EVE 290L

Introduction to Environmental Engineering Laboratory

Experiment #4

Biochemical Oxygen Demand (BOD)

Due: Friday, 2 November, 2012

Background

The oxygen demand of water is commonly used as an indication of its

quality. Biochemical oxygen demand (BOD) is the amount of oxygen

required by organisms to oxidize organic wastes to CO2 and stable end

products.

There are two cases for degradation of organic wastes by

microorganisms:

1. if sufficient oxygen is present, the organic material is degraded to CO2, H2O and stable end products (these end products are

unobjectionable);

2. if there is insufficient oxygen present, the organic material is degraded to CO2, H2O and unstable end products (such as H2S and CH4,

which are objectionable).

The overall point is that the more oxygen required to degrade an

organic waste, the more likely objectionable products will result.

BOD has typically been the most common parameter used to determine the

concentration of organic pollutants in wastewater and to evaluate the

efficiency of treatment processes. The method for determining BOD is

a five-day test (BOD5). For this test, diluted samples are placed in

sealed bottles at a temperature of 20 C for five days. The samples

must be diluted since the oxygen demand of typical wastewater is much

greater than the saturated dissolved oxygen concentration of water at

20 C. The bottles must be stored in darkness to prevent oxygen

addition by photosynthesis. BOD5 (the total amount of oxygen required

by microorganisms during the first five days of biodegradation) is

determined as shown below.

DOi = initial dissolved oxygen concentration [mg/L]

DOf = final dissolved oxygen concentration [mg/L]

Vwaste = volume of wastewater added [mL]

Vwater = volume of dilution water added [mL]

To insure a sufficient number of microorganisms to carry out the

degradation, it is often necessary to seed the dilution water with

bacteria and nutrients. Seeding introduces a BOD of its own and must

be accounted for using the following equation

waste

waterwastefi

V

VVDwhereDDODOBOD ;*5

Equations 1, 2.

I,F = initial and final DO conc. [mg/L] of sample bottle

I,F = initial and final DO conc. [mg/L] of blank bottle

x = volume of seeded dilution water in sample bottle

y = volume of seeded dilution water in blank bottle

D = dilution factor, as defined above

BOD may be modeled as a 1st order reaction as shown below

Lt = remaining oxygen demand left after time t [mg/L]

k = BOD reaction rate constant [time-1]

Integration of Equation 4 results in Equation 5 shown below

Lo = ultimate carbonaceous oxygen demand = BODt + Lt

BODt = biochemical oxygen demand at time t

These equations are used to determine the reaction constant k, which

indicates the rate of biodegradation of the waste.

Procedures

1. Calibrate the DO meter.

2. Determine the DO of the aerated, unseeded dilution water (perform measurement in triplicate). Add the Hach nutrient pillow to the

dilution water (the Hach pillow has been pre-packaged to make 19

liters of dilution water). Determine the DO of the dilution water

after the nutrients have been added (again, in triplicate). Do the

values make sense?

3. Seed the dilution water by adding settled sewage water to the dilution water you created in Step 2. You should add 3 ml of

settled sewage per liter of dilution water. Pipet from the top of

the settled sewage. Determine the DO. This value is DOi for all

subsequent calculations.

4. A sample has been provided to you which was created using 50 mg of glucose (C6H12O6) and 50 mg of glutamic acid (NH3-C5H6O4). The sample

has a volume of 1.5 liters. Stoichiometric calculations indicate an

approximate ultimate BOD of 55 mg/l. Use this BODult to determine a

dilution factor. Each group should use a different factor.

Remember: There should be at least 2 mg/l of DO uptake, 1 mg/l of

DO residual, and the volume of the BOD bottle is 300 ml.

3.Equation''5 DyxFIFIBOD

4.Equationtt kL

dt

dL

5Equation)1( ktotkt

ot eLBODeLL

5. Use the data in the Hach Water Analysis Handbook (Table 2, pp. 917) to estimate a dilution factor for the settled sewage. Each group

should use a different factor.

6. Each group should create 7 samples of the glucose-glutamic acid mixture, 2 samples of the settled sewage, and one blank (a bottle

with seeded dilution water only). One glucose-glutamic acid sample

will be analyzed daily for the next 7 days. The settled sewage

samples will be analyzed on day 5. Measure and record an initial DO

concentration for each bottle, stopper, and water seal (as

instructed in the Hach Water Analysis Handbook, BOD section, pp.

912-926). Important: Clearly label all of your samples.

7. Once the samples have been created, place them in the incubator.

8. Analyze samples. The daily data collected should include time of sampling, temperature, and DO. For the Saturday and Sunday

measurements, students must arrange a way to get in the lab.

Clean all glassware as you go.

Calibrate DO meter before each use.

Students should exchange data and include all data sets in their final reports.

Important: DO Meters are very fickle instruments. Be sure to think about the data you collect; if it does not seem logical

(i.e., DO should decrease with time), recalibrate the DO meter

and take the measurement again

The lab report should include the following:

1. Plots of daily DO and BOD for the glucose-glutamic acid mixture. 2. Tabular presentation of BOD5 for the glucose-glutamic acid

mixture and the settled sewage.

3. Calculation of rate constants.

Note: BOD is a very important measurement to environmental engineers.

Include an introduction section in your report that has a good

discussion of the theory behind BOD and why it is important and

meaningful to an environmental engineer (Do not regurgitate the

background section provided above).