Purification Of Water

Purification of Water

The water supply of a community should, as for as possible, be kept free from all foreign & polluting ingredients. The aim of purification is to protect the water supply by removing the organic matter, suspended & dissolved matter in water. Impure water may be purified by two main methods

• Natural methods
• Artificial methods

Natural Methods

The natural methods can be any of the following:

• Evaporation & condensation.
• Storage & settlement.
• Air, sunlight & oxidation.
• Filtration through earth.
• Biological purification.

Artificial Methods

Based on the size of the process, artificial water purification can be divided into different categories. The goal of water treatment is to create wholesome, safe water. The type of raw water and the desired standards for water quality will determine the type of treatment to be used. There are two scales used:

• Purification on a large scale.
• Purification on a small scale.

Purification on a Large Scale: This method is suitable when supply is to be done for large communities as in urban town or city. Generally this method consists of three stages:

• Storage
• Filtration
• Chlorination
   

Storage

By storage in reservoirs or lakes, gross turbidity is removed and colour is improved, abrupt changes in quality are prevented & bacteriological flora inhibited. Storage reduces the load on the later stages of purification usually water is stored as a preliminary to filtration. The actions are suspension i.e. sedimentation in setting tanks reduces the suspended solid & bacterial contents still further. This is natural purification; we may look at it from three aspects:

Physical: Just by storing alone the quality of water can be improved. About 90% of the suspended impurities settle down in 24 hours due to gravity. Thus water becomes clear. This allows sunlight to penetrate & thereby reduces the work of filters.

Chemical: Some chemical changes take place during storage as the aerobic bacteria oxidize the organic matter with the help of dissolved oxygen, which reduces the content free ammonia & increases nitrates.

Biological: Storage helps in reducing the amount of bacteria present in water. The pathogens die out. It is found that when river water is stored the total bacterial count drops by as much as 90% in the first 5-7 days. 

Filtration:

Filtrations the second stage in the purification of water & quite an important stage because 98-99% of the bacteria are removed by filtration, apart from other impunities. There are two types of filters commonly used:

• Slow sand or biological filtration.
• Rapid sand or mechanical filtration.

Slow Sand or Biological Filtration: Slow sand filters were first used for water treatment is 1804 in Scotland & subsequently in London. During the 19th century their use spread throughout the world. Even today, they are generally accepted as the standard method of water purification. Water is drawn from rivers, streams or lakes into the filter beds. These filter beds consist of water tight rectangular basins usually made of masonry. Each unit has to be at least 50 ^ prime *40^ prime *15 at the bottom, an acre (0.405 hectares) or more. Several such units are required in order to carry out continuous filtrations, without closing down the plant.

Advantages of slow sand filter

• Simple to construct & operate.
• The cost of construction is cheaper than that of rapid sand filters.
• The physical, chemical & bacteriological quality of filtered water is very high as 99.9-99.99% pathogenic free.
• There is no need for chemical coagulation of water prior to filtration.

Disadvantages

• Large area required.
• Sedimentation has to take place before the process of filtration.
• The rate of filtration is slow as 2 gallons per square foot per hour or 50 gallons per square ft. / per day.
• Less feasible in operation than rapid sand filter.

Rapid Sand Filtration: In 1985, the first rapid sand filters were installed in the USA. As the name indicates they are really 100 times faster than slow sand filters. They are of two types

• Gravity type (Paterson's filter).
• Pressure type (Candy's filter).

Gravi Type: The gravity type consists of aunit of filter bed with a surface of 900sq. ft. sand is the filtering medium. Each unit consists of under drains with perforated pipes that are covered with layer of graded gravel upon which a layer of coarse sand is laid. In this type of rapid filtration, the steps involved are:

1. Coagulation
2. Mixing
3. Flocculation
4. Sedimentation
5. Filtration

Coagulation: The raw water brought in from lakes streams or river is led to setting tanks where it is first treated with chemical coagulant such as alum aluminum or sulphate. Dosage of this chemical is 5-40 mg/liter or more depending upon the turbidity, colour, temperature & pH of the water.

Mixing: The treated water is then passes down into the 'mixing chamber' causing violent agitation for a few minutes. This process allows the chemical to get well mixed with water which is very necessary.

Flocculation: The next stage is the gentle & slow stirring of the treated water in a 'flocculator chamber' for about 30 minutes. During this time a flocculent material i.e. aluminum hydroxide is formed. This is due to the action of alum on the calcium carbonate that is already present in the raw water. 

Sedimentation: Following this, the coagulated water is led into sedimentation tanks for a period of 2-6 hours. At this time, the precipitate along with other impurities including bacteria  gets settled down in the tank

Filtration: As filtration proceeds, the 'alum- floc' not removed by sedimentation are held back on the sand bed. It forms a slimy layer comparable to the zoogleal layer in the slow sand filters. It absorbs bacteria from the water & effects purification. Oxidation of ammonia also takes place during the passage of water through the filters. As filtration proceeds, the suspended impurities & bacteria clog the filters. The filters soon become dirty & begin to lose their efficiency. 

Advantages

• Filtration is rapid, 40-50 times than that of a slow sand filter.
• Rapid sand filter can deal with raw water directly. No preliminary storage is needed.
• More flexibility in operation.
• The washing of filter is easy.
• The filter beds occupy less space.

Pressure Type (Candy's filter): The pressure type filter also needs mentioning though its use suits small installations such as swimming pools & commercial instillations where turbid water is to be cleaned &rendered fit for use in boilers.

Disinfection:

Chlorination is one of the most widely used disinfectants for killing pathogenic organisms but has no effect on spores & certain viruses (polio, hepatitis). It is a supplement not a substitute to sand filter. Chlorine is an oxidizing agent. It oxidizes iron, manganese & hydrogen sulphide. It destroys some taste & odour-producing constituents; it controls algae & slime organisms; & aids coagulation.

Principles of Chlorination

There are certain rules which should be obeyed in order to ensure proper chlorination:

• Water should be clear & free from turbidity.
• The chlorine demand of water has to be estimated to prevent over doses.
• Contact period should be at least 30 minutes is essential to kill bacteria & viruses.
• The minimum recommended concentration of free chlorine is 0.5mg / h * our This should be in addition to chlorine demand.

Method of Chlorination

For disinfecting large scale water, chlorine is used as:

• Chloromine or perchloron.
• Chlorine gas

Purification of Water in Small Scale:

Small scale purification is usually done for domestic purposes. Thee commonly used methods for small scale purification are as follows:

House Hold Method 

• Chemical disinfection
• Filtration
• SODIS method
• Disinfection of Wells
• Boiling: Boiling is a satisfactory method of purifying water for household purposes. To be effective, the water must be brought to a "rolling boil" for 5-10 minutes. It kills all bacteria, spores, cysts & ova. Boiling also

Chemical Disinfection

Bleaching Powder: Bleaching powder or chlorinated lime (CaOC*l_{2}) is a white shapeless powder with a strong smell of chlorine. When freshly made, it contains 33% of "available chlorine". It is, however, an unstable compound, on exposure on air, light & moisture liberates chlorine so it should be stored in dark, cool, dry places in closed containers. 1 once (about 28 grams) of bleaching powder is required to disinfect 1000 gallons of water...

Chlorine Solution: Chlorine solution may be prepared from bleaching powder. If 4 kg of bleaching powder with 25% available chlorine is mixed with 20 liters of water, it will give a 5% solution of chlorine. Ready- made chlorine solutions in different strengths are also available in the market (eg. piyush, water guard). Like bleaching powder, the chlorine solution is subject to losses one exposure to light or on prolonged storage. The solution should be kept in a dark, cool & dry place in a closed  container.

High Test Hypochlorite (HTH): High test hypochlorite (HTH) or perchloron is a calcium compound which contains 60- 70% available chlorine. It is more stable than bleaching powder & deteriorates much less on storage.

Chlorine Tablets: Under various trade names (eg. halazone tablet) are available in the market. They are quite good for disinfecting small quantities of water. A single tablet of 0.5 gm is sufficient to disinfect 20 liters of water.

lodine: lodine maybe used for emergency disinfection of water. Two drops of 2% ethanol solution of iodine will sufficient for one liter of clear water. A contact time of 20-30 minutes is needed for effective disinfection. lodine does not react with ammonia or organic compounds to any great extent. lodine is unlikely to become a municipal water supply disinfectant in a broad sense. High costs & the fact that the element is physiologically active (thyroid activity) are its major disadvantages.

Potassium Permagnate: Potassium permagnate is an oxidizing agent used for disinfection of water. The dose is 0.5 gm /1 liter water. Once widely used it is no longer recommended for water disinfection. Although a powerful oxidizing agent, it is not a satisfactory agent for disinfecting water. It may kill vibrio cholera SO commonly used during cholera epidemic. It has other draw-backs too, such as altering the colour, smell & taste of water.

Piyush or Waterguard: For water disinfection 0.5% chlorine solution prepared piyush can use by putting 3 drops in 1 liter of water. Shake well & drink after 30 minutes. Waterguard can use by putting 2 ml in 10 liters of water. Stir well with clean utensil & drink after 30 minutes. These methods are very easy to use & save fuel too.

Filtration: It removes 98 -99\% of bacteria. Water can be purified on a small scale by filtering through:

• Slow Sand Filter
  • Charcoal Filter
  • Bio-sand Filter
• Mechanical or Rapid San (domestic) Filter
  • Charcoal Filter: In this system also three pots is required. Take one pot & place pebbles, sand & charcoal in container to make a filter which is set on top of second receptacle & a hole is made at the bottom of first receptacle. Pour water into top filter & collect drinking water from the bottom.
  • Bio-sand Filter (kanchan arsenic filter): The kanchan arsemic filter is an innovative house hold drinking water treatment device for removing arsemic, pathogens, iron, turbidity, odour and some other contaminants in drinking water. Bio-sand filter uses slow sand filtration process to remove water contaminants including bacteria, iron, turbidity & odour.
• Mechanical or Domestic Filter: In this filter water is filtered by filter candle. The principles of these filters is that water is made to pass through porous "candles" which consist of coarse grained or fine grained according to the material of which they are made. They filter bacteria but not the filter- passing viruses. Filter candles are liable to be logged with impurities & bacteria. So, they should be cleaned by scrubbing with a hard brush under running water & bailed at least once a week. Only clean water should be used with ceramic filter.

SODIS (Solar Water Disinfection):

SODIS is a low-cost (free) and straightforward technique of water filtration that utilizes soft drink bottles and sunshine. It simply needs sunshine, empty soft plastic bottle, and a black surface (if possible). It was developed by the Swiss in 1991 and used to treat waterborne infections including cholera in Columbia, Bolivia, Togo, Burkina Faso, Indonesia, Thailand, and China. It is currently a strategy that many households utilize.

Process of SODIS

• Take 1-2 liters of transparent plastic bottle & remove the label. The bottle should be clean & not crushed.
• Fill the bottle half with clean water.
• Fill the bottle with full volume of water.
• Now, put the bottle horizontally on a flat position in direct sunlight from sunrise to sunset at least 6 hours) so that the bottle can get maximum amount of sunlight. The process is even more effective if the bottom is painted black or placed on a black sheet of plastic, which absorbs more heat thus helping to kill more microbes.
• If there is any cloudiness, the bottle should be kept for 2 days.
• Now, the water is safe & potable for drinking. In this method, 80-85% of microorganisms present in water are killed by means of ultraviolet rays of sunlight. In order to be effective, the water should be free from turbidity. This water can be used up to 1 week.

Principles

• The sunlight is most essential factor.
• The turbidity should be eliminated.
• Pet bottles of 1 or 1.5 liters not more than 10 mm diameter are used.
• We should shake the half filled bottle for 20 times. It increases the amount of O_{2} inside the bottle which kills micro-organisms & improves the taste.
• UV rays can pass only upto 10 cm of water level so large bottle is not useful.
• UV rays can not pass in crashed, dirty & glass bottle so clean plastic bottle is needed.
• During cloudy day, UV rays enter in between cloud it also works during cloudy day but takes a long time.

Advantages

• It is inexpensive, simple & easy to use.
• Safe, wholesome & having good taste.
• The disinfected water can be used for a week.
• The bottle can be reused.

Limitations

• This process is not use full without sunlight.
•  It reduces the amount of micro-organisms but will not purify chemically contaminated water.
• SODIS is not useful for treating large volume of water as it generates only a few liters per day.
• It requires minimum of 5 hours of full sunlight to effectively kill microbes.