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The process is continued until the rate of removal of the water becomes very slow, It requires approximately 1 hour in a laboratory-scale apparatus to dehydrate the 85 per cent acid and somewhat longer for more dilute acids. The last traces of the entraining liquid dissolved in the concentrated acid may be removed by aeration under reduced pressure while heated to a temperature of about 60” C. When dilute lactic acid containing impurities from manufacture (such as calcium sulfate and empyreumatic substances yielding disagreeable odors:) is concentrated by this process, a partial purification is effected; the salts precipitate out of the concentrated acid, and the odors are swept away during the removal of the entraining agent. The total acidity of the concentrated acids is approximately 105 per cent, calculated as lactic acid. The figure is
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in excess of 100 per cent tliie to the piesence of lactyllactic acid and a small amount of lactide, which I S generally below 1 per cent. The acid is practically anhydrous, since it may contain less than 1 per cent of water. The rienly concentrated acid contains about 50 or GO per cent of free lactic acid, the remainder consisting chiefly of lactyllactic acid. However, as explained previously, the water content increases somewhat on storage because of changes in the acid until equilibrium is reached. The product is obtained as a water-white, rather viscous liquid.
Literature Cited (1) Eder, R., and Kutter, F.. H e h . Chim. Acta, 9, 567 (1926). (2) Kef, J. U., Ann., 403. 319 (1914).
Buffer Values of Sewage during Purification ROBERT S. INGOLS AND H. HEUKELEKIAN New Jersey Agricultural Experiment Station, New Brunswick, N. J. From the earliest work on the activated sludge process i t has been realized t h a t changes in the alkalinity and pH values frequently occur during the aeration of sewage with activated sludge. The difficulties of potentiometric titrations in the past have prevented a systematic study of the buffer value of sewage and its changes during purification. The buffer value of sewage is present in the soluble A comparison of the fraction (Seitz filtrate). buffer values of several sewages shows t h a t they differ somewhat in quantity b u t not in quality. The importance of the ammonium bicarbonate i n the sewage buffer value is shown i n two ways: by adding an equivalent amount of this compound to the tap water of a sewage, and by aerating sewage continuously and studying the buffer values a t intervals until the ammonia has been nitrified.
KLY comparatively recently have accurate, rapid, and
0
easily operated potentiometers been sold commercially; the various instruments using glass electrodes amply fulfill these requirements. It is not surprising, therefore, that studies of the purification of sewage have not included potentiometric titrations. The authors find this method very useful in interpreting some results. The paper presents a study of the buffer value of sewage and its changes during purification; only fragmentary statements and studies have been reported in the literature on alkalinity and pH. Ardern and Lockett (1) state: “In order that the nitrification change may proceed without hindrance, it is necessary that the alkalinity or basicity of the sewage should be rather more than equal to the nitric acid resulting from nitrification of the ammonium salts.” Baker (2) in discussing the above statement as to the drop in p H said: “Our experiments indicate that this is not the case.” The evidence for the first statement is not given; the evidence for the second is
Aeration of activated sludge-sewage mixtures under laboratory conditions shows t h a t the buffer values of the mixtures decrease rapidly and t h a t changes in pH are due to losses of volatile acids, to the reduction in the buffer values, and to the oxidation of ammonia to nitrate, and t h a t the relative importance of these three processes changes with time. The reduction of the buffer values in a plant aeration tank is less than the reduction of the buffer values of sewage-sludge mixtures in the laboratory during the same period of time. These and published results indicate a marked short circuiting in the aeration tank a t the plant. Reduction in the buffer values between the influent and effluent of a plant trickling filter is similar to the reduction in the buffer values obtained with activated sludge.
apparently the result of a p H study of sewage aerated for 6 days. Kitrification had probably not set in by the sixth day. I n 1927 Parsons and Wilson (6) showed a large reduction in the alkalinity of a mixture of sewage and sludge aerated under laboratory conditions, roughly paralleling the decrease in ammonia nitrogen. A decrease in alkalinity in sewage by activated sludge is frequently given by English authors. Haseltine (4) recently included some results on the changes in p H and alkalinity which take place through the aeration tanks of a number of activated sludge plants. The figures indicate that there is very little change in p H a t most plants, that the p H of three plants falls, but that the p H of two others which are quite acid at first rises; the drop in alkalinity can be roughly correlated with the nitrates produced by activated sludge. I n 1922 Smith (7) showed that a drop in alkalinity from 236 to 155 p. p. m. was accompanied by a drop in p H from 7.6 to 7.4 with a production of 7.5 p. p. m. nitrate nitrogen be-
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tween influent and effluent trickling filter samples from Plainfield, K,J. Other results showed lower nitrates, less drop in alkalinity, and no drop in pH. I n 1923 Wagenhals, Theriault, and Hommon (8) in a survey of plants in the United States showed a correlation between loss of alkalinity and nitrates in the effluents of trickling filters. Forman in 1935 (3) published a survey of the trickling filters in New Jersey; he showed that the pH values of the influent and effluent of the filters varied very much as the pH values through the aerators of activated sludge plants, as noted by Haseltine (4). These results in the literature should be more fully explained by the work of this paper.
Methods All of the potentiometric titrations were made (a) by adding to 100-ml. samples, 0.02 N acid a t suitable increments up to 25 ml. or pH 3.5, and (b) by adding to another 100-mi. sample, 0.02 N sodium hydroxide in like manner up to 25 ml. or pH 10.5. A Beckman pH meter with glass electrode (laboratory model) was used for all pH determinations. A study of the results obtained from three fractions of sewage (a) raw, (b) settled, and ( c ) Seitz filtrate, indicated that none of the buffer values was present in the material which settled out of the sewage and that only very little buffer value was present in the nonsettleable suspended fraction. The Seitz filtrate had a higher pH value due to the loss of carbon dioxide. Because of the similarity of the various portions, settled sewage was chosen for subsequent studies.
Buffer Values
OF FIGURE1 (above). ANALYSIS
TEE
BUFFERVALUEOF SEWAGE
FIGURE 2 (center). BVFFERVALUESOF A SEWAGE AERATEDFOR SEVERAL DAYS
VALUESOF A MIXTURE OF SEWAGE AND ACTIVATED FIGURE 3 (below). BUFFIPR SLUDcYEl
Figure 1 shows the difference in the buffer value of a sewage as compared with the local water supply. The higher buffer value in sewage is caused by the presence of several materials in varying quantities such as ammonium bicarbonate, fatty acids (soaps), phosphate, humic acids, uric acid, amino acids, and peptones or peptides. An approximation of the relative importance of the various buffer material is evident in Figure 1 where a solution in tap water of an equivalent amount of ammonium bicarbonate is compared to the ammonia in the sewage. A large part of the buffer value of the sewage is caused by this type of compound. The remainder of the sewage buffer value must be the result of the other inorganic and organic materials mentioned above. The similarity in the types of the curves of several sewages is shown in Figures 1, 2, 3, 6, and 7. There is some variation in the magnitude of the buffer values, but curves a t similar magnitudes are very much alike, although the sewage samples were secured from different sources. Many other curves were constructed illustrating this similarity, and some of these samples were taken from plants treating small amounts of industrial
MARCH, 1940
IXDUSTRIA4LAPiD ENGINEERIKG CHEMISTRY
wastes. A brief description of the sewages used in this paper is given in Table I. When sewage alone v a s aerated (Figure 2), samples were taken daily a t first. The aeration of sewage for the first day brought about little change and no change for the next 2 days in the buffer value of the sewage, but aeration during the first few hours expelled some free carbon dioxide and caused a shift upward in pH. During the next 6 days there was practically no nitrification, but the oxidation of a part of the organic matter reduced some of the acid buffer material. At the end of 12 days when nitrification of the ammonia was complete, most of the buffer value had been lost.
i
-PH *---*pH
601
A S FOUYD T H A T WOULD HAVE BY AD91NG; 0 E E N OBTAINED
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TO T H E N I T R A T E S
ly
FORMED TO T H E
ORIGINAL M I X T U R E 5.50
2
HOURS
AERATION
e
a
FIGURE 4. ANALYSISO F THE EFFECT OF NITRATEPRODUCTION UPOX CHANGES IN PH IN A hfIXTURE O F ACTIVATED SLCDGE AND SEWAGE
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the ratio is the reverse of that for the sixth hour so that only 25 per cent of the drop in pH is due to the nitrates present. The changes in p H value during aeration of a sludge and sewage a t different sludge concentrations and with normal and diluted sewage are shown in Figure 5. The amount of air was the same and in excess for each experiment. The lolyer part of this figure shows changes in the pH, the upper part shows the production of nitrates. The results indicate that with the highest sludge concentration the pH drop is most rapid, and that with the more dilute sewage the rate of pH drop is more rapid at the same sludge concentration. With sewage of half strength a t the sixth hour, the two higher sludge concentrations have the same nitrate content but the more rapid oxidation of the carbonaceous material with the higher sludge concentration produces a p H value of 6.1, while the lower sludge concentration yields a pH value of 6.5. These results indicate that one cannot predict the degree of nitrification from the pH value, or the degree of purification from either the nitrates or a given pH value. However, a continuous study of pH values may indicate the degree of purification. These results further indicate that it is necessary to use either a strong sewage or a low sludge concentration or both to maintain a constant pH during the aeration of a mixture under laboratory conditions. If it is desired to study the effect of adding more ammonia to an aerating mixture of sewage and sludge, it is very necessary not to add an ammonium compound with a stable anion such as ammonium chloride, for the chloride ion and the nitrate ion are both anions of highly ionized acids and their accumulation causes a rapid drop in pH. TABLE I. DESCRIPTION OF SEWAGES Looation of Curve Fig. 1 Fig. 2 Fig. 3 Fig. 6 Fig. 7
Source of Sewage
Description of Sewage
When a strong sewage is mixed with activated sludge to Highland Park Domestic. fresh, medium strength give a final concentration of 1500 p. p. m. suspended solids Madison-Chatham Domestic, strong, stale (Figure 3), the buffer value of the sewage accounts for nearly South River Domestic, strong, stale Hillsdale State Hospital Institution, fresh, weak all of the buffer value of the mixture. K h e n the mixture is Domestic, strong, stale Plainfield aerated, the buffer value decreases gradually until a t the sixth hour there is no flattening of the curve. The course of the p H is shown by the upper line of Figure 4. The important processes taking place simultaneously are: ( a ) the loss of The changes just discussed are those which occur with free carbon dioxide by aeration, (b) the oxidation of organic activated sludge-sewage mixtures under laboratory conditions. acids, (c) the conversion of ammonia to nitrates, and ( d ) the A study of the buffer values of sewage and aeration tank production of more free carbon dioxide as its cation (ammonia) disappears. The concurrent loss of carbon dioxide-and organic acids must account for the higher p H during the first 2 hours, for there is also a production of 5 p. p. m. 20 nitrate nitrogen within the same period. The z lower line of Figure 4 shows that p H value which the addition of acid equivalent to the nitrates at IO that hour produced upon the original mixture. ? A t the fourth hour enough labile acids are being 0 lost to more than balance the nitrates formed. At the sixth hour 1.30 milliequivalents of nitric 8.0 acid have been produced while 1.7 milliequivalents of acid are required by the original mixture to reach the p H value of 6.50. This means that 7D 75 per cent of the drop in pH is due to the 2 nitrates formed and 25 per cent to the loss of buffers. At the eighth hour 1.8 milliequivalents 60 of nitric acid are present and the p H has dropped to 5.67; the original mixture requires 3.6 milli50 equivalents of acid to reach this pH. Thus, only 2 4 6 2 4 0 e HOURS A E R A T I O N H O U R I AERATION 50 per cent of the drop in p H is due to the formation of nitrates a t this time. When this process FIGURE5. NITRATEFORMATION AND PH VARIATIOXS WITH VARIOUS is continued for 48 hours of continuous aeration, SLUDGE AND SEWAGE CONCENTRATIONS IL
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,1-:..-":i
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OF AERATION T A N K rF R O M M THE T y INLET OUTL;
LIQUOR e 0 --09
-
ao /
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la./
2 SEWAGE
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I
ACTIVATEDSLUDGE FIGURE 6 (above). BUFFERVALUESAT THE HILLSDALE PLANT FILTER FIGURE 7 (below). BUFFER~'ALUES AT THE PLAINFIELD TRICKLIXG PLAXT
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although the liquor a t the inlet end contained more than twice that amount; the B. 0. D. of the liquor a t the inlet end was less than one tenth that of the B.O.D. of the sewage entering the tank. These plant experiments were repeated a t Hillsdale and a t the activated sludge plant of the joint meeting of hIadison-Chatham with similar results. It is of interest that the aeration tanks of the Chatham plant have baffles. The rapid purification of sewage in the laboratory awaits some new engineering development for transference to regular plant operation. %-hen purification of sewage is carried out on a trickling filter, the same phenomena observed with activated sludge are present. The reductions in buffer value for two different samples from the trickling filter a t the Plainfield sewage treatment plant are shown in Figure 7. The sewages were obtained a t an interval of several weeks but their buffer values were nearly identical. The effluent with 2.5 p. p. m. nitrate nitrogen was obtained soon after the beds had been flooded for Psychoda fly control. The effluent with the 10 p. p. m. nitrate nitrogen was obtained from the trickling filter after a period of operation without flooding. I n the sample with less nitrates, there was a loss of some volatile acid, probably carbon dioxide, accompanied by a rise in pH and a slight reduction in the buffer value. These changes corresponded to those obtained with a short period of aerating sewage with activated sludge under laboratory conditions. The reduction in' the buffer value of the effluent sample containing 10 p. p. m. nitrate nitrogen was much larger and corresponds to the reduction in buffer value obtained with the aeration of sewage with activated sludge for several hours. Since both trickling filters and activated sludges oxidize carbonaceous and nitrogenous matter, the similarity of these results is readily understood. Literature Cited (1) Ardern, E
,
a n d L o c k e t t , W . T . , J . Soc. Chem.
Ind . 3 3 . 5 2 3 (1914).
Liquor a t the inlet and outlet ends of activated sludge plant at Hillsdale are shown in Figure 6. Ridenour ( 6 ) ,describing the sewage plant a t Hillsdale, states that the aeration tank has a nominal detention period of 15 hours and that there are no baffles in the aeration tank. The striking point about the results sho6n is the marked difference between the amount of reduction obtained in the buffer value at the plant and in the laboratory. This small reduction in the buffer value a t the plant takes place in spite of a nominal detention period of 15 hours. Only 0.7 p. p. m. nitrate nitrogen was found in the liquor of the outlet sample in contrast to the almost complete nitrification of the sewage-sludge mixture in the laboratory. This is a further indication of the difficulty in reproducing plant conditions in the laboratory and the differences in the behavior of sewage-activated sludge mixtures. The major reason for these differences is probably short circuiting which occurs in the plants and its positive absence in laboratory experiments with the batch process. Ridenour (6) published figures which indicate a large amount of short circuiting in the aeration tanks a t Hillsdale. Only 5.5 p. p. m. of nitrate nitrogen were formed through the tank,
(2) B a k e r , A. C., Illinois S t a t e iVate; Survey, B 7 d . 16 (1920). (3) F o r m a n , L., a n d Shaw, R. S.,S e w Jersev Sewage W o r k s Assoc. Proc., 20, 68 (1936). (4) Haseltine, T. R., Sewage T o r k s J., 10, 1017 (1938). (5) Parsons, A. S., a n d Wilson, N., Surveuor, 72, 221 (1927). (6) Ridenour, G.hl., a n d H e n d e r s o n , C. K.,Sewage Works J . , 8,766
(1936). (7) S m i t h , R. O., N. J. Agr. E x p t . Sta., Ann. R e p t . , 1922, 490. (8) Wagenhals, H. H., T h e r i a u l t , E. J., a n d H o m m o n , H . B., Pub. Health Bull. 132 (1923). P R E S E N T E D before the Division of Water, Sewage, and Sanitation Chemistry before the 97th Meeting of the American Chemical Society, Baltimore, Md. Journal Series Paper, Department of Water and Sewage Research,LN. J. Agricultural Experiment Station.
