Determination of Bio-Chemical Oxygen Demand (BOD)

Objective

To determine the Biochemical Oxygen Demand (BOD) of Domestic Wastewater Sample.

Related Theory

Biochemical Oxygen Demand (Bod)

“Biochemical oxygen demand (BOD) is usually defined as the amount of oxygen required by bacteria to oxidize decompose-able organic matter under aerobic conditions.”

BOD is a quantitative expression of microbes ability to deplete the oxygen content of a wastewater. This depletion takes place due to the microbes consuming organic matter in the water via aerobic respiration. We have these three types of bacteria in wastewater.

1.      Aerobic Bacteria

These are the bacteria which require oxygen for their survival/metabolism.

2.      Anaerobic Bacteria

These are the bacteria which do not require oxygen for their survival/metabolism.

3.      Facultative Bacteria

These are the bacteria which utilize the oxygen if present but can also survive in the absence of oxygen.

Environmental Significance of Bod Test (Applications)

1-      Oxygen Requirement of Wastewater for its Stabilization

It is the principle test applied to find out the strength of wastewater in terms of oxygen requirements for its stabilization.

2-      For Design of Treatment Facilities

It is an important test for the design of treatment facilities for wastewater.

3-      Stream Pollution Control

The BOD test is the major criterion used in the stream pollution control where organic input must be restricted to maintain desired DO levels.

4-      Efficiency of the Treatment Plants

It is used to evaluate the efficiency of various treatment units.

5-      Choice of Treatment Method

It is a factor in the choice of treatment method.

Environmental significance

In the land-based treatment industry BOD is a parameter of concern. In extreme cases, high concentrations of organic matter can result in almost complete depletion of oxygen in the soil-water matrix. Wastewater with high BOD discharged into streams, causes the dissolved oxygen (DO) content of the water to drop. If DO drops to below a critical level, the ecology of the stream gets disturb. This condition leads to an increase in anaerobic bacteria and foul-smell. 

Typical BOD values

Most pristine rivers  have BOD below 1 mg/l. Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/l. Municipal sewage that is efficiently treated by a three stage process usually has a value of about 20 mg/l. Untreated sewage varies, but averages around 600 mg/l in Europe and as low as 200 mg/l in the U.S. 

Why 5 days?

We calculate BOD because maximum amount of organic matter present in any wastewater gets treated by bacteria in first 5 days. Around 70% of organic matter gets degraded in first 5 days and after that degradation is very slow and no clear changes in BOD values are observed. 

Principle

The BOD test is carried out by diluting the sample with distilled water saturated with oxygen, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen and sealing the sample (to prevent further oxygen dissolving in). The sample is kept at 20°C in the dark to prevent photosynthesis (and thereby the addition of oxygen) for five days, and the dissolved oxygen is measured again. The difference between the final DO and initial DO is the BOD. The apparent BOD for the control is subtracted from the control result to provide the corrected value.

Interference

Nitrogenous demand can interfere with the BOD test. This interference comes about through the inclusion of ammonia in the dilution water and high levels of nitrogenous compounds in wastewater. While it is important to know the nitrogenous demand of the wastewater, it is considered interference when included with the BOD test. To prevent this problem from occurring inhibitory chemicals can be added to keep nitrogenous chemicals from interfering. Results in which these chemicals have been added are usually reported as CBOD (carbonaceous).

Preparation of Samples

Sampling & Storage

Samples for BOD test may degrade significantly during storage between collection and analysis resulting in low BOD values. The samples should be analysed immediately, if not they should be cooled to near freezing temperature during the storage.

Seeding

Seeding is defined as “the introduction of microbial/bacterial culture in the wastewater sample”. It is necessary to have a population of micro-organisms capable of oxidizing the biodegradable organic matter in the sample.

• Domestic wastewater and surface waters receiving wastewater discharges contain, adequate microbial population and may not be seeded.
• Some industrial wastewater, particularly having, high temperature and extreme pH values may not contain significant microbial population and therefore require seeding.

Preparation of Dilution Water

Place desired volume of distilled water in a suitable bottle and add Phosphate Buffer, Magnesium Sulfate, Calcium Chloride, Ferric Chloride solutions per liter of the distilled water. Add appropriate seeding material in the sample. Aerate the dilution water to saturate it with DO.

Apparatus

1.       Burette, graduated to 0.1 mL

2.       Burette stand

3.       300 mL glass stoppered BOD bottles

4.       500 mL wide‑mouthed Erlenmeyer flasks

5.       Pipettes with elongated tips and minimum volume of 1.0 mL (+/‑ 0.1 mL)

6.       Pipette bulb

7.       250 mL graduated cylinders

8.       Distilled water rinse bottle

9.       Air incubator

Reagents

1.       Distilled Water

2.       Manganese Sulfate solution (MnSO₄)

3.       Alkaline Potassium Iodide‑ Sodium Azide solution (NaI + NaN₃)

4.       Sulfuric acid (H₂SO₄) concentrated

5.       Starch indicator solution

6.       Sodium Thiosulfate (Na₂S₂O₃ 5H₂O), 0.025 N

Procedure

1. First of all it is important to know the amount of sample to be used for the test. For this purpose, the source of the sample is to be recorded.

2. Take 9 BOD bottles – note their numbers and arrange them in three groups.
3. Fill each bottle half with the dilution water ensuring that no air gets mixed with it while filling as in DO test. 
4.  Add 2 ml sample in each of the three bottles marked as 1^st group (A1, B1, C1); 5 ml in each bottle of 2^nd group (A2, B2, C2)  and 10 ml in each bottle of 3^rd group (A3, B3, C3) .
5. Fill the bottle completely with dilution media and place the stopper such that no air bubbles are trapped.
6. Now take one bottle from each set and determine their DO. This will be DO initial or D_0. 
7. For comparison, prepare two more bottles with blank solutions media (without sewage sample) and find the DO from one bottle.
8. Place the rest of the six bottles with sewage samples and one bottle for blank in the incubator at 20+2⁰C for 5 days.
9. After 5 days, find out DO in all bottles (we take the next reading after 5 days because after 5 days almost 70–80% of the sample is degraded).
10. That value of oxygen depletion should be considered correct which gives an oxygen depletion of at least 2 mg/L and which has at least 0.5 mg/L DO after 5 days of incubation.
11. Calculate BOD₅ at 20+2⁰C for the sample using following relationship

Observations and Results

BOD table for 0 day

Bottle #	Bottle Name	Volume of Fixed Sample	Volume of Na2S2O3 Used	Dissolved Oxygen
	(ml)	(ml)	(ml)	(mg/l)
	Blank	0ml
	A1	2ml
	A2	5ml
	A3	10ml

BOD table for 5 days

Bottle #	Bottle Name	Volume of Fixed Sample	Volume of Na2S2O3 Used	Dissolved Oxygen	Mean DO
	(ml)	(ml)	(ml)	(mg/l)	(mg/l)
	Blank	0ml
	B1	2ml
	C1
	B2	5ml
	C2
	B3	10ml
	C3

DO Depletion Table

Sample Added	Dissolved Oxygen		DO Depleted
	D0	D5
(ml)	(mg/L)		(mg/L)	(mg/L)
2ml
5ml
10ml
Blank