Lab Module BOD

Wastewater Engineering (EAT 303/4)

Laboratory Module

EXPERIMENT 1BIOCHEMICAL OXYGEN DEMAND (BOD)

OBJECTIVES

To understand the nature of oxygen demand in water by determining the degree of water pollution caused by microorganisms through biodegradation.To determine the biochemical oxygen demand (BOD) of the given wastewater samples.

2.0 INTRODUCTION

BIOCHEMICAL OXYGEN DEMAND (BOD)

BOD is the amount of oxygen required by bacteria to stabilize organic matter under aerobic conditions. The BOD test involves the determination of oxygen uptake by bacteria under standard conditions (5 day incubation at 20 C). Strong wastewater samples must be diluted in keeping with the solubility of oxygen to ensure that DO will be present throughout the test period. The following guide may be used for preparing dilutions:

Typical Amount of Resulting Source

SampleBOD5 rangemg/L sample to add (mL)Concentration

(%)

Clear creek water0 to 5300100



Weak sewage, polluted creek water10 to 256020



Weak sewage, polluted creek water25 to 62.5248


Strong sewage, industrial waste 50 to 125124




Strong sewage, industrial waste 400 to 10001.50.5




3.0 MATERIALS AND APPARATUS

3.1 Reagents

a) Phosphate buffer solution

b) Magnesium sulphate solution

c) Calcium chloride solution

d) Ferric chloride solution

3.2 Apparatusa) BOD sample bottle with stopper

b) 50 mL pipettesc) 1 mL pipettesd) 10 L water container

e) DO meter4.0 PROCEDURE4.1 Set out ten cleaned 300 mL BOD bottles for the test and log the bottle numbers on the labsheet. The sample will be prepared as a duplicate as the Winkler DO method will be used. Six of the bottles will be used for the three different dilutions of the sample and two for the blanks.4.2 Add some dilution water to each BOD bottle, and then add the necessary amount of well-mixed sample using a pipette. Record the amount on the labsheet. Prepare each dilution in the following manner; 1 in 1 (100% sample concentration),1 in 2 (50% sample concentration), 1 in 4 (25% sample concentration) and 1 in 5 (20% sample concentration). For blanks, fill the BOD bottles full of only dilution water.* The dilution water is readily available for use. The dilution water is prepared by adding 1 mL of the following per litre of distilled water and then aerate the mixture to oxygen saturation (approximately 2 h) at room temperature.

- 1 mL phosphate buffer solution

- 1 mL magnesium sulphate solution

- 1 mL calcium chloride solution

- 1 mL ferric chloride solution

4.3Fill each bottle to the top with dilution water. Stopper each BOD bottle so that no bubbles are visible in the sample. Place a water seal on each bottle and a aluminium foil cap over the stopper.4.4Determine and record the DO on the duplicate set bottles with the Winkler titration. Place the duplicate set of bottles in the incubator in the dark for 5 days at 201C. Record the temperature, date and time of the beginning of the incubation on the labsheet. 4.5At the end of the incubation period, remove the BOD bottles from the incubator and record the temperature, date and time on the labsheet.4.6Determine the DO on each bottle with the DO meter. Record the results on the labsheet.4.7Calculate the BOD for the samples.

When dilution water is seeded,

BOD (mg/l) = [(D1 D2) (B1 B2) x f] P (1)

When dilution water is not seeded,

BOD (mg/l) = [(D1 D2)] P (2)

where,D1=DO of diluted sample immediately after preparation, mg/l

D2=DO of diluted sample after 5-day incubation, mg/l

B1=DO of seed control before incubation, mg/l

B2=DO of seed control after incubation, mg/l

f=fraction of seeded dilution water volume in sample to volume of seeded dilution water in seed control (volume of seed in sample/volume of seed control)

P=fraction of wastewater sample volume to total combined volume (volume of sample/volume of mixture)

5.0 RESULTSRecord your sample details. Fill in your DO data to the table below.

Table 1: DO concentration

Bottle *Dissolved oxygen (DO)

(mg/L)

BlankDay 0

Day 5

Seed controlDay 0

Day 5

Sample dilution

100% sample concentrationDay 0

Day 5


Day 5


Day 5


Day 5

BOD can be calculated using the formula given below:BOD (mg/L) = [ (D1 D2) (B1 B2) x f ] / P

Table 2: BOD calculationBlankSeed CtrlSample dilution

100%50%25%20%

(A) Sample vol. in vol. of bottle (mL)

(B) Seed vol. in vol. of bottle (mL)

(C) Initial DO, Day 0 (mg/L)

(D) Final DO, Day 5 (mg/L)

(E) DO depletion (mg/L), (C) - (D)

(F) f, ((E) x [(B) / vol. of bottle]

(G) P, (A) / vol. of bottle

(H) BOD5 (mg/L), [(E) - (F)] / (G)

6.0 DISCUSSION

Briefly discuss your experiments and results.7.0 CONCLUSION

Conclude your experimental results.

8.0 REFERENCES

8.1Dunnivat, F.M. (2004). Environmental Laboratory Exercise for Instrumental Analysis and Environmental Chemistry, Wiley, Interscience.

8.2APHA., AWWA., & WPCF. (1992). Standard Methods for Examination of Water and Wastewater. 16th Edition. American Public Health Association, Washington.9.0 QUESTIONS

9.1What is the need for using seeded dilution water in the BOD test?

9.2What are the main sources of error in a BOD test?

9.3Why is a nitrification inhibitor used in the test?

9.4What is the significance of BOD?

9.5A 20 mL wastewater sample is placed in a standard 300 mL BOD bottle and the bottle is filled with dilution water. The bottle had an initial dissolved oxygen concentration of 10 mg/L and a final concentration of 3 mg/L. A blank (a BOD bottle filled with dilution water) run in parallel showed no changes in dissolved oxygen over the 5 day incubation period. Calculate the BOD5.9.6 In a test for determination of BOD of a sample of water from a polluted stretch of river, three dilutions were used. The observations are given below. Calculate the BOD5 of the sample. Then, from the result discuss what is the discharge limit for BOD according to Environmental Quality Act (EQA) ?Sample NoDilution, % mixtureInitial DO, mg/LFinal DO, mg/L

1507.52.5

2207.55.2

3107.76.9

Lab Module BOD

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