Investigation Of Oxidation Ditch Performance In Treatment ...system, it is possible to achieve...
Transcript of Investigation Of Oxidation Ditch Performance In Treatment ...system, it is possible to achieve...
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INVESTIGATION OF OXIDATION DITCH PERFORMANCE IN lXEATMENT OF
DOMESTIC WASTEWATER
Edward C. Fiss, Jr. Robert M. Stein
George P. lptan
AWARE Environmental Inc. 9305 Monroe Road, Suite J
Charlotte, North Carolina 28270
t
Presented at
1989 N.C. W C F Conference
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INVESTIGATION OF OXIDATION DITCH PEFORMANCE
IN TREATMENT OF DOMESTIC WASTEWATER
Edward C. Fiss, Jr. Robert M. Stein George P. Tyrian
The oxidation ditch technology offers an innovative approach to achieve
tertiary treatment.
now utilized all over the world.
concepts for oxidation ditches.
The use of oxidation ditches originated in Europe and is
This paper reviews the tertiary treatment
The oxidation ditch is a variation of the activated sludge process.
system consists of a closed-loop aeration channel through which mixed liquor
is continuously recirculated. The heart of the oxidation ditch technology is the aeration system.
recirculation of the mixed liquor.
system, it is possible to achieve organic removal, ammonia removal (nitrifi-
cation), and nitrate removal (denitrification) in a single sludge system.
The oxidation ditch concept also has the potential for phosphorus removal.
The
I The aerator provides for oxygen transfer, mixing, and l
Through the proper design of the aeration 1.
There are a number of types of aeration units which have been utilized in
oxidation ditches. This includes turbine aerators, jet aerators, surface
aerators, and brush aerators. Manufacturers have developed a number of
proprietary systems geared to the oxidation ditch process.
i5 the "barrier" ditch. As the name implies, this includes a concrete or
earthen barrier in the channel in which a draft-tube (turbine type) aerator
is installed.
One such approach
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The draft-tube aerator serves to pump water through the draft-tube providing
single-point aeration and positive wastewater recirculation through the
ditch.
of mixed 1 iquor mixing/recirculation and aeration.
The barrier arrangement is unique in that it allows separate control
A second method for implementation of the oxidation ditch process is the
"carousel process". In the carousel arrangement, vertical shaft mechanical
aerators are positioned in the oxidation ditch channel at the two ends of the
race track configuration.
oxygen transfer and mixed liquor recirculation/mixing.
The rotating action o f the aerators provides
The most common method of oxygen transfer and mixing is the installation of a
horizontal shaft, brush rotor in a shallow channel.
bridge mounted or floating and are normally installed in the "straightawaytt
portion of the channel.
ments, a single brush rotor or multiple units in series may be installed in
the channel.
The brush rotors may be
Depending on oxygen transfer and mixing require-
PROCESS CHARACTERISTICS
The ability to provide aerobic/anoxic/anaerobic conditions within an oxida-
tion ditch allows a condition conducive for carbonaceous BOD removal,
nitrification, and denitrification with a single sludge system.
BOD removal or oxidation o f organics is achieved in both the aerobic and
anoxic zones of the channel. Nitrification or oxidation o f ammonia to
Carbonaceous
nitrate occurs only in the aerobic' portion of the channel.
or conversion o f nitrate to nitrogen gas occurs only in the anoxic portion of
the channel.
Denitrification
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1 .
Carbonaceous BOD removal in the ditch process is achieved by facultative
heterotrophic bacteria. The reaction occurs in two phases. The overall
oxidation reactions are presented as Equation 1 and Equation 2.
1.
2.
Organics + 02 + N + P-New Cells + C02 + H20 f Nondegradable
Cells + 02 -C02 + H20 + N + P + Nondegradable Cellular Residue
Cellular Residue
In the aerobic portion of the channel, organic materials (BOD, COD, TOC) are
oxidized by the bacteria using oxygen as an electron acceptor.
portions o f the basin, the organic materials are oxidized by the bacteria
using nitrate (NO3) as an electron acceptor.
aerobidanoxic oxidation o f organic materials results in reduced power
requirements for aeration and a reduction in capital and operational cost.
In the anoxic
Consequently, the alternating
Nitrification is the two-step biological oxidation of ammonia (NH3) to
nitrate (NO3). The oxidation is performed by aerobic autotrophic bacteria
frequently called nitrifiers. The predominant species responsible are
nitrobacter and nitrosomonas. Equations describing the oxidation of amnonia
to nitrite (N02) and oxidation of nitrite to nitrate are presented in
Equations 3 and 4, respectively.
3. 2NH4' + 302-2N02' + 2H20 + 4H+ + New Cells
4. .2N02' + O2-2NO3- + New Cells
Nitrification occurs only under aerobic conditions. Temperature, pH, and
alkalinity are primary factors in biological nitrification. Alkalinity is
. consumed at a rate of approximately 7.14 pounds per pound of amnonia nitri-
This alkalinity reduction causes the pH of the mixed liquor to drop. fied.
The rate of nitrification is pH dependent. The optimum pH for nitrification
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is approximately 8.4.
levels of less than 7. There is also a significant drop in nitrification
rates at temperatures less than 15OC.
The rate of nitrification drops off rapidly at pH
Denitrification or nitrogen removal is the biological reduction of nitrate
(NO3) to nitrogen gas (N2).
by facultative heterotrophic bacteria.
i s presented as Equation 5.
The process is performed under anoxic conditions
The formula which represents reaction
5. 6NO3- + 5CH30H -3N2 + 5C02 + 7H20 + 60H' + New Cells
A carbon source (shown as CH30H in Equation 5) is required for denitrifica-
tion to occur. In the oxidation ditch process, the carbonaceous BOD in the
wastewater is utilized as the carbon source. Denitrification is an alka-
linity producing process whereby approximately 3.57 pounds of alkalinity are
released per pound of denitrified nitrate.
the lowering of pH caused by nitrification in the mixed liquor.
Denitrification therefore slows
Denitrification occurs only under anaerobic or anoxic conditions and there-
fore occurs only in the anoxic portions of the oxidation ditch. Denitrifi-
cation normally will begin occurring when the bulk mixed liquor dissolved
oxygen concentration is 0.5 mg/l or less. A dissolved oxygen gradient is
present in each biological floc particle composing the mixed liquor as shown
in Figure 1. This gradient causes the dissolved oxygen concentration in the
center of the biofloc to be zero when the bulk mixed liquor dissolved
concentration may be above zero.
under low mixed liquor dissolved oxygen conditions.
As a result, denitrification can occur
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OPERATION AND DESIGN CONSIDERATIONS
The oxidation ditch can be operated under entirely aerobic conditions to
obtain organic removal and nitrification. However, in order to operate an
oxidation ditch process and achieve both nitrification and denitrification,
alternating aerobic and anoxic conditions are necessary.
The typical variation in dissolved oxygen concentrations along the length of
the oxidation ditch channel and, from another perspective, over time is
presented in Figure 2.
point of aeration. The dissolved oxygen concentration then declines over the
length of the channel. The rate o f oxygen depletion or the slope of the
dissolved oxygen versus time line is the oxygen uptake rate expressed in
units of mg/l per minute.
The dissolved oxygen concentration is highest at the
The oxygen uptake rate is dependent on several parameters including waste-
water characteristics, temperature, F/M level, and the mean cell residence
time or sludge age.
suspended sol ids (MLVSS) concentrations within a given aeration basin volume
will change the slope of the dissolved oxygen versus time line and the
re1 ative proportions o f aerobic-anoxic basin volumes.
In other words, variation o f the mixed liquor volatile
If an anoxic zone is not provided, then denitrification will not occur.
loss of an anoxic zone may result from the process being operated under a
very low F/M condition (weekends), very low oxygen uptake rates, excessive
aeration, or excessive recirculation.
The
Conversely, higher F/M conditions and
the resulting higher oxygen uptake rate may cause the detention time o f the
aerobic portion of the basin to be insufficient for complete nitrification.
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Without a source of nitrate created by nitrification, denitrification cannot
take place.
The following section presents an approach to utilize in the design o f a
nitrification system. The minimum detention time for nitrification can be
calculated using Equation 6 as follows:
Where: ;N = Maximum nitrified growth rate, Days-1 T = Basin Temp., OC
pH = Basin pH
D.O. = Basin D.O. concentration, mg/l
1.3 = Monod half saturation constant for oxygen, mg/l
From the maximum growth rate, the minimum nitrifier mean cell residence time
(MCRTN) can be calculated by:
1 7. 8" = 7
UN
Where: Q" = Minimum nitrifier solids retention time, days
The design MCRTN ( Q N ~ ) is determined by:
Where: 2.5 is a safety factor.
.The required hydraulic detention time of the aerobic zone can now be
calculated as follows:
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Where: YH = Heterotrophic yield constant (typically 0.6)
X v = MLVSS, mg/l
QN = Design MCRTN, days
KD = Decay constant, l/days (typically 0.05)
So = Influent BOD
SE = Effluent soluble BOD, mg/l
The detention time for denitrification is determined assuming all influent
TKN is oxidized to NOs-N, and therefore the nitrate concentration to be
reduced is equal to the influent TKN.
The minimum MCRT required for denitrification is calculated from:
Where: YDN = Heterotrophic yield constant o f denitrification, lb MLVSS/lb BOD
Decay constant of denitrification, l/days
Peak rate o f denitrification, l b NO3/lb MLVSS-day
Minimum solids retention time, days
The design MCRTD is determined using a safety factor of 2.5, similar to the
nitrification MCRT.
11. QcD = 2.5 Qcm
Where: Qc = Design Heterotrophic MCRT, days
Using equation 10, solve for the specific denitrification rate:
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I' . . "
The required hydraulic detention time in the anoxic zone can now be
calculated by:
12. D.T. = (No - NE)
Where:
xv qDN No = Influent TKN, mg/l
NE = Effluent NO3-N, mg/l
Once the required detention times have been established, the selection and
placement of aeration devices must be determined.
The rate of recirculation or the velocity of the mixed liquor flowing in the
channel determines the slope of the dissolved oxygen versus feet of channel
line. The slope of the dissolved oxygen gradient in the channel is
represented by Equation 13:
13. SDO = OU/V
Where: SDO = Slope of the dissolved oxygen gradient, mg/l/ft
OU = Oxygen uptake, mg/l per minute
V = Bulk mixed liquor velocity, ft/min
Increasing the recirculation rate reduces the slope o f the dissolved oxygen
gradient and, therefore, reduces the detention time of the mixed liquor in
each pass of the anoxic zone.
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The ditch aeration system is sized based on the oxygen demand which will be
exerted on the aeration system.
determining the process oxygen demand in an aerobidanoxic ditch process:
Equation 14 represents a method for
14. AOR = at SR + b' Xv + c'Ng - d' NOR Where: AOR = Process Oxygen Demand, lbs 02/day
SR = BOD removal, lbs/day
Xv = MLSS, lbs
No = Ammonia oxidized, lbs/day
NOR = Nitrate reduced to nitrogen gas, lbs/day
a' = Organic oxygen utilization
b' = Endogenous oxygen utilization
c ' = Nitrification oxygen utilization
d' = Denitrification oxygen credit
The amount o f oxygen required for the aerobic portion of the system is
normally a function of BOD removal, MLVSS in the system, and the ammonia
loading. Normally, 4.5
removed. In the anoxic
the oxygen source, cred
may be taken for oxygen
pounds o f oxygen credit
pounds of oxygen are required per pound o f amnonia
portion of the system where nitrate is utilized as
t in calculation of oxygen required to satisfy BOD
supplied through denitrification. Normally, 2.6
may be expected per pound of nitrate reduced.
Since the mixed liquor is recirculated continuously around the race track
channel, both the level o f aeration and the placement of aeration devices is
critical.
allow variation in the level of oxygen transfer, level of MLVSS concentra-
tion, and aeration volume.
Sufficient flexibility should be incorporated into any design to
Provision of at least two aeration basins allows
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the process to be operated at both high F/M (high oxygen uptake rate) and low
F/M (low oxygen uptake rate).
TYPICAL PROCESS PERFORMANCE
The oxidation ditch process is capable of achieving consistently high levels
of BOD, suspended solids (TSS), and nitrogen removal. A telephone survey was
undertaken in November 1986 to determine the levels of effluent BOD and TSS
which are routinely achieved in oxidation ditch wastewater treatment plants
in the U.S.
presented in Table 1.
achieving nitrification/denitrification is presented in Table 2.
Plant performance data from the surveyed facilities are
In addition, plant operating data for a ditch system
SUMMARY AND CONCLUSIONS
In the oxidation ditch process, the activated sludge mixed liquor undergoes
continuous alternation of aerobic/anoxic conditions enabling a wide variety
of microorganisms to survive. Consequently, oxidation ditches provide
favorable conditions for simultaneous removal of carbonaceous BOD, nitrifi-
cation, and denitrification. Because an oxidation ditch process utilizes a
single sludge system for three processes, and because carbonaceous BOD
removal occurs i n both aerobic and anoxic conditions, oxidation ditches are
usually characterized by capital and operational costs lower than a
traditional activated sludge treatment plant achieving similar performance.
The oxidation ditch process can achieve consistently high levels of BOD,
suspended solids, and nitrogen removal.
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TABLE 1
SUMMARY OF D I T C H ACTIVATED SLUDGE PERFORMANCE DATA
C1 ari f i er Sol ids E f f 1 uent FLOW O v e r f l o w RAS Clarifi r Loading BOD TSS sv I
Location (mg/l) (mg/l) (mg/l) (mg/ l ) (gpd/ft2) MLSS Q mgd Area ft5 lbs/hr/ft2 ~
3000 1.0 3040 0.69 Immokcalee, FL 2 5 1.2 395
Holdenville, OK 2 12
16
30
200
223
.6
1.01
0.26
565
35 1
310
2340 0.3 1062
3060 1.75 2880
3000 0.22 840
0.69
1.02
0.60
Thompson, NY 9
Dawson, MN 5
Presque Isle, 4 ME
28 200 1.3 230 3200 2.7 5652 0.79
Foley, AL 5 -8 10
4
69-70
95
0.7
2.0
220
157
4500 0.7 3180
6500 1.5 12723
0.69
0.62 Clayton, GA 3 (N.E. Plant)
Clayton, GA 4 (Jackson Plant)
13 121 0.44 187 3400 2.29 2353 1.37
S o u t h Florida 6 8 115 0.908 1620 0.82 6720 0.15
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TABLE 2
TY P I CAL OPERATING DATA
Effluent Influent Total Total
Month mg/l mg/l TKN* NH3* MGD mg/l mg/l pH mg/l mg/l BOD TSS F1 ow BOD TSS N P
July
Aug . Sept.
Oct.
Nov.
Dec.
Jan.
Feb.
March
Apr i 1
May
June
Avg .
103 124
81 98
171 87
208 118
226 134
216 150
199 142
215 179
230 141
196 143
226 141
188 132 32* 25.6*
0.928
0.844
1.170
0.908
1.155
0.957
1.126
0.908
1.167
0.98
0.89
0.750
0.976
7 2.5 6.8 4.65
10 32 6.8 5.88
4 8 6.9 1.89
6 8 6.9 4.92
9 6 7.0 4.79
12 10 6.8 5.97
6 10 6.8 5.53
7 9 6.9 3.14
5 8 6.7 2.69
6 10 6.9 1.91
3 6 6.8 3.14
4 4 6.8 2.14
6.6 9.5 - 3.89
6.42
6.46
4.22
6.66
4.92
6.34
4.52
6.43
3.77
6.2
10.40
7.9
6.19
*Long-term avg. only
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Representation of FLOC - Figure 1
Portion of Floc t
R
Aerobic Port ion of Floc
A co
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a.0- - .4 03 E 2.5- C 0 03 s 2 . 0 -
8 x 1.5- >
(D 1.0-
0
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0.5 -
Existing aerator
\
Mechanical ,erator #l
3.0 E
Mechanical Aerator Y2
I I 0 100 200 300 400 500 600 700 800 SO0 1000 1100
g 2.0 Existing > e
Influent Pipe Channel ler,3t!1, Ft
Mechanical Aerator #1
0, Uptakez0.43 mg/l/min.
Mechanical Aerator #2
I
Influent Pipe
Time (Min)
Holdenville OKPresque IsleClayton GAClayton GASouth Florida