INDUSTRIAL AND ENGINEERING CHEMISTRY
242
Vol. 20, No. 3
Technic and Significance of the Biochemical Oxygen Demand Determination F. W. Mohlman, G. P. Edwards, and Gladys Swope SAN~TAXY DISTRICT OF CHICAGO, CHICAGO, ILL.
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URING the past fifteen years much of the scientific work of the laboratories of the Sanitary District of Chicago has been based on the biochemical oxygen demand determination. The value of such an analytical method was first appreciated by Langdon Pearse, sanitary engineer for the District, as a basis for the study of the unique conditions that obtain a t Chicago with regard to disposal of sewage by dilution. The principles of oxygen balance so frequently discussed by Phelps' have a particular bearing on the problems encountered a t Chicago. The importance of the B. 0. D. determination as a fundamental basis for the past, present, and future oxygen requirements of the District has stimulated continuous research on the technic of the method and interpretation of the results. METHODS FOR DETERMINING BIOCHEMICAL OXYGEN DEMAND
The B. 0. D. determination has been in use in several forms for many years.* Three methods of analysis have been in general use: RELATIVE STAEZLITY METHOD-This method3 was used only
t o a limited extent. It consisted in making dilutions of the sample with water of known oxygen content, adding methylene blue,. and incubating until the methylene blue was decolorized. The initial oxygen was multiplied by the dilution factor, then this oxygen value divided by the relative stability, giving the complete demand. This method was very inaccurate owing t o toxic effect of the methylene blue, doubt as t o whether the relative stability curve was applicable, and impossibility of observing intermediate stages of oxygen demand. The method was included in the 1917 edition of Standard Methods of Water Analysis, but was dropped from the 1920 edition. NITRATE METHOD-This method' was developed in the laboratories of the Sanitary District of Chicago by Arthur Lederer, former chief chemist. A solution of sodium nitrate is added t o the undiluted sewage in a 250-cc. bottle, the treated sample is incubated for the desired period, and residual nitrite and nitrate nitrogen are determined. The oxygen equivalent of the residual nitrogen is subtracted from the oxygen equivalent of the nitrate added, on the basis that nitrite nitrogen is equivalent t o 1.71 oxygen (2N:30) and nitrate nitrogen t o 2.86 oxygen (2N:50). This method has the advantage that samples can be taken and prepared for incubation by inexperienced samplers who are not chemists, a number of individual samples can be composited after incubation, and absorption of oxygen during incubation introduces very little error since the sample is not diluted. These advantages do not warrant the use of the method, however, if the results do not check with the dilution method, as now revised. DILUTION M E T H O D L Tis~ ~now S so well known that no extended description of its technic is necessary. The sample is diluted with stored water, the initial dissolved oxygen determined, and after incubation the residual oxygen is determined. A blank of the dilution water is also incubated. The loss in the sample, corrected by the loss in the blank, multiplied by the dilution factor, gives the oxygen demand.
This brief description of the dilution method gives no idea of the many precautions that must be observed in using it, or the many details of technic that are necessary to get A m . J . Pub. Health, 5, 324 (1913). A complete bibliography of the development of the method has been compiled by E. J. Theriault, of the U. S. Public Health Service, who has been foremost in the United States in research on the dilution method. A monograph including this bibliography may soon appear; consequently this paper will contain little reference to the historical basis for the determination. 8 Standard Methods for Examination of Water and Sewage, p. 71 (1917). 4 I b i d . , p. 76 (1925). 1
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accurate and satisfactory results. These details form part of the subject matter of this paper. Nitrate Method
The dilution method was used by Hoover6 a t Columbus in 1911, but mainly as a rough measure of plant operation, with incubation at 37" C. for 1 day. Other investigators attempted to use the English method, with incubation for 5 days at 18" to 20" C., but found it impossible to obtain check results in different dilutions. Lederera compiled the results obtained under his direction by a number of investigators selected from the laboratory section of the American Public Health Association. The conclusions were that the oxygen demand varied with the dilution, and that the discrepancies were so great as to cast doubt on the advisability of the use of the dilution method. Lederer7 had been working on the nitrate method since 1913, and suggested that it might be preferable to the dilution method. Lederer concluded that when proper dilutions were used the two methods did not differ greatly, but a study of his results indicates that those by the dilution method were consistently higher than those obtained by the nitrate method. The values obtained by the dilution method were erratic, and he assumed that the variations were due to inaccuracies in the method. The errors were probably due to the use of unsatisfactory dilution water. In 1918 Theriault and Hommons explained the source of difficulty in the dilution method and suggested an improved technic. Since then the dilution method has become far more accurate. In 1925 this laboratory again compared the two methods. Over a period of 6 months daily samples of raw 39th Street sewage were incubated at 20" C. The results (Table I) show that for 5-day incubations the dilution results were 13 per cent higher than the nitrate ones, and for a 10-day period 28.4 per cent higher. Fifteen-day incubations did not materially increase the nitrate demand. The dilution method results seemed to follow the stability curve, but the nitrate results followed a consistently lower rate of reaction. of D i l u t i o n a n d Nitrate Methods 39th Street Sewage (Monthly averages, p. p. m. at 20' C.) DILUTION METHOD NITRATE METHOD &Day 10-Day &Day 10-Day 136 185 121 133 99 117 144 108 121 127 160 100 110 145 95 115 109 102 132 92 99 128 96 102 113 149 100 116
Table I-Comparison
1925 January February March April June Tulv
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