The principal types of analysis required in the treatment of sewage are: (A) Physical Analysis (B) Chemical Analysis (C) Bacteriological (or Biological) Analysis.
(A) Physical Analysis:
This includes tests for determining physical characteristics of sewage viz., colour, odour, temperature, turbidity and total solids content.
The tests for determining total solids, volatile and fixed as well as suspended dissolved and settleable solids are described below:
(a) Determination of Total Solids:
(i) Take a dry constant weight crucible or dish. Let W1 mg be the weight of the empty crucible.
(ii) Take a known volume (about 50 ml to 100 ml) of well mixed sample of sewage and place it in the above crucible. Let V ml be the volume of sample taken.
(iii) Evaporate the sample to dryness in an oven at 103° to 105° C.
(iv) Cool the crucible in a desicator and determine its weight along with residue. Let W2 mg be the weight of the crucible with residue.
(b) Determination of Total Volatile and Fixed Solids:
(i) Ignite the residue obtained in (a) at 600°C in a muffle furnace for about 15-20 minutes. (ii) Cool the crucible in a desicator and determine its weight along with remaining residue. Let W3 mg be the weight of the crucible with remaining residue.
(c) Determination of Suspended (or Non-Filterable) and Dissolved (or Filterable) Solids:
(i) Take two samples of the same sewage and from one sample determine the total solids as indicated in (a).
(ii) Filter the other sample through a standard asbestos filter or Whatman filter paper no 44.
(iii) Take a known volume V ml of the filtrate (effluent) obtained in step (ii) and place it in a dry constant weight crucible of weight W1 mg.
(iv) Evaporate the filtrate to dryness in an oven at 103° to 105°C.
(v) Cool the crucible in a desicator and determine its weight along with residue. Let W2 mg be the weight of the crucible with residue.
(d) Determination of Settleable Solids:
Total suspended solids are subdivided as settleable and non-settleable solids. The amount of settleable solids in sewage is determined by the use of Imhoff cone. As shown in Fig. 8.7, Imhoff cone is a specially designed glass vessel having a capacity of 1 litre and graduated upto about 50 ml.
One litre of sewage is filled in the cone and it is allowed to stand there for a period of two hours. During this period the settleable solids will settle at the bottom of the cone and their amount in ml can be directly recorded from the graduated scale.
For expressing the amount of settleable solids in mg/l, the solids settled at the bottom of the cone are collected and dried in an oven at 103° to 105°C, and weighed. The amount of non-settleable solids can be estimated by deducting the total settleable solids from the total suspended solids obtained in (c).
The settleable solids are called sludge. By recording the amounts of settleable solids for different settling periods, it is possible to predetermine the sedimentation efficiencies. The data so collected will serve as a basis for the design of sedimentation tanks and also the sludge digestion units. The effluent from sedimentation tank may be considered satisfactory if the contents of settleable solids in it do not exceed 0.5 ml in a further settling period of 2 hours.
(B) Chemical Analysis:
This includes the following tests:
(1) Tests for determining oxygen consumed, dissolved oxygen, oxygen demand and relative stability.
(2) Test for determining chloride content.
(3) Tests for determining nitrogen content.
(4) Tests for determining hydrogen ion concentration or pH value.
(5) Test for determining fats, oils and grease content.
(6) Tests for determining surfactants.
(7) Tests for determining pesticides and agricultural chemicals.
(1) Tests for determining oxygen consumed, dissolved oxygen, oxygen demand and relative stability:
These tests are carried out to determine the amount of organic matter present in sewage, and the same are described below:
(a) Oxygen Consumed Test:
The oxygen consumed test is adopted to determine the amount of oxygen taken up in the oxidation of the readily oxidizable carbonaceous matter present in sewage. In this test standard amount of potassium permanganate (KMn04) is added with dilute sulphuric acid (H2SO4) to a sample of sewage.
The reaction is allowed to take place for 15 minutes, and 4 hours periods, at a constant temperature of 18°C. After this period the equivalent amounts of oxygen used up from KMnO4 are estimated by determining analytically how much available oxygen remained unused. The oxygen consumed test (or KMnO4 test) is a short period test which gives only the amount of oxygen that sewage will absorb from potassium permanganate and will be consumed mainly in the oxidation of carbonaceous matter.
This test, however, does not give an idea regarding the total oxygen required for biological and chemical oxidation of all or bulk of the organic matter for which biochemical oxygen demand (BOD) and chemical oxygen demand (COD) tests are carried out.
(b) Test for Determining Dissolved Oxygen:
The amount of Dissolved Oxygen (DO) present in sewage may vary to a large extent from the state of no oxygen to saturation level.
The dissolved oxygen content of sewage may be determined by Winkler’s method which has been modified by Alsterberg and is as described below:
Collect the sample of sewage to be tested in a narrow mouth flat stoppered reagent bottle of known capacity (usually 300 ml capacity). The bottle should be completely filled. Add 1.0 ml manganous sulphate solution by a pipette, dipping the end below the surface.
Some amount of sample would overflow. Add 1.0 ml alkaline potassium iodide solution. Insert the stopper and mix thoroughly. Allow the precipitate to settle. Add 2.0 ml concentrated sulphuric acid. Dissolve the precipitate by vigorous shaking.
Take the calculated amount of decoction worked out as below:
Titrate with N/40 sodium thiosulphate (Na2S2O3) solution using starch as indicator. Record the ml of titrant used. Since 1 ml of N/40 sodium thiosulphate solution is equal to 1 mg/l of dissolved oxygen, the ml of this solution used is equivalent to mg/l of dissolved oxygen.
(c) Tests for Determining Oxygen Demand:
The presence of oxygen is essential for the livelihood of aerobic organisms present in sewage. The aerobic action continues only till oxygen is present in sewage, and after that anaerobic action begins resulting in putrefaction. Thus oxygen is demanded in sewage for the oxidation of both organic as well as inorganic matter.
The demand of oxygen may be expressed in the following ways:
(i) Biochemical Oxygen Demand (BOD)
(ii) Chemical Oxygen Demand (COD)
(iii) Total Oxygen Demand (TOD)
(iv) Theoretical Oxygen Demand (Th OD)
In addition to these, the amount of organic matter present in sewage may also be determined by the total organic carbon (TOC) test.
(2) Test for Determining Chloride Content:
The chloride content of sewage can be determined by titrating the sample of sewage with standard (N/35.5) silver nitrate (AgNO3) solution using potassium chromate (K2Cr2O7) as indicator.
The silver first reacts with all chlorides and silver chloride is formed as indicated by the following equation-
NaCl + AgNO3 → AgCl + NaNO3
The silver chloride so formed then reacts with potassium chromate to form silver chromate as indicated by the following equation-
2AgCl + K2Cr2O7 → Ag2Cr2O7 + 2KCl
The silver chromate appears as reddish precipitate and the amount of silver nitrate required to produce such reddish precipitate determines the amount of chlorides present in sewage by using the following equation-
(3) Tests for Determining Nitrogen Content:
The various tests conducted for determining the different forms of nitrogen content present in sewage are as indicated below:
(a) Test for Determining Organic Nitrogen:
The amount of organic nitrogen present in sewage is determined by Kjeldahl method. In this method the aqueous sample of sewage is first boiled to drive off the free ammonia, and then it is digested. During the digestion the organic nitrogen is converted to ammonia which is measured, and from the same the amount of nitrogen is calculated which gives the amount of organic nitrogen present in sewage.
Total Kjeldahl nitrogen, which is the sum of organic nitrogen and ammonia nitrogen, is determined in the same manner as the organic nitrogen, except that the ammonia is not driven off before the digestion step.
(b) Test for Determining Albuminoid Nitrogen:
The amount of albuminoid nitrogen present in sewage is determined by first boiling the sample of sewage to drive off the free ammonia, and then adding strong alkaline solution of potassium permanganate (KMnO4) to the boiled sample and again boiling the same. This results in liberation of ammonia (also called albuminoid ammonia) which is measured and from the same the amount of nitrogen is calculated which gives the amount of albuminoid nitrogen present in sewage.
(c) Test for Determining Nitrite Nitrogen:
The amount of nitrite nitrogen present in sewage is determined by colourimetric method. In this method by adding sulphanilic acid, napthylamine solution and sodium acetate solution to a sample of sewage, reddish-purple colour is developed which is measured with a photoelectric colourimeter to get the nitrite concentration of the sewage from standard calibration.
Alternatively the reddish-purple colour developed is compared visually with standard colours of known concentrations of nitrite to get the nitrite concentration of the sewage. For the use of this method the best concentration range of nitrite is below 2mg/l, but higher concentrations can also be determined by appropriate dilution with distilled water.
An alternative method for the higher concentration ranges of nitrite uses the yellow-brown colour of the reactant of nitrite with meta-phenylene diamine in acid solution. Both these methods are pH dependent but otherwise suffer few significant interferences except for chlorides when present in concentrations greater than 15000 mg/l.
(d) Test for Determining Nitrate Nitrogen:
The amount of nitrate nitrogen present in sewage is also determined by colourimetric method. In this case by adding phenol disulphonic acid and potassium hydroxide to a sample of sewage, yellow colour is developed which is measured with a photoelectric colourimeter to get the nitrate concentration of the sewage from standard calibration.
Alternatively the yellow colour developed is compared visually with standard colours of known concentrations of nitrate to get the nitrate concentration of the sewage. In this case chlorides—even at 10 mg/l—cause a severe negative interference and to avoid this silver sulphate is added to completely precipitate chlorides.
The hydrogen ion concentration or pH value of sewage can be measured with a pH meter. Alternatively various indicator solutions that change colour at definite pH values are also used to determine the pH value of sewage. In the later method indicator solution is added to a sample of sewage and the colour developed is compared with the colour of standard tubes or discs of known pH values. However, this method can be used only for relatively clear liquids.
Fats, grease, mineral oils and lubrication oils are soluble in ether or hexane, and hence their content is determined as ether-soluble matter. In order to determine their content, the sample of sewage is first treated with dilute hydrochloric acid which results in liberation of fatty acids. The water content of the treated sample of sewage is evaporated and the residue containing the liberated fatty acids is mixed with ether. The ether is then driven off, leaving behind the ether-soluble matter, which represents the fats, grease and oil content.
The content of surfactants in sewage is determined by measuring the colour change in a standard solution of methylene blue dye.
The concentration of pesticides and agricultural chemicals in sewage is measured by the carbon- chloroform extract method. This method consists of separating the pesticides and agricultural chemicals from the sewage by passing the sample of sewage through an activated-carbon column and then extracting these substances from the carbon using chloroform.
The chloroform can then be evaporated and the weight of these substances can be determined. Pesticides in concentrations of 1 part per billion (ppb) and less can be accurately determined by several methods, including gas chromatography and electron capture or coulometric detectors.
The bacteriological (or biological) analysis of sewage is not of much importance and hence it is rarely carried out. This is so because the presence of bacteria in sewage has no effect on the choice or selection of the sewage treatment method.
Moreover, the presence of bacteria in sewage is essential for the efficient working of the treatment units. Absence of bacteria in a sample of sewage indicates the presence of industrial wastes which are harmful to the bacterial life in sewage.
The results of bacteriological tests conducted for sewage may be used to determine the degree of pollution of water bodies such as natural streams or rivers, tanks, etc., in which treated or untreated sewage is discharged. The bacteriological tests mainly consist of detecting the presence of coliform bacteria and measuring their concentration in sewage.