Following tests are carried out in the chemical examination of water: 1. Presence of Solids 2. Biochemical Oxygen Demand 3. Chemical Oxygen Demand 4. Stability and Relative Stability 5. Chlorides and Sulphides 6. Chlorine Demand 7. Nitrogen 8. Determining pH-Value 9. Grease, Oil and Fats.
Test # 1. Presence of Solids:
It is most essential to know the quantity of suspended, dissolved and colloidal solids in sewage, to have some concept of organic and inorganic matter in the sewage.
The quantity of suspended solids is determined by filtering a measured volume of sewage through a weighed Gooch crucible containing a mat of asbestos fibre. The solids remaining on the asbestos fibre are dried and weighed, which gives the quantity of suspended solids.
Total solids present in sewage are obtained by evaporating a measured volume of sewage at (103°C± 1°C) and weighing the residue. The residue remaining after evaporation is ignited in an electric muffle furnace; the loss in weight will be the quantity of volatile suspended solids.
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The settle able solids are suspended solids, which will settle in one hour to the bottom of the cylinder of specific height. The quantity of settle able solids can be measured gravimetrically or volumetrically. Practically, it is determined by using an Imhoff cone, Fig. 10.4. an Imhoff cone is a conical glass of one litre capacity, graduated at its bottom in millilitres.
The sewage is filled in the cone; the volume of solids settled in the bottom after one hour is directly noted which gives the quantity of settle able solids. The settle able solids usually indicate the volume of sludge which will settle in the tanks.
Suspended solids are normally larger than 1.0 micron (10-3 mm size) and are visible by the naked eye. Suspended solids which settle under quiescent condition in two hours are called settle able solids. The homogeneous and molecularly dispersed solids in the water are called dissolved solids.
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There is 0.2 m to 1 m size and are not visible to the naked eyes nor they can be filtered out. They can be removed by precipitation with the help of chemicals or biological activities. Colloidal solids are the solids in between the suspended and the dissolved solids.
They do not settle by gravity due to Brownian motion. There are no standard tests to determine their quantity. These are removed by chemicals or biological treatments in similar way to dissolved solids.
Procedure of Determining Total and Volatile Solids:
1. Take a known volume of a well-mixed sample in a tarred (W1) dish ignited to constant weight.
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2. Evaporate the sample to dryness at 103°C for 24 hrs.
3. Cool in desiccator, weigh and record the reading (W2)
4. Ignite the dish at 600°C for 30 minutes in a muffle furnace.
5. Cool in desiccator and record final weight (W3).
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The total and the total volatile solids are expressed as follows:
Where
W1 W2 and W3 are recorded in mg
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Procedures for determining non-filtrable solids and volatile non filtrable solids are as follows:
1. Filter a suitable aliquote of sample through a tarred Gooch crucible ignited to constant weight (W1), applying suction.
2. Dry the crucible along with the retained matter at 103°C for 24 hrs.
3. Cool in desiccator and record its weight (W2).
4. Ignite the crucible in a muffle furnace at 600°C for 30 min.
5. Cool in dessicator and record the weight (W3).
Dissolved Oxygen Determination:
Living organisms require oxygen to maintain their metabolic process. Dissolved oxygen (D.O.) is very important in precipitating and dissolution of inorganic substances in water. The solubility of oxygen in water depends upon its temperature. Analysis of dissolved oxygen is the main key test in sanitary engineering.
Due to the following reasons it is necessary to know the dissolved oxygen:
1. To determine the quality of raw water and for keeping proper check on stream pollution.
2. D O. test is the basis of B.O.D. test, which is an important parameter to evaluate pollution potential of the wastes.
3. D. O. is necessary for all aerobic biological waste water treatment processes.
4. Oxygen is very important factor in corrosion. D.O. test is used in controlling the amount of oxygen in boiler feed waters.
Principle of the test:
Oxygen present in the sample oxidizes the divalent manganous to its higher valency which precipitates as a brown hydrated oxide after addition of NaOH and KI. Upon acidification, manganese reverts to divalent state and liberates iodine from KI equivalent to D.O. content in the sample. The liberated iodine is titrated against Na2S2O3 (N/80) with strach as indicator.
Interference:
Ferrous ion, ferric ion, nitrite, microbial mass and high suspended solids constitutes the main sources of interference. Modifications in the estimation procedure to reduce these interferences should be used.
Reagents:
1. Manganese sulphate:
Dissolve 480 g tetrahydrate manganous sulphate and dilute to 1000 ml. This solution should not give colour with starch when added to an acidified solution of KI.
2. Alkali-iodide-azide reagent:
Dissolve 500 g. NaOH and 150 g. KI and dilute to 1000 ml. Add 10g. NaN3 dissolved in 40 ml distilled water.
3. H2SO4 concentrated.
4. Starch indicator:
Prepare paste or solution of 0.5 g. starch powder in distilled water. Pour this solution in 100 ml boiling water. Allow to boil for few minutes. Cool and then use.
5. Stock sodium thiosulphate 0.1 N. Dissolve 24.82 g., NaS2O3 5H2O in boiled cooled distilled water and dilute to 1000 ml. Preserve by adding 5 ml chloroform per litre.
6. Standard sodium thiosulphate 0.025 N. Dilute 250 ml stock Na2S2O3 solution to 1000 ml with freshly boiled and cooled distilled water. Add 5 ml chloroform per litre as preservative.
Test Procedure:
1. Collect sample in a SOD bottle using D.O. sampler.
2. Add 2 ml MnSO4 followed by 2 ml of NaOH +KI+ NaN3. The tip of the pipet should be below the liquid level while adding these reagents. Place stopper immediately.
3. Mix well by inverting the bottle 2-3 times and allow the precipitate to settle leaving 150 ml clear supernatant.
4. Now add 2 ml conc. H2SO4 Mix well till precipitate goes into solution.
5. Talk 203 ml in a conical flask and titrate against Na2S2O3 using starch as an indicator. When 2 ml MnSO4 followed by 2 ml NaOH+KI+NaN3 is added to the sample as in (2) above 4.0 ml of original sample is lost. Thus 203 ml taken for titration will correspond to 200 ml of original sample. Thus
Calculation:
1 ml of 0.025 N, Na2S2O3 = 0.2 mg of O2
Since the sample volume is 200 ml= 0.2 x 1000/200
1 ml of thiosulphate – D. O. mg/1
Each mole of gas at NTP occupies 22.4 litre.
Since the mole of oxygen weighs 16, multiply this value (D. O. mg/l) by 0.698 to give ml of O2 present in the sample at 0°C and 760 mm.
Modifications of D.O. Estimation Procedure:
1. Alsterberg Azide Modification:
The method outlined earlier is known as Winkler Modification and also as Alsterberg Azide Modification. The reagent NaOH+KI+NaN3, is used in the method to eliminate interference caused by NO2. This method also reduces interference due to higher concentration of ferric ion.
2. Rideal Stewart Modification:
This modification is used when the sample contains ferrous ion. Add 0.7 ml conc. H2SO4 followed by 1 ml of 0.63% KMnO4 immediately after collection in the B.O.D. bottle itself. When ferric ions are present in large conc. add 1 ml of 40% KF solution. Now remove excessive KMnO4 using potassium oxalate just sufficient to neutralize KMnO4 as excess oxalate give negative error.
3. Alum Flocculation Modification:
Samples containing high suspended solids consume appreciable amount of 1° in acid condition. Therefore, the samples are treated as follows. Add 10 ml of 10% alum solution followed by 1.2 ml of conc. NH4OH to 1000 ml of the sample. Now allow to settle for 10 minutes and siphon the clear supernatant for D.O. determination.
4. Copper Sulphate Sulfamic Acid Flocculation Modification:
Activated sludge contains biological floes having high demand for O2 Samples from such treatment plants are fixed by adding 10 m copper sulphate sulfamic acid reagent to 1000 ml of the sample. The reagent is prepared by adding 32 g suIfamic acid to 475 ml distilled water plus 50 g CuSO4 in 500 ml D. w. + 25 ml acetic acid.
Test # 2. Biochemical Oxygen Demand (B.O.D.):
In article 10.6 it has been stated that the presence of oxygen is necessary for the livelihood of organisms. The aerobic action continues till the oxygen is present in sewage. As the oxygen exhausts the anaerobic action begins due to which foul smell starts coming. Therefore, indirectly the decomposable matters require oxygen, which is used by the organisms.
The length of aerobic action can be increased if the percentage of oxygen is increased in the beginning and the BOD is satisfied. The Biological Oxygen Demand of a sewage is the quantity of oxygen required for the biochemical oxidation of the decomposable organic matter at specified temperature within the specified time.
During natural decomposition the life activities of organisms are stimulated by high temperatures and decreased at low temperatures, therefore, the temperature and time during BOD tests are specified. The standard time and temperature for this test in America is 5 days and 20°C respectively.
Tests revealed that the aerobic decomposition of organic matters is done in two stages. The carbonaceous matters are first oxidised and the oxidation of nitrogenous matters takes place in the latter stages. Fig. 10.5 shows the curves plotted during the aerobic decomposition of the sewage.
The B. O. D. of sewage for the first stage is about 90% of the total B. O. D. Therefore, the sanitary engineers are mostly concerned with this first stage B.O.D. The oxygen consumed in the aerobic decomposition in the first stage is not recoverable; therefore, the B.O.D. is usually the oxygen demand for the first stage decomposition. The first stage aerobic decomposition takes about 5 days at 30°C temperature.
B.O.D. Test:
The complete oxidation of organic matter takes about 2-3 months but within 10 days nearly 90% biological oxygen demand is satisfied after which the rate of depletion of oxygen is very slow. In laboratory usually 5-days B.O.D. is tested within which 70% B.O.D. is satisfied.
For performing this test, the sample of sewage free from preservative is diluted in water in 1: 100 ratio (i.e. 1 part of sewage and 99 parts of water). The water used for dilution has excess of oxygen, which is determined before dilution. The diluted sample is kept in incubator at average temperature 20°C for 5 days.
The quantity of dissolved oxygen in sample is determined after incubation. The difference between the quantity of oxygen present in water in the beginning and the end of incubation is the amount of oxygen consumed by the sewage.
In India and other hot countries to conduct the test at 20°C requires costly equipment, therefore, the test should be charged to some other suitable standard usually 37°C.
Sometime for more accurate results a sample of diluting water is also incubated in the same incubator along with the sewage for 5 days. The difference in dissolved oxygen content in the incubated sewage sample, and plain water is the B.O.D. of the sewage. This test is very delicate and much care is required while conducting the test.
Interference:
Since D.O. estimation is the basis of B.O.D. test sources of interference in B.O.D. test are the same as the D.O. tests, described above. In addition lack of nutrients in dilution water, lack of an acclimated seed organisms and presence of heavy metals or other toxic materials such as residual chlorine are other sources of interference in the B. O. D. test.
Reagents:
1. Phosphate Buffer:
Dissolve 8.5 g KH2O4, 21.75 g K2HPO4, 33.4 g Na2HPO4. 7H2O and 1.7 g NH4Cl in distilled water and dilute to 1000 ml Keep pH-value to 7.2.
2. Magnesium Sulphate:
Dissolve 82.5 g MgSO4.7H2O and dilute to 1000 ml.
3. Sodium thiosulphate solution 0.025 N. Dissolve 1.575 g Na2SO3 and dilute to 100 ml.
4. Ferric Chloride:
Dissolve 25 g FeCl3H2O and dilute to 1000 ml.
5. Calcium Chloride:
Dissolve 27.5 g anhydrous Cal2 and dilute to 1000 ml.
Test procedure:
Following is the test procedure:
(a) Preparation of Dilution Water:
1. In a container by bubbling compressed air for 1-2 days to attain D.O. saturation, aerate the required volume of distilled water. Temperature should be maintained near 20°C.
2. Now add 1.0 ml each of magnesium sulphate, phosphate buffer, ferric chloride and calcium chloride solutions for each litre of dilution water and mix it well.
3. If the waste water is not expected to contain sufficient bacteria population, add seed to the dilution water. Generally, 2 ml settled sewage is considered sufficient for 1000 ml of diluted water.
(b) Pretreatment and Dilution of Sample:
1. Neutralize the sample around 7.0 pH value, if it is highly alkaline or acidic.
2. The sample should be free from residual chlorine. If it contains residual chlorine remove it by using Na2SO3 solution as follows.
Take 50 ml of the sample and acidify with addition of 10 ml 1 + 1 acetic acid. Add about 1 g KI. Now Titrate with 0.025 Abusing starch indicator. Determine the volume of Na2SO3 required per ml of the sample and add accordingly to the sample to be tested for B.O.D.
3. Samples which have high D.O. content, i.e., D.O. 9 mg/l due to either algal growth or some other reason, reduce the D O. content by aerating and agitating the samples.
4. Make several dilutions of the prepared sample so as to obtain about 50% depleting of D.O. in dilution water but not less than 2 mg and the residual oxygen after 5 days of incubation should not be less than 1 mg/l.
Dilutions should be prepared as follows:
Siphon out seeded dilution water in a measuring cylinder or volumetric flask half the required volume. Now add the required quantity of carefully mixed sample. By siphoning dilution water, dilute to the desired volume and well mix.
Following dilution percentages may be used:
0.1 % to 1 % – Strong trade waste water.
1.0 % to 5% – Raw or settled sewage
5% to 25% – Treated effluent
25% to 100% – River water.
5. Siphon the dilution prepared as above in 4 labelled B. O. D. bottles as demonstrated and put stopper immediately.
6. Keep 1 bottle for determination of the initial D.O. and incubate 3 bottles at 20°C for 5 days. The bottle should have a water seal.
7. Prepare a blank in duplicate by siphoning plain dilution water (without seed) to measure the oxygen consumption in dilution water.
8. Fix the bottles kept for immediate D. O. determination and blank by adding 2 ml in MnSO4 followed by 2 ml of NaOH+KI+NaN3 as described in the previous article of D.O.
9. Determine D.O. in the sample and in the blank as first day and after 5 days.
10. Determine B.O.D. of the sample as follows:
When the sample is seeded, determine B.O.D of seed in above manner and apply corrections as required.
B.O.D. Rates:
The B. O. D. rate at any moment depends upon the temperature and the demand remaining to be satisfied.
Test # 3. Chemical Oxygen Demand (C.O.D.):
This test is a measure of the amount of carbon in organic matter of sewage. It is useful in identifying the performance of the various steps of treatment plants. It is also useful in determining the strength of industrial waters in sewage, which cannot be determined by B.O.D. test.
Chemical Oxygen Demand (COD) test determines the oxygen required for chemical oxidation of organic matter with the help of strong chemical oxidant. This test can be used for the same purpose as the B.O.D. test, after taking into account its limitations.
The limitations of this test are its inability to differentiate between the biologically oxidizable and biologically inert material. COD determination has an advantage over BOD determination that it takes only 5 hours as compared with BOD which require 5 days. This test is also easy and also not affected by interferences as in BOD test.
Principle:
The organic matter gets oxidized completely by K2Cr2O7 remaining after the reaction is titrated with Fe (NH4)2 (SO4)2. The consumption of dichromate gives the O2 required for oxidation of organic matter.
Interference:
Fatty acids, straight chain aliphatic compounds, nitrites, chlorides and iron are the main interfering radicals. The interference of chlorides can be removed by adding H2SO4 to the sample before addition of other reagents. About 0.4 g of H2SO4 is sufficient to complex 40 mg CI ions in the form of poorly ionized H2Cl2.
Addition of Ag2SO4 to conc. H2SO4 (22 g/9lb acid) as a catalyst stimulates the oxidation of straight chain aliphatic and aromatic compounds. NO2 exerts a COD of 1.14 mg NO2. Sulphamic acid in the amount 10 mg NO2 may be added to K2Cr2O7 solution to prevent interference caused by NO2.
Reagents:
1. Sulphuric Acid Reagents:
Add 22 g of Ag2SO4 to 9 lbs. conc. H2SO4 and keep overnight for dissolution.
2. Standard potassium dichromate N/4 Dissolve 12.259 g K2Cr2O7 dried at 103°C for 24 hours in distilled water and dilute to 1000 ml Add about 120 mg sulphuric acid to take care of 6 mg/l NO2-N.
3. Ferroin indicator. Dissolve 1.485 g 1-10 phenanthroline monohydrate and 695 mg FeSO4. 7H2O and dilute to 100 ml with distilled water.
4. Standard ferrous ammonium sulphate (N/10). Dissolve 39 g Fe (NH4)2(SO4)2. 6H2O in about 400 ml distilled water. Add 20 ml conc. H2SO4 and dilute to 1000 ml
5. H2O4. Analytical grade.
Test Procedure:
1. Place 0.4 g H2SO4 in a reflex flask.
2. Add 20 ml sample or an aliquot of sample diluted to 20 ml with distilled water and thoroughly mix.
3. Add pumice stone or glass beads followed by 10 ml standard K2Cr2O7 solution.
4. Slowly add 30 ml H2SO4 containing Ag2SO4 mixing thoroughly. It will prevent fatty acids to escape out due to high temperature. Now if the colour turns green either take fresh sample with lesser aliquot or add more discromate and acid.
5. Connect the flask to condenser. Mix the contents properly before heating, because improper mixing will result in bumping causing accident.
6. Reflux for atleast 2 hours. Cool and then wash down the condenser with distilled water. Dilute to about 150 ml cool and titrate excess K2Cr2O7 with (N/10), Fe(NH4)2 (SO4)2 sing erro in indicator. Sharp colour change from blue green to wine red indicates end-point or completion of the titration.
7. Reflux blank in the same manner using distilled water instead of sample.
Calculation:
COD is calculated from formula:
Where
a = Fe(NH4)2(SO4)2 for blank in ml
b = Fe(NH4)2(SO4)2 for sample in ml
N = Normality of Fe(NH4)2(SO4)2
For standardization of ferrous ammonium sulphate, use 10.0 ml standard K2Cr2O7 acdify by adding 10 ml H2SO4 and titrate with Ferric Ammonium sulphate to be standardized using ferroin indicator. Calculate N by N1 V1 = N2V2 formula.
Test # 4. Stability and Relative Stability:
The stability of sewage is just the opposite to putrescibility. It is defined as the percentage ratio of available oxygen to required oxygen satisfying I-stage B.O.D. It can be best measured by B.O.D. test.
In the simple test a glass-stoppered bottle is filled with sewage and is allowed to remain at room temperature until an odour of hydrogen sulphide starts. The number of days after which the odour starts is the measure of the stability of the sewage.
Usually the stability is determined by methylene blue test, in which dye (0.4 ml of a 0.05% aqueous solution in a 150 ml glass stoppered bottle) is used in determining the start of hydrogen sulphide or disappearance of available oxygen at room temperature 20°Cor 37°C. The number of days taken for the appearance of hydrogen sulphide are noted. Then the relative stability is calculated by the formula.
SR = 100(1 -0.794T20)
Or =100(1-0.605T37)
Where, Sr = relative stability.
T20 and T37 are the number of days of incubation at 20°C and 37°C respectively. Relative stability test is not very useful in the study of raw sewage, because some dissolved and collided solids affect in precipitating of the colour. The main aim of this test is to study the quality of the sewage effluents coming from the sewage treatment plants and their effect on the condition of surface streams, into which the sewage is disposed off.
Table 10.3 gives the value of the relative stability in days required for decolourization.
Test # 5. Chlorides and Sulphides:
The sources of chlorides in sewage are urine, common salt from kitchens and other industrial wastes. In practice these are present in small quantity which do not cause any harm. The presence of sulphides in sewage cause objectionable odours. The sources of sulphides are industries and decomposition of organic and inorganic matters of sewage.
The urine of people and animals contains chlorides. These are in dissolved state and are of inorganic nature, therefore not affected by the sedimentation or biological treatment. Sudden increase in the chloride contents of a stream or river directly indicates its pollution by sewage.
The chloride contents of the sewage are directly affected by the addition of some industrial wastes such as meat salting plants and ice-cream plants or ice factories etc. where large quantity of common salt is used.
Method for Determination of Chlorides:
Chloride ion is determined by titration with standard AgNO3 in which AgCl precipitates out. The end of titration is indicated by formation of red silver chromate from excess AgNO3 and potassium chromate used as an indicator in neutral to slightly alkaline solution.
Bromide, iodide, cyanide, sulfide, thiosulphate, sulphite, iron, phosphate are the prime sources of interference.
Reagents:
1. Potassium chromate indicator. Dissolve 50 gK2CrO4 in distilled water. Add AgNO3 till definite red precipitate is formed. Allow to stand for 12 hrs. Filter and dilute to 1000 ml.
2. Silver nitrate 0.0141 N. Dissolve 2.395 g AgNO3 and dilute to 1000 ml. Standardise against NaCI, 0.0141 N.l.ml of 0.0141 N. AgNO3 = 0.5 mgCI.
3. Sodium chloride 0.0141 N. Dissolve 824.1 mg NaCl (dried at 140°C) and dilute to 1000 ml 1ml = 0.5 mg CI.
4. Special reagent to remove colour and turbidity. Dissolve 125 gAl.K(SO4). 12H2O or AI.NH4(SO4)2 12H2O and dilute to 100 ml Warm to 60°C and add 55 ml concentrated NH4 slowly. Allow to stand for 1 hr. Solution should be free from cl.
Procedure:
1. Pretreatment. Take 100 ml sample and add 3 ml special reagent. Mix well and allow to stand. Filter the supernatant for titration.
2. To the samples having sulphate neutralize the sample and add about 1 ml H2O2.
3. In case the sample has sulphide or thiosulphate, raise the pH-value of the sample to 8.3 or more. Add H2O2, 1.0 ml and then again neutralize the sample.
Titration:
1. Adjust the pH-value of sample between 7.0-8.0.
2. Take 50 ml well mixed sample adjusted to pH 7.0-8.0 and add 1.0 ml K2CrO4
3. Titrate with standard AgNO3 solution, till Ag2CrO4 starts precipitating.
4. Standardize AgNO3against standard NaCl.
5. For better accuracy titrate distilled water (50 ml) in the same way to establish reagent blank.
Calculation:
Where A = ml AgNO3 for sample
B = ml AgNO3 for blank
N = Normality of AgNO3 used.
Test # 6. Chlorine Demand:
It is measure of the amount of organic matter present in the sewage. This test is done during the disinfection of sewage. The chlorine demand of sewage is the amount of chlorine in mg/litre that must be added to it so that after a definite contact time, the residual chlorine must be sufficient to kill the pathogens.
Test # 7. Nitrogen:
Figs. 10.1 and 10.2 sow how waste nitrogenous matter is broken down into simpler compounds such as free ammonia, albuminoid ammonia, nitrites and nitrates. The form of nitrogen directly indicates the state of sewage and the methods required for treating at the treatment works.
Sewage contains various organic nitrogenous compounds which represent various degrees of protein decomposition. The anerobic decomposition liberates free ammonia and ammonia nitrogen from the sewage. The type of treatment to be adopted is also determined from the presence of the organic nitrogen and ammonia taken together.
Nitrites and nitrates are formed by the oxidation of the nitrogen. Nitrites are unstable and break into ammonia or are oxidized and become nitrates. Nitrates help in the growth of algae, which is objectionable and may cause obstruction in the flow of sewage.
Test # 8. Determining pH-Value:
The pH-value of sewage is determined for regulating the various operations of treatment works. It also indicates the capacity to neutralize base or acid and the activity of hydrogen ions. The activities of some organisms are more in a specific pH-value, and similarly the chemical precipitation also depends on pH-value.
Test # 9. Grease, Oil and Fat:
These cause trouble in the flow of sewage and are detrimental to the activities of the bacterial life. Usually these are not allowed to enter in sewers and are removed by traps. Grease oil, waxes, mineral oil etc. are soluble in solvents such as other.
House kitchens, restaurants, hotels, garages, oil industries are the main source of fats, grease and other soluble materials in the sewage. As these have tendency to clog the sewers, these are trapped in the catch-pits specially designed for them.
But sometimes they enter the sewer even through the traps and reach the treatment plants. At the treatment plants these are removed by means of skimming tanks or troughs before the biological treatment units. Even though these are carbonaceous matters, but cannot be easily oxidised, and it is not possible to determine their quantity by oxygen tests.