DESCRIPCIÓN DEL MÉTODO Y DE LA FECHA DE VENTA,

It may be considered under two headings: 1. Purification of water on a large scale 2. Purification of water on a small scale

Purification of Water on A Large Scale

It comprise of one or more of the following measures: 1. Storage

2. Filtration 3. Disinfection

Storage

Water is drawn out from the source and impounded in natural or artificial reservoirs. As a result of storage, a very consider- able amount of purification takes place. It can occur by three processes:

a. Physical: By mere storage, the quality of water improves. About 90 percent of the suspended impurities settle down in 24 hours by gravity.

b. Chemical: The aerobic bacteria oxidize the organic mat- ter present in the water with the aid of dissolved oxygen. As a result, the content of free ammonia is reduced and a rise in nitrates occurs.

c. Biological: The pathogenic organisms gradually die out. Total bacterial count drops by as much as 90 percent in the first 5 to 7 days.

Filtration

By filtration apart other impurities, 98-99 percent of the bacteria are removed. Two types of filters used are:

1. Slow sand or biological filters 2. Rapid sand or mechanical filters

1. Slow sand filter

Slow sand filtration is a simple and reliable process. They are relatively inexpensive to build, but do require highly skilled operators. The process percolates untreated water slowly through a bed of porous sand, with the influent water intro- duced over the surface of the filter, and then drained from the bottom (Fig. 4.1).

Properly constructed, the filter consists of a tank, a bed of fine sand, a layer of gravel to support the sand, a system of under drains to collect the filtered water, and a flow regulator to control the filtration rate. No chemicals are added to aid the filtration process.

Elements of a slow sand filter

Supernatant water: The raw water flows into the upper tank

region in such a manner as to avoid disturbing the scmutzdecke (defined below); flow near that surface must be very gentle. The water in this compartment must have sufficient depth to drive through the schmutzdecke, the filter bed and into the support gravel - and initially should be about 2 to 3 meters, or 7 to 10 feet. The lower limit of the depth is somewhat contro- versial but 1.5 meters, or about 4 feet, should be a reasonable value. There is waiting period of 3 to 12 hours for the raw water which helps it to undergo partial purification by sedi- mentation, oxidation, and particle agglomeration.

A bed of graded sand: The thickness of sand bed is approxi-

mately 1 meter. The effective diameter of sand grain should be 0.15 to 0.30 mm. The sand bed is supported by layer of graded gravel, 30 to 40 cm deep. This prevents the fine grains being carried into the drainage pipes.

The newly laid filter soon gets covered with a slimy growth. This layer is called as ‘Schmutzdecke’, vital or biological layer. It is the “heart” of slow sand filter. It removes the organic bac- teria and holds back bacteria. It oxidizes ammoniacal nitrogen into nitrates and helps to yield bacteria free water (Fig. 4.2).

An under drainage system: It consists of perforated pipes

through which filtered water is collected and it supports the filter medium above.

A system of filter control valves: The outlet pipe system is

equipped with valves, which helps to maintain a constant rate of filtration.

A slow sand filter must be cleaned when the fine sand be- comes clogged, which is measured by the head loss. The length of time between cleanings can range from several weeks to a year, depending on the raw water quality. The operator cleans the filter by scraping off the top layer of the filter bed. A ripen-

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below 1.0 nephelometric turbidity units (NTU), achieving 90 to 99 + percent reductions in bacteria and viruses, and pro- viding virtually complete Giardia lamblia cyst and

Cryptosporidium oocyst removal. Limitations

Slow sand filters do have certain limitations. They require a large land area, large quantities of filter media, and manual labor for cleaning. Water with high turbidity levels can quickly clog the fine sand in these filters.

Slow sand filters do not completely remove all organic chemicals, dissolved inorganic substances, such as heavy metals, or trihalomethane (THM) precursors—chemical compounds that may form THMs when mixed with chlorine. Also, water with very fine clays are not easily treated using slow sand filters. Slow sand filters are less effective at removing microorganisms from cold water because as temperature decreases, the biological activity within the filter bed declines.

2. Rapid sand or mechanical filters

The following steps are involved in this process (Fig. 4.3). 1. Coagulation: The raw water is first treated with a chemi-

cal coagulant such as alum.

2. Rapid mixing: The treated water is then subjected to vio- lent agitation in a “mixing chamber” for a few minutes. This allows a quick and thorough dissemination of alum throughout the bulk of water.

Fig. 4.2: Slow sand filter (Sketch)

Fig. 4.1: Design of slow sand filter

ing period of one to two days is required for scraped sand to produce a functioning biological filter. The filtered water qual- ity is poor during this time and should not be used.

Advantages

Design and operation simplicity—as well as minimal power and chemical requirements— make the slow sand filter an ap- propriate technique for removing suspended organic and in- organic matter. These filters also may remove pathogenic or- ganisms.

Slow sand filtration reduces bacteria, cloudiness, and or- ganic levels—thus reducing the need for disinfection and, con- sequently, the presence of disinfection byproducts in the fin- ished water.

Other advantages include:

• Sludge handling problems are minimal. • Close operator supervision is not necessary.

• Systems can make use of locally available materials and labor.

Slow sand filters also provide excellent treated water qual- ity. Slow sand filters consistently demonstrate their effective-

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Environment and Health

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3. Flocculation: It involves a slow and gentle stirring of the treated water in a flocculation chamber for about 30 min- utes. This slow and gentle stirring results in the formation of a thick, copious, white flocculent precipitate of alu- minium hydroxide.

4. Sedimentation: The coagulated water is now led into sedi- mentation tanks where it is detained for periods varying from 2 to 6 hours when the flocculent precipitate together with impurities and bacteria settle down in the tank.

Disinfection

Disinfection is accomplished both by filtering out harmful microbes and also by adding disinfectant chemicals in the last step in purifying drinking water. Water is disinfected to kill any

pathogens which pass through the filters. Possible pathogens

include viruses, bacteria, including Escherichia coli,

Campylobacter and Shigella, and protozoans, including G lamblia and other Cryptosporidia. In most developed countries,

public water supplies are required to maintain a residual disinfecting agent throughout the distribution system, in which water may remain for days before reaching the consumer. Following the introduction of any chemical disinfecting agent, the water is usually held in temporary storage - often called a

contact tank or clear well to allow the disinfecting action to

complete.

For a chemical or an agent to be potentially useful as a disinfectant in water supplies, it has to satisfy the following criteria:

a. It should be capable of destroying the pathogenic organ- isms present.

b. Should not leave products of reaction which render the water toxic.

c. Have ready and dependable availability at reasonable cost permitting convenient, safe and accurate application to water.

d. Possess the property of leaving residual concentration to deal with small possible recontamination.

e. Be amenable to detection by practical, rapid and simple analytical techniques in the small concentration ranges to permit the control of the efficiency of the disinfection process.

Chlorination

Chlorination is one of the greatest advances in water purification. Chlorine kills pathogenic bacteria, but it has no effect on spores and certain viruses except in high doses. It has limited effectiveness against protozoans that form cysts in water. (Giardia lamblia and Cryptospo-ridium, both of which are pathogenic).

When chlorine is added to water there is formation of hydrochloric and hypochlorous acids. The hydrochloric acid is neutralized by the alkalinity of water. The hypochlorous acid ionizes to form hydrogen ions and hypochlorite ions:

H2O + Cl2 → HCl + HOCl

HOCl → H + OCl

The disinfecting action of chlorine is mainly due to the hypochlorous acid and to a small extent due to the hypochlorite ions.

Chlorine acts best as a disinfectant when the pH of water is around seven because about 90 percent of the hypochlorous acid gets ionized to hypochlorite ions.

Method of chlorination

For disinfecting large bodies of water, chlorine is applied either as (1) Chlorine gas (2) Chloramines (3) Perchloron.

Chlorine gas is a toxic gas, hence there is a danger of a

release associated with its use. This problem is avoided by the use of sodium hypochlorite, which is a relatively inexpensive solution that releases free chlorine when dissolved in water.

Chloramines are chlorine-based disinfectants. Although

chloramine is not as strong of an oxidant, it does provide a long- lasting residual than free chlorine, and it does not form THMs or haloacetic acids. It is possible to convert chlorine to chloramine by adding ammonia to the water after addition of chlorine. The chlorine and ammonia react to form chloramine. Water distribution systems disinfected with chloramines may experience

nitrification, wherein ammonia is used as nutrient for bacterial

growth, with nitrates being generated as a byproduct.

Forms of chlorination:

1. Plain chlorination: When raw water is supplied to con- sumer by applying chlorine treatment only.

2. Pre-chlorination: When raw water is suspected to be highly contaminated, then a dose of chlorine is added to the raw water before it enters the sedimentation chamber. 3. Post-chlorination: When chlorine is added to water after

all the treatment is over, just before it enters the distribu- tion system to prevent contamination in the distribution line.

4. Double chlorination: When pre and post chlorination are both adopted.

5. Break point chlorination: The addition of chlorine to am- monia in water produces chloramines which do not have the same efficiency as free chlorine. If the chlorine dose in water is increased, a reduction in the residual chlorine occurs, due to the destruction of chloramines by the added chlorine. The end products do not represent any residual chlorine. This fall in residual chlorine will continue with further increase in chlorine dose and after a stage, the residual chlorine begins to increase in proportion to the added dose of chlorine. This point at which the residual chlorine appears and when all combined chlorines have been completely destroyed is the breakpoint and corre- sponding dosage is the breakpoint dosage.

6. Superchlorination: It is followed by dechlorination and comprises the addition of large doses of chlorine to the water, and removal of excess of chlorine after disinfec- tion, this method is applicable to heavily polluted water whose quality fluctuates greatly.

All forms of chlorine are widely used despite their respective drawbacks. One drawback is that chlorine from

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any source reacts with natural organic compounds in the water to form potentially harmful chemical byproducts

trihalomethanes (THMs) and haloacetic acids (HAAs),

both of which are carcinogenic in large quantities and regulated by the United States Environmental Protection

Agency (EPA). The formation of THMs and haloacetic

acids may be minimized by effective removal of as many organics from the water as possible prior to chlorine addition. Although chlorine is effective in killing bacteria, it has limited effectiveness against protozoans that form cysts in water. (Giardia lamblia and Cryptosporidium, both of which are pathogenic).

Methods of testing residual chlorine Orthotoluidine (OT) Test

This test enables both free and combined chlorine in water to be determined with speed and accuracy.

Orthotoluidine-arsenite (OTA) Test

This is a modification of the OT test to determine the free and combined chlorine residuals separately

Other agents for disinfection

Ozone (O₃): Ozone is a relatively unstable molecule “free

radical” of oxygen which readily gives up one atom of oxygen providing a powerful oxidizing agent which is toxic to most water borne organisms. It is a very strong, broad spectrum disinfectant that is widely used in Europe. It is an effective method to inactivate harmful protozoans that form cysts. It also works well against almost all other pathogens. Ozone is made by passing oxygen through ultraviolet light or a “cold” electrical discharge. To use ozone as a disinfectant, it must be created on site and added to the water by bubble contact. Some of the advantages of ozone include the production of relatively fewer dangerous byproducts (in comparison to chlorination) and the lack of taste and odor produced by ozonation. Although fewer byproducts are formed by ozonation, it has been discovered that the use of ozone produces a small amount of the suspected carcinogen Bromate, although little Bromine should be present in treated water. Another one of the main disadvantages of ozone, is that it leaves no disinfectant residual in the water. Ozone has been used in drinking water plants since 1906 where the first industrial ozonation plant was built in Nice, France. The US Food and Drug Administration has accepted ozone as being safe; and it is applied as an antimicrobiological agent for the treatment, storage, and processing of foods

Ultraviolet irradiation: UV irradiation is effective against most

microorganisms known to contaminate water supplies includ- ing viruses. UV radiation (light) is very effective at inactivating cysts, as long as the water has a low level of color so the UV can pass through without being absorbed. The main disad- vantage to the use of UV radiation is that, like ozone treat- ment, it leaves no residual disinfectant in the water. Because neither ozone nor UV radiation leaves a residual disinfectant in the water, it is sometimes necessary to add a residual disin- fectant after they are used. This is often done through the addi-

tion of chloramines, discussed above as a primary disinfec- tant. When used in this manner, chloramines provide an effec- tive residual disinfectant with very little of the negative aspects of chlorination.

In document FONCAIXA PYMES 4, FONDO DE TITULIZACIÓN DE ACTIVOS EMISIÓN DE BONOS DE TITULIZACIÓN EUROS (página 156-164)