechlorination

Gelatinous Slime

a) Destroy iron bacteria with a solution of hydrochloric acid, then constant chlorination, followed by activated carbon filtration or calcite filter.

b) Potassium permanganate chemical feed followed by MnZ/anthracite filter

Hydrocarbon Sheen

[Same as Petroleum]


a) For mud, clay, and sediment - use a calcite or pumicite filter, up to 50j)pm

^ For sand, grit, or clay - use a hydrocyclone, sand trap, and/or install new well screen



a) There is no commercial residential treatment for sodium over 1,800 ppm

b) Deionize drinking water only with disposable mixed bed-anion/cation resin; or

c) Reverse osmosis for drinking and cooking water only; or

d) Home distillation system for drinking water.


Single faucet activated carbon filter or whole-house tank-type activated absorption filter

Chemical Tastes (Other)

Pesticides-herbicides: Activated carbon filter will absorb limited amount. Must continue to monitor the product water closely

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  • daniel
    Is trichloroisocyanuric acid bleaching action save the colour of fabric materials?
    7 months ago
  • simon
    How does Trichloroisocyanuric acid disinfect water?
    6 months ago

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