The biochemical oxygen demand (BOD) of sewage can be determined by the following two methods: 1. Dilution Method 2. Direct Method.
1. Dilution Method:
This is the most commonly used method, as it represents the conditions that can be applied to sewage to be discharged into natural water bodies (disposal by dilution). The method is based upon determination of dissolved oxygen originally present in undiluted sample of sewage, and the dissolved oxygen present in diluted sample of sewage after it is subjected to incubation at a constant temperature of 20°C for a period of 5 days.
Thus dissolved oxygen originally present in undiluted sample of sewage is determined. The sample of sewage is then suitably diluted with a specially prepared dilution water so that adequate nutrients and oxygen will be available during the incubation period. The dilution water may be prepared by adding 1.0 ml each of phosphate buffer solution, magnesium sulphate solution, calcium chloride solution and ferric chloride solution in 1.0 litre of distilled water.
The phosphate buffer is added to maintain pH value between 7.0 and 7.6 which is the optimum pH value for the biological activity, and the other salts are added to provide nutrients necessary for the biological activity. Further the dilution water is aerated to saturate it with oxygen before mixing it with the sample of sewage for dilution.
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For BOD tests 300 ml size BOD bottles are used. By adding required quantity of sample of sewage in a known quantity of dilution water, diluted sample of desired percent mixture is prepared and it is filled up in a BOD bottle. Alternatively a known quantity of sample of sewage is pipetted into a BOD bottle to which dilution water is added to completely fill the bottle.
Another BOD bottle is filled up with only dilution water (blank). Both these bottles, one containing diluted sample of sewage and the other containing dilution water, are incubated at a constant temperature of 20°C for a period of 5 days. The dissolved oxygen present in the dilution water (blank) and in the diluted sample of sewage at the end of incubation period is determined.
The BOD of the sample of sewage is then calculated using the following equations:
Where
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DOb, DOi = dissolved oxygen values found in blank (containing dilution water only) and diluted sample, respectively, at the end of incubation period; and
DOs = dissolved oxygen originally present in undiluted sample.
It may be noted that the term (DOb – DOs) represents correction for dissolved oxygen of the sample. Further as the value of DOs approaches DOb, or when the BOD is over 200 mg/l, the term (DOb – DOs) becomes negligible.
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Instead of the above indicated elaborate procedure for the determination of BOD of sewage, the following simplified procedure may also be adopted. The sample of sewage is diluted in the same manner as indicated above. The dissolved oxygen present in the diluted sample of sewage before incubation is determined. The diluted sample of sewage is then incubated at a constant temperature of 20°C for a period of 5 days. The dissolved oxygen present in the diluted sample of sewage at the end of incubation period is determined.
The BOD of the sample of sewage is then calculated using the following equations:
It is evident that the term (DO1 – DO2) represents the amount of oxygen consumed during incubation of the diluted sample of sewage. Further the term (100 / % mixture) or (Vol. of bottle/ ml of sample) or dilution factor.
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The dilution ratio is selected on the basis of expected range of BOD5, so that adequate nutrients and oxygen will be available during the incubation period. Normally several dilutions are prepared to cover the complete range of possible values. Table 8.3 gives the range of BOD that can be measured with various dilutions expressed as percentage mixtures and as ml directly pipetted into 300 ml BOD bottles.
BOD test results are used as a measure for determining the strength of sewage. The relation between BOD and the strength of sewage is indicated by typical values of 5-day BOD of raw and treated sewage given in Table 8.4.
2. Direct Method:
In the direct method a suitable quantity of sample of sewage is kept in contact with a definite volume of air or oxygen in a specially prepared vessel having an arrangement to absorb any CO2 produced. The oxygen absorbed by the sample of sewage is measured manometrically from which BOD of the sample is determined. By this method it is possible to determine BOD at intervals during the storage period of the sample of sewage.
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Expression for First-Stage BOD:
The expression for the first-stage BOD is usually derived on the basis of the assumption that at a given temperature, the rate at which BOD is satisfied at any time (i.e., rate of deoxygenation) is directly proportional to the amount of organic matter present in sewage at that time. Thus-
Where
Lt = amount of first stage BOD remaining in sewage at any time t (or oxygen equivalent of carbonaceous oxidizable organic matter present in sewage at any time t) expressed as mg/l; t = time in days; and
K’ = rate constant (or reaction constant or deoxygenation constant) which signifies the rate of oxidation of organic matter. Its value depends on the nature of organic matter present in sewage and temperature during the reaction. Its unit is per day or (day)-1.
Minus sign is introduced in equation 8.11 because with the increase in time t the value of Lt decreases.
Where C is a constant of integration, which may be evaluated from the initial boundary condition i.e.,
When t = 0, Lt = L0
Where L0 is the total or ultimate first-stage BOD initially present in sewage, or it is the oxygen equivalent of carbonaceous oxidizable organic matter initially present in sewage.
By substituting this boundary condition in equation 8.12, we get-
In the above equation K is known as base 10 rate constant (or reaction constant or deoxygenation constant) in order to distinguish it from K’ which is known as base e rate constant (or reaction constant or deoxygenation constant).
It may, therefore, be concluded that the ultimate first-stage BOD yu of a given sewage is equal to the oxygen equivalent of the carbonaceous organic matter initially present in the sewage. yu is thus a fixed quantity for a given sewage and it does not depend on the temperature during the reaction.
The value of K (i.e., base 10 rate constant or deoxygenation constant) determines the speed of BOD reaction without influencing the ultimate BOD. The value of K depends on (i) type of sewage, and (ii) temperature during the reaction. The range of variation in the value of K, at 20°C temperature during reaction is from 0.05 to 0.3 (day)-1 or more depending on the type of sewage.
However, a typical value of K is usually taken as 0.1 (day)-1 at 20°C temperature during reaction. For the same ultimate BOD, the oxygen uptake will vary with time and with the value of K, as shown in Fig. 8.9. It may be seen from the figure that higher the value of K, faster will be the reaction. Table 8.5 gives typical values of K at 20°C for various types of waters and sewages.
The BOD of a sample of sewage is usually determined at a temperature of 20°C. However, BOD of a sample of sewage may also be determined at a temperature other than 20°C and hence it is necessary to determine the value of rate constant K at a temperature other than 20°C.
The rate constant Kr at any temperature T°C is related to the rate constant K20 at a temperature of 20°C by the following approximate equation which is derived from the van’t Hoff-Arthenious relationship:
The value of θ has been found to vary from 1.056 in the temperature range between 20°C and 30°C to 1.135 in the temperature range between 4°C and 20°C. A value of Soften quoted in the literature is 1.047, but it has been observed that this value does not apply at cold temperature (e.g., below 20°C).
Determination of K and L0:
The value of K is needed if the BOD5 is to be used to obtain L0, the ultimate BOD which is approximately equal to 20-day BOD. The usual procedure followed is to determine K and L0 from a series of BOD measurements.
There are several methods of determining K and L0 from the results of a series of BOD measurements which include:
(i) Method of least squares,
(ii) Method of moments,
(iii) Daily difference method,
(iv) Rapid-ratio method, and
(v) Thomas method.
The method of least square and the Thomas method are discussed below:
(i) Method of Least Squares:
The method of least squares involves fitting a curve through a set of observed data points, such that the sum of the squares of the residuals (the difference between the observed value and the value of the fitted curve) is a minimum. Using this method a curve can be fitted through a set of data points.
Thus for a time series of BOD measurements on the same sample of sewage if there are n observed data points, i.e., there are n values of BOD yt viz., yt1 , yt2 , yt3 ,… ytn , measured at the end of t1, t2, t3, ….. tn days, then the following equation may be written for each of the various n observed data points-
Now, if the sum of the squares of the residuals R is to be a minimum, the following equations must hold good:
To use this method, several observations of yt as a function of t are needed. The data observations should be limited to the first 10 days because of nitrogenous interference.
Second-Stage BOD:
Carbonaceous matter is oxidized in the first stage of biochemical reaction, while nitrogenous matter is oxidized in the second stage. Some of the autotropic bacteria are capable of using oxygen to oxidize noncarbonaceous matter, such as ammonia to nitrites and nitrates.
This second-stage reaction is called nitrification. Thus the nitrogenous oxygen demand caused by the autotrophic bacteria is called the second-stage BOD. Fig. 8.11 shows the normal progression of each stage of BOD for a domestic sewage. At 20°C the reproductive rate of the nitrifying bacteria is very slow, and it takes from 6 to 10 days them to reach significant numbers and to exert a measurable oxygen demand.
The interference caused by their presence can be eliminated by pretreatment of the sample of sewage, such as pasteurization, chlorination or acid treatment. Alternatively certain inhibitory agents such as methylene blue, thiourea and allylthiourea, etc., may be used.
