Iron in Water (exp:9) - assignment - Q-A

Determination of Iron in Water
(1) Q: Discuss the environmental significance of “iron” in water.
Ans: As far as is known, human suffer no harmful effects from drinking waters containing iron and manganese. Such waters, when exposed to the air so that oxygen can enter, become turbid and highly unacceptable from the aesthetic viewpoint owing to the oxidation of iron and manganese to the Fe3+, and Mn4+ states which form colloidal precipitates. The rates of oxidation are not rapid, and thus reduced forms can persist for some time in aerated waters. This is especially true when the pH is below 6 with iron oxidation and below 9 with Manganese oxidation. The rates may be increased by the presence of certain inorganic catalysts of through the action of micro-organisms.
(2) Q: Why hydrochloric acid and potassium permanganate are added to the sample in the determination of iron?
Ans: Hydrochloric acid and potassium permanganate are added to the sample in the determination of iron because, some portion of iron may exist as iron hydroxide precipitates, it is necessary that all iron is in soluble condition. Treating the water to be tested with hydrochloric acid to dissolve ferric hydroxide does this. For determination of total iron, it should be ensured that all iron exist in ferric form Fe3+. this is most readily accomplished by using potassium permanganate, an oxidizing agent.
(3) Q: Ground water with high dissolved iron concentration often becomes after extraction (in contact with air). Explain why.
Ans: Ground water with high dissolved iron concentration often becomes turbid after extraction (in contact with air) because, such waters, when exposed to the air so that oxygen can enter, become turbid. Iron in water is frequently accompanied by heavy growths of iron bacteria. Iron is common in the earth and water-containing CO2, which seeps through iron bearing material, dissolves it to ferrous bicarbonate form Fe (HCO3) 2. The ferrous bicarbonate easily oxidized into ferric hydroxide Fe (OH) 3. Which is precipitated as rusty sediment.

Alkalinity (exp:6) - assignment - Q-A

ASSIGNMENT:
(1) Q: Discuss the environmental significance of “alkalinity”.

Ans:
The alkalinity of waters is due principally to salts of weak acids and strong bases, and such substances act as buffer to resist a drop in pH resulting from acid additions. Alkalinity is thus a measure of the buffer capacity and in this sense is used to a great extent in wastewater treatment practice. Alkalinity is very important in iron removal from water by oxidation precipitation process. It has been found that rate of oxidation of dissolved ferrous iron (Fe2+) into insoluble ferric iron is vary slow for low alkaline waters (alkalinity less than 130 mg/L as CaCO3). Because the alkalinity of many waters is primarily a function of carbonate, bicarbonate and hydroxide content, it is taken as an indication of the concentration of these constituents. Alkalinity of water has little public health significance. Highly alkaline waters are usually unpalatable.
Although Bangladesh Environment Conservation Rule (1997) does not set any limit for its presence in drinking water, limits for carbon dioxide has been prescribed for many industrial uses.
(2) Q: Define total alkalinity, phenolphthalein alkalinity and methyl orange alkalinity.
Ans: phenolphthalein alkalinity: When alkalinity is measured to the phenolphthalein end point (pH 4.5), it is called phenolphthalein alkalinity. Which is due to the presence of either hydroxide or carbonate or both.
CO2 + CaCO3 + H2O = Ca2+ + 2HCO3-
Methyl orange alkalinity: When alkalinity is measured to the methyl orange (or bromocresol green) end point (pH), it is called Methyl orange alkalinity.
CO3 + H+ = HCO3-
Total alkalinity: Which alkalinity is due to the presence of hydroxide, carbonate and bicarbonate, it is called Total alkalinity.
HCO3- + H+ = CO32-
(3) Q: Calculate hydroxide, bicarbonate and carbonate alkalinities of a water sample with a total alkalinity of 210 mg/L as CaCO3 and a pH of 7.7. Which type of alkalinity dominates the total alkalinity? Also calculate the concentrations of carbonate (CO32-) and bicarbonate (HCO3-) ions.
Ans:
Carbonate alkalinity = [50,000{alkalinity/50,000 + (H+) - Kw/(H+)}]/ [1+(H+)/2KA2}]
mg/L as CaCO3 ----- (1)
Bicarbonate alkalinity = [50,000{alkalinity/50,000 + (H+) - Kw/(H+)}]/ [1+2KA2/(H+)}]
mg/L as CaCO3 ----- (2)
Kw =[H+][OH-] =10-14 (at 250C)
KA1 = [H+][HCO3-]/ [H2CO3] = 10-6.3 (at 250C)
KA2 = [H+][CO32-]/ [HCO3-] = 10-10.3 (at 250C)
Total alkalinity = 210 mg/L as CaCO3
pH = 7.7
As pH value is less than 8.3
Carbonate alkalinity = 0
Hydroxide alkalinity = 0
Bicarbonate alkalinity dominates Total alkalinity
From equation (2)
Bicarbonate alkalinity = [50,000{210/50,000 + 10-7.7 - 10-14 /10-7.7]/ [1+2*10-10.3 /10-7.7]
mg/L as CaCO3
= 208.93 mg/L as CaCO3
Bicarbonate [HCO3-] ions concentration = Bicarbonate alkalinity × 1.22
= 208.93 × 1.22 mg/L = 254.89 mg/L
Carbonate alkalinity = 0
Hydroxide alkalinity = 0
Bicarbonate alkalinity = 208.93 mg/L as CaCO3
Bicarbonate [HCO3-] ions concentration = 254.89 mg/L

Alum Coagulation (exp:12) - assignment - Q-A

Alum Coagulation
(1) Q: What do you understand by coagulation and flocculation? Which coagulants are most commonly used for water and wastewater treatment.
Ans:
Coagulation: The term coagulation is used to describe the process by which the charge on particles is destroyed.
Surface water generally contains wide variety of colloidal impurities that may cause the water to appear turbid and may impart color to the water. Colloidal particles that cause color and turbidity are difficult to separate from water because the particles will mot settle by gravity and are so small that they pass through the pores of most common filtration media. In order to be removed, the individual colloids must aggregate and grow in size so that they can settle by gravity. Chemical agents are used to promote colloid aggregation by destroying the forces that stabilize colloidal particles. The process of destroying the stabilizing forces and causing aggregation of colloids is referred to as chemical coagulation.
Flocculation: The term flocculation is used to determine the aggregation of particles into larger units.
The chemicals used for the purpose of coagulation is called coagulants.The most common coagulants used in water and wastewater treatment are aluminum and ferric salts as alum, ferric chloride and ferric sulfate.
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(2) Q: Why addition of alum may result in a drop in pH value. Discuss the affect of alum dose on pH from your experimental results.
Ans:
The common metal salt alum (aluminum sulfate) is a good coagulant for water containing appreciable organic matter. The chemical formula used for commercial alum is Al2(S04)3 . 14 H2O. Once dissolved in water, aluminum forms hydroxo-complexes and solids [e.g., Al(OH)3(s), Al(OH) +2, Al(OH)2+, Al(OH)4- ; Eqs. 1-5] and as a result pH of water is lowered, especially if alkalinity of water is low.
Al2(SO4)3 . 14 H2O (alum) = 2Al +3+ 3 SO4 –2 (1)
Al +3+ 3H2O = Al(OH)3(s) + 3 H+ (2)
Al +3+ H2O = Al(OH) +2 + H+ (3)
Al +3+ H2O = Al(OH)2+ + 2 H+ (4)
Al +3+ H2O = Al(OH)4- + H+ (5)
Theoretically, each mg/l of alum consume approximately 0.50 mg/l (as CaCO3) of alkalinity. For water with low alkalinity, this may result in significant reduction in pH that may interfere with formation of aluminum hydroxide flocs. In water treatment with alum coagulation, residual aluminum that may be present in water after the coagulation process is a cause of concern.
Remedy: If the alkalinity is insufficient, coagulant aids such as *lime ,Ca(OH)2, *soda ash (Na2CO3), *activated silica and *poly-electrolytes are used to provide the necessary alkalinity.
(3) Q: What is the primary mechanism by which heavy metal ions are removed during coagulation.
Ans: Coagulation with alum and ferric chloride or ferric sulfate is also widely used for removal of heavy metal ions (e.g., lead, arsenic) from water. In this process heavy metal ions are primarily removed by adsorption (and subsequent precipitation) onto coagulated flocs of metal (either aluminum or iron) hydroxides. Coagulation with alum and ferric chloride/sulfate has been successfully used for removal of arsenic from water.

Arsenic in Groundwater

Arsenic is both toxic and carcinogenic. Inorganic forms of arsenic dissolved in drinking water are the most significant forms of natural exposure. Organic forms of water of arsenic that may be present in food are much less toxic to humans. Clinical manifestation of arsenic poisoning begins with various forms of skin diseases and proceeds on via diseases to internal organs ultimately to cancer and death. To be safe from these diseases arsenic free water is must. WHO guideline value for arsenic in drinking water is upto .01 mg/l. Bangladesh standard for arsenic in drinking water is 0.05 mg/l.
Groundwater of Bangladesh is contaminated with arsenic, which occurs naturally in alluvial and deltaic sediments. First official detection was in 1993 and subsequent confirmation after 1995 of levels of arsenic in shallow and deep wells in various parts of Bangladesh has raised serious health concerns. Resent statistics says that about 59 districts of Bangladesh have affected by arsenic above maximum permissible limit Total 1.12 million tube-wells are found to be affected. As a result 75 million people are in the risk and 1.2 million people are exposed to arsenic poisoning with 24 million people potentially exposed.
Methods of arsenic determination:
Three methods are used to determination of arsenic. They are as follows:
a. Field Kit method
b. SDDC method
c. Atomic Absorption US method.
Among these methods Field Kit method is an approximate determination. There a calibration curve is used. In SDDC, Spectrophotometer of 535 NM color spectrum is used. We use graphite furnace in our lab. Atomic absorption method is more accurate. Calibration curve is used here. But stability is less here. In our lab we have two computerized machines to determine arsenic. In our lab. We observed atomic absorption machine of old and modern version. We watched testing arsenic by new computerized machine which analyses the sample by burning it at about 2500 C. Then results are shown on screen both in graphically and chart.
Results & comments:
In field Kit method arsenic content of water is found as 225 microgram/l.
In SDDC method arsenic content of water is found as 26 microgram/l.
There is a great difference between the results of the two methods. This may be held due to the approximate analysis of field kit method. Here we use zinc, which is not pure itself; it contains a significant amount of arsenic as impurity. Here we observed color of the solution and then we compare this color with a standard color disk to measure arsenic content. But in our test color was not properly matched with standard disk. We assumed an approximate color, which may be wrong. So result is incorrect. In SDDC method we knew color from color spectrum. So, this result is more reliable than before.
Environmental significance:
Arsenic is naturally occurring element that is tasteless and odorless. As compound of underground rock and soil, arsenic works its way into groundwater and enters food chains through either drinking water or eating plants and cereals that have absorbed the mineral. Daily consumption of water with greater than 0.01 mg/l of arsenic. Less than 2 percent of fatal dose can lead to problems with the skin and circulatory and nervous systems. If arsenic builds up to higher toxic levels, open lesions, organ damages (such as deafness), neural disorders and organ cancer often fatal can result.
Groundwater is the preferred source of drinking water in rural areas, particularly in developing countries, because treatment of the same, including disinfections. The problem of arsenic contamination of groundwater is more serious in Bangladesh, where the groundwater in 59 out of the 64 districts is contaminated with arsenic. Since the removal of arsenic is difficult and an alternate source of water is expensive, people still use tube well water.
According to Bangladesh Environment Conservation rules (1997), drinking water standard for Arsenic is 0.05 mg/l; but for WHO guideline of 0.01 mg/l is adopted.
Available methods of testing:
There are few tests for determination of arsenic concentration in water. Those are as follows:
1. The Field Kit Method
2. Silver Diethyldithiocarbamate (SDDC) Method
3. Atomic Absorption Spectrometric (AAS) Method
The Field Kit Method may be following types:
• Merck Kit
• IAN Kit
• Nipson Kit
• BUET Kit
Silver Diethyldithiocarbamate (SDDC) Method may be following types:
* Using Zinc
* Using Sodium Borohydrate
Atomic Absorption Spectrometric (AAS) Method may be following types:
* Hydride Generation
* Graphite Furnace

Break Point Chlorination (exp:13) - assignment - Q-A

Break Point Chlorination

(1) Q: Why chlorination is necessary?
Ans: chlorination is necessary for any treatment of domestic water because , the chlorination of supplies and polluted water serves primarily to destroy or deactivate disease producing microorganisms. A secondary benefit is the overall improvement in water quality resulting from the reaction of chlorine with ammonia, iron, manganese, sulfide and organic substances.
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(2) Q: What is dechlonation and why it is necessary?
Ans: This is required when residual chlorine is so high that water is rendered aesthetically or otherwise objectionable. Dechlorination is the partial or complete reduction of residual chlorine in water by chemical or physical means, e.g. SO2, activated carbon or aeration.
SO2 +Cl2 + H2O = 2HCl +H2SO4
C + Cl2 + H2O = HCl +CO2
Dechlonation is necessary because, chlorination may produce adverse effects. Taste and odor characteristics may be changed, phenol and other organic compounds present in water supply may be intensified. Combined chlorine formed on chlorination of ammonia or amine bearing waters adversely affects some aquatic life. To fulfill the primary purpose of chlorination and to minimize any adverse effects, it is essential that proper treatment procedures be used with a foreknowledge of the limitation of analytical determination.
(3) Q: What is Break point chlorination?
Ans: Some free available chlorine is desired as residual in water supply system to safe guard the water from the water from any contamination that may occur through leaky joints or cracks in the pipes. Accomplishment of the desired residual chlorine depends upon the characteristics of water.
If the water is free of ammonia or other compounds that may reacts with chlorine, the application of chlorine will yield free available chlorine of same concentration. This is denoted by no demand or zero demand line.
If the water contain only such inorganic ,matter like H2S, Fe, Mn etc, that reduces chlorine then an application of chlorine would yield residual of less concentration than that applied. The bellow the zero demand line denotes this.
If the water contains ammonia that results in the formation of co,bined available chlorine residual, it must be destroyed before a free chlorine residual is obtained. This destruction is brought about by application of excess chlorine. Initially, when molar ratio of chlorine to ammonia remains bellow 1, formation of monochloramines predominates with only a small consumption of the total available chlorine. As the dosage of chlorine is increased the production of dichloramiens is enhanced with corresponding decomposition to nitrogen with compensating consumption of available chlorine and formation of hydrogen chloride. The probable reaction in the destruction of ammonia or chloramines are:
2NH3 +3Cl2 =N2 + 6HCl
4NH2Cl +5CL2 +H2O =N2 + N2O +10HCl
2NHCl3 + HOCl + H2O = 2NO2 +5HCl
at the end of destruction of the chloramines N2, N2O, NO2 etc are mainly gaseous products. HCl being produced as the pH gradually decreases with the application of chlorine and its consumption in the destruction of ammonia decreases till a minimum residue is reached. Amount of chlorine used to reach the minimum free available chlorine residue is called “Break point dose” or “break point chlorination”. At this point the curve bends to a minimum value. Further chlorination above this point gives as equal amount of free available chlorine and the curve become parallel to “no demand line”.

Carbon dioxide (exp:5) - assignment - Q-A

Determination of Carbon-di-oxide in Water
(1) Q: Discuss the environmental significance of “carbon dioxide” water.
Ans: Corrosion is the principal difficulty caused by carbon dioxide. This gas on solution in water producing carbonic acid resulting in lowering of pH value and thus produces corrosive characteristic in water. Severe corrosion of heat exchanger, pipes, valves etc. can result where appreciable quantities of free carbon dioxide are present in water. Corrosion from carbon dioxide is boiler system is most frequently encountered in steam and return lines. Even though the boiler feed water contains no free carbon dioxide, appreciable concentration of this corrosive gas may come from carbonate and bicarbonate of the boiler feed water.
Although Bangladesh Environment Conservation Rule (1997) does not set any limit for its presence in drinking water, limits for carbon dioxide has been prescribed for many industrial uses.
(2) Q: Why groundwater usually contains more carbon dioxide compared to surface water?
Ans: Water is a universal solvent. Carbon dioxide is highly soluble in water. Carbon dioxide enters water in appreciable quantities as the water percolates through soil in which plants are growing. Dissolved in water, it forms carbonic acid which, together with the carbonates and bicarbonates. Surface water is exposed to the atmosphere. In surface water carbon dioxide easily escapes to the atmosphere. So groundwater usually contains more carbon dioxide compared to surface water
(3) Q: Why the test for carbon dioxide should be performed immediately after collection of water sample? Explain.
Ans: The test for carbon dioxide should be performed immediately after collection of water sample because, if there is present carbon dioxide in sample water, easily escaped to the atmosphere.

Chemical Oxygen Demand of Water (exp:11) - assignment - Q-A

Determination of Chemical Oxygen Demand of Water
(1) Q: What are the principal advantages and disadvantages of the COD test over the BOD test.
Ans: The major advantage of COD test is the short time required for evaluation. The determination can be made in about 3 hours rather than the 5 days required for the measurement of BOD . for this reason, it is used as a substitute fir the BOD test in many instances.
One of the chief limitations of COD test is its inability to differentiate between biologically oxidizable and biologically inert organic matter. In addition, it does not provide any evidence of the rate at which the biologically active material would be stabilized under conditions that exist in nature.
(2) Q: Explain why COD values are usually greater than BOD values.
Ans During the determination of COD, organic matter is converted to carbon dioxide and water regardless of the biological assimilability of the substance. For example, glucose and lignin are both oxidized completely. As a result, XOD values are greater than BOD values, especially when biologically resistant organic matter (e.g., lignin) is present.
(3) Q: What could be inferred from the following analytical results concerning the relative case of biodegradability of each waste?
Waste 5-day BOD (mg/L) 5-day COD (mg/L)
A 240 300
B 100 500
C 120 240
Ans:
Waste 5-day BOD (mg/L) 5-day COD (mg/L) Ratio of BOD&COD Comment
A 240 300 0.8 Highly Biodegradable
B 100 500 0.2 Slightly Biodegradable
C 120
240 0.5 Moderately Biodegradable

Chloride in Water (exp:8) - assignment - Q-A

Determination of Chloride in Water
(1) Q: Discuss the environmental significance of “chloride” in water.
Ans: Chlorides in reasonable concentrations are not harmful to human. At concentrations above 250 mg/L they give a salty taste to water that is objectionable to many people. For this reason, chlorides are generally limited to 250 mg/L in supplies intended for public use. In many areas of the world where water supplies are scarce, source containing as much as 2000 mg/L are used for domestic purposes without the development of adverse effects, once the human system becomes adapted to the water. According to Bangladesh Environment Conservation rules (1997), drinking water standard for chloride is 150-600 mg/l; but for coastal regions of Bangladesh, the limit has been relaxed to 1000 mg/l.
Before the development of bacteriological testing procedures, chemical testes for chloride and for nitrogen, in its various forms, served as the basis of detecting contamination of groundwater by wastewater. Chlorides are used to some extent as tracers in sanitary engineering practice; however, they have been replaced to a great extent by organic dyes.
(2) Q: What precautions should be taken in the determination of chloride concentration by Mohr method? Explain why.
Ans: Several precautions should be taken in the determination of chloride concentration by Mohr method:
A uniform sample size should be used, preferably100 ml (or 50ml), so that ionic concentrations needed to indicate the end point will be constant.
The pH should be in the range of 7 to 8 because Ag+ is precipitated as AgOH at high pH levels and the CrO42- is converted to Cr2O72- at low pH levels.
A definite amount of indicator should be used to provide a certain concentration of CrO42- otherwise AgCr2O4may form too soon enough.
(3) Q: In determination of chloride, why an indicator blank or error is subtracted from the amount of silver nitrate used in titration? Explain.
Ans: In titration, chloride is precipitated as a white silver chloride as equ.
Ag+ + Cl- = AgCl (white precipitate)
The end point of titration cannot be detected visually unless an indicator capable of demonstrating the presence of excess Ag is present. The indicator normally used in potassium chromate, which supplies chromate ions. As the concentration of Cl- ions becomes exhausted, the silver ion concentration increases and a reddish brown precipitate of silver chromate is formed
2Ag+ + CrO42- = Ag2CrO4 (reddish brown precipitate)
This taken as evidence that all chloride has been precipitated. Since an excess Ag+ is needed to produce a visible amount of Ag2CrO4, the indicator error (blank or error) is subtracted from all titration.

Color-of-Water (exp:2) - assignment - Q-A

(1) Q: Discuss the environmental significance of “color”.
Ans: Colored water is not always harmful to man, but in most cases it is. Even if the water is not harmful, people for aesthetic reasons do not prefer it. Also disinfection by chlorination of waters containing natural organics (which produces color) results in the formation of chloroform, other trihalomethanes, and a range of other chlorinated organics, leading to problems which is a major concern in water treatment. So it is important to limit the color of water for domestic supplies.
According to Bangladesh Environment Preservation Act (1997), drinking water standard for color is 15 units.
(2) Q: Discuss briefly the causes of color in water.

Ans: most water available to us are colored to some extent due to the presence of various impurities (i.e., iron and manganese in association with organic matter from decaying vegetation). Impurities may be in the colloidal form in water or it may be in suspended state. Color caused by dissolved and colloidal form of impurities is called true color and that caused by suspended matter, in addition to dissolved and colloidal compound, is called apparent color. Ground water may show color due to the presence of iron compound.
(3) Q: Write down the methods that are commonly used for removing color from water and wastewater.
Ans: By the following methods are commonly used for removing color:
- Sedimentation with Coagulation.
- Filtration.
- By absorption and chemical precipitation & sand filtration

Discussion : Arsenic

In the laboratory there is used Membrane Filter Method& Multiple Tube Method. Besides Membrane Filter Method is widely used In the laboratory, because this method is comparatively easy and less costly and time consuming for the determination of Total Coliform & Facal Coliform in the water sample than Multiple Tube Method. Direct result is found for Total Coliform & Facal Coliform in Membrane Filter Method..
The value of Total Coliform & Facal Coliform obtained from Membrane Filter Method& Multiple Tube Method should be identical for the same sample. The value of Total Coliform & Facal Coliform is equal to TNTC.
According to Bangladesh Environment Conservation rules (1997), drinking water standard for Total Coliform & Facal Coliform is 1 and 0 /l00ml respectively; according to WHO drinking water standard for Total Coliform & Facal Coliform is fully absent condition. In the sample Total Coliform & Facal Coliform concentration is not within acceptable range. So the water sample may be required additional treatment for highly Total Coliform & Facal Coliform contamination.

Discussion : Coliform in Water

In the laboratory there is used Membrane Filter Method& Multiple Tube Method. Besides Membrane Filter Method is widely used In the laboratory, because this method is comparatively easy and less costly and time consuming for the determination of Total Coliform & Facal Coliform in the water sample than Multiple Tube Method. Direct result is found for Total Coliform & Facal Coliform in Membrane Filter Method..
The value of Total Coliform & Facal Coliform obtained from Membrane Filter Method& Multiple Tube Method should be identical for the same sample. The value of Total Coliform & Facal Coliform is equal to TNTC.
According to Bangladesh Environment Conservation rules (1997), drinking water standard for Total Coliform & Facal Coliform is 1 and 0 /l00ml respectively; according to WHO drinking water standard for Total Coliform & Facal Coliform is fully absent condition. In the sample Total Coliform & Facal Coliform concentration is not within acceptable range. So the water sample may be required additional treatment for highly Total Coliform & Facal Coliform contamination.

Discussion : Fecal Coliform

In the laboratory there is used Membrane Filter Method& Multiple Tube Method for the determination of Facal Coliform. Besides Membrane Filter Method is widely used In the laboratory, because this method is comparatively easy and less costly and time consuming for the determination of Facal Coliform in the water sample than Multiple Tube Method. Direct result is found for facal Coliform in Membrane Filter Method. The suitable temperature is about 440 c for the determination of facal Coliform in water.

The value of Facal Coliform obtained from Membrane Filter Method& Multiple Tube Method should be identical for the same sample. The value of Facal Coliform is equal to TNTC.
According to Bangladesh Environment Conservation rules (1997), drinking water standard for Facal Coliform is 0 /l00ml; according to WHO drinking water standard for Facal Coliform is fully absent condition. In the sample Facal Coliform concentration is not within acceptable range. So the water sample may be required additional treatment for highly Facal Coliform contamination.

Hardness of Water (exp:7) - assignment - Q-A

Determination of Hardness of Water

(1)Q: What is hardness and how it is caused? Discuss the environmental significance of “hardness”.
Ans: Hard water are generally considered to be those waters that require considerable amounts of soap to produce a foam or leather and that also produce scale in hot water pipes, heaters, boilers and other units in which the temperature of water is increased substantially. Such property of water is called Hardness.
Hardness is caused by multivalent metallic cations. Such cations are capable of reacting with soap to form precipitates and with certain anions present in water to form scale. The principal hardness causing cations are the divalent calcium, magnesium, strontium, ferrous iron and manganous ions.
These cations and the important anions with they are associated are shown in the following table in the order of their relative abundance in natural waters. Aluminum and ferric ions are sometimes considered as contributing to the hardness of water. However, their solubility is so limited at PH values of natural waters that ionic concentrations are negligible. The hardness of water is derived largely from contact with the soil and rock formation.
Table: Principal cations causing hardness and major anions associated with them
Cations causing hardness Anions
Ca2+ HCO3-
Mg2+ SO42-
Sr2+ Cl-
Fe2+ NO3-
Mn2+ SiO32-
Environmental significance:
Hard waters are as satisfactory for human consumption as soft waters. Because of their adverse action with soap, however, their use for cleaning purpose is quite unsatisfactory, unless soap costs are disregarded. Soap consumption by hard waters represents an economic loss to the water user. Sodium soaps react with multivalent metallic cations to form a precipitate, thereby losing their surfactant properties. In recent years these problems have been largely alleviated by the development of soaps and detergents that do not react with hardness.
Boiler scale, the result of the carbonate hardness precipitation, may xause considerable economic loss through fouling water heater and hot water pipes. A change is pH in the water distribution systems may also result in deposits of precipitates.

(2)Q: A water has the following analysis:
Na+ : 20 mg/L Cl- : 40 mg/L
K+ : 30 mg/L HCO3- : 67 mg/L
Ca2+ : 5 mg/L CO32- : 0 mg/L
Mg2+ : 10 mg/L SO42- : 5 mg/L
Sr2+ : 2 mg/L NO3- : 10 mg/L

What is the total hardness; carbonate hardness and noncarbonate hardness in mg/L as CaCO3?
Ans: Ca2+, Mg2+ and Sr2+ are hardness producing agents
Hardness as mg/l CaCO3 = M2+(mg/l) X 50 / (equivalent weight of M2+)
Divalent
Cation Equivalent weight of M2+ mg/l of divalent cation (M2+) Hardness as
mg/l of CaCO3
Ca2+
Mg2+
Sr2+ 20
12
43.8 5
10
2 12.5
41.67
2.283
Total hardness as mg/l CaCO3 = 56.45
The anions only HC03- and C03- are alkalinity-producing agents.
Carbonate alkalinity mg/l as CaCO3 =(mg/l C03- )/ .6 = 0/. 6 = 0
Bicarbonate alkalinity mg/l as CaCO3 =(mg/l HC03- )/ 1.22
= 67/1.22 = 54.92
Total alkalinity mg/l as CaCO3 = 54.92
As alkalinity< total hardness
Carbonate hardness (mg/l as CaCO3) =alkalinity (mg/l as CaCO3)
= 54.92 mg/l as CaCO3
Noncarbonate hardness (NCH) =total hardness _ Carbonate hardness
= (56.45 – 54.92) mg/l as CaCO3
= 1.53 mg/l as CaCO3
Total hardness as mg/l CaCO3 = 56.45
Carbonate hardness = 54.92 mg/l as CaCO3
Noncarbonate hardness (NCH) = 1.53 mg/l as CaCO3

(3) Q: Calculate hydroxide, bicarbonate and carbonate alkalinities of a water sample with a total alkalinity of 210 mg/L as CaCO3 and a pH of 7.7. Which type of alkalinity dominates the total alkalinity? Also calculate the concentrations of carbonate (CO32-) and bicarbonate (HCO3-) ions.
Ans: If alkalinity< total hardness
Carbonate hardness (mg/l) =alkalinity (mg/l)
If alkalinity> total hardness
Carbonate hardness (mg/l) = total hardness (mg/l)
alkalinity of our sample = 260 mg/l as CaCO3
Total hardness of our sample = 315 mg/l as CaCO3
As alkalinity< total hardness
Carbonate hardness (mg/l) =alkalinity (mg/l)
= 260 mg/l as CaCO3
Noncarbonate hardness (NCH) =total hardness _ Carbonate hardness
= (315 – 260 ) mg/l as CaCO3
=55 mg/l as CaCO3
Carbonate hardness (mg/l) = 260 mg/l as CaCO3
Noncarbonate hardness (NCH) = 55 mg/l as CaCO3

Oxygen Demand of Water (exp:10) - assignment - Q-A

Determination of Biochemical Oxygen Demand of Water
(1) Q: A sample of sewage is mixed with water( no seeding done) in the ratio of 1: 20 (i.e., 1 ml of sewage diluted to 20 ml by adding water) for BOD test. The initial DO is 8.5 mg/l and final DO after 5 days, is 3.1 mg/l. calculate BOD5 of the sewage.
Ans: The five-day BOD of a diluted sample is given by
BOD5 = [DOI - DOf ] X D.F. - - - - - - - - - - - - (1)
Where
D.F. = Dilution factor = (vol. of wastewater + dilution water) / (vol. of
wastewater)
= (1+19)/ 1
= 20
Here, DOI =initial DO = 8.5 mg/l
DOf =final DO = 3.1 mg/l
By using equation (1)
BOD5 = (8.5 – 3.1) X 20
= 5.4 X 20
= 108 mg/l
Therefore, BOD5 of the sewage is 108 mg/l
(2) Q: A test bottle containing just seeded dilution water has its DO level drop by 0.8 mg/l in a 5-day test. A 300 ml BOD bottle filled with 30 ml of wastewater and the rest with seeded dilution water experiences a drop of 7.3 mg/l in the same period (5-day). Calculate the BOD5 of the wastewater.
Ans: The oxygen demand of the wastewater (BODw) is determined by the following equation
BODm Vm = BODw Vw + BODd Vd - - - - - - - - ( 1 )
D.F. = Dilution factor = (vol. of wastewater + dilution water) / (vol.
of wastewater)
= (30 + 270)/ 30
= 10
Where,
BODm = the BOD of the mixture of wastewater = 7.3 mg/l
BODd = the BOD of the dilution water alone = 0.8 mg/l
BODw = the BOD of the of wastewater
Vm = the volume of wastewater and dilution water = 300 ml
Vw = the volume of wastewater = 30 ml
Vd = the volume of dilution water = 270 ml
By using equation (1)
BODm Vm = BODw Vw + BODd Vd
7.3 X 300 = BODw X 30 + 0.8 X 270
2190 = BODw X 30 + 216
BODw = (2190 – 216)/ 30
BODw = 1974/30
BODw = 65.8 mg/L
Therefore, BOD5 of the wastewater is 65.8 mg/l
(3) Q: In a BOD test on a diluted wastewater sample ( 1: 20 dilution, but not needed), the initial DO is 8.4 mg/L, and final DO after 5 days is 4.2 mg/L. If the reaction rate constant is 0.22/day, calculate
(a) 5- day BOD of the wastewater,
(b) Ultimate carbonaceous BOD of the wastewater
(c) Remaining Oxygen demand after 5-days
(a) Ans: determination of 5- day BOD of the wastewater,
The five day BOD of a diluted sample is given by
BOD5 = [DOI - DOf ] X D.F. - - - - - - - - - - - - (1)
Where
D.F. = Dilution factor = (vol. of wastewater + dilution water ) / (vol.
of wastewater)
= (1+19)/ 1
= 20
here, DO0 = initial DO = 8.4 mg/l
DO5 = final DO = 4.2 mg/l
By using equation (1)
BOD5 = (8.4 – 4..2) X 20
= 4.2 X 20
= 84 mg/l
5- day BOD of the wastewater is 84 mg/L
(b) Determination of ultimate carbonaceous BOD of the wastewater. Lo
BOD5 = L0 (1 – e-kt )
84 = L0 (1 – e-0.22 X 5 )
L0 = 125.91= 126 mg/L
ultimate carbonaceous BOD of the wastewater.Lo = 126 mg/L
(c) remaining Oxygen demand after 5-days, L5
L0 = BOD5 + L5
L5 = L0 – BOD5
L5 = 126- 84
L5 = 42 mg/L
remaining Oxygen demand after 5-days, L5 = 42 mg/L

PH (exp:1) - assignment - Q-A

(1) Q: Discuss the environmental significance of “pH”.
Ans: A controlled value of pH is desired in water supplies, sewage treatment and chemical process plants. In water supply pH is important for coagulation, disinfection, softening and corrosion control. In biological treatment of waste pH is the most significant. Organisms involved in treatment plants are operative within certain pH range.
According to Bangladesh Environment Conservation Rule (1997), drinking water standard for pH is 6.5 to 8.5.
(2) Q: Define pH in terms of hydrogen-ion (H+) concentration and hydroxyl-ion
(OH-) concentration. A decrease in pH of one unit represents how much increase in hydrogen-ion concentration?
Ans: pH is a measure of the acid or alkaline condition of water. It is way of expressing the hydrogen ion concentration, or more preciously, the hydrogen ion activity. pH is define as follows:
pH = -log{H+}
Where, {H+} is the concentration (or activity) of hydrogen ion (or proton) in moles per liter (M)
Water dissociates to form hydrogen ion ( H+) and hydroxyl ion (OH-) according to following equation:
H2O = H+ + OH-
At equilibrium, it can be written,
Kw = {H+} {OH-} / {H2O}
But , since concentration of water is extremely large (approximately 55.5 mol/L) and is diminished very little by the slight degree of ionization, it may be considered as a constant and its activity is taken as 1.0. Thus
Kw = {H+} {OH-}
Where Kw = Equilibrium Constant
For pure water at 250C, Kw = 10-7×10-7 = 10-14 . This known as the ion product of water or ionization constant for water. In other words, water (de-ionized or distilled water) at 250C dissociates to yield 10-7 mol/L of hydrogen ion (H+) and 10-7 mol/L f hydroxyl ion (OH-). Hence, according to equation pH = -log {H+}
, pH of deionized water is equal to 7.0.
(3) Q: At 250C, hydroxyl-ion concentration of a solution is 10-5.7. Determine the pH of the solution.
Ans: For pure water at 250C, Kw = 10-7×10-7 = 10-14 and given {OH-} = 10-5.7
{H+} = Kw / {OH-}
= 10-14 / 10-5.7
= 10-8.3
pH = -log{H+}
= -log 10-8.3 = 8.3

Report Writing : Sample

OBJECTIVE :
The objective of this test is to determine the chemical coagulation of water sample and thereby
• to determine the optimum doses of coagulant which wll be use to remove colloidal particles by forming larger flocs.
REAGENTS :
• Standard Alum solution
• Standard Arsenic solution (if as removed is to be demonstrated)
APPARATUS :
• Coagulation (stirring) device
• PH meter
• Turbidity meter
• Glass beakers (1000 ml; 6 nos)
PROCEDURE :
• We have to determine PH and turbidity of the water to be treated.
• We have to fill six 1000 mL beakers each with 500 mL water to be treated.
• We have to add required coagulant (standard alum soluiton) to each beaker.
• We have to mix the samples in the beaker with the help of the stirring device. The sample has to be subjected to one minute of rapid mixing followed by 15 minutes of slow mixing ( about 40 rpm)
• We have to allow the flocs to settle down for about 15 minutes. We have to observe the characteristics of the flocs and the settling rates.
• We have to collect the supernatant from each beaker and measure PH and turbidity of each.
• We have to plot PH versus alum dose in a graph paper and we have to observe the effect of alum dose on PH.
• We have to plot turbidity versus the coagulant(alum) dose in a graph paper. We have to determine optimum dose of alum from the plot.
ASSIGNMENT ON DETERMINATION OF CHEMICAL COAGULATION (ALUM COAGULATION)
QUESTION NO. 1:
What do you understand by coagulation and flocculation? Which coagulants are most commonly used for water and wastewater treatment.
ANSWER :
COAGULATION :
The term coagulation is used to describe the process by which the charge on particles is destroyed .
Surface water generally contains wide variety of colloidal impurities that may cause the water to appear turbid and may impart color to the water. Colloidal particles that cause color and turbidity are difficult to separate from water because the particles will mot settle by gravity and are so small that they pass through the pores of most common filtration media. In order to be removed, the individual colloids must aggregate and grow in size so that they can settle by gravity. Chemical agents are used to promote colloid aggregation by destroying the forces that stabilize colloidal particles. The process of destroying the stabilizing forces and causing aggregation of colloids is referred to as chemical coagulation.
FLOCCULATION :
The term flocculation is used to determine the aggregation of particles into larger units.
The chemicals used for the purpose of coagulation is called coagulants.The most common coagulants used in water and wastewater treatment are aluminum and ferric salts as alum, ferric chloride and ferric sulfate.
ASSIGNMENT ON DETERMINATION OF BIOCHEMICAL OXYGEN DEMAND (BOD)
QUESTION NO. 1: What do you understand by coagulation and flocculation? Which coagulants are most commonly used for water and wastewater treatment.
ANSWER:
Various types of color and turbidity producing colloidal impurities do not settle by gravity and are so small that they pass through the pores of most common filtration media. The process of removing these by destroying their stabilizing forces and causing aggregation using chemical agents is referred to as chemical coagulation.
"Coagulation" is the process by which the charge on particles is destroyed, and 'flocculation" is the aggregation of particles into larger units.
The chemicals used for the purpose of coagulation are called coagulants. The most common coagulants used in water and wastewater treatment are -¨ aluminum and ferric salts such as alum,ferric chloride and ferric sulfate.
QUESTION NO. 2:
Why addition of alum may result in a drop in pH value. Discuss the affect of alum dose on pH from your experimental results.
ANSWER :
The common metal salt alum (aluminum sulfate) is a good coagulant for water containing appreciable organic matter. The chemical formula used for commercial alum is Al2(S04)3 . 14 H2O. Once dissolved in water, aluminum forms hydroxo-complexes and solids [e.g., Al(OH)3(s), Al(OH) +2, Al(OH)2+, Al(OH)4- ; Eqs. 1-5] and as a result pH of water is lowered, especially if alkalinity of water is low.
Al2(SO4)3 . 14 H2O (alum) = 2Al +3+ 3 SO4 –2 (1)
Al +3+ 3H2O = Al(OH)3(s) + 3 H+ (2)
Al +3+ H2O = Al(OH) +2 + H+ (3)
Al +3+ H2O = Al(OH)2+ + 2 H+ (4)
Al +3+ H2O = Al(OH)4- + H+ (5)
Theoretically, each mg/l of alum consume approximately 0.50 mg/l (as CaCO3) of alkalinity. For water with low alkalinity, this may result in significant reduction in pH that may interfere with formation of aluminum hydroxide flocs. In water treatment with alum coagulation, residual aluminum that may be present in water after the coagulation process is a cause of concern.
Remedy: If the alkalinity is insufficient, coagulant aids such as *lime ,Ca(OH)2, *soda ash (Na2CO3), *activated silica and *poly-electrolytes are used to provide the necessary alkalinity.
We did not measure pH during our experiment. So we cannot discuss the affect of alum dose on pH in this experiment.
QUESTION NO. 3: What is the primary mechanism by which heavy metal ions are removed during coagulation.
ANSWER:
Coagulation with alum and ferric chloride or ferric sulfate is also widely used for removal of heavy metal ions (e.g., lead, arsenic) from water. In this process heavy metal ions are primarily removed by adsorption (and subsequent precipitation) onto coagulated flocs of metal (either aluminum or iron) hydroxides. Coagulation with alum and ferric chloride/sulfate have been successfully used for removal of arsenic from water.
DATA & CALCULATION SHEET:
EXPT NO. : 12
EXPT NAME : DETERMINATION OF CHEMICAL COAGULATION(ALAM COAGULATION)
Alum Dose
mg/L Alum Dose
mg/500 ml Alum solution Added ml Turbidity
NTU
10 5 0.5 4.6
20 10 1.0 4.9
30 15 1.5 5.8
50 25 2.5 6.5
80 40 4.0 7.1
100 50 5.0 12.7
150 75 7.5 4.6
200 100 10.0 3.9
250 125 12.5 3.5
300 150 15.0 1.14
From the plot of the turbidity versus the coagulant (alum) dose,(in a plain graph paper) the optimum dose of alum =
Result :
The optimum dose of alum =
DISCUSSION :
On the basis of experiment and probable sources of errors :
• In this experiment we are asked to determine PH, color and arsenic concentration of the water to be treated. But we did not measured these parameters.
• The whole experiment was done by the lab technicians, so less error might occur.
• The lab technicians did not take precise measurement of volume of sample, coagulant added the sample.
• Collecting the supernatant from each beaker we measured only the turbidity, but not the PH
• During the 15 minutes of settling down, the sample might be disturbed by some means. For this reason, the result was subjected to error.
On the basis of Result:s
Optimum dose of alum =
Comments :

• The shape of the plot of measured turbidity and the dose of coagulant is almost opposite to that of the typical shape of the curve.
• From the curve we cant get the optimum dose of alum.

Sample Collection

Sampling Data:
Locality: Dhaka University campus
Sample Site: Pond of Shahidullah Hall
Date of Collection: 01-08-2000
Time of Collection: 12.30 PM
Date of Testing: 01-08-2000
Time of Testing: 2.45 PM
Temperature: about 29 0C
Weather Condition: quite Cloudy
Wind Blow: From South to North
Report:
For accurate analysis of the water sample water should have been collected from different points of the pond. But we collected water from only one point. So it might not be a representative one.
Water sample is collected from a pond, which is approximately a rectangular in shape and its length is about 200 m, and width is approximately 100 m. There is surrounded a road. The sample was collected about 30m apart from the east bank and about 50m from the north bank. The color of water in the pond is greenish. Trees surround the sides of the pond. Leaves falling from the trees become rotten and pollute the water. There are many fishes in the pond. There are some water plants surrounding the pond. The pond is paved at one side of the bank. There are buildings at all sides. People through garbage into the pond, such as (plastic bag, bottle, paper etc). There is a paved floor beside the pond to wash clothes and utensils. This water gets mixed with pond water and makes it dirty.

Solids (exp:4) - assignment - Q-A

Determination of Total Solids, Total Dissolved Solids and Suspended Solids in Water

(1) Q: Discuss the environmental significance of “solids content” in water.
Ans: To determine whether a water is suitable for domestic purpose, it is required to know how much solid it contains. According to Bangladesh environment preservation act (1997), potable water should not contain more than 1000 mg/l of total dissolved solids (TDS). Water with high dissolved solids generally are of inferior palatability and may induce unfavourable physiological reaction in the user. Water high in suspended solids may be aesthetically unsatisfactory for such purposes as bathing.

(2) Q: How dissolved solids concentration can be estimated from measurements of specific conductance?
Ans: A rapid assessment of the dissolved solids content of water can be obtained by specific-conductance measurements. Such measurements indicate the capacity of a sample to carry an electrical current, which in turn is related to the concentration of ionized substances in the water. Most dissolved inorganic substances in water are in ionized form and so contribute to the specific conductance. Although the nature of the various ions, their relative concentrations, and the ionic strength of the water affect this measurement, such dissolved solids content can be approximated by multiplying the specific conductance (in μS) by an empirical factor varying from about 0.55 to 0.90.

(3) Q: Explain why groundwater typically contains high total dissolved solids compared to surface water. Why is the situation reverse for the case of suspended solids?
Ans: Groundwater typically contains high total dissolved solids compared to surface water because, when water infiltrate through the earth various type of minerals dissolved with groundwater.
The situation reverse for the case of suspended solids because, the opening through which water flows in ground are very small. This considerably restricts the rate of floe while at the same time providing a filtering action against particles originally in suspension in the water.

Turbidity of Water (exp:3) - assignment - Q-A

(1) Q: Discuss the environmental significance of “turbidity”.

Ans: Turbidity is important for water supply engineers as turbid water is not aesthetically acceptable to people. There is always a fear among the people that turbid water may cause diseases. for filtration, turbid water is not suitable as it causes quick clogging of filter bed which necessitates the use of pre-treatment plant. Turbidity is also an important parameter in disinfection process.

Disinfection is usually accomplished by means of chlorine, ozone, or chlorine dioxide. To be effective, there must be contact between the agent and the organisms to be killed. However , in case which turbidity is cause by municipal wastewater solids, many of the pathogenic organisms may be encased in the particles and protected from the disinfectant. Hence USEPA has placed a maximum level of 0.5 to 1.0 units of turbidity depending on the disinfection process used, as the maximum amount allowable in public water supplies.

According to Bangladesh Environment Preservation Act (1997), drinking water standard for Turbidity is 10 NTU.

(2) Q: Why turbidity is important in “filtration” and “disinfection” ?

Ans: Turbidity is important in “filtration” and “disinfection” because; for filtration, turbid water is not suitable as it causes quick clogging of filter bed which necessitates the use of pre-treatment plant.

Turbidity is also an important parameter in disinfection process. Disinfection is usually accomplished by means of chlorine, ozone, or chlorine dioxide. To be effective, there must be contact between the agent and the organisms to be killed. However , in case which turbidity is cause by municipal wastewater solids, many of the pathogenic organisms may be encased in the particles and protected from the disinfectant.

(3) Q: Write down the methods that are commonly used for removing turbidity from water.

Ans: By the following methods are commonly used for removing turbidity:
- Plain Sedimentation.
- Sedimentation with Coagulation.
- Filtration.

BUET- CE:332 (Environment Engg Sessional)

CE 332 – ENVIRONMENT ENGINEERING SESSIONAL -1

 

INDEX PAGE : 

Name of the Experiment

1 Determination of the pH of Water

2 Determination of Color of Water

3 Determination of Turbidity of Water

4 Determination of Total Solids, Total Dissolved Solids and Suspended Solids in Water

5 Determination of Carbon-di-oxide in Water

6 Determination of Alkalinity of Water

7 Determination of Hardness of Water

8 Determination of Chloride in Water

9 Determination of Iron in Water

10 Determination of Biochemical Oxygen Demand of Water

11 Determination of Chemical Oxygen Demand of Water

12 Alum Coagulation

13 Break Point Chlorination

14 Demonstration on the Methods of Determination of Arsenic in Water

15 Determination of Total Coliform in Water

16 Determination of Fecal Coliform in Water