WATER THE PROBLEM OF PURITY

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2 WATER THE PROBLEM OF PURITY In its pure state, water is one of the most aggressive solvents known. It is called as the Universal Solvent, water, to a certain degree, will dissolve virtually everything to which it is exposed. Contaminants found in water include atmospheric gases, minerals, organic materials (some naturally occurring, others man-made) plus any materials used to transport or store water.

3 The Hydrologic Cycle illustrates the process of contamination and natural purification. 3

4 Natural Contamination and Purification Water evaporates from surface supplies and transpires from vegetation directly into the atmosphere. The evaporated water then condenses in the cooler air on nuclei such as dust particles and eventually returns to the earth s surface as rain, snow, or other precipitation. It dissolves gases such as carbon dioxide, oxygen, and natural and industrial emissions such as nitric and sulfuric oxides, as well as carbon monoxide. Typical rain water has a ph of 5 to 6. The result of contact with higher levels of these dissolved gases is usually a mildly acidic condition what is today called acid rain that may have a ph as low as

5 As the precipitation nears the ground, it picks up many additional contaminants - airborne particulates, spores, bacteria, and emissions from countless other sources. Most precipitation falls into the ocean, and some evaporates before reaching the earth s surface. The precipitation that reaches land replenishes groundwater aquifers and surface water supplies. The water that percolates down through the porous upper crust of the earth is substantially filtered by that process. Most of the particulate matter is removed, much of the organic contamination is consumed by bacterial activity in the soil, and a relatively clean, mildly acidic solution results. This acidic condition allows the water to dissolve many minerals, especially limestone, which contributes calcium. Other geologic formations contribute minerals, such as magnesium, iron, sulfates and chlorides. The addition of these minerals usually raises groundwater ph to a range of 7 to

6 This mineral-bearing water is stored in natural underground formations called aquifers. These are the source of the well water used by homes, industries and municipalities. Surface waters such as rivers, lakes and reservoirs typically contain less mineral contamination because that water did not pass through the earth s soils. Surface waters will, however, hold higher levels of organics and undissolved particles because the water has contacted vegetation and caused runoff to pick up surface debris. 6

7 Bacterial Contamination One difficulty of water purity is bacterial contamination and control of bacterial growth. Water is essential for all life. It is a necessary medium for bacterial growth because it carries nutrients. It is an essential component of living cells. Its thermal stability provides a controlled environment. Water will support bacterial growth with even the most minute nutrient sources available. 7

8 IDENTIFYING IMPURITIES The impact of the various impurities generated during the hydrologic cycle and/or bacterial colonization depends upon the water user s particular requirements. In order to assess the need for treatment and the appropriate technology, the specific contaminants must be identified and measured. 8

9 Specific Impurities Many individual impurities can be quantified through water analysis techniques. Below is a discussion of most ionic individual contaminants. Common Ions A number of terms are used to express the level of mineral contamination in a water supply. 9

10 Examples Iron Manganese Sulfate Chloride Alkalinity ((CO3)2,(HCO3-) and (OH-)). Nitrate/ Nitrite Silica Aluminum Phosphate Sodium Potassium 10

11 Dissolved Gases Carbon Dioxide Dissolved carbon dioxide (CO 2 ) associates with water molecules to form carbonic acid (H 2 CO 3 ), reducing the ph and contributing to corrosion in water lines. Oxygen Dissolved oxygen (0 2 ) can corrode water lines, boilers and heat exchangers, but is only soluble to about 14 ppm at atmospheric pressure. Hydrogen Sulfide The infamous rotten egg odor, hydrogen sulfide (H 2 S) can contribute to corrosion. It is found primarily in well water supplies or other anaerobic sources. 11

12 Heavy Metals Heavy metals such as lead, arsenic, cadmium, selenium and chromium when present above certain levels can have harmful effects on human health. In addition, minute concentrations may interfere with the manufacture and effectiveness of pharmaceutical products, as well as laboratory and industrial processes of a sensitive nature. 12

13 Dissolved Organic Compounds Dissolved organic materials occur in water both as the product of material decomposition and as pollution from synthetic compounds such as pesticides. Naturally-Occurring Tannins, humic acid and fulvic acids are common natural contaminants. They cause color in the water. They have no known health consequences in normal concentrations. In the presence of free halogen compounds (principally chlorine or bromine), they form chlorinated hydrocarbons and trihalomethanes (THM s), which are suspected carcinogens. Synthetic Organic Compounds (SOC s) A wide variety of synthetic compounds which are potential health hazards are present in water systems due to the use of industrial and agricultural chemicals. These compounds are not readily biodegradable and leach from soil or are carried by runoff into water sources. 13

14 Volatile Organic Compounds (VOC) Due to relatively low molecular weight, many synthetic organic compounds such as carbon tetrachloride, chloroform and methylene chloride will easily volatilize. Volatility is the tendency of a compound to pass into the vapor state. Most are introduced into the water supply in their liquid phase. If ingested they may be absorbed into the bloodstream. Radioactive Constituents Water in itself is not radioactive but may contain radionuclides. They are introduced either as naturally-occurring isotopes (very rare) or refined nuclear products from industrial or medical processes, radioactive fallout or nuclear power plants. 14

15 Microbiological Contamination Microbiological contamination can be classified as viable and nonviable. Viable organisms are those that have the ability to reproduce. Nonviable organisms cannot reproduce or multiply. Bacterial Contamination Bacterial contamination is quantified as Colony Forming Units (CFU), a measure of the total viable bacterial population. CFU s are typically determined by incubating a sample on a nutritional medium and counting the number of bacterial colonies that grow. 15

16 Pyrogenic Contamination Pyrogens are substances that can induce a fever in a warm-blooded animal. The most common pyrogenic substance is the bacterial endotoxin. Pyrogens are quantified as Endotoxin Units per milliliter (EU/mL). Total Organic Carbon (TOC) TOC is a direct measure of the organic, oxidizable, carbon-based material in water. TOC is a vital measurement used in sophisticated water treatment systems such as electronics grade where any amount of contamination can adversely affect product quality and yield. 16

17 Biological Oxygen Demand (BOD) BOD is a measure of organic material contamination in water, specified in mg/l. BOD is the amount of dissolved oxygen required for the biochemical decomposition of organic compounds and the oxidation of certain inorganic materials (e.g., iron, sulfites). Typically the test for BOD is conducted over a five-day period. Chemical Oxygen Demand (COD) COD is the amount of dissolved oxygen required to cause chemical oxidation of the organic material in water. Both BOD and COD are key indicators of the environmental health of a surface water supply. They are commonly used in waste water treatment but rarely in general water treatment. 17

18 General Qualitative Identification Qualitative identification is usually used to describe the visible or aesthetic characteristics of water. Among others these include: Turbidity (Clarity) Taste Color Odor 18

19 Turbidity Turbidity consists of suspended material in water, causing a cloudy appearance. This cloudy appearance is caused by the scattering and absorption of light by these particles. The suspended matter may be inorganic or organic. Generally the small size of the particles prevents rapid settling of the material and the water must be treated to reduce its turbidity. Turbidity can be measured by different optical systems. Such measurements simply show the relative resistance to light transmittance, not an absolute level of contamination. 19

20 A candle turbidimeter is a very basic visual method used to measure highly turbid water. Its results are expressed in Jackson Turbidity Units (JTU). A nephelometer is more useful in low turbidity water, with results expressed in Nephelometric Turbidity Units (NTU). Suspended matter can also be expressed quantitatively in parts per million (ppm) by weight or milligrams per liter (mg/l). This is accomplished by gravimetric analysis, typically filtering the sample through a 0.45-micron membrane disc, then drying and weighing the residue. 20

21 Taste The taste sense is moderately accurate and able to detect concentrations from a few tenths to several hundred ppm. However, taste often cannot identify particular contaminants. A bad taste may be an indication of harmful contamination in drinking water, but certainly cannot be relied on to detect all harmful contaminants. 21

22 Color Color is contributed primarily by organic material, although some metal ions may also tint water. While not typically a health concern, color does indicate a certain level of impurities, and can be an aesthetic concern. True color refers to the color of a sample with its turbidity removed. Turbidity contributes to apparent color. Color can be measured by visual comparison of samples with known concentrations of colored solutions. Color can also be measured using a spectrophotometer. 22

23 Odor The human nose is the most sensitive odordetecting device available. It can detect odors in low concentrations down to parts per billion (ppb). Smell is useful because it provides an early indication of contamination which could be hazardous 23

24 General Quantitative Identification ph The relative acidic or basic level of a solution is measured by ph. The ph is a measure of hydrogen ion concentration in water. Since ph is expressed in log form, a ph of 6.0 is 10 times more acidic than a ph of 7.0, and a ph of 5.0 is 100 times more acidic than a ph of 7.0. The ph level can be determined by various means such as color indicators, ph paper or ph meters. A ph meter is the most common and accurate means used to measure ph. 24

25 Total Solids Total Solids (TS) (Table 1) is the sum of Total Dissolved Solids (TDS) and Total Suspended Solids (TSS). In water analysis these quantities are determined gravimetrically by drying a sample and weighing the residue. 25

26 Conductivity/Resistivity Ions conduct electricity. Because pure water contains few ions, it has a high resistance to electrical current. The measurement of water s electrical conductivity, or resistivity, can provide an assessment of total ionic concentration. Conductivity is described in microsiemens/cm (µs) and is measured by a conductivity meter. Resistivity is described in megohm-cm, is the inverse of conductivity and is measured by a resistivity meter. 26

27 Water Purification Water purification is the removal of contaminants from raw water to produce drinking water that is pure enough for human consumption or for industrial use. Substances that are removed during the process include bacteria, minerals and man-made chemical pollutants. A small amount of disinfectant is usually intentionally left in the water at the end of the treatment process to reduce the risk of re-contamination in the distribution system. 27

28 Sources of drinking water 1. Deep groundwater 2. Shallow groundwater (wells) 3. Upland lakes and reservoirs 4. Rivers, canals and low land reservoirs 5. Atmospheric water generation 6. Rainwater harvesting or fog collection 28

29 Municipal Water Treatment There are three principal stages in water purification:- 1. Primary treatment 2. Secondary treatment 3. Tertiary treatment 29

30 Primary Treatment Pumping and containment - The majority of water must be pumped from its source and directed into pipes or holding tanks. Screening - The first step in purifying surface water is to remove large debris such as sticks, leaves, trash and other large particles which may interfere with subsequent purification steps. Storage - Water from rivers may also be stored in bankside reservoirs for periods between a few days and many months to allow natural biological purification to take place. Pre-conditioning - Many waters rich in hardness salts are treated with sodaash (Sodium carbonate) to precipitate calcium carbonate. 30

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32 Secondary Treatment This includes the techniques that can be used to remove the fine solids, micro-organisms and some dissolved inorganic and organic materials. The choice of method will depend on the quality of the water being treated, the cost of the treatment process and the quality standards expected of the processed water. ph Adjustment If the water is acidic, lime or soda ash is added to raise the ph. Lime is the more common of the two additives because it is cheaper, but it also adds to the resulting water hardness. 32

33 Coagulation and Flocculation Together, coagulation and flocculation are purification methods that work by using chemicals which effectively "glue" small suspended particles together, so that they settle out of the water. Many of the suspended water particles have a negative electrical charge. The charge keeps particles suspended because they repel similar particles. Coagulation works by eliminating the natural electrical charge of the suspended particles so they attract and stick to each other. The joining of the particles so that they will form larger settleable particles is called flocculation. The larger formed particles are called flocs. 33

34 The coagulants include Fe and Al salts Alum K 2 (SO 4 ) 3. 24H 2 O Ferrous Suphate Fe 2 SO 4.7H 2 O Sodium Aluminate NaALO 2 The chosen coagulant and the raw water is slowly mixed in a large tank called a flocculation basin. The flocculation paddles turn very slowly to minimize turbulence. The principle involved is to allow as many particles to contact other particles as possible generating large and robust floc particles. 34

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36 Sedimentation Water exiting the flocculation basin enters the sedimentation basin, also called a clarifier or settling basin. It is a large tank with slow flow, allowing floc to settle to the bottom. The outflow is typically over a weir so only a thin top layer-furthest from the sedimentexits. The amount of floc that settles out of the water is dependent on the time the water spends in the basin and the depth of the basin. As particles settle to the bottom of the basin a layer of sludge is formed on the floor of the tank. This layer of sludge must be removed and treated. 36

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38 Filtration After separating most floc, the water is filtered as the final step to remove remaining suspended particles and unsettled floc. The most common type of filter is a rapid sand filter. Water moves vertically through sand which often has a layer of activated carbon above the sand. The top layer removes organic compounds affecting taste and odour. Most particles pass through surface layers but are trapped in pore spaces or adhere to sand particles. This property of the filter is key to its operation: if the top layer of sand were to block all the particles, the filter would quickly clog. 38

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40 Ultrafiltration Membranes Ultrafiltration membranes are a relatively new development; they use polymer film with chemically formed microscopic pores that can be used in place of granular media to filter water effectively without coagulants. The type of membrane media determines how much pressure is needed to drive the water through and what sizes can be filtered out. 40

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42 Tertiary Treatment Disinfection is normally the last step in purifying drinking water. Water is disinfected to destroy any pathogens which passed through the filters. Following the introduction of any chemical disinfecting agent, the water is usually held in temporary storage - often called a contact tank or clear well to allow the disinfecting action to complete. 42

43 Chlorine The most common disinfection method is some form of chlorine which is a strong oxidant that kills many micro-organisms. Because chlorine is a toxic gas, there is a danger of a release associated with its use. This problem is avoided by the use of bleaching powder which is a relatively inexpensive solid that releases free chlorine when dissolved in water. CaOCl2 + H2O Ca(OH)2 + Cl2 Cl2 + H2O HCL +[O] Germ + [O] oxidized germ Although chlorine is effective in killing bacteria, it has limited effectiveness against protozoans that form cysts in water. Another drawback of chlorine gas is that it reacts with organic compounds in the water to form potentially harmful levels of the chemical byproducts trihalomethanes (THMs). The formation of THMs is minimised by effective removal of as many organics from the water as possible before disinfection. 43

44 Chloramines Chloramines are another chlorine-based disinfectant. Although chloramines are not as effective as disinfectants, compared to chlorine gas or sodium hypochlorite, they are less prone to form THMs. ClNH2 + H2O HOCL +NH3 HOCl HCl + [O] 44

45 Ozone (O 3 ) Ozone is made by passing oxygen through ultraviolet light or a electrical discharge. To use ozone as a disinfectant, it must be created on site and added to the water by bubble contact. Some of the advantages of ozone include the production of relatively fewer dangerous by-products (in comparison to chlorination) and the lack of taste and odor produced by ozonation. But disadvantages include high cost and no residual disinfectant in the water. O O2 + [O] 45

46 UV Radiation UV radiation is very effective at inactivating cysts, as long as the water has a low level of colour so the UV can pass through without being absorbed. 46

47 Because neither ozone nor UV radiation leaves a residual disinfectant in the water, it is sometimes necessary to add a residual disinfectant after they are used. This is often done through the addition of chloramines, discussed above as a primary disinfectant. When used in this manner, chloramines provide an effective residual disinfectant with very little of the negative aspects of chlorination. 47

48 What Is Hard Water? Hardness is that characteristic of water which prevents the lathering of soap. It is caused by compounds of calcium and magnesium. Water dissolves, suspends, or exchanges certain trace elements and compounds from many things that it contacts on its travels. Total water 'hardness' (including both Ca++ and Mg++ ions) is reported as ppm w/v (or mg/l) of CaCO3. Water hardness usually measures the total concentration of Ca and Mg, the two most prevalent divalent metal ions, although in some geographical locations iron, aluminium, and manganese may also be present at elevated levels. 48

49 Temporary or Carbonate Hardness Temporary hardness is hardness that can be removed by boiling or by the addition of lime (calcium hydroxide). It is caused by a combination of calcium ions and bicarbonate ions in the water. Boiling, which promotes the formation of carbonate from the bicarbonate, will precipitate calcium carbonate out of solution, leaving water that is less hard on cooling. Ca (HCO3)2 Mg (HCO3)2 Heat > CaCO3 + H2O + CO2 Heat > MgCO3 + H2O + CO2 49

50 Permanent or Non-Carbonate Hardness Permanent hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water, which become more soluble as the temperature rises. Despite the name this can be removed using a water softener, or ion exchange column. 50

51 Hardness of water is usually reported as: Units of Hardness Parts per million (ppm) is parts of CaCO3 equivalent hardness per million parts of water Milligrams per litre (mg/litre) is the no. of mg of CaCO3 hardness per litre of water Degree Clark is the no. of grains of CaCO3 hardness per gallon of water. 51

52 Problems caused by hard water While hard water is not generally unhealthy, it can cause many potentially costly problems. Hard water causes scaling, which is the precipitation of minerals to form a rockhard deposit called lime scale. For domestic purposes, hard water requires more soap and synthetic detergents for laundry and washing. Using soap on the body in hard water can cause the formation curd. This curd remains on the skin even after rinsing, clogging pores serving as a medium for bacterial growth, causing skin irritation and dry skin. In industry, hard water contributes to scaling in boilers, cooling towers and other industrial equipment. 52

53 WATER SOFTENING The process of removing hardness producing salts from water is known as softening of water and can be accomplished by one of the following methods. Lime-soda Process Zeolite or Permutit Process. Deionization Process 53

54 Lime-soda Process Lime-soda Process is the most widely used method of softening water. It is based on converting the dissolved calcium and magnesium salts into insoluble salts, which are then allowed to settle and filtered off. The added ingredient lime Ca(OH) 2 precipitates temporary hardness, permanent magnesium hardness, iron and aluminium salts and free acids (like CO2, H2S- etc). Ca(HCO3)2+ Ca(OH) > 2CaCO3 + 2H2O Mg(HCO3)2 + 2Ca(OH) > 2CaCO3 +Mg(OH)2 +2H2O MgSO4 + Ca(OH) > Mg(OH)2 +CaSO4, FeSO4+ Ca(OH) > Fe(OH)2 +CaSO4 54

55 Types of Lime-soda Process Lime-soda process may be carried out in the following ways Cold lime-soda process Hot lime-soda process 55

56 Cold lime-soda process. This method is usually applied for municipal water treatment. In this method calculated quantity of chemicals (lime and soda) are mixed with water at atmospheric temperature. At room temperature the precipitates formed are finely divided, so, they do not settle down easily, nor can they be filtered easily. Consequently, it is essential to add small amounts of coagulants. These coagulants forms flocs of of aluminium hydroxide, which entraps the fine precipitates formed by the reactions of lime and soda. This process provides water containing a residual hardness of 50 to 60 ppm. 56

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58 Hot lime-soda process This method is applied for boiler feed-water treatment. This involves treating water with chemicals at a temperature of 80 to 150 o C. 58

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60 Comparison of HLSP & CLSP Since hot process is operated at a temperature close to the boiling point of the solution, so, the reaction proceeds faster, while in hot softening the reactions are completed in 15 minutes; whereas in the cold process several hours are needed, the softening capacity of hot process is increased to many fold, the precipitate and sludge formed settle rapidly and hence no coagulants are needed, much of the dissolved gases (such as CO2 and air) are driven out of the solution, viscosity of softened water is lower, so, filtration of the water becomes, much easier. This in turn increases the filtering capacity of filters, and hot lime-soda produces water of comparatively lower residual hardness of 10 to 20 ppm. 60

61 Zeolite or Permutit Process Zeolites (or Permutits) are Complex silicates consisting of macromolecules of several metallic and non-metallic oxides, which form a rather crystalline structure of approximate chemical formula Na2O. Al2O3. 2SiO2. 6H2O and they are simply represented by Na2Ze. They hold sodium ions in loose fashion and consequently when they are treated with a solution, an equilibrium is formed between the sodium ions held by the zeoiite and positive ions present in the solution. Thus, if the zeolite bed is kept in contact with a solution containing heavy metal ions such as Ca +2 Mg +2, etc., there will be a tendency for these ions to be exchanged by the sodium ions contained in zeolite. 61

62 Natural and Synthetic Zeolites Zeolite used for water-softening purposes can be natural or synthetic. The former are mined as such and are more durable. The later are manufactured by heating china clay and soda ash together and cooling and crushing the resulting glass. Artificial zeolites have a greater exchange capacity per unit weight but are less durable and more easily effected by acids than natural zeolites. 62

63 Process details For softening of water by zeolites process, hard water is percolated at a specified rate through a bed of zeolites kept in a cylinder. The hardness causing ions (Ca +2, Mg +2, etc.) are retained by the zeolite, while the outgoing water contains sodium salts.reactions taking place in the softening process are : 63

64 Regeneration. After some time, when the zeolite is completely converted into calcium and magnesium zeolites and it ceases to soften water, i.e., it gets exhausted. At this stage the supply of hard water is stopped and the exhausted zeolite is reclaimed by treating the bed with a dilute (10%) brine (NaCl) solution, CaZe(or MgZe)+ 2NaCl >Na2Ze + CaCl2(orMgCl2) (Exhausted zeolite) (Brine) (Reclaimed zeolite) (Washings) The washings (containing CaCl2 and MgCl2) are led to drain and the reclaimed zeolite bed is used again for softening. 64

65 Zeolite process of softening. 65

66 Advantages of zeolite process. It removes the hardness completely, ie., nearly zero hardness water is produced. The equipment used is compact occupying a small space. No impurities are precipitated so there is no danger of sludge formation in the treated water at a later stage. The process automatically adjusts itself for different hardness of incoming water. 66

67 Disadvantages of zeolite process. The treated water contains more sodium salts than in lime-soda process. Higher capital cost as compared to LSP but lower operational cost. Zeolite treatment only replaces the cations (like Ca 2+, Mg 2+, etc.) with Na+, but leaves all the acidic ions (like HCO - 3" and CO 3 2- ) in the softened water. When such softened water is used in steam boilers, free CO 2 is released. Free CO 2 thus set free is weakly acidic and is corrosive to boiler materials. 67

68 Osmosis Osmosis is the spontaneous net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides. 68

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70 Reverse Osmosis Reverse osmosis is a separation process that uses pressure to force a solvent through a semipermeable membrane that retains the solute on one side and allows the pure solvent to pass to the other side, forcing it from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. 70

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