Respiratory Quotient of Activated Sludge and of Activated SludgeSewage Mixtures C. M. SAWYER AND M. STARR NICHOLS State Laboratory of Hygiene, University of Wisconsin, Madison, Wis.
Respiratory quotients for activated sludges obtained from four different cities varied from 0.59 to 0.94. The R. Q . values of activated sludge-sewage mixtures in which the amount and kind of food material were held constant and the amount of activated sludge was varied showed no definite trend of variation. Variation of temperature between the limits of 10 to 25” C. had no effect on the R. Q. values. Sewages from seven separate sources were studied; the source had an effect on the magnitude of the R. Q. values. The character of the activated sludge from different sources was also a factor. Addition of glucose to activated sludgesewage mixtures generally resulted in higher R. Q. values than those given by controls; addition of peptone or soap and especially urea or ammonium chloride reduced the R. Q. values. Sludge storage without aeration and age of sewage had no effect. The R. Q . values determined fell between the limits of 0.51 and 1.18, with the majority between 0.70 and 0.95. EASUREMENT of the rate of oxygen utilization by activated sludge-sewage mixtures has come into prominence in recent years as a criterion of sludge activity and other characteristics of the activated sludge method of sewage treatment, but little attention has been paid to the quantity or rate of carbon dioxide evolved or to its correlation with the oxygen used. Wooldridge and Standfast (S), using Barcroft differential manometers in pairs and arranged so that both carbon dioxide and oxygen could be measured, obtained respiratory quotient (R. Q.) values for activated sludge-sewage mixtures of 0.37 for the first hour to 0.63 during the fifth hour. The pH was controlled a t 7.0 with a phosphate buffer. Heukelekian ( 1 ) suggested the use of carbon dioxide production as a possible means of following the course of activated sludge-sewage oxidations. He aerated the sludge-sewage mixture and recovered the carbon dioxide produced by means of a barium hydroxide absorption chain. The oxygen utilized was measured by the conven-
M
tional B. 0. D. method. Respiratory quotient values calculated from his data are from 0.83 to 0.91 for sewage, from 0.96 to 1.56 for activated sludge, and from 0.59 to 0.66 for fresh sewage solids. Oxygen values obtained by Heukelekian from B. 0. D. data cannot in any way be considered comparable to the oxygen utilization of an activated sludgesewage mixture, and this comparison may account for R. &. values greater than 1.0 in some cases. Heukelekian and Ingols ( 2 ) believe that carbon dioxide production by activated sludge-sewage mixtures may offer “a more reliable way of determining the load on the aeration tank than B. 0. D.” The possibility of the use of carbon dioxide production as a measure of sewage oxidation and the need for data concerning the mechanism of activated sludge-sewage oxidation prompted us to attempt a correlation of the actual usage of gaseous oxygen with the concomitant production of carbon dioxide when sewage undergoes oxidation in an aerated activated sludge-ewage mixture.
Methods The four instruments used in previous simultaneous studies on oxygen utilization were adapted for these experiments on the respiratory quotients (i. e., volume of carbon dioxide produced divided by volume of oxygen used). These instruments were operated simultaneously under controlled temperature conditions by one operator. Figure 1 shows the four instruments as used, and Figure 2 gives the details of the construction of one of them. It has been named OxyUtilometer, abbreviated hereafter to Oum: A represents the rotary compressor from which the air, under pressure, travels t o tube B where it bubbles through alkali solution to remove the carbon dioxide. On leaving B, the air passes through trap C t o prevent mechanical entrainment of alkali. The air, freed of its carbon dioxide, passes to diffuser plate D where it is broken up into fine bubbles; in its rise t o the surface it agitates the mixture and at the same time supplies it with oxygen. The air, reaching the surface of the mixed liquor in bottle E, has lost some of its oxygen and has picked up a new load of carbon dioxide, but it is again at atmospheric pressure. It now flows back t o compressor A by way of the wide tube, F , and the trap, G . This movement of the air constitutes the cycle through which it k constantly recirculated. As oxygen is removed by the mix in E and the carbon dioxide produced is removed in trap B, the amount of oxygen in the air in the system decreases and the pressure drops. Such a change is indicated on manometer I of the measuring device. To bring the pressure in the system back to normal, water is added from buret J into bottle H which is filled with pure oxygen; an equivalent volume of oxygen is displaced through the series of 1-mm. tubes at N into the circulating system to replace the oxygen used and maintain the same partial pressure relation between oxygen and nitrogen. Thus, by maintaining a constant level in the manometer by addition of water from the buret and by noting the time elapsing between readings on the buret, the rate at which oxygen is being 1042
AUGUST. 1939
INDUSTRIAL AND ENGINEEIIINC, CHEMISTRY
1043
i.
1.
TJTIU
used can be computed when thc proper corrections are made for the volumc of the samplc in E, tbe temperaturc of the oxygen at H , ltnd t.he barometric pressure. When bottle H becomes fdcd with xater. it is refilled with oxygen by displacement of t.he water with pure oxygen stored in balloon K by opening or closing the necewar? valves 01 scre\v clamps at,0, P,R, L, and S. of apparatus indicated aica is imrnersnc1 as complctelv m possible in a eonstant-temperahire bath. dioxidk evolves durinr th;?course of thc Iuu was determined by titratinp the content; of the alkali t,raps with normal sulfuric acid; phenolphthalein mid then methyl orange v-ore used as indicators. From t,he titrat.ion valucs the volume of carbon di-
sludge (rlln 10). ~i~~~~ rUns on sludge a,,d sewage (2 and 5 ) which show the greatest rise in pII value during aeration also show the bigbest respiratory quotient. Tlris rise in pI1 was due not to ti;e libcra&n of ammonia nitrogen, krlltto a loss ,,f dioxide from tlic llicarbonates of the carriage water of the sewage. Figure 3 presents simple acration erperiirients on Madison city water and sewage. When aeration is conducted unt.il tlic pII reaches 8.6, approximately 5 mg. (3.5nil.) US carbon diuxide are evolved. This vohune amounts to about 5 rar cent OS the total carbon dioxide evolved during a run and was considered ait,hin experimiental limits; since t,lie vise in pH was variable, 110 cor-
of oxygen used to obtain thevalue for t,he respirat,ory quotient.
ISffect of Activated Sludge Conerntration in Shidgc-Sewage ;Mixtures The sospeitded solids concentratioii was varied within each run (except No. 5 ) by diluting concentrated activated sludge with distilled water. To thcse adjusted sludges m r e added a roilstant valume (usually 2200 ml.) of food material in tire form of sewage so tbat t,lre final volurrrc of iriixt.ure was 3200 ml. and the suspended solids concentration equaled that sbo~vnin Table I. In ruir 5 activated sludge and sewage wcre mixed in varying proportions so as t o siniulnte plant operation. In runs 1 to 5 sewage A and activated sludge A were used. 111 run 10 sewage 9 was aerated in contact with activat,ed sliudge C . The average R. Q. d u e for the first five runs, inclridiiig all eighteeir separate values, is 0.86. If the five \ d u e s for lowest suspenrled solids are averaged, t.be R. Q. is found t o he 0.91; t.tie average value for t.he fire values of highest sludge concentration is 0.83 for the respiratory quotient. Undoubtedly sludge concentration has some influence on R. Q. values, bot the effect is slight and not OS a rnagnitode t o br rampnred with t,he effect shown by n diKerent
OXYis
FIG.2 . DETAILOF OXY-UTILOMETER
INDUSTRIAL AND ENGINEERING CHEMISTRY
1044
VOL. 31, NO. 8
TABLE I. EFFECTOF CONCENTRATION OF ACTIVATED SLUDGE ON RESPIRATORY QUOTIENT OF ACTIVATED SLUDGE-SEWAGE MIXTURES Run No.
Vol. COP
Vol.
M2 *
MZ.
25 25
8 8
107 144
128 186
0.84 0.78
...
7.6 7.5
8.2 8.2 8.2 8.2
7.9 7.9 7.9 7.9
8.8 8.7 8.6 8.6
1120 1680 2200 2700
25 25 25 25
5 5 5 5
63 68 73 74
66 74 83 85
0.96 0.92 0.88 0.87
I C 8.2 2 C 8.2 3 C 8.2 4 C 8.2 At standard temperature and pressure.
8.2 8.2 8.2 8.2
8.6 8.6 8.7 8.7
2040 2680 3400 4180
20 20 20 20
5 5 5 5
47 50 61 70
42 48 54 59
1.12 1.04 1.12 1.18
A A A A A A
4
4 3 2 1
...
10
Q
Time
HOUTS
...
3
PH
Final pH
' C.
...
2
----SludgNo.
p H of Sewage A
Suspended Solids P . p . m. 1340 2680
1
Oum No.
Temp.
On"
R. Q.
TABLE 11. EFFECTO F TEMPERATURE ON RESPIRATORY QUOTIENT Run
Oum No.
pH Values
Vol.
con
VOI. Oa
MZ.
Ml.
15 20
Hours 8 8
67 95
116
90
0.70 0.82
11
3 4
... ...
...
...
...
Suspended Solids P. p . m. 2420 2360
12
1 2 3 4
8.2 8.2 8.2 8.2
8.1 8.1 8.1 8.1
8.2 8.5 8.5 8.5
3820 3820 3780 3820
10 15 20 25
6 6 6 6
66 78 90 102
81 96 115 132
0.82 0.81 0.78 0.77
13
1 2 3 4
8.2 8.2 8.2 8.2
8.4 8.5 8.5 8.5
3900 3820 3740 3640
10 15 20 25
6 6 6 6
59 69 80 104
72 85 102 122
0.82 0.81 0.78 0.85
14
1 2 3 4
8.2 8 2 8.2 8.2
8 1 8 2 8 3 8.1
4200 4250 4140 4120
10 15 20 25
7 7 7 7
76 86 102 120
94 109 134 154
0.81 0 79 0.76 0.78
Vol. On M1.
R. Q.
74 118 73 156 214
0.84 0.78 0.94 0.73 0.59
42 63 108 53
0.90 0.89 0.78 0.88
No.
Sludge A
Sewage A
Final
...
8.1 8.1 8.1
8.1
Temp.
Time
c.
R. Q.
TABLE 111. RESPIRATORY QUOTIENTS OF ACTIVATED SLUDGESALONE^ Run
No. 18 19 21 23
0
Oum No. 3 2 4 2 1
- S l u d g w
No.
A AA
c
D
E
PH 8.2 7.8 7.9 8.1 7.9
Find
PH
8.2 7.0 7.7 8.2 8.0
7.0 7.1 8.1 8.4 7.9 8.3 8.0 8.3 A Determined a t 20' C. after 24 hours of aeration without feeding. 24
1 2
26
1 3
2A 2E 2c
Effect of Temperature Simultaneous studies were made upon identical activated sludge-sewage mixtures a t four different temperaturesnamely, lo', 15", 20', and 25' C. The temperatures were selected to cover approximately the normal range of sewage temperatures in temperate climates. Results of these studies (Table 11)indicate that temperature variations have no effect upon the magnitude of the R. Q. values.
Respiratory Quotients of Activated Sludge Alone Oxy-Utilometers were charged with sludge only. The results of these experiments are given in Table I11 and show that the R. Q. values for the various sludges a t their base rates of oxidation vary from 0.59 to 0.94. I n some cases a drop in pH of the sludge occurred during the course of the experiment; in this respect, the sludge differed from the activated
Suspended Solids P . p . m. 9400 4300 10100 5200 5360 5560 4480 8720 10240
Time
Hours 5 5 5 5 6 3 3 4 4
Vol. Con MZ. 62 92 69 114 126 38 56 84 47
sludge-sewage mixtures which in practically all cases increased in alkalinity.
Effect of Variation in Sewage and Activated Sludge In the performance of these experiments, the results were obtained in two ways. Either different sewages were oxidized by the same sludge or the same sewage was oxidized by different sludges. I n runs 20, 22, and 25 both were accomplished simultaneously (Table IV) . Results of run 15 show that the R. Q. values obtained by the oxidation of the same sewage with different sludges may differ widely. Oxidation with sludge A results in an R. Q. of 0.79; oxidation with sludge C gives an R. Q. of 0.88. Similar differences are shown by the results of runs 19 and 25. The possible influence of the sewage upon the R. Q. value obtained when oxidation is accomplished by the same ac-
TABLE Iv. EFFECTO F Run No.
Oum No.
-Sludge---No.
VARIATION IN SEWAGE AND
,----Sewage---
PH
NO.
PH
Final PH
ACTIVATEDSLUDGE Suspended Solids
P. p. m.
...
...
A A A A
A A
8.2 8.2 8.2 8.2
D A D
1 2 3 4
A
E
A E E
7.8 7.8 7.2 7.2
23
3 4
25
1 2 3 4
A A A A C C
8.1 8.1 7.7 7.7 7.9 7.9
6.9 8.0 7.4 8.1 7.4 8.1
30
1 3 2 3
A A
... ...
JJW A C A C A B A
7.6
...
8.7 8.3
A A
7.7 7.7
PP
FO
8.2 8.2
7.6 8.2
19 20
22
36
1 3 1
A C AA A
1 2 3 4
3
D D
...
7.9 7.9 7.8 8.0 7.8 8.0
A
7.0 7.6 7.0 7.6
A
E A
2820 2300 2020 2900 2820 2940 1460 1480 3040 2980 3160 3160 3540 3260 3720 3760 5040 4920 2430 2430 2170 2170
8.2 8.5 7.3 8.2 8.5 8.5 8.6 8.7 8.1 8.5 7.9 7.9 8.5 8.6 8.3 8.5 8.1 8.2
7.7 7.8 7.8
15
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INDUSTRIAL AND ENGINEERING CHEMISTRY
AUGUST, 1939
ON
RESPIRATORY QUOTIENT
Temp. O
VOl.
Vol.
Hours
M1.
ME.
4 4 5 5 7 7 7 7 6 6 6 6 6 6 6 6 6 6 4 4 5 5
73 52 97 76 74 80 83 80 121 85 222 187 109 81 94 88 123 128
92 59 121 83 87 102 100 99 186 118 353 284 215 88 124 118 181 195
0.79 0.88 0.80 0.92 0.85 0.78 0.83 0.81 0.65 0.72 0.63 0.66 0.51 0.92 0.76 0.75 0.68 0.66
46 76 106 66
53 92 136 103
0.87 0.83 0.78 0.64
Time
c.
25 25 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 25 25 25 25
COz
0 2
R. Q.
-
TABLE V.
EFFECTOF ADDITION OF PURESUBSTANCES ON RESPIRATORY QUOTIENTOF ACTIVATEDSLUDGE-SEWAQE MIXTURE s Oum No.
-SludgNO.
pH
43
1 2 3 4
A A A A
8.1 8.1 8.1
7.8 7.8 7.8 7.8
8.3
6
8.2 8.2 8.4
6 6 6
44
1 2 3 4
C
7.7
7.7
8.0 7.9 8.0 8.1
5
5
7.7 7.7
7.5 7.5 7.5 7.5
7.9 7.7 7.8 8.1
5 5 5 5
7.7 7.1 8.3
7 7 7
Run
No.
45
46
C C
C C
8.1
pH of Sewage A
1 2 3 4
C
C C
7.3 7.3 7.3 7.3
7.9 7.9 7.9 7.9
1 3 4
A A A
7.8 7.8 7.8
7.9 7.9 7.9
Final
PH
Time
Hours
5 5
R. Q.
Substance Added
Vol. c02
Val.
P. p . m.
MI.
MI.
90 110 110 93
108 128 138 119
0.83 0.B6 0.80 0.78
GlLbose Peptone
300 100
0 2
Soap
100
Glucose Peptone Soap
300 100
112 88 90 83
122 102 112 88
0.92 0.86 0.80 0.94
Glucose Glucose Glucose
272 544 816
100 118 137 67
116 139 148 82
0.86 0.85 0.93 0.82
Urea NHaCl
50 135
119 142 92
177 192 110
0.68
... ...
...
0.74
0.84
Effect of Addition of Pure Substances to SludgeSewage Mixtures Pure substances were added to sewage in order to study their effect upon the R. Q. when such mixtures are oxidized by activated sludge; glucose was selected to represent the carbohydrates, peptone the proteins, and Castile soap the fats. Peptone and soap were chosen because they are readily soluble in water. An attempt was made to take roughly equivalent quantities of each of the materials on the basis of the oxygen necessary to convert it to the stable products of aerobic oxidation. I n the case of urea and ammonium chlo-
0 MADISON C I T i WATER p H 74
A MADISON SWAGE
pH8.I
TIME IN MINUTES
tivated sludge is best shown by the results of runs 23 and 36. I n run 23 oxidation of J J W , a malthouse waste, by sludge A gave a n R. Q. value of 0.51; oxidation of sewage from source A by another portion of sludge A gave an R. Q. value of 0.92. I n run 36 oxidation of sewage FO by sludge A gave a n R. Q. value of 0.78; oxidation of sewage from source PP with sludge A gave a value of 0.64. Other results illustrating the same difference in a less emphatic way are shown in the results of runs 20, 22, and 30.
1046
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 31, NO. 8
ride, the amounts used were calculated so as to give a decided increase in the oxygen requirements over that needed by the sewage alone. Results of runs 43 and 45 (Table V) show that the addition of glucose increases the R. Q. value over that given by the control containing sewage alone; in run 44,which has a relatively high R. Q. value for the control itself, the value is slightly lower. The addition of peptone and soap reduces the value of the R.Q. as shown in the results of runs 43 and 44. The influence of nitrogen compounds upon the R. Q. values is shown by the results of run 46. The addition of 50 mg. of urea per liter of mixture caused a decided drop in the R. Q. value from 0.84 given by the control sample to 0.68. Ammonium chloride caused a similar decrease in the R. &. value but not as great so urea. This difference may have been partially due to the lower final pH of the mixture containing ammonium chloride. The oxidation of ammonia to nitrate and water by activated sludge requires 4.0 atoms of oxygen for each molecule of ammonia, with the concomitant liberation of one molecule of carbon dioxide by the action of the nitric acid formed on the bicarbonates of the sewage. For each molecule of urea oxidized to nitric acid and carbon dioxide there would be evolved, therefore, one molecule of carbon dioxide through hydrolysis of this compound to ammonium carbonate and two molecules of carbon dioxide liberated from the bicarbonates of the sewage through the formation of nitric acid by oxidation of the ammonia. Thus 4.0 molecules of oxygen are used and three molecules of carbon dioxide are produced in the hydrolysis and oxidation of one molecule of urea, with 0.75 as the theoretical respiratory quotient. From a similar calculation the theoretical R. Q. of the ammonium chloride is found to be 1.0. When these compounds are mixed with sewage that is undergoing biological oxidation (run 46), it is not possible from the data to explain the low R. Q. values obtained. Incomplete oxidation, neutralization of nitric acid by ammonia, or the liberation of free nitrogen from nitrites may play a part in this unexpected result.
in this paper in which R. Q. values were determined on runs varying from 3 to 8 hours do not shorn such a relation. I n order to determine whether the R. Q. value changes during the course of a particular run, the R. Q. was determined a t the end of 3 hours and again after 5 hours on two separate runs. On the first run the 0-3 hour R. Q. was found to be 0.82 and the 3-5 hour R. Q., 0.80. On the second run the 0-3 hour R. Q. was 0.85 and the 3-5 hour R. Q. was 0.84. The problem was attacked in still another manner by equipping one of the Oxy-Utilometers with two alkali traps with valve connections such that the circulating air could be passed through one or the other as desired, and the one not in use could be removed and its carbon dioxide content determined by titration. I n this manner determinations of carbon dioxide were made at 10-minute intervals a t the beginning of the run while the carbon dioxide evolution was the most rapid, and a t 15- and 20-minute intervals as the rate decreased. Simultaneously, on another Oxy-Utilometer containing an identical mixture, the oxygen utilization was determined by readings over short (5-10 minute) intervals. The results of these runs are given in Figure 4 (sludge A and sewage A used). For convenience of comparison the carbon dioxide was calculated to its oxygen equivalent and these values were plotted instead of those for carbon dioxide. The R. Q. value during the first 40 minutes apparently varies considerably. It is believed that this anomaly is caused by saturation phenomena on the part of both the oxygen and the carbon dioxide. From the period of 40 to 120 minutes the ratio between the two remains about constant. Shortly after 2 hours have elapsed, the ratio apparently becomes greater than unity but drops below unity again before the third hour has elapsed. These variations are believed to be due to similar saturation phenomena. If these saturation phenomena could be eliminated, a fairly uniform respiratory quotient would undoubtedly he found throughout the course of the oxidation.
Effect of Storage of Activated Sludge without Aeration Prior to Mixing with Sewage
The term “respiratory quotient” is commonly used in biological investigations to indicate a comparison between the volume of carbon dioxide produced and the volume of oxygen used in metabolic processes. Since the activated sludge process is fundamentally biological in nature, the determination of respiratory quotients for its metabolism is permissible. That the R. Q. is influenced markedly by the character of the metabolite is shown by the following theoretical considerations:
Portions of a stock supply of activated sludge undergoing aeration were removed at different times prior to the time the run was started and allowed to stand for periods varying from 0 to 9 hours, without agitation or aeration. I n preparation for a run, all instruments were charged with sludge at the same time and treated with equal quantities of the same sewage. R. Q. values were not affected, but the utilization of oxygen and the production of carbon dioxide were increased with storage. Therefore, it appears that prolonged storage of activated sludge can increase the load upon the aeration facilities to a marked extent.
Effect of Age of Sewage in a Sludge-Sewage Mixture Because of the nature of this study, simultaneous runs could not be made. T o eliminate as much as possible the effect of the increasing age upon the activated sludge, a stock supply was used that had been aerated but had not been fed for 24 hours prior to the start of the run. It is believed that by this method the influence of the changing age of the sludge was reduced to a minimum. The results showed that the R. Q. values remained practically constant even when the sewage was stored over a period of 12 hours.
Effect of Time on a Sludge-Sewage Mixture Wooldridge and Standfast (8) reported a gradual increase in the R. Q. values of activated sludge-sewage mixtures as the oxidation progressed. Results of the work already shown
Theoretical Considerations
Carbohydrates (glucose as example) : CsHizOs
+ 6 0,
R.Q. = COz/Oz
+
6 COS 6 HzO = 6 / 6 = 1.0
(1)
Fat (olein as example) :
+
+
CsHs(CisHaa0z)a 80 0 2 = 57 COz 52 HzO R. Q. COz/Oz = 57/80 = 0.71
(2)
Protein (albumin as example, formula by Lieberkiihn):
R. Q.
COz/Oa = 63/77 = 0.82
Because of the fact that the metabolism of proteins by activated sludge may also involve the oxidation of the nitrogen to nitrates, the following equation is probably preferable to represent the metabolic process:
R. Q.
=i
COz/O,
72/113
= 0.64
AUGUST, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
I n the activated sludge process it is reasonable to assume that the nitrogen pentoxide and the sulfur trioxide liberated in this oxidation would react with the water to form nitric and sulfuric acids. This would result in a change of the p H of the mixture, which would be resisted by the buffering effect of the bicarbonates and the organic matter of the mixture with the formation of an approximately equivalent quantity of carbon dioxide as shown in the following equations:
+ 9 HzO = 18 H + + 18 NOS+ HzO = 2 H + + SO4--
9 N2Os SO8
Considering all buffering due to bicarbonates : 20 HCOa20 H + = 20 Ha0 20 CO,
+
+
The addition of this amount of carbon dioxide to the amount indicated in Equation 4,illustrating the oxidation of protein by activated sludge, would increase the total amount from 72 to 92 and result in a higher R. Q.:
R.Q. = COz/Og
92/113
=i
0.81
I n consideration of the theoretical R. Q. values for the various classes of food materials, it seems reasonable that the R. Q. of an activated sludge-sewage mixture can be varied by adding different food materials; also, since sewage itself is of such variable composition, we cannot necessarily expect different samples to give the same results. I n the matter of the variation of R. Q. values obtained by treating like samples of sewage with different activated sludges, we must look for another explanation of the difference since the available food material is the same except for those food substances stored in the different sludges. Aside from this slight difference in food supply, the variation in biological life composing the sludges seems to be the deciding factor.
1047
Experiments conducted in this laboratory have shown that the nature of the oxidation produced by two different activated sludges when fed the same sewage may be radically dissimilar. The nature of these experiments is too extensive to present here but will be given in a later publication. I n the measurement of carbon dioxide production over a particular period of time, it must be remembered that the carbon dioxide evolved is not necessarily a true indication of the amount being produced a t that particular interval of time. The high solubility a t ordinary temperatures and the danger of incomplete removal by underaeration coupled with the tendency to decomposition of the bicarbonates present in the sewage by overventilation makes the exact determination of the rate of production extremely difficult. Since the respiratory quotient varies so widely for different sludges and sludge-sewage mixtures, the authors doubt whether carbon dioxide measurements as a means for the determination of oxidation rates can be justified. They do believe, however, that further studies involving the simultaneous determination of both oxygen used and carbon dioxide produced under the same environmental conditions may show interrelations between the respiratory quotient and such sludge characteristics as activity, biological composition, ash content, bulking, and dewatering. Such relations will be of value in developing a better understanding of the mechanism of the activated sludge process of sewage treatment.
Literature Cited (1) Heukelekian, H., Sewage Works J., 8, 210 (1936). (2) Heukelekian and Ingols, Ibid., 9, 717 (1937). (3) Wooldridge and Standfast, Biochem. J., 30, 156 (1936). P R E S E N Tbefore ~ D the Division of Water, Sewage, and Sanitation Chemistry a t the 96th Meeting of the American Chemioal Society, Milwaukee, Wis.
The Action of Filter Aids This paper shows quantitatively that, when kieselguhr is added as a filter aid to a filter cake consisting of rigid quartz particles, the resulting increase in permeability is due solely to the corresponding change in the porosity of the cake. The changes i n resistance on adding different proportions of filter aid are similar to those observed on adding kieselguhr to compressible cakes and substantiate the theory that, i n these cases too, the main function of the filter is to increase porosity. It foHows that the most important property of a filter aid is its high porosity, and that this should not be sacrificed by seeking high adsorptive or high coagulating power.
T
HE paper is presented to amplify the conclusions drawn in a previous article ( I ) . It was there stated that the chief function of a filter aid such as diatomaceous earth was to give a filter cake a highly porous texture and hence a high permeability. The experimental data also indicated that positively charged colloidal particles are probably ad-
P. C. CARMAN University of Cape Town, Rondebosch. South Africa sorbed on the negatively charged surface of siliceous filter aids such as kieselguhr; but that, even though this effect increased the efficiency of the kieselguhr, its contribution appeared to be of minor importance compared with the purely mechanical effect of the change in texture. If we assume that the action of kieselguhr is essentially mechanical, it should be possible to substantiate this assumption quantitatively and mathematically in the case of ideal cakes, provided we can find the correct relation between cake permeability and cake porosity. Recently this has become possible, owing to advances in the theory of the permeability of beds of rigid particles ( 2 ) .
Experiments on Permeability I n the present experiments the substances to be removed by filtration were two finely divided quartz powders, consisting of particles in the size range 2 to 10 microns, since they are rigid and not small enough to introduce colloidal phenomena. To these were added as filter aid a special grade of kieselguhr sold commercially as Celite 503, which contains particles in the same size range and is completely free from colloidal matter.
'
