EPA-670/2-75-051
June 1975 Environmental Protection Technology Series
SINGLE STAGE NITRIFICATION-DENITRIFICATION
National Environmental Research Center
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
-------
EPA-670/2-75-051
June 1975
SINGLE STAGE NITRIFICATION-DENITRIFICATION
By
Dolloff F. Bishop, James A. Heidman,
and
John B. Stamberg
EPA-DC Pilot Plant
Washington, D.C. 20032
Program Element No. 1BB043
Project Officer
Dolloff F. Bishop
Advanced Waste Treatment Research Laboratory
National Environmental Research Center
Cincinnati, Ohio 45268
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
For »ale by the Superintendent of Document!, U.S. Government
Printing Office, Washington, D.C. 2O402
-------
REVIEW NOTICE
The National Environmental Research Center—Cincinnati has reviewed
this report and approved its publication. Approval does not signify that
the contents necessarily reflect the views and policies of the U.S.
Environmental Protection Agency, nor does mention of trade names or
commercial products constitute endorsement or recommendation for use.
-------
FOREWORD
Man and his environment must be protected from the adverse effects of
pesticides, radiation, noise and other forms of pollution, and the unwise
management of solid waste. Efforts to protect the environment require a
focus that recognizes the interplay between the components of our physical
environment—air, water, and land. The National Environmental Research
Centers provide this multidisciplinary focus through programs engaged
in
o studies on the effects of environmental contaminants on man
and the biosphere, and
o a search for ways to prevent contamination and to recycle
valuable resources.
This work describes an alternate method for removing organic (BOD),
nitrogen (NHa), and phosphorus (P) pollutants from the aqueous environment.
This wastewater treatment approach achieves significant removals of organics
and nitrogen pollutants from wastewater with minimum use of energy and
scare chemicals such as methanol.
A. W. Breidenbach, Ph.D.
Director
National Environmental
Research Center, Cincinnati
111
-------
ABSTRACT
The removal of 75 to 84 percent of nitrogen from primary wastewaters has
recently been achieved in a single stage activated process at the EPA-DC
pilot plant in Washington, D.C., without the use of supplemental organic
carbon. The removal was achieved in a two pass biological reactor in which
the D.O. concentration was varied from 0 to 2 to 3 mg/1. The air was applied
on a 30-min cycle, first to one reactor pass then to the other pass. Mechan-
ical mixers suspended the mixed liquor solids when the air was not applied to
the pass. At F/M of approximately 0.1 gm BOD /day/gm MLVSS, a mixed culture
of carbonaceous (BOD,, removal), nitrifying, and denitrifying organisms devel-
oped. With the D.O. at 2 or above, the nitrification readily occurred. When
the D.O. decreased to near zero, denitrification occurred. The process also
produced efficient organics removal with more than 85 percent of the COD re-
moved from primary wastewater.
The COD/TKN ratio in the wastewater entering the reactor controlled the
amount of nitrogen removal. With a COD/TKN ratio of 10, 84 percent removal
of total nitrogen from the primary effluent was achieved in the summer and
75 percent removal in the winter. When the COD/TKN ratio was reduced to
about 7.5 by FeCl_ treatment in the up-stream primary process, the nitrogen
removal across the biological process in the summer decreased to 67 percent.
The data suggest that increases in the COD/TKN ratio would further increase
nitrogen removal.
Laboratory kinetic studies on the mixed liquor solids revealed that the
denitrification kinetic rate controlled the reactor design. The nitrification
rate constants varied from about 0.03 gm of NHr-N removed/gm of MLVSS/day
at 15.5°C to 0.11 gm of NH--N removed/gm of MLVSS/day at 27°C. The denitri-
fication rate constants during the initial anaerobic cycle varied from about
0.03 gm NO--N removed/gm MLVSS/day at 15.5°C to 0.055 gm NO_-N removed/gm
MLVSS/day at 25°C. The denitrification rate constant then declined to lower
levels after the organic carbon was consumed by the biological reaction. In
the winter with District of Columbia wastewater, bulking growth occurred in
the reactor and required low overflow rates of 12.2 m/d (300 gpd/ft ) to
capture the solids in the clarifier. Bulking conditions did not occur during
summer operations. The mixed liquor solids in the summer exhibited initial
settling velocities of 3.7 to 4.3 m/hr (12 to 14 ft/hr).
This report was submitted in partial fulfillment of Contract No. 68-01-0162
by the Department of Environmental Services, Government of the District of
Columbia, under the sponshorship of the Environmental Protection Agency.
Work was completed as of September 1973.
IV
-------
CONTENTS
Page
Abstract iv
List of Figures vi
List of Tables vii
Acknowledgements viii
Sections
I Conclusions 1
II Recommendations 3
III Introduction 4
IV Treatment Approaches for Nitrogen Removal 5
V Experimental Plan and Procedures 7
VI Process Performance 10
VII Nitrification-Denitrification Kinetics 24
VIII References 27
v
-------
FIGURES
No. Page
1 Pilot Plant System for Single Stage Nitrification- 8
Denitrification
2 Start-up of Aerobic-Anaerobic Cycle 11
3 Initial Settling Velocity and the SVI 21
4 Methanol Denitrification of Residual (N0$ + N03)-N 22
5 Cyclic Nitrification and Denitrification 25
-------
TABLES
No. Page
1 Operating Conditions 12
2 Organic Removal 14
3 Solids and Phosphorus Removal 15
4 Nitrogen Removal 16
5 Nitrogen Removal and the F/M and COD/TKN Ratios 17
6 Effect of Adding Fed. and of Tertiary Filtration 19
7 Settling and Solids Production 20
8 Batch Kinetics for Cycling Aerobic-Anaerobic Conditions 26
Vli
-------
ACKNOWLEDGMENTS
The assistance of the operators, technicians, and laboratory staff at the
EPA-DC pilot plant is gratefully acknowledged.
vill
-------
SECTION I
CONCLUSIONS
The removal of 75 to 84 percent of nitrogen from primary wastewaters has
recently been achieved in a single stage activated sludge process (30,000
to 50,000 gpd) at the EPA-DC pilot plant in Washington, D.C., without the
use of supplemental organic carbon. The removal was achieved in a two pass
biological reactor in which the dissolved oxygen concentration was varied
from 0 to 2 to 3 mg/1. The air from the blower was applied on a 30-min
cycle, first to one reactor pass then to the other pass. Mechanical mixers
suspended the mixed liquor solids when the air was not applied to the pass.
At an F/M ratio of 0.1 gm BOD5/day/gm MLVSS, a mixed culture of carbonaceous
(BOD5 removal), nitrifying and denitrifying organisms developed and essen-
tially complete nitrification was achieved. With the dissolved oxygen at
2 or above, the nitrification readily occurred. When the dissolved oxygen
decreased to near zero, denitrification occurred.
The COD/TKN ratio in the wastewater entering the reactor controlled the
amount-of nitrogen removal. With a COD/TKN ratio of 10:1, 84 percent of
the total nitrogen, based on the primary effluent, was removed in the summer
and 75 percent in the winter. When the COD/TKN ratio was reduced to about
7.5 by FeCl3 treatment in the up-stream primary process, the nitrogen re-
moval across the biological process in the summer decreased to 67 percent.
The increased removal of particulate organics with the FeClj treatment in
primary sedimentation reduced the relative amount of the organic carbon
source available for denitrification and left more NO^-N in the process
effluent. The data suggest that increases in the COD/TKN ratio would
further increase nitrogen removal.
Laboratory kinetic studies on the mixed liquor solids revealed that the
denitrification kinetic rate controlled the reactor design. Nitrification
rate constants at a given temperature generally remained the same during
repetitive aerobic-anaerobic cycles. The denitrification rates, however,
decreased as the residual carbon was consumed by the biological reactions.
In the winter with District of Columbia wastewater, bulking growth occurred
in the reactor and required low overflow rates of 12.2 m/d (300 gpd/ft ) to
capture the solids in the clarifier. Bulking conditions disappeared during
summer operations. The mixed liquor solids in the summer exhibited initial
settling velocities of 3.7 to 4.4 m/hr (12 to 14 ft/hr).
Since the nitrate from nitrification was used to remove BOD5 from the waste-
water, the process advantages for this single stage two basin nitrification-
denitrification process included:
o the reduction in the volume of air needed to achieve
nitrification and BOD5 removal
-------
o The minimizing and, with further study, potential elimination
of supplemental organic carbon sources required for complete
denitrification
o substantial nitrogen removal without special recycle
of the mixed liquor
o the elimination of intermediate clarifiers required
in staged nitrification-denitrification
Even if bulking limits winter operations, application of the dissolved
oxygen cycle to existing extended aeration plants in warm weather reduce
requirements to achieve nitrification and BOD 5 removal. The reduced air re-
quirement potentially lowers conventional operating costs.
-------
SECTION II
RECOMMENDATIONS
The process applied to the District of Columbia wastewater produced
bulking sludge conditions in the winter operations. Other continuously
aerated activated sludge processes at the District of Columbia have also
exhibited bulking sludge conditions for various operating approaches,
especially in the winter. Work is needed to determine, in general, why
bulking sludge conditions occur and, specifically, whether the anaerobic-
aerobic operation of the single stage nitrification-denitrification
approach contribute to the bulking. Additional work is also needed on
the kinetics of the nitrification, denitrification, and carbonaceous
utilization to develop an optimum anaerobic-aerobic process configuration.
-------
SECTION III
INTRODUCTION
Nitrification by autotropic bacteria (Nitrosoma and Nitrobacter) achieves a
maximum rate1 in the activated sludge process at dissolved oxygen concentra-
tions of approximately 2 mg/1 or above. The rate decreases to zero as the
dissolved oxygen concentration decreases to zero. In contrast, while denitri-
fication by facultative bacteria occurs in both anoxic and aerobic systems,
the most rapid denitrification occurs with dissolved oxygen concentrations of
zero and with ample sources of readily available organic carbon to serve as
electron donors for the bacterial reduction of the nitrate or nitrite.
In biological treatment of wastewaters, oxidation of carbonaceous material,
nitrification, and devitrification all occur within a single process if suffi-
cient bacterial solids retention time is provided for development of the
nitrifying organisms. Unfortunately, optimum process operating conditions for
oxidation and for subsequent denitrification are thermodynamically antago-
nistic. That is, the presence of the more powerful oxidant oxygen (electron
acceptor) suppresses the use of NOj (electron acceptor) in the biological
oxidation of the carbonaceous material in the wastewater. In conventionally
aerated biological systems, efficient oxidation of the carbonaceous and
nitrogenous materials, in either a single or two stage process is achieved
under aerobic reactor conditions. The aerobic conditions produce the nitrate
product but also remove the organic materials that act as readily available
electron donors for rapid denitrification. In addition, the dissolved oxygen
in the water significantly suppresses or minimizes the biological reduction
of the nitrate or nitrite as they are produced.
Clearly, while in municipal wastewaters, the biological potential exists for
both nitrification and denitrification without supplemental organic carbon
sources, the operating conditions usually employed in aerobic biological
processes produce efficient carbonaceous removal and nitrification but not
efficient denitrification. Thus, the purpose of this work was to select and
to demonstrate the operating conditions for the activated sludge process at
the EPA-DC Pilot Plant in Washington B.C., that would produce efficient nitri-
fication and denitrification with the indigenous carbonaceous material in the
wastewater acting as the electron donor during denitrification.
-------
SECTION IV
TREATMENT APPROACHES FOR NITROGEN REMOVAL
As early as 1962, Ludzack. and Ettinger2 recognized the possibility of using
the organic carbon in wastewater as the electron donors for the bacterial
reduction of the nitrate or nitrite products of biological oxidation, They
used alternate anaerobic and aerobic zones within a single activated sludge
reactor to provide for a period of low or zero dissolved oxygen for denitri-
fication. In the process, effluent reactor mixed liquor with the nitrate
product from the aerobic Cor second) zone was recycled to the reactor influent
zone, which was operated under anaerobic conditions. With ample supply of
influent carbonaceous material, the nitrate acting as an electron acceptor
was reduced to nitrogen gas during the biological oxidation of the carbona-
ceous material. The amount of denitrification was controlled by the rate of
recycle of the mixed liquor from the aerobic zone to the anaerobic zone. In
the study, the maximum nitrogen removal exceeded 60 percent.
In an alternate approach to achieve nitrification-denitrification, Wuhrman,3
in a two zone reactor, located the anaerobic zone after the aerobic zone. In
this approach which did not require recycle of mixed liquor, the endogenous
activity of the activated sludge mass provided the organic carbon (electron
acceptors) for the denitrif ication. Wuhrman reported nitrogen removals of
more than 90 percent in his approach. According to Barnard,1* others studies,
however, could not repeat Wuhrman's removal. Apparently the endogenous
activity of the activated sludge mass provided low amounts of available
carbonaceous material for the denitrification and produced a low endogenous
denitrification rate.
Since these early approaches, various biological systems for maximum nitrogen
removal have been evaluated. Earth et al., developed a reliable three stage
approach that consisted of three independent activated sludge reactor-clarifi-
ers for sequential carbonaceous removal, nitrification, and finally denitri-
fication. Since essentially all of the carbonaceous material was removed
from the wastewater after nitrification, a supplemental organic carbon
source, such as methanol, was required to produce denitrification in the final
stage. A short aerobic zone or basin was employed between the denitrification
reactor and the final clarifier. The aerobic zone prevented clarifier sludge
bulking, which is caused by the N£ flotation of the sludge from the denitri-
fication reactor, and aerobically removed small amounts of excess methanol
from the water. Earth's three stage approach applied at the Environmental
Protection Agency-District of Columbia (EPA-DC) pilot plant in Washington,
B.C., provided very efficient all-seasons operations,6 with average total
nitrogen residuals in the filtered (dual media) final effluent of 1.5 mg/1 as
nitrogen.
-------
Balakrishman and Eckenfelder7 proposed a contact stabilization modification
of the three stage system to eliminate the need for methanol in denitrifica-
tion. In their approach, the nitrified effluent from the second stage and the
solids from the first stage (contact-aeration) clarifier were mixed in a basin
for stabilization and denitrification before final clarification. The NOf in
the nitrified effluent acted as an electron acceptor in the biological stabi-
lization (oxidation) of the active contact-aeration sludge and, thus, was
reduced to nitrogen gas without a supplemental organic carbon source. Nitro-
gen removals of approximately 80 percent have been reported"* for this
approach.
In 1972, Matsctue8 reported significant removals of nitrogen in an "oxidation
ditch" activated sludge plant at Vienna-Blumental. The plant employed two
basins in series with rotors for aeration and mixed liquor recirculation.
Examination of the detailed data9 on the plant clearly revealed the required
high solids retention time and operating conditions for efficient nitri-
fication-denitification. The dissolved oxygen concentration at the rotors
were above 2 mg/1 and essentially zero before the next rotor. The wastewater
recirculation, inherent in rotor operation, provided a supply of carbonaceous
material for the denitrification. The alternating high-low dissolved oxygen
tension, the appropriate solids retention time, and the presence through
recirculation of readily available organic carbon, provided the conditions for
efficient nitrogen removal.
Recently, three studies, including the work reported here, have been inde-
pendently completed in developing a conventional single reactor-clarifier
system for removing nitrogen without supplemental organic carbon. Barnard"'10
developed a single reactor-clarifier system using four basins alternating
from anaerobic to aerobic conditions in series with a single final clarifier.
In the system, the first two basins employed the Ludzack-Ettinger approach.
Mixed liquor from the second basin (nitrifying aerobic conditions) was
recycled at rates up to four times the influent plant flow to the first basin
(anaerobic conditions) for partial denitrification. The third basin (anaero-
bic conditions) employed Wuhrman's endogenous denitrification. The final basin
(aerobic conditions) prevented final clarifier sludge bulking, potentially
caused by the N2 production in the third basin. Barnard's single stage
(multi-basin) reactor clarifier approach provided more than 90 percent removal
of nitrogen without the use of methanol.
Christensen11 proposed a two basin, single, reactor-clarifier process with
appropriate solids retention for nitrification- denitrification without a sup-
plemental carbon source and without mixed liquor recirculation. The process
employed alternating aeration in first one basin and then the other to
produce the alternate high-low oxygen concentration needed for efficient
nitrification-denitrification. To provide readily available organic carbon,
the influent wastewater and the settled activated sludge,recycled from the
clarifier, was alternately added to either of the two basins during the basin's
anaerobic portion of the plant's aeration cycle. In the system, the process
flow always proceeded from the anaerobic (nonaerated) to the aerobic basin to
the clarifier, but the physical direction of the flow through any basin
reversed itself with the change from anaerobic to aerobic operating conditions.
Reportedly,11 the approach consistently produces effluents with total nitrogen
content of 2 to 5 mg/1.
-------
SECTION V
EXPERIHENTAL PLAN AND PROCEDURES
This approach to achieve nitrification-denitrification in a single, activated
sludge reactor-clarifler with the wastewater organic carbon used for
denitrification consisted, simply, of providing alternate periods of aerobic
and anaerobic conditions within the reactor and operating the reactor at
sufficiently low food-to-microorganism ratios (high solids retention times) to
ensure a nitrifying population within the mixed liquor culture. To provide
the alternating aerobic-anaerobic conditions while continuously operating the
air compressor, an activated sludge reactor (Figure l) in the EPA-DC pilot
plant was divided into two equal basins operating in series. Each basin, with
a water depth of approximately 3.35 m (11 ft), provided a detention time of
3.55 hr at a process flow of 189 m3/d (50,000 gpd). Air was supplied
alternately to each basinj first to one basin and then to the other on a
30-min cycle. The dissolved oxygen in the basins was manually controlled at
between 2 and 3 mg/1 during aeration and rapidly decreased to zero during the
anaerobic period.
In each basin, two mechanical mixers, operated at 30 rpm, suspended the mixed
liquor solids without any significant surface aeration of the wastewater.
Air was supplied through a 0.95 cm (3/8-in.) orifice below a 30.5 cm-diameter
(12-in.) by 1.53 m long (5 ft) Kenics* static mixer.
For additional experimental options, an effluent mixed liquor recycle pump
and two other small basins with water depth of 3.35 m (11 ft) were also
provided. The effluent mixed-liquor recycle pump was used to increase the
amount of NO? in the first basin of the reactor, through recycle of the
second basin mixed liquor. It was only used during the first month of
operation. The first optional basin with a 0.89-hr detention at 189 m3/d
(50,000 gpd) and with a 30-rpm mixer was covered for anaerobic operation and
equipped with a methanol feeding system for denitrification of any residual
nitrate. The second optional basin with a Kenics static mixer and a 0.89-hr
detention at 189 m3/d (50,000 gpd) provided continuous aeration to remove
excess methanol and to prevent Na-induced sludge bulking in the clarifier.
The optional basins were used only in the last month of the experimental work.
For most of the work, the mixed liquor was settled in a center feed clarifier,
which was a convented thickener with a conventional thickening mechanism.
The clarifier, with a water depth of 3.35 m (11 ft), provided an overflow rate
of 21 m/d (520 gpd/ft2) at a 189 m3/d (50,000 gpd) process flow. To permit
relevelling of the clarifier, for about one month of the experimental work. An
alternate center feed clarifier without a thickening mechanism was used with
an overflow rate of 27 m/d (665 gpd/ft2) at a 189 m3/d (50,000 gpd) flow.
The pilot plant start-up began in December 1972. The study continued through
September 1973. For most of the study, primary effluent from the District of
*Mention of trade names or commercial products does not constitute endorsement
or recommendation for use.
-------
AIR
REACTOR
CLARIFIE
RETURN
SLUDGE
Figure 1. Pilot plant system for single stage
nitrification-denitrification
8
-------
Columbia Water Pollution Control Plant was fed at various steady flows
to the activated sludge reactor. During July through September, however,
pilot plant primary effluent was fed on a diurnal cycle with a 2:1
maximum-to-minimum flow to the biological reactor. In July and August,
45 mg/1 of FeCl3 was added for phosphorus removal to the influent raw
wastewater. Also in July and August, the secondary effluent was
filtered through dual media filters at an average loading (2:1 diurnal
flow cycle) of 7.8 m/hr (3.2 gpm/ft2). Alum at a 20 mg/1 dose was
added to the secondary effluent in a mixing chamber immediately ahead of
the filters. The filter media consisted of 61 cm (24 in.) of 1.2- to
1.4-mm coal over 30.5 cm (12 in.) of 0.6- to 0.7-mm sand.
In evaluating the process, appropriate samples were manually composited
over a 24-hr period on Tuesday, Wednesday, and Thursday. Samples collected
on Friday-Saturday and Sunday-Monday, except those for BODs analysis, were
composited over a 48-hr period. BOD5 samples were always composited over
a 24-hr period. All samples were stored at 3°C to minimize biological
activity. All samples, except those for BOD$ and suspended solids analyses,
were preserved with one drop of I^SOi, per 30 ml of sample while they were
held in storage.
The BODs was determined by the dissolved oxygen probe method;12 the
ammonia12 and the nitrate-nitrite,13 by the Technicon autoanalyzer; and
the total ptiosphorus, by the persulfate method.1** All other analyses
employed "Standard Methods."
Batch settling tests on the mixed liquor were periodically conducted in
a stirred (10 rph) settling column, 15.3 cm (6 in.) in diameter and 2.3 m
(8 ft) long. Batch kinetic studies were performed in the laboratory to
determine kinetic rates for nitrification and denitrification. The
studies were performed by mixing 1 to 2 liters of the recycle solids with
the primary effluent in a ratio similar to that existing in the process.
A water bath was used to ensure that the temperature of the kinetic test
remained the same as that in the process. The mixture was aerated,
usually for 30 min, and then mixed with a magnetic stirrer under anaerobic
conditions, usually for 30 min, for several cycles. The changes in NHs-N
NO^-N were monitored by Technicon analyses to determine the kinetic rates
of the alternating nitrification and denitrification.
-------
SECTION VI
PROCESS PEFORMANCE
In mid-December 1972, the operation of the system was started with District
of Columbia primary effluent as feed and with a seed of partially nitrifying
activated-sludge. The start-up system consisted of two basins in series and
the final clarifier with a standard sludge recycle system. On January 1, 1973
with complete nitrification established, the controlled dissolved oxygen cy-
cling was applied across the two reactor basins. The steady influent flow
was increased from 110 m3/d (29,000 gpd) to 163 m3/d (43,000 gpd) on January
3. The internal recycle of the mixed liquor was started on January 5, at a
recycle flow equal to 50 percent of the influent flow. The process behaved
as expected. With the cycling of the dissolved oxygen concentration, the
denitrification rapidly increased and the nitrate residual (Figure 2) de-
creased to about 4 mg/1 as nitrogen. With the initiation of the recycle of the
mixed liquor from the second basin to the first, the residual nitrate concen-
tration continued to decrease to less than 2 mg/1 as nitrogen, but the NHs-N
gradually increased to about 2 mg/1. The denitrification did not cause any
immediate sludge bulking in the process. Unfortunately, the process,
operating at a low F/M ratio, began to develop a poorly settling sludge that,
by January 19, forced a reduction in process flow to 110 m3/d (29,000 gpd)
to prevent excessive solids carryover.- Microscopic examination of the sludge
indicated the probable presence of both Sphaerotilus and Nocardia organisms.
(Both organisms have been identified at various times in other activated
sludge processes at the EPA-DC pilot plant.)
On January 19, the internal recycle of the mixed liquor also was discontinued.
With the elimination of internal mixed liquor recycle and with the lower flow,
the effluent NH3-N decreased to less than 0,5 mg/1 and the effluent NOa-N
increased to about 4.5 mg/1. The poorly settling sludge, however, continued
as a major operating problem throughout the winter months and persisted into
spring. In spite of the process settling characteristics, the first month's
operation clearly demonstrated process nitrogen removal potential without a
supplemental organic carbon source and without mixed liquor recycle.
The average monthly operating conditions are summarized for the 9 months of
the study in Table 1, The average operating conditions for January represent
only the low flow operation used for about one half of the month. In addition,
from mid-March through April 6, an alternate clarifier without a thickening
mechanism was employed with about a 28 percent increase in the overflow rate.
The average clarifier overflow rates given for March and April, however, are
those of the original clarifier.
During the 9 months, the reactor was operated with a F/M ratio of approximately
0.1 gm BOD5/day/gm mixed liquor volatile suspended solids (MLVSS). In the
winter, long detention periods (12 hr) and somewhat lower F/M ratios provided
time for the slower biological reactions and permitted low overflow rates
needed for solids separation of the bulking mixed liquor in the clarifier. In
April and May, attempts were made to eliminate the bulking growth. From about
mid-April to May 20, the process was operated with continuous aeration (thus,
10
-------
10-
0)
CD
O)
E
ui
O
O
DC
8-
6-
4-
2-
I
5
I
10
I
15
1 Effluent (NOj + NO2-)-N, mg/l
• Effluent NH4-N, mg/l
• Effluent, TKN, mg/l
Avg. Influent TKN 25.7 mg/l
Avg. Influent NH3-N 16.7 mg/l
20
I
25
I
30
Figure 2.
Days in Jan.1973
Start-up of aerobic-anaerobic cycle
-------
Table 1. OPERATING CONDITIONS
Jan.
Feb.
March
April
May
June
July
August
Sept.C
Detention
Time,
hr
12.3
12.3
12.3
12.4
10.5
8.8
6.8
6.6
8.7
SRT,
days
_a
33
24
a
21
18
15
_a
a
F/M,
qm BOD
(gm MLVSS) (Day)
.072
.066
.10
.081
.089
.105
.093
.089
0.11
MLSS,
mg/1 (% vol.)
3510(74)
3980(73)
2950(73)
3540(67)
4270(67)
4010(69)
3040(64)
3200(57)
3700(65)
Overflow
Rate,
m/d
12.3
12.6
12.3
12.3
14.1
17.4
22. 2b
23.l"
23.1
unknown solids losses.
Average overflow; 2:1 maximum-to minimum flow variation.
•»
"Two optional basins (one for methanol denitrification, the other for re-
aeration) were placed on stream.
12
-------
without anaerobic periods) to "burn out" the bulking growth. The attempt was
not totally successful but, with increasing water temperatures and summer
operating conditions, by June the solids settling characteristics had improved.
Summer operation was not plaqued with bulking growth. In the July and August
operation, the Fed3 added to the primary process reduced the COD and total
Kjeldahl nitrogen (TKN) as well as the phosphorus entering the biological
process. The process flow and reactor solids were altered to maintain the
desired operating F/M,
The organic, solids and phosphorus removals during the study are summarized
in Tables 2 and 3. Clearly the process provided excellent organic and solids
removal. Winter operation with bulking growth (January-April) did not prevent
efficient organic and solids removal but did require low clarifier overflow
rates. Since the process effluent contained nitrifying organisms, both the
standard BOD5 and the BOD5 with nitrification inhibition by 0.5 mg/1 of
thiourea are included in Table 2. The COD removals ranging from 83 to 91
percent across the biological reactor were both consistent and excellent, even
in September when methanol was added for residual nitrate removal.
The suspended solids removals (Table 3) ranged from 85 to 93 percent removal
across the biological reactor-clarifier. With the FeCl3 added in the primary
in July and August, the removals across primary and secondary treatment
averaged 95 percent. The phosphorus removals without specific chemical treat-
ment varied from 25 to 47 percent across the biological reactor. These phos-
phorus removals were unusually high for biological treatment especially for
operation at low F/M. Approximately 45 mg/1 of FeCl3, added into the primary
process in July and August, produced approximately 85 percent removal of the
phosphorus across primary and secondary treatment.
Nitrogen removals (Tables 4) revealed that the alternating nitrification-
denitrification process in the winter (15°C) produced about 75 percent
nitrogen removal across the biological system without methanol. In June at
higher twmperatures (25°C), the removal increased to 84 percent of the nitrogen
entering the biological reactorv The lower removals in April and May represent
operation for a portion of each month (early April and late May) with aerobic
and anaerobic conditions and for a portion of each month with continuous
aerobic conditions. The lower nitrogen removals across the biological process
in July and August (67 percent) occurred with altered operating conditions
(Table 4). The operation for January through June was performed at steady
flow and at COD/TKN ratios of approximately 10:1. With the addition of FeCl3
in the primary process and with diurnal flow variation, the nitrogen removals
across the biological process decreased significantly. In the most signif-
icant change, the FeCl3 addition decreased the COD/TKN ratio from 10:1 to
between 7:1 and 8:1. Although not shown in Table 5, the COD/NH3-N ratio
decreased even more sharply from approximately 18:1 to about 9:1. At present,
more work is required over a range of F/M and COD/TKN ratios, preferably with
automatic dissolved oxygen control, to determine more clearly the effect of
the controlling variables [F/M ratio, COD/TKN or COD/NH3-N ratios, and
dissolved oxygen cycle (time and dissolved oxygen values)] on the process
design and operation.
Filtration of the secondary effluent in July and August with 20 mg/1 of alum
13
-------
Table II. ORGANIC REMOVAL
Jan.
Feb.
March
April
May
June
Julyb
August
Sept.c
Primary
Influent ,
mg/1
96.5
99
110
98.8
115
107
51
44.2
99
BOD
Secondary
Effluent,
mg/1
20.4
14.0
16.3
10.0
8.4
7.1
7.7
6.2
15.4
Sec. (Inh)
Effluent,3
mg/1
-
-
6.5
5.3
3.3
3.2
3.8
2.6
7.2
Primary
Influent ,
mg/1
246
234
262
230
234
238
119
112
225
COD
Secondary
Effluent,
mg/1
40
25.1
30.6
27.7
24.3
23.4
20.9
17.3
31.6
% Removal
84
89
88
88
90
91
83
85
86
aBOD- test with 0.5 ng/1 of thiourea for Inhibition of nitrification.
•L
FeClq in primary process reduced influent organic load.
'llethanol addition (18.6 mg/1) during September 5-12, with two optional
reactor basins.
14
-------
TaBle III. SOLIDS AND PHOSPHORUS REMOVAL
Suspended Solids
Jan.
Feb.
March
April
May
June
July*
a
August
Sept.
Primary
Influent ,
mg/1
110
108
128
120
109
112
153
197
110
Secondary
Effluent ,
rag/1
15.4
14.3
15
13
11.8
7.8
9
10
16
% Removal
85
87
88
89
89
93
94
95
85
Primary
Influent ,
mg/1
7.25
7.12
7.10
6.22
7.24
7.14
5.88
5.78
6.77
P
Secondary
Effluent,
mg/1
4.83
4.33
3.76
3.69
5.03
5.32
1.14
0.88
3.80
% Removal
33
39
47
41
27
26
81
85
45
with FeCl3 in t'ie Primary process, the removals of solids and phosphorus
is based upon the raw wastewater rather than on the primary effluent.
15
-------
Table IV. NITROGEN REMOVAL
Secondary Effluent
1973
Jan.
Feb.
March
April
May
June
July*
August3
Sept.
Primary
TKN,
mq/1
25.7
23.2
24.8
21.7
23.3
24.0
15.0
14.9
22.6
TKN,
mq/1
2.28
1.52
4.20
5.20
1.36
1.51
2.14
1.23
10.2
NH4 + -N,
mq/1
0.53
0.31
2.40
3.90
0.31
0.45
1.63
0.59
9.4
(N03- + N02-)-N,
mq/1
3.99
4.41
2.30
6.03
8.25
2.30
2.72
3.74
0.22
mq/1
6.27
5.94
6.5
11.2
9.61
3.81
4.86
4.97
10.4
Total N
% Removal
76
75
74
49 b
59 b
84
67 (74)'C
67 (74)°
54 d
a
The FeCl3 in the primary process reduced the TKN in the influent to the biological
reactor.
Portions of each month operated with continuous aeration.
% Removal based upon the raw wastewater.
Methanol addition inhibited nitrification.
-------
Table V. NITROGEN REMOVAL AND THE F/M AND COD/TKN RATIOS
Period
Jan.
Feb.
March
June
July
August
Sept.
Temp.,
c
14.0
14.2
15.5
23.0
25.0
25.5
26a
F/M,
gm BOD/gm MLVSS/day
0.072
0.066
0.100
0.105
0.093
0.089
0.11
COD/TKN
9.6
9.9
10.5
10.3
7.9
7.5
10
% Removal
Total N
76
75
74
84
67
67
54
l-Iethanol addition inhibited nitrification.
17
-------
[A12(80^)3 . 14 H20] added ahead of the filters (Table 6) was chiefly effec-
tive for phosphorus removal. Average phosphorus residuals decreased by more
than 50 percent with filtered residuals of 0.4 to 0.5 mg/1 as phosphorus. The
accumulated removals increased from approximately 85 to 93 percent. Fil-
tration only modestly decreased other pollutant concentrations. The filtration
removals of these pollutants from the secondary effluent ranged from 17 to 35
percent.
The settling characteristics and sludge production in the biological process
is summarized in Table 7. The low production in April reflected solids accu-
mulation to replace unknown solids losses in the transfer of operation from
one clarifier to another in March and April. A high solids production in
July represents excess solids wasting to maintain the desired F/M ratio for
the decreased food input to the reactor during FeCls addition in primary
treatment. Solids production for August is not given because of an unknown
solids loss' (mechanical failure). The changes in the solids concentration of
the recycled sludge revealed the presence or absence of the thickening device
within the clarifier. The sludge volume index (SVI) (Table 7) and the initial
settling velocities of the mixed liquor (Figure 3) clearly indicated the poor
winter solids settling characteristics and the gradual disappearance of the
bulking sludge in the summer. Because the bulking sludge problem occurred at
low F/M operating conditions in several "continuously aerated activated sludge
studies in the EPA-DC pilot plant, the winter settling problems may not be
related to the on-off aeration sequence for nitrification-denitrification.
Laboratory denitrification tests (Figure 4) revealed that methanol addition
to the m^xed liquor leaving the second reactor basin removed the residual
nitrate from the wastewater. In September, the single stage nitrification-
denitrification system was operated with methanol addition for complete
nitrogen removal. The Fed3 addition for phosphorus removal, employed in pri-
mary treatment in August, was discontinued to increase the COD/TKN ratio
entering the biological reactor and to ensure maximum nitrogen removal.
Methanol was added on September 5 into the third, previously unused, anaerobic
basin of the biological reactor and was continued until September 12, at an
average dosage of 18.6 mg/1. The methanol was sufficient to supply the organic
carbon needed for complete denitrification. The small aerobic basin following
the anaerobic basin was also placed on stream.
With methanol addition, the residual TKN and ammonia in the effluent immedi-
ately increased. The TKN residual continued to increase throughout the month
of September, even after the CH3OH dosage was discontinued. Because of the
observed loss of nitrification in the single stage nitrification-denitrifi-
cation system, laboratory studies on methanol addition were performed on
samples of nitrifying sludge from the nitrification process in the pilot
plant's three stage activated sludge system. The methanol addition clearly
inhibited the batch nitrification kinetics of the nitrifying sludge and con-
firmed the loss of nitrification observed in the single stage nitrification-
denitrification system. It is not known whether acclimation of the nitrifiers
to methanol could be developed. Since the glucose added as a control in the
laboratory nitrification tests did not alter the laboratory nitrification
kinetic rate, organics compatible with the nitrifying organisms can be sub-
stituted for methanol to complete the denitrification. These organics, such
18
-------
Table VI. EFFECT OF ADDING Fed-}3 AND OF TERTIARY FILTRATION
COD
Effluent
Raw
Primary
Secondary
Filtered13
S
S
P
N
mg/1
July
250
119
21
15
Aug.
319
112
17
14
July
153
73
9
5.8
Aug.
197
58
10
7.4
July
5.88
3.10
1.14
0.46
Aug.
5.42
2.67
0.83
0.40
July
18.7
15.1
4.9
4.3
Aug.
19.1
14.9
6.0
4.4
45 mg/1 of FeCl3 added ahead of primary clarifier.
Dual media filters: 24 in. of 1.2- to 1.4-mm coal over 12 in. of 0.6- to
0.7-mm sand.
19
-------
Table VII. SETTLING AND SOLIDS PRODUCTION
Jan.
Feb.
March
April
May
June
July
August
Sept.
Overflow
Rate,
ffl/d
12.3
12.6
12.3
12.3
14.1*
17.4
22.2
23.1
23.1
Underflow
Solids,
mg/1
10,200
10,100
6,720
6,760
10,600
11,500
9,894
8,927
9,940
SVI,
ml/gm
245
250
330
277
227
188
133
134
121
Solids Produced,
gm/gm Applied BOD
Total
_a
.63
.92
_a
.80
.77
1.11
_a
_a
Wasted
_a
.49
.71
_a
.70
.70
0.94b
_a
_a
Solids production not given because of solid accumulations or unknown
solids losses. In September, optional reactor basins were placed on stream.
The solids production reflects increased wasting to control F/M for decreased
influent food.
20
-------
v. 12-
510-
O 8-
LLJ
> 6-
O
Z
LU
CO
-I
< 2H
50 100 200 300 400
SLUDGE VOLUME INDEX (SVI), ml/gm
Figure 3. Initial settling velocity and the SVI
-------
5-1
K3
IN)
~ 4H
O)
E
JL 3-
lot
O
z
& 2-
1-
20
Temp., 15.5°C.
MLSS, 4292
% Vol., 73.5%
CH3OH, 20 mg/l
8=0.023 g
(gm MLVSS)(day)
40
9*0
100
60 80
TIME, minutes
Figure 4. Methanol denltrification of residual (N01+ N03)-N
-------
as primary sludge, may be added either in the second basin of the process or
into the optional small anaerobic basin to increase residual nitrogen removal,
23
-------
SECTION VII
NITRIFICATION-DENITRIFICATION KINETICS
Batch laboratory nitrification-denitrification kinetic tests were performed
periodically during the study. The typical cycling of the nitrogen species
during the laboratory tests with on-off aeration is shown in Figure 5. The
laboratory batch kinetic rate constants from these laboratory tests are
summarized in Table 8. In the winter (15°F»), the initial nitrification and
denitrification rate constants were approximately similar in value and suggest
equal time periods of aerobic and anaerobic conditions for most efficient
operation. In contrast in summer, the nitrification rate constant was clearly
higher than the denitrification rate constant. Thus, denitrification required
more time than nitrification and controlled the reactor size.
Careful examination of the rate constants and the rate plots (Figure 5) also
revealed that the nitrification rate constant tended to be similar through
complete removal of the ammonia. The initial denitrification rate constant
tended to decrease as the batch reaction continued. The maximum batch
denitrification rate constant persisted for about one anaerobic period in the
warm weather and, in winter with slower carbonaceous activity, extended into
the second anaerobic period of the on-off cycle. Lower rates occurred for
subsequent anaerobic periods and tapered off to immeasurable levels even
though substantial C3 to 4 mg/1 NOa-N remained in the water, as shown in
Figure 5.
An effect of the COD/TKN ratio also occurred in the batch kinetic studies.
In June at 25°C with a COD/TKN ratio of approximately 10:1, the denitrifi-
cation rate constants, although lower than the nitrification rate constant
exhibited easily measurable values through four aerobic-anaerobic cycles.
Through July and August with COD/TKN ratios of about 7.5:1, the initial cycle
denitrification rate constant exhibited a decreasing value. The decreasing
value of the denitrification rate constant probably was produced by a gradual
shift in the organism population distribution of the mixed liquor and by the
decreased availability of indigenous organic carbon. At the low F/M condi-
tions. A shift in the population distribution should occur with both the
decreasing COD/TKN or COD/NH3-N ratio and with continued summer operation.
The subsequent values of the denitrification rate constants during repetitive
aerobic-anaerobic cycling were also difficult to measure at the low COD/TKN
ratio. With the cycle time in an August test (26.5°C) providing 30 min of
aeration and 60 min of anaerobic activity, the endogenous denitrification rate
constant was estimated at 0.0075 gm N/gm MLVSS/day.
Clearly, as the organics in the water during the batch test were depleted by
the biological activity, the denitrification rate decreased. Thus the
denitrification kinetic behavior in the batch laboratory reactor supported the
observation that the decrease in nitrogen removal during July and August was
significantly related to the decrease in the COD/TKN concentration ratio in
the wastewater entering the biological reactor.
24
-------
N)
cn
O)
E
LU
o
o
DC
TEMP. 15.5°C
MLSS 3710 mg/l
TIME, hours
Fifure 5. Cyclic nitrification and denitrification
-------
Table VIII. BATCH KINETICS FOR CYCLING AEROBIC-ANAEROBIC CONDITIONS
Nitrification
2/22/73
6/28/73
7/20/73
8/24/73
Denitrifi cation
2/22/73
6/28/73
7/20/73
8/24/73 b
Temp. ,
°C
15.5
25.0
27.0
26.5
15.5
25.0
27.0
26.5
qm N/qm MLVSS/Dav
kl k2 k3 k4 k5
0.032 0.042 0.016 0.026 0.035
0.083 0.095 -
0.11 0.11 -
0.12 -
0.032 0.029 0.021 0.019
0.055 0.030 0.033 0.030
0.042 -
0.026 0.0075b -
30-min cycle of aerobic-anaerobic operation.
Cycle altered; 30 min aerobic, 60 min anaerobic-
26
-------
SECTION VIII
REFERENCES
1. Downing, A. L., and Hopwood, A. P., "Some Observation on the Kinetics
of Nitrifying Activated-Sludge Plants," Schweig. Z. Hydrol., 26, 271
(1964).
2. Ludzack, F. J., and Ettinger, M. B., "Controlled Operation to Mini-
mize Activated Sludge Effluent Nitrogen," Jour. Water Poll. Control
Fed., 34, 920 (1962).
3. Wuhrman, K. Effects of Oxygen Tension on Biochemical Reactions in
Biological Treatment Plants. In: Advances in Biological Waste
Treatment. Proceedings of the 3rd Conference at Manhattan College,
New York City, 27, April 1960.
4. Barnard, J. L., "Biological Denitrification," South African Branch
Water Pollution Control, 705 (1973).
5. Earth, E. F., Brenner, R. C., and Lewis, R. F., "Chemical Control of
Nitrogen and Phosphorus in Wastewater Effluent," Jour. Water Poll.
Control Fed., ^6, 2040 (1968).
6. Heidman, J. A., Bishop, D. F., and Stamberg, J. B., "Carbon, Nitrogen
and Phosphorus Removal in Staged Nitrification-Denitrification
Activated Sludge Treatment," AIChE Symposium Series 145, 71, 264 (1975)
7. Balakrishman, S., and Eckenfelder, W. W., "Nitrogen Removal by
Modified Activated-Sludge Process," Jour. Sanit. Engrng., Div. Am.
Soc. Civ. Engrs., 96, SAZ (1970).
8. Matsche, N., "The Elimination of Nitrogen in the Treatment Plant of
Vienna-Blumental," Water Res., ^, 485 (1972).
9. Von de Emde, W., Personal Communication.
10. Barnard, J. L., "Cut P and N without Chemicals," Water and Wastes
Engrg., 11, Part I, 33 (July 1974), Part II, 41 (August 1974).
11. Christensen, M. H. Denitrification of Sewage by Alternating Process
Operation. In: Proceedings of the 7th International Conference on
Water Pollution Research, Pergammon Press Ltd., London, 1413 (ii)
(1974).
12. "FWPCA Methods for Chemical Analysis of Water and Wastes." U.S. Dept.
of the Interior, Fed. Water Poll. Control Adm., Cincinnati, Ohio
(November 1969).
13. Kamphake, L., Hannah, S., and Cohen, J., "Automatic Analysis for
Nitrate by Hydrozone Reduction," Water Res., _!, 205 (1967)
27
-------
14. Gales, M., Julian, E., and Kroner, R., "Method for Quantitative
Determination of Total Phosphate in Water," Jour. Am. Water Wks.
Assoc., 58, 1363 (1966).
15. "Standard Methods for the Examination of Water and Wastewater."
12th ed., American Public Health Association, New York (1965).
28
-------
TECHNICAL REPORT DATA
(Please read iHstructions on the reverse before completing}
REPORT NO.
EPA-670/2-75-051
2.
3. RECIPIENT'S ACCESSION-NO.
TITLE ANDSUBTITLE
SINGLE STAGE NITRIFICATION-DENITRIFICATION
5. REPORT DATE
June 1975; Issuing Date
6. PERFORMING ORGANIZATION CODE
Uollott F.Bishop
James A. Heidman
John B. Stamberg
8. PERFORMING ORGANIZATION REPORT NO.
PERFORMING ORGANIZATION, NAME AND ADDRESS
Government of the District of Columbia
Department of Environmental Services
EPA-DC Pilot Plant
5000 Overlook Avenue, S.W.
Washington, D.C. 20Q32
10. PROGRAM ELEMENT NO. 1BB043
ROAP 21-ASO Task 026
11. CONTRACT/BTOUSX NO.
68-01-0162
2. SPONSORING AGENCY NAME AND ADDRESS
National Environmental Research Center
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final Report-12/72 to 9/73
14. SPONSORING AGENCY CODE
5. SUPPLEMENTARY NOTES
6. ABSTRACT
The removal of 75-84% of the nitrogen from primary wastewaters was achieved in a
single stage activated sludge process. The two-pass reactor was operated with a food
to mass ratio (F/M) of approximately 0.1 gm BOD5/day/gm MLVSS. The air was applied
on a 30-minute cycle first to one reactor pass then to the other pass. Mechanical
mixers uspended the mixed liquor solids when the air was not applied. The D.O. con-
centration varied from 0.0 mg/1 without air to 2-3 mg/1 during aeration. In June with
a 9-hour detention time in the reactor, the residual pollutant concentrations averaged
23 mg/1 of COD (90% removal), 3.8 mg/1 of total nitrogen (84% removal) and 7.8 mg/1 of
suspended solids (93% removal). In February, with a 12-hour reactor detention time,
the residual pollutants averaged 25 mg/1 of COD (89% removal), 6.0 mg/1 of Total N
(75% removal) and 14 mg/1 of suspended solids (87% removal). Since the nitrate from
nitrification was used to remove BOD5 from the wastewater, the process advantages
for single stage nitrification-denitrification included the reduction in the amount
of air needed to achieve 6005 removal and nitrification; the minimizing of supple-
mental organic carbon sources for denitrification; and the elimination of clarifiers
in staged nitrification-denitrification.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
*Nitrification
Nitrobacter
Nitrosomonas
*Activated Sludge Process
Aeration
*Anaerobic Processes
Dissolved Gases
Sewage
Oxygen
Treatment
Nitrogen Removal
*Denitrification
*0n-0ff Aeration
Single Stage
Nitrification-
Denitrification
EPA-DC Pilot Plant
TJaoVi-i nctt-rm P.P.
13B
not*
Try"
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
37
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
29 •% I).S. GOVERNMENT PKINTING OFFICE: 1975-657-593/5388 Region No. 5-11
------- |