BOD – Biological Oxygen Demand

The Biological Oxygen Demand (BOD) is the measure of the oxygen require to oxidize the organic matter present in a sample. It indicates the organic pollution in waste water as well as surface water. The BOD may define as the oxygen require for the micro-organisms to carry out biological decomposition of dissolve solids or organic matter in the wastewater under aerobic conditions at standard temperature.

Generally, we choose a 5 days period for standard BOD test, during which oxidation is 60% – 70% complete. While within 20 days period, the oxidation is about 95% – 99% complete. A constant temperature of 20 degree cel. is maintain during the incubation. The BOD value of 5 days incubation is commonly written as BOD5 or 5-day BOD.

Test for Biological Oxygen Demand

BOD test is perform by two tests

  1. Direct Method – The test consists of keeping the sample of waste water in contact with a definite air or oxygen, in a specifically prepared vessel. The BOD is then measured manometrically.
  2. Dilution Method – The water aerated to saturate with oxygen before mixing with sewage sample. The initial DO of this diluted sample measured. The diluted sample then incubated for 5 days at 20 degree Celsius in an air tight glass vessel. The DO of the sample again measured. Then BOD therefore computed as –

BOD5 = [oxygen consumed] X dilution ratio

where,

oxygen consumed = difference between initial and final content of DO

dilution ratio =

volume of diluted sample / volume of undiluted sewage sample

First stage Biological Oxygen Demand formulation

At given temp. the rate of BOD at any time given to be directly proportional to the amount of organic matter present in sewage. In other words, the exertion of BOD is

extertion of BOD

The amount of BOD remaining at any time t is

BOD remaining

Hence Yt is the amount of BOD that has been exerted at any time t is

Yt of BOD

Therefore BOD of 5 days is evidently given by

BOD5
First Stage BOD Curve
First Stage BOD Curve

To conclude we may say that the ultimate first stage BOD of a given wastewater is equal to the initial oxygen equivalent of organic matter present in the sample of wastewater. The value of K depends upon: (i) type of waste water (ii) temperature during reaction.

Effect of K on BOD for given Lo value
Effect of K on BOD for given Lo value

For the same ultimate BOD, the oxygen uptake will vary with the value of K as illustrated in above curve (Effect of K on BOD for given Lo value).

Typical values of K at 20 degree celsius
Typical values of K at 20 degree celsius

K value also depends on temperature, thus K varies with the change in temp.

Temperature K

Using the above equation, we can compute different values of K with different value of T temperature. The values of Ѳ is 1.056 for the temp. range 20°C to 30°C and for Ѳ is 1.135 for the temp. range 4°C to 20°C. Sometimes Ѳ is 1.047 for any other temperature such as cold temperatures.

Determination of K and Lo

For the computation of K and Lo, we can use any of the following methods –

  1. Method of least squares
  2. Method of moments
  3. The daily difference method
  4. The rapid ratio method
  5. Thomas method

we will only discuss here the important method only i.e. Thomas method.

Thomas method graph
THOMAS METHOD

This Thomas method is a graphical procedure based on similarity of series expansion. he gave the general expression : Z = a – bt. where

General experession

by knowing the values of a and b , we get

Thomas method

Its observed that not all experimental data will fit the theoretical curve. The data which is best is generally that which falls between t = 2 to t = 10 days.

Second stage Biological Oxygen Demand

As stated earlier, carbonaceous matter oxidized in first stage of biochemical reaction, while nitrogenous matter oxidized in the second stage. Some of the autotrophic bacteria are capable of using oxygen to oxidize non carbonaceous matter, such as ammonia to nitrites and nitrates. This second stage reaction is called nitrification.

COMBINED CARBONACEOUS AND NITRIFICATION DEMAND
COMBINED CARBONACEOUS AND NITRIFICATION DEMAND

Thus, the nitrogenous oxygen demand caused by the autotrophic bacteria is called the second stage BOD.

Limitations of BOD test

  1. BOD test measures only biodegradable organics.
  2. Pre-tretment is needed if the sample contains toxic wastes.
  3. The effects of nitrifying bacteria should be reduced or eliminated by pre-treatment or by use of inhabitory agents before the test is applied.
  4. It is essential to have high concentration of active bacteria present in the sample wastewater.
  5. The test looses its stoichiometric validity after the soluble organic matter present in the solution has been used or exhausted.
  6. The test uses an arbitrary long period of time, to obtain results.
  7. For the BOD5 test, the 5-day period may or may not correspond to the point where soluble organics matter present has been used.

Why we do BOD test ? What is the use of BOD test ?

The BOD test results are useful for the following purposes –

  • determination of approximate quantity of oxygen requires for the biological stabilization of organic matter present in the wastewater.
  • determination of size of wastewater treatment facilities
  • measurement of efficiency of some treatment processes.
  • determination of strength of sewage.
  • determination of amount of clean water required for the efficient disposal of wastewater by dilution.

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