water manuscript
Content
(2) Cd pollution and itai-Itai disease. When the wastewater floats through the land, the Cd compound has been absorbed into the soil and so that the plant in the soil will absorb the Cd compound which in the end will be eaten by creature. The crops has the great ability to absorb the Cd in the water or in the soil, and the soil may be has the great ability absorbing the Cd. The rice, one of the crops, its ability of absorbing the Cd is more greater than the other crops, so when the Cd being absorbed into the human body through all kinds of gateway, it may cause the itai-itai disease. It major symptom is the pain of bone through out the whole body. The Cd compound destroys the metabolism of the Ca. It cause the result that the person of which it Ca is lost and bone become soft, when they cough or were touched accidently, their bone will be broken (fracture) and they often malnutrition and scrawny, in the end die because of the other compounding disease. Its some symptoms are similar to endemic Fluorosis, but its mechanism is not similar. The mechanism of this disease is not clear up to now. It’s likely because the cadmiurn can damage the function of kidney, then inhibit the activity of VitD, and then hinder the Ca, P reserving in the skeleton, at last lead to osteomalacia. The heavy metals discharged from the Kamioka Mines into the Jinzu River were deposited in the river bed, and accumulated especially at the bottom of Jinzu River Dams . The cadmium in the river bed was dissolved in river water and polluted the agricultural water used in Osawano-cho and Fucyu-machi where the disease broke out . The cadmium dissolved in the agricultural water was absorbed by the soil in the rice patties, thereby contaminating it. Of the heavy metals that contaminated the soil, cadmium was absorbed by the rice some 3 times more than zinc, lead and copper, and concentrated inside unpolished rice. The cadmium was introduced into the human body by eating rice of a high cadmium concentration.
1
WATER QUALITY MANAGEMENT
Four aspects:
Man’s Need for Water Water Systems Sanitary Standard for Drinking Water Quality Water Treatment 1. The quantities required for each use vary substantially. The uses of water include many aspects, for the people: ⑴ drinking and culinary purposes; ⑵ personal cleanliness, including bathing and laundering; ⑶ household cleanliness, including washing dishes, flushing toilet and so on; ⑷ heating and air conditioning; For the community or for the city ⑸ urban irrigation; ⑹cleaning streets; ⑺ recreational purposes, including swimming pools and the watering of playing fields; ⑻ amenity purposes, such as public fountains and ornamental ponds; ⑼ fire protection; For the production ⑽ power production from hydropower and steam power; ⑾ commercial and industrial purposes, including industrial process waters and cooling; ⑿ carrying away wastes from all manner of establishments. ⒀ uses for agricultural production, such as raising cattle, irrigating farmland and so on. The uses of water in a community are many, and the requirements in quantity and quality are varied. Conventionally, it has been convenient and economical to provide a single water supply sufficient in quantity to serve all uses and suitable in quality to meet drinking requirements, even though only a small fraction of the total water supply is actually used for drinking. 2. Water systems To serve these uses, communities require sources of water, transmission pumps
2
and mains, treatment plants, and distribution systems for delivering water to each user. Transmission systems and treatment plants should be designed for the maximum day, which occurs generally in summer and is about 150% of the average demand. The distribution system should meet the peak demand during the day, which may be 150% to 300% of the maximum day demand, being larger for smaller communities where the peak is determined by requirements for fire protection.
3.
The sanitary Standard for Drinking water Quality
For China, to reduce the spreading and the occurring of the disease, the resource water must be dealt with through necessary hygienic steps, as a result that it can accord with the demands of 《 the life-drinking water-quality standard》to be used for drinking. the government set the （一）the basic sanitary demands for the drinking water. 1. Security in the epidemiology. There should contain no pathogenic microorganism or sponge in the water, not leading to the spreading of the waterborne disease, ie. to prevent the transmission of waterborne disease.. 2.The chemical components should do no harm to the body and should do benefit to the body. In the water, there should contain necessary trace elements for sustaining the health such as zinc, copper, iron and so on. They are very important for sustaining our health. For example, the purified water sold in the supermarket or market does not often as our drinking water because it does no harm for us but its trace element is scarce. So if we use it as drinking water for a long time (including cooking, drinking), There are lack trace elements in our body, and lead to anemic caused by iron lacking and other disease caused by lacking trace element. As for the harmful chemical components, they should be controlled under threshold limit value. 3. The sensory nature of the water should be good. It should be clear, uncolorful distubidiful, smeltless, and it should not contain anything that can be seen. 4.The quantity of water should be enough, and it should be used conveniently. The quantity should satisfy the total demands for the whole people of the
3
town(community). And we should also consider the short-term and the longtime development of water quantity. (The rainwater is not as the source of drinking water, because its quantity is not guarantee) (二)Drinking water quality standard. The water quality standard means Maximum allowable concentration (MAC). It’s just the standard for promising the water quality for directly drinking and it’s also the principle for the sanitary (Hygienic) department to carry out, the water relating work and the principle for the judgement of the water-purification. There are some different between the standard of American and the standard of China.. Here, I’ll introduce Chinese drinking water quality standard simply. In China the standard includes 4 items such as the sensory nature and chemical items, the toxic items, the bacteria items and the radioactive items. The sense nature of the water items is for keeping the good nature of the water. The toxic items and the radioactive items are for keeping the healthy nature of water and the bacteria items are for keeping the security of the water in the epidemilolgy. 1. The sense nature and the chemical items is to assure esthetic quality Color: The cleaning water is generally uncolorful. If its color changed, there must be the effect of the pollutant. For example:①There are many algals in the water and then it will be green. ②There are many humus in the marsh water and it will be yellow. ③Big amounts of Fe2+ substance, and it will be brown for the reason of Fe2+ substance is oxidized to Fe3+ substance. ④If the water is polluted by the industrial wastes then it will be the special color of which the industrial wastes. So the standard of drinking water is that the color of water will not exceed 0 15 and not contain any other color if the water has been dealt with. (2) Turbidity Degree: It just means the amount of the floating substance in the water. If the water is turbiditiful, it is the scattering of the colloid. The cleaning water should be clear, unturbiditiful, it should not exceed 30. On the special conditions it should not exceed 50. Of course, some certain instruments measure it. (3) Stink and Taste: Cleaning water should be unstinkable and untastful. If it is just stinkable and tastful, it just means that it has been polluted by nearby life
4
waste water such as the feces of human and animals, so in the cleaning water, there should has not any stink and taste. (4) The seen particles should not exist in the drinking water. (5) pH limit: the range of pH is from 6.5 to8.5. If the pH limit is too high, the salt will be separated out from the water, deteriorating the sense nature, affecting the disinfecting effect. If the pH limit is too low, it will erode the water-vessel. (6) Total hardness: The hardness means the total amount of Ca and Mg salt in the water in the form of CaCO3 content (mg/L). In general, it can be classified into the hardness of the carbonate, and the hardness of non-carbonate. The carbonate hardness can be called temporary hardness, this kind hardness can be removed through boiling. The non-carbonate hardness can be called the lasting hardness. If we use the high hardness water to cook, the nutrition value of the food will be reduced. The cardiac-system disease incidence rate has some contrary relationship with high hardness water. If somebody who is used to living in the soft water area goes into the hardness water area, there will appear some unregulated confusion of the gastrointestinal tract such as diarrhea, indigestion. The standard set formulates that the total hardness of the drinking water will not exceed 450mg/L. (7)The other chemical elements including Fe、 Cu、 and the vaporized Mn、 Zn、 2phenol、So 4、Cl dissolved solid and so on . When the chemical components in the water exceed certain limit value, the outside nature of the water will be affected, the color, the strange smell and turbidity may be appear, and cause the disorder of the gastrointestinal tract. Much chemical components can also cause the harmful to the body. 2. The toxicological index (1) Fluorides: Fluorides is a normal constituent of all diets and is an essential nutrient, it can help the form and the metabolism of the bone and the tooth. Moderate fluorides were absorbed by certain substances in enamel, and form a hard chemical substance. This substance spread tooth’s surface and forms a protecting layer of teeth. This layer can resist the eroding of acid and prevent (dental caries) decayed tooth. But if we intake excessive F-, it can lead to the poison of Fluorine. So we should control the content of F- in the water to prevent all kinds of disease relating to F-, such as excessive fluorides in drinking water supplies produce unsightly dental fluorosis, which incidence rate increases with increasing fluoride concentration. The optimal fluoride level in drinking water
5
differs from place to place, varying with amounts of fluorides in food and with climate conditions. In China, incidence rate of the endemic fluorosis is very high in some areas such as Henan, but maybe the mountain areas has much higher incidence rate probably because the ground water contains more Fluoride in the mountain areas. Dental Fluorosis makes the teeth being yellow or brown, or fracture easily. It often occurs in childhood. When endemic fluorosis influence the adult, it can make the bone transforming its shape such as “O” leg (bowlegs, bandy legs) or “X” leg. Person with these disease will be painful very much and it is incurable. (We will talk in detail next lesson) (2)CN、 Se、 Cd、 Pb, AgNo3:All of these chemical substances have As、 Hg、 Cr、 some toxicity. If the content in the water is too high, we’ll be affected if we drink the water for such long time. The standard set formulates for these substances the MAC. The America also made the standard for these substances. (Maximum contaminant levels MCLS). (3)CCl3(chloroform)、CCl4 (tetrachloride)、PHA、DDT(dichloro-diphenyltrichloroethane)、666. These five chemical compounds can be often found in the water and they’re threading the body. But up to now there is no convenient way to detect them, so in China, we classify them into temporary standard. CCl3、CCl4、PHA are all carcinogens. In America there has the standard for them. 3. Bacteriological quality index: There are many kinds of microorganism in the water, but the amount of them are so small, Five categories of pathogens are found in water: bacteria, viruses, protozoa, worms, and fungi .The principal bacterial waterborne disease are typhoid , cholera, and shigellosis (bacillary dysentery), all of which attacked the industrialized countries of the world in the mid-nineteenth century and are still major causes of morbidity and mortality in Asia, Africa, and Latin America. If they have over polluted the water, there will exist a lot of microorganisms. So the test of the bacteria in the water can help us judge if the water has ever been polluted and the degree by feces of animal and human. In our everyday work, we often use three indexes as the standard of bacteriological index: It’s total bacteria count, the total coliform group and the free residual chlorine. (1)Total bacteria count: It means the total bacteria in 1 ml water which has been cultured in common nutritious agar medium for 24 hours under 37℃. It can be used for the judgement of the cleaning degree and the purification
6
effect .The drinking water quality standard set formulates that it cannot exceed 100 CFU/L. (2) Total coliform group: It means total coliform counts not exceeded in 1ml water. It is the better index for the judgement of the water quality. It can tell us if the excrement and the urine have polluted the water. The coliform determination that has been used as a measure of fecal contamination since the early twentieth century is valuable, not because the coliform organisms are pathogenic but because they are always present in the normal intestinal tract of humans and other warm-blooded animals and are found in great numbers fecal wastes. Thus, while their presence may not signify that a water is a health hazard, their absence provides reasonable evidence of bacteriological safe water. The predominant coliform in humans and other warm-blooded animals is Escherichia coliform. Various coliforms have different survival times in the environment, and assessment of the presence of these different species may indicate the nature of the water contamination. In waters recently contaminated by fecal discharges, however, the fecal coliform organisms are present in larger numbers and the fecal coliform organisms are present in larger numbers and the fecal coliform count is useful test to affirm the presence of fecal pollution. Nevertheless, the presence of any type of coliform organism in treated water suggests either inadequate or contamination of the treated water. Cliform bacteria, whether fecal or non-fecal, should not be present in significant numbers in any potable water supply. The standard sets formulate that the total coliform group will not exceed 3 CFU/L . (3) Free residual chlorine: For quality-control purpose and for the purpose of the security of the water, it must be disinfect by using chlorination. The amount of the chlorinate Cl- should be enough, so as to the free residual chlorine in the water has been left. After the course of disinfecting, many bacteria has been killed, keeping certain concentration of free residual chlorine can make the disinfecting effect steady and lasting. It should be 0.3~0.5mg/L in the drinking water. Many students may forget the free residual chlorine when talking of bacteriological index. So l hope you don’t forget this point. 4.Radioactive index The grossα(alpha) radioactive indwell in the nature, such as groundwater. The grossβ(beta) radioactive is produced by environmental pollution. In our
7
country, the total α radioactive will not exceed 0.1Bq/L (becquerel) ,and the total radioactive β will not exceed 1Bq/L.
Water treatment
The treatment of waters to make them suitable for subsequent use requires physical, chemical, and biological processes. These processes may take place in nature. Where natural processes cannot assure a desired quality, these processes need to be engineered in water treatment plants. Engineering processes are increasingly necessary, in part because the contamination that impairs the quality of water is increasingly man-made and resistant to nature’s purification process and in part because of growth of population and its activity in the face of fixed natural resources. The conventional sequence of processes for the purification of surface water for potable purposes includes flocculation and coagulation, sedimentation, filtration, and disinfection. This treatment removes color, turbidity, microorganisms, colloidal particles, and some dissolved substances. Some of these processes may be omitted where the waters are drawn from a protected source and are free of color and turbidity. On the other hand ,the conventional treatment is not directed to dissolved synthetic organic chemicals and is only moderately effective in the removal of heavy metals and radioactivity. If these constitute a problem, additional processes, such as adsorption on granular activated carbon, may be required.
Coagulation The processes described here include chemical addition, rapid
mixing, and flocculation. The purpose is to remove finely divided suspended material, colloidal material, microorganisms, and to some extent dissolved substances, particularly those of larger molecular size, by bringing them together into flocs sufficiently large to be removed by sedimentation, filtration, or both. The raw water may be highly colored and free of turbidity or turbid and free of color. The particles responsible for the color and turbidity are not discernible to the naked eye. After coagulation, however, the individual floc(絮状物) particles
8
are easily observed, being on the order of 1 to 2 mm in diameter.
The principal Coagulants are alum, aluminum sulfate available in solid or
liquid form, and ferric salts such as ferric sulfate. These aluminum and iron salts, on solution, form trivalent aluminum and ferric ions. These ions react with alkalinity, which may be naturally present or, if not, may be provided through the addition of lime or soda ash. The coagulation process is aided by flocculation produced in spedial tanks, where mechanical paddles or diffused air stirs the water gently, promoting the conjunction of suspended particles. The resulting large flocs then settle easily.
Sedimentation
encouraged to settle.
the effluent from the flocculation tanks with large but
variable-sized flocs is led into sedimentation tanks, where the flocs are
Filtration--hrough layers of sand to improve turbidity, odour, colour
floc particles that escape the sedimentation tank are removed in filters. The conventional filter is about 1 m in depth and is made up of sand grains varying in size from 0.5 to 1.0 mm. As water passes down through the filter beds, the floc settles in the intestices or is adsorbed onto the surface of the sand grains. When the amount of floc accumulated in the filter is sufficiently great to impede(i ： 阻 止 ) the flow of water by increasing the head loss to 2 to 3 m, the filter is backwashed. Disinfection
9
1
WATER QUALITY MANAGEMENT
Four aspects:
Man’s Need for Water Water Systems Sanitary Standard for Drinking Water Quality Water Treatment 1. The quantities required for each use vary substantially. The uses of water include many aspects, for the people: ⑴ drinking and culinary purposes; ⑵ personal cleanliness, including bathing and laundering; ⑶ household cleanliness, including washing dishes, flushing toilet and so on; ⑷ heating and air conditioning; For the community or for the city ⑸ urban irrigation; ⑹cleaning streets; ⑺ recreational purposes, including swimming pools and the watering of playing fields; ⑻ amenity purposes, such as public fountains and ornamental ponds; ⑼ fire protection; For the production ⑽ power production from hydropower and steam power; ⑾ commercial and industrial purposes, including industrial process waters and cooling; ⑿ carrying away wastes from all manner of establishments. ⒀ uses for agricultural production, such as raising cattle, irrigating farmland and so on. The uses of water in a community are many, and the requirements in quantity and quality are varied. Conventionally, it has been convenient and economical to provide a single water supply sufficient in quantity to serve all uses and suitable in quality to meet drinking requirements, even though only a small fraction of the total water supply is actually used for drinking. 2. Water systems To serve these uses, communities require sources of water, transmission pumps
2
and mains, treatment plants, and distribution systems for delivering water to each user. Transmission systems and treatment plants should be designed for the maximum day, which occurs generally in summer and is about 150% of the average demand. The distribution system should meet the peak demand during the day, which may be 150% to 300% of the maximum day demand, being larger for smaller communities where the peak is determined by requirements for fire protection.
3.
The sanitary Standard for Drinking water Quality
For China, to reduce the spreading and the occurring of the disease, the resource water must be dealt with through necessary hygienic steps, as a result that it can accord with the demands of 《 the life-drinking water-quality standard》to be used for drinking. the government set the （一）the basic sanitary demands for the drinking water. 1. Security in the epidemiology. There should contain no pathogenic microorganism or sponge in the water, not leading to the spreading of the waterborne disease, ie. to prevent the transmission of waterborne disease.. 2.The chemical components should do no harm to the body and should do benefit to the body. In the water, there should contain necessary trace elements for sustaining the health such as zinc, copper, iron and so on. They are very important for sustaining our health. For example, the purified water sold in the supermarket or market does not often as our drinking water because it does no harm for us but its trace element is scarce. So if we use it as drinking water for a long time (including cooking, drinking), There are lack trace elements in our body, and lead to anemic caused by iron lacking and other disease caused by lacking trace element. As for the harmful chemical components, they should be controlled under threshold limit value. 3. The sensory nature of the water should be good. It should be clear, uncolorful distubidiful, smeltless, and it should not contain anything that can be seen. 4.The quantity of water should be enough, and it should be used conveniently. The quantity should satisfy the total demands for the whole people of the
3
town(community). And we should also consider the short-term and the longtime development of water quantity. (The rainwater is not as the source of drinking water, because its quantity is not guarantee) (二)Drinking water quality standard. The water quality standard means Maximum allowable concentration (MAC). It’s just the standard for promising the water quality for directly drinking and it’s also the principle for the sanitary (Hygienic) department to carry out, the water relating work and the principle for the judgement of the water-purification. There are some different between the standard of American and the standard of China.. Here, I’ll introduce Chinese drinking water quality standard simply. In China the standard includes 4 items such as the sensory nature and chemical items, the toxic items, the bacteria items and the radioactive items. The sense nature of the water items is for keeping the good nature of the water. The toxic items and the radioactive items are for keeping the healthy nature of water and the bacteria items are for keeping the security of the water in the epidemilolgy. 1. The sense nature and the chemical items is to assure esthetic quality Color: The cleaning water is generally uncolorful. If its color changed, there must be the effect of the pollutant. For example:①There are many algals in the water and then it will be green. ②There are many humus in the marsh water and it will be yellow. ③Big amounts of Fe2+ substance, and it will be brown for the reason of Fe2+ substance is oxidized to Fe3+ substance. ④If the water is polluted by the industrial wastes then it will be the special color of which the industrial wastes. So the standard of drinking water is that the color of water will not exceed 0 15 and not contain any other color if the water has been dealt with. (2) Turbidity Degree: It just means the amount of the floating substance in the water. If the water is turbiditiful, it is the scattering of the colloid. The cleaning water should be clear, unturbiditiful, it should not exceed 30. On the special conditions it should not exceed 50. Of course, some certain instruments measure it. (3) Stink and Taste: Cleaning water should be unstinkable and untastful. If it is just stinkable and tastful, it just means that it has been polluted by nearby life
4
waste water such as the feces of human and animals, so in the cleaning water, there should has not any stink and taste. (4) The seen particles should not exist in the drinking water. (5) pH limit: the range of pH is from 6.5 to8.5. If the pH limit is too high, the salt will be separated out from the water, deteriorating the sense nature, affecting the disinfecting effect. If the pH limit is too low, it will erode the water-vessel. (6) Total hardness: The hardness means the total amount of Ca and Mg salt in the water in the form of CaCO3 content (mg/L). In general, it can be classified into the hardness of the carbonate, and the hardness of non-carbonate. The carbonate hardness can be called temporary hardness, this kind hardness can be removed through boiling. The non-carbonate hardness can be called the lasting hardness. If we use the high hardness water to cook, the nutrition value of the food will be reduced. The cardiac-system disease incidence rate has some contrary relationship with high hardness water. If somebody who is used to living in the soft water area goes into the hardness water area, there will appear some unregulated confusion of the gastrointestinal tract such as diarrhea, indigestion. The standard set formulates that the total hardness of the drinking water will not exceed 450mg/L. (7)The other chemical elements including Fe、 Cu、 and the vaporized Mn、 Zn、 2phenol、So 4、Cl dissolved solid and so on . When the chemical components in the water exceed certain limit value, the outside nature of the water will be affected, the color, the strange smell and turbidity may be appear, and cause the disorder of the gastrointestinal tract. Much chemical components can also cause the harmful to the body. 2. The toxicological index (1) Fluorides: Fluorides is a normal constituent of all diets and is an essential nutrient, it can help the form and the metabolism of the bone and the tooth. Moderate fluorides were absorbed by certain substances in enamel, and form a hard chemical substance. This substance spread tooth’s surface and forms a protecting layer of teeth. This layer can resist the eroding of acid and prevent (dental caries) decayed tooth. But if we intake excessive F-, it can lead to the poison of Fluorine. So we should control the content of F- in the water to prevent all kinds of disease relating to F-, such as excessive fluorides in drinking water supplies produce unsightly dental fluorosis, which incidence rate increases with increasing fluoride concentration. The optimal fluoride level in drinking water
5
differs from place to place, varying with amounts of fluorides in food and with climate conditions. In China, incidence rate of the endemic fluorosis is very high in some areas such as Henan, but maybe the mountain areas has much higher incidence rate probably because the ground water contains more Fluoride in the mountain areas. Dental Fluorosis makes the teeth being yellow or brown, or fracture easily. It often occurs in childhood. When endemic fluorosis influence the adult, it can make the bone transforming its shape such as “O” leg (bowlegs, bandy legs) or “X” leg. Person with these disease will be painful very much and it is incurable. (We will talk in detail next lesson) (2)CN、 Se、 Cd、 Pb, AgNo3:All of these chemical substances have As、 Hg、 Cr、 some toxicity. If the content in the water is too high, we’ll be affected if we drink the water for such long time. The standard set formulates for these substances the MAC. The America also made the standard for these substances. (Maximum contaminant levels MCLS). (3)CCl3(chloroform)、CCl4 (tetrachloride)、PHA、DDT(dichloro-diphenyltrichloroethane)、666. These five chemical compounds can be often found in the water and they’re threading the body. But up to now there is no convenient way to detect them, so in China, we classify them into temporary standard. CCl3、CCl4、PHA are all carcinogens. In America there has the standard for them. 3. Bacteriological quality index: There are many kinds of microorganism in the water, but the amount of them are so small, Five categories of pathogens are found in water: bacteria, viruses, protozoa, worms, and fungi .The principal bacterial waterborne disease are typhoid , cholera, and shigellosis (bacillary dysentery), all of which attacked the industrialized countries of the world in the mid-nineteenth century and are still major causes of morbidity and mortality in Asia, Africa, and Latin America. If they have over polluted the water, there will exist a lot of microorganisms. So the test of the bacteria in the water can help us judge if the water has ever been polluted and the degree by feces of animal and human. In our everyday work, we often use three indexes as the standard of bacteriological index: It’s total bacteria count, the total coliform group and the free residual chlorine. (1)Total bacteria count: It means the total bacteria in 1 ml water which has been cultured in common nutritious agar medium for 24 hours under 37℃. It can be used for the judgement of the cleaning degree and the purification
6
effect .The drinking water quality standard set formulates that it cannot exceed 100 CFU/L. (2) Total coliform group: It means total coliform counts not exceeded in 1ml water. It is the better index for the judgement of the water quality. It can tell us if the excrement and the urine have polluted the water. The coliform determination that has been used as a measure of fecal contamination since the early twentieth century is valuable, not because the coliform organisms are pathogenic but because they are always present in the normal intestinal tract of humans and other warm-blooded animals and are found in great numbers fecal wastes. Thus, while their presence may not signify that a water is a health hazard, their absence provides reasonable evidence of bacteriological safe water. The predominant coliform in humans and other warm-blooded animals is Escherichia coliform. Various coliforms have different survival times in the environment, and assessment of the presence of these different species may indicate the nature of the water contamination. In waters recently contaminated by fecal discharges, however, the fecal coliform organisms are present in larger numbers and the fecal coliform organisms are present in larger numbers and the fecal coliform count is useful test to affirm the presence of fecal pollution. Nevertheless, the presence of any type of coliform organism in treated water suggests either inadequate or contamination of the treated water. Cliform bacteria, whether fecal or non-fecal, should not be present in significant numbers in any potable water supply. The standard sets formulate that the total coliform group will not exceed 3 CFU/L . (3) Free residual chlorine: For quality-control purpose and for the purpose of the security of the water, it must be disinfect by using chlorination. The amount of the chlorinate Cl- should be enough, so as to the free residual chlorine in the water has been left. After the course of disinfecting, many bacteria has been killed, keeping certain concentration of free residual chlorine can make the disinfecting effect steady and lasting. It should be 0.3~0.5mg/L in the drinking water. Many students may forget the free residual chlorine when talking of bacteriological index. So l hope you don’t forget this point. 4.Radioactive index The grossα(alpha) radioactive indwell in the nature, such as groundwater. The grossβ(beta) radioactive is produced by environmental pollution. In our
7
country, the total α radioactive will not exceed 0.1Bq/L (becquerel) ,and the total radioactive β will not exceed 1Bq/L.
Water treatment
The treatment of waters to make them suitable for subsequent use requires physical, chemical, and biological processes. These processes may take place in nature. Where natural processes cannot assure a desired quality, these processes need to be engineered in water treatment plants. Engineering processes are increasingly necessary, in part because the contamination that impairs the quality of water is increasingly man-made and resistant to nature’s purification process and in part because of growth of population and its activity in the face of fixed natural resources. The conventional sequence of processes for the purification of surface water for potable purposes includes flocculation and coagulation, sedimentation, filtration, and disinfection. This treatment removes color, turbidity, microorganisms, colloidal particles, and some dissolved substances. Some of these processes may be omitted where the waters are drawn from a protected source and are free of color and turbidity. On the other hand ,the conventional treatment is not directed to dissolved synthetic organic chemicals and is only moderately effective in the removal of heavy metals and radioactivity. If these constitute a problem, additional processes, such as adsorption on granular activated carbon, may be required.
Coagulation The processes described here include chemical addition, rapid
mixing, and flocculation. The purpose is to remove finely divided suspended material, colloidal material, microorganisms, and to some extent dissolved substances, particularly those of larger molecular size, by bringing them together into flocs sufficiently large to be removed by sedimentation, filtration, or both. The raw water may be highly colored and free of turbidity or turbid and free of color. The particles responsible for the color and turbidity are not discernible to the naked eye. After coagulation, however, the individual floc(絮状物) particles
8
are easily observed, being on the order of 1 to 2 mm in diameter.
The principal Coagulants are alum, aluminum sulfate available in solid or
liquid form, and ferric salts such as ferric sulfate. These aluminum and iron salts, on solution, form trivalent aluminum and ferric ions. These ions react with alkalinity, which may be naturally present or, if not, may be provided through the addition of lime or soda ash. The coagulation process is aided by flocculation produced in spedial tanks, where mechanical paddles or diffused air stirs the water gently, promoting the conjunction of suspended particles. The resulting large flocs then settle easily.
Sedimentation
encouraged to settle.
the effluent from the flocculation tanks with large but
variable-sized flocs is led into sedimentation tanks, where the flocs are
Filtration--hrough layers of sand to improve turbidity, odour, colour
floc particles that escape the sedimentation tank are removed in filters. The conventional filter is about 1 m in depth and is made up of sand grains varying in size from 0.5 to 1.0 mm. As water passes down through the filter beds, the floc settles in the intestices or is adsorbed onto the surface of the sand grains. When the amount of floc accumulated in the filter is sufficiently great to impede(i ： 阻 止 ) the flow of water by increasing the head loss to 2 to 3 m, the filter is backwashed. Disinfection
9
