EPA-670/2-73 058
September 1973
Environmental Protection Technology Series
Ammonia - Nitrogen Removal By
Breakpoint Chlorination
Office of Research and Development
U.S. Environmental Protection Agency
Washington, D.C. 20460
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Monitor.thg, Envizon-
mental Protection Agency, have been grouped into five series. These
five broad categories were established to facilitate further develop-
ment and application of environmental technology. Elimination of
traditional grouping was conscious2y planned to foster technology
transfer and a maximum interface in related f.i el ds. The Live series
are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the Environmental Protection Technology
Series. This series describes research performed to develop and demon-
strate instrumentation, equipment and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution.
This work provides the new or improved technology required for the
control and treatment of pollution sources to meet environmental
qual.i ty standards.
ERA REVIEW NOTICE
This report has been reviewed by the Office of Research and Development,
EPA, and approved for publication. Approval does not signify that the
contents neaesgaril reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or cozmnercia.1
products constitute endorsement or recommendation for uae.
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EPA-670/2-73-058
September 1973
AMMONIA-NITROGEN REMOVAL
BY
BREAKPOINT CHLORINATION
BY
Thomas A. Pressley
Dolloff F. Bishop
Adolf P. Pinto
Alan F. Cassel
Contract No. 14-12-818
Project 11010 EYM
Program Element 1B2033
Project Officer
Dolloff F. Bishop
Advanced Waste Treatment Research Laboratory
National Environmental Research Center
Cincinnati, Ohio 45268
Prepared for
Office of Research and Development
U.S. ENVIRONMENTAL PROTECTION AGENCY
Washington, DC 20460
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402 - Price 95 cents
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ADS TRACT
In labora tory breakpoint ch.lorina ti on, sod urn hypochi on te oxi di zed the
ammonia in buffered p,ater (with 20 mg/i of NH -N) and in raw, secondary
and lime clarified and filtered municipal wa ewa tens chiefly to 5 gas
with only small amounts of NQ ,—N (0.3mg/i at pH 5 to 2.0 mg/i at pH 8)
a.nd NC1 3 —N (0.3 rag/i at pH 5 o 0.05 mg/i at pH 8) The oxidation of
ammonia to nItrogen gas was best accomplished in a pH range 6-8 and for
water or highly pretreated wastewater required a Cl:NH 3 -N weight ratio
of app.i ximateiy 8:1. Higher Cl :NH 3 —N ratios (8-10 :1) were needed as
the degree of pretreatment of the wastewa tens decreased. The reaction
of intermediate monochloramine and free chlorine was complete in less
than one minute at pH 7.0.
In the pilot plant, gaseous chlorine with sodium or calcium hydroxide
for pH control was efficiently mixed with filtered secondary and with
lime clarified and filtered raw wastewater. The NH was oxidized with
greater than 95 conversion to N 2 gas. Operation a controlled p11
greater than 6.0 and with low residuals of free Cl 2 (2 mg/l)_produced
essentially no nitrogen trichloride and modest amounts of N0 3 -N (0.6 mg/i).
In the breakpoint chlorination of wastewaters, a split chlorine feed
technique to utilize the alkalinity of the water and decrease the amount
of base required in the process is described.
The instrumentation used in the control of the process and additional
instrumentation designed and built by the EPA-EC Pilot Plant staff to
fully automate the process is also described.
This report was submitted in partial fulfillment of Project 11010 EYM
and Contract No. 14-12-818 by the Department of Environmental Services,
Government of the District of Columbia under the sponsorship of the
Environmental Protection Agency. Work was completed as of September 1971.
.1.2
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CONTENTS
Page
Abstract
List of 2’igures iv
List of Tables
Acknowledgments
Sections
I Conclusions
II Recommendations 3
III Introduction 4
IV Experimental 6
V Analytical Procedures 9
VI Reaction Products, pH and Temperature 11
VII Laboratory Chlorination of Wastewaters 19
VIII Pilot Plant Chlorination 28
IX Instrumentation and Process Control 36
X Costs 42
XI References 44
XII Publications and Patents 45
iii
-------
FIGURES
No. Page
1. Breakpoint Ch1orinc tion Reactor 8
2. Breakpoint in Buffered Aqueous SWstems 13
3. Ammonia Removal at Breakpoint 14
4. pH and Nitrite # Nitrate Formation 15
5. pH and Nitrogen Trichloride Formation 16
6. pH and Byproducts after Breakpoint 17
7. Breakpoint in Raw Wastewater 20
8. Breakpoint in Secondary Effluent 21
9. Breakpoint in Lime Clarified Filtered Secondary Effluent 22
10. Breakpoint in Lime Clarified Filtered Raw Was tewate.r 23
11. Byproducts in Raw Wastewater 24
12. Byproducts in Lime Clarified Raw Wastewater 25
13. Byproducts in Secondary Effluent 26
14. Byproducts in Lime Clarified Filtered Secondary Effluent 27
15. Daily NH 3 -N Removal Variations 32
16. Breakpoint Chlorination Instrumentation 37
17. Breakpoint Computer Controller 40
iv
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TABLES
No. Page
1. Breakpoint Chlorination - Temperature Studies 18
2. Breakpoint Chlorination of Filtered Secondary Effluent 29
3. Base Requirement in Breakpoint Chlorination of Filtered 29
Secondary Effluent
4. Nitrate Formation with pH in Filtered Secondary Effluent 30
5. Nitrogen Trichioride Formation with pH in Filtered 30
Secondary Effluent
6. Continuous Breakpoint of Lime Clarified and Filtered 33
Raw Wastewater
7. Breakpoint Chlorination Costs 43
V
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ACKNOWLEEGI IENTS
Walter W. Schuk of the Environmental Protection Agency developed the
control system described in the study and is acknowledged with sincere
thanks.
The operation of the pilot system with analytical support was performed
by the EPA—DC Pilot Plant staff under the supervision of Chief Operator,
George D. Gray, Chief Mechanic, Robert A. Hallbrook, and Chief Chemist,
Howard P. Warner.
v i
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SECTION I
CONCLUSIONS
1. Ammonia in wastewater and water was oxidized by chlorine to
chiefly nitrogen gas with a residual NH 3 -N concentration of less than
0.1 mg/i. The chlorine was applied in the gaseous state to a stream
of water and base before mixing with the wastewater, or as a solution
of hypochlorite. The base was required to neutralize the acid produced
in the reaction. The chlorine and base were premixed before addition
to the wastewater and then completely and rapidly mixed with the waste-
water in a breakpoint reactor.
2. In the laboratory on buffered aqueous systems, monchioramine con-
centrations increased with chlorine doses of up to about a 5:1 weight
ratio of C1:NH 3 -N and then decreased to zero at the breakpoint dosage
of at least 8:1 Cl:NH 3 —N. Qualitatively, the formation of monochlor-
amine and the reaction between intermediate rnonochloramine and free
chlorine at pH 7.0 was completed in less than 1 minute.
3. In the aqueous systems, traces of dichioramine were produced in the
5-8 pH range. Nitrate and nitrogen trichioride were also produced.
Potential products of N 2 0, NO, and NO 2 were not detected.
4. In the buffered aqueous systems at the breakpoint, NO 3 formation
increased from 1.5 of the influent NH 3 -N at pH 5 to about i09 at pH 8.
Simultaneously, the Nd 3 formation decreased from l.59 of the influent
NH 3 —N at pH 5 to 0.259 at pH 8.
5. In wastewater, the amount of NO 3 formation was less (40-50?6) than
that in the aqueous system but varied with pH similar to the buffered
aqueous system. Above pH 6.5 with proper mixing, the nitrogen
trichloride formation in wastewaters and aqueous systems did not occur
until after the chlorine dosage exceeded a 7.6:1 weight ratio of
Cl:NH 3 -N. In general in the pH range of 6-8, increasing the amount of
excess free chlorine in the wastewater after breakpoint increased the
amount of Nd 3 produced. Also, as in the aqueous system after the
breakpoint, the formation of Nd 3 increased as the pH of the wastewater
during the breakpoint process decreased.
6. In wastewater the formation of Nd 3 decreased with decreasing pre-
treatment. Indeed, in raw wastewater, Cl:NH 3 -N dosages up to 12:1 did
not produce Nd 3 .
7. The Cl:NH 3 -N weight ratio at breakpoint varied from approximetely
8:1 to 10:1 for the wastewaters tested. Increased pretreatment reduced
the amount of chlorine with lime clarified and filtered secondary
effluents requiring an 8:1 Cl:NH 3 -N dosage.
1
-------
8. A temperature effect on the twpe and amount of reaction products
did not occur in the 5°C to 40°C range.
9. With gaseous chlorine, the base required increased with final
reaction pH. At pH 7 in secondary effluent, 1.3 pounds of NaOH were
used per pound at Cl 9 (1.25 equivalent per equivalent of chlorine).
Stoichiometrically, 1.5 pounds of NaOH per pound of Cl 2 were required
to neutralize all the acid produced by the breakpoint. The difference
between the base theoretically needed and that actually used was
attributed to acid neutralization by the alkalinity of the secondary
was tewa ter.
10. Since NC1 9 formation occurred when the C1:NH 3 -N ratio was greater
than that needed for breakpoint, especially under acidic conditions,
and NO 3 production increased at higher pH (8), care must be taken
during the addition of base and chlorine to produce conditions that
will minimize the production of either of these undesirable residuals.
Breakpointing at a p11 of approximately 7 with very careful control of
excess free chlorine provided the best overall operation.
2
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SECTION II
RECOMMENDATIONS
The stirred tank reactor employed in the study should be replaced by
inline mixers which can efficiently and rapidly mix the chlorine with
the wastewater. The chlorine dosing and the stability of the pH control
loops should be evaluated with the inline mixer reactor.
Kinetic studies on the oxidation of nonochlorarnine to nitrogen gas and
on the by—product reactions are recommended to optimize reactor design.
The use of chlorine in the pH reduction step (l0. O-7.0) liberates CO 2
which can be stripped out of the water. If the CO 2 is left in the
water, part of the base added will react with it. Hence, the decrease
in base usage with two stage chlorination with the intermediate CO 2
stripping should be evaluated.
Chlorine and CO 2 may be used to reduce the pH of the first stage lime
clarified effluent from pH 11.5 to 7. For initial ammonia concentrations
of up to 20 mg/i, the alkalinity (‘ 300 mg/i as CaCO, ) of lime clarified
water at pH 11.5 is sufficient to neutralize the acid produced by the
breakpoint reaction. Thus the second stage clarification in the two
stage lime clarification can be eliminated. This process alternate
requires evaluation.
Instrumentation described in the report has been designed and built by
the Pilot Plant Staff to completely automate the process. Evaluation
of the instrumentation under continuous operating conditions is needed.
3
-------
SECTION III
INTRODUCTION
Breakpoint chlorination, as practiced for many years in the water treat-
ment industry provides a physical—chemical means for removing ammonia
from wastewaters. In water, at NH 3 -N concentrations usually below
1 mg/i, chlorine reacts with the ammonia to form various chioramines:
Cl 2 + 1120 hOd HC1 (1)
NH + IIOC1 - NH C1 ÷ H 2 0 + H (2)
NH 2 C1 4 - hOd NHC1 + 1120 (3)
NHC1 2 + HOC1 - NC1 + H 2 0 (4)
Chlorine is added to process waters until a point is reached where the
total dissolved residual chlorine has reached a minimum (the breakpoint)
and the NH 3 -N has disappeared.
In water at NH 3 —N concentrations of less than 1 mg/i, and before the
breakpoint, the type of chioramine formed depends upon the pH.
Spectrophotometric analyses (2, 4, 5, 12, 14) indicates that the chief
constituent is monochioramine in the pH range of 7-8.5. As the pH
decreases below 7, increasing amounts of dichioramine appear. In the
ph’ range of 4.5-5.0, dichioramine is the chief product; below pH 4,
nitrogen trichioride is the chief product.
Breakpoint chlorination studies (17) on buffered sunthetic ammonia
samples at pH 7.0 reveal that the monochiorarnine concentration reaches a
maximum at the 5:1 weight ratio of Cl:NH 3 -N. As the weight ratio of
Cl:NH 3 —N exceeds 5:1, the monochioramine breaks down (6) to form
dichiorarrdne and ammonia according to equation 5.
2NH 2 C1 > NHC1 + NH 3 (5)
The dichloramine reaches a maximum concentration at the Cl:NH 3 —N weight
ratio of about 7.5:1.
The literature (7) indicates that in water with less than 1 mg/i of
1 ’111 3 -N, the reactibn proceeds in competition with monochioramine
formation (equation 2) until the chlorine dosage reaches the breakpoint
at approximately a 10:1 weight ratio of Cl:NH —N. Other studies, (3, 14)
however, indicate that monochioramine is oxidized by excess chlorine under
slightly alkaline conditions to nitrogen gas as shown in equation 6.
4
-------
2NH 2 C1 ÷ Hod ) - N ÷ 3HC1 ÷ H 2 0 (6)
Stoichiometrically, the ammonia oxidation through monochioramine to N 2
corresponds to a 7.6:1 weight ratio of Cl:NH 3 -N. The literature
(2, 4, 7, 14) also suggests the occurence of other end—products, in-
cluding nitrate as shown in equation 7 and nitrogen trichloride. In
fact, the nitrogen trichoride produced (equation 4) in water treatment
plants (10) during breakpoint chlorination has been a serious problem.
4HOC1 + NH 3 > - HNO ÷ 4HC1 ÷ Ff20
In wastewaters, the NF l 3 —N concentration may be more than an order of
magnitude higher than those normally encountered in natural waters.
For use of breakpoint chlorination for nitrogen removal at the NH 3 -N
concentrations encountered in wastewaters, the end products of the
reaction need to be known. This study identified the major products of
the breakpoint reaction in synthetic buffered solutions and in waste-
waters and determined the formation and behavior of the the undesirable
residuals of Nd 3 and NO 3 as a function of pH, chlorine dosage, and
temperature in both laboratory the pilot plant studies.
5
-------
SECTION IV
EXPERIMENTAL
The laboratory chlorination studies ere performed in synthetic buffered
aqueous systems, to eliminate effects from other substances reacting with
chlorine, usually with initial NH 3 -N concentrations of 20 mg/i, and in
raw, secondary and lime clarified municipal wastewaters with NH 3 -H con-
centrations ranging from 8-15 mg/i.
In end product identification studies on buffered aqueous systems, the
gaseous products were qualitatively analyzed for N ‘ °2’ and N 2 0. The
procedure involved purging the aqueous sample, con aining 20 mg/i NH -N
and buffered at pH 7.5, with helium to remove atmospheric nitrogen iA a
closed system attached to a gas chromatograph. The sample was then
treated with nitrogen-free standard sodium hypochiorite at a dosage equal
to a 10:1 weight ratio of Cl:NH 3 -N. The gaseous products of the reaction
were then flushed through the gas chromatograph with helium.
A test was also performed to identify the presence of any nitric oxide
(NO) or nitrogen dioxide (NO 2 ). An enclosed breakpoint chlorinated
sample was purged with air into a cold solution of 0.2N H 2 S0 4 to remove
any NO or NO 2 formed during the chlorination. In the procedure, (13)
the NO was oxidized to NO 2 by the oxygen in the air and absorbed in the
O.2N H 2 SQ as nitrous and nitric acids. The 0.2N H 2 S0 4 solution was then
analyzed or (NO 3 ÷ N0 2 )-Th
Spectrophotometric scanning of both chlorinated aqueous samples and
their CC1 4 extracts was also performed to determine other end-products.
The samples, after two hours of contact with the chlorine, were placed
in a recording spectrophotometer and scanned over the spectral range of
200-500 mg/i.
In the quantitative studies, aqueous ammonia or wastewater samples were
manually mixed in separatory funnels with increasing dosage of standard
sodium hypochlorite over the pH range of 5-8. A two hour contact time
(17) was provided for_all systems. The samples, after chlorination,
were analyzed for (NO 3 + N0 2 )-N, NC1,N, NH 3 -N, NH 2 C1 and NHC1 2 -N, TKN,
total residual chlorine, free avai1a le chlorine and pH.
Quantitative temperature studies were also conducted at 5, 15, 25, and
40°C on aqueous ammonia samples containing 20 mg/i NH 3 -N and buffered
at pH 6.0 to determine temperature effects on the end-products. Multiple
samples in separatory funnels were collected at the designated temperature
in a constant temperature bath and treated with increasing dosages of
pre-cooled sodium hypochlorite. The samples were then manually mixed
and allowed to stand at the selected temperature for two hours before
analysis.
6
-------
The pilot chlorination reactor, designed after completion of the laboratory
studies, was a 1200 gal. vessel, 4 ft. in diameter and 12 ft. tall.
This reactor is shown in Figure 1. The reaction chamber in the bottom
3 ft. of the vessel (310 gal.) was divided into three equal sections
separated by epoxy coated plywood inserts. In each section, a propeller
mixer mounted on a common shaft, driven by a 3 h.p. motor, was rotated
at 224 r.p.m. The influent entered at the top of the reactor. A pump
on the effluent stream recycled 11 gal .1mm into the first mixing stage.
Chlorine was added into the recirculation loop through a Wallace and
Tiernan V-Notch chlorinator and injector nozzle. The dosage rate could
be manually set to provide up to 12:1 Cl:N ratio by weight. The reaction
pH was maintained automatically by pumping a 1 to 22 NaOH solution into
the line ahead of the chlorine injector. The base and C1 , were premixed
before addition to the wastewater. The NaOH feed was con rolied by an
inline pH probe on the discharge effluent using a variable stroke
positive displacement pump.
Initial runs were made on filtered secondary effluent at constant flow
rates that ranged between 25 and 35 gpm. The average feed concentration
of BOD, COD, TOC and suspended solids was 16 mg/l, 41 mg/i, 18 mg/i and
25 mg/i respectively. The pH ranged from 7.0 to 7.3 with an alkalinity
of 120-150 mg/i as CaCO . Later the system was run continously, usually
at 25 gpm on lime c1ari ied and filtered wastewater having a pH 9.3-9.9,
phenolphthalein alkalinity in the 50-70 range, and methyl orange alkalinity
in the range of 80-140 mg/l as CaCO 3 .
The study required field measurements for NH 3 -N which were made using
a specially programmed Technicon Automatic Analyser. Free and combined
chlorine were measured by the Modified Palm Method. During the later
part of the study the free residual chlorine was monitored continuously
using another Technicon Automatic Analyser. Nitrogen trichioride and
nitrate formation were measured periodically in the laboratory.
7
-------
TO
CARBON
COLUMNS
FIGURE 1 Breakpoint Chlorination Reactor
FEED
CI
Ca(OH) 2
8
-------
SECTION V
ANALYTICAL PROCEDURES
A modified automated hydrazine reduction method (9) with an alkaline
digestion to eliminate chioramine interferences was used for the
(NO 3 ÷ N0 2 )-N analysis. In this procedure, the sample was made
alkaline with NaOH, digested to dryness, redissolved and neutralized
with HC1 and diluted to volume for analysis. This modified procedure
was reliable to + .05 mg/i.
Nitrogen trichioride was analyzed in the synthetic amnvnia samples and
in wastewater samples by both the spectrophotometric procedure of
Czeh et al. (5) and the Modified Palm Method (16). The results of
spectrophotometric analysis of Nd 3 on wastewater samples were repro-
ducible to ÷ .05 mg/i NC1 3 -N. While analyses from the Palm Method
were similar to those from the spectrophotometric procedure, they were
poorly reproducible; therefore, the analyses from the spectrophotometric
procedure were employed in the evaluation of the breakpoint studies
on wastewaters. The spectrophotometric procedure for the analysis of
Nd 3 , however, represented all compounds extracted from the was tewater
that absorb at 265 and 345 in CC1 4 .
Free available chlorine, mono and di-chioraraine were analyzed according
to the Modified Palm Method (16) with N, N-diethyl-p-phenylenediamine
oxalate as the indicator, and by arnperometric titration (15). All
other analyses employed the procedures of “Standard Methods for the
Examination of Water and Wastewaters” (15).
A Varian Aerograph Gas Chromatograph (Model 1532-2B) equipped with an
ionization detector (with a tritium source) and a molecular sieve
type column, designed to detect N 2 , 2 ’ and N 2 0, was used to
qualitatively analyze the gaseous product. A Forma (Model 2095)
circulating—constant temperature bath was used to control the temp-
erature of the studies to ÷ 0.1°c U.V. absorbancies were determined
on a Beckman DU Spectrophotometer or on a Beckman DBG recording
Spectrophotometer, both with 10 mm quartz cells.
The Nd 3 standard was prepared by the method of Noyes (1) and assayed.
liquots of the stock were diluted in carbon tetrachioride to make
standard solutions ranging from 0-100 mg/l Nd 3 . The Nd 3 standards
scanned in the spectrophotometer produced peaks only at 265 and 345
m, o with linear absorbancies for increasing Nd 3 concentrations. The
molar absorptivities calculated from the standards were 228 for the
peak at 345 mp and 445 for the peak at 265 m,.i. The molar
absorptivitieS reported by Czeh, Fuchs, and Antczak (5) were 232 for
the peak at 345 mj and 450 for the peak at 265 m i.
9
-------
Spectroqrade Cd 4 was used throuqhout the study for extraction and
• dilution of standard and unknown samples. All other reagents used
in the study were of reagent quality.
10
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SECTION VI
REACTION PRODUCTS, pH AND TEMPERATURE
Reaction Products
In laboratory studies on buffered solutions at pH 7.5 with 20 mg/i
of NH, -N, spectrophotometric scans (200-500 m ,.u) of the buffered
solution treated with increasing dosages of chlorine up to Cl:NH 3 -N
weight ratio of 7.5:1 revealed adsorption peaks at 243 m,. for
monochloramine (11) and at 205 m t1 for nitrate (15). Above the 7.6:1
ratio the peak at 243 m v disappeared and a peak at 287 m, u appeared
and increased with increasing chlorine doses. The control sample
containing only the buffered solution with free Chlorine and without
NH 3 -N, produced a peak only at 287 m,u. Thus, the peak at 287 m,u
above the 7.6:1 ratio of Cl:NH 3 -N was produced by free chlorine.
The absorbance produced by monochioramine at 243 m, increased
linearly with increasing chlorine dosages to a maximum at the 5:1
weight ratio of C1:NH 3 -N and then decreased to near zero at approxi-
mately the 7.6:1 ratio. The strong peak produced by NO at 205 m ’
Lncreased with increasing chlorine dosage through the l :l weight
ratio of Cl:NH 3 -N and confirmed the formation of NO 3 during
chlorination. In summary, spectral scanning of the aqueous ammonia
solutions with different dosages of sodium hypochiorite indicated
only the formation and decomposition of monochloramine, the gradual
formation of N0 3 -N and the presence of free available chlorine after
the breakpoint.
The CC1 4 extracts, scanned in the spectrophotometer in the range of
200-500 m .z against a reference control blank, produced strong Nd 3
peaks (5) at 265 and 345 mAi only after the chlorine dosage exceeded
the 7.6:1 weight ratio of Ci:NH -N. At pH 6 and below measurable
but small amounts of Nd 3 were detected below the 7.6:1 weight ratio
of Cl:NH 3 -N.
Gas chromatographic analysis for N , O and N 2 0 at the breakpoint
detected only P 1 2 . Since 02 was no detected, the formation of N 2 O
with subsequent decomposition to N 2 and 02 did not occur. The tests
for nitric oxide (NO) and nitrogen dioxide (NO 2 ) revealed that neither
of these compounds were present. Qualitatively, the reaction of
intermediate monochloramine and free chlorine was completed in less
than one minute. Rate of formation of N 2 was not determined.
pH and Temperature
The addition of chlorine to buffered aqueous samples containing 20 mg/i
N11 3 -N at pH 7.0 produced a typical breakpoint curve with the complete
11
-------
removal of the ammonia and a minimum total residual chlorine concen-
tration of about 0.6 mg/i at approximately 8:1 weight ratio of
Cl:2VH 3 -N (Figures 2 and 3). The chlorination studies of buffered
solutions at pH 7.0 containing 1 mg/i Nil 3 -N also reached the break-
point at approximately an 8:1 weight ratio of Ci:N11 3 -N. In all tests
on buffered aqueous systems in the pH range of 6-7, the breakpoint
occurred at chlorine doses approximately equal to an 8:1 (stoichio—
metric ratio 7.6:1 equations 2 and 6) weight ratio of Cl:Niq 3 -
Outside of the 6—7 pH range, the chlorine dose required for the break-
point increased as shown in Figure 2.
In pH studies on synthetic samples containing 20 mg/i NH -N, the
formation of N0 3 -N at the breakpoint (Figure 4) increased with increas-
ing pH from about 0.3 mg/i (i.5°/ of the NH -N) at pH 5.0, to about
2.0 mg/i (l0 of the NH 3 -N) at pH 8.0. With increasing chlorine
dosages above the breakpoint, the N0 3 -N formation increased sharply at
pH 6.0 and above, but increased only slightly in the pH range of 5-6.
Thus, low pH (5-6) produced minimum amounts (0.3 mg/i) of N0 3 -N.
In contrast, the amount of NC1 3 -N formed at the breakpoint decreased
from approximately 0.3 mg/i (l.5’ ) at pH 5.0 to 0.05 mg/i (0.25’s) at
pH 7 or 8 as seen in Figure 5. In the pH range of 7-8, a Nd -N
concentration of less than 0.1 mg/i occurred at the breakpojn . As
chlorine dosages exceeded the breakpoint, the formation of Nd 3
increased sharply at and below pH 7.0. At pH 8.0, however, the Nd
concentration increased but never exceeded approximately 0.3 mg/i f r
chlorine dosages up to a 12:1 weight ratio of Cl:NH 3 -N. With NC1
which has a greater nuisance potential than nitrate, chlorination 3 at
the 9:1 weight ratio of Cl:NH. -N (slightly above the breakpoint)
minimized Nd 3 residuals at pfl 8 but maximized nitrate formation as
shown in Figure 6.
The 0.05 mg/i of Nd 3 at the breakpoint (produced by the oxidation
of dichioramine) (6) also indicated only small amounts of dichioramine
formed above pH 7. The modified Palm Analysis for dichloramine
revealed less than 0.1 mg/i NHC1f N above pH 7 after 2 hours of contact
time for all chlorine dosages.
Breakpoint Chlorination studies On buffered aqueous systems at pH 6.0
were conducted in the temperature range of 540 0 C. These tests did
not reveal significant changes in the reaction products after the
2 hour contact time. Complete removal of the NH -w was achieved at
all temperatures as seen in Table 1. 3
12
-------
E
LU
z
0
-J
I
(-)
-J
0
(I)
LU
-J
0
F-
120
Ho
100
90
80
70
60
50
40
30
20
I0
0
I 2 3 4 5 6 7 8 9 10 11 12
CL:NH 3 -N
WI. RATIO
FIGURE 2 Breaktoint in Buffered Aaueous Systems
-------
5
4
3
2
E
2
I
z
0
I 2 3 4 5 6 7 8 9 10 H
CL:NH 3 -N
WI. RATIO
12
FI(URE 3 Ammonia Removal at Breakpoint
-------
I I I I I I I F I I I
3
2
E
z
I J
0
z
•1-
‘In
0
z
0
.;
7.
H60
p1-15.0
—.
j I I I I - I I I
I 2 3 4 5 6 7 8 9 10 II
2
CL:NH 3 -N
WT RATIO
pH 8.0
U i
pH 7.0
.
S
FIGURE 4 pH and Nitrite ± Nitrate Formet 4 on
-------
4
CLNH 3 -N WT RATIO
FIGURE 5 pH and Nitrogen Trichioride Formation
3
2
N
E
z
-J
0
z
0
-------
3
I I
I I I
5 6 7 8
pH
• NO - NO -N.
NCL 3 -N U
N
0 )
E
I .
z
LIJ
0
a
a:
H
z
2
0
.- .
FIGURE 6 pH and Byproducts After Breakpoint
-------
TABLE 1
BREAKPOINT CHLORINATION
TEMPERATURE STUDIES
pH = 6 Contact Time = 2 hours
Temp.
Cl :NH 3 —N
wt. ratio
NH 3 -N
mg/i
(NO 3 ÷ N0 2 )-N
mg/i
NC1 3 -N
mg/i
5°C
0
5:1
6:1
7:1
8:1
9:1
10:1
11:1
20.0
9.20
6.10
1.19
0.00
0.00
0.00
0.00
0.00
0.10
0.18
0.27
0.43
0.50
0.60
0.70
0.00
0.00
0.00
0.00
1.10
1.89
3.91
5.67
15°C
0
5:1
6:1
7:1
8:1
9:1
10:1
11:1
20.0
9.69
6.00
1.22
0.00
0.00
0.00
0.00
0.00
0.05
0.05
0.16
0.30
0.43
0.52
0.60
0.00
0.00
0.00
0.00
1.10
1.90
3.90
5.67
40°C 0 20.0 0.00 0.00
5:1 9.20 0.00 0.00
6:1 6.70 0.10 0.00
7:1 3.90 0.15 0.00
8:1 0.00 0.42 1.10
9:1 0.—U 0.47 1.92
10:1 0.00 0.55 4.00
11:1 0.00 0.66 5.70
18
-------
SECTION VII
LABORATORY CHLORINATION OF WAS TEWATERS
In laboratory breakpointing of unclarified and lime clarified raw and
secondary wastewaters (NH 3 -N concentrations of 8-15 mg/i) with sodium
hypochiorite, the CaC0 alkalinities of 80-120 mg/i in the water main-
tained the pH of all tFie samples between 6.5 and 7.5. The chlorine
demand required for the breakpoint decreased and approached the
stoichiometric amount (equation 2 and 6) for oxidation of NH 3 to N 2
as the degree of wastewater pretreatment increased. As an example,
a chlorine dosage equivalent to a 10:1 weight ratio of Cl:NH 3 was
required to breakpoint raw wastewater (Figure 7), while a 9:1 weight
ratio was required in the secondary effluent (Figure 8), and about
an 8:1 weight ratio of Cl: NH 3 -N for lime clarified and filtered
secondary effluent (Figure 9).
Total kjeldahl nitrogen analyses revealed nearly complete removal of
the NH 3 -N, but only a slight reduction of the organic nitrogen within
the two hour contact time (Figures 7, 8, 9, 10). The nuisance residuals
of nitrate and nitrogen tn chloride-nitrogen at the breakpoint were
always less than 1 mg/l (Figures 11, 12, 13, 14). In the 6.5-7.5 PH
range, Nd 3 formation did not occur until the Cl:NH 3 -N weight ratio
exceeded a 7.6:1 dosage. The amount of Nd 3 also decreased with
decreasing wastewater pretreatment, and did not occur in the raw waste—
Water. The breakpoint process removed only ammonia nitrogen. There
was essentially no change in the organic nitrogen concentration.
If chlorine is employed rather than sodium hypochlorite, the breakpoint
chlorination of arm onia concentrations normally encountered in waste-
waters may produce more acid than can be neutralized by the buffer
capacity of the wastewater, Stoichiometrically (equation 1, 2, and 6)
14.28 mg/i of CaCO 3 alkalinity (1.5 lb of NaOH/lb of Cl 2 used) are
required to neutralize the acid produced by the oxidation of one mg/i
NH 3 —N to N 2 . Therefore, a lime clarified secondary was tewater
containing 20 mg/i NH 3 -N requires approximately 160 mg/i Of chlorine
for breakpointing and an alkalinity of about 286 mg/i. Since the
amount of Nd 3 fonned before the breakpoint increases with decreasing
pH, any excess acid produced must be neutralized with base and proper
mixing to avoid both excess local chlorine concentrations and low pH.
19
-------
26
CL:NH 3 -N W. RATIO
60
5O
40
30
20
-J
10 1—
20
N
E
I ’ )
24
22
20
(8
16
14
12
JO
8
6
4
2
0
z
w
(9
0
I—
z
FIGURE 7 Breakpoint in Raw Wastewater
-------
N
E
z
Lii 7
0
I-
z
6
5
30
E
U i
0
—J
20 r
0
-J
0
(I )
w
0
I0
Q
cUNH 3 —N WI. RATIO
14
13
II
I0
9
8
4
3
2
0
I a 3 4 5 6 7 8 9 JO II 12
FIGURE 8 Breakpoint in Secondary Effluent
-------
N)
N)
N
o
E
z
w
0
z
16
15
14
13
12
II
I0
9
8
7
6
5
4
3
2
0
40
30
20
l0
120
N
E
Lu
z
0
-J
I
U
-j
(I )
Lu
-J
4
F-
0
I-
2 3 4 5 6 7 8 9 10 II
CUNH 3 -N
WT. RATIO
FIGURE 9 Breakpoint in Lime Clarified Filtered Secondary Effluent
-------
16
14
12
N
E
z
LU
C!,
0
I—
z
4
2
0
8
6
CLNH N
WT. RATIO
80
70 z
E
60 w
z
0
.J’J
I
0
40 -J
4
30 5
ILl
20 -J
4
I—
0
IC I-
0
FIGURE 10
Breakpoint in Lime Clarified Filtered Raw Wastewater
-------
0 6 1
. N0 ÷NO -N
0.5
A NCL 3 - N
— 0.4
N
/
E
O.3
0
0
0.2 s/ / ’
0.I - /
.
S _— I
A A A A A A A
I 2 3 4 5 6 7 8 9 10 II 12
CUNH 3 -N WI. RATIO
FIGURE 11 Byproducts in Raw Wastewater
-------
I -— I I I I I F
A NO -f-NO - N
• NCL 3 - N
A A a
I 2 3 4 5 6 7 8 9 0 II 12
CL;NH 3 -N WT. RATIO
FIGURE 12 Byproducts in Lime Clarified Raw Wastewater
N
E
z
uJ
0
z
1.4
‘.3
1.2
I. I
I.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.I
QO
A
A
•-
i—- I I I
-------
0.6
0.5
0.4
E
0.3
0.2
0.I
0.0
CLNH 3 N
ii 2
2
w
0
0
H
z
2 3 4 5 6 7 8 9 10
WI. RATIO
FI( URE 13 By roducts in Secondary Effluent
-------
I I I I - I F I
• NO +NC N
Ł NCL 3 -N
—•
A A_—--t
I 2 3
S
4 5 6 7 8 9 10 II 12
CLNH 3 -N WT RATIO
0.8
0.7
0.6
0.5
z
u 0.4
(7
0
0 : : 03
z
0.2
0.l
O.O
A Ł
FIGURE 14
Byproducts in Lime Clarified Filtered Secondary Effluent
-------
SECTION VIII
PILOT PLANT CHLORINATION
During construction of the breakpoint chlorination system, chlorination
was attempted in the pilot plant using an eductor to mix chlorine water
and wastewater in a 2 inch diameter pipe. The NH 3 -N ranged from 3.0 to
9.4 mg/i and the flow rate was 20 gal../min. Base was prernixed in the
water upstream of the eductor to maintain the effluent in the 7—8 pH
range. Samples were taken 6 ft. downstream of the eductor. Breakpoint
was not achieved and excessive Nd 3 formation occurred. A comparative
test in the laboratory with good mixing achieved breakpoint. Thus,
the eductor did not provide adequate mixing.
Initial runs on the breakpoint chlorination system (Figure 1) in the
Pilot Plant were made with filtered secondary effluent. Breakpoint
occurred at a Ci:NH 3 -N weight ratio between 8:1 and 9:1. In repeated
tests (Table 2) the influent NH 3 -N concentrations ranging from 12.9
to 21.0 mg/i, were reduced to less than 0.1 mg/i.
Typi cal ca us ti c requirements for filtered secondary effluent in the pH
range 6.0 to 7.9 (Table 3) at approximately a 9:1 Cl:NH 3 —N dosage ratio
increased with increasing reaction pH from 0.88 pounds of NaOH per
pound of Cl 2 at pH 6.0 to 1.67 pounds per pound at pH 7.9. At pH 7,
1.3 pounds of NaOH were used per pound of Cl 2 . Calcium hydroxide
solution was also used and the quantities of calcium hydroxide needed
were found to be in stoichiometric proportion to the NaOH requirement.
Since the breakpoint stiochiometry to neutralize the acid produced at
pH 7 requires 1.5 pounds of NaOH per pound of Cl 2 , the neutralization
at pH 7 with 1.3 pounds of NaOFI per pound of Cl 2 must have used some of
the was tewater alkalinity.
The secondary wastewater was analyzed to determine the amounts of
The amount of nitrate produced (Table 4) did not
formation of nitrate was minimized by operating
near pH 6. The amount of nitrogen trichloride
0.43 mg/i. As in laboratory tests, however, the
trichioride increased with decreasing pH. More
concentration increased sharply with increasing
the breakpoint (Table 5). Because Nd 3 formation
chlorine dosage than on pH, operation at above
undesirable products.
exceed 1.2 rng/l. The
in the lower pH range
formed did not exceed
formation of nitrogen
importantly, the Nd 3
chlorine dosage above
was more dependent on
pH 7, with good mixing and good chlorine dosage control, however,
produced only small amounts of NC1 3 and NO 3 . (If good chlorine dosage
control is not achieved, operation at pH 8 can be employed to minimize
the noxious NCl 3 formation but will result in slightly or higher N0 3 -N
residuals and increased usage of base).
Chloride ion, a direct product of the reaction, was added to the
product water as NaC1 or CaC1 2 . The Stiochiometry revealed that with
28
-------
Table 2. Breakpoint Chlorination of filtered Secondary Effluent
Table 3. Caustic Requi rexcen ts for Breakpoint Chlorination of
Filtered Secondary Effluent
pH
6.0
7.0
7.9
pounds NaOH
pound Cl 2
0.88
1.3
1.67
Ci :NH 3 -N
wt. ratio
8.8:1
9.0:1
9.0:1
NH 3 -N
- mg /1
nfluent
13.9
20.3
21.0
12.9
17.0
15.4
Effluent
0.1
0.1
0.1
0.1
0.1
0.1
pH
6.0
6.0
7.0
7.0
7.5
8.0
Cl :NH 3 —N
Wt. Ratio
8.4:1
8,8:1
8.5:1
9.0:1
9.2:1
8.4:1
Free Cl
mg/i
7.0
6.5
2.0
3.4
8.5
2.5
29
-------
Table 4. Nitrate Formation
6.0
6.0
7.0
7.0
7.9
8.0
in
Filtered Secondary Effluent
13.9
20.3
17.0
16.1
20.3
15.4
0.5
0.8
0.9
0.8
1.2
0.9
% N0 3 —N
of in.fluent NH 3 -N
3.6%
3.9%
5.3%
5.0%
5.9%
5.9%
Table 5. Nitrogen Tn chloride Formation
in
Filtered Secondary Effluent
6.0
7.0
7.0
7.5
7.9
NH 3 -N in
mg/i
20.3
17.0
16.1
17.0
20.3
Ci :NH 3 -N
wt. ratio
8.75:1
10.0:1
12.0:1
9.2:1
9.0:1
NC1 3 -N
mg/i
0.33
0.05
0.43
0.05
0.05
pH
NH 3 -N in
mg/i
NO-N
rag/i
pH
30
-------
an influent NH 3 -N concentration of 15 mg/i and at C1:NH 3 —N ratio of
9:1, approximately 220 mg/i of NaCI was produced.
The lime clarified and filtered raw wastewater was continuously break-
pointed over a 5 month period in the pilot system. The feed from
filtration to the chlorination process had a pH of 9-10.
During the study, TKN and P111 3 —N removals (Table 6) generally revealed
that breakpoint chlorination removed chiefly NH 3 -N with the organic
nitrogen little affected by chlorine. The dissolved organics were
likewise unaffected by chlorination as the TOC did not change
appreciably through the system.
In the first nnnth of operation with an influent pH of 9 to 10 and
without pH preadjustment, the average NH 3 —N concentrations in the
influent and effluent were 12.5 mg/i and 1.2 mg/i, respectively (Table 6).
The WaOH usage was 0.9 pounds per pound of chlorine. NO 3 in the
effluent was 0.8 mg/i.
Control of pH was difficult with an influent pH of approximately 10.
Instead of maintaining a steady value of 7.0, the pH periodically
cycled between 6.0 and 8.0 with occasional high pH above 8. The daily
NH 3 variations in the effluent (Figure 15) revealed that a P itt 3
breakthrough occurred periodi call y. The NH 3 -N breakthrough coincided
with periods of high pH.
The operating difficulties occurred because of instability in the
process pH control loop. The instability was caused by the large
detention time of about 8 minutes in the reactor with poor mixing in
the upper part of the tank. The instability on p11 control could be
minimized by reducing with either CO 2 or Cl 2 the pH of the influent
feed to approximately that desired in the breakpoint before entering
the breakpoint reactor.
In the next three months with CO 2 preadjustment of the pH to 7, the
controller consistently maintained the pH at 7.0 for a steady flow rate
of 25 gprn. With effective pH control, the daily NH 3 -N variation in the
effluent (Figure 15) revealed consistently good NH 3 -N removal. The
average influent and effluent concentrations for this period were 11.0
mg/l and 0.3 mg/l of NH 3 -N. The NO 3 in the effluent averaged 0.6 nig/l.
The NH 3 —N removals represented a significant improvement over the first
month’s operation. However, the NaOH usage increased to 1.8 pounds
per pound of chlorine (Table 6). The increased base requirement
occurred because the CO 2 converted the OH alkalinity at p11 10 to
HCO 3 alkalinity at pH 7. With the system controlled at pH 7, the
bicarbonate alkalinity was not available for neutralizing the acid
produced in the breakpoint reactions. The stiochimetric base
requirement is 1.5 pounds of NaOH per pound of Cl 2 . Thus excess CO 2
also appeared to exert an additional base demand.
31
-------
DAILY VARIATIONS
‘
INFLUENT
4
13
2
II
I0
9
2
E
z
F()
I
z
EFFLUENT
I
APRIL
10 20 30 0 20 30
MAY
JUNE
DAYS
10 20 30 (0
JULY
AUG
FIGURE 15 Daily N1L -N Removal Variations
-------
TABLE 6
Continuous Breakpoint
of
Lime Clarified and Filtered Raw Was tewater
Month
April
MaW
June
Ju 1W
August
Flow, gpm
25
25
25
25
22.5—55.5
Influent
pH 9.9 9.3 9.3 9.2 9.0
P. alk, mg/i 70 45 54 31 30
M.O., alk., mg/i 140 84 96 82 80
NH 3 -N mg/i I2. 5 11.6 10.8 10.4 9 7
TKN-N, mg/i 14.2 14.1 12.2 11.9 10.7
NO 3 —N, mg/i 0.16 0.02 0.0 0.0 0.0
TOC, mg/i 22.6 19.9 16.7 17.0 18.3
pH Adjustment None CO 2 CO 2 CO 2 Cl 2
pH after Adj. 9.9 7.0 7.0 7.0 7.0
Reaction Conditions
pH 7.0 7.1 7.2 7.3 1
Cl:N, lb/lb. 9:1 9:1 9:1 9:1 9:1
NaOH:C1, lb/lb. 0.9 1.8 1.8 1.8 1
Effluent
M.O., alk., mg/i 52 100 124 118 60
NH 3 -N mg/i 1.2 0.4 0.4 0.4 0.5
TKN, mg/i 4.8 2.5 2.0 1.8 2.11
N0 3 —N, mg/i 0.8 0.7 0.6 0.5 0.5
TOC, mg/l 23.1 20.1 18.8 18.8 16.9
1 pH cycled with poor process control; breakpoint was frequently
lost. Data represents periods of successful breakpoint; base
requirement and pH not accurately determined.
33
-------
In further evaluation of these results, a brief review of the
neutralization reactions is needed. The breakpoint chlorination
reactions produce acid which must be neutralized to maintain a neutral
reaction medium and to prevent the formation of undesirable side
products. To reduce process cost, the use of the alkalinity present
in the wastewater to neutralize part of the acid produced is needed.
The types of alkalinity present in the wastewater include OH alkalinity
at high pH and 11CC 3 alkalinity at neutral pH. The hydroxide alkalinity,
a strong base, neutralizes the acid directly without buffer effects.
OH + It > 1120 (8)
The 11 (03 alkalinity, a weak base, neutralizes the acid by forming CO 2
(equation 9). At the reaction pH of 7.0,C0 2 is reconverted by the
base added for pH control to 11003 ions. Thus, little 11C0 3 alkalinity
is available for neutralization in a single stage breakpoint reactor
controlled at pH 7.
OH * 002 ) - 11(03 (9)
The alkalinity values in the influent and effluent (Table 6) indicated
whether the alkalinity present in the wastewater was utilized for acid
neutralization. In the first month of operation at pH 10, with the
stoichiometric requirement of 1.5 pounds of NaOH per pound of chlorine,
only 0.9 pounds of NaOH were used per pound of chlorine. Thus, the
OH alkalinity present in the wastewater at p11 9-10 satisfied part of
the base demand and was indicated by a decrease in alkalinity across
the system (Table 6). In the next three months of operation, CO 2
needed in p11 preadjustment for effective control converted the OH
alkalinity into 11C0 3 alkalinity (equation 9) which was unavailable for
acid neutralization in the breakpoint chlorination system operated at
pH 7.0. Hence, NaOH usage increased to 1.8 pounds per pound of
chlorine. In this case, the base requirement was 0.3 pounds more than
the stoichiometric value of 1.5 pounds because of the additional base
demand exerted by excess CO 2 . The increase in alkalinities across the
system (Table 6) confirmed the conversion of excess 002 to bicarbonate
ion.
The 11003 alkalinity present in the wastewater, however, may be used
for neutralizing acid (equation 10).
HCO3*H - > - 11 2 0+C0 2 (10)
If chlorine is used in a two-step breakpoint system at a dosage not to
exceed that for the formation of nonchioramine (5:1 C1 2 :NH 3 -1J) in the
first stage, and acid formed during the reaction produces CO 2 at a
lower pH such as 6. The CO 2 is then stripped from the water before
additional chlorine and base is added in a second reactor operated at
34
-------
pH 7.0. This two stage approach has its greatest potential in a
wastewater with appreciable 11C0 3 alkalinity such as secondary effluent
with a pH near 7 but may also be applied to the lime clarified raw
wastewater.
During the last stage of operation with chlorine to preadjust the pH
a diurnal flow variation was also applied to the system. With the
system on a diurnal flow, pH control was very difficult. The daily
average NH 3 -N variation in the effluent (Figure 15) revealed the
highest NH 3 -N breakthrough during the entire 5 months of operation.
The monthly average NH 3 -N concentrations in the influent and effluent
were 10.2 mg/i and 2.1 mg/i. While the use of Cl 2 in pH preadjustment
probably reduced base requirements, the base usage during this stage
of operation could not accurately be determined because of frequent
loss of breakpoint as the pH cycled. Thus, an improved control system
was needed to operate the split chlorine treatment approach and to
handle the diurnal cycle. The chlorine reactor with large detention
time and inefficient mixing caused the pH to cycle.
To provide improved process control and a flexible system for use on
both lime treated (pH 10) and secondary was tewaters (pH 7), a two
stage chlorination system is proposed to replace the existing pilot
system. The proposed system may be used as either single or two stage
operation on the limed wastewaters (pH 10 and above). The two stage
system includes influent pH adjustment in a 1.6 second (average)
static mixer by prechiorination followed by CO 2 stripping in the
existing reactor tank and a 1.6 sec static mixer as the breakpoint
reactor. The complete system also includes the complex process
control system as described in the subsequent section. The proposed
system with process control instrumentation needs to be installed
and evaluated.
35
-------
SECTION IX
INSTRUMENTATION AND PROCESS CONTROL
The amount of chlorine required for breakpoint depends upon the ammonia,
and non—ammonia chlorine demand, and ‘the amount of residual free chlorine
desired in the wastewater.
The control of breakpoint chlorination in wa tewater is more complex
than most industrial control systems because of the inherent
variability of the wastewater and the wastewater flow. The inlet
ammonia concentration, wastewater flow, and wastewater alkalinity, will
vary sharply with time. The chlorine dosage must be set and controlled
in proportion to the ammonia feed. With too low a chlorine dosage, the
reaction will not go to completion and chioramines will be the end
product. Nitrogen trichioride will be formed if more than a little
excess chlorine (beyond that needed for breakpoint) is added to the
system, especially if the pH is allowed to drop to below 6, or if
complete and rapid mixing is not achieved.
The instrumentation system employed in the study is shown in Figure 16.
The flow to the system is measured by the magnetic flow meter. The
NH 3 —N concentration in the influent is measured by the Technicon Auto
Analyser #1. The Cl 2 dose to the reactor is set manually at approxi-
mately a 9:1 Cl:NH 3 -N weight ratio. The free residual chlorine in the
effluent Is measured by the Technicon Auto Analyser #2. This reading
is used to manually trim the initially set (9:1 weight ratio) Cl 2 dose
so as to constantly maintain a small smount (2-4 mg/i) of free residual
chlorine in the effluent.
There are two control loops in the system. The first loop is for the
pH reduction from 10.0 to 7.0. The response time is of the order of
one-half second and the pH control is very good. The second pH control
loop is the 1 to 2% NaOH solution feed to maintain a pH of 7.0 in
the reactor. The response time in the second pH loop is of the order
of 8 minutes and a steady pH is maintained with difficulty.
Thus, the EPA-DC Pilot Plant staff proposed a controller design to
completely automate and to improve control of breakpoint chlorination.
The proposed control system establishes an approximate chlorine dosage
for breakpoint chlorination of ammonia and medifies the approximate
chlorine dosage to the optimum chlorine dosage required for complete
removal of ammonia by breakpoint chlorination from a specific wastewater.
The function of the breakpoint computer controller is described by the
following equation:
(A B . C)(f ED-El) = F
A = Process flow in liters per minute
36
-------
INFLUENT ___
M E T E R 1
CO 2 OR C —
I
I
I 0
I-
I
I
I
I
CONTROL SENSOR
_r NaOH
I
0
I- I
U
4
L U
I
I
— —
fl __
SENSOR CONTROL
AFj LY
EFFLUENT
I
FIGURE 16
BREAKPOINT CHLORINATION INSTRUMENTATION
-------
B = NH 3 -N concentration, mg/i
C = Preselected Cl:NH 3 -N weight ratio
D Set point free chlorine residual, rag/i
E = Measured free chlorine residual, mg/i
F = Total chlorine dosage, grams/mm.
f [ D—E} = Controller signal based upon deviation
of measured free chlorine residual
from setpoint
As shown in Figure 16, process flow (A) is measured by a flow meter.
A continuous measurement of the ammonia concentration (B) in the
influent stream is made by an automatic analyzer. This automatic
analyzer may operate colorimetrically or use a specific electrode. The
two linear signals (A & B, flow and ammonia concentration) are
multiplied by a manually preselected value (C) by the breakpoint control-
ler to establish an approximate chlorine requirement (A-B.C) to achieve
breakpoint chlorination. The manually preselected value (C) corre-
sponds to the Cl:NH 3 -N weight ratio required for the ammonia oxidation
to N 2 and has a theoretical value of 7.6:1. The selected value depends
upon the wastewater, and usually varies from 8—10:1.
The free chlorine concentration in the process effluent (E) is
automatically measured and the free chlorine signal thus determined is
compared to a setpoint free çi 2 residual (D) by a standard process
controller. The difference signal (D—E) is used by the breakpoint
computer-controller in a feedback loop to modify the chlorine require-
ment established by the mass signal (A•B•C) to the optimum chlorine
dosage (F) required for complete ammonia remeval and for the desired
free chlorine residual.
Careful pH control is necessary to prevent the formation of noxious
by-products and to stabilize the process pH for efficient breakpoint
operation. Control of pH is achieved by the proper addition of a
base to neutralize the acid formed by addition of chlorine to the
process. The pH of the system depends upon both the alkalinity of
water, the flow, and the total chlorine dose. Simple feedback pH
control does not prevent cycling in the was tewater pH because of the
wide variations in flow and operating conditions. In the proposed
system, the pH may be maintained by a combined feed forward and feed-
back control system as described by the following equation:
FG = H
F = Signal proportional total chlorine dosage
G = Output pH error signal (difference from manually
selected pH setpoint)
38
-------
= Signal proportional to change in base dosage
The p1! of the process effluent is measured and transmitted to the pH
controller with an automatic pH analyzer. The pH controller produces
a pH error signal proportional to the difference be1 een the measured
pH and the selected pH control point (setpoint). The pH error signal
is multiplied by the signal from the chlorine breakpoint controller
that is proportional to the total chlorine dose. The product signal
is proportional to the amount of base required for pH control and
controls the base feed to the reactor. The control system thus
minimizes upsets in process pH caused by changes in chlorine dose, flow
and alkalinity of the wastewater.
As may be seen in Figure 17, the breakpoint computer—controller is
basically a series of multiplying and dividing function generators and
a process controller. These operations can be performed pneumatically
or electronically by analog or digital equipment. The breakpoint
computer—controller receives signal A from the flow meter, signal B
from the amrionia analyzer and mul tiplies the product of A and B times
a manually selected factor C corresponding to the Cl:NH 3 -N weight ratio
required for the ammenia oxidation to N 2 . The resulting signal from
the product of A, B and C is used to control the chlorine feed to the
process until a free chlorine residual is sensed. When a free residual
chlorine signal is sensed and compared to free chlorine setpoint in a
process controller, the controller feedback signal f [ D—E] is combined
with that of the feed forward chlorine demand (A.B.C) to control the
chlorine dosage and maintain the desired free chlorine residual. As
long as a free chlorine residual is not sensed (i.e., initial start—up),
the Cl:NH 3 —N weight ratio (C) must be manually increased until a free
chlorine residual is obtained.
After a free chlorine residual has been obtained and the appropriate
Cl:NH 3 -N weight ratio selected (for the specific wastewater), the
fluctuations in the flow or in fluent ammonia concentrations will auto-
matically cause a corresponding change in chlorine dosage computed by
the ABC signal. Fluctuations in the process chlorine demand other
than flow or amnvnia fluctuations (i.e., organic chlorine demand) will
cause a corresponding change in the [ D-E] signal and thus maintain the
selected free chlorine residual.
In an alternate embodiment of the control system, the alkalinity of the
inifluent wastewater is utilized to neutralize the acid formed by the
addi tion of chlorine, reducing both base requirements for pH control
and effluent wastewater alkalinity. The effect is achieved by predosing’
with chlorine to a manually selected p1-1 level.
The breakpoint computer-controller establishes a theoretical chlorine
dosage for breakpoint chlorination of ammonia, as described in the
previous system, subtracts the chlorine predose required for pH
adjustment (if any) and modifies the approximate chlorine dosage to the
39
-------
(A - B - C - K)( f( D - E)) = F
F
APROCESS FLOW RATE,1/min.
B=NH 3 -N CONCENTRATION,
mg/I
C=PRESELECTED CI:NH 3 -N
WEIGHT RATIO
D=PREDETERMINED FREE
CHLORINE RESIDUAL,mg/I
E=MEASURED FREE CHLORINE
RESIDUAL,mg/I
F TOTAL CHLORINE DOSAGE,
grams/rn in.
KCHLORINE PREDOSE,
grams/rn in.
f(D-E)ANALOG P.1.
CONTROLLER OUT PUT
L ’PNEUMATIC OR ELECTRONIC
FUNCTION GENERATOR
Q
A
B
C
K
D
E
D-E)
FIGURE 17 BREAKPOINT COMPUTER CONTROLLER
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optimum dosage required for complete removal of ammonia by breakpoint
chlorination of a specific wastewater. The function of this alkalinity-
breakpoint computer-controller is described by the following equation:
(A B C- K) (if fD-Ej) = F
A = Process flow rate, 1/rain.
B = NH 3 -N concentration, mg/i
C = Preselected Cl:NH 3 —N weight ratio
V = Predetermined free chlorine residual, mg/i
E = Measured free chlorine residual, mg/i
F = Total chlorine dosage, grams/rain.
K = Chlorine predose, grams/mm . ,
f [ D—E] = Controller signal based upon deviation
of rreasured free chlorine residual from
setpoin t.
Prechiorination dosage (K) is a function of the pH and buffer capacity
of the particular wastewater influent , An approximate predose is either
calculated for a specific wastewater or determined empirically. This
approximate amount is refined by an error signal from a flow pro-
portioned p H controller at the effluent side of the predose reactor.
The chlorine predose (K) is continuously measured and subtracted from
the product of the A-B-C signal by the breakpoint computer-co%2tr.oller..
The resulting (A.B.C-K) signal is then modified by the [ v—B] signal as
described in the first embodiment.
With reference to the control system described, the quantities A, B, C
K and f [ v-sJ are available in the form of electronic signals.. The
evaluation of the proposed control systems in the pilot plant only
requires that the computer—controller be constructed and installed to
operate on the signal from existing sensors.
41
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SECTION X
COSTS
Itemized costs are given in Table 7. The calculations are based on
a 300 MGD plant assuming an average ammonia—nitrogen concentration
in the influent of 15 mg/i. Chlorine and lime costs are $75 and $24
per ton, respectively. Annual capital costs are computed at an annual
rate of 8 including interest and azmjrtization.
42
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Table 7. Breakpoint Chlorination Costs
Cents/1000 gal.
ElectricitW (Mechanical Mixing) .50
Cherni cals
Chlorine (135 mg/i) 4.22
Lime (1 lb/lb Cl 2 ) 1.35
Supplies .05
O&MLabor .60
Capital Charges .60
Total 7.32
43
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SECTION XI
REFERENCES
1. Booth, U.S., ed. Inorganic Syntheses, McGraw Hill, New York,
New York, 1, 65, (1939).
2. Chapin, R.M., JACS, 53, 912 (1931).
3. Cole, S.A., Taylor, W.C., TAPPI, 39, 62)1 (1956).
4. Corbett, R.E., Metcalf, W.S., and Soper, F.G., J. Chem. Soc.,
London, 1927 (1953).
5. Czeh, F.W., et. al, Anal. Chern., 33, 705 (1961).
6. Faust and Hunter, Principles and Applications of Water Chemistry ,
John Wiley and Sons, Inc., New York, 23, (1967).
7. Griffin, A.E., and Baker, R.J., J. New England Water Wks. Assoc.,
55, No. 3 (1941).
8. Griffin, A.E., and Chamberlain, N.S., J. New England Water Wks.,
ASSOC., 55, No. 3 (1941).
9. Kamphake, L.J., Hannah, S.A., Cohen, J.M., Water Res., 1, 205 (1967).
10. Kirk, R.E., and Othmer, D. F., Encyclopedia of Chemical Technology,
John Wiley and Sons, New York, New York, 2nd Rev. Ed., 4, 916,
(1964).
11. Kleinberg, J., Tecotzky, M., and Audrieth, L.E., Anal. Chem.,
26, 1388, (1954).
12. Moore, E.W., Water and Sew. Wks., 98, No. 3 (1951).
13. Nerbergall and Schmidt, College Chemistry, D.C. Health and Company,
Boston, Massachusetts, 361, (1957).
14. Palm, A.T., JAWWA, 44, 48 (1952).
15. Standard Methods for the Examination of Water and Wastewater,
12th Ed., American Public Health Assoc., New York, (1965).
16. Water Chlorine Residuals #1, Study 1/35, Analytical Reference
Service, Department of Health, Education and Welfare, Public
Health Service, (1969).
17. Yutaka, I., et al., Bull. Chem. Soc. Jap., 40, 835 (1967).
44
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SECTION X li
PUBLICATIONS AND PATENTS
Pressley, T.A., Bishop, D.F.,, and Roan, S.G., “Ammonia-Nitrogen Removal
by Breakpoint Chlorination”, Environmental Science and Technology,
6, 622 (1972).
Cassel, A.F., Pressley, T.A.., Schuk, W.W., and Bishop, D.F., “Physical
Chemical Nitrogen Removal from Municipal Wastewater”, AIChE Symposium
Series, 124, Water 1971, 68, 56 (1971).
Patent Pending: Serial No. 175902, Thomas A. Pressley and Thlloff F.
Bishop, “Nitrogen Removal from Wastewaters by Breakpoint Chlorination,
August 30, 1971.
Patent Pending: Serial No. 251777, Walter W. Schuk, Thomas A. Pressley,
and Dolloff F. Bishop, “An Automatic Control System for the Safe and
Ectnomical Removal of NH 3 by Breakpoint Chlorination”, May 9, 1971.
45
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SELECTED WATER 1 Repo:tNo. 2. 3. 4ceessionNo.
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
4. Title Ł A portDate
c .
AMMONIA-NITROGEN REMOVAL BY BREAKPOINT CHLORINATION 8. .rlorrni ..gOrgamzatian
Report No.
7. Author(s) Pressley, Thomas A. , Bishop, Doiloff F.,
Pinto, Adolf P., and Cassel, Alan F.
9. Organization 11010 EYM
EPA—DC Pilot Plant I I. Contract/GrantNo.
5000 Overlook Avenue S.W. 14-12-818
Washington, D.C. 20032 j .JRepo and
,,. Period Covered
12, Sp sorü Organ. ENVIRONMENTAL PROTECTION AGENCY
15. Supplementary Notes
Environmental Protection Agency report number EPA 670/2—73-058
76. Abstract
In laboratory studies of breakpoint chlorination, sodium hypochiorite oxidized
the ammonia in buffered distilled water (with 20 mg/i of NH 3 -N), raw w.astewater,
lime clarified and filtered raw wastewater, secondary effluent and lime clarified
and filtered secondary effluents to chiefly nitrogen gas. In the_oxidation of
this buffered water •and the wastewaters, only small amounts of N0 3 -N ‘(0.3 mg/i
at pH 5 to 2.0 mg/i at pH 8) and NC1 3 —N (0.05 mg/i at pH 8 to 0.3 mg/l at pH 5)
were produced. The oxidation of ammonia to N 2 gas was best accomplished in the
pH range 6-8. A Cl :NH 3 -N ratio of approximately 8:1 was sufficient for the
buffered water and the highly treated wastewater. However, increasing Cl :NH 3 -N
ratios (8-10:1) were required as the degree of pretreatment of the was tewater
decreased.
In the pilot breakpoint operation, chlorine gas with sodium or calcium hydroxide
for pH control was efficiently mixed with filtered secondary effluent and with
lime clarified and filtered raw wastewater. Greater than 95 of the NH -N was
converted to N 2 gas. Operation at controlled pH greater than 6.0 and wfth low
residuals of free Cl 2 (2 mg/i) produced essentially no nitrogen trichloride and
modest amounts of N0 -N (0.6 mg/i).
17a. Descriptors
* Wastewater Treatment Oxidation Nutrients
* Ammonia * Chlorination
Nitrogen Chlorine
Filtration Alkalinity
1 7b. Identifiers
* Breakpoint Chlorination
Lime Clarification
Free Chlorine
Nitrogen Trichloride
17c. COWRRF1e!d& Group 05D
18 A .ailsb;lrt JP Security Class 2! Na 0f Send To
(Repo :)
I D WATER RESOURCES SCIENTIFIC IN FORMATION CENTER
cc r:ty - s. . She US. DEPARTMENT OFTHE INTERIOR
(Page) WASHINGTON. D. C. 20240
Abstractor Kent S. Ki enhau r in s titutio n nvirnnmental Prnf rtinn Aq nt-y
02 .V,.JUNE 971
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