EPA-625/4-73-004a Revised
Nitrification and
Denifrification
Facilities
Wastewater Tfeatmen
ERMechnotogy Transfer Seminar Publication
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EPA-625/4-73-004a
NITRIFICATION AND DENITRIFICATION FACILITIES
WASTEWATER TREATMENT
ENVIRONMENTAL PROTECTION AGENCY • Technology Transfer
August 1973
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ACKNOWLEDGMENTS
This seminar publication contains materials prepared for the
U.S. Environmental Protection Agency Technology Transfer Program
and has been presented at Technology Transfer design seminars
throughout the United States.
The information in this publication was prepared by Clair N.
Sawyer, Harry E. Wild, Jr., and Thomas C. McMahon, representing
Metcalf & Eddy, Inc., Consulting Engineers, Boston, Mass.
NOTICE
The mention of trade names or commercial products in this publication
is for illustration purposes, and does not constitute endorsement or recom-
mendation for use by the U.S. Environmental Protection Agency.
Revised February 1974
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CONTENTS
Page
Introduction
Chapter I. Factors Affecting Nitrification Kinetics ................. 3
Nitrification and Population Dynamics ................... 4
Nitrification Kinetics .......................... 5
Investigations ............................. °
Oxygen Nitrification Systems ....................... 16
References .............................. 17
Chapter II. Design Criteria of Nitrification Systems ................ 19
Nitrification Tanks ........................... 19
Settling Tanks ............................. 25
References .............................. 26
Chapter III. Denitrification by Suspended Growth Systems ............. 27
Denitrification Tanks .......................... 27
Settling Tanks ............................. 32
Sludge ................................ 33
Effluent Quality ............................ 33
111
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INTRODUCTION
Nitrification and denitrification have been well-recognized phenomena in wastewater treatment
for many years. The former occurred to the greatest degree during the warmer months of the year
and was considered highly beneficial in most instances because of the oxygen resource that the
nitrates provided. Because of additional capital and operating costs required to produce nitrates,
American engineers generally attempted to design or use processes that minimized nitrification.
The problems of rising sludge in conventional activated-sludge and standard-rate trickling-
filter plants were shown to be due to denitrification. The common way of controlling the problem
was to limit nitrification.
The Michigan studies on the significance of nitrogenous oxidation (NOD) in creating oxygen
sag in receiving streams and other studies showing the role of ammonia and nitrate nitrogen in
stimulating algal blooms have demonstrated the need for information on how wastewater-treatment
plants can be designed to accomplish nitrification and denitrification. Figure 1 shows the facilities
required to accomplish both in a controlled manner.
A three-stage biological system is considered necessary in northern climates where wastewater
temperatures drop below 65° F (18° C). The first stage is necessary to remove carbonaceous BOD5
to levels of about 50 mg/1. The second stage is needed to accomplish nitrification and should be
designed to employ the plug-flow principle as closely as possible. The third stage accomplishes
denitrification. A source of carbonaceous BOD must be added to reduce the nitrates to nitrogen
gas in a reasonable period of time.
CARBONACEOUS
BOD
NITRIFICATION
DENITRIFICATION
Figure 1. Model system for nitrification and denitrification.
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Chapter I
FACTORS AFFECTING NITRIFICATION KINETICS
The nitrification phenomenon has been studied intensively by soil scientists for the past
century. With the advent of biological wastewater-treatment systems, chemists and engineers
were impressed by the fact that the same phenomenon occurred in their treatment plants.
Originally, in the absence of biological methods of assessing degrees of purification, chemical
analyses served as the major means of evaluation. Experience soon taught that highly nitrified
effluents were immune to putrefaction. As a result, wastewater-treatment plants before 1930
were designed as standard or conventional plants intended to accomplish a relatively high degree
of nitrification, at least during the summer months.
With the development and widespread application of the BOD (biochemical oxygen demand)
test, it became apparent to many designing engineers that high degrees of waste treatment, in
terms of BOD removal, could be accomplished at marked savings in capital and operating costs
by designing to avoid nitrification. Thus, from 1940 until the late 1960's the main objective
in the United States was to design to minimize nitrification.
Many of the newly designed high-rate or modified plants, and some older plants suffering
from overloads, were plagued with denitrification and resultant "rising-sludge" problems in
the final clarifiers. These problems stimulated numerous studies on how to control nitrification,
since it was a physical impossibility to accomplish high degrees of nitrification, in most cases,
without expansion of the plant facilities.
Although the NOD of unnitrified effluents was well understood, sanitary engineers usually
dismissed this matter from their minds on the basis of three premises.
• Nitrification is caused by special organisms, the population of which is minimal in surface
waters.
• The reaction constant for nitrogenous oxidation is small in relation to the constant for
carbonaceous matter.
• Oxidation of ammonia to nitrates simply converts dissolved oxygen to a form from which
it is still available to prevent formation of anaerobic conditions.
The philosophy that unnitrified effluents are not damaging to receiving streams has been
undermined by biologists and conservationists, who point out that nitrates will not satisfy the
oxygen requirements of fish and many other aquatic organisms, and by the river and stream
investigations of Gannon1 and of the Michigan Water Resources Commission, reported by
Courchaine.2
As a result of the studies conducted in Michigan, many States are now requiring that NOD
be considered as well as BOD in any analysis of pollutional loads that streams can bear. This
requirement will undoubtedly mean that many plants of the future will be designed to accomplish
extensive nitrification, at least during the warmer months of the year when-oxidation rates are
highest and stream flows are apt to be minimal.
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With regard to eutrophication of surface waters, nitrogen in the fixed forms of ammonium
and nitrate ions is considered to be one of the major nutrients supporting blooms of green and
non-nitrogen-fixing blue-green algae. Nitrogen removal from wastewaters is being requested in
some areas and considered in many others. Where discharge is to lakes or reservoirs with significant
detention times, seasonal removal will not suffice and performance 365 days per year will be
expected. Removal through nitrification followed by denitrification represents the most promising
method at this time. It has the advantage of returning nitrogen to the atmosphere in its natural
form.
NITRIFICATION AND POPULATION DYNAMICS
It seems certain at this time that nitrification will play a greater and greater role in wastewater
treatment in the future because of anticipated increased NOD-removal requirements and possible
use of systems employing nitrification-denitrification for nitrogen removal.
It is conceivable that NOD removal will be a seasonal requirement in most locations and will
occur during the warm months of the year. If so, then conventional designs of biological systems
similar to those used before 1930, or those capable of maintaining conditions so that the reciprocal
growth rate of the nitrifying bacteria is less than the mean cell residence time or sludge-retention time
as described by Jenkins and Garrison,3 will be required. In simple terms, this means that nitrification
in plants can be maintained only when the rate of growth of nitrifying bacteria is rapid enough to
replace organisms lost through sludge wasting. When these bacteria can no longer keep pace, the
ability to nitrify decreases and may become extinct.
It has been well established that no treatment plants, including those of the extended aeration
type, are capable of accomplishing complete nitrification, year round, in our Northern States. In
situations where nitrogen removal is required and the nitrification-denitrification route is preferred,
it will be mandatory to accomplish nitrification in a separate biological system where the reciprocal
growth rate can be kept less than the mean cell residence time at all times. Owing to this necessity
a large part of the normal BOD will have to be removed before the wastewater enters the nitrification
unit. A system for such removal is shown as figure 1-1. It is believed that a BOD of 40 or 50 mg/1
can be tolerated in the feed to the nitrification unit; consequently either high-rate activated-sludge
or trickling-filter systems should be acceptable for the first stage of treatment.
NITRIFICATION
BOD
REMOVAL
— SLUDGE RECYCLE—'
— SLUDGE RECYCLE-
WASTE WASTE
Figure 1-1. Two-stage biological system required to guarantee complete nitrification.
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NITRIFICATION KINETICS
The response of both nitrite- and nitrate-forrnii;.j bacteria in pure culture to various environ-
mental conditions has been studied extensively. The effect of pH on the respiration rate of
Nitrosomonas, as reported by Meyerhof4 and Engel and Alexander,5 is shown in figure 1-2; the
effect on Nitrobacter, as reported by Meyerhof,6 is shown in figure 1-3.
Early studies on nitrification in wastewater treatment were related mainly to its control to
prevent rising-sludge problems in the activated-sludge process. These studies brought dissolved
oxygen (DO) under close scrutiny, since DO was the only environmental condition that could be
considered readily controllable under normal operating conditions. Bragstad and Bradney7
reported that DO must be kept below 0.5 mg/1 to control nitrification. Recently, Downing et al.8
and Jenkins and Garrison3 have reported on other aspects affecting nitrification, and Zanoni9
investigated the effect of temperature on the velocity constant for nitrification in treated effluents.
A portion of this study was prompted by three major considerations.
• A paper by Borchardt1 ° indicated that temperature had little effect on nitrification in the
range of 15° to 35° C (see fig. 1-4), in opposition to published data.1 l
• Information is lacking on sludges in systems receiving feed stock containing relatively low
BOD.
• There is a considered need to establish a quantitative basis for evaluating the ability of
nitrifying sludges to convert ammonia to nitrate under various temperature and pH
conditions.
INVESTIGATIONS
Method of Study
The investigations to be described were conducted at Marlborough, Mass., where a 10-gpm
pilot nitrification unit, receiving settled high-rate trickling-filter effluent, was operated, open to
the weather from October 1969 through April 1970. All ooservations on the effects of DO were
made in the pilot plant. The studies on the influence of temperature and pH were made in the
laboratory using return sludge from the nitrification unit and settled trickling-filter effluent in the
apparatus shown in figure 1-5. The batch studies on pH and temperature were conducted with DO
levels above 2 mg/1 to insure that DO would not be an inhibiting factor.
Experimental Results
Pilot Plant. The results of measuring the DO in the aeration tank of the pilot plant and the
resulting effluent quality are indicated on figure 1-6. The DO concentration was measured twice
daily and found not to have a significant variance during any given day. The effluent ammonia-
nitrogen concentration was taken from a 24-hour composited sample.
The wide range of DO concentration resulted from breakdowns in one of the two available
compressors and from a varying demand for air at other locations.
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Figure 1-6 indicates that there was apparently no inhibition of nitrification occurring at DO
levels exceeding 1 mg/1.
Laboratory. The laboratory studies were concerned with determining the effect of tempera-
ture and pH under carefully controlled conditions. The procedure used involved collection of
samples of return sludge from the nitrification pilot plant and of settled trickling-filter effluent,
determination of suspended and volatile suspended solids in the return sludge, and adjustment of
portions of each to definite pH and temperatures before making the desired mixtures in the aeration
units. In most instances, the trickling-filter effluent was supplemented with a dilute aqueous solu-
tion of ammonium chloride in order to give runs of sufficient duration to obtain three or more
experimental values.
The rate of nitrification was determined by measuring residual ammonia nitrogen on grab
samples of mixed liquor that were filtered immediately after collection. DO, pH, and temperature
were monitored continuously during the course of each study. Dilute sodium hydroxide was
added to control pH as needed. The system of study may have involved some slight loss of ammonia
at pH levels above 8.5, but such losses were too insignificant to be detected from a plotting of the
data.
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DO PROBE
pH PROBE
THERMOMETER
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02 4 68 10 12
NITRIFICATION UNIT AVE. AERATION TANK DO, mg/L
Figure 1-6. Relationship ot residual ammonia to dissolved oxygen.
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It was assumed that the relative population of nitrifiers in the total MLVSS (mixed-liquor
volatile suspended solids) concentration for the duration of the study remained constant. It is
felt that this assumption was justified due to the long duration of the pilot studies run under the
same conditions, employing settled trickling-filter effluent as feed stock.
MLVSS. The nitrification studies were conducted with MLVSS concentrations within the
range of 800-6,000 mg/1.
A sample of two of the experiments run at the same pH and temperature conditions but
with two different mixed-liquor volatile suspended solids is shown in figure 1-7. It was observed
that the time to completely nitrify the same amount of ammonia nitrogen per gram of MLVSS
was constant given the same environmental conditions. This observation allows direct comparisons
to be made for different MLVSS concentrations in the study and permits subsequent data to be
expressed in terms of milligrams of ammonia nitrogen per milligram of MLVSS.
Ammonia. The augmented ammonia-nitrogen concentrations for the studies varied from 6
to 60 mg/1. The ammonia-nitrogen concentration had to be augmented on many occasions because
the time required for complete nitrification of lov/ levels was so short that only one or two samples
could be analyzed before attaining the zero level. Two sample results are shown in figure 1-8. Both
of the experiments were conducted at the same pH and temperature conditions. As can be seen
from the figure, the slopes of the lines are parallel and constant for all residual concentrations of
ammonia nitrogen regardless of the initial concentration. This result would indicate that nitrification
CONDITIONS
pH 8.5
TEMPERATURE 2I°C
TIME.HRS
Figure 1-7. Effect of variation in mixed-liquor volatile
suspended solids.
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Figure 1-8. Effect of variation in ammonia concentration.
is not inhibited at concentrations normally found in a domestic wastewater system, and also allows
adjustment of other data for different ammonia-nitrogen concentrations by constructing a line
parallel to the experimental line at the desired concentration.
BOD. A special study was made to determine the effect of variable BOD upon the rate of
nitrification. In the study three different samples were nitrified. The temperature and pH for
all three units were the same. The wastewater in the first unit was primary effluent with a BOD of
110 mg/1. The second unit contained settled trickling-filter effluent with a BOD of 45 mg/1. The
third unit contained nitrification effluent from the pilot plant with a BOD of 5 mg/1. All samples
were augmented with enough ammonium chloride to give a reasonable duration of the test.
Figure 1-9 shows the results of this special study. Within the limits of the study, there was no
apparent inhibition of nitrification for the various BOD concentrations. It should be realized that
this study was undertaken to determine the reaction of the nitrifiers to a shock loading of BOD, and
that any sustained high-BOD loading would eventually cause nitrification to cease, owing to the
washing-out effect of sludge wasting on the nitrifiers. This conclusion was reached because of the
low growth rate exhibited by the nitrifiers as compared to those organisms utilizing carbonaceous
BOD and because of the established fact that an increased BOD loading in a conventional system
leads to greater sludge production.
pH. The pH range investigated in these studies was from 6.0 to 10.5. The samples were
adjusted to the desired pH level and maintained at that level for the duration of the experiment.
The ammonia-nitrogen weight per MLVSS weight ratio of the grab samples was plotted against
time, and all the other variables were noted. The time plot allowed calculation of the exact time
of complete nitrification, i.e., complete oxidation of ammonia. A sample graph (see fig. 1-10)
shows two sample results, both of which were obtained at a temperature of 20° C. The pH of one
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TEMPERATURE 20° C
TIME,MRS
Figure 1-9. Effect of variation in BOD.
sample was 8.5 and the other 6.5. The figure also shows an adjusted line to compensate for different
initial concentrations of ammonia.
Three factors are immediately evident from figure 1-10.
• There was no apparent initial uptake of ammonia nitrogen by the nitrifiers.
• There was no lag time involved in the rate of nitrification.
• The rate was uniform and constant for the entire length of the experiment. This result
indicates that the nitrifiers work at maximum efficiency at all times independent of the
residual concentration of ammonia nitrogen.
The studies indicate an optimum pH for nitrification to be 8.4. Figure 1-11 shows that 90
percent of the maximum rate occurs in the range of 7.8 to 8.9, and that outside the ranges of 7.0
to 9.8 less than 50 percent of the optimum rate occurs.
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Temperature. The temperature studies covered the range from 5° to 30°C, and nitrification
occurred at all temperatures investigated. The rate of nitrification increased with temperature
throughout the full range. Figure 1-12 indicates the straight-line relationships for two sample exper-
iments run at different temperatures but the same pH. One adjusted line is shown to offset the
initial ammonia concentration difference. There was no lag period observed nor any decrease in the
rate of nitrification as the residual ammonia concentration decreased.
12
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Figure 1-12. Effect of variation in temperature.
Figure 1-13 indicates the relationship of the rate of nitrification at all temperatures studied to
the rate at 30° C. Since 30° C is a very high wastewater temperature for all but the most southerly
States in the United States, a summary of relative rates in terms of other maximum temperatures
is as shown in table 1-1, based on the data of figure 1-13.
These data indicate, on the basis of temperature alone and the most adverse conditions con-
sidered possible, that up to five times as much detention time may be needed to accomplish com-
plete nitrification in the winter as is needed in the summer. Temperature effects can be overcome
to a considerable degree, however, by increasing mixed-liquor suspended solids (MLSS) and adjusting
pH to more favorable levels. Optimum design for complete nitrification will depend on the best
combination of aeration tank capacity, MLSS, and pH for winter operating conditions. Under
summer conditions, operation will be possible at less favorable pH levels and lower MLSS.
Discussion
When all of the foregoing information is evaluated, rates of nitrification can be computed.
The rate of nitrification has been defined as the weight ratio of ammonia nitrogen oxidized per
day to the MLVSS.
The rates for any pH within the range of 6.0 to 10.5 are shown in figure 1-14. All rates are for
a temperature of 20° C. As the figure shows, the rate varies from a maximum of 0.185 gram
NH3-N nitrified per day per gram MLVSS at a pH of 8.4 to a minimum of 0.020 gram NH3-N
nitrified per day per gram MLVSS at a pH of 6.0.
The results obtained in this study with respect to pH show good correlation with the results
indicated in the section of the paper on work performed by others.
This study obtained results on temperature effects opposed to those observed at Ann Arbor,
Mich., reported by Borchardt,10 and shown in figure 1-4. Borchardt's low-temperature observa-
tions were made in extended aeration studies by measuring the ammonia and nitrate oxygen
in the effluent of the units. It is felt that the apparent effects of temperature were observed because
the units were not being stressed to their limit of nitrification at the higher temperatures and
13
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Table 1-1 .—Relative rates of nitrification at various
temperatures
30° C
100
25° C
80
100
20° C
60
75
100
15° C
48
60
80
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127
34
45
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112
116
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Figure 1-13. Rate of nitrification at all temperatures
compared to rate at 30 C.
14
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75% OPTIMUM RATE
7.5. 9.3
20 25 30 35
TEMPERATURE 8C
Figure 1-15. Rate of nitrification versus temperature at various pH levels.
• Temperature did affect the rate of nitrification. The rate increased through the range of
5° to 30° C, in reasonable agreement with the van't Hoff-Arrhenius law.
• The time required for nitrification is directly proportional to the amount of nitrifiers
present in the system.
• Instantaneous increases (from 50 to 110 mg/1) or decreases (from 50 to 5 mg/1) in BOD
concentration did not affect the rate of nitrification. It would be expected, however,
that a change in the average BOD concentration of the feed would affect that percentage
of MLVSS which is composed of nitrifiers, and as a result would affect the time to achieve
complete nitrification.
OXYGEN NITRIFICATION SYSTEMS*
From 1971 through 1973, Union Carbide Corporation conducted numerous nitrification
studies utilizing pure oxygen. Included were pilot-plant operations at Tampa, Fla., Amherst, N.Y.,
and Brockton, Mass., and a number of laboratory treatability studies. Data from the studies have
been incorporated ir-to the development of a design approach for oxygen nitrification systems.
These oxygen nitrification studies have shown that the concentration of Nitrosomonas is pro-
portional to the concentration of influent ammonia. As the ammonia loadings to the system
increase, the relative population of nitrifying organisms to total volatile suspended solids will also
increase. Because the nitrifying organisms are a small fraction of the total volatile suspended solids,
the nitrifying capacity of a system can increase substantially owing to an increase of ammonia in
the influent without a noticeable change in the MLVSS.
*Abstracted from M. J. Stankewich, Jr., Union Carbide Corporation, "Biological Nitrification with the High
Purity Oxygenation Process," presented at the Annual Meeting of the Purdue Industrial Waste Conference,
Lafayette, Ind., May 1972.
16
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Although nitrifying organisms initially prefer a pH environment of 7.0 to 8.0, the Union
Carbide oxygen studies reveal that they will acclimate to lower pH environments and approach
the ammonia removal capacity attainable at the higher pH levels. Acclimation was demonstrated
by the fact that, after a period of time, consistent nitrification was achieved at operating
conditions at which nitrification would fail before acclimation.
REFERENCES
!J. J. Gannon, "River BOD Abnormalities," Bull. 05168-1-F, University of Michigan, Office
of Research Administration, Ann Arbor, Mich.
2Robert J. Courchaine, "Significance of Nitrification in Stream Analysis—Effects on the
Oxygen Balance," J. Water Pollut. Cont. Fed., 40, 835,1968.
3D. Jenkins and W. E. Garrison, "Control of Activated Sludge by Mean Cell Residence Time,"
J. Water Pollut. Cont. Fed., 40, 1905,1968.
4Meyerhof, Arch. f. die ges Physiologie, 166, 255,1917.
5M. S. Engel and M. Alexander, "Growth and Autotrophic Metabolism of Nitrosomonas
europaea," J. Bact, 76, 217,1958.
6Meyerhof, Arch. f. die ges Physiologie, 164, 416,1916.
7R. E. Bragstad and L. Bradney, "Packing House Waste and Sewage Treatment at Sioux Falls,
South Dakota," Sewage Works J., 9, 959, 1937.
8 A. L. Downing, H. A. Painter, and G. Knowles, "Nitrification in the Activated Sludge
Process," J. Inst. Sewage Purif. (Brit.),pt. 2, 130, 1964.
9A. E. Zanoni, "Secondary Effluent Deoxygenation at Different Temperatures," J. Water
Pollut. Cont. Fed., 41, 640, 1969.
10J. A. Borchardt, "Nitrification in the Activated Sludge Process,"Bull. The Activated Sludge
Process, University of Michigan, Division of Sanitary and Water Resources Engineering, Ann Arbor,
Mich.
11C. N. Sawyer and G. A. Rohlich, "The Influence of Temperature upon the Rate of
Oxygen Utilization by Activated Sludge," Sewage Works J., 11, 946,1939.
17
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Chapter II
DESIGN CRITERIA OF NITRIFICATION SYSTEMS
This chapter discusses the design criteria that appear to be reasonable at this time (October
1972). It must be emphasized that these criteria are based solely on pilot-plant experience.
NITRIFICATION TANKS
Tank Layout
Because the rate of oxidation of ammonia is essentially linear (zero-order reaction), short
circuiting must be prevented. The tank configuration should insure that flow through the tank
follows the plug-flow mixing model as closely as possible. Such configuration can be accomplished
by dividing the tank into a series of compartments with ports between them. Figure II-l shows
three compartments as a minimum number. Tanks can be designed for either diffused-air or
mechanical-aeration systems.
Since the oxidation rate of the process varies widely with temperature, special provisions
may be necessary to incorporate the necessary flexibility in the oxygen supply system, as discussed
hereinafter.
pH Control
Nitrification tanks should be sized to permit complete nitrification under the most adverse
combination of ammonia load and temperature expected, and at a pH as near optimum as feasible.
The range of 7.6-7.8 is recommended in order to allow carbon dioxide to escape to the atmosphere.
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Figure 11-1. Model nitrification system.
19
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The nitrification process destroys alkalinity and the pH may fall to levels that will inhibit nitri-
fication unless excess alkalinity is present in the wastewater or lime is added to maintain favorable
pH levels.
2NH4HCO3 + 4O2 -> 2HNO3 + 4H2O + 2CO2
2HNO3 + Ca(HCO3)2 -» Ca(NO3)2 + 2CO2 + 2H2O
2NH4HCO3 + 4O2 + Ca(HCO3)2 -» Ca(NO3)2 + 4CO2 + 6H2O
Theoretically, 7.2 pounds of total alkalinity are destroyed per pound of ammonia nitrogen oxidized
to nitrate. One-half of this destruction is due to loss of alkalinity caused by ammonia and the remain-
der is due to destruction of natural alkalinity, as shown in the foregoing equations.
Whether lime additions will be required depends upon the alkalinity of the wastewater and the
desired pH of operation. For operation under the most adverse temperature conditions and at operat-
ing pH, sufficient lime must be added initially to raise the pH into the desired range, and then 5.4
pounds of hydrated lime per pound of ammonia nitrogen will be required to maintain the pH. An
actual titration test should be conducted to obtain design criteria. In Boston sewage, about 250
pounds of hydrated lime are needed per million gallons to raise the pH initially to optimum pH range,
and an additional 700 pounds are needed to hold it there during the course of oxidation of the
ammonia. The total hydrated lime requirements are estimated to be about 115 mg/1. Additional
amounts of lime may be required if chemicals, such as alum, have been added previously for phos-
phorus removal.
Marked reductions in lime requirements will result in any system that can be designed to oper-
ate at pH levels of 7.8 or less, because carbon dioxide resulting from destruction of alkalinity and
organic matter will be washed out of the liquid phase by air contact. The pH of such systems will
vary somewhat with the rate of aeration (ventilation).
The type and sensitivity of the pH control system will depend on the character of the waste-
water and the variations in the ammonia load fed to the system. Figure II-2 shows a proposed system
for pH control in the most demanding situation. In many situations, a lesser degree of control will
be feasible; in some none will be needed.
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LURRY
re -•>• i
1 o
PH
-? $-
pH
1 6
pH
Figure 11-2. pH control for nitrification system, plan view.
20
-------
Although studies by the authors showed the optimum pH for the nitrification reaction to be in
the range of 8.4 to 8.6, other workers have reported lower optimum levels more in keeping with those
found by Engle and Alexander, as shown in Figure 1-2.
Downing and Knowles1 have reported that pH levels above 7.2 do not increase the rate of nitri-
fication. They presented an equation
Activity = 1 - 0.83(7.2 - pH)
for calculation of activity at ph levels below 7.2. Using this equation the activities in Table II-l can
be determined.
Haug and McCarty2 recently have reported upon nitrification in submerged filters and observed
pH phenomena. They found that the nitrifying organisms were able to acclimate to low pH levels of
6.0 within about 10 days, and develop the ability to oxidize ammonia as rapidly as at pH levels of 7.0
or more. They made no studies, however, on nitrifying populations in the fixed growths, so it was
not proven whether the increased activity was due to acclimation or an increased population of organ-
isms, resulting in the availability of excess ammonia.
The information provided by Downing and Knowles and Haug and McCarty offers some hope
that the addition of lime for initial pH elevation may not be necessary. The need for lime then re-
volves around having sufficient alkalinity present to allow complete nitrification at satisfactory
kinetic rates at the lowest temperatures and highest mass loadings of ammonia expected.
In any event, sufficient alkalinity should be present to leave a residual of from 30 to 50 mg/1
after nitrification is completed. As a general rule, where phosphorus removal is accomplished in
the first stage of a two- or three-stage system by use of alum or ferric salts, it will be necessary to
provide lime-feeding facilities, and the optimum pH of operation becomes more or less an academic
matter. In situations where feeding of lime is not essential, good engineering normally will indicate
that additional tankage be provided to overcome the limitations of reduced activity, as opposed to
providing lime-feeding facilities to keep the tankage at a minimum.
Table 11-1 .-Activities at pH 7.2 and below
pH
7.2
7.0
6.8
6.6
6.4
6.2
Activity
1.00
.83
.67
.50
.34
.17
21
-------
MLSS and MLVSS Concentrations
Designs based on MLSS concentration alone should be avoided, because MLSS will not truly
reflect the biological mass in the system. The ratio of MLVSS to MLSS may vary depending on the
nonvolatile suspended solids (including residual chemical precipitates) in the feed. The fraction of
the MLVSS attributable to nitrifying organisms is as yet unknown; however, for nitrification sys-
tems receiving normal secondary effluents, MLVSS concentrations of 1,500-2,000 mg/1 appear to be
safe for design.
Tank Capacity
The choice of the design-peak load depends on the circumstances of the specific project, and
need not necessarily be the absolute maximum expected load. For many projects, the use of a peak-
load factor of 1.5 represents a reasonable peak at low-temperature conditions.
Figure II-3 shows the permissible volumetric loading of the nitrification tanks at a pH of 8.4
and at various temperatures and MLVSS concentrations, based on the nitrification kinetics studies at
Marlborough, Mass, (see chapter I).
U-
d
O
O
g
§
o
tO
I
30
25
20
IO
\ I 1
BASED UPON NITRIFICATION RATES
OBSERVED AT MARLBORO, MASS
5 10 15 20
TEMPERATURE, ° C
Figure 11-3. Permissible nitrification-tank loadings.
25
22
-------
•s.
I
X
<
UJ
CK
tu
0.
\J\J
90
80
70
60
50
40
30
20
io*
0
6.
X
^
X
^
r
/
AT 20° C
/
'
I/
X
^
^^
X
x
N
\
s
S
vk
\
L
\
1
0 7.0 8.0 9.0 1C
pH
Figure 11-4. Percent of maximum rate of nitrification at constant temperature versus pH.
Figure II-4 shows the corrections that must be applied to the permissible loadings when the pH
is different from 8.4. In plants with well-buffered wastewater, it may be more economical to provide
the additional tankage to permit operation at a lower pH, rather than to add an alkaline material.
The following is a sample calculation for computing the tank size :
Given:
Design flow =10 mgd
Average concentration to nitrification tanks = 15 mg/1
Minimum temperature = 10° C
Operating pH = 7.8
MLVSS concentration = 1,500 mg/1
Computed:
1. NH3 load
a. Average = W X 8.34 X 15 = 1,250 Ib/day
b. Maximum = 1,250 XI.5 = 1,870 Ib/day
2. Tank volume at 10° C, MLVSS = 1,500 mg/1
a. From figure II-3, volumetric loading = 8.2 Ib per 1,000 ft3
b. Tank volume = 1870/8.2 X 103 = 228,000 ft3
23
-------
3. Tank volume adjusted to pH 7.8 (see fig. III-4) = 228,000/0.88 = 260,000 ft3
4. Check detention period = (260,000 X 24 X 7.48)/(10 X 106) = 4.65 hr
Oxygen Requirements
Stoichiometrically, each pound of ammonia nitrogen that is nitrified requires 4.6 pounds of
oxygen. (The amount of ammonia nitrified is usually slightly more than the amount of nitrate
measured because some denitrification occurs.) Usually, it is assumed that all of the ammonia fed
will be nitrified. An additional oxygen allowance must be made for carbonaceous BOD that escapes
from the secondary treatment process.
Nitrification appears to be uninhibited at DO concentrations of 1 mg/1 or more. Design based
on maintaining 3 mg/1 of DO in the mixed liquor under average loading conditions includes a reason-
able factor of safety. Under peak loading the DO concentration may be permitted to fall somewhat,
but not below 1 mg/1.
There follows a sample calculation for oxygen requirements:
Given:
Design flow =10 mgd
Average NH3-N concentration = 15 mg/1
Average BOD = 30 mg/1
Computed:
1. NH3 load
a. Average = 1,250 Ib/day
b. Maximum = 1,870 Ib/day
2. BOD load = 2,500 Ib/day
3. Oxygen requirement
a. NH3 oxidation = 1,870 X 4.6 = 8,650 Ib/day
b. BOD requirement = 2,500 X 1.5 = 3,750 Ib/day
c. Total requirement = 12,400 Ib/day
To design the aeration system, the total oxygen requirement must be corrected to actual operat-
ing conditions by the use of well-known equations incorporating such factors as
• Critical wastewater temperature
• Minimum DO concentration
• Coefficient of wastewater oxygen-uptake rate (alpha)
24
-------
• Coefficient of wastewater DO saturation (beta)
« Altitude of plant
The rate of nitrification will vary significantly with temperature and pH, and compensation for
this variation must be made in the design of the plant. During the summer, the following methods
can be used to match the oxygen demand rate to the oxygen supply rate:
• Reduce MLSS concentration
• Reduce pH by reducing chemical supply
• Reduce tankage in service while increasing oxygen supply to the tanks remaining in service
Miscellaneous
Although the nitrification process will handle the normal variations in ammonia load found in
raw wastewater, experience at the Washington, D.C., pilot plant indicates that nitrification in the
carbonaceous removal units must be carefully controlled to insure stable operation. Experience at
South Lake Tahoe, Calif., indicates that the addition of 2-8 mg/1 of chlorine to the effluent of the
carbonaceous aeration tank effectively will prevent nitrification. In addition, excessive amounts of
carbonaceous BOD and suspended solids that escape from the carbonaceous treatment process, such
as those associated with "bulking" sludge caused by filamentous growths, must not be so great that
sludge wasting from the nitrification process causes a washout of the nitrifying organisms. Carbona-
ceous-BOD concentrations higher than 50 mg/1 in the nitrification influent may interfere with winter
operation.
Foam spray systems have not been found necessary where the MLSS concentration was greater
than 2,000 mg/1.
The following substances have been shown3 to have an inhibiting effect on the nitrification proc-
ess in concentrations greater than those indicated:
Halogen-substituted phenolic compounds, 0 mg/1
Thiourea and thiourea derivatives, 0 mg/1
Halogenated solvents, 0 mg/1
Heavy metals, 10-20 mg/1
Cyanides and all compounds from which hydrocyanic acid is liberated.on acidification, 20 mg/1
Phenol and cresol, 20 mg/I
SETTLING TANKS
Design information on settling tanks serving nitrification systems is limited generally to pilot-
plant research studies. The criteria given herein represent what has been determined as of October
1972.
aE. A. Drew, Chief Engineer, Middle Regional Drainage Scheme, England.
25
-------
Surface Loadings
The maximum permissible hydraulic surface loading appears to be approximately 1,000 gpd/ft2.
Average surface loadings should be in the range of 400-600 gpd/ft2. It may be necessary to reduce
this loading somewhat if the MLSS concentration is greater than 2,500 mg/1, because of limiting
sedimentation-tank-solids loadings.
Mulbargerb noted at the Manassas, Va., pilot plant that settling improved in the nitrification
settling tanks when alum was added to upstream processes, probably due to carryover of alum floe.
It has also been noted that the periodic addition of waste sludge from the carbonaceous treatment
process improves settling. In cases where nitrification units follow, addition of waste sludge from
them may facilitate a more rapid buildup of nitrifying organisms.
Number of Tanks
Because of the relatively slow growth and settling rates of nitrifying sludges, it is desirable to
provide more than two settling tanks to insure that the sludge is kept within the system when a tank
is down for maintenance and repair. Four tanks are a desirable minimum number.
Depth
Depths of 12-15 feet are recommended.
Sludge-Collection Equipment
Experiences to date have shown no evidence of rising-sludge problems, probably due to com-
plete nitrification and very low residual BODC (oxidizable carbonaceous matter) levels. Use of rapid-
removal suction-type sludge-collection equipment is not mandatory, but it may be desirable in large
circular tanks. The settling tanks should be equipped with skimmers and provision should be made
to use the scum system to pump floating sludge, should it ever occur, to the nitrification tank influent.
Sludge
It is recommended that capacity be provided for a return-sludge rate of 50-100 percent of aver-
age flow, since the nitrification sludge is lighter and does not compact as well as carbonaceous sludges.
Continuous sludge wasting was not normally necessary at the pilot plants at Washington, D.C.,
and Marlborough, Mass. Periodic adjustments of MLSS concentration are necessary, however, and
provisions should be made to dispose of waste nitrification sludge with the waste sludge from the
carbonaceous treatment process.
REFERENCES
1 Downing and Knowles, Proceedings of the Third International Conference on Water Pollution
Research, vol. 2, p. 117, Munich, 1966.
2Haug and McCarty, J. Water Pollut. Cont. Fed., 44, 2089, 1972.
"M. C. Mulbarger, private communication.
26
-------
Chapter III
DENITRIFICATION BY SUSPENDED GROWTH SYSTEMS
Only pilot-plant data are available at the present time to serve as the basis of suspended growth
denitrification systems. It appears that the most valid information that can serve as a basis of
rational design comes from the pilot-plant studies at Manassas, Va.,a and the investigations at Washing-
ton, D.C.k Figure III-l shows the kinetics of the denitrification reaction in relation to temperature
for a given pH range, as reported by Mulbarger and as observed at Washington, D.C. The data from
which the figure was developed were obtained in laboratory studies in a manner comparable to those
shown in figure 1-15, and are considered fully as reliable. The reasons for the difference between the
two curves has not been determined fully; but this unexplained difference points to the need for
additional kinetic studies on other wastewaters.
DENITRIFICATION TANKS
The tank layout should assure that the plug-flow mixing model is followed as closely as possible,
because nitrates are not adsorbed by biological growths and detention periods may be quite short.
Whether covered tanks are required to minimize absorption of oxygen from the atmosphere is a
matter of conjecture. There is some evidence to indicate that properly designed denitrification units
can be made to seal themselves by formation of a floating scum. In any event, airtight or walk-in
covers are to be avoided, because nitrogen and carbon dioxide are both released during the denitrifi-
cation reaction.
pH
Studies by Mulbargerc have indicated that optimum pH for the denitrifying organisms is in the
range of 6.5-7.5, the same as for most saprophytic bacteria. Figure III-2 shows the corrections that
must be applied to the permissible tank loading when the pH is different from the optimum range.
That the pH of the effluent from the nitrifying units may exceed 7.5 at some time during a year
is no particular problem, because carbon dioxide generated from oxidation of carbonaceous matter
in the denitrification unit quickly reduces the pH into the favorable range below 7.5. There is no
need for addition of chemicals to control pH.
MLSS and MLVSS
The limited experience available has shown that denitrifying sludges have settling properties
comparable to good activated sludges. It seems reasonable to assume, therefore, that mixed-liquor
solids in the range of 2,000 to 3,000 mg/1 can be maintained without excessive rates of returning
sludge. The volatile matter in the denitrifying sludges at Manassas, Va., and Washington, B.C., is
about 65 percent.
aReported by M. C. Mulbarger, private communication.
J. Stamberg, private communication.
CM. C. Mulbarger, private communication.
•27
-------
RANGE OF APPLICATION
<
0
LU
O
O
O
Of
£-
z
O
LU
N
O
X
O
l/>
oo
Z
O
Z
LU
O
(MULBARGER)
REF. MULBARGER
OPTIMUAA pH RANGE
REF. STAMBERG
pH 7.0-7.5
TEMPERATURE, C
Figure 111-1. Effect of temperature on rate of denitrification.
28
-------
REF. MULBARGER
100-
90
z
LU
u
80-
6.0 7.0 8.0
PH
Figure 111-2. Percent of maximum rate of denitrif ication versus pH.
Size
Reference to figure III-l will show that the minimum temperature to be allowed for will play a
great role in determining the size of the denitrification tanks, as well as the MLVSS that can be
carried in the system. Figure III-2 and figure III-3 may be used to compute the size of the denitrifi-
cation tanks as follows (sample calculation based on kinetic data from Manassas, Va.):
Given:
Design flow = 10 mgd
Average NO3-N + NO2-N concentration = 15 mg/ld
Minimum temperature = 10° C
Expected operating pH = 7.3
this example problem, assume complete conversion is desired.
29
-------
BASED ON DEIMITRIFICATION RATES
OBSERVED AT MANASSAS, VA.
(REF: MULBARGER)
200 -i
u
O
O 150-
z
I
ro
O
Z
~ 100-
50-
OPTIMUM pH RANGE
\ I
10 15
TEMPERATURE
3000 mg/L
MLVSS
2500 mg/L
MLVSS
2000 mg/L
MLVSS
1500 mg/L
MLVSS
1000 mg/L
MLVSS
\
20
T
25
Figure 111-3. Permissible denitrification-tank loadings.
MLVSS = 2,000 mg/1
Computed:
1. NO 3 -N + NO 2 -N loading
a. Average = 10 X 8.35 X 15 = 1,250 Ib/day
b. Peak = 1,250 X 1.5 = 1,870 Ib/day
2. Tank loading at 10° C, optimum pH (from fig. III-3) = 26.8 lb/1,000 ft3
30
-------
3. Tank volume at MLVSS = 2,000, optimum pH = 1,870/26.8 X 103 = 70,000 ft3
4. Check detention period = (70,000 X 7.48 X 24)/10 mgd = 1.25 hr
Such a system would have over twice the tankage needed at 20° C. For this reason, good design
will allow for idle operation of part of the capacity during the warm months of the year. A design
similar to that shown for the nitrification system in figure II-l is recommended.
At Washington, D.C., the results of Stamberg indicate that the tankage allowance must be con-
siderably more generous—possibly three or four times as great if complete denitrification is to be
required in the winter months. It is questionable whether denitrification will be needed during the
low-temperature months of the year, because of the flushing action of high river flows during the
spring months.
Carbonaceous Matter
Effluents from nitrifying units are exceptionally free of BODC. For this reason, denitrification
is very slow unless a readily oxidizable source of carbonaceous matter is added. Methyl alcohol
(methanol) is the cheapest commercial source of carbonaceous matter at this time. Glucose (com
sugar) is the next cheapest source. Methanol is preferable because it is more completely oxidized
than glucose and, consequently, produces less sludge for disposal.
In some areas, nitrogen-deficient industrial wastes, such as brewery wastes, might be available
and suitable for use. All such waste materials should be employed before considering methanol be-
cause it is produced from natural gas, which is not an unlimited resource.
When methanol is used for denitrification the basic reaction involved is
5CH3OH + 6H+1 +6NO3" *- 5CO2 + 3N2 + 13H2O
(5 X 32) = 160 (6 X 14) = 84
From the foregoing equation and weight relationships, it might be concluded that each pound of
nitrate nitrogen would require about 2 pounds of methanol for its reduction, which is true, but some
of the methanol is used to produce new cell growth (sludge) as follows:
(CHgOH),, —-*- CO2 + (CH2O)X + H20
Also, nitrified effluents normally carry some DO into the denitrification tank and some DO may
enter the mixture as a result of agitation. This increases the amount of methanol required. An equa-
tion commonly used to estimate methanol requirements is: pounds per day methanol = 2.47 Ib
NO3-N + 1.53 Ib NO2-N + 0.87 Ib DO.
Reports indicate that from 3 to 4 pounds of methanol per pound of nitrate nitrogen are required
to consume DO and leave sufficient to reduce the nitrate to nitrogen gas.
The amount of methanol fed must be very closely controlled by a system such as that shown
in figure III-4 to insure that enough is fed to reduce the nitrates and to avoid an excess. Any excess
is not only a waste of chemical; it creates an undesirable residual BOD.
Equipment
The contents of the denitrification tanks are mixed with underwater mixers comparable to
those used in flocculation tanks in water-treatment plants. The energy provided must be sufficient
31
-------
CH3 OH (METHYL ALCOHOL)
NITRATE
ANALYZER
/
\
i
CONT
o
_i
>
;
/
NITRIFIED WASTEWATER
AIR
WASTE
T
-i — •— •
f * S^
- -( CLt
V.
1 RETURN SLUDGE
Figure 111-4. Model system for feeding methyl alcohol to denitrification tanks.
to keep the MLSS in suspension, but must be controlled to prevent pickup of atmospheric oxygen
as much as possible, unless the tanks are covered or some other method is used to exclude contact
with the air.
Power requirements of %-% hp per thousand cubic feet have been found to be adequate.
Nitrogen Release
The denitrification reaction results in the formation of carbon dioxide and nitrogen gas. Both
have limited solubility in water, especially the latter. Because of the gentle mixing used in the
denitrification tanks, the mixed liquor leaving the tanks is supersaturated with nitrogen, and possibly
carbon dioxide. As a result, gas bubbles tend to form and adhere to the MLSS and inhibit settling
in the final clarifier. Supersaturated conditions can be relieved by employing an aeration tank or
aerated open tanks. It is recommended that from 5 to 10 minutes detention be provided at peak
flow. Such a facility will also provide the ability to remove small amounts of excess methanol.
SETTLING TANKS
The limited experience available indicates that the settling properties of denitrification sludge,
following relief of supersaturation, are very similar to conventional activated sludge.
Tank depths of 12-15 feet are recommended, and surface overflow rates should not exceed
1,200 gallons per square foot per day at peak flows. MLSS concentrations greater than 2,500 mg/1
may require larger tanks owing to the higher settling-tank solids loadings.
A suction-type sludge collector is recommended for large circular tanks. Long rectangular
tanks should be equipped with midtank sludge-drawoff systems.
Skimming facilities should be provided on the settling tanks and provisions should be made
for returning the scum to the denitrification tank when desired.
32
-------
SLUDGE
Return
Capability of returning sludge to the denitrification tank of up to at least 50 percent and
preferably of up to 100 percent of average flow is recommended.
Waste
Provision should be made for periodic wasting of sludge from the denitrification systems similar
to that employed for carbonaceous systems. Normally, the sludge should be wasted to mix with
primary and/or waste-activated sludge and be disposed of with them. The waste-sludge line, however,
should be designed to transport sludge to the nitrification tank when desired. In the event that
nitrifying sludge is lost from the nitrifying system, it is normally captured by the denitrifying system.
It can be returned to its normal home, at least in part, by using denitrifying sludge to reseed the
nitrification tank.
Quantity of Waste Sludge
It is reported that about 0.2 pound of sludge will be generated for each pound of methanol fed.
This would correspond to about 0.7 pound per pound of nitrate nitrogen reduced.
EFFLUENT QUALITY
Based on pilot-plant studies operating under steady-state conditions, in table III-l effluent
quality is predicted from a nitrification-denitrification system designed for operation at 10° C waste-
water temperatures. At warmer temperatures improved quality can be expected.
Thus, it appears that 90 percent removals of total nitrogen can be achieved in actual practice.
Table 111-1. Predicted effluent quality at 10° C wastewater temperatures
Component
Suspended solids
BOD
Organic-N
NH3-N
NO,-N
Total-N ...
Milligrams per liter
10
5
1 0
5
K
2 0
ftUS GOVERNMENT PRINTING OFFICE 1977—757-056/5613
33
-------
METRIC CONVERSION TABLES
Recommended Units
Description
Length
Area
Volume
Mass
Time
Force
Moment or
torque
Stress
Unit
metre
kilometre
millimetre
micrometre
square metre
square kilometre
square millimetre
hectare
cubic metre
litre
kilogram
gram
milligram
tonne or
megagram
second
day
year
newton
newton metre
pascal
kilopascal
Symbol
m
km
mm
jjm.
m2
km2
mm2
ha
m3
1
kg
9
mg
t
Mg
s
d
year
N
N-m
Pa
kPa
Application
Description
Precipitation,
run-off,
evaporation
River flow
Flow in pipes,
conduits, chan-
nels, over weirs,
pumping
Discharges or
abstractions,
yields
Usage of water
Density
Unit
millimetre
cubic metre
per second
cubic metre per
second
litre per second
cubic metre
per day
cubic metre
per year
litre per person
per day
kilogram per
cubic metre
Symbol
mm
m3/s
m3/s
l/s
m3/d
m3/year
I/person
day
kg/m3
Comments
Basic SI unit
The hectare (10 000
m2) is a recognized
multiple unit and
will remain in inter-
national use.
The litre is now
recognized as the
special name for
the cubic decimetre.
Basic SI unit
1 tonne = 1 000 kg
1 Mg = 1 000 kg
Basic SI unit
Neither the day nor
the year is an SI unit
but both are impor-
tant.
The newton is that
force that produces
an acceleration of
1 m/s2 in a mas:
of 1 kg.
The metre is
measured perpendicu-
lar to the line of
action of the force
N. Not a joule.
of Units
Comments
For meteorological
purposes it may be
convenient to meas-
ure precipitation in
terms of mass/unit
area (kg/m3).
1 mm of rain =
1 kg/m2
Commonly called
the cumec
1 l/s = 86.4'm3/d
.
• • •
• ~ * , .. *
The density of'""
wate"r understand!
ard conditions is
1 000 kg/m3 or
1 000 gfl or
1 g/ml.
Customary
Equivalents
39.37 in.=3.28 ft=
1.09yd
0.62 mi
0.03937 in.
3.937 X 10'3=103A
1 0.764 sq ft
= 1.196 sq yd
6.384 sq mi =
247 acres
0.001 55 sq in.
2.471 acres
35.314 cu ft =
1.3079cuyd
1. 057 qt = 0.264 gal
= 0.81X10^ acre-
ft
2.205 Ib
0.035 oz=1 5.43 gr
0.01543 gr
0.984 ton (long) =
1.1 023 ton (short)
0.22481 Ib (weight)
= 7.233 poundals
0.7375 ft-lbf
0.02089 Ibf/sq ft
0.14465 Ibf/sq in
Description
Velocity
linear
angular
Flow (volumetric)
Viscosity
Pressure
Temperature
Work, energy.
quantity of heat
Power
Recommended Units
Unit
metre per
second
millimetre
per second
kilometres
per second
radians per
second
cubic metre
per second
litre per second
pascal second
newton per
square metre
or pascal
kilometre per
square metre
or kilopascal
bar
Kelvin
degree Celsius
joule
kilojoule
watt
kilowatt
joule per second
Symbol
m/s
mm/s
km/s
rad/s
m3/s
l/s
Pa-s
N/m2
Pa
kN/m2
kPa
bar
K
C
J
kJ
W
kW
J/s
Comments
Commonly called
the cumec
Basic SI unit
The Kelvin and
Celsius degrees
are identical.
The use of the
Celsius scale is
recommended as
it is the former
centigrade scale.
1 joule = 1 N-m
where metres are
measured along
the line of
action of
force N.
1 watt = 1 J/s
Customary
Equivalents
3.28 fps
0.00328 fps
2.230 mph
15,850 gpm
= 2.120 cfm
15.85 gpm
0.00672
poundals/sq ft
0.000145 Ib/sq in
0.145 Ib/sq in.
14.5 b/sq in.
5F
7 -17.77
3
2.778 X 10'7
kwhr =
3.725 X 10 7
hp-hr = 0.73756
ft-lb = 9.48 X
10'4Btu
2.778 kw-hr
Application of Units
Customary
Equivalents
35.314 cfs
15.85 gpm
1.83X103gpm
* » * **^f **f* ;•'*
0.264 gcpd
0".06»'lb/eufi " -
i '
- -- 1 i
Description
Concentration
BOD loading
Hydraulic load
per unit area;
e.g. filtration
rates
Hydraulic load
per unit volume;
e.g., biological
filters, lagoons
Air supply
. , (
Pipes
diameter
length
Optical units
Unit
milligram per
litre
kilogram per
cubic metre
per day
cubic metre
per square metre
per day
cubic metre
per cubic metre
per day
cubic metre or
litre of free air
per second
millimetre
metre
lumen per
square metre
Symbol
mg/t
kg/m3d
m3/m2d
m3/m3d
m3/s
l/s
mm
m
lumen/m2
Comments
If this is con-
verted to a
velocity, it
should be ex-
pressed in mm/s
(1 mm/s = 86.4
m3/m2 day).
Customary
Equivalents
1 ppm
0.0624 Ib/cu-ft
day
3.28 cu ft/sq ft
0.03937 in.
39.37 in. =
3.28ft
0.092 ft
candle/sq ft
------- |