Investigation Of Oxidation Ditch Performance In Treatment Of by fdh56iuoui



              Edward C. Fiss, Jr.
               Robert M. Stein
               George P. lptan

           AWARE Environmental Inc.
           9305 Monroe Road, Suite J
         Charlotte, North Carolina 28270


                  Presented at

          1989 N.C. W C F Conference


                              Edward C. Fiss, Jr.
                                Robert M. Stein
                               George P. Tyrian

The oxidation ditch technology offers an innovative approach to achieve
tertiary treatment. The use of oxidation ditches originated in Europe and is
now utilized all over the world. This paper reviews the tertiary treatment
concepts for oxidation ditches.

The oxidation ditch is a variation of the activated sludge process. The
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. The aerator provides for oxygen transfer, mixing, and
recirculation of the mixed liquor. Through the proper design of the aeration       .

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.

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.     One such approach
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.

                                      - 1 -
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.   The barrier arrangement is unique in that it allows separate control
             of mixed 1 iquor mixing/recirculation and aeration.

             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.    The rotating action o f the aerators provides
             oxygen transfer and mixed liquor recirculation/mixing.

             The most common method of oxygen transfer and mixing is the installation of a
             horizontal shaft, brush rotor in a shallow channel.     The brush rotors may be
             bridge mounted or floating and are normally installed in the "straightawaytt
             portion of the channel.     Depending on oxygen transfer and mixing require-
             ments, a single brush rotor or multiple units in series may be installed in
             the channel.


             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.     Carbonaceous
             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
             nitrate occurs only in the aerobic' portion of the channel.    Denitrification
             or conversion o f nitrate to nitrogen gas occurs only in the anoxic portion of
             the channel.

                                                    - 2 -
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.   Organics + 02 + N + P-New         Cells + C02 + H20   f   Nondegradable
                                                                               Cellular Residue
                2.    Cells + 02 -C02         + H20 + N + P + Nondegradable 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.           In the anoxic
            portions o f the basin, the organic materials are oxidized by the bacteria
            using nitrate (NO3) as an electron acceptor.       Consequently, the alternating
            aerobidanoxic oxidation o f organic materials results in reduced power
            requirements for aeration and a reduction in capital and operational cost.

            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.    2H'
                       N4   + 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-
            fied.    This alkalinity reduction causes the p H of the mixed liquor to drop.
            The rate of nitrification is pH dependent. The optimum pH for nitrification

                                                     - 3 -
is approximately 84
                  ..    The rate of nitrification drops off rapidly at pH
levels of less than 7 There is also a significant drop in nitrification
rates at temperatures less than 15OC.

Denitrification or nitrogen removal is the biological reduction of nitrate
(NO3) to nitrogen gas (N2).    The process is performed under anoxic conditions
by facultative heterotrophic bacteria.     The formula which represents reaction
i s presented as Equation 5.

    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.     Denitrification therefore slows
the lowering of pH caused by nitrification in the mixed liquor.

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.     As a result, denitrification can occur
under low mixed liquor dissolved oxygen conditions.

                                       - 4 -

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.   The dissolved oxygen concentration is highest at the
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 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.    In other words, variation o f the mixed liquor volatile
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.

If an anoxic zone is not provided, then denitrification will not occur. The
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.    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.

                                      - 5 -
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:

    7.   8
         "    =   7
         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:

                                               - 6 -
         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:

                                           - 7 -
. "

               The required hydraulic detention time in the anoxic zone can now be
               calculated by:

                  12.    D.T.   = (No   - NE)
                               x v qDN
                        Where: 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.

The ditch aeration system is sized based on the oxygen demand which will be
exerted on the aeration system.          Equation 14 represents a method for
determining the process oxygen demand in an aerobidanoxic ditch process:

   14.     AOR = a t 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 pounds of oxygen are required per pound o f amnonia
removed.     In the anoxic portion of the system where nitrate is utilized as
the oxygen source, cred t in calculation of oxygen required to satisfy BOD
may be taken for oxygen supplied through denitrification.                     .
                                                                   Normally, 2 6
pounds o f oxygen credit 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.     Sufficient flexibility should be incorporated into any design to
allow variation in the level of oxygen transfer, level of MLVSS concentra-
tion, and aeration volume.         Provision of at least two aeration basins allows

                                            - 9 -
*   -

        the process to be operated at both high F/M (high oxygen uptake rate) and low
        F/M (low oxygen uptake rate).


        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.   Plant performance data from the surveyed facilities are
        presented in Table 1.     In addition, plant operating data for a ditch system
        achieving nitrification/denitrification is presented in Table 2.


        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.

                                              -   10   -
, -

                                                          TABLE 1

                                  E f f 1uent                C1 ari f i er                           Sol ids
                     BOD        TSS        sv I    FLOW      Overflow               RAS   Clarifi r Loading
      Location       (mg/l)    (mg/l) (mg/l)      (mg/l)      (gpd/ft2)      MLSS   Q mgd Area ft5  lbs/hr/ft2

 Immokcalee, FL         2         5                1.2           395         3000    10
                                                                                      .      3040      0.69

 Holdenville, OK        2        12        200      .6           565         2340    .
                                                                                    03       1062      0.69
 Thompson, NY           9        16        223     1.01          351         3060    1.75    2880      1.02
 Dawson, MN             5        30                0.26          310         3000    0.22     840      06

 Presque Isle,          4        28        200     1.3           230         3200     .
                                                                                     27      5652      0.79
 Foley, AL            5-8        10     69-70       .
                                                   07            220         4500    07
                                                                                      .      3180      0.69
 Clayton, GA            3         4         95      .
                                                   20            157         6500    1.5    12723      0.62
 (N.E. Plant)

 Clayton, GA            4        13        121     0.44          187         3400    2.29    2353       1.37
 (Jackson Plant)
 S o u t h Florida      6         8        115     0.908                     1620    0.82    6720       0.15

                                                           - 11 -
                                          TABLE 2
                                   TY P ICAL OPERATING DATA

                  Influent                                                  Total    Total
          BOD    TSS                    F1ow          BOD    TSS              N        P
Month     mg/l   mg/l   TKN*     NH3*    MGD          mg/l   mg/l        pH mg/l     mg/l

July       103    124                   0.928           7      2.5   6.8      4.65    6.42
Aug   .     81     98                   0.844          10      32    6.8      5.88    6.46
Sept.      171     87                   1.170           4       8    6.9      1.89    4.22
Oct.       208    118                   0.908           6       8    6.9      4.92    6.66
Nov.       226    134                   1.155           9       6    7.0      4.79   4.92
Dec.       216    150                   0.957          12      10    6.8      5.97    6.34
Jan.       199    142                   1.126           6      10    6.8      5.53    4.52
Feb.       215    179                   0.908           7       9    6.9      3.14    6.43
March                                   1.167           5       8    6.7      2.69    3.77
Apr i1     230    141                   0.98            6      10    6.9      1.91    6.2

May        196    143                   0.89            3       6    6.8      3.14   10.40
June       226    141                   0.750           4       4    6.8      2.14    7.9
Avg   .    188    132   32*    25.6*    0.976         6.6     9.5         -   3.89    6.19

*Long-term avg. only

                                         -   12   -
 Representation of FLOC   -   Figure 1

                       Port ion
                       of Floc

of Floc

     a.0-                        Mechanical
                                  ,erator #l
E 2.5-
8                   Existing                      Mechanical
                    aerator                       Aerator Y2



     0.5    -       \
                                200   300   400    500   600     700    800    SO0   1000   1100
                                                               Influent Pipe
                                            Channel ler,3t!1, Ft

E                               Aerator #1

                                            0, Uptakez0.43 mg/l/min.
g    2.0
>                                                 Mechanical
e                   ExistingI
                                                  Aerator #2

                                                               Influent Pipe
                                                  Time (Min)

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