"Determination of Biochemical Oxygen Demand (BOD)"

Objective: The objective of this experiment is to determine the biochemical oxygen demand (BOD) of given water sample. BOD may have various units, but most often it is expressed in mg of oxygen required per litter of water/wastewater (mg/1).

Biochemical Oxygen Demand: The biochemical oxygen demand (BOD) determination is an empirical test in which standardized laboratory procedures are used to determine the pollution strength of domestic and industrial wastewater. BOD is defined as the amount of oxygen required by microorganisms while oxidizing decomposable organic matter under controlled aerobic conditions. The BOD test relies on measurable depletion of dissolved oxygen (D0) over a specified period of time, generally 5 days at 20°C incubation. The complete degradation of organic matter may take 20 to 30 days. Simple organic compounds like glucose are almost completely oxidized in 5 days while domestic sewage is oxidized to 65% in this period. Therefore, it has become a standard practice to measure the oxygen demand over a 5-day period, realizing that the ultimate oxygen demand (for complete degradation of organic matter) is considerably higher and would take a much longer time to determine in the laboratory. BOD in general gives a qualitative index of organic substances which are degraded quickly in a short period of time.

Principle: The BOD is measured by determining the oxygen consumed (by the bacteria) from a sample placed in an air-tight container and kept in a controlled environment for a preselected period of time. The 5-day BOD or BOD, is the amount of oxygen consumed by microorganisms during the first 5 days of biodegradation. In its simplest form, a BODS test would involve putting a sample of water/wastewater into a stoppered bottle, measuring the dissolved oxygen (DO) of the sample at the beginning of the test and again at the end of five days. The difference in DO would be the BOD, of the water/wastewater. Light must be kept out of the bottle to keep algae from adding oxygen by photosynthesis and the stopper is used to keep air from replenishing DO from outside. To standardize the procedure, the test is run at a fixed temperature of 20°C. Since the oxygen demand of typical waste is several hundred milligram per litter, and since the saturated value of DO for water at 20°C is only 9.1 mg/1, it is usually necessary to dilute the sample to keep final DO above zero. If during the five days of experiment the DO drops to zero, then the test b invalid since more oxygen would have been removed had more been available.


The five-day BOD of a diluted sample is given by

BOD5= [DO, - DO x D.F.

where. D.F = Dilution Factor = (Vol. of wastewater + dilution water) / (Vol. of wastewater)
In some cases, it becomes necessary to seed the dilution water with microorganisms to ensure that there is an adequate bacterial population to carry out the biodegradation. In such cases, two set of BOD bottles must be prepared. One for just the seeded dilution water (called the "blank") and the other for the mixture of wastewater and dilution water. The change in D0 in both is measured. The oxygen demand of the wastewater (BOD) is then determined from the following relationship:
BODm Vm = BOD, Vw + BOD Vd
where,
BOD = BOD of the mixture of wastewater and dilution water
BODd= BOD of the dilution water alone;
Vm = volume of wastewater; Va = volume of dilution water; Vm = Vw + Vd

According to the Environment Conservation Rules (1997), drinking water standard for BOD is 0.2 mg/l (at 20°C). For wastewater effluent allowable concentration of BOD varies from 50-250 mg/l depending on discharge point of the effluent (e.g., inland water, irrigation land, public sewer etc.).

Reagents:
1. Ferric chloride solution
2. Calcium chloride solution
3. Magnesium sulfate solution
4. Phosphate buffer solution

Apparatus:
1. Incubator
2. BOD bottle
3. Beaker (250 ml)
4. Measuring cylinder
5. Dropper
6. Stirrer

The 5-day BOD is calculated according to the following formula:

BOD (mg/I) = ( initial DO - final DO/P)
where P = fraction of sample used

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