The design of a water treatment plant is usually dependent uponthe quality of raw water. However, the production of water fit for human consumption, is not as easy as it sounds. The desired "end-product" comes into existence because coagulation is a commonly used treatment method, apart from flocculation, sedimentation, filtration and disinfection.
In fact, in case of rapid-sand filtration plants, it isn't really possible to achieve crystal-clear water, in absence of the coagulation unit. The plant would be unable to remove viruses and other water-borne contaminants.
Coagulation and flocculation units are used in conjunction with each other. In the former, coagulants are added to incoming water and intense mixing is done for few seconds. Water then flows to the subsequent flocculation unit, where floc formation is achieved through slow and gentle mixing.
The purpose of adding coagulants to water, is to agglomerate colloidal particles to floc particles, so that they settle readily.
Some of the factors, influencing the coagulation process are water temperature, particle size, concentration of particles, type of coagulant used, water pH, electrolyte concentrations, exchange capacity and electrical charge on particles.
Coagulants used in water treatment are aluminum sulfate (commonly known as alum), copper sulfate, ferrous sulfate, quicklime, hydrated lime, soda ash, ferric chloride and others. However, the most common and widely used coagulant is aluminum sulfate. The main reason for this is its free availability, ease in handling and relatively lower costs.
Aluminum sulfate has the tendency to absorb moisture from the air. This, however, does not affect its efficiency. A mixture of aluminum sulfate and quicklime in enclosed containers would cause an explosion, as calcium in lime liberates hydrogen from water at high temperatures - about 550 degrees centigrade.
The handling of dry aluminum sulfate is not free from hazards. Its dust exhibit properties of acidic chemical and, is relatively more hazardous. Protective clothing, goggles, gloves, neck cover and respirators are used by many chemical operators in water treatment plants.
The optimum pH range of water for use of aluminum sulfates is 5.5 to 8.0 for turbid water. For coagulating highly coloured water, it ranges from 4.4-6.0. Typically, aluminum sulfate is added at the rate of 1 grain per gallon of water (20mg/1). Experience indicates that under some conditions, the amount of coagulant required may increase with an increase in water temperature. Additional coagulation will be required, if the concentration of surfactants in water increase beyond 15mg/1. At temperatures below 20 degrees centigrade, difficulty may be encountered in achieving good floc formation.
Some water treatment plants use coagulants at two or more points in the treatment process. This is known as double coagulation, but is rarely practiced.
According to the WHO's 1984 "Guidelines for Drinking Water Quality" a guideline value of 0.2mg/l of aluminum in drinking-water has been recommended, based on aesthetic considerations, The European Economic Community's (EEC) maximum admissible level of aluminum is also 0.2mg/l. USA's Environmental Protection Agency (EPA), under the Safe Drinking Water Act, has set a secondary maximum level of aluminum as 0.05-0.2mg/l.
In water treatment, most of the aluminum used as coagulant settles down and is removed as waste sludge. Some of it is also removed by filtration units in the plant. In spite of this, traces of aluminum always remains in finished water.
All well-designed and properly-operated water treatment plants are technically able to meet the WHO guideline value. Higher aluminum level in finished waters (greater than 0.2mg/1), would signify a defect in coagulation, flocculation, sedimentation or filtration units. This in turn, would lead to aluminum deposition in the distribution system (post-flocculation), during periods of low water flows, reducing the hydraulic capacity of the distribution system.
Subsequent high flows in the system would re-suspend aluminum sediments, conveying it all to the consumers' taps. Hence, some consumers may, at some time, get higher levels of aluminum in drinking water.
The level of residual aluminum in finished waters is greatly influenced by the presence of fluoride and alkalinity. Research indicates that, presence of 0.8mg/1 of fluoride will cause elevated levels of aluminum in finished waters. High levels of residual aluminum in finished waters would also be caused by the presence of low levels of alkalinity (8-10mg/1 as calcium carbonate) in the water. This is probably because of poor floc formation.
Though the WHO's guideline value is 0.2mg/1, the intake of aluminum through drinking water in people living in hot climates, is slightly more, as compared to people living in cold regions. This is because of a higher intake of drinking water hot areas (3 litres, as against 1.5 litres per day, consumed in cold regions).
It is assumed here that drinking water contains a maximum of 0.2mg/1 of aluminum. In Karachi, every third person is under stress. This, together with the diet, consisting of a high amount of red chilies, has made "acidification" a common health problem.
Regular use of "antacids," containing aluminum hydroxide, is another source of intake of aluminum. Not only this but also the use of aluminum utensils has its share, in the intake of aluminum on a diurnal basis.
There is a high level of turbidity in drinking water, greater than 5JTU (Jackson Turbidity Units), in Karachi, especially during rainy season. This causes people to use alum to coagulate and removeturbidity, increasing the daily intake of aluminum.
Research conducted during the last two decades appears to implicate the intake of aluminum through drinking water, with the etiology of several neurological diseases, like Alzheimer's, pre-senile dementia and Amyotrophic Lateral Sclerosis (ALS) and Parkinsonism Dementia (PD).
As with all epidemiological studies, some unknown confounding variables may be operating, suggesting caution in the interpretation of results. Studies conducted by different researchers show a fairly strong relationship between higher intake of aluminum through drinking water and the occurrence of neurological disorders.
According to a study, there is significant difference in aluminum metabolism between young and old animals and that, 25 per cent of the older animals have a greater ability to absorb aluminum into tissues, including the brain. Since aluminum is toxic to brain cells, it also poses a potential health problem to older people.
Another research study indicates that people with Alzheimer's disease and Down's Syndrome have a defective form of the protein transferrin. This protein normally binds aluminum in the blood and prevents it from entering the brain.
Less aluminum is bound in people with this problem. This means that more is available to enter the brain, causing neurotoxic changes, as seen in Alzheimer's disease.
There is some evidence suggesting that, individuals suffering from chronic renal problems absorb aluminum more readily than normal people. Aluminum once absorbed appears to bind to serum proteins, in particular, transferrin. Absorbed aluminum appears to be excreted from the body by the kidneys. People with renal insufficiency tend to accumulate aluminum, because of their inability to eliminate it through the kidneys.
People, who require renal dialysis, can suffer severe health complications, due to the presence of aluminum in dialysis fluids. The main risk of dialysis to patients exposed to high aluminum levels is dementia.
Water quality guidelines are currently being reviewed by international agencies. While the guideline values of aluminum in drinking water are based on aesthetic conditions to prevent post-flocculation and discolouration of in distribution systems, it appears unlikely that the current review may change aluminum guidelines from aesthetic considerations to health-related ones.
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