TREATMENT OF WATER

The available raw water has to be treated to make it fit, i.e. potable, means safe for human consumption. It should satisfy the physical, chemical and bacteriological standards as specified above. The various methods of purification of water are

1. Screening
2. Plain  sedimentation
3. Sedimentation  aided  with  coagulation
4. Filtration
5. Disinfection
6. Aeration
7. Softening
8. Miscellaneous treatments like defluoridation, recarbonation desalination etc.

Screening

Screens are provided before the intake works so as to prevent the entry of big objects like debris, branches of trees, part of animals etc. Screens may be of two types, coarse screen and fine screens. Coarse screens are parallel iron rods placed vertically or at a small slope at about 2.5 cm to 10 cm apart. The fine screens are made up of fine wire or perforated metal with small openings less than 1 cm size. Finer is the screen more are the chances of clogging so generally only coarse screens are used. The screens may be manually cleaned or mechanically cleaned depending upon the requirement i.e. the size of the treatment plant.

Plain Sedimentation

Sedimentation is done to remove the impurities which have specific gravity more than that of water and are settleable. When water is moving these impurities remain in suspension due to the turbulence and as the velocity is reduced they settle down. It is not necessary to stop the motion of water completely as it will require more volume of  the  sedimentation  tanks.  As  per  the  theory  of  sedimentation  the  settlement  of  a particle depend upon the velocity of flow, the viscosity of water, the size shape and specific gravity of particle. The settling velocity of a spherical particle is expressed by Stoke’s law which gives the final equation as follows,

Knowing the settling velocity of particle, that is intended to be settled, the design of the settling tank is done.

Sedimentation Aided with Coagulation

The fine suspended particles like mud particles and the colloidal matter present in water cannot settle down by plain sedimentation with ordinary (lesser) detention periods. Some of the colloidal impurities will not settle even if the water is detained for long periods in the sedimentation tanks as the same charge on the clay particles repel each other and do not allow them to settle down. So the sedimentation is aided with  coagulation.  Coagulation  is  a  process  in  which  some  chemical  like  alum  or ferrous sulphate is mixed in water resulting in particle destabilization. Operationally this is achieved by the addition of appropriate chemical like alum and intense mixing for achieving uniform dispersion of the chemical.

These chemicals are more effective when the water is slightly alkaline. Sometimes sodium carbonate or lime is to be added to achieve the suitable pH of water. Flocculation is the second stage of the formation of settleable particles (or flocs) from destabilized (neutral) colloidal particles and is achieved by gentle (slow) mixing. So in flocculation the alum is first mixed rapidly for dispersion and then slow mixing produces flocks. Both these stages of flocculation are greatly influenced by physical and chemical forces such as electrical charge on particles, exchange capacity, particle size and concentration, pH, water temperature and  electrolyte  concentration.

Filtration

Filtration is a physical and chemical process for separating suspended and colloidal impurities from water by passage through a porous bed made up of gravel and sand etc. Actually the sedimentation even aided with coagulation and flocculation cannot remove  all  the  suspended  and  colloidal  impurities  and  to  make  water  (specially surface water) fit for drinking filtration is a must. The theory of filtration includes the  following  actions:

• Mechanical  straining,  the suspended  particles  present  in  water  that  are  of bigger size than the voids in the sand layers are retained their itself and the water becomes free of them. The sand layer may get choked after some time and then it is to be cleaned for further action by washing it back.

• Sedimentation, the small voids in the sand act as tiny sedimentation tanks and the colloidal matter arrested in these voids is a gelatinous mass and thus attracts other finer particles. These finer particles are thus removed by the sedimentation.

• Biological metabolism, certain micro-organisms are present in the sand voids.

• They decompose the organic matter like the algae etc. and thus remove some of the impurity.

• Electrolytic  change,  according  to  the  theory  of  ionization  a  filter  helps  in purifying the water by changing the chemical characteristics of water. The sand grains of the filter media and the impurities in water carry electrical charge of opposite nature which neutralizes each other and forces the paticles to settle now by gravity.

Disinfection

The filtration of water removes the suspended impurities and removes a large percentage of bacteria but still some of the bacteria remain there in the filtered water. This bacteria may be harmful bacteria (disease producing bacteria) known as pathogenic bacteria. The process of killing these bacteria is known as disinfection. There are many diseases like cholera, gastro entities, infectious hepatitis (jaundice), typhoid etc., the bacteria or virus of which transmits through water. It is necessary to make water free from any micro-organism before human consumption.

Contamination (mixing of pathogenic micro-organism) may take place in the water supply at any time (because of leakage etc.) so proper measures must be taken to stop it at all levels. Generally the disinfection is done by adding chlorine to water. There should be a residual amount of chlorine after the disinfection to fight with any probable contamination in the route of water to the consumer. Following are some of the methods of disinfection

• Boiling of water
• Treatment with excess lime
• Use of ozone
• Treatment  with  ultraviolet  rays
• Use  of  potassium  permanganate
• Treatment with silver
• Use of bromine, iodine and chlorine

Out of the above, treatment with chlorine is the most popular and economically effective. Actually the criteria for a good disinfectant as per the Manual are,

• It should be capable of destroying the pathogenic organisms present, within the contact time available and not unduly influenced by the range of physical and chemical properties of water encountered particularly temperature, pH and  mineral  constituents.

• It should not leave products of reaction which render the water toxic or impart colour or otherwise make it unpotable.

• It should have ready and dependable availability at reasonable cost permitting convenient,  safe  and  accurate  application  to  water.

• It should possess the property of leaving residual concentrations to deal with small  possible  recontamination.

• It should be amenable to detection by practical, rapid and simple analytical techniques in the small concentration ranges to permit the control of efficiency of the disinfection process.

The factors affecting the efficiency of disinfection are

• Type, condition and concentration of organisms to be destroyed
• Type  and  concentration  of  disinfectant
• Contact  time  and  concentration  of  disinfectants  in  water  and
• Chemical and physical characteristics of water to be treated particularly the temperature,  pH  and  mineral  constituents.

Potable water should always have some amount of residual chlorine, as there are all chances of contamination at all levels. This may be 0.2 ppm. to 0.3 ppm. depending upon the requirement (rainy season or enhanced chances, more Cl required). To make sure the presence of chlorine some tests are done out of which Orthotolodine test is the most common one.

Orthotolidine Test: In this test 10 ml of chlorinated sample of water is taken after the required contact period (say half an hour) in a glass tube. 0.1 ml of orthotolidine solution is added to it. The colour formed is noted after 5 minutes and compared with the standard coloured glasses. Darker is the yellow colour formed more is the residual chlorine. The test is very simple and even a semi- skilled employee can perform it satisfactorily and it can be done at the site itself and accordingly corrective measures can be taken.

For example if there is a complaint from a hostel mess. Test is performed for the tank water and if no residual chlorine is found, bleaching powder (a good source of chlorine) is mixed with some water and added to the tank water in paste form and stirred. The test is again performed after half an hour till it shows the required residual chlorine.

Aeration

• Taste and odour, both are undesirable in water. Aeration is done to remove taste and odour. Aeration is done to promote the exchange of gases between the water and the atmosphere. In the water treatment, aeration is performed for the following purposes,

• To add oxygen to water for imparting freshness, for example    water from underground sources may have lesser oxygen.
• For expulsion of carbon dioxide, hydrogen sulphide and other volatile sustances causing  taste  and  odour.
• To precipitate impurities like iron and manganese specially from undeground water.

• In aeration gases are dissolved in or liberated from water until the concentration of the gas in the water has reached its saturation value. The concentration of gases in a liquid generally obeys Henery’s law which states that the concentration of each gas in water is directly proportional to the partial pressure (product of the volume percentage of the gas and the total pressure of the atmosphere.) or concentration of gas in the atmosphere in contact with water. The saturation concentration of a gas decreases with temperature and dissolved salts in water. Aeration accelerates the exchange of gas. To ensure proper aeration it is necessary to,
  • Increase the area of water in contact with the air. The smaller are the droplets produced the larger will be the area available.
  • Keep the surface of the liquid constantly agitated so as to reduce the thickness of the liquid film which would govern the resistance offered to the rate of exchange  of the  gas.
  • Increase the time of contact of water droplets with air or increase the time of flow which can be achieved by increasing the height of jet in spray aerators and increasing the height of tower in case of packed media.
  • Where oxygen is to be dissolved in water, the concentration or partial pressure of the oxygen may be increased by increasing the total pressure of the gas in contact with water. For this purpose air injected into a main under pressure is a reasonably efficient method of increasing the amount of dissolved oxygen.

Water Softening

The reduction or removal of hardness from water is called as water softening. For domestic water supplies the softening is done to reduce the soap consumption, to ensure longer life to washed fabric, to lower the cost of maintaining plumbing fixtures and to improve the taste of food preparations and improve palatability (good taste). For industrial supplies softening is done for reducing scaling problems in boilers and the interference in the working of dyeing systems.

Usually a total hardness of 75 to 100 mg/l  (as CaCO3) would meet these requirements. The magnesium hardness should not exceed 40 mg/l  to minimize the possibility of magnesium hydroxide scale in domestic water heaters. Calcium and magnesium associated with bicarbonates are responsible for carbonate hardness and that with the sulphates, chlorides and nitrates contribute to non carbonate hardness. Normally the alkalinity measures the carbonate hardness unless it contains sodium alkalinity. The non carbonate hardness is measured by the difference between the total hardness and the carbonate hardness. Carbonates and bicarbonates of sodium are described as negative non carbonate hardness.

The temporary hardness or bicarbonate hardness can be removed by boiling or by adding lime. The following reactions take place by boiling:

As it is difficult to boil the water at large scale the addition of lime is done. The following reaction takes place when lime is added to water,

The carbonates of calcium and magnesium are removed by sedimentation being insoluble in water.
The permanent hardness is removed by:

1. Lime-soda process
2. Zeolite process
3. Demineralization  or  deionization  process. Lime-soda  process

In the lime soda process lime and soda ash (Na2CO3) are added which removes both the  temporary  and  permanent  hardness. The additional reactions with soda are as follows

Using the above reactions the hardness is removed in the lime soda plants

Zeolite  process

The lime soda method is a precipitation method in which some chemicals or reagents are added to the water. In the zeolite process no such chemical is added and the hard water is passed through a bed of special material called as the zeolite. The zeolite has the property of removing calcium and magnesium from the water and substituting sodium in their place by ion exchange phenomenon. Zeolites are complex compounds of  aluminum,  silica  and soda,  some  times  synthetic  and  otherwise  natural.  Natural zeolites are mainly processed from green sand (glauconite). It   has an exchange value of  about  8000  gm  of  hardness  per  m3   of  zeolite.  The  common  artificial  zeolite  is permutit. It has larger grains with white colour. Permutit (SiO2  Al2O3  Na2O) has a high exchange value of 35000 to 40000 gm of hardness per m3   of zeolite.

When hard water passes through a bed of permutit the following reactions take place

Similar reactions take place with compounds of magnesium hardness can be reduces to almost zero by this method. Due to continuous use of the zeolite the sodium gets exhausted  and  then  the  zeolite  has  to  be  regenerated  by  passing  a  solution  of  salt through it. The sodium in the brine (salt water) replaces the calcium and magnesium in the exhausted zeolite which is thus restored and the calcium and magnesium are discharged  to  waste  with  the  wash  water.

Demineralization or deionisation process

The conventional zeolites exchange the sodium cations (positive in electric charge) for the cations of calcium magnesium and iron in water and the softening is essentially a cation exchange process. In the demineralization or deionisation process, some other zeolite is used which exchanges all the cations (including sodium) for hydrogen. Some of the  zeolites are patented as zeo-karbs, catex organolites etc. are carbonaceous zeolites since they are made from materials like coal and lignite. They are also known as hydrogen exchanger. The water treated with hydrogen zeolite has the sulphates, chlorides and bicarbonates transformed into sulphuric, hydrochloric and carbonic acids. Actually the demineralization is done mainly to make the brackish water (as sea water) useful for drinking. It is a costly process.