Laboratory Report

Experiment # 6

Biochemical Oxygen Demand

Group #1

Jonathan Damora

Tuesday March 11, 2014

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Purpose

The purpose of this experiment is to perform a Biochemical Oxygen Demand test on primary clarifier

effluent from a wastewater treatment plant to determine a BOD versus time curve. This curve can then

be used to determine the Ultimate BOD of the wastewater sample and the rate constant for its decay.

Introduction

Biochemical Oxygen Demand (BOD) is a representation of dissolved oxygen content in relation to the

speed at which bacteria aerobically decompose, oxidize, organic matter within a sample. Thus it is a

method of defining the strength of wastewater, which itself depends on the level of easily

biodegradable organic matter within a given sample. If there is dissolved oxygen present and organic

matter that can be oxidized by bacteria, the organic matter will be decomposed by the bacteria

aerobically until the oxygen is low enough that anaerobic bacteria becomes dominant. BOD testing was

developed over 100 years ago, with one of the earliest standardizations of BOD testing on sewage

coming from the Royal Commission on Sewage Disposal in 1912, which states their regulations on BOD

of effluent at 65 degrees Fahrenheit. BOD testing has since been standardized at 20 oC.

The testing of BOD is important throughout many fields, as BOD is listed as a conventional pollutant in

the Clean Water Act, passed in 1972. It is used to measure the effectiveness of a wastewater treatment

plant, the pollutional strength of industrial wastewater, and is commonly used as a measure of the

strength of any fresh water sample or purification strength of a body of water. The United States uses

BOD to manage the quality of secondary treatment, which is expected to remove 85% of BOD measured

in the sewage and produce effluent that has a 30-day average BOD below 30 mg/L and a 7 day average

BOD below 45 mg/L. Industrial pre-treatment of wastewater uses BOD to both determine the level and

type of treatment needed. If the BOD to COD ratio is greater than .4 or .5 then biological treatment of

the wastewater is possible, as long as there are no biological toxins present. The industrial BOD testing

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may require the addition of seed bacteria, as there may be no living bacteria present depending on the

prior use of the water.

BOD testing specifically measures the dissolved oxygen concentration at different points to determine

the rate at which oxygen is used. The metabolism of organic compounds by bacteria can be represented

by the below equation, with the bacteria acting as the host for the oxidation of organic compounds to

CO2, water, and ammonia. In this case the organic compound is oxidized by oxygen.

Aerobic decomposition is not the only source of oxygen demand within a sample, as there are

chemoautotrophic bacteria nitrifying ammonia, ammonium, and nitrite into nitrate. This splits a

measurement of BOD into carbonaceous BOD (cBOD) and nitrogenous BOD (nBOD), unless a nitrification

inhibitor is used, such as the 2-chloro – 6- (trichloro methyl) pyridine used in this experiment. Usually

for a BOD5 test done at 20 oC the small number of nitrifying bacteria present do not add significantly to

the oxygen demand, but our experiment goes to 9 days which allows the population of nitrifying

bacteria to grow to an appreciable level. The demand due to nitrifying bacteria is not considered as

important in determining pollution strength as cBOD since nBOD does not involve metabolism of organic

matter. Other interferences include bio-inhibition from toxins, non-bioassimilable organic matter, and

other forms of oxygen demand.

The values obtained by a BOD test are used to determine the ultimate BOD of the wastewater sample

and the rate constant for its decay. These factors arise from empirical studies of the decay of

biodegradable matter in wastewater, which show that it usually follows first order kinetics. This means

specifically that the rate at which organic matter is oxidized depends on the amount of organic matter

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present. Thus the BOD at a specific time, t, can be calculated from

the above equation once ultimate BOD and the rate constant, k, are determined.

Procedure

To perform our BOD test we made 48 identical 300-mL BOD bottles containing 8 mL of settled

wastewater (primary clarifier effluent) and 292 mL of a solution containing: aerated distilled water, ferric

chloride, magnesium sulfate, calcium chloride, a phosphate buffer, and the nitrification inhibitor (2-

chloro – 6- (trichloro methyl) pyridine). The other chemicals are added to provide nutrients and buffer

pH in order to facilitate bacterial growth. These bottles were then placed into the BOD incubator at 20

oC except for 4 of the bottles which are used to determine the initial DO concentration using the Winkler

titration technique. The DO concentration is determined each day for 9 days, except the 6 th and 8th day,

with each day’s titration being replicated using 6 separate BOD bottles with the DO being averaged for

that day.

The DO concentration is determined using the Winkler Titration technique outlined here. First we

added 1 mL of manganese (II) sulfate and alkali-iodide-azide solution respectively to our BOD bottle and

thoroughly mixed it. A brown precipitate is formed in the presence of dissolved oxygen, or a white

precipitate of Mn(OH)2 is formed in the absence of DO. If there is no DO, then the process is complete

since the purpose is to test the concentration of DO. If there is oxygen and a brown manganese

precipitate forms, add 1mL of sulfuric acid and mix thoroughly. The brown precipitate dissolves and the

Iodide ion (I-) is converted to iodine. Titration is then performed using a sodium thiosulfate solution and

a starch indicator, with the DO concentration equal to the volume of sodium thiosulfate used. As stated

above, these steps to determine DO concentration were replicated up to 6 times each day and average

values are presented.

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Figure 1. BOD-Time Curve

Results

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0 1 2 3 4 5 6 7 8 9 100.00

50.00

100.00

150.00

200.00

250.00

300.00

BOD-Time Curve

Time (days)

BOD

(mg/

L)

Figure 2. Graphical Determination of k

0 1 2 3 4 5 6 7 8 9 100.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35f(x) = 0.015135716075873 x + 0.206535753832616R² = 0.96908644428739

Graphical Determination of Rate Constant, k

Time (days)

(t/B

OD)

^(1/

3)

Table 1. BOD Results

Time (day)

Average DO (mg/L)

BOD (mg/L) (t/BOD)^(1/3)

Ultimate BOD

0 7.70 0.00 1 4.80 108.75 0.21 305.62282 3.80 146.25 0.24 249.98043 3.00 176.25 0.26 240.54894 2.30 202.50 0.27 244.62045 2.10 210.00 0.29 236.19857 1.90 217.50 0.32 227.99369 1.20 243.75 0.33 248.4959

The intercept of the best fit line for Figure 2 is 0.2065 and the slope is 0.0151, thus

k=2.61*(.0151/.2065)=0.191. This rate constant can be plugged into this equation.

Along with the k value determined above I will plug t and BOD at each time. The results are tabulated in

Table 1, with the average ultimate BOD equal to 250.5 mg/L.

The BOD in the above table was calculated using this equation, . Where D1 is

the initial DO concentration, D2 is the DO at time t, P is equal to the decimal fraction of wastewater (8ml

wastewater / 300ml BOD bottle volume), and BOD is the calculated BOD at time t.

Discussion

The results fit what was expected from a sample containing organic pollutants. The BOD-time curve

displays the distinctive plateau as the microbial population shifts towards predominantly protozoa

predation and then increases again as the dead protozoa cells are decomposed by more bacteria. The k

value of 0.191 is around the expected value, although slightly higher, which means that decomposition

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occurred rapidly and there was not any biological growth inhibitors present. The ultimate BOD result of

250.5 mg/L is possibly misleading since there is much debate over the use of first order chemical kinetics

on BOD. When using BOD = BODo (1-e-kt) the ultimate bod value is much higher, around 450-550 mg/L.

And when using the method given in Snoeyink and Jenkins’ Water Chemistry, the Ultimate BOD is 594.

The higher values seem more likely since the BOD-time curve shows that the BOD is climbing again at

day 9. There is most likely no nitrification interference due to the nitrification inhibitor added. It is

possible that other interferences occurred, such as microbial inhibition, although unlikely since the

result was fairly high.

Discussion Questions

1. What are the applications of BOD test data? Discuss BOD test limitation in terms of the

biodegradability of wastes.

BOD testing gives data that shows the relative organic strength of that water sample as well as the decay

rate of said organic matter. BOD testing is applicable throughout industrial and public wastewater

treatment. The US Government has even put regulations on the General Pollutant, BOD, within the

effluent discharged from wastewater treatment plants. For example, testing the BOD of wastewater

prior to treatment and then again after treatment will tell you the efficacy of your treatment process, in

terms of organic pollutional strength. BOD testing attempts to focus specifically on oxidation of organic

compounds using oxygen, but there are many different forms of bacteria that use different oxidizing

agents. For example, there are species of bacteria that oxidize ferric iron into ferrous iron. The BOD

results cannot show you exactly what contributed to the DO demand, thus it is difficult to determine

whether there is any interferences that contributed to your data other than the biodegradation of

organic waste.

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2. What purposes are served by ferric chloride, magnesium sulfate, phosphoric acid, ammonium

chloride, and calcium chloride?

Ferric Chloride is used as a coagulant to settle suspended solids, as well as being an important mineral in

aerobic processes. Magnesium Sulfate is a common form of the mineral Magnesium. Phosphoric acid is

used as a pH buffer to ensure it stays constant, as well as a way to add phosphate to the solution which

allows any ferric oxide within the solution to be precipitated out as ferric phosphate. Ammonium

Chloride is used to buffer the pH of the solution. Calcium Chloride adds bioavailable calcium ions into

the solution, which is a necessary mineral for many forms of life.

3. How do the BOD results relate to COD data from previous Lab?

We could compare the ration of BOD/COD to determine the level of oxygen demand from metabolism

of organic products, which would give us an indication whether biological treatment would be efficient

to use on the wastewater. The wastewater in the previous lab gave a COD value of approximately 415

mg/L, and this experiment determined that the BOD of the wastewater was 250.5, thus the ration of

BOD to COD is .6, if the two samples are from the same source. This means that biological treatment

would still be effective on this water. The COD test was much less time consuming but gave less detailed

data.

4. List the major factors influencing the rate of biological oxidation in the BOD test.

The major factors for BOD testing are of course first of all the test parameters, meaning the organic

matter present and the dissolved oxygen concentration. Then the largest factors on BOD results are

nitrifying bacteria, since they will increase the demand for DO, and toxic compounds or residual chlorine

from treatment, which will inhibit microbial life within the sample thus lowering BOD results incorrectly.

Other factors include the pH of the sample, which effects the equilibrium of many reactions and changes

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species concentration, temperature of the sample and the presence of certain non-assimilable organic

material such as wood pulp, which will increase demand for DO.

Conclusion

BOD testing is a useful way to determine the level of biological activity and the concentration of organic

matter that is useful to microorganisms. The long testing time detracts from the attraction of this test

over COD, since it focuses specifically on the organic matter that bacteria can decompose. Despite its

limitations it is clear that BOD testing is essential as it allows us to quantify organic pollution within a

water sample, without expensive chromatography or other methods used to determine composition.

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Works Cited

Snoeyink, Vernon L., and David Jenkins. Water Chemistry. New York: Wiley, 1980. Print.

"BOD." Wikipedia. Wikimedia Foundation, 10 Mar. 2014. Web. 11 Mar. 2014.

"Winkler Titration Technique." Wikipedia. Wikimedia Foundation, 10 Mar. 2014. Web. 11 Mar.

2014.

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