EPA-600/2-76-251
November 1976
Environmental Protection Technology Series
CONVENTIONAL TERTIARY TREATMENT
PRO
Municipal Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S, Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five 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
demonstrate 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 quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-76-251
November 1976
CONVENTIONAL TERTIARY TREATMENT
by
Thomas P. O'Farrell
and
Dolloff F. Bishop
EPA-DC Pilot Plant
Contract Number 14-12-818
Project Officer
Dolloff F. Bishop
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental
Research Laboratory, U.S. Environmental Protection Agency, and approved
for 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 r»r recommendation for use.
ii
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solution
and it involves defining the problem, measuring its impact, and searching for
solutions. The Municipal Environmental Research Laboratory develops new and
improved technology and systems for the prevention, treatment, and management
of wastewater and solid and hazardous waste pollutant discharges from
municipal and community sources, for the preservation and treatment of public
drinking water supplies, and to minimize the adverse economic, social, health,
and aesthetic effects of pollution. This publication is one of the products
of that research; a most vital communications link between the researcher and
the user community.
This report describes pilot testing of physical-chemical treatment for
the reduction of residual pollutants in secondary effluent. The residual
nutrients and organic materials are efficiently removed to low levels to
prevent accelerated eutrophication.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
iii
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EXECUTIVE SUMMARY
The objective was to evaluate conventional-tertiary treatment
consisting of primary sedimentation, step aeration, two-stage or
single-stage (lime-sodium carbonate) high pH lime clarification, ammonia
stripping, filtration, neutralization, and activated carbon adsorption
for potential application as an AWT Plant in the District of Columbia.
The secondary effluent from the EPA-DC Pilot Plant's step aeration
process was fed, usually with an average flow of 58,000 gpd and a diurnal
variation of 33,000 to 100,000 gpd, to a physical-chemical treatment
system consisting of either one or two lime treatment solids contactors
(with a recarbonation tank in the two-stage operation), an air cooling
tower (for ammonia stripping), dual-media filter, a neutralization tank
and a small activated carbon column (laboratory pilot scale - 0.32 gpm
flow). Each clarifier was divided into 4 zones; a primary mixing zone
(rapid mix), a secondary zone (flocculation), a clarification zone and a
sludge densification zone. Solids from the densification zone from each
clarifier were externally recycled at 10 percent of average flow to either
the primary mixing zone or to the recarbonation tank. The clarification
system was designed for a maximum flow of 100,000 gal/day with a maximum
overflow rate of 2,000 gal/day/ft^. The system was usually operated on
a diurnal cycle with a maximum to minimum flow rate of about 3:1.
During the two-stage clarification operation, the pH was reduced
to approximately 10.5 in the recarbonation tank where the average contact
time was 15 minutes. Five mg/1 of ferric ions were added to the primary
mixing zone of the second clarifier to improve the flocculation of the
precipitation column carbonate. Single-stage operation consisted of
pumping sodium carbonate solution to either the primary or secondary
mixing zone of the first clarifier to precipitate the excess column ions.
The recarbonation tank and second-stage clarifier were eliminated.
Ammonia air stripping was applied to the second-stage clarifier
effluent at pH 10.5. The stripping system included five crossflow cooling
towers, each packed with polyproplylene grids. The water was pumped to
a distribution box located on the top of each tower and flowed downward
over the grids. The air was drawn countercurrent to the flow of water,
between towers and crossflow within the packing of each tower.
The filtration system consisted of a splitter box followed by two
gravity fed dual-media filters. The filters were backwashed for 10
minutes by a sequential surface-wash backwash system. The flows of the
surface-wash and backwash system were 3 and 20 gal/min/ft^, respectively.
iv
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A side stream of the filtered effluent was neutralized to pH 7-8
and pumped through four 3-inch activated carbon adsorption columns. At
a constant rate of 0.32 gal.min, the loading to the column was 7 gal/min/
ft^ with a total empty bed contact time of 30 minutes. The columns were
manually backwashed every day at a rate of 15 gpm/ft^.
While the tertiary treatment demonstrated excellent removals of
phosphorus and organic carbon, the nitrogen removal by ammonia stripping
was not satisfactory for the District of Columbia treatment requirements
because of air temperature effects on the stripping process. Specifically,
two-stage lime precipitation of a good quality step aeration effluent in
the solids contactor units produced low residuals of 0.13 mg/1 of phos-
phorus and 2.1 mg/1 of BOD. Dual-media filtration of the chemically
clarified effluent reduced phosphorus and BOD concentrations to 0.09 mg/1
P and 1.4 mg/1, respectively.
Single-stage clarification (ph 11.5, sodium carbonate addition)
with a good quality secondary effluent produced a phosphorus residual
of 0.53 mg/1 as P in the clarified effluent. The carryover precipitated
phosphorus, however, was filterable as filtration produced an effluent
with an average residual phosphorus concentration of 0.10 mg/1 P.
Operational difficulties in the pumping of sodium carbonate solution
produced a carryover of excess soluble calcium ion into the filter. The
post precipitation of these ions as CaCO_ in the filters caused cementation
and channeling of the filter.
Recalcined lime, with recoveries up to 75% of the total solids slurry,
was recycled to the clarification system without a loss in efficiency of
the tertiary system. With filamentous growths in the activated sludge
system, high organic solids concentrations entered the first-stage
clarifier, produced slurry pool instability and reduced the treatment
efficiency of the system.
In the ammonia stripping studies with inlet water and air temperatures
of 25°C (77°F) and 24.4°C (76°F), respectively and an air to liquid rate of
327 ft air/gallon liquid, 80 percent of the available ammonia was air
stripped from the effluent for the two-stage lime system at pH 10.5. A
reduction in inlet air temperature to 6°C (43°F) reduced the removal ef-
fiency to 56 percent. At pH 10.5, the calcium carbonate scale rate was 16
mg/1 CaCO . Activated carbon adsorption removed 55 percent of the total
organic carbon from the neutralized filter effluent with an average residual
of 3.7 mg/1.
The results on the District of Columbia wastewater further confirm
earlier work on the treatment capability and reliability of tertiary
physical-chemical treatment for the removal of phosphorus and carbon. The
work also demonstrated the limitations of ammonia stripping for nitrogen
removal. The design data, the operating experiences, and the process
limitations are thus valuable in the area of production of high quality
water for very high water pollution control standards and for water reuse.
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This report was submitted in partial fulfillment of Contract No.
14-12-818, by the Department of Environmental Services, Government of
the District of Columbia, under the sponsership of the Office of Research
and Development, U.S. Environmental Protection Agency. Work was completed
as of October 1971.
vi
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CONTENTS
Page
Foreword m
Executive Summary i-v
List of Figures viii
List of Tables ix
Acknowledgments x
I Introduction 1
II Conclusions 4
III Recommendations 5
IV Experimental 6
V Operation and Results 12
VI Overall System Performance 25
VII References
VIII Bibliography
vii
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LIST OF FIGURES
Number Page
1 Tertiary Pilot Plant System 7
2 Solids Contactor 8
3 Ammonia Air Stripping System H
4 Ammonia Concentration as Affected by Biological
Nitrification 21
5 Calcium Carbonate Scale Affect on Air Flow Rate 24
viii
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LIST OF TABLES
Number Page
1 Tertiary Operating Conditions 9
2 Operating Variables 13
3 Phosphorus Removal for Single and Two-Stage Lime
Precipitation of Secondary Effluent 15
4 BOD Removal for Single and Two-Stage Lime Precipitation
of Secondary Effluent 16
5 TOC, COD and Suspended Solids Removal foi Single and
Two-Stage Lime Precipitation of Secondary Effluent 17
6 Solids Balance Data for Secondary Effluent Operation
(October 1970) 19
7 TOC Removal by Activated Carbon Adsorption 23
8 Tertiary System Performance 26
ix
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ACKNOWLEDGMENTS
The pilot system was constructed, maintained and operated by the EPA-DC
pilot plant staff under the direction of Robert A. Hallbrook, chief
mechanic; Walter W. Schuk, chief instrument technician; George D. Gray,
chief operator; and Howard P. Warner, chief analytical chemist. Alan B.
Hais maintained and operated the carbon adsorption system.
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SECTION I
INTRODUCTION
As a result of the Potomac River-Washington Metropolitan Area Enforce-
ment Conference, 1969 (1) the District of Columbia's Water Pollution
Control Plant was required to attain high removals of carbon, phosphorus
and nitrogen prior to discharge to the Potomac River. To meet these
requirements, various treatment systems for the removal of C, P and N
were studied at the EPA-DC Pilot Plant in Washington, D. C. One of the
systems studied consisted of primary, secondary and tertiary (physical-
chemical) treatment and is described as conventional-tertiary treatment.
Tertiary treatment has been applied to secondary wastewaters for the
removal of phosphorus, nitrogen and organic carbon (2) (3) (4)(5)(6), and
usually includes the following unit processes: chemical clarification
(alum, iron or lime), filtration, ammonia removal (ion exchange, breakpoint
chlorination or air stripping) and activated carbon adsorption. Lime
precipitation has generally been selected, over use of alum and iron salts,
as the prime chemical clarification process since it produces a treatable
sludge with a high pH which can be used in conjunction with ammonia air
stripping. Alternate methods of chemical precipitation have been used
depending on the relative hardness of the wastewater and on the relative
phosphorous removal required. A high pH ( ~ 11.5) lime process with one or
two-stages is generally applied to waters of moderate alkalinities (100-200
mg/1 CaCO ), while in areas of higher alkalinities (~ 250 mg/1 CaCO,), a low
pH (~9.5; single-stage system may be used. The high pH (~ 11.5) operation
produces water from which ammonia may be air stripped (7). Experimental
studies reveal that phosphorus residuals of less than 0.2 mg/1 as P (8)
have been obtained with the high pH lime process while residuals in the
vicinity of 0.5 mg/1 as P (9) are typical with the low pH process.
In the high pH lime process, lime is added to the wastewater to increase
the pH above 11.5 and to precipitate the bicarbonate, phosphorus and
magnesium ions according to the following reactions .
Ca + HO>3 + OH •* CaCO, + H20 (1)
5Ca + 3HP0 4- 70H~ •* CaOH(P0) + 6E (2)
20H~ -» Mg(OH)2 (3)
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The magnesium hydroxide formed is an excellent coagulant aid and flocculates
the organic solids and the precipitated inorganic solids. However, the
amount of lime required to attain pH 11.5 produces an excess of calcium ions
in the wastewater. Two methods can be used for the removal of the calcium
ions. One consists of recarbonation of the first-stage clarified effluent to
pH 9.5-10.5 with carbon dioxide to precipitate the excess ions as calcium
carbonate according to Equation 4. This precipitation is settled in the
second-stage clarifier. Alternately the addition of sodium carbonate into
the first-stage clarifier can be used to precipitate the calcium ions as
calcium carbonate without requiring a second sedimentation stage. This
reaction is described by Equation 5.
Ca + 20H + CO + CaCO. + H-0 (4)
Ca++ + 2N3+ + C°3 + CaC03+2Na+ <5>
Thus the high lime process has two basic modes of operation, single-
stage sedimentation with sodium carbonate addition and two-stage
sedimentation with carbon dioxide addition between the stages. In either
the single or the two-stage system, the effluents may be neutralized by
additional carbon dioxide as shown in Equation 6.
OH + C02 •* HC03 (6)
In the low pH system, lime is added to increase the pH to approximately
9.5 and precipitate a portion of the bicarbonate ions (Equation 1)
as calcium carbonate and most of the phosphate (Equation 2). The mag-
nesium ions generally are not precipitated. Calcium ions are not in excess
and recarbonation is not required unless neutralization of the effluent
is desired. The sludge produced during any of the lime precipitation pro-
cesses is thickened, dewatered and fed to furnaces for recalcination. The
recalcined lime is returned to the system while the stack gas containing
approximately 10 percent carbon dioxide is used for recarbonation and
neutralization.
Following chemical clarification, filtration is applied for additional
solids and phosphorus removal. Mixed bed filtration (sand and coal, or
garnet, sand and coal) has been successfully applied to lime treated
secondary effluents. To prevent the possibility of calcium carbonate scale
within the media, the clarified effluent should be neutralized to less
than pH 8 prior to filtration.
Three methods are available for removal of nitrogen by tertiary treatment
and are: breakpoint chlorination, selective ion exchange and air stripping
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of the airanonia. All of these processes remove nitrogen only in the form
of ammonia, and therefore, in the tertiary mode must be applied to a non-
nitrified secondary effluent. Breakpoint chlorination requires contacting
a neutralized or non-neutralized filtered effluent in a highly mixed,
pH controlled system with chlorine at a chlorine to nitrogen ratio of
approximately 9:1 C1:N (10). Selective ion exchange is applied to a
neutralized filter effluent for anmonia removal in packed bed pressurized
control units containing a natural zeolite, clinoptilolite, which is
selective for the ammonium ion (10).
Air stripping of ammonia (2) (4) is used in conjunction with the lime
precipitation process which raises the pH and converts the ammonium ion
to ammonia according to the following equation:
NH^ + OH + NH4OH *; NH3 + H20 (7)
Unfortunately, ammonia is highly soluble in water, and its volatility
decreases markedly with decreasing temperature. As a result, effective
air stripping requires warm temperatures and the use of large volumes of
air per unit volume of water.
For the removal of soluble organic carbon in tertiary treatment systems,
filtered effluents have been applied to activated carbon systems operating
as packed bed downflow or suspended bed upflow units. Packed bed downflow
carbon columns are generally operated under pressure.
This report details the evaluation of tertiary treatment of a conventional
secondary effluent (step-aeration) pilot scale process to produce an
effluent with low phosphorus, and particulate organic and inorganic con-
centrations that meet the requirements for discharge to the Potomac River.
The tertiary system consisted of lime precipitation (two-stage with inter-
mediate recarbonation or single-stage with sodium carbonate addition),
ammonia stripping, dual media filtration, neutralization and activated
carbon adsorption. Ammonia stripping was applied only during portions of
the study to determine the effect of temperature on ammonia removal and
the calcium carbonate scale rate at pH 10.5.
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SECTION II
CONCLUSIONS
Two stage lime precipitation of a good quality (suspended solids less than
25 mg/1) step aeration effluent produced low phosphorus and BOD residuals
of 0.13 mg/1 and 2.1 mg/1 respectively.
Dual media filtration of the chemically clarified effluent reduced
phosphorus and BOD concentrations to 0.09 mg/1 P and 1.4 mg/1,
respectively.
Activated carbon adsorption removed 55 percent of the total organic
carbon from the neutralized filter effluent. This removal resulted in
an average residual TOG of 3.7 mg/1.
Single stage clarification (pH 11.5, sodium carbonate addition) with a
good quality secondary effluent produced a phosphorus residual of 0.53
mg/1 as P in the clarified effluent. The carryover precipitated phosphorus,
however, was filterable as filtration produced an effluent with an average
residual phosphorus concentration of 0.10 mg/1 P.
Operational difficulties in pumping the sodium carbonate solution produced
a carryover of excess soluble calcium ion into the filter. The post
precipitation of these ions as CaCO_ in the filters caused cementation of
the media and channeling in the filter.
Recalcined lime, with recoveries up to 75 percent of the total solids slurry,
was recycled to the clarification system without a loss in efficiency of
the tertiary system.
Filamentous growths in the activated sludge system caused high organic
solids concentrations to enter the first-stage clarifier, this produced
slurry pool instability and reduced the efficiency of the system.
At inlet water and air temperatures 25.0 and 24.4°C (77°F and 76°F),
respectively, and an air to liquid rate of 2.45 M air/liter water
(327 ft air/gallon liquid), 80 percent of the available ammonia was
stripped from the effluent of the two-stage lime system at ph 10.5. A
reduction in inlet air temperature to 6°C (43°F) reduced the removal
efficiency by 30 percent. At pH 10.5, the calcium carbonate scale rate
was 16 mg/1 CaCO-.
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SECTION III
RECOMMENDATIONS
Tertiary treatment has demonstrated high removals of phosphorus and
organic carbon. Future work in tertiary treatment should be related
to nitrogen removal since air stripping of ammonia is too restrictive
because of air temperature effects. Breakpoint chlorination and selective
ion exchange are alternative ammonia processes and should be demonstrated
in tertiary applications.
The use of sodium carbonate in conjunction with the high lime process
should be studied to determine the kinetic rates of reaction. The
kinetic rate of reaction of the lime and carbon dioxide should be deter-
mined to provide better design criteria.
The two-stage lime system, although producing excellent quality of water,
produces a large amount of solids. Alternatives, such as low lime pH
treatment with ferric chloride should be investigated.
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SECTION IV
EXPERIMENTAL
Pilot System
The system for the tertiary treatment of the effluent from a 265 liters/rain
(70 GPM) step aeration pilot plant is shown in Figure 1. The two clari-
fication units (solid contactors) were identical in design with a side
wall depth of 2.9 m (9.5 ft.) (Figure 2). Each clarifier was divided into
4 zones; a primary mixing zone (rapid mix), a secondary mixing zone (floccula-
tion), a clarification zone and a sludge densification zone. The clearance
between the bottom (discharge) of the secondary mixing zone and the flat
bottom of the clarifier was .91 meters (3 ft.). Solids from the densifi-
cation zone were externally recycled at 10 percent of average flow to the
primary mixing zone to increase the rate of precipitation and produce
particles of increased size and weight during flocculation. The variable
speed turbine mixer in the primary and secondary mixing zones were operated
at 36 and 15 rpm, respectively.
3
The clarification units were designed for a maximum flow of 378 m d
(100,000 gpd) with a maximum overflow rate of 81.5 m/d (200 gpd/ft ).
The system was operated on a diurnal cycle with the minimum, average, and
maximum flows shown in Table 1 along with other operating bath. At an
average flow of .15 liter/rain (40 GPM) the clarification system treated
218 m /day (57,600 gpd) at an overflow rate of 45.6 m/d (1120 gpd/ft).
During the two-stage operation, the pH was reduced to approximately 10.5
in the recarbonation tank where the average contact time was 15 minutes
and a turbine mixer rotating at 75 rpm provided mixing.
Solids from the second clarifier were recycled to the recarbonation tank
at 10 percent of the average flow. Five mg/1 of Ferric ions were added
to the primary mixing zone of the second clarifier to improve the floccu-
lation of the precipitated column carbonate.
Single-stage operation consisted of pumping sodium carbonate solution
to either the primary or secondary mixing zone of the first clarifier
to precipitate the excess column ions. The recarbonation tank and
second-stage clarifier were eliminated.
During both the single and two-stage operation, solids were wasted at
approximately 2 percent of average inlet flow from the first-stage
clarifier. Solids were wasted at similar rates from the second clarifier
to the first clarifier during the two-stage operation.
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INFLUENT
r n i i i L|
FILTERS
L
NEUTRALIZATION
EFFLUENT
CO2
LIME
CI2
CARBON COLUMNS
FIGURE 1. TERTIARY PILOT PLANT SYSTEM
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INFLUENT
00
EFFLUENT
TURBINES
AID
SLUDGE
FIGURE 2. SOLIDS CONTRACTOR
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TABLE 1. TERTIARY OPERATING CONDITIONS
Lime Precipitation
Stage 1 and 2
Recarbonation
Primary Zone
Secondary Zone
Air Stripping
5 Towers
Filtration
2 Filters
Carbon Adsorption
4 Columns
Flow Rate
liter/gal
87 min
151 ave
204 max
265 rain peak
151 ave
151 ave
151
121 constant
87 min
151 ave
204 max
265 rain peak
1.21 constant
Loading
26.1 m/d
45.6 m/d
61.5 m/d
79.4 m/d
.0815 m/min
.0803 m/min
.139 m/min
.188 m/TTin
.244 m/min
.285 m/min
Detention Time
minutes
154
15
14
20
30
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Ammonia air stripping was applied to the second-stage clarifier effluent
at pH 10.5. The stripping system (Figure 3) included five crossflow
cooling towers each packed with forty 1.2m by 1.5 m by 1.3 cm (4 ft by
5 ft by 0.5 in.) polypropylene grids. The layers of each grid were spaced
every 6.4 cm (2.5 in.) and the grids were packed to promote waxed air flow
through the system. The water was pumped to a distribution box located on
the top of each tower and flowed downward over the grids. The air was
drawn countercurrent to the flow of water between towers and crossflow
to the water flow within the packing of each tower by 3.7 Kw (5 hp)
centrifugal fan.
The effluent from the first stage clarifier during the single-stage
operation and the effluent from the second-stage clarifier during the
two-stage operation were fed to the filtration system. The filtration
system consisted of a splitter box followed by two gravity fed dual-media
filters (3.0 m [10 ft.] of hydraulic head). Each filter was packed with
46 cm (18 in.) of 0.9 mm anthracite coal on 15 cm (6 in.) of 0.45 mm sand.
Fused aluminum oxide plates were used to support the media and provide
flow distribution during backwash. The filters were backwashed for 10 minutes
by a sequential surface wash-backwash system. The flows of the surface™
wash and backwash systems were 0.12 and 0.81 m/min (3 and 20 gal/min/ft ),
respectively.
A side stream of the filtered effluent was neutralized to pH 7-8 and
pumped through the activated carbon adsorption system. The system
included four 7.6 cm (3-inch) columns packed to a depth of 2.1 m (7 feet)
per column with 8 x 30 mesh Calgon activated carbon. At a constant rate
of 1.2 liters/min (0.32 gal/min) the loading to the column was 0.285 m/min
(7 gal/min/ft ) with a total empty bed contact time of 30 minutes. The
columns were manually backwashed every day at a rate of .61 m/min (15 gpm/ft ).
Analytical Procedures
The system operated 24-hours a day, 7 days a week, with samples taken
every four hours and composited for 24 or 48 hours, depending on the
nature of the sample. Samples were stored at 3°C to minimize biological
activity. Effluent samples were analyzed for TOC, BOD, COD, total
phosphorus, TKN, ammonia, calcium, magnesium and total solids.
Total organic carbon was determined by the Beckman Carbonaceous Analyzer
(11); and the 5-day BOD, by the probe method (12). Calcium and magnesium
concentration were determined by the Perkin and Elmer Model 303 Atomic
Adsorption Unit (12). Ammonia (12), nitrite and nitrate (13) were deter-
mined on a Technicon Autoanalyzer. Total phosphorus was measured by the
persulfate method (14) and the methods for all other analyses were taken
from "Standard Methods" (15).
10
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AIR
OUT
ttt
WATER
IN
I
AIR
IN
BLOWER
WATER
OUT
FIGURE 3. AMMONIA AIR STRIPPING SYSTEM
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SECTION V
OPERATION AND RESULTS
On February 9, 1970, tertiary treatment of the secondary effluent from
the Pilot Plant step aeration process was initiated with the start-up
of the lime precipitation process. Dual media filtration of the chemically
clarified secondary effluent began on February 27, 1970. The clarifi-
cation system was operated in two stages with intermediate recarbonation
at a constant rate of 40 GPM until June 18, 1970. Then the system was
placed on a 2.3:1 diurnal flow variation. On July 23, 1970, a single
stage lime clarification with sodium carbonate was substituted for the
two-stage lime clarification with intermediate recarbonation, and operated
until December 23, 1970. The effluent from the second-stage clarifier
was pumped to the air stripping system from March 24, 1970 until July 14,
1970. The carbon adsorption system was placed in operation on April 16,
1970 and operated until October 13, 1970.
Process flows with corresponding hydraulic loadings and detention times
are summarized in Table 1; the operating variables in Table 2. As seen in
Table 2, the nitric acid produced by nitrification in the step aeration
process during the summer months reacted with the alkalinity and lowered
the buffer capacity of the secondary effluent. In addition, the conversion
of ammonia to nitrate restricted the operation of the air stripping system.
Clarification and Filtration
During the study, four alternate methods controlled the lime feed:
conductivity control, alkalinity-flow proportional control (Calgon
Chemonitor), pH-flow proportional control and fixed dose-flow proportional.
The pH-flow proportional control system provided the most reliable operation
(16). The apparent increase in the lime dose shown in Table 2 for the month
of June and later was caused by recycling the recalcined lime into the
clarification system. The recycled lime was obtained from the lime sludge
which was dewatered on a vacuum filter and recalcined (17) with the non-
carbonate (inert) solids in the cake.
During two stage operation, five mg/1 of ferric ions were added to the
primary mixing zone of the second clarifier to improve the flocculation
of the calcium carbonate. The waste solids from the second clarifier
were pumped to the first clarifier with wasting of solids from the total
system through the blowdown of the first clarifier. The waste rate from
the first clarifier at approximately 1.5 percent of total flow produced
a total solids concentration of approximately 5 percent in the blowdown
and maintained a solids balance in the system.
12
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TABLE 2. OPERATING VARIABLES
Month Alkalinity
mg/1 CaCO
March
April
May
June
July
August
September
October
November
December
117
79
79
54
70
84
115
144
146
143
Lime Dose
mg/1 CaO
390
330
353
450
466
342
542
546
430
409
1st Stage 2nd Stage Na CO 1st Stage Waste
pH pH mg/1 % of influent
11.8 10.4
11.8 10.3
11.8 10.3
11.7 10.2
11.5 10.3
11.7 273
11.7 271
11.6 272
11.6 194
11.5 210
1.59
1.46
1.47
1.06
1.49
2.09
2.06
2.20
1.54
1.10
2nd Stage
% of inf
3.50
3.24
2.36
2.16
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In the two-stage operation, the recycle of solids from the second
clarifier to the first clarifier and the relatively low amount of
organic solids in the step aeration effluent produced slurry pool
stability in the clarifiers. Initiation of the diurnal cycle with
a dry weather peak overflow rate of 61 m/d (1,500 gal/day/ft ) did
not decrease the process removal efficiencies or the slurry pool
stability in either clarifier. In addition, the use of recalcined
lime with recycled inerts in June and July did not alter the process
removal efficiencies.
The efficiencies of removal of phosphorus, BOD, TOC, COD, and
suspended solids by lime precipitation are summarized in Tables 3, 4
and 5. Two-stage lime precipitation and filtration, operated until
July 23, 1970 on high quality secondary effluent from the step aeration
process, produced excellent removals of BOD and phosphorus. The average
residual phosphorus concentrations following clarification and filtration
were 0.13 and 0.09 mg/1 as P, respectively. The efficient step aeration
process converted the soluble BOD materials to insoluble particulates
which were captured in the tertiary chemical clarification and filtration
systems. After two-stage lime clarification and filtration, the average
residual BOD's were 2.1 and 1.4 mg/1, respectively. The phosphorus and
BOD residuals after filtration corresponded to an accumulated removal
from the raw wastewater of approximately 99 percent. The average residual
TOC and COD after clarification were 9.1 and 18.4 mg/1 respectively, and
after filtration were 7.5 and 17.5 mg/1. The average residual suspended
solids concentrations in the clarified and filtered effluents were 17.3
and 3.8 mg/1.
Single-stage lime clarification at pH 11.5 with sodium carbonate addition
and filtration was operated from late July to late December. The operation
included a period of seasonal change in water temperature. In single
stage lime precipitation, sodium carbonate was added to the secondary
mixing zone of the first clarifier to precipitate the excess calcium
ions. The effluent from the first-stage clarifier was fed directly to
the filters, by-passing the recarbonation tank and the second clarifier.
During the first three months of operation, the waste rate was maintained
at approximately 2 percent of influent flow and produced a solids concentra-
tion in the bottom of the clarifier of approximately 3.5 percent. Later,
the waste rate was gradually reduced to approximately 1 percent to increase
the solids concentration in the blowdown. As a result of this change in
wasting rate, the solids in the sludge blowdown increased to greater than
5 percent. The single stage system test was interrupted during late October
when filamentous organic solids from the upstream step aeration process over-
flowed the secondary weirs into the lime clarification process and caused
instability of the slurry pool in the lime clarifier.
14
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TABLE 3. PHOSPHORUS REMOVAL FOR SINGLE AND TWO-STAGE
LIME PRECIPITATION OF SECONDARY EFFLUENT*
Month
(1970)
March
April
May
June
July
August
September
October
November
December
Influent
mg/1
6.2
6.1
6.4
6.8
7.2
6.7
6.7
7.1
7.3
6.6
Clarification
mg/1 % Removal
0.11
0.13
0.11
0.16
0.14
0.41
0.72
0.46
0.59
0.59
98
98
98
98
98
94
89
93
92
91
Filtration
mg/1 % Remov;
0.11
0.10
0.06
0.11
0.09
0.08
0.07
0.16
0.36
0.29
98
98
99
98
99
99
99
98
95
96
*Phosphorus concentrations reported as P
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TABLE 4. BOD REMOVAL FOR SINGLE AND TWO-STAGE LIME
PRECIPITATION OF SECONDARY EFFLUENT
Month
(1970)
March
April
May
June
July
August
September
October
November
December
Influent
mg/1
10.4
12.1
10.2
20.7
20.5
17.6
10.7
29.8
31.8
33.0
Clarification
mg/1 % Removal
1.9
2.5
2.7
1.6
1.8
1.8
2.1
4.4
13.0
10.4
82
79
74
92
91
89
80
85
59
68
Filtration
mg/1 % Removal
1.6
1.6
1.3
1.3
1.3
1.5
1.8
3.5
8.3
10.5
85
87
87
94
94
91
83
88
74
68
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TABLE 5. TOC, COD AND SUSPENDED SOLIDS REMOVAL FOR SINGLE AND
TWO-STAGE LIME PRECIPITATION OF SECONDARY EFFLUENT
Influent
Two-Stage* 19.2
Single-Stage** 23.0
Clarification
Two-Stage*
Single-Stage** 11.9
Filtration
Two-Stage*
Single-Stage** 11.2
* March - July, 1970
** August - December, 1970
TOC
mg/1
19.2
23.0
8.1
11.9
7.5
11.2
COD
% REMOVAL mg/1
48.6
69.7
58 18.4
48 27.0
61 17.4
51 26.0
SUSPENDED
SOLIDS
% REMOVAL mg/1
33
34
62 17.3
61 16.3
64 3.8
63
% REMO
48
52
89
77
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As seen in Table 3, the conversion of the two-stage system to single
stage produced an increase in the residual phosphorous concentration
following clarification. However, during the first three months of
operation, filtration successfully moved the precipitated phosphorus
to an average residual of 0.10 mg/1 as P.
During the first three months of single stage operation, the residual
BOD following clarification and filtration averaged 2.6 and 2.3 mg/1,
respectively (Table 4). The TOC and COD concentrations after filtration
averaged 9.1 and 21.2 mg/1, respectively, during the first three months
of single stage operation and represented a slight increase as compared
to the two-stage system. An increase in the filtered effluent suspended
solids concentration from 3.8 mg/1 during the two-stage operation to 6.2
mg/1 during the first three months of the single-stage operation may be
caused by the use of the recycled lime or the lack of the ferric chloride
addition within the single-stage system. The low phosphorus concentration
of 0.10 mg/1 as P for the first three months of operation indicates that
the increased suspended solids concentrations did not contain calcium
hydroxylapatite. The average TOC, COD and suspended solids concentrations
after single stage clarification and filtration as shown in Table 5 for
the full five months (Aug.-Dec.) were moderately higher than those of the
two stage operation and reflected sludge bulking conditions in the upstream
biological (step aeration) process during November and December.
A solids balance for the month of October on the chemical clarification
system is shown in Table 6. The balance was performed with recalcined
lime equal to 74 percent of the total weight of slurried solids added
to the system to maintain a pH of 11.6. The solids production was based
on the average phosphorus, calcium, magnesium and suspended solids concen-
trations in the influent and effluent to the clarifier. The excess calcium
was assumed to be calcium carbonate. The so^.ds l^the recycle lime were
calculated from average concentrations of Ca , Mg and phosphorus in the
recalcined lime. The magnesium in the recycled solids was assumed to be
insoluble MgO. With these_assumptions, the total solids production was
calculated to be 0.80 kg/m (6.7 lbs/1000 gal). Based on the total waste
flow and the average total solids of the waste flow, the measured total
solids production of 0.86 kg/m (7.1 lbs/1000 gal) was within 5.6 percent of
the calculated solids production.
When water temperatures began to sharply decrease in late October, the step
aeration process developed filamentous sludge, and the conversion of the
soluble BOD to particulate BOD gradually decreased. The filamentous sludge
with its poor settling characteristics produced bulking sludge which over-
flowed the weirs of the secondary settler into the tertiary chemical clari-
fication system. Thus the amounts of organic materials (BOD, TOC, and COD)
increased in the secondary effluent (Tables 4 and 5). Although the lime
system clarified the secondary effluent during the final two months of
operation (the clarified effluent suspended solids concentration averaged
13.5 mg/1), the BOD following clarification increased to 11.7 mg/1. Likewise
18
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TABLE 6. SOLIDS BALANCE DATA FOR SECONDARY EFFLUENT OPERATION
(OCTOBER 1970)
Production Recycled Total
kg/month kg/month kg/month
Ca5(OH)(P04)3 236 572 807
CaCO 3538 305 3538
Mg(OH)2-MgO 139 305 444
Suspended Solids 249 249
Unknowns 191 191
4162 1068 5230
3
Volume of wastewater in October: 6567m
Measured solids production: 5265 kg
Solids production per unit volume:
Calculated 5230 kg 0~ . / 3 ,£ .,,,,, nnn ,,.
TTZy 3 = .80 kg/m (6.7 lb/1,000 gal)
m
Measure 5265 0, . , 3 ,-,-.-.,/-. nr>n T\
= .86 kg/m (7.1 lb/1,000 gal)
* Calculated from measured concentrations of Ca ' 4'
suspended solids and stoichiometry of precipitation reactions.
Calculation assumes complete calcination of CaC07-
** Based upon complete measured values of total solids concentration
and volume of waste solids slurry.
19
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the TOC and COD following chemical clarification increased to 15.1 and
34.9 mg/1, respectively. With the increase in biological solids in the
clarifier, the TOC of the wasted solids also doubled and slurry pool
instability occurred.
The decrease in water temperature also disrupted pumping of the sodium
carbonate and as a result adversely affected the filtration process.
Soda ash crystallization in the pump and associated piping prevented
uniform addition of sufficient sodium carbonate. As a result, calcium
ion concentrations in the clarified effluent increased from approximately
40 mg/1 to 70 mg/1. The excess clacium ions post-precipitated as CaCO
in the filters and converted the filter media to solids chunks within a
month. Channeling of the filters occurred, reducing filter efficiency.
Backwashing the filter was ineffective and filter runs decreased from the
normal 50 to 10 hrs. This poor filtration produced an increase in
phosphorus concentration following filtration from the normal 0.1 mg/1
to an average of 0.33 mg/1 as P and in suspended solids concentration
from the normal 6 mg/1 to 10.3 mg/1.
Ammonia Air Stripping
During the operation of the ammonia stripping process, the entire tertiary
treatment system was operated at a constant rate of 3,2 gpm. The loading
to the air stripping system was .081 m/min3(2 gpm/ft ) with an initial
G'/L' value of approximately 2600 m air/m liquid (350 cu. ft air/gal
liquid).
The step aeration process fluctuated through various stages of nitrifi-
cation and thus, after chemical clarification, provided an influent to
the ammonia stripping column of varying ammonia concentration (Figure 4).
During the first 17 days of operation (Figure 4), nitrification did not
occur in the step aeration process. The average concentration of ammonia
in the inlet and outlet of the tower were 9.5 and 4.2 mg/1 NH.J-N, respectively,
for an average removal of 56 percent. During this period, the average out- 3
let water temperature was 7.2°C (45°F) with an average G'/Lf value of 2600 m
air/m liquid(347 cu ft air/gal liquid). Between the 35th and 45th days of
operation, the step aeration system produced little nitrification, and the
percent removal of ammonia averaged 66 percent (3.1 mg/1 NH»-N average
residual) with an average outlet water temperature of 13.9°C (57°F).
During previous studies (7), nearly 80 percent of the ammonia was removed
under similar operating conditions with an outlet water temperature of 20.5°C
(57°F).
While stripping efficiencies were affected by temperature, the rate of
calcium carbonate scaling and its effect on the reduction in the air-
to-liquid ratio were significantly affected at pH 10.5. The calcium
carbonate scale in the pH 10.5 tests reduced the air-to-liquid
20
-------
120
FIGURE 4. AMMONIA CONCENTRATION AS AFFECTED BY
BIOLOGICAL NITRIFICATION
-------
3 3
ratio from an initial 260 m air/m liquid (350 cu ft air/gal liquid)
to approximately 330 cu ft air/gal liquid (Figure 5) over the 112 day
period. During the operation with the effluent at pH 10.5, inlet and
outlet calcium ion concentrations averaged 48.3 and 41.8 mg/1 of Ca ,
respectively. The calcium ion decrease corresponded to a calcium car-
bonate scaling rate of 16 mg/1 of CaCO_.
In earlier work (7) with an influent pH of 11.5 and air temperature of
(78°F) 25.6°C, 90 percent of the ammonia was removed at an air to
liquid rate of 3.75 m air/liter of water (500 cu ft air/gal liquid).
In similar tests at pH 11.5 with an air temperature of 6°C (43°F)
the removal efficiency of the air stripping system was reduced to 60
percent. At an inlet pH of 11.5, the calcium carbonate scale rate on
the packing was found to be 125 mg/1 of CaCO..
Carbon Adsorption
The small carbon adsorption system was fed lime clarified neutralized
(carbon dioxide addition) filtered effluent at a constant rate of 1.21
liter/min (0.32 gpm). The 4 column system was operated from April 16
through October 13, chiefly to demonstrate final product quality. Although
a daily backwash schedule was maintained, biological activity in the last
month of operation within the columns increased the pressure drop across
the adsorption system. The differential pressure losses exceeded the head
capacity of the small positive displacement and during the final 30 days
of operation a reduced flow of .91 liter/min (0.24 gal/min) was maintained.
The efficiency of the system for total organic carbon removal is shown
in Table 7. The effluent TOC increased from 2.4 mg/1 to 4.8 mg/1
with a corresponding percent removal decrease from 65 to 49 percent
during the 6 months of operation. The estimated loading on the two lead
columns was 0.12 kg TOC/kg carbon applied.
22
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TABLE 7. TOC REMOVAL BY ACTIVATED CARBON ADSORPTION
Month
(1970)
April*
May
June
July
August
September
October**
Influent
mg/1
6.9
7.7
7.5
7.7
9.9
7.8
9.5
Effluent
mg/1
2.4
2.9
3.1
3.5
4.5
4.9
4.8
Removal
%
65
62
59
55
55
37
49
* April 16-30
**' October 1-13
23
-------
370
340
O
O
-J
<
O
c
310
N>
280
250
220
• '
24
48 72
DAYS
96
120
FIGURE 5. CALCIUM CARBONATE SCALE AFFECT ON
AIR FLOW RATE
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SECTION VI
OVERALL SYSTEM PERFORMANCE
The operation of a tertiary treatment system on a good quality secondary
effluent has demonstrated that a high quality effluent is possible.
The results of the operation which included two-stage lime precipitation,
air stripping of ammonia, dual-media filtration and activated carbon
adsorption are presented in Table 8. The nitrogen residual, which
includes ammonia and organic nitrogen, was attained only during warm
temperatures ( 24°C [ 75°F]). To meet the Potomac River discharge
requirement, breakpoint chlorination (10) of the residual nitrogen could be
used for additional ammonia removal and would reduce the final nitrogen
level to approximately 2 mg/1.
25
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TABLE 8. TERTIARY SYSTEM PERFORMANCE
TOC Phosphorus Nitrogen
mg/1 mg/1 mg/1
Raw 104 8.4 22
Secondary 19.2 6.4 17
Clarification 8.1 0.13 15
Air Stripping* 5.0
Filtration 7.5 0.09
Carbon Adsorption 3.7
*Air Stripping represents studies at inlet air temperatures of
76°F (24.4°C).
26
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SECTION VII
REFERENCES
1. Recommendation of the Conference, Potomac River - Washington Metropolitan
Area Enforcement Conference, Washington, D,C., May 8, 1969,
2, Gulp, R,, "South Lake Tahoe Water Reclamation System", £, 36, 1969.
3. Weber, W.J., C.B. Hopkins, and R, Bloom, "Physicochemical Treatment of
Wastewater", Journal Water Pollution Control Federation, 42, 83, 1970.
4. Stander, G.J., and L.R.J. Van Vuuren, "The Reclamation of Potable Water from
Wastewater", Journal Water Pollution Control Federation, 41, 355, 1969.
5. Hager, D.G., and D.B. Reilly, "Clarification Adsorption in the Treatment of
Municipal and Industrial Wastewater", Journal Water Pollution Control
Federation, _42_, 794, 1970.
6. Stamberg, J.B., D.F. Bishop, H.P. Warner, and S.H. Griggs, Chemical
Engineering Progress Symposium Series, No. 107, 67, 310, 1970.
7. O'Farrell, T.P., P.P. Frauson, A.F. Cassel, and D.F. Bishop, "Nitrogen
Removal by Ammonia Stripping", Journal Water Pollution Control Federation,
44, 1527, 1972.
8. O'Farrell, T.P., D.F. Bishop, and S.M. Bennett, "Advanced Waste Treatment at
Washington, D.C.", Chemical Engineering Progress Symposium Series, No. 97,
65^, 251, 1969.
9. Villers, R.V., E.L. Berg, C.A. Brunner, and A.N. Masse, "Treatment of Primary
Effluent by Lime Clarification and Granular Carbon", presentation at the 47th
Annual ACS Meeting, Toronto, Canada, May 1970.
10. Cassel, A.F., W.W. Schuk, T.P. Pressley, and D.F. Bishop, "Physical Chemical
Nitrogen Removal from Municipal Wastewater", AIChE Symposium Series No. 124,
68» 56, 1972.
11. Schaeffer, R.B., et al., "Application of a Carbon Analyzer in Waste Treatment",
Journal Water Pollution Control Federation, 37, 1545, 1965.
12. FWPCA Methods for Chemical Analysis of Water and Waste, U.S. Dept. of the
Interior, FWPCA, Cincinnati, Ohio, November 1969.
27
-------
13. Kamphake, L.S., S. Hannah, and J. Cohen, "Automated Analysis for Nitrate by
Hydrazine Reduction", Water Resources, J^ 205, 1967.
14. Gales, M., E. Julian, and R. Kroner, "Method for Quantitative Determination
of Total Phosphorus in Water", Journal of American Water Works Association,
58, 1363, 1966.
15. Standard Methods for the Examination of Water and Wastewater, 12th ed.,
American Public Health Association, New York, 1965.
16. Schuk, W.W., H.P, Warner, and D.F, Bishop, "Control System in Advanced
Waste Treatment", presentation at the 68th National AIChE Meeting,
Houston, Texas, March 1971,
17. Bennett, S.M., and D.F. Bishop, "Solids Handling and Reuse of Lime Sludge",
presentation at the 68th AIChE Meeting, Houston, Texas, March 1971.
28
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SECTION VIII
BIBLIOGRAPHY
O'Farrell, T.P., and D.F. Bishop, "Lime Precipitation in Raw, Primary, and
Secondary Wastewater", AIChE Symposium Series No. 124, 68, 43, 1972.
O'Farrell, T.P., F.P. Frauson, A.F. Cassel, and D.F. Bishop, "Nitrogen Removal
by Ammonia Stripping", Journal Water Pollution Control Federation, 44, 1527, 1972.
29
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 . REPORT NO.
EPA-600/2-76-251
3. RECIPIENT'S ACCESSION«NO.
4. TITLE AND SUBTITLE
Conventional Tertiary Treatment
5. REPORT DATE
November 1976 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Thomas P. O'Farrell and Dolloff F. Bishop*
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Government of the District of Columbia
EPA-DC Pilot Plant
5000 Overlook Ave. SW
Washington, B.C. 20032
10. PROGRAM ELEMENT NO.
1BC611
11. CONTRACT/GRANT NO.
14-12-818
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
*Presently with MERL, Cincinnati
16. ABSTRACT
Tertiary treatment on effluent from the EPA-DC Pilot Plant's step aeration activated
sludge system included two-stage or single-stage lime clarification, air stripping
of ammonia, dual-media filtration, neutralization, and activated carbon adsorption.
With a good secondary effluent to the lime clarification units and the dual-media
filtration system, the two-stage process produced residual BOD and phosphorus (as P)
concentrations of 1.4 mg/1 and 0.09 mg/1, respectively. With fresh carbon, the TOG
was less than 3 mg/1 after treatment by carbon adsorption. Single-stage operation
with sodium carbonate addition and with a good quality secondary effluent as feed
produced an average phosphorus residual of 0.53 mg/1 as P after clarification.
Following dual-media filtration, the phosphorus residual as P was reduced to 0.10 mg/L
With a poor quality secondary effluent (filamentous growth), the slurry pool in the
first-stage solid contactor unit was unstable and produced lower quality effluents.
At warm air temperatures (greater than 75 F), 80 percent of the ammonia was air
stripped from the effluent of the two-stage lime system. Recalrined lime was
recycled to the clarification system without a reduction in the efficiency of the
tertiary system.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
*Sewage Treatment
Calcium oxide
Calcium phosphates
*Clarification
*Filtration
*Activated carbon
Ammonia
b.IDENTIFIERS/OPEN ENDED TERMS
Two-stage lime
clarification
Lime reuse
Phosphorus removal
*Tertiary treatment
ammonia stripping
COSATl Held/Group
13B
3 DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS /This Report/
Unclassified
21. NO. OF PAGES
40
2O. SECURITY CLASS (This page>
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
30 £.U.S. 60VHJUKNT KNTING OFFICE: 1976-757-056/5*33 Region No. 5-11
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