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S. Suzanne Nielsen (ed.)Food AnalysisFood Science Text Serieshttps://doi.org/10.1007/978-3-319-45776-5_28

28. Determination of Oxygen Demand

Yong D. Hang¹

(1)

Department of Food Science and Technology, Cornell University, Geneva, NY 14456, USA

Yong D. Hang

Email: ydh1@cornell.edu

Keywords

Oxygen demandBiochemical oxygen demandChemical oxygen demand

28.1 Introduction

Oxygen demand is a commonly used parameter to evaluate the potential effect of organic pollutants on either a wastewater treatment process or a receiving water body. Because microorganisms utilize these organic materials, the concentration of dissolved oxygen is greatly depleted from the water. The oxygen depletion in the environment can have a detrimental effect on fish and plant life.

The two main methods used to measure the oxygen demand of water and wastewater are biochemical oxygen demand (BOD) and chemical oxygen demand (COD). This chapter briefly describes the principles, procedures, applications, and limitations of each method. Methods described in this chapter are adapted from Standard Methods for the Examination of Water and Wastewater, published by the American Public Health Association (APHA) [1]. The book includes step-by-step procedures with equipment for BOD, COD, and other tests for water and wastewater.

28.2 Methods

28.2.1 Biochemical Oxygen Demand

28.2.1.1 Principle

The biochemical oxygen demand (BOD) determination is a measure of the amount of oxygen required by microorganisms to oxidize the biodegradable organic constituents present in water and wastewater. The method is based on the direct relationship between the concentration of organic matter and the amount of oxygen used to oxidize the pollutants to water, carbon dioxide, and inorganic nitrogenous compounds. The oxygen demand of water and wastewater is proportional to the amount of organic matter present. The BOD method measures the biodegradable carbon (carbonaceous demand) and, under certain circumstances, the biodegradable nitrogen (nitrogenous demand).

28.2.1.2 Procedure

Place a known amount of a water or wastewater sample that has been seeded with an effluent from a biological waste treatment plant in an airtight BOD bottle, and measure the initial dissolved oxygen immediately. Incubate the sample at 20 °C and, after 5 days, measure the dissolved oxygen content again (APHA Method 4500-0). The dissolved oxygen content can be determined by the membrane electrode method (APHA Method 4500-O G) or the azide modification (APHA Method 4500-0 C), permanganate modification (APHA Method 4500-0 D), alum flocculation modification (4500-0 E), or copper sulfate-sulfamic acid flocculation modification (APHA Method 4500-0 F) of the iodometric method (APHA Method 4500-0 B) to minimize interference by nitrite or ferrous or ferric iron. The iodometric method is a titrimetric procedure that is based on the oxidizing property of dissolved oxygen, while the membrane electrode method is based on the diffusion rate of dissolved oxygen across a membrane. A dissolved oxygen meter with an oxygen-sensitive membrane electrode made by Fisher, Orion, YSI, or other companies is used to measure the diffusion current, which is linearly proportional to the concentration of dissolved oxygen under steady-state conditions. It is important to change frequently and calibrate the membrane electrode to eliminate the effect of interfering gases such as hydrogen sulfide. The azide-modified iodometric procedure, for example, is used to remove interference of nitrite, which is the most commonly interfering material in water and wastewater. The alum flocculation modification method is commonly used to minimize the interference caused by the presence of suspended solids. The BOD value, which is expressed as mg/L, can be calculated from the difference in the initial dissolved oxygen and the content of dissolved oxygen after the incubation period according to the following equation (APHA Method 5210 B):

$\mathrm{BOD}\kern0.5em \left(\mathrm{mg}/\mathrm{L}\right)=100/ P\times \left(\mathrm{DOB}-\mathrm{DOD}\right)$

(28.1)

where:

DOB = initial oxygen in diluted sample, mg/L
DOD = oxygen in diluted sample after 5-day incubation, mg/L
P = mL sample × 100/capacity of bottle

28.2.1.3 Applications and Limitations

The BOD test is used most widely to measure the organic loading of waste treatment processes, to determine the efficiency of treatment systems, and to assess the effect of wastewater on the quality of receiving waters. The 5-day BOD test has some drawbacks because:

1.

The procedure requires an incubation time of at least 5 days.
2.

The BOD method does not measure all the organic materials that are biodegradable.
3.

The test is not accurate without a proper seeding material.
4.

Toxic substances such as chlorine present in water and wastewater may inhibit microbial growth.

28.2.2 Chemical Oxygen Demand

28.2.2.1 Principle

The chemical oxygen demand (COD) determination is a rapid way to estimate the quantity of oxygen used to oxidize the organic matter present in water and wastewater. Most organic compounds are destroyed by refluxing in a strong acid solution with a known quantity of a strong oxidizing agent such as potassium dichromate. The excess amount of potassium dichromate left after digestion of the organic matter is measured. The amount of organic matter that is chemically oxidizable is directly proportional to the potassium dichromate consumed.

28.2.2.2 Procedure

A known quantity of sample of water or wastewater is refluxed at elevated temperatures for up to 2 h with a known quantity of potassium dichromate and sulfuric acid using an open reflux method (APHA Method 5220 B), a closed reflux titrimetric method (APHA Method 5220 C), or a closed reflux colorimetric method (APHA Method 5220 D). The amount of potassium dichromate left after digestion of the organic matter is titrated with a standard ferrous ammonium sulfate (FAS) solution using orthophenanthroline ferrous complex as an indicator. The amount of oxidizable organic matter, determined as oxygen equivalent, is proportional to the potassium dichromate used in the oxidative reaction. The COD value can be calculated from the following equation (APHA Method 5220 B):

$\mathrm{COD}\kern0.5em \left(\mathrm{mg}/\mathrm{L}\right)=\left( A- B\right)\times M\times 8000/ D$

(28.2)

where:

A = mL FAS used for blank
B = mL FAS used for sample
M = molarity of FAS
D = mL sample used
8000 = milliequivalent weight of oxygen × 1,000 mL/L

28.2.2.3 Applications and Limitations

Potassium dichromate is widely used for the COD method because of its advantages over other oxidizing compounds in oxidizability, applicability to a wide variety of waste samples, and ease of manipulation. The dichromate reflux method can be used to measure the samples with COD values of greater than 50 mg/L.

The COD test measures carbon and hydrogen in organic constituents but not nitrogenous compounds. Furthermore, the method does not differentiate between biologically stable and unstable compounds present in water and wastewater. The COD test is a very important procedure for routinely monitoring industrial wastewater discharges and for the control of waste treatment processes. The test is faster and more reproducible than the BOD method. The obvious disadvantages of the COD method are:

1.

Aromatic hydrocarbons, pyridine, and straight-chain aliphatic compounds are not readily oxidized.
2.

The method is very susceptible to interference by chloride, and thus the COD of certain food processing waste effluents such as pickle and sauerkraut brines cannot be readily determined without modification. This difficulty may be overcome by adding mercuric sulfate to the sample prior to refluxing. Chloride concentrations greater than 500–1000 mg/L may not be corrected by the addition of mercuric sulfate. A chloride correction factor can be developed for a particular waste by the use of proper blanks.

28.3 Comparison of BOD and COD Methods

The BOD and COD analyses of water and wastewater can result in different values because the two methods measure different materials. As shown in Table 28.1, the COD value of a waste sample is usually higher than its BOD because:

table 28.1

Oxygen demand of tomato processing wastes

Item	1973	1974	1975
BOD, mg/L	2,400	1,300	1,200
COD, mg/L	5,500	3,000	2,800
TOC, mg/L	2,000	1,100	1,000

From [2]

1.

Many organic compounds that can be chemically oxidized cannot be biochemically oxidized. For example, cellulose cannot be determined by the BOD method but can be measured by the COD test.
2.

Certain inorganic compounds such as ferrous iron, nitrites, sulfides, and thiosulfates are readily oxidized by potassium dichromate. This inorganic COD introduces an error when computing the organic matter of water and wastewater.
3.

The BOD test can give low values because of a poor seeding material. The COD test does not require an inoculum.
4.

Some aromatics and nitrogenous (ammonium) compounds are not oxidized by the COD method. Other organic constituents such as cellulose or lignin, which are readily oxidized by potassium dichromate, are not biologically degraded by the BOD method.
5.

Toxic materials present in water and wastewater that do not interfere with the COD test can affect the BOD results.

The COD has value for specific wastes since it is possible to obtain a direct correlation between COD and BOD values. Table 28.2 shows the COD and BOD values of waste effluents from fruit and vegetable processing factories. The BOD/COD ratios of these processing waste effluents varied considerably and ranged from 0.50 to 0.72 [2]. The BOD/COD ratio can be a useful tool for rapid determination of the biodegradability of organic matter present in the wastes. A low BOD/COD ratio indicates the presence of a large amount of nonbiodegradable organic matter. Samples of wastewater with high BOD/COD ratios have a small amount of organic matter that is nonbiodegradable.

table 28.2

COD and BOD values of selected fruit and vegetable processing wastes

Product	COD (mg/L)	BOD (mg/L)	Mean ratio (BOD/COD)
Apples	395–37,000	240–19,000	0.55
Beets	445–13,240	530–6,400	0.57
Carrots	1,750–2,910	817–1,927	0.52
Cherries	1,200–3,795	600–1,900	0.53
Corn	3,400–10,100	1,587–5,341	0.50
Green beans	78–2,200	43–1,400	0.55
Peas	723–2,284	337–1,350	0.61
Sauerkraut	470–65,000	300–41,000	0.66
Tomatoes	652–2,305	454–1,575	0.72
Wax beans	193–597	55–323	0.58
Wine	495–12,200	363–7,645	0.60

From Splittstoesser and Downing [2]

28.4 Sampling and Handling Requirements

Samples of water and wastewater collected for oxygen demand determinations must be analyzed as soon as possible or stored under properly controlled conditions until analyses can be made. Samples for the BOD test can be kept at low temperatures (4 °C or below) for up to 48 h. Chemical preservatives should not be added to water and wastewater because they can interfere with BOD analysis. Untreated wastewater samples for the COD test must be collected in glass containers and analyzed promptly. The COD samples can be stored at 4 °C or below for up to 28 days if these are acidified with a concentrated mineral acid (sulfuric acid) to a pH value of 2.0 or below.

28.5 Summary

Oxygen demand is most widely used to determine the effect of organic pollutants present in water and wastewater on receiving streams and rivers. The two important methods used to measure oxygen demand are BOD and COD. Table 28.3 summarizes the principle, advantages, and disadvantages of BOD and COD. The BOD test measures the amount of oxygen required by microorganisms to oxidize the biodegradable organic matter present in water and wastewater. The COD method determines the quantity of oxygen consumed during the oxidation of organic matter in water and wastewater by potassium dichromate. Of the two methods used to measure oxygen demand, the BOD test has the widest application in measuring waste loading to treatment systems, in determining the efficiency of treatment processes, and in evaluating the quality of receiving streams and rivers because it most closely approximates the natural conditions of the environment. The COD test can be used to monitor routinely the biodegradability of organic matter in water and wastewater if a relationship between COD and BOD has been established.

table 28.3

Comparison between biochemical oxygen demand and chemical oxygen demand

	Principal	Advantages	Disadvantages
Biochemical oxygen demand (BOD)	Measure amount of oxygen required by microorganisms to oxidize the biodegradable organic matter present in water and wastewater (i.e., correlation between amount of organic matter and amount of oxygen used to oxidize pollutants)	Measures true compounds of interest (i.e., organic matter). Less expensive. No interference from certain compounds that affect COD results (see text for details)	Slower; less precise. Gives low values if poor seed material. Requires inoculum
Chemical oxygen demand (COD)	Organic compounds are destroyed by refluxing in strong acid solution with a known excess of an oxidizing agent (potassium dichromate) (i.e., correlation between amount of organic matter chemically oxidized and amount of oxidizing agent consumed)	Faster. More precise	Not a direct measurement of organic matter. More expensive. Can overestimate organic matter if sample is high in certain compounds and can underestimate if high in other compounds (see text for details)

28.6 Study Questions

1.
In your new job as supervisor of a lab that has previously been using the BOD method to determine oxygen demand of wastewater, you have decided to change to the COD method.
1. (a)
  
  Differentiate the basic principle and procedure of the BOD and COD methods for your lab technicians.
2. (b)
  
  In what case would they be instructed to use mercuric sulfate in the COD assay?
3. (c)
  
  How do you justify making the change from the BOD method to the COD method?
2.
In each case described below, indicate if you would expect the COD value to be higher or lower than the results from a BOD test. Explain your answer.
1. (a)
  
  Poor seed material in BOD test
2. (b)
  
  Sample containing toxic materials
3. (c)
  
  Sample high in aromatics and nitrogenous compounds
4. (d)
  
  Sample high in nitrites and ferrous iron
5. (e)
  
  Sample high in cellulose and lignin

28.7 Practice Problems

1.
Determine the BOD value of a sample given the following data (see Eq. 28.1):
- DOB = 9.0 mg/L
- DOD = 6.6 mg/L
- P = 15 mL
- Capacity of bottle = 300 mL
2.
Determine the COD value of a sample given the following data (see Eq. 28.2):
- mL FAS for blank = 37.8 mL
- mL FAS for sample = 34.4 mL
- Molarity of FAS = 0.025 M
- Sample = 5 mL

Answers

1.

BOD = 48 mg/L

Calculation:

$\begin{array}{c}\mathrm{BOD}\left(\mathrm{mg}/\mathrm{L}\right)=100/ P\times \Big(9.0\mathrm{mg}/\mathrm{L}-6.6\mathrm{mg}/\mathrm{L}\Big)\\ {}=100/ P\times 2.4\\ {}=240/ P\\ {}=240/\left(15\kern0.5em \mathrm{mL}\times 100/300\kern0.5em \mathrm{mL}\right)\\ {}=240/5\\ {}=48\end{array}$
2.

COD = 136 mg/L

Calculation:

$\begin{array}{c}\mathrm{COD}\kern0.5em \left(\mathrm{mg}/\mathrm{L}\right) = \left(37.8\mathrm{mL}-34.4\mathrm{mL}\right)\times 0.025\times 8000/ D\\ {}=3.4\times \kern0.5em 0.025\times 8000/ D\\ {}=680/ D\\ {}=680/5\\ {}=136\end{array}$

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