Chemical Oxygen Demand (C.O.D.)

Chemical Oxygen Demand (C.O.D) 

 

Aim:To determine chemical oxygen demand (C.O.D.) of the given water sample as per IS: 3025 (Part58)

Apparatus: C.O.D. digester, COD vials, Glassware

Chemicals: 

• Potassium dichromate
• Sulfuric acid
• Ferrous Ammonium Sulphate (FAS)
• Silver Sulphate
• Mercury sulphate
• Ferroin indicator
• Organic free distilled water

Preparation of reagents:

• Standard Potassium Dichromate reagent (used as digestion solution). It is prepared by:
• 4.913 g K2Cr2O7 + 33g Mercuric sulphate + 167 ml conc.H2SO4 + make-up this mixture to 1000 ml using distilled water. This is standard potassium dichromate solution used for dilution

• Sulphuric acid reagent - catalyst solution. It is prepared by:
• 5.5g silver sulphate crystals + 500 ml conc.H2SO4 for 24 hours to dissolve cilver sulfate crystals completely 

• Standard Ferrous Ammonium Sulphate Solution
• Dissolve 39.2g Ferrous ammonium Sulphate crystals in 1000ml distilled water

Procedure:

• Take two COD vials with stopper (One for sample and one for blank)
• Add 2.5 ml sample and 2.5 ml D.W to each vial respectively
• To each vial, add 1.5 ml potassium dichromate solution (digestion solution)
• Add 3.5 ml sulphuric acid reagent (catalyst solution)       [COD vials are HOT now]
• Cap the vials TIGHTLY and place them in the digester with the temperature set to 150C for 120 minutes. Start the digester.
• After two hours, transfer the cooled digested solution to a conical flask
• Add few drops of ferroin indicator to the flask. The solution turns bluish green
  
• Fill the burette with Ferous Ammonium sULPHATE (FAS)
• Titrate the solution in the flask with FAS (in the burette)
• Appearance of reddish brown colour indicates end point of the titration. Note the volume of FAS consumed.
  

COD is determined using the formula:

COD in mg/L = [(A-B) * N * 8 * 1000] / [volume of sampletaken]

(OBSERVATIONS)

Result:

 

 

Notes:

1. COD does not differentiate between biologically available and inert organic matter.
2. COD is a measure of total quantity of oxygen needed to oxidise ALL organic matter into carbondioxide and water
3. COD values are ALWAYS GREATER THAN BOD values
4. Potassium dichromate is considered the best oxidising agent
5. Mercuric sulphate is added to reduce the interference of chlorides
6. For industrial wastewater COD is almost 2.5 times BOD
7. 
   

Answers to viva questions

ANSWERS TO VIVA QUESTIONS

1. Hardness is the amount of dissolved calcium and magnesium ions in water (Ca2+, Mg2+). Hardness in water is caused due to contact of flowing water with soil and rocks. It is expressed as ppm of CaCO3. Hard water is unfit for bathing and laundry purposes due to high consumption of soap and residue on utensils. Hard water is unfit for use in textile and beverage industries and should not be used for boiler feed.
2. Temporary hardness is due to dissolved Calcium and Magnesium ions (Calcium hydrogen carbonate and Magnesium hydrogen carbonate). It is called temporary because it can be removed by boiling
3. Permanent hardness is hardness due to presence of Chlorides, Nitrates and Sulphates of Calcium and Magnesium which cannot be removed by boiling. Permanent hardness can be removed by base-exchange process or by the use of silicon zeolites
4. Dissolved Oxygen is the amount of gaseous oxygen (O2) dissolved in the water. Oxygen enters the water by direct absorption from the atmosphere, by rapid movement, or as a waste product of plant photosynthesis
5. At 0℃ the saturation value of DO in water is 14.6 PPM
6. Biochemical Oxygen Demand or BOD refers to the amount of biologically degradable organic matter present in the water. It is a measure of the strength of the sewage and indicates organic matter present in the water sample. It takes five days to determine BOD. This is known as BOD₅ which reflects about 70 - 75% BOD. This is adopted as a standard. However, complete oxidation takes more than three weeks and is known as ultimate BOD and is represented as BODu
7. Chemical Oxygen Demand (COD) refers to the amount of chemically oxidizable matter present in the water. It is ALWAYS greater than BOD
8. Alkalinity refers to the capacity to neutralize acids. It occurs due to carbonate (CO₃²⁻), bicarbonate (HCO₃⁻) and hydroxides (OH⁻)
9. Turbidity is defined as the extent to which light is either scattered or absorbed by suspended suspended matter in water.
10. Residual chlorine refers to the excess amount of chlorine in water after break-point chlorination
11. The significance of jar test is that it helps to determine the optimum dosage of coagulant to be added to the water for obtaining the most clear water after sedimentation and coagulation processes
12. Sedimentation refers to the removal of suspended matter in water by allowing it to stand still in a quiescent pond for period known as 'detention period'
13. Coagulation refers to the addition of a chemical called "coagulant" that traps the very finely suspended particles (that do not settle easily) in a gel. Examples of coagulants are alum and ferric chloride
14. When the coagulated chunks called flocs in the coagulation tank are stirred, all the flocs come together to form bigger flocs. This process is called flocculation
15. The ill-effects of hard water in domestic use are it forms precipitates requiring excessive use of soap in washing and bathing. In industries, hard water causes encrustation and formation of scales in pipes and boilers
16. BOD is Biochemical Oxygen Demand and it is the amount of Oxygen required to decompose organic matter
17.  BOD is the amount of Oxygen required while DO is the amount of Oxygen present in water in a dissolved state. If BOD is high, DO is low (Inversely proportional)
18. BOD and COD refer to the amount of Oxygen REQUIRED for oxidation of organic matter biologically and chemically respectively. COD is always higher than BOD because COD oxidizes organic matter BOTH biologically and chemically active organic matter while BOD refers to only biologically active organic matter
19. Turbidity plays a very important role in wastewater treatment. It indicates the amount of suspended sediment in water. Water with high turbidity block sunlight to aquatic plants and carry contaminants and pathogens
20. Dissolved Oxygen (DO) is measured using a method called "Azide modification method" or 'winkler's method'
21. Coagulation refers to aggregation of particles. It is mainly used in the treatment of water containing colloids
22. The permissible value of residual chlorine is 0.1 to 0.2 ppm or mg/l
23. The permissible value for chloride content in water is 250 ppm
24. The amount of chlorine in water decreases initially till ALL the bacteria are killed and then starts increasing as chlorine appears as free chlorine. This point is known as break point chlorination
25. A graph is plotted in jar test to accurately determine the optimum dosage of coagulant
26. The source of chlorides in water is the dissolution of rocks as water flows on land
27. Minimum DO to support aquatic life is 2 ppm
28. The saturation value of DO at 25 C is 8.1 ppm
29. Alkalinity is the capacity to neutralise an acid. Alkalinity plays an important role in chemical coagulation and biological waste treatment processes
30. Standardization is the process of determining the exact concentration of a solution. 
31. Normality is the ratio of molecular weight of a substance to the number of gram equivalents of that substance
32. Dilution ratio is the ratio of volume of the mixture to volume of the concentrate. It is used in determining BOD of very strong wastes
33. Hypochlorous acid is a weak acid formed during the process of chlorination. It is mainly used for disinfection of water,
34. During the determination of residual chlorine, in one of the steps, potassium dichromate is added as a reagent and the solution is kept in the dark to prevent photochemical oxidation of potassium dichromate 
35. Standardization of a reagent is done to adjust the concentration of a reagent to the specific value required for the reaction. Usually stock solutions required for a reaction are prepared in bulk of a concentration greater than the requirement and hence it is required to calculate their precise concentration and adjust it accordingly as per the requirement for the particular experiment
36. Alkalinity is caused due to carbonate, bicarbonate and hydroxide ions while hardness is caused due to divalent cations of Calcium and Magnesium
37. Hardness and Alkalinity are both expressed in mg/L of CaCO3
38. EDTA - Ethylene Diamine Tetra Acetic acid; EBT - Eriochrome Black T
39. Winklers test is conducted to determine dissolved oxygen in water and it is also known as Alkali Azide Modification Method
40. Saturation value for DO depends on temperature and its value is 8.1 at 25 C
41. Permissible value for chlorides in water is 250 ppm
42. The types of alkalinities are phenolphthalein alkalinity and methyl orange alkalinity
43. Carbonate, bicarbonate and hydroxide ions contribute to alkalinity in water
44. Turbidity is measured in NTU (Nephelo Turbidity Units)
45. 'Stock solution' refers to a standard solution prepared in bulk and used repeatedly. Stock solutions are prepared for frequent use in labs and they are usually of a higher concentration than needed. They should be standardised before use
46. If the solution has a higher value than the range for which the instrument is calibrated, the solution is diluted and the formula used is (A * (B+C))/C where A = Turbidity of diluted solition, B = Volume of dilution water in ml, C = Volume of sample that had been diluted
47. Alum is chemically called 'Aluminium Sulphate' and is used as a floccculant to remove unwanted colour and turbidity from water. It is used in combination with filtration in conventional water treatment processes
48. Coagulants function to neutralize negatively charged particles and destabilize the forces that keep the particles apart.
49. If too little of coagulant is added, all the suspended particles in water do not coagulate and if coagulant is added in excess, water acquires a bitter taste
50. Alkalinity and Hardness, both are expressed as mg/l of Calcium Carbonate

INDEX

ENVIRONMENTAL ENGINEERING LAB

1. Determining the concentration of sodium and potassium in water sample using flame photometer
2. Determination of optimal coagulant dosage using jar test
3. Determination of turbidity in water sample using nephelo turbidity meter
4. Determination of alkalinity
5. Determination of residual chlorine
6. Estimation of chloride
7. Determination of Dissolved Oxygen (D.O)
8. Estimation of total hardness and calcium hardness in water sample
9. Determination of Total Solids
10. Determination of B.O.D.
11. Determination of C.O.D
    

Questions for viva voce

Answers

Questions suggested for viva voce

QUESTIONS & ANSWERS

1. What is hardness of water 
2. What is temporary hardness and how can it be removed
3. What is permanent hardness and how can it be removed
4. What is meant by dissolved oxygen
5. What is the maximum value for dissolved oxygen in water
6. What is B.O.D (Biochemical Oxygen Demand)
7. What is C.O.D (Chemical Oxygen Demand)
8. What is alkalinity
9. Define turbidity
10. What is meant by residual chlorine
11. What is the significance of jar test
12. What is sedimentation
13. What is coagulation
14. What is flocculation
15. What are the ill-effects of hard water
16. What is BOD ?
17. What is the difference between BOD and DO(Dissolved Oxygen). If BOD is high (~300 ppm), what is the value of DO (low or high)
18. What is the difference between BOD and COD. Which is higher and why
19. What is the significance of turbidity in water treatment
20. Name the method used to determine DO
21. What is a coagulant and why is it used
22. What is residual chlorine and what is its permissible value
23. What is the permissible limit of chloride content in water
24. What is break-point chlorination
25. Why is a graph plotted to determine the optimum dosage of coagulant
26. What is the source for chlorides in water
27. What is the minimum DO content to support aquatic life
28. What is the saturation value for DO
29. Define alkalinity and its significance in water treatment
30. What is standardisation of titrant
31. Define Normality
32. What is dilution ratio and where is it used
33. What is Hypochlorous acid and what is its application
34. In the Starch-Iodide method for determination of residual chlorine, why is the flask kept in a dark place for five minutes during the standardisation of sodium thio sulphate using potassium dichromate
35. What is meant by standardisation of a reagent and why is it done
36. How are alkalinity and hardness related
37. How are hardness and alkalinity expressed
38. Expand EDTA and EB-T
39. What is 'Winkler's Test' conducted for and what is it also known as
40. What is the saturation value for DO
41. What is the permissible limit for chloride content in water
42. What are the types of alkalinities
43. What are ions contributing to alkalinity in water
44. What are the units of turbidity
45. What is meant by a 'stock solution' and what are the chemicals used to prepare stock turbidity solutions 
46. Explain how the turbidity of a solution is calculated if the meter reading is beyond the range calibrated
47. What is alum and where is it used in water treatment
48. What is the function of a coagulant
49. What happens if too little or too much of a coagulant is added to water
50. In what way are hardness and alkalinity expressed and why

ANSWERS

Determining the concentration of sodium and potassium in water sample using flame photometer

DETERMINING THE CONCENTRATION OF SODIUM AND POTASSIUM IN WATER SAMPLE USING FLAME PHOTOMETER

AIM:-To determine the concentration of sodium and calcium using flame photometer.

APPARATUS:- Flame Photometer.

THEORY:- Sodium, potassium and calcium are abundantly available in nature and their salts are highly soluble in water. Hence most of the water samples contain these elements in varying quantities depending upon the source of water. Even though, they are harmless in small quantities, excessive concentrations may impart a bitter taste to water and may make the water hazardous  to the health of cardial and kidney patients. Sodium is corrosive to metals surface and in large concentrations it is toxic to plants.

            The principle of working of flame photometer is very simple. The sample is sprayed as fine mist into non-lumbinous flame which becomes coloured depending upon the characteristics of the emissions of the elements present in the sample water. The equipment has two channels which work simultaneously. Each channel has a detective device which views the flame through a narrow band optical filter that only passes the wave lengths centered around the characteristics emissions of the selector element. For sodium, this wave length is 589nm and for potassium it is 768nm. The output of the detector is passes on to an electronic metering device which converts the input to a digital readout

            Concentrations in the range of 0-2m.eq. per lit of sodium and 0-0.1m.eq.per lit of potassium can be measured with the equipment after calibration.

RELAVANCE:- Potassium is a major chemical constituent of potable water. The average level of sodium is greater than 100mg/lit. Higher amounts of sodium render boiler operations difficult. High concentration of sodium in blood leads to hyper tension. Soil permeability can be affected by concentration of sodium. Potassium is generally not found in water in high concentration. The ratio of sodium to potassium is generally 10:1 to 20:1.

CHEMICAL REAGENTS:

i)                    Deionized water

ii)                  Sodium chloride

iii)                Potassium chloride

PROCEDURE:-

a) Calibration of the flame photometer:-

i)                    Prepare 1000ppm Nacl solution by dissolving 2.5416 grams of Nacl salt in 1.0 lit of deionozed water. Using this stock solution, prepare a standard solution of 1.0m.eq.per lit (1.0m.eq./lit=23ppm)

ii)                  Prepare 1000ppm Kcl solution by dissolving 1.907 grams of Kcl salt in 1.0lit of deionized water. Using this stock solution, prepare standard solution of 0.08m.eq.’lit(1.0m.eq/lit=39ppm). These two standard solutions are used for the calibration of equipment.

b) Procedure:-

1. Switch on the power supply and allow the instrument to warm up for about 5 mins.
2. Switch on the air compressor and set the pressure between 0.3-0.6 kg/cm2
3. Light the flame and adjust the gas supply so that a low intensity blue flame is obtained.
4. Set all the coarse and fine controls to the maximum position and select the sodium and potassium filters.
5. Feed deionized water to atomizer and wait atleast for 30 seconds.
6. Adjust the “Set Reference” course and fine controls for zero readout for potassium and sodium.
7. Aspirate 1.0m.eq/lit. of sodium solution and wait at least 30 seconds and then adjust the “set F.S” course and fine controls for a readout of 100 for sodium only.
8. Aspirate 0.08m.eq/lit. of potassium solution and wait atleast 30 seconds and then adjust the “set F.S” coarse and fine control for a readout of 80 for potassium only.
9. Repeat steps 6 to 8 until the readings are stabilized the equipment now stands calibrated.

c) Determination of sodium and potassium concentration in the given sample water:-

Feed the sample to the atomizer and obtain the readings corresponding to sodium and potassium. If the concentrations of sodium and potassium in the sample water is more than the measurable limit of the equipment, the unit will not show a meaningful display. In that case the sample has to be diluted so as to get a meaningful digital read out. Note down the extent of dilution of the sample (i.e. dilution factor). Let 10ml of sample water is diluted to 50ml by adding deionozed water. Let the readings be 62 and 31 for sodium and potassium respectively.

Specimen calculations:

Dilution factor = ml of diluted sample   =  50  =  5

                             ml of sample                   10

Concentration of sodium

in mg/lit                                   =  62*1*23*5 = 71.3 mg/lit

                                                   100

Concentration of potassium

in mg/lit.                                  = 31 *0.08*39*5  = 6.045 mg/lit

                                                    80

OBSERVATIONS:-

Sample No.	Digital readout for		Dilution factor
	Na	K

CALCULATIONS:-

RESULTS:-

INTERPRETATION OF RESULTS:-

Determination of optimal coagulant dosage using jar test

DETERMINATION OF OPTIMUM COAGULANT DOSE USING JAR TEST APPARATUS

AIM:- To determine the optimum coagulant dose.

APPARATUS:- Jar test apparatus and Nephelo turbidity meter.

THEORY:- In plain sedimentation, very fine suspended particles of size 0.006mm to 0.002mm are not removed, since they required a detention period of 10 hours to 4 days which is impracticable. In addition to this fine suspended particle, water also contains electrically charged colloidal particles which are continuously in motion and never settle down due to gravity. It has been found that, the above mentioned impurities can be removed by sedimentation with coagulation.

            It has been found that when certain chemicals (i.e. coagulants) are added to water an insoluble, gelatinous precipitate is formed. This precipitate during its formation and descent through the water absorbs very fine suspended and colloidal impurities there by reducing the turbidity of the water.

RELEVANCE:-

            Coagulation of raw water using the optimum coagulant dose removes colloidal impurities from the water. These colloidal impurities are normally associated with organic matter containing pathogenic bacteria which are responsible for water borne diseases. The chemical coagulation also makes the process of disinfection more effective. Coagulation also removes objectionable colour, taste and odour’s from water. Usually the dose of Alum varies between 5mg/lit for relatively clear water to about 85 mg/lit for very turbid waters. The average dose is about 20mg/lit.

CHEMICAL REAGENTS:-

ALUM SOLUTIONS:- Dissolve 1.0 gram of Alum in 1 lit of distilled water so that each ml. of Alum solution contains one milligram of Alum.

PROCEDURE:- Take 2 lit of sample water in all the six jars of the apparatus. Then add Alum solution in each of the six jars in varying amounts. The range of Alum dose depends upon the turbidity of the raw sample water. The normal range of Alum dose varies between 15ppm to 60ppm. Add the Alum solution to each of the six jars as per the tabular column shown in observation sheet. After adding different amounts of Alum solution in the all six jars, place the jars on the platform provided and fix the stirring paddle to the connecting rod which rotates by a gear and spindle system, with thee help of electric motor, the paddles are rotated at a speed of 30-40 rpm for about 2 minutes. The speed of the paddles are then reduced to a minimum so as to cause flocculation and stirring is continued to about 20-30 min. the rotation of paddles are then stopped and floc is allowed to settle for 30min. then pippet out the supematant from each jar and measure the turbidity using nephelometer.

GRAPH:- Plot a graph between the coagulant dose applied and turbidity of coagulated sample, by taking turbidity value on Y-axis and Alum dose on X-axis; as shown below. Then determine optimum coagulant dose from the graph which corresponds to minimum turbidity.

OBSERVATIONS:

Jar No.	Alum dose in mg/lit	ml of Alum solution to be added in 2000ml

CALCULATIONS:-

RESULTS:-

INTERPRETATION OF RESULTS:-

Determination of turbidity in water sample using nephelo turbidity meter

MEASUREMENT OF TURBIDITY USING NEPHELOMETER

AIM:- To determine the turbidity of the given sample water by Nephelometric method.

APPARATUS:- Nephelo turbidity meter.

THEORY:- Turbidity is a measure of the extent to which light is either absorbed or scattered by suspended material present in the water. Turbidity is surface waters results from the erosion of colloidal material such as clay, slit, rock fragments and metal oxides from soil, vegetable fibers and micro-organisms may also contribute to turbidity. Drinking water supplies requires special treatment by chemical coagulation and filtration before it may be used for public water supply.

            This turbidity can be brought down to required level by adding coagulants. Coagulants when added to water it will form a geletaneous substance known as floc and this will arrest the fine suspended and colloidal particles. These arrested particles will settle down rapidly because of increase in their size.

RELEVANCE:- Turbidity waters are aesthetically displeasant and are not accepted for domestic use. The colloidal matter associated with turbidity provides adsorption sites for chemicals and biological organisms that may be harmful or cause undesirable tastes and odour. Disinfection of the turbid waters is difficult and unsatisfactory, since the colloids partially shield organisms from the disinfectant. This IS values for drinking water is 10 to 25 NTU.

REAGENTS:-

1. Turbidity free water:- Pass distilled water through a lower turbidity than distilled water, discard the first 200ml, collected. If filtration does not reduce turbidity use distilled water. 
2. Stock turbidity solutions:-

i)                    Solution 1:- Dissolve 1.0 grams hydrazine suplate (NH₂)₂.H₂So₄ in distilled water and dilute it to 100 ml in a make up flask.

ii)                  Solution 2:- Dissolve 10.0 grams hexamethylene tetramine (CH₂)₆N₄ in distilled water and dilute it to 100ml.

iii)                Solution 3:- In a 100ml flask, mix 5ml. each of solution 1 and 2. Allow it to stand 24 hours, then dilute it to 100ml and mix thoroughly. The turbidity of this solution is 400 NTU.

iv)                Standard Turbidity Solution:- Take 10.0ml of solution 3 in a 100ml make up flask and dilute it to 100ml. with turbid free water. The turbidity of this suspension is 40 NTU.

PROCEDURE:-

a) Calibration of Nephelometer:-

i)                    Select proper range of NTU on Nephelometer.

ii)                  By placing distilled water in Nephelometer test tube, set the Nephelometer reading to zero by using the knobs provided for zero setting.

iii)                Using the standard turbid solution (i.e. 40 NTU), calibrate the Nephelemeter (i.e. adjust the Nephelemeter reading to 40 NTU using calibration knob)

b) Determination of turbidity of sample water:

i)                    For samples having turbidities less than 40 NTU:  Thoroughly shake the sample so as to remove any air bubbles and pour it into meter cell. Read out the turbidity of the sample from the digital display.

ii)                  For samples having turbidities above 40 NTU:- Dilute sample with 1,2 or 3 volumes of turbidity free water and convert the value obtained as below.

                 

If five volumes of turbidity free water were added to one volume of sample and the diluted sample showed a turbidity of 30 NTU, then the actual value is equal to 180 units. i.e.

                  Nephelometric turbidity units (NTU) = A(B + C)

                                                                                        C

Where

A = Turbidity found in diluted sample, B = Volume of dilution water in ml

C = Sample volume for dilution in ml.

OBSERVATIONS:-

For undiluted sample                                      For diluted sample

Digital read out =                                            Vol. of sample ( C ) =

                                                                        Vol. of dilution water ( B ) =

                                                                        Digital read out ( A ) =

CALCULATIONS:-

For undiluted sample                                      For diluted sample

Turbidity of sample in NTU =                        Turbidity in NTU = A(B + C)

                                                                                                               C

RESULTS:-

INTERPRETATION OF RESULTS:-