March, 1928
INDUSTRIAL, A N D ENGINEERING CHEMISTRY
247
A New Method for Determining Biochemical Oxygen Demand' F. Sierp RUHRYERBASD, Esssr, G E R S I X N Y
TANDARD Methods for the Examination of Water and Sewage" specifies two methods for determining biochemical oxygen demand. According t o the one method a quantity of sewage, predetermined in each case, is diluted with oxygen-saturated water and the oxygen demand measured by the decrease in dissolved oxygen of the diluted mixture. In the second method a nitrate solution is added as a source of readily available oxygen. I n both methods, apart from other difficulties, the quantity of oxygen in the dilution water or the nitrate solution progressively decreases as the organic matter is broken down, without being replenished again. As a result the later stages of decomposition often have available only low oxy3wen concen-
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Figure 1-Apparatus for Determining Biochemical Oxygen Demand with Gaseous Oxygen
trations; the organisms are in an increasingly unfavorable environment and will thus show a corresponding decrease in activity. In order to avoid these difficulties biochemical oxygen demand methods with undiluted sewage, as in the nitrate method, have been studied in this laboratory.2 The best solution of the problem seemed to be the use of pure oxygen. With the cooperation of Mr. Fransenieier, an apparatus was constructed to bring pure gaseous oxygen into contact with undiluted sewage. The volume of oxygen, corrected to 0" C. and 760 mm. pressure, which is absorbed in a given time a t a constant temperature of 20" C., gives the biochemical oxygen demand. Apparatus
The apparatus shown in Figure 1 consists of an Erlenmeyer or Florence flask of about 500 cc. capacity which holds the undiluted sewage. Supported on the flask by a well-fitting rubber stopper is a gas buret, the gas chamber of which is connected by a central diffusion tube to the space above the liquid in the flask. A leveling bulb, attached by rubber tubing to the gas buret, insures constant pressure. 1 Translated by S. L.NEAVE, Illinois State Water Survey, Urbana, Ill.
* Great Britain, Sewage Disposal, Report 5, App. V I , addendum to Sec. 1 , p. 438 (908). Ed.
This bulb contains an alkaline sodium chloride solution, which absorbs carbon dioxide formed in the flask and prevents solution of gas in the confining liquid. Procedure
The apparatus is weighed and then filled in the following manner: The Erlenmeyer flask, after careful cleaning with potassium dichromate-sulfuric acid mixture, is completely filled with the sample of sewage. The stopper carrying the gas buret is now forced into the neck of the flask t o a predetermined mark (which controls the volume of the flask), the liquid meanwhile being forced up into the capillary tube until it is full and any excess allowed to escape through tube A . The leveling bulb is raised until the salt solution almost reaches the tip of the diffusion tube, and the fraction of a cubic centimeter of air remaining in the top of the buret is swept out by oxygen admitted a t C and escaping a t B. Tube B is now closed, A is opened, and from 100 to 200 cc. of sample are forced out by the oxygen into a measuring cylinder, leaving 300 to 400 cc. in the flask. d is closed and the gas buret filled with oxygen by lowering the leveling bulb, and then C also is closed. The quantity of sample retained is now determined by again weighing the apparatus. In order to satisfy the immediate oxygen demand and to saturate the sewage with oxygen, the apparatus is well shaken; this requires 1 to 2 hours. It is then stored in a dark place a t 20" C., the leveling bulb being raised enough to maintain a slight pressure. By shaking the apparatus daily, or, if possible, more frequently a t the start, intimate contact is maintained between the oxygen and the sewage. Likewise, before each reading of the gas volume the apparatus must be shaken to insure a saturation of the liquid comparable to the initial reading.
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Figure 2-Comparison of Direct Absorption and Dilution Methods for Determination of Biochemical Oxygen Demand
In accordance with American practice, oxygen absorptions have been determined over periods of 5 to 20 days. Figure 2 shows a comparison between the direct absorption method and the dilution method; the direct process, since carbon dioxide is continuously removed, gives somewhat higher, but always comparable, results while the values for the dilutions vary widely. This apparatus enables one to read off the oxygen demand hourly or daily, while with the old method this necessitated a large number of bottles.
