Dissolved Oxygen Recordings with t L Dropping Mercury Electrode ROBERT S. INGOLS, New Jersey Agricultural Experiment Station, New Brunswick, N. J. cell as the inert reference electrode and the droaaing m e r c w electrode consisting of a separatory funnel with a long &em and a short length of oapillary tubing of 0.04mm.bore to obtain a drop rate of approximately 1drop per second. For outside work both were encased in a. steel shell t o protect the electrical connections and prevent contamination of either reservoir. A cup was placed 5 cm. (2 inches) below the capillary to catch the used mercury. A long, heavy, high-conductance cable was used to connect the electrodes to the instrument for outdoor work. The unit is illus-
m R E importance of dissolved oxygen in several phases of
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water and sewage chemistry h& long been recognized. The satisfactory use of the dropping mercury electrode for determining dissolved oxygen in sewage and activated sludgesewage mixtures has been demonstrated by the author (2). Petering and Daniels (6) have used the dropping mercury electrode for the determination of dissolved oxygen in studies upon photosyntbesis, while Manning (4) has adapted this method to the study of the dissolved oxygen cycle in lake water throughout a %hour period. Manning was the 6rst to demoustrate that dissolved oxygen can be determined continuously in water by using a constant voltage and noting the shift in current flow with a sensitive recording galvanometer. This paper will demonstrate the use of the constant voltage method to obtain continuous dissolved oxygen values in laboratory experiments and during the regular operation of the aeration tank of activated slndge-sewage plants.
Instrument The instrument supplied by the Cambridge Instrument Company New York, N. Y., for this study uses a simple potentiometer Lircuit with a sensitive, suspension, mirror galvanometer to measwe the ourrent flow m the dropping mercury elertrode circuit. A light beam impinging u on the mirror is reflected to a photoelectric cell. The ourrent gom the photoelectric cell is amplifi-d and then recorded at minute intervals .with a recording
the laboratory principally to obtain type curves, The difference in the rate of increase of dissolved oxygen in activated sludge-sewage mixtures studied in different laboratory units is illustrated in Figure 2; curve A shows how rapidly the dissolved oxygen rises when 1 liter of the activated sludgesewage mixture is in a tall glass tube agitated with air at the rate of 80 ml. per liter per minute (4 cubic feet per gallon per 6 hours) through a sintered glass diffuser at the bottom. Each accentuated horizontal line is equivalent to 1.0 p. p. m. of dissolved oxygen, while each vertical division is 1 hotv. Curve B shows how the dissolved oxygen in the same aPparatus is affected by lowering the oxygen in the diffusii gas from 25 per cent (air) to 5 per cent (air and nitroger so that it is equivalent to 20 ml. per liter per minute (1 cut foot of air per gallon per 6 hours). Curve C shows how t he dissolved oxygen increases in a shallow aeration tank, 30 X 30 X 60 cm. (1 X 1 x 2 feet), with spiral flow using a sm:dl amount of air for agitation. It was not possible to measure the air used in the &mk, but it was adjusted visually to the minimum needed to keep the sludge in motion. The results in the aeration tank (curve C ) with a slow rise in dissolved oxygen are more comparable to plant conditions than the r e sults in curve A with the tall glass tube with the rapid rise in dissolved oxygen. Since much laboratory work upon activated sludge has been done with efficient aeration vessels, it is small wonder that the results have not been transferable to the less efficient aeration tanks of sewage plants. The important effect of the dissolved oxygen level upon nitrification h m hem shown tm and auv study which includes rates of dissolved he use of approxiefficient, olled ap3 not use r space. ,"d R TlPP ~..
FIQW 2. DISSOLVED ..
OXYQEN
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CURVES OBTAINED IN L4BORATORY VAQE SLUDGEPLUSSET
UNITS WITH ACTIVATED
f Sewsge-si"dze m'xt"TB in cy1inder agitated with sir cylinder agitated with 5 pey
