BOD LOADING AS A ACTIVATED
BASIS OF DESIGN FOR SLUDGE PLANTS
by
JOSEPH F. BERBERICH B. C. E.
Rensselaer Polytechnic Institute
1944
SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE IN SANITARY ENGINEERING from the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Signature of Author
1947
Signature redacted
Department of Civi nd Sanitary EngineeringCertified
by
Signature redacted
Thesis Supervisd
Chairman of Departmental Comittee
on Graduate Students
pnature
redacted
Cambridge, Mass. May 1947
Professor Joseph S. Newell, S.B., Secretary of the Faculty,
Massachusetts Institute of Technology, Cambridge 39, Mass.
Dear Sir:
In partial fulfillment of the requirements for the degree of Master of Science in Sanitary Engineering from the Massachu-setts Institute of Technology, I present this thesis entitled
"BOD Loading As A Basis of Design For Activated Sludge Plants."
Signature redacted
Fh . Brberich
ACKNOWLEDGEMENT
The author is deeply indebted to the various State Sanitary Engineers and the operating officials of the plants which provided source material for this thesis and to Professor William E. Stanley for his kindly help and suggestions during
TABLE OF CONTENTS
Page
I.
Synopsis . . .1
II. Previous Work on BOD Loading For Activated Sludge Plants ...
3
1. History of Activated Sludge . . . . .
3
2. Activated Sludge Process Theories . . .
3
3. Oxygen Requirements . . .
4
4.Design
Factors . . .6
III. Fundamental Considerations . . . 9
1. Criterion of Loading - BOD . . . 10
2. Air Requirements of the Process . . . . 12
3. Importance of SS Concentrations . . . . 14
4. Nitrification . . . 15
IV. Investigational Procedure . . .
16
1. General Method . . . 16
2. Scope of Investigation . . . . . . . 18
3. Method of Making Computations . . . . 20
4. Creditability of Data and Results . . . 22
V. Results . . . 23
1. Air Quantities . . .
24
2. BOD Removal Related to Duration of Aeration Period . . . 32
3. Effect of Per Cent of Returned Sludge and Suspended Solids Concentration . . , . .
36
4. Nitrification . . . . . . . . . . 48
5. Loadings and Plant Performance . . . .
56
6. Design Charts. . . . . . . . . .
63
VI. Discussion-of Results. . . .
69
1. Air Quantities . . . 69
2. Effect of Aeration Period . . . 70
3. Effect of Returned Sludge and Suspended Solids Concentration . . . 71
4.Nitrification
. . . 745. Loadings and Plant Performance . . . . 77
6.
Design Charts. . . . 79TABLE OF CONTENTS (CONT.)
Page
VIII. Bibliography . . . . . . . . .. ...6
84IX.
References . . . . . . .86
X. Appendices . . .
88
A. Plants Used In This Study . . . 88
B. Extracts of Operating Data . . . 99
C. Extracts of Replies From State Sanitary Engineers . . .
105
D. Correspondence from Designing Engineers and Plant Officials . . . . . . . . 122
Figure
1
Relation
#
BOD Applied per 1000 cu ft of Air vs.
Effluent BOD - PPM . . . . . . . . . . .
2
BOD Loading
-
#
BOD Applied per 1000 cu ft of Air
vs. Per Cent Reduction of BOD . . . . . .
3
Relation Between BOD Removal and Air Used
. .4
Relation Between Air Supply and Effluent BOD
5
Relation Between BOD Removal and Aeration Period
6 Aeration Period vs. Reduction of BOD . .7
Relation Between Per Cent Returned Sludge and Per
Cent Reduction of BOD . . . . . . . . . .8
SS Mixed Liquor per PP$1 of BOD Applied vs. Per Cent
Reduction of BOD . . . . . . . . . . *
9
SS Mixed Liquor per PPM BOD Applied vs. Effluent
BOD - PPM . . . . . . . . . . . . . .
10
Relation Between SS Mixed Liquor and Per Cent
Reduction of BOD . . . . . . . . . . . .
11
Relation Between SS Mixed Liquor and BOD Removal
12
BOD Loading
-
# BOD Applied Daily per PPM SS per
1000 cu ft of Tank Capacity x 1000 vs. Per Cent
Reduction of BOD . . . . . . . . . . .
13
BOD Loading
-
# BOD Applied Daily per PPM SS per
1000
cu
ft of Tank Capacity x 1000 vs. Effluent
BOD -
PPM
. . . . . . . . . . .14
Relation Between Nitrification and Per Cent
Reduction of BOD
.
. . . . . . . . . . .15
Relation Between Aeration Period and Effluent
Nitrate Content . . . . . . . . . . .
16
Relation Between BOD Loading-K"" and Nitrification
17
Relation Between BOD Loading-K"' and
Nitrification
18
Relation Between BOD Loading-K" and Nitrification
Page
25
26
28
30
33
34
37
39
40
41
42
43
48
49
51
52
54
INDEX OF FIGURESFigure
INDEX OF FIGURES (CONT.)
19 BOD Loading -
#
BOD Applied vs. Effluent BOD 20 BOD Loading - # BOD Applied per 1000 cu ft vs.Per Cent Reduction of BOD . . . . . . . 21 BOD Loading - K"' -
#
BOD per day per 1000 cuft of Tank per Hour Detention Time vs. Per CentReduction of BOD . . . . .
22 Relation Between BOD per day per 1000 cu ft of Tank Capacity per Hour Detention Time- K"' vs. BOD of Effluent-. PPM . . . . 23 Relation Between BOD Loading - K' and Effluent
BOD-
PPM
. . . . . . . *24
K' vs. Kn
25
K" vs. K"'
57
58
59
60
64
65
66
67
* . . . . 0 6 0 0 * 0 * * S 0 0 0 0 6 0 0 0 0 026
K"' vs K""
. . ..
.
.
C-1 Letter to State Sanitary Engineers . . * . D-1 Relation of Concentration of Activated Sludgeto Sludge Activity- Plate No. 18 . . . D-2 Aeration Tank Loading Studies - Raw Sewage
Population Equivalent vs. Loading - lb. BOD/
1000 cu ft/Day (Peoria,
Ill.)
. . . . .D-3 Aeration Tank Loading Studies- Final Effluent BOD and Ou Ft - Air per lb. Applied BOD vs. Aeration Tank Loading- lb. BOD/1000 cu ft/Day
(Peoria,
Ill.) .
.
.
.
.
.
.
.
.
.
.*
106
125
126
127
PageINDEX OF TABLES
Table
A Effect of Air- Parameter
#
B.O.D. Applied per1000 cu ft of Air . . . . * . * .
27
B Effect of Air- cu ft of Air per#
of B.O.D.Removed . . . 29 C Effect of Air- cu ft of Air per gallon of Sewage 31 D tffect of Aeration Period . . . . . . . . 35 E Effect of Returned Sludge and Suspended Solids
Concentration per PPM BOD Applied . . .
44
F Effect of Suspended Solids Concentration InAeration Tank Liquor- BOD Removal PPM per PPM S.S.
Mixed Liquor
.
. . . , . . , . . 45G BOD Loading Parameter Including the Effect of Suspended Solids Concentration in Relation to the
Purification Effected by the Process . . .
46
H Relation of Aeration Period, Nitrification andPer Cent Reduction of
BOD . . .50
I Nitrification and BOD Loading Using Parameters
K"" and K"' * . - - . * . * - . - a . 53
J Nitrification and BOD Loading Using Parameters
K"' and K" - . . . - - - . 0 . * . . 55
K BOD Loading and Plant Performance Using Parameter K"" . a
..0 ..
61
L BOD Loading and Plant Performance Using Parameter
K"t . . .
62
M Basic Data for Design Charts Shown in Figures 22
thru 26 .. . . . . .
68
Extracts of Operating Data - Appendix B . . . 100-104 Page
I. SYNOPSIS
The activated sludge method of sewage treatment has been accepted and used in the field of sanitary engineering. However, the process has not yet been completely understood especially with respect to loadings which may be imposed upon the several plant elements which comprise a sewage treatment plant employing this process. It has been the purpose of this thesis to investi-gate the loadings of various representative operating activated sludge plants with the objective of determining the factors which influence the permissible loading and their effects upon plant operation and from the study to resolve a method of design for activated sludge plants based upon BOD loading.
Correspondence was carried on with the various State Sanitary Engineers for tie purpose of determining the results of their experience and if any regulations have developed employing the cncept of BOD loading as applied to activated sludge type of sewage treatment plants. Also it was desired to obtain from the State Sanitary Engineers lists of those activated sludge plants in their respective states which were under good technical control and supervision and which might provide reliable data from which analyses could be made. Such data were secured and analyzed for thirty operating reports from plants which ranged in capacity from 0.348 to 280.0 MD located in 12 states.
Various loading parameters have been developed including pounds of BOD applied per day per 1000 cubic feet of aeration tank capacity as the basic unit. This has been further modified
by the effect of detention time, quantities of air used and the suspended solids concentration in the mixed liquor through the aeration tank.
From this investigation, it has been found that, in con-ventional types of plants, the BOD loadings in terms of pounds of BOD per 1000 cubic feet of aeration tank for different plants has varied from about 12. to about 52. Minimal per cent reduction of BOD has been obtained with loadings in the magnitude of 30 to 35
lbs of BOD per 1000 cubic feet of tank capacity. On the basis of the present BOD Test, this appears to indicate that these usual loadings may not be the most effective loadings.
Decreases in nitrification have resulted from increased BOD loading. In the conventional type of plant definite increases in purification have resulted from increased aeration periods. Increased purification can be expected if the suspended solids concentration in the mixed liquor is increased, though the BOD loading is held constant.
It appears that activated sludge plants may be designed and their conditions of operation indicated by utilizing the functional inter-relation of complex loading parameters.
It has been determined by the replies from State Sanitary Engineers that BOD loadings have not yet become a standard utilized by the state departments of health in reviewing the design of acti-vated sludge type of sewage treatment plants.
iiwiima
3
II. PREVIOUS WORK ON BOD LOADINGS FOR ACTIVATED SLUDGE PLANTS
1 - History of Activated Sludge.
Activated sludge, as a treatment process, is relatively new when compared to the other common methods of sewage treatment. The purifying effect of organism growths in sewage was first noticed by Clark at the Lawrence Experiment Station in 1912 when he found that containers holding sewage with growths on the side thereof' caused more purification than those containers without such growths. However, he poured out the contents each time and thus missed the discovery of the added purification effect of the accumulated solids. Ardern and Locket in England studied this phenomena but they let the solids accumulate and thus developed the true activated sludge process.
In further studies the carbon and nitrogenous stages of stabilization were discovered. The relative accelerating in-fluence of activated sludge upon the purification of sewage was
studied and reported by the Illinois State Water Survey in
November 1914. Since about 1925 a great many activated sludge type sewage treatment plants have been built and are in successful
operation.
2 - Activated Sludge Process Theories.
Various theories have been presented to explain the mechanism of the activated sludge process. Theriault (1,2,3,4) proposed the idea that the clarification accomplished by the process can be explained on the basis that the gelatinous film or covering of the clumps of activated sludge act in the manner of zeolites. He based
this on the finding of inorganic zeolite in activated sludge capable of adsorbing ammonia and organic compounds.
Buswell (5) proposed that the process is of a biological nature with the bacteria in the sludge either absorbing the soluble forms of organic matter directly into the cell body or depending upon extra cellular enzymes to render dialyzable those colloidal particles which are too large to be directly absorbed.
Wooldridge and Standfast (6) held that the purification process depends upon the presence of oxidative enzymes which may be effective in oxidizing the constituents of sewage even though the organisms producing the enzymes are dead; providing that the manner in which the organisms were killed did not destroy the enzymes.
Parsons and Wilson (7) ascribed definite importance to adsorption phenomena in reference to the clarification ability of activated sludge. They found application of the Freundlich equation based on their experiments.
Other investigators have proposed that iron salts form the nucleus of the activated sludge floc in which the iron acts as a coagulant. The purification effected by the activated sludge process was explained by Baly on the basis of a physiochemical reaction depending upon the neutralization of the charges on the sewage colloids.
3 - Oxygen Bequirements.
Bloodgood (8) has done considerable work on the oxidation of activated sludge. He developed a machine to measure the oxidizing capacity of activated sludge and developed two concepts. One
hour by a 0.50 per cent solids sludge which has had its normal period of aeration since being mixed with sewage.
Another concept is "sludge activity" or the rate of oxygen consumption, less the sludge demand, when synthetic sewage and sludge are combined to form a 0.50 per cent mixture, also expressed
in ppm per hour. Bloodgood concluded that the oxygen demand of a sludge is a measure of the unoxidized matter that it contains and also that the sludge activity gives the rate at which sludge can be expected to purify and whether or not it can be expected to settle well.
In the consideration of air requirements for BOD reduction, Miles and Porges (9), working with sulfur dye wastes, found that the maximum reduction of BOD in
4
hours was 84% whereas the same amountof air applied over a period of 6 hours provided a maximum reduc-tion of 97.5% of the BOD. Studying samples and various amounts of air, it was found that the effect of the different quantities of air, as meaoured by the BOD removal, was very slight. It is of interest to note that they were working with air quantities in excess of that of actual practice i.e. quantities in the order of 2.5 cubic feet of air per gallon of sewage.
Freese (10) divided the aeration part of the activated sludge process into two stages: 1) a short period of about one hour during which there is rapid coagulation and adsorption, which reactions are proportional to the amount of biologically active sludge that has been returned; 2) a second stage of purification much slower and retarded by the oxygen demand of the return sludge in the mixed liquor.
e
Freese stated that the optimum per cent of returned sludg6 would be that which results in the minimum time of aeration for the sum of the above two stages, but states that too low a return will render the mixed liquor inactive. He also states that the quantity of air required for the process depends upon the air required to agitate the sewage sludge mixture and the amount needed to satisfy and maintain the sludge activity. This latter amount is approxi-mately 10 times the amount needed to satisfy the BOD of the sewage.
He further states that for spiral flow the total air required ' amounts to 0.0038 to 0.0054 cubic feet per gallon per ppm of 5 day BOD removed in the overall treatment process. He deemed the above requirements to be independent of the aeration period, amount of sludge return, depth of tank, reaeration and preliminary treat-ment by settling or screening.
4 - Design Factors.
Greeley (11) in reviewing the then (1945) current basic factors used in the design of activated sludge plants cites the following loading yardsticks:
a. Pounds of BOD applied to the aeration tanks per 1000 cubic feet of tank capacity.
b. Pounds of BOD applied per 1000 cubic feet of air supplied or 1000 cubic feet of air supplied per pound of applied BOD.
c. The ppm of suspended solids in the mixed liquor in the aeration tank.
1. With average domestic sewage- 25 to 30 pounds of
applied BOD/1000 cubic feet of tank.
2. With normal sewage- 1 to 2 pounds of BOD applied
per 1000 cubic feet of air.
3. Suspended solids in mixed liquor from 1,500 to
2,000
ppm
with tendencies toward lower
concentra-tions of say 1,000 to 1,500
ppm.
Edwards (12) statedin regard to suspended solids content
of the mixed liquor, that no more should be carried than the
amount needed to purify the sewage properly; that the
concentra-tion of
S.S.
in aeration tanks seems to be related to the
volatile solids in the sewage. Evidence seems to show that if
the per cent of volatile solids is high the concentration of S.S.
in aeration tanks should be low since sludge with high volatile
content is bulkier and settles less rapidly and less readily
than one with a high ash content and for that reason may yield
a poorer effluent. Sawyer (13) found that the lower the
volatile solids are the higher the concentrations of solids that
may be carried and vice versa.
The National Research Council (14) reported that, among
the sewage treatment plants on military
installations utilizing
the activated sludge method of sewage treatment, the air supply
per square foot of tank surface varied from 6.1 to 13.0 cu. ft.
The BOD loading in terms of cubic feet of air per pound of BOD
applied varied from 1,230 to
1,330,
excepting Shelby No. 2
plant in which 2,470 cu. ft. of air were supplied per pound of
to those per pound of BOD applied ranged from 1.07 to 1.29. Relative to returned sludge, the N.R.C. report shows that at Fort Monmouth the most concentrated returned sludge had a S.S. content as much as 3,848 ppm and that this sludge constituted 31.9% of the total flow. The returned sludges at other plants contained 1,191 to 2,560 ppm of S.S. and constituted between
49.2 and 57 per cent of the total flow. These last mentioned sludges gave between 740 and 790 ml per liter of settleable solids in 30 minutes. Suspended solids content of the mixed liquor ranged from 592 to 1,497 ppm. The average settleable solids in the mixed liquor ranged from 260 to 830 ml.
The National Research Council further reported that the mixed liquor, as calculated by the addition of the BOD of the primary effluent and the BOD of the final effluent as that of the returned sludge, varied from 127 to 217 ppm.
In regard to the loading of the aeration tanks, the National Research Council stated that they varied from 7.0 to 31.5 pounds BOD applied per 1000 cu. ft. of aeration tank
volume. They also presented curves which show a relation between efficiency of aeration and secondary settling considered to-gether and BOD loading expressed on a "SS-hour" basis. The loading unit used was pound BOD aeration tank influent/day/ 1000 pound of SS in aeratin tank/hour aeration. "This method of rating loading reflects both the amount of biologically active material and the effective time for biological reaction" (14).
Harrison, Cockburn and Anderson (15) proposed a concept similar to the one used in the N.R.C Report which they called the
9
coefficient of interfacial contact.
III. FUNDAMENTAL CONSIDERATIONS
In the consideration of a subject such as this it is necessary to examine the many underlying factors which influence the results which can be expected from the process under con-sideration and for which bases of design are being searched. In the process of sewage treatment by the activated sludge process many interrelated factors are encountered. It is the purpose of this section to discuss the many interrelated factors,
the manner in which they enter the process, how they are inter-related, and how they affect the process as a whole.
Briefly, the factors which should be considered may be listed as follows: 1) the organic loading on the process, 2) the amount of air supplied to the process, 3) the detention period which is employed, 4) the suspended solids content of the mixed liquor, and 5) the character of the activated sludge under
con-sideration i.e. its oxidation velocity and nitrification. Sewage treatment, in general, has as its objective the economical removal of putrifactive objectionable matter from
sewage so that the purified fluid may be discharged with impunity into receiving bodies of water.
Just what is the nature of this objectionable matter which constitutes the accepted idea of the loading of a sewage treatment plant? Is it always the same? Does it vary from location to location? Briefly, it is composed of the wastes usually organic,
10
which result from the metabolic activities and the industrial activities of human beings.
The latter consideration is an important one for though there are variations in the composition of the excreta of various population categories, these are small compared to the problem of plant loading variations due to the composition of wastes of industrial origin. These considerable variations in the composi-tion or nature of plant loading is of importance and will be discussed hereinafter.
1 - Criterion of
Loading
- B DThe present criterion of activated sludge plant loading, now widely accepted, is the Biochemical Oxygen Demand of the
sewage, hereinafter referred to as the BOD. The BOD is a measure of the oxygen required to oxidize the organic matter present in a sewage by means of the action of bacteria.
The BOD test, as now performed according to Standard Methods for the cxamiion of W~a d Sewge, does not take into
con-sideration the effects of concomitant nitrification or lack of it in evaluating the loading and performance of a sewage treatment works. New techniques are being studied (16,17) which it is
expected will improve the value of the BOD test.
Notwithstanding the present deficiency of the BOD test, the data found by it constitutes the best available measure of the strength or more specifically the organic loading that is imposed upon the aeration process of the activated sludge treatment
process. More complete knowledge of the extent of the organic loading by a sewage or waste may ultimately be obtained by
utilizing data obtained from the Chemical Oxygen Demand Test the procedure for which is as yet unpublished.
In reference to biochemical oxidation in relation to activated sludge, Ruchoft (18) reported that, for aeration periods usually found in practice in the activated sludge process, the oxidation velocity was much greater than that which occurs in natural bio-chemical oxidation. With the temperature held constant, it was found that the oxidation rate was apparently dependent upon the BOD of the sewage added to the sludge under study and the condi-tion of the sludge.
This was confirmed by Sawyer (19) who found that the sludge treating the sewage with the higher BOD gave the higher rate of
oxygen utilization. This concept is an important one and should be borne in mind in the consideration of BOD as the basis of design of activated sludge plants.
In so much as the oxidizing capacity of the activated sludge is important in the stabilization of organic matter, it is of importance to note that various factors are important in prolonging the initial high rate of oxygen utilization. Sawyer
(20) showed that the high rates of certain activated sludge
sewage mixtures was due to the oxidation of nitrogenous materials and that the duration of the high rate could be prolonged by the addition of materials which have readily available nitrogen
such as urine, urea etc. He also reported that those sludge-sewage mixtures which have a low degree of nitrification were found to have a high initial rate of oxygen utilization due to carbonaceous matter and that this initial high rate of oxygen
12
utilization could be prolonged by the addition of glucose.
In line with the above data relating to the development of different types of activated sludges it was found that the nitri-fying ability of activated sludges was dependent upon the types of diet upon which they were fed. Sawyer (21) stated that the BOD to ammonia nitrogen ratio in the various diets of the sludges he had under study changed the nitrifying ability of the sludges.
Sludges fed on a diet containing only a low ratio of BOD to ammonia nitrogen developed the greatest ability to oxidize nitrogen while those with a high BOD to ammonia nitrogen ratio
lost most of their ability to oxidize nitrogen. 2 - Air Requirements of the Process
The quantity of air needed for the successful operation of the activated sludge process, at least in relstion to the diffused type of activated sludge plant with which this work is concerned, is dependent upon satisfying two basic needs namely the applica-tion of sufficient air to supply the oxygen required for the
stabilization of the organic matter undergoing oxidation; and secondly that amount of air which is needed to impart motion to the mixed liquor and thus bring new surfaces in contact with the dissolved oxygen.
The first requirement can be further broken down into 1) the oxygen needed to satisfy the BOD of the sewage itself and 2) the oxygen needed to maintain the activity of the sludge. Studies show that if an activated sludge is over aerated the oxidation rate of the sludge slowly falls to that of natural biochemical oxidation. If an activated sludge is under aerated
it has been found that the oxygen requirement of the sludge itself is increased. Also in this case its ability to oxidize new food at high rates is impaired.
In regard to the actual quantity of air required to main-tain sludge activity Freese (10) states that the amount needed for this purpose is approximately ten times that needed to satis-fy the BOD of the sewage. He also states that for a spiral type flow installation with complete treatment, the total air required amounts to 0.0038 to 0.0054 cubic feet per gallon per ppm of 5 day BOD removed in the overall treatment process.
The attempt should be made in the activated sludge process to obtain as fine a division of air into bubbles as is possible in so much as this provides more contact surface for the taking up of oxygen by the mixed liquor. However, there is exerted a negating force constituted by the presence of colloidal
particles which attach themselves to the surface of the bubbles and thus render the dissolving of the oxygen of the air into the mixed liquor more difficult. Freese (10) reported there is
little difference in size of bubbles obtained from plates having a permeability of 15 compared to those obtained from plates having a permeability of 60.
The duration of the aeration period is of basic importance when considering the design of aeration tanks. Miles and Porges
(9) showed the increased value of the same quantity of air when applying it over a longer period of time. Also to be considered is the fact that the aeration phase of the activated sludge process of sewage treatment may be divided into two stages, the first
14
consisting of clarification and the second of oxidation of the adsorbed colloidal organic particles. Sufficient aeration time must be allowed for both stages of the process.
3
- Importance of S.S. Concentrations.The amount or concentration of suspended solids carried in the mixed liquor is another basic factor. This is a measure of the adsorbing material which will be active in the purification
process. It is composed of zoogleal material and forms the matrix for the bacteria which oxidize the organic material in the sewage.
Outside of the suspended solids content of the sewage it-self and the tendency for the agglomeration of the sewage solids into masses which in turn adsorb colloidal particles from the substrate, the suspended solids content of the mixed liquor is largely determined by the per cent of activated sludge returned to the aeration tanks from the final clarifiers.
Freese (10) opined that the short period of coagulation and adsorption of the first stage of the aeration process is dependent upon the amount of biologically active sludge that has been returned to the aeration tanks. The purification phase of the aeration process takes more time than the first stage of coagula-tion and adsorption. This purification is hindered by the oxygen demand of the returned sludge in the mixed liquor. Thus there is an optimum amount of sludge to be returned which results in a minimim time of aeration which will take care of both stages of the aeration process. Both Heukelekian (24) and Ridenour (25)
found that the reduction of BOD increased with increasing sus-pended solids concentrations.
Sawyer's work (22) indicated that comparable BOD removals can be accomplished with a wide range of activated sludge concen-trations; that the use of low concentrations causes sludge to develop with a high volatile solids content and high activity; that high concentrations of activated sludge results in the forming of sludges with lower volatile solids and lower activities. Thus, there is a definite relation between air supply needed and the amount of suspended solids carried in the aeration tank, in as much as the actvity of a sludge can be measured in terms of the
rates in which it utilizes oxygen. Low concentrations of activated sludge solids in the aeration tanks permit larger quantities of BOD to be stabilized by the same quantity of oxygen than if higher concentrations of solids were carried in the aeration tanks.
4 - Nitrification.
The phenomena of nitrification, noted previously is worthy of further consideration. Alert activated sludge sewage treatment plant operators have long realized the effect of nitrification
in relation to the apparent treatment plant efficiency. In the early days of sewage treatment, nitrification was considered the criterion of good sewage treatment. Nitrification indicates that the purification stabilization process of the sewage treatment has been carried to its ultimate point.
However, operating experience has shown that it is possible to decrease the aeration period and air supply and obtain a
resulting effluent with a BOD materially less than obtained with longer detention periods and greater quantities of air.
le
The explanation for this apparent anomoly lies in the two stage nature of the BOD satisfaction curve. If the plant had been operating to produce an effluent at the incipient nitrification point or slightly above, the period of further biochemical oxida-tion now at the natural oxidaoxida-tion rate would carry on into the nitrification stage.
Hence, for best plant efficiency as indicated by the present BOD test, the accelerated biochemical processes of the treatment plant should be stopped at a point such that the further 5 days 200C biochemical oxidation of the sample of effluent in the BOD bottle would carry only to the beginning of nitrification.
This modus operandi provides material savings in the way of power costs otherwise expended for added blower capacity. Reduction in detention time also would increase the capacity of the plant, and further remove the time when additional tank capacity would be required.
IV. INVESTIGATIONAL PROCEDURE
1. General Method.
In an attempt to derive a means of designing aeration tanks on the basis of BOD loading, it is entirely reasonable to draw upon the experience of the presently operating activated sludge plants in as much as the data secured from this source would be representative of the actual conditions which obtain in the field and any conclusions which might be drawn from the analysis of such data would be typical and could be logically applied to new and equally varied situations.
In line with this letter Appendix C was sent to 39 State Sanitary Engineers in states having activated sludge plants with the purpose of obtaining a list of those activated sludge sewage treatment plants being operated under good technical supervision from which might be obtained good operating data. The names and addresses of the operating officials, as well as the designing engineers were requested so that more complete information might be obtained. Replies were received from 34 states or a 87.3%
response.
Also to determine the status of the use of BOD as a basis of design of activated sludge plants as recommended by the
several state departments of health, information was requested of these agencies pertaining to any standards of design which they might have developed. Appendix C.
In accordance with the information received from the Ptate Sanitary Engineers a letter was written to the operating officials of 43 activated sludge sewage treatment plants in the United States. A listing of these plants will br found as a
part of Appendix "D". To supplement the information received from the responses of operators, information was also requested of several of the designing engineers of the plants under considera-tion. In some cases it was necessary 46 continue correspondence with the plant officials to procure information which could not be
found in their routine operating reports. One such item was the nitrate content found in their plant effluent.
Pertinent responses from the designing engineers as well as from those operating officials who exhibited active interest
in the problem of activated sludge plants from the viewpoint of BOD loading will be found as part of Appendix "D".
2 - Scope of Investigation
The operating data secured from the several plants in the form of annual reports proved to be much too voluminous to be in-cluded as such in this thesis. These annual reports also contained much information that did not apply to the phase of the problem being investigated. Accordingly, a careful segregation of the data contained in the reports was made for each of the plants and the resulting selection of data appears as appendix "B". A tabulation of the following data relative to the several plants has been included in this investigation:
1 - Period covered by report 2 - Sewage flow, MGD
3 - Raw sewage BOD 4 - Raw sewage SS
5 - Primary effluent BOD - 6 - Finml effluent BOD
7 - Final effluent SS
8 - Nitrate content of final effluent, ppm 9 - Sludge index of returned sludge
10 - SS of returned sludge
11 - Per cent of sludge returned (of sewage flow) 12 - SS mixed liquor
13 - Detention time of aeration tanks (based on mixed liquor)
14 - Aeration tank capacity - 1000 cu. ft. 15 - Depth over diffusers - feet
16 - Air - 1000 cu ft/day
17 - Dissolved oxygen in final tank effluent 18 - Dissolved oxygen in the mixed liquor of the
aeration tank
From the original
43
plants written to in soliciting data, 27 plants have been selected for study due to the completeness and availability of data there from. The 27 selected plants are located in 12 states and comprise the following:California: Michigan:
Pasadena Ann Arbor
Jackson
Illinois: Pontiac
Chicago, S.W.
Decatur Nebraska:
Peoria Grand Island
Springfield Omaha
Indiana: New York:
Fort Wayne Syracuse
Gary
Hammond Ohio:
Marion Cleveland, Easterly
Richmond
Texas:
Iowa: Austin
Marshalltown San Antonio
Louisiana: Wisconsin:
Lake Charles Madison
Milwaukee
Maryland: Richland Center
Baltimore Two Rivers
Hagerstown
The plants listed above give a fairly good representation of the various different sizes of activated sludge type of plants as found in this country. The plants were selected with the end in mind of obtaining the most reliable data available. These plants were all among those recommended by the State Sanitary Engineers
as being under good technical supervision and control and as such might be expected to produce good, desireable, trustworthy data.
All the plants with the exception of Richmond, Indiana, are
straight diffused air type activated sludge plants. The Richmond plant in addition to using diffused air employs gravity aspirators to supply air to the mixed liquor. So with this one exception, all the plants conform to the standards of the conventional type of activated sludge plant, and it is believed they are very repre-sentative plants. A short description of each one of the above
plants with some of their outstanding features, if any, is presented hereinafter as Appendix "A".
3 - Method of Making Cmputations.
In the calculA'tions .involved in an investigation, such as this it is necessary to start with some specific units of load, time, air, and solids concentration. The BOD unit was taken as the pounds of BOD applied or removed per day. This calculation has been made as follows:
Given: a primary tank effluent containing 154 ppm BOD and a sewage flow of 6.598 MGD
Find: The pounds of BOD applied per day to the aeration process.
Solution:
pa BOD X
1,000,000 6,598,000 x 8.34
X
=
6.598
x154
x8.34
= 8,460 lbs. BOD applied per day
In the calculations made in this investigation the decrease in volume of sewage flow due to the effect of solids removal in the primary sedimentation process has been considered negligible
and has been omitted from consideration in the above and other calculations.
Tank capacities have been expressed in 1,000 cu. ft. for all calculations in which tank capacities appear.
In the course of this study various compound loading units have been developed. The mathematical form might be described
by the loose application of the term parameter in as much as they are functions of variables or parameters. This can be seen from the analogous situation presented by the following equation:
y = ax + b
Here y and x are basic variables and the values of a and b deter-mine the values of y and x. A and b may be termed variable para-meters in so much as they may determine different values of x and
y depending upon their own value. In an analogous way the function of a sewage plant expressed say in the percent reduction of BOD is a function of the BOD load or any more complex unit such as BOD/1000 cu ft of tank. Here the various values of efficiency will be determined by or rather be a function of the complex
loading unit and as such a function of the parameter loading unit.
The theoretical implications of the various parameters
developed will be explained in those sections of this investigation which follow and in which they apply.
Another facet of the problem of BOD loading is what constitutes the total BOD in the aeration tanks. Some investi-gators working with this problem have attempted a solution by using the BOD of the effluent as the BOD of the returned sludge. The National Research Council (14) in their report has done this.
In the absence of data on the BOD of returned sludge in the various plants under consideration, this element of loading is not included in this investigation, so that the loading of an aeration tank has been considered as consisting entirely of the
BOD applied thru the medium of the primary settled sewage or fine screened sewage as the case may be.
4 - Creditabilityo f Data and Results.
It must be borne in mind that the data on which this inves-tigation is based was secured or determined in each case under the direct supervision of the plant operator. So the actual relative value of data stems from the technical qualifications of the operators plus contributing local factors. It is believed that due degree of credence may be given to the data from each plant and the results of analysis thereof in as much as care was taken in selecting the sources of the data. A large amount of
confi-dence was placed in the judgment of the State Sanitary Engineers in their evaluation of the creditability of the reports of the various activated sludge plants. The opinions of the designing engineers were also sought on this matter since it was a physical impossibility for personal inspection to be made of these plants and their testing techniques.
In the case of the securing of nitrate data, a suggested method of analysis was cormunicated to the plant operator. For the case in point, the method presented on page 66 of "Analysis of Water and Sewage" by Theroux, Eldrige and Mallman was
suggested. It is appreciated that this procedure is subject to error due to possible interference with chlorides. However, the data secured by this simplified procedure enables classification of the activated sludge plants into those with strong nitrifi-cation and those with weak nitrifinitrifi-cation.
V. RESULTS
In spite of the complexity of the problem of loading of activated sludge plants an attempt has been made in this inves-tigation to group the avenues of approach into a small number of main ones. Without a doubt the ones which the author have
selected are not mutually exclusive and therefore when one avenue is investigated there will be evident encroachment into other avenues.
In order that the results of this investigation may be more easily understood, two representations of results are being utilized; one being a tabulation form and the other a graphical
or chart form. The chart form will be found to be a pictorial representation of the tabular results.
24
1. Air guantities
A prime factor that affects the operation of an activated sludge plant is the quantity of air supplied to the process. Figures #1 and #2 following and the tabulated data therewith, Table A, illustrate the effect on the process of BOD load per 1000 cubic feet of air used.
Figure 3 and Table B show air quantities used in cubic feet per pound of BOD removed in relation to the 5 day BOD removed.
Figure 4 and Table C represent the conditions obtained in the several plants under investigation concerning the cubic feet of air used per gallon of sewage in relation to the effluent BOD of the several plants.
7 7I
7___'
ib
--- - ---7 '-7r_ _ _ ITvfl7I
___ I -. I___
_ L _ : 1 7L 7. _15-10I t*
1---I ~--~ -* I-'a-
~
<1
I! %3 i-i- 2$-6-~ lll t.1 + irtI -4 f --.. -.. -. --- --1 t7-7 t -. -.- . .. . . * -.- -71 -4 U T t~ --- ---- -1 -4 + t- --- --1 t T te + -~- 4 4 -. -.
-2
T -W- ILE -1-T 7'T -- q W ~ ~ T -t J"--H-H- -.-.-.-7 7 T.; -. . -. . . .- --. t. . . 7 . . . . -. -.. -.. --.--7_7--y
*emEo
wo
om0Ia
TABLE A EFFECT OF AIR
Parameter # B.O.D. Applied/1000 Cu.Ft. Air
ai) 4 . Plant k 0 4-H04! Austin, Tex. Decatur, Ill. Peoria, Ill. Ann Arbor, Mich. Baltimore, Md.-Fort Wayne, Ind. Gary, Ind.
Milwaukee, Wisc.(W) Omaha, Neb.
Pasadena, Cal.(MP) Grand Island, Neb. Hagerstown, Md. Hammond, Ind. Jackson, Mich. Lake Charles, La. Marion, Ind. Marshalltown, Ia. Madison, Wisc. Pontiac, Mich.
Richland Center, Wisc. Richmond, Ind.
San Antonio, Tex. Springfield, Ill. Syracuse, N.Y.
Two Rivers, Wisc.(E) Two Rivers, Wisc.(V) Cleveland, 0.(Easterly) Milwaukee, Wisc.(E) Chicago, Ill. (S.V.) Ann Arbor, Mich.,Jan.'L7
27.1 20 17.8 19 16
9.52
18.313
15.78
26 134
21 1641
40
11 12.25
118
27.22.59
11.29.5
17 15 128,460
5,000
31,7007,500
19,*270 .
29,350
14,450
181,0003,150
16,100
2,1555,670
22,7006,300
1,390
5,240
13,600
1,006 4,700 28,0005,670
7,360
411
52066,800
155,200308,000
8.1005,909
5,250
17,7006,300
13,20013,030
8,689
124,000 2,876 16,2301,950
3,470 11,9704,560
1,8703,740
3,234
6,280 1,150 2,918 29,500 5,7205,400
533
660
91,000
70,600 213,000 L. 800 1.43 82.2.95
87.0
1.79 92.6
1.19 88.5
1.46 93.6
2.25 91.51.66 86.8
1.46 94.5
1.10 89.90.99 20.5
1.10 91.61.64 85.8
1.90 88.81.38 96.3
.74 82.5 1.40 87.8 - 70.8 2.17 79.0.874
1.610.95
0.99
1.36
0.77 0.790.73
2.201.45
1.6995.0
95.0
89.3
89.3
89.5
91.390.4
89.4
94.788.6
93.3 0) +3' P4C 0t 4t, r. 1. 2.3.
4.
5.
6.
7. 8.9.
10. 11. 12. 13. 14.15.
16. 17. 18. 19. 20. 21. 22. 23. 24. 26. 27. 28. 29.30.
t t --i
~t-44+4
411 -A-4H>T
i
VP~~
.>.-~
.P . ---: T4 T 7t L+.2
_~~~~
I I_ IN__ __ -I em P in. ~ . -mu .mE 4 -44' -- ---- 4A ++4+4 1 i i ' 11J
itTABLE B EFFECT OF AIR
Cu. Ft. of Air/# B.O.D. Removed
-P.0 0 410 Plant 0 0 0 6.~~~~P For ranIn.14 69013 C45 7. Austin, Tex. 126.9 6,980 5,909 847 2. Decatur, Ill. 133 4,345 5,250 1,210 3. Peoria, Ill. 226.2 29,400 17,700 602
4. Ann Arbor, Mich. 147 6,640 6,300 950
-5. Baltinore d, 2 3. 18203 10,909
6. Fort Wayne, Ind. 174 26,900 13,9030 485
7. Gary,
mI.
62 12,060 8,689 6958. Milwaukee, Wisc.(W) 314 171,200 124,000 725
9. Omaha, Neb. 115 2,800 2,826 1,026
11. Grand Island, Neb. 86 1,972 1,950 989
12. Hagerstown, d. 158 4870 3,2470 713
13. Hamond, mId. 103- 20,160 11,970 594
14.
Jackson, Mich. 103 6,060 4,560 75325. RhLa Crle , La. c 23. 12 9D56- 1,870 t,20
16. Marion, Ind. 115 4,600 3,1740 814
17. Marshalton, Ia. 99 - 3,234
-18. Madison, Wise. 150 10,750 6,280 585
129 8,9280 6,9160 745
19. Pontiac, Mich. 99 - -
-20. Richland Center- Wisis. Q2.7 375 3 1,20
21. Richmond, Ind. 94 4,9465 2,9918 653
22. San Antonio, Tex. 113 25,5301 29,500 1,156
23. Springfield,, Ill 66 5,060 5,o720 1,v130
24. Syracuse, N. Y, 232.8 6,9600 5.,00 818
26. Two Rivers, Wisc.(W) 104.5 474 660 1,393
27. Cleveland, 0. (Easterly) 79.5 59,700 91,000 1,525 28. Milwaukee, Wisc.(E) 313.3 147,000 70,600 481 29. Chicago, Ill. (S.W.) 117 273,200 213,000 780 __0. Ar Aborch.Jan. 147 165_- 6,890 , 6297
-I
__I11
-4--.. 1.It'
-~- -~.4
171.7
__ -~ ~ I~
~
-- -__ -- I--- __r
177
-4- --I IK
--r
-____ -4 ~eV --- I -J .-I--...-- -i-.,---\
I
_-1
-1
{uI~L -r .1--*'~
\.~.
Stp
__ I1
I I ____ k ~ -7 4-a
T4 ,.,~., ,~,~7A~7fL,7,rtLr7 -... . . I I.7-.-Li
-4
~tI V N '4 pA4W1
-.. jTABLE C EFFECT OF AIR
Cu. ft. of Air/gallon of Sewage .. 9 27.) 4ID r~C-4 4HA 4-Q 1Austin, Tex. .897 27.1 2. Decatur, Ill. 1.34 20 3. Peoria, Ill. 1.13 17.8
4. Ann Arbor, Mich. 1.16 19
5.
Baltimore, Md. 1.24-6. Fort Wayne, Ind. .705 16
7. Gary, Ind. .358 9.52
8. Milwaukee, Wisc.(W) 1.9 18.3
9.
Omaha, Neb. .985-10. Pasadena, Cal.
1.39-11. Grand Island, Neb. .71
-12. Hagerstown, Md. .94 26
13. Hammond, Ind. .51 13
14. Jackson, Mich. .645 4
15. Lake Charles, La. 1.35 21
16. Marion, Ind. .78 16
17. Marshalltown, Ia. .73
-18. Madison, Wisc. - 40
19. Pontiac, Mich. -
-20. Richland Center, Wisc. 2.41
21. Richmond, Ind. .513
-22. San Antonio, Tex. 1.09 11
23. Springfield, Ill. .622 8
24. Syracuse, New York 1.59 27.2 25. Two Rivers, Wisc.(E) 1.22
9.59
26. Two Rivers, Wisc.(W) 1.21 11.2 27. Cleveland, 0. (Easterly) 1.01 9.5
28. Milwaukee, Wisc.(E) 1.26 17
29. Chicago, Ill. (S.W.) .76 15
2. BOD Removal Related to Duration of Aeration Period.
In Figures 5 and 6 following with appended tabulation, Table D, will be found the relation between aeration period and purification expressed as 5 day BOD removed and percent reduction of 5 day BOD.
... .... ... .. . .... .... ... . ... . . .. .. .. .. ... . .. .. .. T 7-F T 1 E 4-_L L 4 71 :-T-:. TT __ T 4,r .4- T4-4 4H t -T 4-H t 41, I T -r- 7-TT-+ T
4:
X. T T4 LH T 4- 4 T 41 kill 417 T I .... .... ... ;tT_1 T1-11-T 4N T-IT, -441 _t . i! .... .... .... . -1 T T -r4 TF-t T-t Ii4 + 4-t -t F- -4-r t Tl 17T _H_ _T 4, =+ ,4 T II . .. . . ... . J4 -... .... .... .... .... TT 1H hA t141- -4 t i i t i Ft 1+ -T- _F 4 jt . . .. . . . .. . .. . Li t , 7 4 . . . . . . . . . . . . . . . . . . . . . . . . t 7 . . . . . . . . . . . . 07 "ll -+ -. : _LtT. . . . . . . . . . . . . . 44 t 7 1 7 : r rlft* t.II I ___
L
1
-1
-77
77
...{~i
7-71
{~~ivt<7
-t-.77V
:~
K
~~~i
-fI
I
-+--1---
77-4
"1 O
V I-t- -G _
.
TABLE D
EFFECT OF AERATION PERIOD
0
Plant
Austin, Tex.
Decatur, Ill.
Peoria, Ill. Ann Arbor, Mich. Baltimore, Md. Fort Wayne, Ind. Gary, Ind.
Milwaukee, Wisc.(w) Omaha, Neb.
Pasadena, Cal,(MP) Grand Island, Neb. Hagerstown, Md. Hammond, In*. Jackson, Mich. Lake Charles, La. Marion, Ind. Marshalltown, Ia. Madison, Wisc.
Pontiac, Mich.
Richland Center. Wisc. Richmond, Ind.
San Antonio, Tex. Springfield, Ill. Syracuse, N.Y. Two Rivers, Wise.(E) Two Rivers, Wisc.(W) Cleveland, 0. (Easterly) Milwaukee, Wisc. (E)
Chicago, Ill. (S.W.) Ann Arbor, Mich.,Jan.'47
126.9
133
226.2
147
20174
62
314
115
149.3
86
158
103
103
99
115
99
150
99
2U9.8
94
113
66
232.8
102.7
104.5
79.5
315.3
117.
165
5.8
4.7
5.28
5.18
7.3
4.92
8.45
6.71
6.5
7.72
5-4
3 approx.
5.7
6
5.5
7.77
9.22
7.55
5.9
9.55
5
L.k
4-;'0
r4 1, 0 *1'4 4, 0 rd1.
2.3.
4.
5.
6.
7.
8.
9.
10.
11.
12.13.
14.
15.
16.
17.
18.19.
20. 21. 22. 23. 24. 25. 26. 27. 28. 29.30.
82.287.0
92.6
88.5
D3.6
91.5
86.8
94.5
89.9
90.5
85.8
88.8
96.3
82.587.8
79.0
91.1
89.3
89.5
91.3
90.4
89.4
94.7
88.6
93.3
I
3. Effect of Per Cent of Returned Sludge and Suspended Solids Concentration.
Since the zooglea acting as adsorptive material and also matrix for the bacteria performing the biochemical exodations is represented by the returned sludge or suspended solids in the aeration tank, investigation of these factors is presented in Figures #7 through #13 and in tabulations E through G following.
The change in the effect of the parameter by the inclusion of tank capacity in 1000 cu ft is shown specifically in Figures #12 and #13 and by tabulation G.
7= TT 14-4 liv- ALI I tt+ Jj +77 I 44-.11 N .... .... . . . . ... . .. . . . .. . . .. . . .. . . . . . . . .. .. .. .... .... .... .... 77:7 .... . . . . . . I s .... .... .... .... .... ... .... .... .... .... .... .... .... ..
...
...
r . . . . . . . .. . I f -T -; -4t ' ... .... .... .. tit a i4T ,Up
t +-Tl 77 ET t I ;:T Ll q 7 -7 ... .... 41 . . . . . . . . . . . . . 77W 7. .. .. ... .. . 7 7 7 J A, .7 t W oo .4, wr 71 t +H+ H+ 44 -tt 44T t-4r-" Lj t ; 11 f - .-' I .- -4- -, f+ i LL -1 -4, i, Ti f: +-*4 11.4 = ''I -.. ... .... .. .. .. . .... .. .. ... . . 7 .. .... .... .. .... .... .. .. .. . . . ... ... . .... . . . . . .. .... .... . ... .. . .... 7 .7 .7 . . .... .... .... .... .. .. .. .. . .... .... .... .. 7-71 7-. . . . . . . . . . . . a a . . . .... .... ... r% .. .... 77-.. ... ... .. . ... .. .. .. .... .... .. ... .... .... . .... .... .... .... .... .... .... . . . . . . . . . . . . . . . . ' ' -: :1 . : : : : ' : _ I . . . . . . : -* . .. . . . . .... . .. .... -j: MR: _LLL _LL I:-... .... Ty lilt -77 .... .... .... 04-... ... .. .. . .. .. .. .. .. .. .. ... ... . 777 7. . . . . : -. . . . . . . . . . .. : .-t4 .1 ... Iq ... .... . .... .... . ... .... .... .. .... .... . All 7. -7 7 :7 -77- 7 .... .... .. -7 77- 7: 4 77 . . . . . . _14 7T _77 cjsy _ WCT
ij. t-4 t4 T f T q m Ni PT rf4" 1 i r -T 7 :1 7:. cz J-: 77--' . . . . . . . . . . . 7 7 7---t .... ... -. . . . . . . ... m66"L
