Conditions i Activate Sludge wing Frothin GAIL
P.
EDWARDS AND MARTIN E. GI"'
New York University, New York 53,
N. Y.
In batch-operated activated sludge experiments, the detergent content of the liquid, oxidationreduction potentials, and surface tension were measured before, during, and after aeration. In a typical experiment, about 37% of the added detergent was adsorbed b y the sludge as soon as the sludge and detergent were mixed, and 4270 was removed from the liquid after 5 hours' aeration. During aeration, the surface tension and the oxidation potential of the liquor gradually increased. Under the conditions of the experiment, frothing seemed independent of surface tension and oxidation potential.
ROTHING in the activated sludge process continues t o be F an important complaint against the presence of detergents in sewage. Experiments have been carried out t o determine some of the conditions that exist during frothing. In this study, batch process experiments were set up and anionic detergent concentration in the liquid, surface tension, and oxidation potential were measured under various methods of operation to determine whether these are important factors in causing frothing. I n the course of these experiments, the need for a satisfactory method of expressing the concentration of detergents in sewage arose because the composition of detergents used as standards in the preparation of calibration curves is unknown. For example, the standard detergent, Nacconol NRSF, used in these studies has been described as 92.7% pure with an average alkyl chain length of 13 carbons. The formula of this detergent might be given a8 C13H27C6H4S03Na. Actually it is a mixture of compounds. I n sewage, where various types of anionic detergents may be found, the difficulty in reporting is even greater. Fortunately, cetyltrimethylammonium bromide (CTAB), the titrating agent used in the two-phase titration method ( I ) , is relatively stable and may be standardized by the Volhard method for bromides, so that the results of the two-phase titration method may be expressed as milliequivalents if the purity of the cetyltrimethylammonium bromide is determined. Pure cetyltrimethylammonium bromide, obtained from Fine Organics, Inc., was found by analysis t o have a n equivalent weight of 362.4. The equivalent weight computed from the formula is 364.4, a difference of 0.54%. The value of 362 was used in these experiments t o calculate the results as milliequivalents of anionic detergent. This procedure eliminates the need for making a calibration curve and expresses the result in terms of the active ingredient contained in the detergent. It is not based on compounds of uncertain composition or purity. All determinations of detergent content and surface tension in mixed liquor samples m-ere made on the supernatant liquor after settling. Determinations of oxidation-reduction potentials and suspended solids were made on the whole sample. Several experiments were made with sewage in a batch process activated sludge process and typical results from different sewages are shown in Table I. I n these experiments, the suspended solids in the aeration process were maintained between 1150 and 1350 p.p.m. With a fairly high detergent concentration in the sewage, some detergent was quickly adsorbed by the sludge when 1
Present addreas, Monsanto Chemical Co., Dayton, Ohio.
246
sewage and sludge were mixed. When the sewage contained little detergent, considerable detergent moved back into the liquid portion of the system when the sludge and sewage were mixed. The adsorption of detergent by the sludge appears to be reversible and depends on the concentration of detergent as well as the degree of saturation in the sludge, This reversibiiity of adsorption of detergent by the sludge may offer a clue as to the mechanism of foaming.
Table I.
Effect of Aeration on Sewage-Sludge Mixtures
Anionic Detergent, Meq. X 1O-3/ Liter Arerage Suspended Solids in Aerator 1350 P.P.X. Sewage with High Detergent Content 8.40 On Aeration mixture 418 64.9 7.72 50 (settling) Effluent 410 66.5 50 (settling Sludge 40.5 13: 24 Sewage A 330 53:3 13.68 53.5 60 Aeration mixture 374 13.24 54.0 75 Aeration mixture 390 12.16 58.7 120 Aeration mixture 392 11.48 57.0 180 Aeration mixture 385 10.60 60.0 240 Aeration mixture 390 9.92 62.8 300 Aeration mixture 391 8.84 67.0 360 Aeration mixture 395 7.72 67.5 410 (50-mjn. settljng.) Effluent 350 410 (50-inin. settling) Sludge 300 1i:44 50:9 Sewage B 340 14.36 420 Aeration mixture 320 51.3 6.64 17 hr. Aeration mixture 400 64.5 Time, Minutes
Sample
Surface E H ~ Tension, M Y . Dynes/Crn.
Average Siisp ended Solids in Aerator 1150 P.P.hI. with Low Detergent Conte,nt 65.6 Ob Aeration mixture 400 380 65.0 5 0 (settling) 50 (settling 58:s Hn 57.2 57.2 08.0 61.5 Aeration mixture 360 65.2 Aeration mixture 360 66.5 Aeration mixture 300 68.3 Aeration mixture 365 68.0 Effluent 345 Sludge 320 60:O Sewage D 280 57.0 Aeration mixture 305 425 67.0 Aeration mixture 380 +17 hr. a Previous aeration period 18 hours. b Previous aeration period 17 hours.
Sewage 3.96 8 84
...
6.63 10.36 12.80 12.60 11.04 9.92 8.84 7.28 7.72
7'08 10.80 4.40
Certain trends observed during these experiments also occurred in all the later work-for example, as the aeration period progresEed the oxidation-reduction potential of the mixed liquor
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48,No. 2
STREAM POLLUTION PROBLEMS slowly increased. The potentials were measured in millivolts and based on the hydrogen electrode. Concurrently the surface tension rose while the detergent concentration decreased in the supernatant liquor. The inability to repeat experiments with the same sewage made it impossible to substantiate any theories properly. It was also felt t h a t the use of sewage involved too many variables, which tended to complicate the picture. These variables include fat, protein, detergent, and electrolyte concentration, which affect frothing and surface tension.
Table 111.
Oxidation-Reduction Potentials of Glucose and Flour-Glucose Sludge System Aeration Time Hour;
Sample
Glucose Formula Detergent, E H , meq. X lo-%/ mv. liter 490
.. ..
510
..
600
Flour-Glucose Detergent, E H , meq. X mv. liter 510 500 310
4YU
4ou
510
490
At this time a preliminary test was run t o determine the oxidation-reduction potentials of the two systems. As shown in Table 111, the potential of the sludge derived from the flour and glucose formula was lower than the potential of the glucose sludge after a 0.5-hour settling period for both. The aqueous phase of the glucose mixed liquor still contained titratable anionic detergent, even after 20 cycles using the synthetic feed. This is not
“j U
B Affer 6+c. Aerafion A At Sfort o f Aeraticn
: o
0 Feed Bp
I
I
2b
0
40
’
sz?
1
’
80
N A C C O N O L O R I G I N A L L Y PRESENT, P.P.M.
Figure 1.
Adsorption of detergent by flour-glucose feed and sludge
It was decided that more meaningful results could be obtained using synthetic sewages of precisely known compositions. Two feeds were used: one containing dextrose (glucose) and the other wheat flour and glucose as the organic food. The total formulas for 12 liters of feed are as follows: Dextrose Formula Dextrose 400 p.p.m. = 4.8 g r a m d l 2 liters Nitrogen 1 part N per 20 parts B.O.D. Phosphorus 1 part P per 80 parts B.O.D. (Total formula t o be 400 p.p.m. B.O.D.) 3.6 grams/l2 liters Na(NHdHPO4 used Silica 0.1 gram/l2 liters (as fuller’s earth) As KCI, 0.009 g r a m d l 2 liters Potassium
Flour and Dextrose Formula Wheat flour Dextrose Silica NasPOd2HzO
i
U
m
E 2
s8
50.0-
cs
3
c W
U
9 a
v)
400
300 p.p.m. = 3.6 g r a m d l 2 liters 100 p.p.m. = 1.2 grams/l2 liters 0.1 gram/l2 liters fuller’s earth 12.3 p.p.m. = 0.15 gram/l2 liters
The change-over from sewage t o synthetic feed was done simply by adding the synthetic feed t o the activated sludge over many cycles until the sewage sludge was converted into synthetic sludge. After several cycles, the sludges were examined microscopically; the results as shown in Table I1 indicated t h a t the sludges were normal.
0 After Zo./fr Aeration L7
After 5.m
A
At Star/ of Aerotion
Aeration
i 0 feed 20
30’0 0
40
60
80
100
N A C C O N O L O R I G I N A L L Y PRESENT, P.P.M.
Table II. Microscopic Examination of Sludge
Zooglea Stalked ciliates Free swimming ciliates Flagellates Worms Rotifera Filamentous bacteria Supernatant liquor
February 1956
Glucose Sludge Abundant AVany Moderate Few Few Few Absent Slightly cloudy
Flour and Glucose Sludge Abundant Many Numerous Few Few Numerous Absent Clear
Figure 2.
Surface tension of samples derived from glucose-activated sludge samples
the case with the flour and glucose system. Apparently, the wheat flour prevents desorption of detergent by virtue of its strong attraction for such anions. An experiment was carried out measuring the detergent content in the supernatant liquors of both systems, a t various p H values, in order t o obtain more information concerning the desorption of detergent from sludge. Aerated
INDUSTRIAL AND ENGINEERING CHEMISTRY
247
mixed liquors from the two synthetic feeds were titrated for detergent content after the liquors had been adjusted t o various p H values and allowed to aerate for about 15 minutes a t each pH. I t is apparent from the results shown in Table IV that detergent becomes more readily extractable from sludge as the p H is raised. For the glucose system the detergent concentration of the liquid phase doubled as the p H was raised from 6.0 t o 7.4. In view of this sensitivity t o variation in pH, the systems were buffered by using sodium ammoniuni phosphate in the glucose formula and sodium bicarbonate in the flour and glucose formula.
Table IV.
a
e
0-After 5Nc
3
A
- --- A l 5 f o r f
Aetotion Aemtion
o f Aerofion
Effect of pH on Desorption of Detergents
Glucose Formula Detergent i n pH of supernatant, mixed meq. X 10-V liquor liter 4.2 2.65 2 .43 6.0 7.4 5.52 8.3 7.72 E n d point of titration poor hecau-e of
Flour-Glucose Formula Detergent in p H of supernatant, liquor mixed meq. X 10-8/ liter 4,s 1.32 6.0 1.32 6.64a 7.4 6.645 8.0 acciimulation of dye a t interface.
At this point a n investigation of the properties of the flour and glucose-activated sludge system was made. Various amounts of Yacconol were added t o the basic flour and glucose feed. Batchwise treatment cycles were then carried out measuring detergent concentration, surface tension, suspended solids, and oxidationreduction potentials as indicated by Tables V and V I The detergent concentration apparently has no effect on the oxidationreduction potential. Here, as before, the potentials rose slowly as the aeration time increased. The surface tension of the aqueous phase decreased with increasing detergent concentration, but
Table V. Feed Nacoonol Added t o Feed, P.P.hI.
Q -After 2ont
z "01
mv.
Surface tension, dpnes/cm.
520 520 520 514 523 525
44.0 56.0 53.0 43.9 36.8 34.0
EH,
Anionic detergent.
meq. X l o - $ /
liter
I
se
0 Figure
I
'
a
I
I
,
)
'
40 60 80 N A C C O N O L O R I G I N A L L Y PRESENT, P.P.M.
1
IO0
3. Adsorption of detergent by glucose sludge
increased as the aeration progressed. The results as shown i n Figure 1 indicate that although large percentages of detergent were removed by agents in the feed itself, some of this detergent went back into the free detergent state when the feed was mixed with the sludge. This is indicated by the shaded area in Figure I . Although the flour and glucose system offered a considerabl? improvement over sewage, the presence of protein still tended to obscure the singular effects of synthetic detergents because of surface activity of protein molecules. The svnthetic feed based on glucose as the only organic ingredient TT-as adopted with the
Properties of Activated Sludge Systems
(Suspended solids of aeration liquor 1450 p.p.m.) Mixture a t Start of Aeration hfixture after 6-Hour Aeration Anionic Anionic Surface detergent, Surface detergent, E H , tenaion, meq. X lO-s/ EK, tension, meq. X lo-:/ mv. dynes/cm. liter mv. dynea/cm. liter Flour-Glucose System
0 5
10 25 50 100
....
437 453 443 450 434 433
7.72 13.24 26.48 45.20 104.8
71.0 68.0 66.3 58.0 45.9 43.7
3.32 4.40 6.64 17.68 42,40 99.36
455 480 470 472 455 457
76.4 76.0 73.7 63.7 60.5 57.0
3.96 3.96 4.40 15.44 24.28 61.84
Mixture after 5-Hour Aeration Anionic Surface detergent, EH, tension, meq. X lo-&/ mv. dynes/cm. liter
0 5 10 25 50 100
415 410 410 410 410 410
67.0 60.0 52.6 44.8 38.8 34.7
440 445 440 440 440 445
0 8.60 16.36 38.20 78.16 155.4
Table VI. Nacconol
Added Feed to
P.P.L-4. 0
a
248
0
5 4.0 10 7.3 25 16.0 50 28 100 66.5 Previous aeration period 17
i:o
2.7 9.0 22.0 33.5 hours.
Glucose System 0.0 450 3.76 460 8.16 450 19.88 455 39.08 450 77.28 450
67.8 66.0 65.0 59.5 55.6 54.0
0.0 2.64 5.52 17,138 38.64 73,96
470 475 460 470 470 470
75.0 71.4 70.0 65.9 62.0 62.0
0.0 2.20 4.44 15.48 33 I12 64.04
Detergent Removal with Flour-Glucose-Activated Sludge (Suspended solids in aeration liquor 1680 p.p.m.) Mixture a t Start of Aerationa
Feed Detergent, P.P.M. Detd. Removed
63.8 59.5 57.0 54.8 46.4 43.0
Mixture after 20-Hour Aeration Anionic Surface detergent, meq. ?( 10-3,' EII, tension, mv. dynes/cm. liter
Removed,
% 20
27 36 44 34
Detergent, P.P.M. Computed Detd. 0 0 4.0 2.0 8.0 3.5 20.0 10.0 40.0 28.0 80.0 63
p.p.m.
2' 0 4.5 10.0 14.0 17.0
Mixture after 6-Hour Aeration Detergent removed, Removed,
Removed,
Determined,
9%
p.p.m.
p.p.m.
..
0 1.5 2.0 9.0 14.0 39.0
..
50 56 50 35 21
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
2.5 6.0 11.0 26.0 41 .O
%
..
62 75 55 65
51
Vol. 48, No. 2
STREAM POLLUTION PROBLEMS knowledge that no foaming or surface active agent was contained in the feed. This system was studied extensively again as a batch operation. The first study was one in which the suspended solids concentration was constant while the detergent ooncentration was varied. Values typical of several experiments are given in Table V.
Table VII.
Aeration Time, Hours 0
Froth Formation in Glucose-Activated Sludge System
(Suspended solids in aeration liquor 1450 p.p.m.) Nacconol Added t o Feed, P.P.M. 0 5 10 25 50 Height of Froth, Inches 0
0
0
0 0
0
2 3
4
0
5
0
0 0
1
0 0
0 0 0 0 0 0
0 0 0 0.25 0.37 0.50
100
4 2
0 0 . 2 5 collar 0.50 collar 0.62collar 0.50collar 0.50collar
collar
1 . 5 collar
1 . 5 collar 1 . 0 collar 0 . 7 5 collar
Glucose Sysiem
I20 tern#Nacconol ‘Consfon1 ‘I
A - A t S t a r 1 o f Aeration D-Afrer5#.
2.5
Aerotion
~’ -.
r‘
15
0
c
I
05
As shown in Table V, the oxidation-reduction potential of the mixed liquor rises slowly but consistently with increasing aeration time. The concentration of Nacconol in the supernatant liquor had little or no effect on the potential. The surface tension variation of the feeds and supernatant liquors (Figure 2) followed the expected pattern. As the detergent concentrations of the samples were increased, the surface tension values were lowered. Again as the aeration time lengthened and the adsorption of detergent by the sludge progressed, the surface tension went up. Table VI1 shows the froth formation throughout the first 5 hours of the experiment. Within the first hour the mixture containing the 50 p.p.m. Nacconol feed started t o froth, and after the second hour a thin layer of stable froth appeared in the mixture originally having the 25 p.p.m. feed. For these t n o , the froth increased very slowly but steadily during the run. The mixture prepared from the 100 p.p.m. Nacconol feed frothed throughout the entire run, but gradually decreased in height as the aeration continued. Kone of the mixtures containing less than 25 p.p.m. Nacconol showed any froth after 5 hours. The variation of detergent concentration and detergent adsorption is described in Table VI11 and in Figure 3. After an aeration of 20 hours, between 45 and 60% of the Nacconol initially present was adsorbed by the sludge. More than half of this adsorption took place immediately upon mixing the sludge and feed. Thus with all the effects previously noted, particularly the development of foam only after considerable aeration, there is a concurrent decrease in dissolved detergent. Experiments were also run on the glucose system in which the suspended solids concentration was varied while a fixed concentration of 24 p.p.m. Nacconol was included in the feed (Table IX).
February 1956
I 0
-,-.-.-.
, .
/.2140
K , - . 2 8 0 0 1
Allowing for 20% return sludge, this means that each mixture initially had a Nacconol concentration of 19.2 p.p.m. The sludge used had an oxidation-reduction potential of 570 mv. and the dextrose feed with the Nacconol had a potential of 440 mv., so t h a t the introduction of increasing amounts of dextrose sludge raised the potentials of the mixtures. As shown by Figure 4, increasing suspended solids concentration consistently results in the adsorption of larger and larger amounts of Nacconol from the supernatant liquor. The surface tensions of the supernatant liquors correspondingly increased with increasing amounts of suspended solids in the mixtures. Again it was found t h a t the slow, gradual change in oxidation-reduction potential during an aeration period apparently offers little insight into the problem. Even the great differences in suspended solids content studied in this experiment resulted in only small variations in potential. The froth picture described in Table X and Figure 5 for this experiment is interesting because i t confirms results obtained previously. Over the course of the 5-hour aeration period the following trends were observed. The foams of the mixtures of 0 and 360
INDUSTRIAL AND ENGINEERING CHEMISTRY
249
Table VIII.
Detergent Removal with Glucose-Activated Sludge (Suspended solids i n aeration liquor 1450 p.p.m.)
Nacconol Added t o Mixed Liquor, P. P.M.
.4t Start of Aeration P.P ni.
Table IX.
TO.
susp. solids, p.p.m.
Feed 1 2 3 4 5 6
Table X.
After 5-Hour Aeration Removed,
0 280 GOO
1350 2040 2630
Decrease
%
Removed, P.p.m.
Effect of Variation of Suspended Solids on Glucose-Activated Sludge System (19.2 P . P . ~ Kacconol . added t o aeration liquor) At Start of Aeration Surface Detergent, SUSP. EH, tension, meq. X l O - a / solids, En mv. dynes/cm. liter p.p. ni mv. 45.0 37.32 33.12 49.0 20 i30 29.80 50.8 450 520 28.72 530 50.0 1000 22.52 530 52.3 2550 20 96 2240 530 57.1 15.00 2970 60.0 530
..
.
Froth Formation with Variation in Suspended
0 1 2
3 4 5
10
.4verage Suspended Solids Concentrations, P . P . X . 365 830 1450 2140 2800 Height of Froth, Inches
3.0 1.5 2.0 3.5 2.0 3 5 2.0 3.5 2.0 3.0 1.75 3.0 Thin froth
0.37 0 0.75 0 0.75 0 1.25 0.87 0 1 5 0.75 0.12 1.5 1 .o 0.25 Creamy stable froth
0.5
i.0 1.0
0 0 0 0 0
0
p.p.m. suspended solids w r e both higher than the others; but as the aeration continued, they became flimsier and decreased in height. For the 830 and 1450 p.p.m. mixtures, little foam formed initially, but it increased steadily during the aeration. There was no foam initially a t the surface of the mixtures containing 2100 and 2800 p.p.m. of suspended solids and this condition continued throughout the entire aeration period for the 2800 p.p.m. solids mixture; with 2100 p.p.m. suspended solids, a thin collar of foam appeared a t the surface after 3.5 hours. Thus a tendency t o foam upon extended aeration occurs even as detergent is being further adsorbed from the aqueous phase of mixed liquor,
250
I
After 5-Hour Aeration Surface Detergent. tension, iileq X 1 O - a / dynes/cm. liter 5i.6 54 2 53 2 56 2 59 3 63 1
-477. susp.
aolids p.p.m.
2s:72
10
2B 48
365 830 1450 2140 2800
23.40 21.40 15 44 13 92
Conclusions
Solids Seration Time, Hours
%
Decrease
After 20-Hour Aeration Removed, P.p.m. Decrease %
The most satisfactory way t o express detergent content in a sewage is as milliequivalents of anionic detergent per liter, as it makes possible comparison of results given by different experimenters. This is possible through standardization of the cetyltrimethglammoriium bromide. During a n aeration cycle in a n activated sludge plant, much of the detergent is rapidly adsorbed by the sludge, while the surface tension of the supernatant liquor gradually- increases. The oxidation-reduction potential of the mixed liquor rises slowly as the aeration progresses. Under the conditions of experiments frothing seems to be independent of surface tension and oxidation-reduction potential. No explanation of the late appearance of froth in the aeration tank is given by the measurements made up t o this point. literature Cited (1) Edwards, G. P., and Ginn, M. E., Sewage: and Znd. Wastes 26, 945
(1954). RECEIVEDfor review April 2 5 , 1955. ACCEPTED Allgust 9 , 1955. Investigation supported by a research grant f r o m the National Institutes of Health, Public Health Service.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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