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WATER POLLUTION (UNIT -1) : SYLLABUS : Water Pollution: Brief overview of water quality parameters, Classification of water pollutants and their sources, Total Hardness and its determination (EDTA method)- (Numericals), Alkalinity and its determination-(Numericals), DO, BOD and COD their determinati...

WATER POLLUTION (UNIT -1) : SYLLABUS : Water Pollution: Brief overview of water quality parameters, Classification of water pollutants and their sources, Total Hardness and its determination (EDTA method)- (Numericals), Alkalinity and its determination-(Numericals), DO, BOD and COD their determination, Treatment of Water for Domestic use: Disinfection by Breakpoint chlorination. Waste-water treatment: Primary, secondary and tertiary treatment, Water Conservation and Management, Rain-water harvesting 1. Water Pollution Overview : Any contamination of water which alters the physical, chemical or biological properties of water that are detrimental to the ‘whole’ ecosystem e.g., fertilizers and pesticides from agricultural runoff; sewage and food processing waste; lead, mercury, and other heavy metals; chemical wastes from industrial discharges; and chemical contamination from hazardous waste sites. brought about mainly by human activities and natural resources and which cause harmful effects on terrestrial and aquatic life. 2. Sources of Pollutants : Natural sources: decomposed vegetable, animal and weathered products brought into main water resources Anthropogenic activities: domestic as well as industrial pollution Agricultural discharge: pesticides, insecticides, plant nutrients, fertilizers etc. Bacteria, algae, virus also cause water pollution 3. Difference Bw Domestic and Industrial wastes : DOMESTIC INDUSTRIAL Release of huge quantities of municipal Effluents from factories, refineries and a and domestic wastes through drains number of chemical industries. E.g., into the rivers and canals. E.g., human acids, alkalis, detergents, metals, faeces, kitchen wastes, organic water pesticides etc. that provide nutrition to bacteria and fungi 4. Classification of Water Pollutants: i) Oxygen demanding wastes : Organic substances formed due to death and decay of aquatic/terrestrial flora and fauna, from municipal waste water or in effluents from certain industries, like food processing, paper production Organic substances are oxidized by bacteria or micro-organisms consuming dissolved oxygen in water. Produce undesirable odour, tastes and reduce the acceptability of water in domestic supply. Dissolved oxygen (DO) is Essential for sustaining the plant and animal life in any aquatic system Amount of DO in water is reduced because of oxygen demanding wastes that oxidize in water, reducing the amount of DO.  Oxidation of certain inorganic compounds may also contribute to the oxygen demand. Measures of oxygen demand commonly used: A. Biological Oxygen Demand (BOD): measure of biological degradation of waste; most important measure of the strength of organic pollution. B. Chemical Oxygen demand (COD): measure of chemical oxidation of waste. ii) Pathogens (Disease causing agents) Grow and multiply within the host Carried into the water bodies by sewage and wastes from farm and various industries Contaminated water caused by poor sanitation can lead to both, water borne and water contact diseases. Examples of pathogens associated with water borne diseases include bacteria, viruses and protozoa. Common diseases: Cholera, Dysentery, Typhoid etc. Mechanism of infection: Through ingestion or contact A. Larvae attach themselves to human skin, penetrate it and enter the blood stream, Schistosomiasis caused by free swimming larva, cercaria, in water. B. Escherichia coli (E. coli), belonging to the coliform group is harmless bacteria found in large number in human faeces. A large concentration of E. coli in water indicates faecal contamination and evidence of the presence of pathogens. iii) Synthetic organic compounds: Pesticides, synthetic organic chemicals and detergents Non-biodegradable and persistent Accumulative toxic poisons and ultimately reach objectionable levels in water. ➔ Pesticides : Way to water body: surface run off from agricultural lands, waste discharge by pesticide manufacturers and by other means. Effects: Persistent, Bio-accumulate in fatty tissues, Bio-magnifiable, Carcinogenic, Cause birth defects, Severe neurological damage etc. ➔ Detergents – “cleansing agents” : The basic active ingredient in detergents is the surfactant or surface-active agent having hydrophobic (hydrocarbon) and hydrophilic (polar) groups. Surfactants decrease the surface tension of water so that they can penetrate the surface and interstices of the object being cleaned.  Usually contain 10 – 30% surfactant and remaining are polysulphate salts (the builder) and a number of other ingredients. Surfactant concentrations as low as 1 ppm produce foam in rivers and in sewage treatment plants since non-toxic to humans but imparts off-taste to drinking water. It reduces the rate of oxygen absorption in water and may lead to the symptoms similar to asphyxiation in certain kind of fish (trout). Till the early 1960s the surfactants present in synthetic detergent was alkylbenzene sulphonate (ABS) which is non biodegradable. ABS have been now replaced by linear alkyl sulphonate (LAS) which is rapidly biodegradable. iv) Plant nutrients : Chemicals such as nitrogen, phosphorus, carbon, sulphur, calcium, potassium, iron, manganese etc. are essential to growth of living things. These cause excessive growth of aquatic plants, algal bloom. Algae die and decompose leading to reduced DO levels. The gradual accumulation of silt and organic matter in the lake is known as eutrophication, which is serious health hazard. Eutrophied lake gets shallower and warmer with further growth and decay of algae and eventually lake turns into a marsh or bog. Way to water bodies: through municipal waste water, industrial wastes and runoff from fertilized lands. Certain bacteria in the intestines reduce nitrate in water to toxic Nitrites. Nitrites have greater affinity for haemoglobin as compared to oxygen. Thus, nitrite attaches to haemoglobin forming methaemoglobin, producing a condition called methaemoglobin anemia commonly known as blue baby syndrome. In young babies, it is extremely fatal. Nitrates can be converted into nitrosamines in the body leading to gastric cancer. v) Inorganic chemicals and minerals Includes inorganic salts, mineral acids, finely divided metals or metal compounds, trace elements, cyanides, organometallic compounds etc. added to the water bodies through municipal and industrial waste waters and mine runoff. Acid Mine Drainage: The mining of sulphur bearing ores containing lead, zinc and copper lead to acid drainage. Coal mines discharge containing varying amounts of iron sulphide (pyrite) is also a major cause of acid mine drainage. These discharges release considerable quantities of sulphuric acid and ferric hydroxide, (which are formed as result of reactions between air, water and pyrite) into local streams through seepage. Soluble Salts: Water naturally accumulates variety of dissolved salts as it passes through soils and rocks on its way to sea. These salts include cations such sodium, calcium, magnesium and potassium and anions such as chloride, sulphate and bicarbonate.  Concentration of total dissolved solids (TDS) in water is less than 500 mg/L TDS - safer for drinking purpose. WHO recommends 300 mg/L TDS, not to be less than 50 mg/L Heavy Metals : The metals of particular concern in industrial waste waters are cadmium, chromium lead, mercury, silver, arsenic, aluminium, copper, cobalt, manganese etc. Some of these, such as chromium and iron, are extremely toxic in higher doses. Metals may be inhaled or may be ingested. These have a range of adverse impacts on the body, including nervous system and kidney damage, creation of mutations and induction of tumours. The kidneys contain millions of excretory units called nephrons and the chemicals that are toxic to kidneys are called nephrotoxins. Cadmium, lead and mercury are examples of nephrotoxic metals. vi) Sediments : Sediments include soil, sand and mineral particles washed into the aquatic environment by storms and flood waters. These are sources of organic and inorganic matter in the streams, fresh water, rivers, and other water bodies. Soil particles eroded by running water ultimately find their way into water reservoirs via ‘siltation’. This reduces the storage capacity in dams, etc. Blocked sunlight and hence decreased photosynthetic rate of aquatic plants. Evolution of oxygen is decreased. Sediments increase the cost of water treatment. Sediments enter pumping equipment and power turbines and increase the turbidity. vii) Radioactive substances: Enter into aquatic system through number of human activities involving the use of naturally occurring or artificially produced radioactive materials. Activities such as mining and processing of ores to produce radioactive ores, e.g, uranium and thorium. The refining of uranium ore is an important source of radioactive waste, producing radionuclides of radium, bismuth etc. From nuclear power plants and from industrial use of radioactive materials leakage from underground nuclear detonations. Use of radio isotopes in medicine, industry, agriculture and research operations. Radioactive substances can enter living organisms with food and water and get accumulated in blood and certain vital organs like thyroid gland, the liver, bone and muscular tissues. These may cause cancers, leukemia and eye cataract. viii) Thermal Discharges : Industry and power plants (Coal fired power plants, electric powers, steel and chemical industries as well as atomic energy plants) use large quantities of water for cooling purposes and discharged directly into water bodies. This results in increase in temperature of the water bodies which is, in general, called as Thermal Pollution.  Rise in temperature of water decreases dissolved oxygen content of water which effects the aquatic life. At 32°F the DO content of water is 12 ppm (100% Saturation) which is decreased to 6 ppm at 64° F. Fishes are killed due to action of heat on nervous system, inactivation of enzymes and coagulation of cell protoplasm. An increase in temperature also, increase the toxicity of some chemical pollutants. ix) Oil : Oil and oil wastes are added to the water bodies from industries as effluents, oil refineries, storage tanks, automobile waste oil and petrochemical plants. It is insoluble in water and hence floats over it as a thin layer. Oil may penetrate the feather of the birds and effect their insulation and buoyancy. Thus, birds experience difficulty in floating and flying. The birds may ingest oil while they dive to feed. This ingestion may produce toxic effects. Moreover, the oil slick formed on the surface prevents the diffusion of oxygen into water resulting in decreased concentration of DO. x) Volatile Organic Compounds Most commonly found contaminants in water. Often used as solvents in industrial processes. They are volatile and hence their concentrations remain as low as few micrograms per litre in surface water. But in ground water their concentrations can be hundreds or thousands of times higher. These are toxic and carcinogenic, their presence in drinking water is cause of great concern. 5. Sources of water : The chief sources of water fall in two main groups: ❑Surface water Flowing: i) Streams ii) River water iii) Sea water Still Waters iv) Lake water v) Reservoirs ❑Underground water: A part of the rain water, which reaches the surface of the earth, percolates into the earth. It comes in contact with a number of mineral salts present in the soil and dissolves some of them. It comes out in the form of spring or well. It is clear in appearance due to the filtering action of the soil, but contains more of the dissolved salts, thus more hardness. 6. Specification of water :  The main impurities present in water are classified into three types: Physical impurities: Physical Impurities include suspended and colloidal impurities (a) Suspended impurities (Make the water turbid )-Clay, sand, decayed vegetable and animal matter. (b) Colloidal impurities (Impart colour, odour and taste )-Finely divided clay and silica, colouring matter, waste products, etc. Chemical impurities Chemical Impurities include dissolved salts and dissolved gases. (a) Dissolved salts (make water hard)– Chlorides, sulphates, bicarbonates, carbonates of Ca, Mg, Na, K (b) Dissolved gases – Oxygen, carbon dioxide, hydrogen sulphide etc.(acidic and corrosive) Bacteriological impurities : Pathogenic micro-organism-Spread various diseases like typhoid, dysentery, hepatitis etc. 7. WATER QUALITY STANDARDS : Water used for drinking purpose should meet certain quality criteria with respect to appearance (turbidity, colour) potability (taste, odour), health (bacteria, nitrates, chlorides etc.,) and toxicity (metals, organics). These criteria are established by health or other regulating agencies to ensure that the water quality in a resource is suitable for drinking purposes. 8. WATER : Water is defined as amount of calcium and magnesium present in water and is classified into two types based on its property to react with soap solution. SOFT WATER : Soft water is one that gives good lather readily with soap solution. HARD WATER : Water that does not produce lather with soap readily but forms an insoluble precipitate like white scum is known as hard water. The hardness of  water is caused by the presence of dissolved salts such as bicarbonates, sulphates, chlorides and nitrates of divalent metal ions like calcium and magnesium. Soap is sodium or potassium salt of higher fatty acids like stearic, oleic and palmetic acids. When soap is mixed with soft water lather is produced due to stearic acid and sodium stearate. When water with hardness is used for washing, large amount of soap is consumed. Thus, hardness of water can be defined as the soap consuming capacity of water. The reaction can be explained as follows : Common hardness producing salts present in water: chlorides, sulphates and bicarbonates of Calcium and Magnesium. i.e., CaCl2 , CaSO4 , MgCl2 , MgSO4 ,Ca(HCO3 )2 and Mg(HCO3)2. Their quantity only decides the extent of hardness of water. 9. Classification of Hardness of Water : Hardness of water can be classified into two categories : 1. Temporary hardness (or) Carbonate hardness(Bicarbonates , Carbonates & Hydroxide of Ca, Mg) : This hardness is caused by two dissolved bicarbonate salts Ca(HCO3 )2 and Mg(HCO3 )2. The hardness is called temporary because, it can be removed easily by boiling. During boiling, bicarbonates are decomposed to yield insoluble carbonates or hydroxides, which are deposited as a crust at the bottom of vessel. 2. Permanent hardness (or) Non-Carbonate hardness (Chlorides, Sulphates of Ca, Mg): This hardness is due to the dissolved chlorides, sulphates and nitrates of calcium and magnesium. These salts are CaCl2 , MgCl2 , CaSO4 , MgSO4 , Ca(NO3 )2  , Mg(NO3 )2. It cannot be removed easily by boiling. Hence, it is called permanent hardness. Only chemical treatment can remove this hardness. TOTAL HARDNESS = TEMP HARDNESS + PERMANENT HARDNESS 10. Expression of Hardness : Hardness of water is expressed in terms of number of parts of CaCO3 (or) its equivalent present in a particular quantity of water. Calcium Carbonate equivalents Hardness is expressed in terms of CaCO3 (or) its equivalent. If water contains CaCO3 alone the hardness is a measure of number of parts of CaCO3. Usually, water contains some other salts. The amount of these salts is converted in to their CaCO3 equivalent. CaCO3 was selected for expression of the degree of hardness because; 1. It is an insoluble salt, and all the dissolved salts of calcium are precipitated as CaCO3 2. The Molecular Weight of CaCO3 is 100, and also is a primary standard. 11. Units of hardness : (4 QUESTIONS IN PDF(01)) 12. Estimation of Hardness of Water by EDTA Method : It is a complexometric titration of water sample with EDTA using EBT indicator. This is a reliable method because of its greater accuracy. EDTA (Ethylene diamine tetra acetic acid) forms a colourless stable complex with Ca2+ and Mg2+ ions in water at pH 8-10.  Ammonia buffer is used to maintain the pH. In this method EBT (Eriochrome Black-T) is used as an indicator. Initially EBT forms an unstable complex with Ca2+ and Mg2+ ions, giving wine red colour to the solution. During the titration EDTA reacts with this complex (Ca-EBT or Mg-EBT complex), forms a stable complex (Ca-EDTA or Mg-EDTA) and releases the blue EBT into the solution. Hence the end point is wine red to blue colour. EDTA EBT 13. Principle of EDTA Method : Disodium salt of EDTA forms complex with calcium and magnesium ions present in water. The indicator used in this titration is Eriochrome Black-T (EBT)* which also form unstable complex with calcium and magnesium ions present in water, at pH value about 8-10. In order to maintain the pH , buffer solution [ NH4Cl-NH4OH] mixture is added. Only at this pH such a complexation is possible, This wine-red colour solution is titrated against EDTA, EDTA replaces EBT indicator from [Ca / Mg - EBT] complex. The colour of the solution changes from wine red to blue at the end point.  EDTA is tetraprotic acid, can be represented as H4 Y, where H represents the four ionizable hydrogens. The protons liberated considerably alter the pH of the solution and may cause dissociation of metal-EDTA complex. 14. Procedure for EDTA method : (IN PRACTICAL FILE) (4 QUESTIONS IN PDF(02)) 15. ALKALINITY : Alkalinity of water is a measure of its acid neutralizing ability or it is the tendency of water to accept H+ ions in order to neutralize it with the supply of OHions. In water analysis, it is often desirable to know the kind and amount of various forms of alkalinity present in water. Alkalinity in water is due to the presence of bicarbonates, carbonates and hydroxides of Ca, Mg, Na and K. (i) Caustic alkalinity (Due to OH- and CO3 2- ) (ii) Bicarbonate alkalinity (Due to HCO3 - ) Thus, the alkalinity of water may be due to 1. Hydroxide only 2. Carbonate only 3. Bicarbonate only 4. Both hydroxide and carbonate 5. Both carbonate and bicarbonate. 5. Hydroxide and bicarbonate 6. Hydroxide, carbonate and bicarbonate Determination of alkalinity OH- , CO3 2- and HCO3 - can be estimated separately by titration against standard acid using phenolphthalein and methyl orange as indicators. The possibility of hydroxide and bicarbonate existing together in water is ruled out because they combine with each other to form carbonate. Procedure Alkalinity : (IN PRACTICAL FILE) (3 QUESTIONS IN PDF(03)) 16. DISSOLVED OXYGEN : ❑Oxygen is poorly soluble in water. The amount of oxygen in water depends on physical, chemical and biological activities taking place in water. The solubility of dissolved oxygen (DO) in water at saturation at any temperature and pressure is given by Henry’s law. ❑Oxygen itself is not a pollutant in water but its deficiency is an indicator of several types of pollution in water. ❑DO is determined by Winkler’s method or iodometric titration. The DO in water oxidizes KI and an equivalent amount of iodine is liberated. ❑This iodine is titrated against a standard hypo solution. However, since DO in water is in molecular state and is not capable of reacting with KI, therefore an oxygen carrier such as manganese hydroxide is used. ❑The method involves introducing a conc. solution of MnSO4 , NaOH and potassium iodidesodium azide reagent, into the water sample. The white precipitate of Mn(OH)2 which is formed, is oxidized by oxygen in water sample to give a brown precipitate of basic manganic oxide MnO(OH)2. ❑This MnO(OH)2, in acidic medium dissolves and liberates free iodine from the added KI in a equivalent amount of DO in water sample. This liberated I2 is then titrated against Na2 S2O3 solution using starch as indicator. 17. BIOCHEMICAL OXYGEN DEMAND :  When biodegradable organic matter is released into water, the microorganisms feed on the wastes and break it down into simpler organic or inorganic substances. If the decomposition takes place in the presence of oxygen i.e aerobically, the non objectionable , stable end products are formed. On the other hand, if insufficient oxygen is available, the decomposition takes place anaerobically. The micro-organisms causing the decomposition of the organic matter without the presence of oxygen are entirely different from aerobic bacteria and produce highly objectionable end products including H2 S, NH3 and CH4. 4 The amount of oxygen required by micro-organisms to oxidize organic water aerobically is called Biochemical oxygen demand(BOD). The BOD of raw water will indicate the extent of organic matter present, thus indicating the extent of treatment required to make it safe for use. The BOD of treated water should be nil, so as to make it free from any organic matter. If BOD is high, the dissolved oxygen becomes low and this results in greater pollution. Polluted waters will continue to absorb oxygen for many months, till the oxidation gets completed and hence it is not practical to determine this ultimate oxygen demand which would require an extended period of time. As a result, it has become standard practice simply to measure the oxygen over a shorter period of 5 days. This is known as 5 day BOD test. BOD5 is the total amount of oxygen consumed by microorganisms during the first five days of biodegradation. In its simplest form, BOD5 would involve diluting a known volume of sample water with a known volume of pure water, whose oxygen content is already known. This is kept for 5 days at 20◦C in a stoppered bottle, away from light. The dissolved oxygen is measured after period of incubation. The difference between the original oxygen content and residual oxygen will indicate the oxygen consumed by the water sample in five days in oxidizing the organic matter present in water sample. Thus, BOD5 is given by : BOD5 = Loss of oxygen in mg/L * Dilution factor The dilution of the waste water sample with pure water is necessary because waste water contains more of oxygen  demanding material and hence the dissolved oxygen may not be sufficient to decompose it aerobically. 18. CHEMICAL OXYGEN DEMAND : Although BOD test is applicable to organic wastes, there are many drawbacks. Certain organic materials are not biodegradable and hence can give wrong conclusions that less organic matter is present because BOD of such water samples will be low. In this case COD, chemical oxygen demand reveals the real organic content present. Here the oxidation of organic substances present in water is done chemically. COD, is the amount of oxygen required by organic matter in a sample of water for its oxidation by a strong chemical oxidizing agent such as K2Cr2O7. (DETERMINATION OF OD, BOD, COD(PDF(04) 19. WATER TREATMENT FOR DOMESTIC PURPOSE : i) Screening: Screening is the process by which the floating material like wood pieces, leaves etc are removed. Raw water is made to pass through large screens having number of holes which hold back the floating matter and allow the water to pass. ii) Sedimentation: Water is made to stand undisturbed in big tanks for sufficient period of time (2 to 8 hours), so that the suspended particles settle down at the bottom. In sedimentation tank, flow of water is reduced and water is not  brought to complete rest. It consists of a rectangular tank. It is provided with baffle walls to reduce the velocity of the incoming water. Settled sludge is taken out from bottom. iii) Sedimentation Aided with Coagulation: Finely divided silica, clay and organic matter does not settle down easily. These are in the form of colloids and are generally negatively charged. Because of their same charge, these particles do not coalesce together to form larger particles and hence do not settle down easily. They can be removed easily by adding coagulants. Coagulants when added to water, form insoluble gelatinous precipitates which entrap very fine suspended particles, forming bigger sized agglomerates which settle down easily. Cationic coagulants are generally added to bring about coagulation in water. These cationic coagulants neutralize the negative charge on the colloids which there by come together and coalesce to form bigger particles and hence settle down. The most commonly used coagulants in water treatment plants are Alum, sodium aluminate and ferrous sulphate. iv) Filtration: Rapid Sand filters are most widely used and are of two types- a) Rapid Gravity filters: Finely divided particles like silica, clay, and organic matter remain suspended in water as colloids, typically carrying a negative charge. Due to their like charges, these particles repel each other and do not naturally come together to form larger particles, making them difficult to settle. To facilitate their removal, coagulants are introduced. When added to water, coagulants create insoluble gelatinous precipitates that trap the fine suspended particles, forming larger aggregates that settle more easily. Cationic coagulants are commonly used for this purpose, as they neutralize the negative charge on the colloids, allowing the particles to coalesce and settle. The most widely used coagulants in water treatment plants include alum, sodium aluminate, and ferrous sulfate. The rapid gravity filters get clogged very frequently and have to be washed every 24 to 48 hours. b) Rapid Pressure Filters: Water is passed through pressure filters at a pressure greater than the atmospheric pressure in airtight containers. The raw water is pumped into these vessels by means of pumps. v) Disinfection of water by chlorination : It means the chlorination of water to such an extent that living organisms as well as other organic impurities in water are destroyed. Chlorine is cheap, reliable and easy to handle. Moreover., it is capable of providing residual disinfecting effects for long periods and thus prevent future recontamination of water. It involves addition of sufficient amount of chlorine to oxidise organic matter, reducing substances, and free ammonia in raw water, leaving behind mainly free chlorine, which possesses disinfecting properties against pathogenic bacteria. It is also known as free residual chlorine.  When chlorine is added to water, it forms hypochlorous acid or hypochlorite ions which have immediate disastrous effect on most forms of microscopic organisms. The hypochlorous acid is unstable and dissociates as All the three forms HOCl, -OCl, Cl2 existing in a sample of water are termed as free chlorine. HOCl is found to be most destructive according to enzymatic hypothesis given by Gleen and Stumpt. For this reason the pH value of water during chlorination is generally maintained slightly less than 7 so as to prevent the dissociation of HOCl. The HOCl also reacts with ammonia likely to be present in water to form various chloramines as In the usual chlorination process, the pH is kept slightly less than 7 around 6.5 and hence dichloramine is the predominant species. These chloramines so formed are stable and are found to possess disinfecting properties. They act as chlorine reserves. The chlorine in this form is called combined chlorine. As compared to the free chlorine, combined chlorine is less effective. 20. Break point chlorination : The amount of chlorine required for disinfecting water depends upon the inorganic and organic impurities present in water. When chlorine is added to water and its amount is estimated after a few minutes, it is found that the available chlorine is not equal to the amount of chlorine added. A relationship between the amount of chlorine added to water and the free residual chlorine is shown in figure. The curve shown in figure can be divided into four stages. Stage I: Initially for lower doses of chlorine, there is no free residual chlorine since all the added chlorine gets consumed for complete oxidation of reducing substances present in water. This is due to the fact that initially Cl2 reacts with the inorganic impurities present in water forming chlorides. These chlorides do not have any residual oxidizing power. Stage II: As the amount of chlorine dosage is increased, amount of residual chlorine also shows steady increase. This stage corresponds to the formation of chloramines. But chloramines respond to the test for estimation of chlorine in the same way as free chlorine and hence curve represents the chlorine available from combined residuals. Chloramines remain in water body for longer periods which help to kill the micro-organisms even in a distribution system. Simultaneously this stage also corresponds to the formation of chloro-organic compounds without oxidizing them. These are responsible for bad odour and unpleasant taste in water. Stage III: At still higher dose of applied chlorine, the complete oxidation of chloro-organic compounds and partial oxidation of chloramines takes place and accordingly free residual chlorine also decreases and reaches a dip when the oxidative destruction is complete. This dip is known as break point. The chlorination upto breakpoint ensures complete destruction of organic compounds which give unpleasant taste and bad odour. Stage IV: After break point, the added chlorine is not used in any reaction and the residual chlorine agrees fairly well with the quantity of chlorine added. Thus free chlorine is available only if the demand for chlorine by other reacting  substances present in water is met with. Hence, for effectively killing the micro- organisms as well as bad tastes and odours, sufficient chlorine has to be added. Addition of chlorine in such doses is known as break point chlorination. Note : i. Chlorination after break point increases the free residual chlorine (Cl2 , HOCl, -OCl). ii. The free chlorine as well as the combined chlorine will cause germicidal action on micro-organisms. iii. Free chlorine will instantaneously kill the pathogens, while the combined chlorine will provide long term germicidal effect. iv. Hence to use chlorine as an effective disinfectant, the chlorine dosage has to be slightly more than the break point. v. In general, water is satisfactorily disinfected if the free chlorine residual is about 0.2 ppm i.e. the chlorine added in the dosage upto break point + 0.2 ppm. vi. Amount greater than 0.2 ppm of free residual chlorine causes bad taste in water and its harmful as it irritates the membrane of intestine. 21. Wastewater Treatment Process : The purpose of waste water treatment is to remove the contaminants from water so that the treated water can meet the acceptable quality stan- dards. The quality standards usually depend upon whether water will be reused or discharged in to a receiving stream. Available waste water treatment processes can be broadly classified as Physical processes Chemical processes Biological processes These processes consists of a series of unit operations. These are ap- plied in different combinations composition and specifications of the waste water. 1. Primary treatment ➔ Pretreatment Screening Grit Removal ➔ Sedimentation Sedimentation Aids : Mechanical Flocculation, Chemical Coagulation ➔ Equalization ➔ Neutralization  One way to deal with coarse material in waste water is to use a device called comminuter, which grinds the coarse material in to small pieces which flow along with water and handled in Sedimentation tank. A. Pretreatment : Screening Grit removal 1. Removes large floating objects After screening, the waste water passes into a grit chamber 2. Such as rags, sticks, wood and Velocity of water is reduced or it is other large floating and detained for a few minutes suspended solids 3. A typical screen consists of a Grit settling chambers are periodically parallel steel bars. disconnected from the main system to remove grit manually 4. Bars spaced anywhere from 2 For possible use in landfilling, road to 7 cm apart. making and on sludge drying beds. 5. Followed by a wire mesh screen Grit also is a food manure for growing crops B. Sedimentation : ▪ From the grit chamber, the sewage passes to a primary settling tank known as sedimentation basin. ▪ Here the velocity of the water is reduced considerably to allow most of the suspended solids to settle out by gravity. ▪ The most common equipment used include horizontal flow sedimentation tanks. ▪ The water is detained in the horizontal flow tanks for 2-3 hours resulting in removal of 50% of the suspended solid matter. ▪ An efficient sedimentation tank or clarifier removes about 80-90% of the suspended solids and 40% of organic matter. The solids that settle are called primary sludge or raw sludge. ➔ Sedimentation aids : For the removal of finely divided solids mechanical flocculation or chemical coagulation is employed.  C. Equalization and Neutralization: If both acid and alkaline wastes are produced in the nearby plants then mutual neutralization by mixing them is the cheapest method of neutralization. 2. Secondary Treatment: In waste water much of the organic material is dissolved or in colloidal form which is not removed by primary treatment. Thus removed by secondary treatment. secondary treatment is achieved through biological processes: Coagulation of the finely divided or colloidal matter. Oxidation of organic matter to CO2 Conversion of nitrogenous organic matter to ammonia, which is eventually converted into nitrite and nitrate. Anaerobic digestion of the sludge so obtained. Three commonly used approaches are: (i) Trickling filters (ii) Activated Sludge Process (iii) Oxidation Ponds (Lagoons) i) Trickling Filters (Aerobic filteration) : A trickling filter consists of a rotating distribution arm that sprays the liquid over a circular bed of rocks or other coarse material. Individual rocks get coated with layer of biological slime (aerobic microorganisms, zooglea-bacteria, algae, protozoa etc.) that absorbs and consumes wastes through the bed. Biological towers made with plastic media are prevalent. ➔ ADVANTAGES : 1. Simple to operate and can produce BOD removal to the extent of 65 to 85%. 2. 2. Constant monitoring is not required. 3. Effluents so produced are of better quality. ➔ DISADVANTAGES: 1. Microbial film formed is sensitive to temperature changes. 2. Efficiency of the filter is dependent upon the composition of waste, pH, size uniformity of the filtering medium & supply of air. 3. Cost of construction is high. 4. Trickling filters are used for treating industrial waste water from dairy, brewery, food processing, pulp and paper mills, pharmaceuticals, petrochemicals etc. ii) Activated Sludge Process : Most versatile biological oxidation method, employed for the treatment of waste water containing organic matter. Mixture of waste water and activated sludge is agitated and aerated. The activated sludge is the sludge obtained by settling the sewage in presence of excess of oxygen. The activated sludge is biologically active because it is heavily laden with microorganisms which are in active state of growth. ➔ ADVANTAGES : 1. The primary advantage is good effluent quality. The effluent after going through activated sludge has little BOD (< 20mg/L) 2. It takes less area as compared to trickling water filters. 3. The activated sludge process equipment is less expensive ➔ DISADVANTEAGES : 1. For the process to be efficient, at least 0.5ppm oxygen must be present. 2. The optimum pH 6.5 to 9.0 has to be maintained throughout. 3. The presence of detergents (which are not biodegradable) lead to the formation of foam, making the process difficult. 4. The disadvantage of this process is production of a huge amount of sludge, which should be digested and disposed off. iii) Oxidation Ponds (LAGOONS): Shallow ponds, typically 1-2 m deep Organic matter is oxidized by microorganisms present in the pond Waste water enter the pond at one end and treated waste water is collected at the other end  Decomposition of the organic matter near the surface is aerobic (algal photosynthesis), anaerobic near the bottom, hence, called facultative ponds deeper ponds (lagoons )are mechanically aerated. ➔ ADVANTAGES : 1. The process is simple and cheap. 2. Can be used for all types of waste waters 3. Due to the high pH of waste water in the pond, the heavy metal ions present in waste water are precipitated as hydroxides which settle as sludge. ➔ DISADVANTAGES : 1. The oxidation ponds require larger space. 2. Anaerobic conditions may lead to release of bad odours. 3. The main drawback of the above secondary treatment processes is the formation of sludge. 4. The collection, processing and disposal of sludge can be the most costly and complex aspect of waste water treatment. 3. Sludge Treatment and Disposal : Sludge is the watery residue from the primary sedimentation tank and humus tank from secondary treatment. Quantity of sludge produced may be as high as 2% of the original volume of waste water, depending upon the treatment process used. The traditional method of sludge digestion is anaerobic digestion. It involves the microorganisms that thrive in absence of oxygen. The organic material in sludge is digested by these microorganisms under Anaerobic conditions to give carbon dioxide and methane gas. The components of the sewage which can be converted into gases are called volatile solids. ➔ Sludge digestion in digester: Sludge is maintained at 35°C for 30 days at pH 7.0 to 8.0. CH4 , CO2 and NH3 are liberated as the end products. Digested sludge is removed from the anaerobic digester. This sludge contains 90 to 93% water and is dewatered. Dewatering is accomplished by mechanical methods, the most common being centrifugation and filtration, which includes pressure filtration and vacuum filtration. Drying beds are also commonly used. The dewatered sludge is sent for ultimate disposal. Wet sludge is sprayed on to crop land where it functions as fertilizer. Dried sludge may be used as a landfill or a soil conditioner 4. Tertiary Treatment : ▪ The emphasis on recovery of valuables from industrial wastewaters have created the need for tertiary treatment. Tertiary treatment improves the quality of the effluent further. The effluent after secondary treatment plant still contains suspended solids (20- 40mg/L) which may settle on the stream or river bed and inhibit certain forms of aquatic life. Some amount of BOD, significant amount of nutrients, dissolved solids, traces of organic chemicals and other contaminants are also present. Type of tertiary treatment depends upon the specific goal which include removal of: 1. suspended solids 2. bacteria 3. dissolved organic solids 4. toxic substances 5. nutrients (phosphorus and nitrogen) (I) Removal of suspended solids (Micro-straining): This can be achieved by micro-straining. The filter media consists of finely woven stainless steel fabric. The treated waste is allowed to pass through it. The solids retained on the fabric are washed into a trough, which recycles the solids to the sedimentation tank. (II) Removal of bacteria : Chlorination Bacteria are removed by retaining the effluents in maturation ponds or lagoons for specified period of times. (III) Removal of dissolved solids : a)Adsorption: Dissolved solids can be organics or inorganics which are removed by adsorption on activated carbon. Special adsorbents are commercially available for the removal of toxic heavy metals from industrial waste water. b) Solvent Extraction: Used to recover phenolic materials from waste waters of refineries and coke plants. waste water is intimately brought in contact with a solvent having high affinity for the solute. c) Ion Exchange: Used to remove hardness and iron and manganese salts from drinking water. This technique has been extended to waste water treatment for the removal and recovery of waste during water treatment. d) Reverse Osmosis: When waste water containing dissolved solids is allowed to pass through a semi-permeable membrane at a pressure, which is more than osmotic pressure, the water from the waste passes through the membrane. Hence a highly concentrated solution containing dissolved salts is left behind. e) Chemical precipitation: The precipitating agents like lime etc. remove heavy metal ions by precipitating these as hydroxides. Precipitating agents include FeSO4 , alum and ferric chloride. (IV) Removal of Nutrients: (a)Nitrogen Removal: All forms of nitrogen in wastewater are harmful because plants can utilize the inorganic forms as nutrients, NH3 can be utilized by bacteria resulting in reduced oxygen in water. Ammonia stripping: Ammonia is present in natural water as ammonium ion. This NH4 is changed to ammonia gas by raising the pH (the OH– concentration) of the waste water by adding quick lime. The ammonia gas is liberated. NH4 + OH -> NH3 ^ + H2O Another approach of nitrogen removal is nitrification i.e. to convert NH4 + to NO3 – , followed by anaerobic stage in which microorganisms convert nitrates to nitrogen gas (N2 ). (b) Phosphorus Removal (Chemical precipitation): Phosphorus is present in the form of orthophosphates (H2PO4 – , HPO4 2- and PO4 3- ). Phosphates are removed by adding coagulants usually alum [Al2 (SO4 )3 ] or lime [Ca(OH)2]. 22. Conservation of water: A step to conserve water is the step to secure the future. The most essential among all the natural resources on earth is water. A drop of water is worth more than a sack of gold for the thirsty man. Water conservation is what that can reduce the scarcity of water. It aims to improve the efficiency of use of water, and reduce losses and waste. Tips to save water: Avoid leakage of water from the taps. Turn the tap off when not in use especially when you brush your teeth or wash clothes. Rainwater harvesting is the another method to conserve water. The water supply should be limited in those areas which enjoys the unlimited water supplies. Technical methods to conserve water: 1. Rainwater Harvesting 2. Historical Water Bodies 3.Ponds ➔ What is Water Harvesting? The collection of rain and ground water into storage receptacles and using it for municipal purposes. Examples of Water Harvesting: On a small scale you could collect rain water in a container or bucket and use it to water your household plants. Recycle used waste water, treat it, and put it back into the water system into the aquifers and rivers. ➔ Traditional Types of Water Harvesting The type used depends on conditions including both physical and human:- KULS- Water channels found in precipitous mountain areas. E.g.:- Himachal Pradesh, Jammu. VIRDAS- Shallow wells dug in low Depressions called jheels (tanks). E.g.:- Great Rann of Kutch in Gujarat Bamboo drip irrigation in Meghalaya : i. 200-year-old system ii. Used by tribal farmers of Khasi and Jaintia hills iii. Bamboos divert water from perennial springs on hilltops to the lower reaches by gravity iv. Used to irrigate the betel leaf or black pepper crops v. 18-20 litres of water entering the bamboo pipe system per minute gets transported over several hundred meters and finally gets reduced to 20-80 drops per minute at the site of the plant. vi. Attempts made to introduce modern pipe systems but farmers prefer to use their indigenous form of irrigation. –