i. Gravity Settling
Gravity settling is the process in which suspended particles are settled down in large tanks by gravity. It is also called as sedimentation. This process is simple and achieved by detaining water in tanks. After settling is over, the water is withdrawn from the tank without disturbing the settled particles. Settling velocity is an important criterion in sedimentation. All the particles heavier than water will move downward due to gravity force. If we consider a single particle, it experiences drag force while settling down. It also experiences the upward buoyancy force. The particle attains a uniform velocity called settling velocity, when the sum of drag force and buoyancy forces equals the gravity force.
ii. Filtration
Filtration is used to remove microorganisms, colour and odour from water. For large scale water treatment, the filter generally consists of thick layers of fine and coarse layers of sand. Size of the sand used is very important . If very fine sand is used,the sand layer will clog quickly and you need to clean frequently. The washing could be achieved by back flow of water called backwashing or scraping the clogged top layer and washing and then reusing it. If the size of sand is large, the sand layer cant filter the water effectively.
However getting all the sand particle of same size is difficult. The sand is graded using a set of sieves of different sizes. The effective size of sand particles is defined as the size of sieve that allows passage of 10 per cent of sand by weight.The sand used should be hard and should be free from impurities. The depth of sand bed is the next critical parameter in design and operation of filter. The sand bed is supported by layer of gravel. Under drains are installed in this gravel layer to collect the water filtered through sand bed. The sand filters are generally used after sedimentation tanks. Water from sedimentation tank is uniformly distributed over entire sand bed without disturbing it. This water is filtered through sand bed and the filtered water is collected by under drain system. In the initial period of operation, the filtration rate is more. The filtration rate gradually decreases due to clogging of sand bed.
iii. Removal of Specific Contaminants
Neither iron nor manganese in water presents a health hazard. However, their presence in water may cause taste, staining and odour problems. In fact iron is an essential element for the formation of hemoglobin, which is essential in transporting oxygen from the lungs to tissue cells. In addition to stains and taste, iron and manganese decrease the efficiency of ion-exchange softening unit by clogging and coating the exchange medium, interfere with plumbing fixtures by leaving deposits.Excess iron in water will support the growth medium for certain bacteria that require high iron concentrations. These bacteria may increase the deposition problems and may produce sulfides. This could clog pipelines and cause taste and odor problems.
After magnesium and calcium bicarbonates, iron is third in line for causing problems in water supplies. Iron is frequently found in acidic water with manganese and sulfur. This combination makes treatment more difficult. In most cases, the iron content of water is less than 5 mg/l. In severe cases, it can be as high as 60 mg/l. Very low iron concentrations can still cause problems. In concentrations of 0.3mg/l, it can cause stains on equipment, walls, and floors. For final rinse water and sanitizing solutions, iron content in excess of 0.2 mg/l will cause problems.
In well water, iron will be in the ferrous form due to deprivation oxygen in ground water. In surface water, iron is normally in the ferric state. Both the ferrous and ferric states of iron are objectionable. The precipitate formed by the growth of these organisms can cause blocked pipes.
Manganese can be found in well water with iron. In surface water, manganese dioxide is formed and precipitates. Manganese is present in small amounts, normally not exceeding 3 mg/l. Even at that low amount, it will cause black stains that are difficult to remove.The partial removal of iron can be achieved in other treatment processes also. Water softening with lime removes iron and carbon dioxide. Chlorination for disinfection will aid in the iron and manganese removal.
In surface water, due to presence of oxygen, the iron and manganese are found in oxidized form and can be removed relatively easily. Ground water generally is deprived of oxygen and so has high content of reduced iron, which can be removed by aeration or chemical oxidation. Commonly used processes for iron removal include aeration followed by, chemical oxidation, conventional treatment combined with lime softening and biological treatment methods. These processes are usually followed by filtration to remove precipitates.
Among the above techniques, aeration-filtration is more common. Chemical precipitation is more suitable for water with a higher concentration (Fe>5.0 mg/L).The potassium permanganate-manganese greensand method is suitable for low to moderate concentrations of iron and manganese, about 0-5 mg/L. The prechlorination-filtration process is generally recommended for low iron concentrations (e.g., less than 2.0 mg/L). The ion exchange method is used only for small quantities, while activated carbon adsorption is relatively expensive. Biological treatment involves a combination of both physicochemical and biological removal mechanisms.
When raw water contains manganese in addition to iron, the above-mentioned iron removal processes are practically ineffective for efficient removal of manganese.Precipitation in the form of hydroxide, or oxidation by oxygen, are generally feasible only at high pH (at least 9.0-9.5) values; oxidation by chlorine is sometimes possible,but only in the presence of large excess of chlorine, which then requires neutralization.The high residence time required for removal of manganese is also a problem.
Gravity settling is the process in which suspended particles are settled down in large tanks by gravity. It is also called as sedimentation. This process is simple and achieved by detaining water in tanks. After settling is over, the water is withdrawn from the tank without disturbing the settled particles. Settling velocity is an important criterion in sedimentation. All the particles heavier than water will move downward due to gravity force. If we consider a single particle, it experiences drag force while settling down. It also experiences the upward buoyancy force. The particle attains a uniform velocity called settling velocity, when the sum of drag force and buoyancy forces equals the gravity force.
ii. Filtration
Filtration is used to remove microorganisms, colour and odour from water. For large scale water treatment, the filter generally consists of thick layers of fine and coarse layers of sand. Size of the sand used is very important . If very fine sand is used,the sand layer will clog quickly and you need to clean frequently. The washing could be achieved by back flow of water called backwashing or scraping the clogged top layer and washing and then reusing it. If the size of sand is large, the sand layer cant filter the water effectively.
However getting all the sand particle of same size is difficult. The sand is graded using a set of sieves of different sizes. The effective size of sand particles is defined as the size of sieve that allows passage of 10 per cent of sand by weight.The sand used should be hard and should be free from impurities. The depth of sand bed is the next critical parameter in design and operation of filter. The sand bed is supported by layer of gravel. Under drains are installed in this gravel layer to collect the water filtered through sand bed. The sand filters are generally used after sedimentation tanks. Water from sedimentation tank is uniformly distributed over entire sand bed without disturbing it. This water is filtered through sand bed and the filtered water is collected by under drain system. In the initial period of operation, the filtration rate is more. The filtration rate gradually decreases due to clogging of sand bed.
iii. Removal of Specific Contaminants
Neither iron nor manganese in water presents a health hazard. However, their presence in water may cause taste, staining and odour problems. In fact iron is an essential element for the formation of hemoglobin, which is essential in transporting oxygen from the lungs to tissue cells. In addition to stains and taste, iron and manganese decrease the efficiency of ion-exchange softening unit by clogging and coating the exchange medium, interfere with plumbing fixtures by leaving deposits.Excess iron in water will support the growth medium for certain bacteria that require high iron concentrations. These bacteria may increase the deposition problems and may produce sulfides. This could clog pipelines and cause taste and odor problems.
After magnesium and calcium bicarbonates, iron is third in line for causing problems in water supplies. Iron is frequently found in acidic water with manganese and sulfur. This combination makes treatment more difficult. In most cases, the iron content of water is less than 5 mg/l. In severe cases, it can be as high as 60 mg/l. Very low iron concentrations can still cause problems. In concentrations of 0.3mg/l, it can cause stains on equipment, walls, and floors. For final rinse water and sanitizing solutions, iron content in excess of 0.2 mg/l will cause problems.
In well water, iron will be in the ferrous form due to deprivation oxygen in ground water. In surface water, iron is normally in the ferric state. Both the ferrous and ferric states of iron are objectionable. The precipitate formed by the growth of these organisms can cause blocked pipes.
Manganese can be found in well water with iron. In surface water, manganese dioxide is formed and precipitates. Manganese is present in small amounts, normally not exceeding 3 mg/l. Even at that low amount, it will cause black stains that are difficult to remove.The partial removal of iron can be achieved in other treatment processes also. Water softening with lime removes iron and carbon dioxide. Chlorination for disinfection will aid in the iron and manganese removal.
In surface water, due to presence of oxygen, the iron and manganese are found in oxidized form and can be removed relatively easily. Ground water generally is deprived of oxygen and so has high content of reduced iron, which can be removed by aeration or chemical oxidation. Commonly used processes for iron removal include aeration followed by, chemical oxidation, conventional treatment combined with lime softening and biological treatment methods. These processes are usually followed by filtration to remove precipitates.
Among the above techniques, aeration-filtration is more common. Chemical precipitation is more suitable for water with a higher concentration (Fe>5.0 mg/L).The potassium permanganate-manganese greensand method is suitable for low to moderate concentrations of iron and manganese, about 0-5 mg/L. The prechlorination-filtration process is generally recommended for low iron concentrations (e.g., less than 2.0 mg/L). The ion exchange method is used only for small quantities, while activated carbon adsorption is relatively expensive. Biological treatment involves a combination of both physicochemical and biological removal mechanisms.
When raw water contains manganese in addition to iron, the above-mentioned iron removal processes are practically ineffective for efficient removal of manganese.Precipitation in the form of hydroxide, or oxidation by oxygen, are generally feasible only at high pH (at least 9.0-9.5) values; oxidation by chlorine is sometimes possible,but only in the presence of large excess of chlorine, which then requires neutralization.The high residence time required for removal of manganese is also a problem.
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