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

I. .

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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L ' f ,

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

a.0- - .4 03 E 2.5- C 0 03 s 2 . 0 -

8 x 1.5- >

(D 1.0-

0

- -

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)