water treatment and purification
The treatment given to water before it is supplied to the public for use as per required quality standards is known as water treatment. The amount of water purification treatment depends on the impurities present in the water and the standards of water required for public use. The following water treatment filter process is used to remove different types of impurities:
1. Screening application: It is the process of removing large sized particles like leaves, bushes, branches, debris etc. with the help of screen provided in front of the intake works. Screens can be thick or fine or both. In the screening process, the screen is kept tilted at an angle of 45° to 60° with the horizontal. The main purpose of screen tilt is to increase the opening area and reduce the flow velocity. The opening zones should be such that the speed of flow through them does not exceed 0.75 to 1 m/s.
2. plain sedimentation process : This is the process of retaining water in a basin so that the suspended particles settle down as a result of the action of gravity and forces. The rate of settling of a particle in water depends on the viscosity of water, density of water, specific gravity of the particle, size of the particle. It may be noted that the particles settle more rapidly in water at higher temperatures and at lower temperatures. The process of plain sedimentation can remove 60% of suspended matter and 75% of bacteria.
The following important factors should be considered in designing a sedimentation tank:
(a) The period of detention. This is the time required to fill the tank at a certain flow rate. The detention time for a simple plain sedimentation tank is 6 to 8 hours.
(b) the depth of the tank. This is also an important factor and should be 6m times 3m.
(c) speed of flow. The water flow velocity is obtained by dividing the volumetric flow by the vertical cross-sectional area of the tank. Velocity should not exceed 5 mm/s (or 30 cm/min).
(d) sludge capacity. Sludge capacity depends on the duration of the cleaning and the amount of matter removed from the water. Hence proper permission should be given in the design of the size of the tanks.
3. sedimentation with coagulation process : The process of adding certain chemicals (known as coagulants) to water to form a flocculant precipitate to absorb and enter the colloidal substance is called sedimentation with coagulation. When the turbidity of water is about 45 ppm. The period of detention can be kept from 2 to 6 hours. The most common coagulants used for water purification are alum or aluminum sulfate, lime or ferrous sulfate, sodium aluminate, magnesium carbonate, and ferric coagulants.
4. filtration process: The process of purifying water passing through a bed of sand or other fine granular material is called filtration process . Filters are used for this purpose. The sand required for process of filtration in water treatment must have the following properties:
(a) It should be free from clay, loam, lime and organic matter.
(b) It should be of uniform size and nature.
(c) It should neither be too fine nor too thick.
(d) It should be resistant and hard.
(e) Its weight should not be less than 5% when kept in hydrochloric acid for 24 hours.
It may be noted that the rate of decreases filtration process in water treatment when water is filtered through a bed of very fine sand. The effective size of sand particles ranges from 0.30 to 0.50 mm.
Filters can be slow sand filter, fast sand filter and pressure filter as discussed below:
The effective size of sand particles for the slow sand filter is 0.30 to 0.35 mm and the coefficient of uniformity is 1.75. Slow sand filters require a larger area (from 100 to 2000 m) than fast sand filters. Underwater drainage system receives and collects filtered water. These filters operate at atmospheric pressure and do not require any coagulation. The filtration rate of a slow sand filter is between 100 and 200 litres/m²/h. The slow sand filter is more efficient at removing up to 99 percent of bacteria from raw water. Its 50 ppm can remove turbidity.
Slow cleaning of the sand filter is required when the head or filtration head loss exceeds 1.2 m. The cleaning period is carried out from 1 month to 3 months. After cleaning the slow sand filter. Filtered water should not be used for a period of 24 hours to 48 hours. Slow sand filtration process are used for towns and villages.
For rapid sand filtration process the effective size of the particles varies from 0.35 to 0.50 mm and the coefficient of uniformity is 1.6. The under drainage system receives and collects the filtered water and also allows back washing to clean the filter. Water is forced into a rapid gravity filter at a pressure greater than atmospheric pressure. These filters require coagulation. The rate of filtration of the fast sand filter is about 30 times that of the slow sand filter (ie about 3000 to 6000 l/m²/h). Rapid sand filter removes bacteria from 80% to 90% and turbidity from 35 to 40 ppm. limit can be removed. Rapid sand filter cleaning is required when the head or filtration head loss exceeds 2.5 meters and the cleaning period is taken as 2 to 3 days. Fast sand filters are cheap and economical compared to slow sand filters.
Pressure filters are like small rapid sand filters and are placed in cylindrical shape in closed and water-tight steel vessels. Water is forced into the filter at a pressure greater than atmospheric pressure. The rate of filtration varies from 6000 to 15000 litres/m²/h. Pressure filters are less efficient than slow or fast sand filters at removing bacteria and turbidity. These filters are not used to treat municipal water supplies because they cost more. They are best suited for swimming pools, railway stations, personal industries, private estates, etc.
5. chlorine disinfection in water treatment plant : The process of killing pathogenic bacteria from water and making it safe for public use is called disinfection. Chlorine is the most commonly used disinfectant for drinking water worldwide. The process of applying small amounts of chlorine or chlorine compounds to disinfect water is known as chlorination of water. The contact time for chlorination should be at least 20 minutes. Depending on the amount of chlorine there are the following types of chlorination process:
(a) Plain chlorination. When chlorine is added to plain or raw water from tanks or reservoirs, it is called plain chlorination. The amount of chlorine required is 0.5 ppm. Use this method at 20 to 30 ppm. It is used for the treatment of water with less turbidity.
(b) Pre-chlorination. When chlorine is added to raw water before any treatment (i.e. sedimentation, coagulation, filtration etc.), it is known as pre-chlorination. The amount of chlorine should be such that at least 0.2 to 0.5 ppm. The rest of the chlorine comes to the filter plant. Pre-chlorination reduces the bacterial load on the filter thereby increasing their efficiency.
(c) Post-chlorination. Adding chlorine to water after all types of treatment is known as post-chlorination. The amount of chlorine should be such that the residual chlorine is approximately 0.2 ppm. Appears in water after a contact period of 20 minutes.
(d) double chlorination. When chlorine is added to water at more than one point, it is called double chlorination.
(e) Super chlorination. When more chlorine is added than is required for adequate bacterial purification of water, it is called super chlorination. This is done in special circumstances such as epidemics of waterborne diseases. A high dose (ie 2 to 3 ppm) of chlorine is added to the water beyond the break point to protect the public.
(f) Break point chlorination. When the amount of chlorine increases, the combined available chlorine also increases. With further addition of chlorine, the compound is oxidised and the newly formed substances do not react to show any residues. This phenomenon is called the break point. Break point chlorination completely destroys all the bacteria (pathogens) of the disease. It oxidizes water impurities such as ammonia. It also inhibits weed growth and removes taste and odor from the water.
(g) Dechlorination. Removal of unwanted chlorine in water by chemical or physical treatment is known as dechlorination. The most commonly used chemicals for water dechlorination are sulfur dioxide, sodium bisulfate, sodium thiosulfate, activated carbon, potassium permanganate, etc.
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