In this article we will discuss about the various operations involved in water treatment.
1. Sedimentation:
Sedimentation is a process of separating from water by gravitational settling the suspended particles that are heavier than water.
In water there are mainly two types of suspended solids:
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(i) Inorganic solids having specific gravity of about 2.65, and
(ii) Organic solids having specific gravity in the range of 1.0 to about 1.4.
Thus most of the suspended particles present in water have specific gravity greater than 1 (i.e., specific gravity of water), but these are held in suspension because of turbulence in flowing water. However, when water is retained in a tank or basin it is brought to rest and there being no turbulence the suspended particles settle down and get deposited at the bottom of the tank.
The tank or basin used for retention of water for the purpose of sedimentation is known as sedimentation tank or sedimentation basin, or settling tank or settling basin. The time for which water is retained in a sedimentation tank is known as detention period or detention time or retention period. The sedimentation tanks are located near the filter units and in case of variations in demand; these may be used as storage reservoirs.
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Sedimentation may be classified as:
(i) Plain sedimentation, and
(ii) Sedimentation with coagulation (or coagulation, flocculation and sedimentation).
(i) Plain Sedimentation:
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In plain sedimentation suspended particles are separated from water by the action of natural forces alone i.e., by gravitation with or without natural aggregation. Plain sedimentation is usually employed as a preliminary process to reduce heavy sediment loads from highly turbid water prior to subsequent treatment processes such as coagulation/filtration.
(ii) Sedimentation with Coagulation:
In sedimentation with coagulation fine suspended particles and colloidal particles, which cannot be removed by plain sedimentation within commonly used detention periods of few hours, are converted into settle able floes by coagulation and flocculation and subsequently settled in sedimentation tanks.
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The phenomena of settling down of particles at the bottom of sedimentation tank is known as hydraulic subsidence and every particle has its own hydraulic settling value which will cause its hydraulic subsidence.
The hydraulic subsidence or settling of particles in a sedimentation tank is affected by the following factors:
(i) Velocity of flow of water;
(ii) Size and shape of particles;
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(iii) Specific gravity of particles;
(iv) Viscosity of water;
(v) Surface overflow rate;
(vi) Detention period;
(vii) Inlet and outlet arrangements; and
(viii) Effective depth of settling zone.
The particles are moved in the horizontal direction by the velocity of flow of water. As such a higher velocity of flow will oppose the settling down of the particles.
The larger particles will settle more readily than the smaller particles because of larger force of gravity in the case of former than in the case of later.
The particles having specific gravity of about 1.2 or more, readily settle down in the basin, but it is difficult to cause the settling of lighter particles.
The viscosity of water helps settling down of the particles.
The settling of particles of varying hydraulic subsidence is solely a function of surface overflow rate also called ‘surface loading’. A higher surface overflow rate results in a lower detention period.
A longer detention period would be needed for settling of discrete particles as compared to that for flocculent particles forming clusters of different size, shape and weight.
Improper inlet and outlet arrangements would impede the settling of particles.
2. Filtration:
Considerable amount of suspended particles of clay and silt and colloidal matter present in raw water is removed by sedimentation with coagulation, but even after that it may contain certain amount of suspended matter which needs to be removed before water can be supplied to the public. Further sedimentation with coagulation is not effective in removing fine floe particles (which do not settle down in sedimentation tank), colour, dissolved minerals, and micro-organisms or bacteria from water.
As such the operation called filtration is adopted which is one of the most important operations in the purification of water. Filtration is a process of passing water through thick layers of porous media which in most of the cases is a layer of sand supported on a bed of gravel. For filtration of water different types of filters.
It has been noticed that during filtration the following effects are produced in water:
(i) The suspended and colloidal matter present in water is removed to a considerable extent.
(ii) The chemical characteristics of water are changed.
(iii) The number of bacteria present in water is also considerably reduced.
The above noted effects produced in water during filtration may be explained on the basis of the following four actions:
(i) Mechanical straining
(ii) Sedimentation and adsorption
(iii) Biological metabolism
(iv) Electrolytic changes.
(i) Mechanical Straining:
The particles of suspended matter that are of size larger than the size of the interstices or voids between the sand grains cannot pass through these interstices, and are therefore arrested and removed by the action of mechanical straining. It, however, cannot remove colloidal matter or bacteria too small to be strained out.
(ii) Sedimentation and Adsorption:
The interstices between the sand grains act as minute sedimentation tanks in which the particles of the suspended matter settle on the sides of the sand grains.
These particles adhere to the sand grains because of the following two reasons:
(a) Due to the physical attraction between the suspended particles and the sand grains.
(b) Due to the presence of a gelatinous coating formed on the sand grains by previously deposited colloidal matter and bacteria.
Thus by this action of sedimentation and adsorption, colloids, small particles of suspended matter and bacteria are removed.
(iii) Biological Metabolism:
The growth and life processes of the living cells is known as biological metabolism. The bacteria which are caught in the voids of the sand grains require organic impurities such as algae, plankton, etc., as the food for their survival. These organisms, therefore, utilise such organic impurities present in water, and convert them into harmless compounds by the complex biochemical reactions.
The harmless compounds so formed are deposited at the surface of the sand in the form of a layer which contains a zoological jelly in which the biological activities are at their highest. This layer is called the schmutzdecke (dirty skin). This layer further helps in absorbing and straining out the impurities. Moreover, the bacteria not only break-down the organic impurities, and convert them into harmless compounds, but also destroy each other and maintain a balance of life in the filter.
(iv) Electrolytic Action:
The action of filter is also explained by ionic theory. It states that when two substances with opposite electric charges come in contact with each other, the electric charges are neutralized and in doing so, new chemical substances are formed. It is observed that some of the sand grains of filter are charged with electricity of some polarity.
Hence, when particles of suspended and dissolved matter having electricity of opposite polarity come into contact with such sand grains, they neutralise each other and it results in changing the chemical characteristics of water. After some interval of time, the electrical power of sand grains gets exhausted. At that time, it becomes necessary to clean the filter and restore it with this property.
Filter Materials or Filter Media:
Sand is the most commonly used filter material because it is widely available, cheap and effective in removing impurities. The layer of sand is supported on a gravel bed. The sand to be used for filters should be fairly uniform in size.
The uniformity characteristics of sand are expressed in terms of:
(i) Effective size, and
(ii) Uniformity coefficient.
(i) The effective size of sand represents that size of sieve in mm through which 10% of the sample of sand by weight will pass. In other words it represents a size in mm such that 10% of the sand particles are finer than this size. It is denoted by D10 and it is also known as effective diameter of sand.
(ii) The uniformity coefficient of sand is the ratio of sieve size in mm through which 60% of the sample of sand by weight will pass, to the effective size of sand. Thus if D6o denotes the sieve size in mm through which 60% of the sample of sand by weight will pass (or D60 is a size in mm such that 60% of the sand particles are finer than this size), then the uniformity coefficient of sand, Cu, is given by-
For uniformly graded sand, Cu is nearly equal to unity.
Further the sand to be used for filters should satisfy the following norms:
(i) It should be of hard and resistant quartz or quartzite and free of clay, fine particles, soft grains and dirt.
(ii) Ignition loss should not exceed 0.7% by weight.
(iii) Its specific gravity should be in the range of 2.55 to 2.65.
(iv) Its silica content should be not less than 90%.
(v) Its soluble fraction in hydrochloric acid should not exceed 5% by weight.
(vi) Wearing loss should not exceed 3%.
Besides sand anthrafilt has also been used as filter material. It is made from anthracite which is a type of coal that bums nearly without flame or smoke. The other materials which have also been used as filter media include coal, crushed coconut shell, diatomaceous earth, powdered or granular activated carbon, etc. However, none of these filter materials have found wide application as sand and hence sand is the only filter material which is most widely used.
The layer of sand is supported on a bed of gravel. The gravel to be used below the sand should be hard, durable and free from impurities.
3. Disinfection:
A considerable amount of bacteria and other micro-organisms present in raw water are removed by filtration, but the water obtained from filters still contains bacteria and other micro-organisms, some of which may be pathogenic (disease producing). The water obtained directly from filters is therefore not safe for drinking because it may result in spreading of various water borne diseases.
As such in order to make the water obtained from filters safe for drinking purposes, it is necessary to kill the disease producing bacteria and other micro-organisms present in it. The treatment by which the disease producing bacteria and other micro-organisms present in water are killed is known as disinfection.
The substance or agent used for disinfection of water is known as disinfectant. Another term synonymous to disinfection is sterilization which, however, means killing of all types of bacteria and micro-organisms, (i.e., both disease producing and non-disease producing type).
Criteria for a Good Disinfectant:
A water disinfectant should satisfy the following criteria:
(1) It should be capable of killing the pathogenic organisms present in water within the contact time available, the range of water temperatures encountered, and wide range of pH value. Further its effectiveness should not be affected by the mineral constituents of water to be treated.
(2) It should not render the water toxic, or impart colour or otherwise make it unpotable.
(3) It should be readily available at reasonable cost.
(4) It should be safe to handle, and its method of application should be simple.
(5) It should be able to persist in residual concentrations as a safeguard against recontamination.
(6) It should be amenable to detection by practical, rapid and simple analytical techniques in the small concentration ranges to permit the control of disinfection process.
Mechanism of Disinfection:
The mechanism of killing the pathogens depends largely on the nature of the disinfectant and on the type of micro-organisms.
In general the destruction or inactivation of the micro-organisms may be caused by the following four mechanisms:
(i) Damage to cell wall of micro-organisms leads to cell lysis and death.
(ii) Alteration of cell permeability which causes outflow from the cells of vital nutrients such as nitrogen and phosphorous.
(iii) Changing the colloidal nature of the cell protoplasm,
(iv) Inactivation of critical enzyme systems responsible for metabolic activities.
Kinetics of Disinfection:
The kinetics of disinfection is affected by several factors. The effect of some of these factors can be expressed V in terms of mathematical relationships derived for ideal conditions.
The ideal conditions assumed are as follows:
(i) All cells of a single species of organisms are discrete units which are equally susceptical to a single species of disinfectants.
(ii) Both cells as well as disinfectants are uniformly dispersed in water.
(iii) During time of contact the disinfectant stays unchanged in chemical composition and constant in concentration.
(iv) Water contains no interfering substances.
Under the above noted ideal conditions the process of disinfection is a function of the following three factors:
(a) Time of contact
(b) Concentration of disinfectant
(c) Temperature of water.
(a) Time of Contact:
Time of contact is an important factor affecting the rate of destruction of organisms. In general under ideal conditions according to Chick’s law the rate of kill of organisms is proportional to the number of organisms remaining in water at any time t. Thus –
Equation 9.44 indicates that 100% kill of organisms will require an infinite time of contact which is not feasible in practice. As such the usual practice is to determine the time of contact required to achieve slightly less than 100% kill, say 99.7 to 99.9%.
According to Chick’s law rate of kill of organisms remains constant with time. However, rate of kill of organisms have been experimentally observed to increase with time in some cases and decrease with time in other cases. To account for these departures from Chick’s law the following modified equation has been suggested.
(b) Concentration of Disinfectant:
Rate of killing of organisms is affected, within limits, by changes in concentration of disinfectant. The relationship between concentration of disinfectant and time required for killing a desired percentage of organisms is generally expressed by the following equation-
Cntp = constant …(9.48)
where C is concentration of disinfectant;
n is a coefficient of dilution ; and
tp is time required for a constant percentage of kill of organisms.
Values of n greater than 1 indicate rapid decrease in the efficiency of disinfectant as its concentration is reduced; if n is less than 1, contact time is more important than concentration, and for n equal to 1 both concentration and contact time affect the efficiency of disinfectant to the same extent.
Concentration-time relationship for HOCl at 0 – 6°C resulting in 99% kill of E-Coli and several other viruses are as indicated below:
The value of n is less than 1, changes in contact time have more effect on killing of organisms than corresponding changes in concentration of HOCl. Further these equations also indicate that some viruses such as coxsackie virus A2 are more resistant to killing by HOCl than commonly used indicator organisms, E-Coli.
(c) Temperature of Water:
It is observed that increase in temperature results in a more rapid kill.
The relationship between time and temperature, to effect a given percentage of kill can be expressed as:
Equation 9.49 indicates that at lower temperature of water (e.g., in winter season) the time required for achieving the same percentage of kill for the same concentration of disinfectant will be higher than that for higher temperature (e.g., in summer season). However, if time of contact cannot be changed due to design constraints, dose of disinfectants will have to be changed to account for changes in temperature to achieve same percentage of kill.
4. Water Softening:
The reduction or removal of hardness from water is known as water softening. The water to be supplied to the public should not be very hard.
Although there is no fear of any health hazard due to the consumption of hard water, but it is not desirable to supply hard water because it causes the following troubles:
1. It causes more consumption of soap in laundry work and hence proves to be uneconomical for washing processes in textile industries.
2. It leads to the modification of some of the colours and thus affects the working of the dyeing system.
3. It causes serious difficulties in the manufacturing process such as paper making, ice manufacture, canning, rayon industry, etc.
4. It causes corrosion and incrustation of pipes and plumbing fixtures.
5. It causes formation of scales in the steam boilers and other water heating systems.
6. It makes food tasteless, tough or rubbery.
7. It increases the fuel costs.
Types of Hardness:
There are two types of hardness:
(i) Temporary hardness; and
(ii) Permanent hardness.
(i) Temporary Hardness:
Temporary hardness also known as carbonate hardness is caused due to the presence of bicarbonates of calcium and magnesium.
(ii) Permanent Hardness:
Permanent hardness also known as non-carbonate hardness is caused due to the presence of sulphates, chlorides and nitrates of calcium and magnesium.
Removal of Temporary Hardness:
Temporary hardness of water can be removed by the following methods:
(i) By boiling;
(ii) By adding lime.
(i) Boiling:
The following reactions take place during boiling of water:
(ii) Addition of Lime:
When hydrated lime Ca(OH)2 is added to water the following reactions take place-
In both the cases calcium carbonate CaCO3 and magnesium carbonate MgCO3 are formed which are insoluble in water, and hence these can be easily removed in the sedimentation tanks. However, the boiling of water on a large scale is impracticable and uneconomical. Hence addition of lime is preferred to boiling for the removal of temporary hardness.
5. Miscellaneous Methods of Water Treatment:
Besides the various methods of water treatment, certain other methods of water treatment are required to be adopted for specific purposes which include-
I. Removal of colour, odour and taste
II. Removal of iron and manganese
III. Fluoridation and Defluoridation
IV. Desalinization or Desalination
I. Removal of Colour, Odour and Taste:
Some of the methods of water treatment remove colour, odour and taste from water.
Such methods of treatment of water are:
(i) Coagulation followed by filtration
(ii) Pre-chlorination
(iii) Super-chlorination followed by dechlorination; and
(iv) Use of chlorine dioxide.
However, these methods remove colour, odour and taste from water only up to a certain extent.
As such for the removal of colour, odour and taste from water some special methods are usually adopted which are as indicated below:
1. Aeration
2. Treatment by activated carbon
3. Use of copper sulphate
II. Removal of Iron and Manganese:
It is observed that practically all waters whether from surface or underground sources contain at least some traces of iron. This is due to the fact that iron is present in practically all soils and rocks, and rain water in percolating through soils and rocks acquires iron in addition to other mineral constituents according to the character of the geological formation. Further manganese, usually in smaller amounts, may accompany iron in water.
When their content in water exceeds 0.3 p.p.m, they become objectionable due to the following reasons:
(i) If water containing iron and manganese is used in a laundry, it will develop reddish or brownish stains on the clothes. It also creates stains on fabrics in textile industry.
(ii) Iron and manganese may be deposited in distribution pipes. It leads to the blocking of pipes, meters, etc.
(iii) The water becomes unpleasant in taste.
(iv) The water is coloured either red or brown. The reddish tinge is due to the presence of iron and the brownish tinge is due to the presence of manganese.
(v) The water required for certain industries such as paper-making, photographic film manufacturing, ice-making, etc., must be entirely free from iron and manganese.
Iron and manganese may be present in water either without combination with organic matter or in combination with organic matter.
When iron and manganese occur in water without combination with organic matter, they can be removed by aeration followed by coagulation, sedimentation and filtration. During aeration dissolved ferrous and manganese compounds are converted into insoluble ferric and manganese compounds which are then removed in settling tanks or filters.
Iron is present in water mainly as ferrous bicarbonate and during aeration the following reactions take place:
4Fe (HCO3)2 + 2H2O + O2 → 4Fe (OH)3 + 8CO2
The ferric hydroxide Fe (OH)3 is insoluble in water. Similar reactions take place for manganese bicarbonate.
When iron and manganese occur in water in combination with organic matter, it becomes difficult to break the bond between them. Once the bond is broken, the treatment may be carried out. The bond may be broken, either by adding lime and thus raising the pH value of water to about 8.50 to 9.00, or by adding chlorine or potassium permanganate.
While deciding the method of removal of iron and manganese from water, it is desirable to know what other treatment is to be given to water and if possible, duplication of treatment should be avoided. For example, chlorination can be employed to assist removal of iron and manganese and at the same time bacterial protection can be achieved by the same process.
It is to be noted that the removal of iron alone is simpler and more successful than the removal of manganese or iron combined with manganese. As such the possible presence of manganese in combination with iron must be carefully ascertained because special measures are required for its removal.
Manganese is not oxidised and precipitated so readily as iron and if suitable precautions are not taken, the treatment any remove iron, but it may not prove adequate to remove manganese from water, and hence deposits will be produced in mains, reservoirs, etc.
III. Fluoridation and Defluoridation:
Fluoridation:
It is found that a fluoride concentration of about 1 p.p.m in water is beneficial for prevention of dental caries (i.e., decay of teeth) in children. Thus if water has less concentration of fluoride it is essential to added sufficient quantity of fluoride to water to be supplied to bring the fluoride content to the desired level of about 1 p.p.m. The process of raising the fluoride content of water is known as fluoridation.
The fluoride compounds which are usually adopted for fluoridation of public water supplies are sodium fluoride (NaF), sodium hexafluorosilicate or sodium silico fluoride (Na2 Si F6) and hexafluorosilicic acid or hydrofluosilicic acid (H2SiF6). Sodium-fluoride having purity to the extent of 95 to 98% is most commonly used.
The application of fluorides to water may be either in powder form or in solution form. It depends on the characteristics of fluoride compound to be used for fluoridation. For instance it is preferable to apply sodium fluoride in solution form since in powder form it is toxic and must be contained in dust-tight hoppers or containers.
Further the feeding of fluorides is usually done separately and not along with chlorine. It is best to apply fluoride after other treatments, but at point where thorough mixing of fluoride with water can take place before water leaves the treatment works. It is essential to maintain a close control of the dose of fluoride applied to water. The excess fluoride, if any, can be removed by any suitable method of defluoridation.
It is found that if water with excessive concentration (more than 3 p.p.m) of fluoride is consumed it may cause dental fluorosis or mottled enamel in children.
Further when water containing 8 to 20 p.p.m of fluoride is consumed over a long period of time, bone changes may occur, resulting in crippling fluorosis. Hence when concentration of fluoride in water is more than 1 to 1.5 p.p.m., it should be reduced. The process of reducing fluoride concentration in water is known as defluoridation.
Following are the methods of defluoridation:
(i) Activated carbons prepared from various materials can be used as defluoridation agents.
(ii) During lime-soda process of water softening, fluorides are also removed along with the removal of magnesium.
(iii) The materials such as filter-alum, calcium phosphate, bone charcoal, synthetic tri-calcium phosphate, etc., may be added for the removal of excess fluoride content in water.
(iv) Water may be allowed to pass through filter beds containing fluoride retaining materials.
Most of the above mentioned methods of defluoridation suffer from one or the other disadvantages such as high initial cost, expensive regeneration, poor fluoride removal capacity, etc. A new technique known as Nalgonda technique has been tried to overcome all these drawbacks. In this technique sodium aluminate or lime, bleaching powder and filter-alum are added to fluoride water in sequence. Water is then stirred for ten minutes and settled for one hour.
Water is then withdrawn without disturbing the sediments. Sodium aluminate or lime accelerates settlement of precipitate and bleaching powder ensures disinfection. The alum dose required will depend upon the concentration of fluorides, alkalinity and total dissolved solids in raw water. The Nalgonda technique includes coagulation-sedimentation, disinfection and defluoridation. It is found that this technique is simple in operation and economical. It can be used with advantage in villages either on an individual scale or on a mass scale.
IV. Desalinization or Desalination:
It is estimated that about 97% of the total quantity of water available on our planet earth is carried by oceans. Ocean water is highly saline having a dissolved salt content of about 35000 mg/l and hence it is not suitable for consumption. The remaining 2.5% of water is brackish having dissolved salt content of 1000 to 3000 mg/l which is also too salty for consumption.
Thus only about 0.5% of the total available water is fresh water, which being too meagre it is realized that the fresh water supplies will soon be inadequate in many areas. As such there has been a rapidly increasing interest to develop methods by which saline water may be converted to fresh water. The process of converting saline water to fresh water is known as desalinization or desalination.
The various methods used for desalinization are as indicated below:
1. Distillation
2. Reverse osmosis
3. Electro dialysis
4. Freezing
5. Demineralization
6. Solar evaporation.