December, 1930
INDUSTRIAL AND ENGINEERING CHEMISTRY
1343
Comparative Rates of Stream Purification under Natural and Controlled Conditions' H. W. Streeter
u. s. P U B L I C HEALTHSBRVICE,
CINCIXX.4T1, O H I O
Observed r a t e s of progressive reduction in the bionearest approach to an ideal HE natural purification chemical oxygen d e m a n d of t w o highly polluted measure, as it would elimiof all polluted bodies s t r e a m s , one a large natural river ( t h e Illinois) a n d nate all of the uncertainties of water is manifested t h e o t h e r a small artificial s t r e a m , are compared, b o t h and sources of error entailed by two phenomena of paras between themselves a n d w i t h corresponding rates obin applying such a test. It ticular interest to the sanitatained u n d e r laboratory conditions. has not been followed, howrian-namely, by a progresT h e r a t e s observed i n the extreme u p p e r section of ever, to any considerable exsive reduction in the density the Illinois River, a l t h o u g h greater than t h o s e f o u n d in tent, because our knowledge of sewage bacteria; and by a the laboratory, a p p e a r t o b e explained largely by the of the rate of re-aeration of gradual oxidation of certain presence of a considerable proportion of readily oxidiznatural streams under variorganic substances, originatable m a t e r i a l having an i m m e d i a t e oxygen d e m a n d . ous conditions is very limited, ing in polluting wastes, to T h e r a t e s f u r t h e r downstream in t h e Illinois River a n d Although an effort was made fairly simple oxidized prodw i t h i n the p r i m a r y zone of carbonaceous oxidation apseveral years ago, in connecucts of carbon and nitrogen. proached very closely t h e laboratory rate u n d e r similar tion with a study of the natuAs the former is of interest t e m p e r a t u r e conditions. Aside f r o m t h e extreme u p p e r ral purification of the Ohio chiefly to the bacteriologist section of t h e artificial channel, in which t h e r a t e of River (6), to m e a s u r e the and the latter to the chemist, oxidation appears t o approach that of artificial sewagerates of re-aeration in various attention will be confined in t r e a t m e n t processes, t h e rates measured in t h e c h a n n e l stretches of that stream, and this paper to that phase of were of a b o u t t h e same order of m a g n i t u d e as t h o s e estimates of the rate of restream purification which is n o t e d in the u p p e r section of the Illinois River u n d e r aeration of the Illinois River concerned with the oxidation comparable densities of pollution. have been made more recently of organic matter. Various m e t h o d s for measuring s t r e a m oxidation a r e (5),little, if any, further work I n undertaking to evaluate discussed. has been done on this imstream purification, it is a portant question. matter of Drimarv imDortance With the foregoing limitations in mind, it is proposed to to ascertiin, if iossible, the rate at which oxidation occurs. Theoretically, there are three possible methods for measuring discuss briefly the results of some observations which have this rate, two of which depend upon a direct application of been made, in connection with studies of stream pollution, of the well-known laboratory test for biochemical oxygen de- the comparative rates of oxidation in the Illinois River, a mand, whereas the third involves this test only indirectly. highly polluted natural stream, and in an experimental stream channel, in which the density of pollution was of about the Stated briefly, these three methods are as follows: (1) Measurement of the rate of deoxygenation, as observed same order of magnitude as that of the Illinois River and in in samples of the stream water incubated at a standard tempera- which the conditions of flow and pollution were subject to ture for various periods of time, thence assuming that the same artificial control. Although as yet these results have been specific rate holds true in the stream. only partially analyzed, they have afforded interesting evi(2) Determination of the rate of decrease in the observed oxygen demand of the stream water at successive points separated dence relative to the influences modifying oxidation pheby known times of flow, assuming that the reduction observed in nomena in streams. the stream between two given points is a measure of the amount As a measure of the specific rate of carbonaceous oxidation, of oxidation accomplished in that river stretch. the numerical value of a coefficient, hereafter denoted as (3) Estimation of the rate of deoxygenation in the stream as being represented by the algebraic difference between the known "K1," may be used as given in the relation:
T
or assumed rate of re-aeration and the observed rate of change in the dissolved-oxygen content of the stream water.
The first and second methods, it will be noted, involve assumptions relative to the applicability of laboratory tests of oxygen demand as a measure of the extent and rate of deoxygenation in streams. Such assumptions are subject to possible error in some instances, as examination of samples of the supernatant water of streams does not always tell the whole story as to the forces concerned in deoxygenation-as, for example, where sludge deposits occur, or where adsorption of organic matter from the flowing water by plankton growths attached to the bottom takes place. If, moreover, the oxidation in a given test stretch of a stream undergoes a transition from the carbonaceous to the nitrification stage, as noted by Theriault ('7, p. 131), the average rate of deoxygenation in such a stretch may be virtually indeterminate. The third method, which does not involve the direct a p plication of the oxygen-demand test, would be in theory the Received September 20, 1930. Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September S to 12, 1930. 1
log
L -
= -&t
(1) La in which La represents the initial oxygen demand and L the residual demand after time t. The value of K1 a t 20" C., as a measure of the rate of deoxygenation of samples of sewage and sewage-polluted water, was originally formulated by Phelps (4), in connection with the relative stability test as approximating the figure 0.1, when the time t is expressed in terms of days. This figure was closely confirmed by the extensive studies of Theriault. At temperatures other than 20" C. the value of K1 was found experimentally by Phelps and the writer to be governed very nearly by the relation:
This relation also was confirmed closely by Theriault's studies. Rates Observed u n d e r N a t u r a l Conditions
Bearing in mind that the laboratory value of K 1 has been established as being about 0.1 at 20" C., an effort was made,
INDUSTRIAL AND ENGINEERING CHEMISTRY
1344
k s t , to estimate the value of this specific rate coefficient in the Illinois River, from observations of &day oxygen demand made by the Public Health Service in 1921-22 at a series of sampling stations separated by known times of flow and ranging in location from a point immediately below Joliet (Station 286) to a point opposite Chillicothe (Station 179) just above Peoria Lake. A semi-logarithmic plot was made of the average oxygen-demand values observed in this &etch of the river during a period of 12 months extending from Sep(00
-
I
I
I
1
I
,
F/G I
50M E A N T E M P 12 L A B O R A T O R Y VALUE
E
75
0~fi-o.071
-
20-
9
T
I
tember, 1921, to August, 1922, inclusive, with mean times of flow below Station 286 as abscissas. I n this plot (Figure I) the points were shown to follow two distinct slopes, to which lines were fitted. From the respective slopes of these lines the value of K1 in the upper section of the stretch (Stations 286227) was measured as being approximately 0.22, whereas in the lower section (Stations 227-179) its value was much lower, 0.068. At the average river temperature during this year the laboratory value of K1, as computed by Equation 2, would be 0.071, a figure closely approaching that observed in the lower section of the stretch. From the relative values of K1 observed in the upper and lower sections of this river stretch, the specific rate of oxidation in the upper section would appear to have been about three times that observed in the lower section, which in turn was very nearly equal to the expected laboratory rate. A possible explanation of the higher rate in the upper section may be found in two conditions which were present to a much larger extent in this section. One condition was the existence of an immediate oxygen demand in the river water above Station 227, which Theriault estimates has a specific rate coefficient approximating 1.5, or about fifteen times the biochemical (laboratory) rate. If 10 per cent of the total oxygen demand of the river in the upper section were of the immediate variety and if it produced a rate of deoxygenation fifteen times the biochemical rate a t the prevailing average stream temperature (12.7' C.), the weighted mean value of the specific rate coefficient in this section would be expected to be about 0.21, a figure very close to that observed. It is also possible that a second condition-namely, combined sedimentation and adsorption of oxidizable material by attached growths of plankton, which were especially prevalent in the upper section-may have accounted in part for the excess of the observed rate of decrease in this section over that which would correspond to the laboratory rate. During certain individual months, notably in October, 1921, and in May, 1922, the observed rate of decrease in the oxygen demand of the river approached very closely, throughout the greater part of the entire river stretch studied, the rate which would be expected at the prevailing river tempera-
Vol. 22, No. 12
ture if computed from the laboratory value of K1. I n October, for example, the observed rate between Stations 286 and 179 was 0.067, as compared with a computed laboratory rate of 0.074. I n May the observed rate below Station 263 (Morris) was 0.075, as compared with a computed rate of 0.087. I n May the channel of the river was relatively clear of sludge deposits, as the result of floods in the preceding month. Rates Observed under Controlled Conditions
I n considering rates of decrease in oxygen demand observed under controlled conditions, reference may be made to some results obtained in connection with the operation of an experimental stream channel at the Stream Pollution Laboratory of the U. S. Public Health Service a t Cincinnati. This channel comprises a series of forty-eight interconnected metal channels, each 90 feet long, thus providing a total length of 4320 feet. The dimensions are such as to provide a cross-sectional area of flow 2 inches wide and 6 inches deep. During the period of 9 months covered by these observations, extending from November, 1928, to July, 1929, inclusive, a mixture of settled Ohio River water and sewage, in the proportions of 70 per cent of the former and 30 per cent of the latter, was circulated through the channel system at a constant rate of 0.75 gallon per minute, providing a velocity of flow equal to 1.6 feet per minute, which incidentally was sufficiently slow to allow considerable sedimentation. The total period of flow through the entire channel system was 45 hours under these conditions. Within the k s t 90 feet of flow, corresponding to a time interval of about 20 minutes, the extent of bacterial reduction exceeded 90 per cent a t times and averaged over 60 per cent in several months. The conditions of biological growth in this section were very intensive, approaching those of sewage filters in this respect. Sedimentation also was very active i n this section. I n the sections further downstream the rate of bacterial reduction diminished progressively along a curve resembling in form those observed in natural streams (1, 3). Observations of the reduction in the oxygen demand of the channel water were complicated by the unmistakable evidence that the oxidation of organic matter passed through the entire carbonaceous stage in the extreme upper section of the channel and virtually had completed passing through the secondary, or nitrification, stage a t the outlet of the channel system. Thus under the intensive conditions of pollution and selfpurification prevailing in the channel, a cycle of oxidation, requiring more than 20 days (7, Fig. I) under laboratory conditions, was completed in a period of about 2 days. During the period of study when the conditions of flow and initial pollution in the channel system remained constantly as above described, the following mean observations were made at five fixed sampling stations located in the channel, including the inlet and outlet: STATION
TIME OF FLOW
Hours
Av. DisSOLVED
OXYGEN P. Q. m.
Av. DAY COMPUTED B. 0. D. VALUE O F
KI
P. Q. m.
0 (entrance)
0
5.75
16 01
1-90
0.33
6.51
8.15
21.1 0.78
11-75
10
4.19
3.88
28-09
26
5.11
2.04
48-0 (outlet)
45
7.54
1.38
0.42 0.21
The general order of magnitude of the average b d a y B. 0. D. values thus observed in the experimental channel was very similar to those observed in the stretch of the Illinois River embraced in Figure I. I n the channel, for example, they
December, 1930
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ranged from 16.0 p. p. m. a t the entrance to about 1.4 p. p. m. at the outlet. I n the Illinois River they ranged from 14.2 p. p. m. at Joliet to 3.5 p. p. m. a t Chillicothe, the lower terminal of the test stretch. I n the first case, however, the total reduction observed amounted to 91 per cent in less than 2 days, whereas in the second instance it was only 75 per cent in nearly 6 days. The apparent specific rates of reduction in 5-day B. 0. D. in the channels, as measured in terms of the computed value of K1 for each successive interval between sampling stations, ranged from 21.1 in the first 70 feet (over 200 times the laboratory rate) to 0.21 in the lowermost stretch (about twice the laboratory rate). I n the uppermost section, the rate was of about the same order of magnitude as that which has been calculated from the curve of Grant, Hurwitz, and Mohlman (2, Fig. 10) showing the rate of reduction in the B. 0. D. of activated sludge, when aerated for various periods. Values thus calculated ranged from 34.5 to 22.3, for periods of aeration varying from 8 to 40 minutes. I n the lower sections of the channel the apparent rate corresponded roughly to those observed in the upper stretch of the Illinois River. I n Figure I1 is shown a semi-logarithmic plot of the observed rates of reduction in &day B. 0. D. in the experimental channel system, as based on averages for the 9-month test period; also a plot of the corresponding changes observed in the dissolved-oxygen content of the channel water. The proportionate rate of reduction in the oxygen demand of the channel water is shown to diminish progressively in passing downstream. The dissolved-oxygen content followed roughly a typical "sag" curve, reaching a minimum point a t about 10 hours below the inlet to the channel system. That the apparently high rates of satisfaction of oxygen demand in the experimental channel were due in part to sedimentation of suspended oxidizable material and subsequent oxidation of this deposited material over a considerable period of time is shon-n very convincingly by Theriault's analysis of the changes observed in the sludge collected in the uppermost section of the channel (8). That adsorption of such material by attached growths probably also played a part in the changes observed in the oxygen demand of the channel water is suggested in Figure 111, in which data shown in Figure I1 have been replotted on logarithmic paper. The straightline trend of the three points representing observations in the lower three-fourths of the channel thus indicates a rate of abstraction of oxygen demand from the flowing stream corresponding roughly to that which would be given by the Freundlich adsorption equation, y = ax-". The failure of the point representing the average oxygen demand a t Station 1-90 to fall into line with the other three points probably reflects the part played by sedimentation and possibly also by the immediate oxygen demand in the first section of the channel above Station 1-90. Thus, it is conceivable that. in the absence of these factors, the apparent reduction in oxygen demand accomplished in 20 minutes might have required 3 or 4 hours, bringing the observation at this station roughly into line with the other three. As a check on the foregoing observations, an approximate calculation of the rates of deoxygenation occurring in the experimental channel was made according to the third method described in the earlier portion of this paper-namely, by correcting the change in dissolved oxygen observed between successive stations for the amount of oxygen estimated to be absorbed into the stream through re-aeration. Although such a calculation necessarily was a very rough one in the case a t hand, because of the presence of an oxygen demand in the water flowing through all sections of the channel and its continued modification by sedimentation and adsorption, this source of error was reduced t o a minimum by calculat-
1345
ing the specific rate of re-aeration for the lowest section of the channel (Stations 28-09 t o 4&0), in which these two complicating influences were presumably least effective. Except as modified by variations in re-aeration due to biological photosynthesis, the rates of re-aeration thus determined could be assumed to hold approximately for all sections of the channel, in which the flow and other physical conditions affecting the absorption of oxygen from the atmosphere are practically constant throughout the test period. I n making this calculation an equation was used, as originally developed by Phelps and the writer (6, p. 15) defining the curve of resultant progressive change in the dissolvedoxygen content of a stream (in practice, the so-called "oxygen sagJ' curve) in terms of the initial oxygen demand, the initial dissolved oxygen, and the specific rates of deoxygenation and re-aeration, respectively. I n the case a t hand all the terms in this equation were known except the specific rate of reaeration, designated by the symbol K Sin order to distinguish it from the deoxygenation coefficient K1. By substituting the known values for each month, the corresponding value of K1, defining the specific rate of re-aeration, was readily calculated. (For method of calculation see 6, App. C, p. 74.) The results of this calculation gave an average value of Kz approximating 0.25 for the nine individual months of the observations, only two of the nine calculated values diverging widely from this figure. (This divergence apparently was due to the effect of excessive biological photosynthesis during the 2 months in question, giving values of KQapproaching 0.40.) This average value signified that the specific rate of re-aeration in the channel system was about two and onehalf times the laboratory rate of deoxygenation a t 20" C., as defined by a value of K1 equal t o 0.1.
1
0
/O
20
30
J
I
40
50
T / M E O F FLOW / N H O U R 5
Assuming that this re-aeration rate held for other sections of the experimental channel, with the flow and other conditions affecting atmospheric re-aeration remaining constant throughout the channel system, values of the deoxygenation coefficient were calculated for two of the upper sections of the channel (Stations 1-90 to 11-75 and Stations 11-75 to 2&09). Except for 2 of the 9 months, when the calculated rate of deoxygenation was excessively high, the rates calculated for the uppermost section, Stations 1-90 to 11-75, were fairly consistent among themselves and more nearly constant than the corresponding rates based on the observed reduction in oxygen demand. The net reductions in 5-day oxygen demand, as calculated from the former rates, likewise were more nearly constant than were the observed reductions, their average deviation from the mean being 17 per cent as compared with 42 per cent in the latter case. I n the next section of the channel downstream (Stations 11-75 t o 28-09)
INDUSTRIAL AND ENGINEERING CHEMISTRY
1346
the same comparative trend of the data was observed. The average specific rates of deoxygenation for the two sections, as calculated from the corrected rate of re-aeration, were 0.47 and 0.58, respectively, when expressed in terms of the rate coefficient, K1. These apparent rates were about five times the corresponding laboratory rate a t the equivalent mean temperature.
T/ME
OF
FLOW
HOUP.5
The situation thus presented, when interpreted in the light of Theriault’s conclusion that the biochemical oxidation of suspended organic matter deposited or otherwise stored in the channel proceeds a t substantially the same rate as under the conditions of the laboratory test for oxygen demand, affords interesting evidence of the extent to which this prolonged oxidation of such deposited material imposes a burden on the more immediate oxygen resources of the supernatant stream. If, for example, no organic material were deposited or adsorbed in the sections of the experimental channel covered by the foregoing observations and deoxygenation proceeded at the laboratory biochemical rate, the net average reduction in oxygen demand to be expected in the section extending from Stations 1-90 to 11-75 during the 9 months in question would have been about 0.5 p. p. m., whereas the observed mean reduction was 4.2 p. p. m. The excess of the observed over the expected reduction, amounting to 3.7 p. p. m., or 88 per cent of the total, represented the draft imposed on the dissolvedoxygen content of the stream in this section to satisfy both the oxygen demand of the deposited material and a certain residual portion of the immediate demand of the supernatant water. As Theriault’s observations have indicated that the proportion of immediate oxygen demand in the channel water was about 20 per cent of the total demand, this portion, if satisfied at a specific rate about fifteen times the biochemical rate, would be estimated, in the case at hand, as being about 2.0 p, p. m., or 48 per cent of the total reduction observed. The remainder, amounting to 1.7 p. p. m. or 40 per cent of the total, would appear to represent the oxygen demand exerted by the material deposited and adsorbed in the channel. I n this connection it is of interest to note that, in a report made in 1925 by the Engineering Board of Review of the Sanitary District of Chicago, it was estimated (5) that the oxygen demand exerted by the settlable solids in the upper portion of the Illinois River, where sludge deposits were extensive and attained a considerable depth, was about 35 per cent of the total oxygen demand exerted by the sewage discharged into the river, It thus appears that the percentage figure obtained from the observations in the experimental channel, under conditions of pollution roughly similar to those prevailing in the Illinois River, does not differ materially from that estimated for this stream, in spite of the wide difference existing in the two cases with respect to the depth of the sludge deposits. Although the apparently close agreement between the proportions of the total-oxygen demand exerted by sludge deposits in these two instances may be to some extent fortuitous, the general parallelism observed between the two cases in other respects would seem to indicate that it is significant, particularly as regards the relatively small in-
Vol. 22, No. 12
fluence of sludge depth on the oxygen demand exerted by such deposits in a given time. Conclusion
The foregoing observations have included only a small portion of those available for a more thorough and comprehensive analvsis, ” , which it is hoDed to undertake during the next year or two. From these observations the foll&ing tentative conclusions, subject, to future revision, may be drawn: (1) Rates of oxidation observed in a highly polluted natural stream, such as the upper Illinois River, appear to be fairly similar, under approximately parallel conditions with respect to intensity of pollution, t o those observed under controlled conditions in an artificial channel carrying a mixture of sewage and settled Ohio River water. (2) The apparent rate of reduction in the biochemical oxygen demand of the supernatant stream, as measured by the observed reduction in the demand between successive samdinrr stations, does not appear to represent the true rate of deoxygenation, which is compounded of the rates of satisfaction of (a) the biochemical oxygen demand of the material carried in suspension and solution by the stream proper, ( b ) the immediate oxygen demand of the stream, and (c) the oxygen demand of oxidizable material deposited and adsorbed in the stream channel. (3) I n a highly polluted stream receiving “stale” sewage and subject to sludge deposits, a very large proportion of the total draft made on the immediate oxygen resources of such a stream may be required in order t o satisfy both the immediate oxygen demand of the stream and the more prolonged demand of deposited and adsorbed material in the channel bed. (4) Although the data bearing on the true rate of biochemical oxidation under natural stream conditions were not sufficiently extensive t o permit a definite conclusion on this point, such evidence as was available appeared to indicate that, in the carbonaceous oxidation stage, it follows closely the rate observed in the laboratory test for biochemical oxygen demand.
I n connection with these tentative conclusions careful distinction should be made between the true rate of oxidation, as determined by the total amount of material oxidized in the entire stream, including its channel, in a given unit of time, and the rate a t which oxidizable material is removed from the stream proper, as indicated by the observed rate of decrease in its measurable oxygen demand. I n the former case full account must be taken of the progressive oxidation of deposited and adsorbed organic matter during a prolonged period of storage in the channel bed. I n the latter instance we are concerned only with the temporary removal of oxidizable material, both biochemically and physically, from the stream proper. Although this may provide an index of the momentary efficiency of natural purification, it may give a very distorted picture of the true rate a t which the total-oxygen demand of the stream is being satisfied. As this true rate is the key to an accurate measurement of the re-aeration capacities of streams, its more exact definition will be necessary before an effective attack can be made on this problem. It is largely through laboratory studies such as those described by Theriault and by Purdy and Butterfield, in their respective papers, and by systematic efforts to apply the results of such studies to field conditions, that further advances may be made toward a solution of the problem. L i t e r a t u r e Cited Frost and Hoskins, U. S.Pub. Health Service, Pub. Health Bull. 143, 278 (1924). Grant, Hurwitz, and Mohlman, Sewage Wo‘orksJ., 2, 228 (1930). Hoskins, Ruchhoft, and Williams, U. S. Pub. Health Service, Pub. Health Bull. 171, 186 (1927). Phelps, Intern. Cong. Appl. Chem., XXVI, 251. Sanitary District of Chicago, Rept. of Engineering Board of Review, Pt. 111, App. I, Sewage Disposal (February 21, 1925). Streeter and Phelps, U. S. Pub. Health Service, Pub. Health Bull. 146 (1925). Theriault, I b i d . , 178 (1927). Theriault and McNamee, IND.ENQ.CHBM.,22, 1330 (1930).
