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Biochemical Oxygen Demand -B.O.D

February 5, 2014 Microbiology Comments Off on Biochemical Oxygen Demand -B.O.D

Biochemical oxygen demand or B.O.D is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period.

The determination of the Biochemical Oxygen Demand or Biological Oxygen Demand (BOD) evaluates the amount of biodegradable organic material present in waste water, effluent and polluted waters. The BOD test reflects the amount of dissolved oxygen (DO) consumed by bacteria while oxidizing these materials. Dissolved oxygen is essential for the life of aquatic fauna and flora, and the BOD test is a measure of the ecological impact that effluent water may have on the receiving body of water (river, lake, etc.).

Three methods are widely used for BOD measurement:

Dilution method

This is a standard method. As an example, the APHA standard methods 5210B from the American Public Health Association describes in detail this protocol. Dilution water is prepared by adding inorganic nutrients and buffer salts to purified water. If the sample does not contain sufficient amounts of bacteria, or contains compounds toxic to bacteria (e.g. chlorinated effluent), it may be necessary to add microbial seed as well. Various dilution levels of the sample water are then prepared using the dilution water. The BOD bottles are filled to the top, capped and sealed. They are incubated in the dark at 20°C for 5 days. The levels of dissolved oxygen are measured prior to and after the 5-day incubation period. The difference between these two values, corrected for the dilution and the blank, is the BOD5 value. BOD tests results are expressed in mg/L of dissolved oxygen.

Manometric method

In this test, a manometer is fitted into a bottle containing the undiluted sample. It continuously measures the drop in air pressure in the bottle, which reflects the amount of oxygen uptake by the sample. This method is easier than the dilution method because no dilution is necessary, and continuous measurements are obtained.
The presence of toxicants or poor seeding material in water samples may lead to falsely low BOD results. Therefore, it is recommended to regularly use a glucose – glutamic acid (GGA) solution as a standard check solution. The oxygen uptake of this solution should be 198 +/- 30.5 mg/L.

Winkler Method

In Winkler method, filling sample bottles completely with waste water (no air is left to bias the test). The dissolved oxygen is then “fixed” using a series of reagents that form an acid compound that is titrated. Titration involves the drop-by-drop addition of a reagent that neutralizes the acid compound and causes a change in the color of the solution. The point at which the color changes is the “endpoint” and is equivalent to the amount of oxygen dissolved in the sample. The sample is usually fixed and titrated in the field at the sample site. It is possible, however, to prepare the sample in the field and deliver it to a lab for titration.

Winkler method

Dissolved oxygen field kits using the Winkler method are relatively inexpensive, especially compared to a meter and probe. Field kits run between $35 and $200, and each kit comes with enough reagents to run 50 to 100 DO tests. Replacement reagents are inexpensive, and you can buy them already measured out for each test in plastic pillows.

You can also buy the reagents in larger quantities, in bottles, and measure them out with a volumetric scoop. The advantage of the pillows is that they have a longer shelf life and are much less prone to contamination or spillage. The advantage of buying larger quantities in bottles is that the cost per test is considerably less.

The major factor in the expense of the kits is the method of titration they use eyedropper, syringe-type titrator, or digital titrator. Eyedropper and syringe-type titration is less precise than digital titration because a larger drop of titrant is allowed to pass through the dropper opening and, on a micro-scale, the drop size (and thus the volume of titrant) can vary from drop to drop. A digital titrator or a buret (which is a long glass tube with a tapered tip like a pipet) permits much more precision and uniformity in the amount of titrant that is allowed to pass.

If your program requires a high degree of accuracy and precision in DO results, use a digital titrator. A kit that uses an eye dropper-type or syringe- type titrator is suitable for most other purposes. The lower cost of this type of DO field kit might be attractive if you are relying on several teams of volunteers to sample multiple sites at the same time.

How to calculate BOD value in a sample?

Biochemical oxygen demand
if unseeded, BOD = (D1-D2)/P
if seeded, BOD = ((D1-D2)-(B1-B2)f)/P

D1 = DO of diluted seeded wastewater
D2 = DO of wastewater after incubation
B1 = DO of diluted seed sample
B2 = DO of seed sample after incubation
f = ratio of seed volume in seeded wastewater test to seed volume in BOD test on seed
P = decimal fraction of wastewater sample used. (vol. of wastewater)/(vol. of dilution water plus wastewater)

BOD5  is the total amount of oxygen consumed by microorganisms during the first five days of biodegradation

 

BOD5 = (DOi – Dof)/(P)    this is the five-day BOD of a diluted sample

Where,  DOi = the initial dissolved oxygen (DO) of the diluted wastewater

DOf = the final DO of the diluted wastewater, 5 days later

P  = the dilution fraction =   (volume of wastewater)/(volume of wastewater plus dilution water)

A standard BOD bottle holds 300 mL, so P is just the volume of wastewater divided by 300 mL


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Summary of "Biochemical Oxygen Demand -B.O.D"

Biochemical oxygen demand,B.O.D is amount of dissolved oxygen needed by aerobic biological organisms.There are 3 methods for finding Biochemical Oxygen Demand.

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