Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR MONITORING BIOLOGICAL ACTIVITY IN FLUIDS
FIELD OF THE INVENTION
The present invention relates to a method for monitoring metabolically
' significant transition points during the microbial metabolism of organic and
inorganic substrates and controlling the microbiological process.
BACKGROUND OF THE INVENTION
Microbial use of organic and inorganic substrates in metabolic processes
can cause detectable changes in measurable parameters such as pH and oxygen
utilization rates.
If nitrification is a predominant reaction within a microbial culture, the
production of hydrogen ions (H') from the nitrification process would be
expected to decrease markedly upon the exhaustion of readily usable ammonium
(NH4+) below some metabolically critical level. Consequently, the activity of
hydrogen ions in solution, i.e, pH, would also be expected to change.
Similarly, the oxygen utilization of a microbial culture would be expected
to be higher in a condition in which exogenous organic substrates were readily
available and plentiful than in a condition where these substrates were
depleted
below some metabolically significant level. In both examples, the measurable
rate of change in pH, sometimes hereinafter referred to as "pH production
rate"
or "pHPR, " and oxygen utilization, sometimes hereinafter referred to as
"biological oxygen consumption rate" or "BOCR, " would be directly affected by
the rate of substrate metabolism over time. Thus, 'assuming that changes in pH
and oxygen consumption in a medium result from microbial metabolic activity
alone, pHPR and BOCR could theoretically be used to signal metabolically
significant transition points in a microbiological process. pHPR is defined as
d(pH)/dt or -0(pH)/Ot and BOCR is defined as d(DO)/dt or -A(DO)/Ot. A
negative slope of pH and/or DO results in a positive pHPR and/or BOCR
measurement.
SUMMARY OF THE INVENTION
The method of the invention involves the isolation of a fluid sample from
a fluid supply, such as wastewater in a purification process. pHPR is
calculated
from pH measurements taken from the fluid sample and analyzed to quickly
. determine when metabolically significant transition points occur. The
analysis
dictates what control steps are needed and when they should be implemented to
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maximize the efficiency of the process being monitored.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graphic representation describing the Michaelis-Menten theory
of reaction kinetics.
Fig. 2 is a graph depicting theoretical responses of oxygen utilization rate
(BOCR) and rate of change in pH (pHPR) of a mixed liquor sample as =
concentrations of ammonium (NH4') and organic carbonaceous material,
collectively referred to as BOD (biochemical oxygen demand), change over time
in a microbiological process.
Fig. 3 is a graph depicting theoretical responses of oxygen utilization
(BOCR) and rate of change in pH (pHPR) of a mixed liquor sample as the
concentrations of anunonium (NH4') and organic carbonaceous material,
collectively referred to as BOD (biochemical oxygen demand), change over time
in a microbiological process.
Fig. 4 shows a schematic front elevational view of one embodiment of
apparatus which may be used to separate and monitor a fluid sample from a
fluid
supply in a bioreactor tank in accordance with the invention.
Fig. 5 graphically illustrates the relationship between the rate of oxygen
change between the cessation and onset of aeration and BOCR expressed as %
change in oxygen saturation per minute.
Fig. 6 graphically illustrates the relationship between the pH change
between the cessation and onset of aeration and pHPR expressed as change in pH
per minute as ammonia concentration changes.
Fig. 7 graphically shows the relationship between pHPR expressed as
change in pH per minute and ammonia concentration where COD is not a
metabolically limiting factor.
Fig. 8 graphically shows the relationship between BOCR expressed as %
change in oxygen saturation per minute and anunonia concentration where COD
is not a metabolically limiting factor.
Fig. 9 shows the response of pHPR expressed as change in pH per minute
under various conditions of ammonia and COD availability. =
Fig. 10 shows the relationships between pHPR expressed as change in pH
per minute, BOCR expressed as % change in oxygen saturation per minute,
=
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ammonia concentration and COD under various conditions of ammonia and COD
availability.
Fig. 11 is a graph of DO and pH change versus time under continouous
aeration.
Fig. 12 is a graph of pH, NH3-N concentration and d(pH)/dt versus time.
= Fig. 13 is a graph of DO and d(DO)/dt versus time.
DETAILED DESCRIPTION OF THE INVENTION
The mechanistic rate at which biochemical reactions proceed can be
described in part by the Michaelis-Menten theory as illustrated in Fig. 1.
This
theory states that the rate of biochemical reaction is very low at very low
substrate concentrations, but the rate increases as substrate concentration
rises
until a point is reached beyond which there are vanishingly small increases in
the
reaction rate no matter how much the substrate concentration rises. In other
words, no matter how far the substrate concentration is raised beyond this
point,
the reaction rate will approach, but never reach a plateau. This plateau is
the
maximum reaction rate or Vmm. It is a linear extrapolation corresponding to a
substrate concentration equal to 2KS. Kg is the substrate concentration at
which
the metabolic reaction rate is one-half the maximum reaction rate (V.ax).
It, therefore, follows that from a metabolic perspective, 2Ks is a
significant substrate concentration. Microbial metabolism of a substrate
proceeds
above 2K5 at a maximum and nearly constant rate. The metabolic reaction rate
can become variable and limited by substrate availability below 2Ks.
Consequently, changes in certain measurable parameters directly affected by
and/or related to the rate of microbial metabolism of particular inorganic and
organic substrates can be expected to change as the concentration of the
particular substrate changes. Specifically, at a substrate concentration equal
to
or greater than 2K, the dependent measurable parameter and/or the measured
rate of change in this parameter over time would be expected to be relatively
constant. As a substrate concentration decreases to below 2Ks, the dependent
measurable parameter and/or the measured rate of change in this parameter over
time would be expected to differ markedly from the values measured when the
substrate concentration was equal to or higher than 2KS.
= For many biological reactions, it is desirable to determine the point at
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which certain substrates have been depleted below this metabolically
significant
2K5 concentration. It is possible to detect changes in the pattern of
metabolic
behavior of a microbial culture by monitoring changes in certain dependent
measurable parameters, as the concentrations of certain organic and inorganic
substrates change.
For example, in many wastewater purification processes it is an object to
reduce the concentrations of certain organic and inorganic substrates to very
low
levels. These substrates typically include those organic substrates
collectively
referred to and measured as BOD (biochemical oxygen demand) and/or COD
(chemical oxygen demand) and inorganic ammonium (NH4+). Assuming that the
nitrification reaction and the BOD/COD reduction reaction were the two most
predominant reactions, it would be expected that characteristic changes would
be
seen in both the oxygen utilization rate (BOCR) and rate of change in pH
(pHPR) as BOD and ammonia are depleted below their respective 2KS values.
A drawback of utilizing BOCR and pHPR as control parameters is that in
a continuous wastewater purification process, the changes in pH and DO in the
fluid medium are dependent on many factors such as concentration of nutrients
(biodegradable carbonaceous, nitrogenous, phosphorous compounds and the like),
concentration of biomass, alkalinity and the like. Those factors are
constantly
changing as wastewater passes through the treatment facility. Consequently, it
is difficult to obtain the relationship between the measured parameters and
the
performance of wastewater purification due to the interference of too many
unknown and ever changing factors. Unless these interfering factors can be
either detected or maintained as constant during the pH and DO measurement,
pHPR and BOCR measurement will not provide more valuable information on
wastewater treatment performance.
Utilization of a biological activity detecting device such as that disclosed
in U.S. Patent No. 5,466,604, enables in situ
isolation of wastewater samples from the main body of the wastewater under
treatment. Of course, other apparatus may be used in accordance with this
invention. Also, the term "in situ" is used herein to describe any real-time
fluid
sample isolation process, irrespective of whether the sample remains in the
main
body of the fluid, e.g. wastewater. In other words, apparatus may be used that
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physically removes the sample(s) from the fluid main body so long as
measurements may be made substantially in "real-time" and/or "on-line."
Theoretical responses of BOCR and pHPR to concentration changes in
BOD and ammonia (NH4+) are depicted in Figs. 2 and 3 and explained below.
The Figures graphically represent responses from a single sample of mixed
liquor (i.e., wastewater) and microbes for biological nutrient removal (BNR)
isolated from the main body of wastewater. The isolated sample is
alternatively
aerated and not aerated. Aeration begins and continues until a level of
dissolved
oxygen has been reached that is higher than the DO level in the main body of
wastewater by a margin. Once this level is reached, the aeration stops and
only
begins once the level of dissolved oxygen within the sample reaches a level
that
is lower than the level of DO in the main body of wastewater by a margin.
During the periods where aeration is not conducted, both BOCR and pHPR are
evaluated and calculated as follows, in equations (6) and (7):
(6) BOCR = -(ODO)/(Ot)
wherein ADO is equivalent to the change in saturation level of dissolved
oxygen,
expressed as percent saturation, measured over a time period Ot; and
(7) pHPR = -(ApH)/(At)
wherein OpH is equivalent to the change observed in pH over a time period Ot.
As shown in Period A of Figs. 2 and 3, when both the concentrations of
NH4+ and BOD are above their respective 2K5 values, BOCR is constant and at
its highest relative level, since BOD utilization proceeds at maximum rates
and
wins the competition over oxygen consuming reactions of nitrification. Thus
pHPR is constant at a moderate level. This BOCR/pHPR pattern, as well as
those described below, is expected assuming that 1) the nitrification and BOD
utilizing reactions are the predominant reactions ongoing within the
biological
sample, 2) the production and activity of hydrogen ions are related to the
rate of
the nitrification reaction, and 3) the reactions are not limited by the
availability
of oxygen.
Subsequently, continued metabolism depletes the available NH4+ below
its 2K, value and, the rate of nitrification, hydrogen ion production falls
from a
maximum rate to a lower rate where ammonia concentration is a metabolically
limiting factor. As shown in Period B of Fig. 2, pHPR drops significantly to a
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comparatively low level and BOCR drops to a comparatively moderate level to
reflect the decreased demand and use of oxygen caused by the significantly
lower
nitrification reaction rate. The transition from an ammonia concentration
above
the 2K5 value to below the 2KS value is depicted in the transition between
periods
A and B in Fig. 2.
Period C of Figs. 2 and 3 shows that where the concentration of available
NH4' is below its 2Ks value and upon depletion of BOD below its 2Ks value,
pHPR increases very slightly to reflect the change in net metabolic behavior
of
the mixed biological population and BOCR drops to its lowest rate to reflect
the
very low oxygen utilization by BOD consuming and nitrification reactions. This
transition is depicted between Periods B and C in Fig. 2.
Period D of Fig. 3 shows that where the concentration of BOD is below
its 2Ks value, but the concentration of NH4' is above its 2Ks value, pHPR
increases to its highest level reflecting a high rate of nitrification and
BOCR
drops to a moderate level reflecting a net decrease in total oxygen
utilization
caused by the decreased level of BOD consuming reactions. The highest pHPR
is seen under this condition because the buffering effects of the BOD
consuming
reactions are absent. Normally, production of CO2 in the BOD consuming
reactions affords some pH buffering capacity to the sample via a carbonic acid
system. Thus, in the absence of BOD consuming reactions and the resulting
production of CO2, the pHPR is much greater than in the other conditions.
It is possible to determine pertinent information about the biological
sample based on the example provided above, by monitoring and comparing
trends and/or levels of BOCR and pHPR because they represent key measurable,
dependent parameters of microbial metabolic activity. Specifically, this
example
illustrates how a determination can be made as to whether 1) both
nitrification
and BOD removal are occurring simultaneously at maximum rates, 2)
nitrification is occurring while BOD has been reduced to levels below its 2Ks
value, 3) BOD removal reactions are ongoing while ammonia has been reduced
below its 2Ks value, and 4) both ammonia and BOD have both been reduced
below their respective 2K5 values.
Direct and continuous comparison of the measured parameters BOCR and
pHPR leads to several conclusions about the condition of the wastewater. If a
=
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mixed liquor sample is continuously monitored and a large increase in pHPR
occurs simultaneously with a decrease in BOCR, this indicates that BOD has
been depleted below its respective 2Ks value while ammonia is still plentiful.
If
a mixed liquor sample is continuously monitored and BOCR decreases to a
moderate level while pHPR decreases to a near zero level, it indicates that
ammonia has been depleted below its respective 2KS value while BOD is still
plentiful. If a mixed liquor sample is continuously monitored and BOCR
decreases to a low level while pHPR decreases to a low level, it indicates
that
both ammonia and BOD have been depleted below their respective 2Ks values.
This condition is also indicated by a decrease in BOCR to a low level and
slight
increase in pHPR from a near zero level to a slightly higher, but low, level.
Table I summarizes these patterns and illustrates how comparison of the
relative values and patterns of the measured parameters of BOCR and pHPR
yields the pertinent information described above in conjunction with Figs. 2
and
3.
TABLE I
PERIOD CONCENTRATION OF MEASURED RELATIVE
PARAMETER VALUE OF
MEASURED
BOD NH + PARAMETER
BOCR HIGH
A > 2KS > 2KS pHPR MODERATE
BOCR MODERATE
B > 2Ks < 2KS pHPR NEAR ZERO
BOCR LOW
C < 2KS < 2KS pHPR LOW
BOCR MODERATE
D < 2K5 > 2K5 HPR HIGH
Fig. 4 shows an example of a preferred apparatus used to isolate a
wastewater sample. The apparatus 11, immersed in wastewater batch 2 (only a
portion of which is depicted), includes a detection chamber 8 having a movable
cover 32. Movable cover 32 is pushed in the direction of arrow "A" by inner
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TM
shaft 56 driven by an Acme shaft 57 connected to motor 53. At the open
position, rotation of propeller 48 forces an exchange of wastewater between
the
inside and outside of detection chamber 8 and detection chamber 8 is filled
with
a fresh sample of wastewater. After a given period of time, e.g. 30 seconds,
motor 53 is programmed to reverse its rotation direction, movable cover 32 is
pulled in the direction of arrow "B" until detection chamber 8 is fully closed
and
sealed. The movable cover 32 and propeller 48 are driven by the same
reversible low RPM motor 53 which coaxially connects inner shaft 56 and outer
shaft 55. The coaxial assembly is shielded by stainless steel pipe 54.
The DO concentration is detected by DO probe 10 after filling detection
chamber 8 with a fresh sample of wastewater and, if DO is less than the oxygen
concentration in the main body of wastewater by a given margin, air and/or
oxygen is pumped into detection chamber 8 through aeration tube 13 until that
DO concentration is attained. A DO concentration at a level that is higher or
lower than the oxygen concentration in the main body of wastewater by a given
margin will ensure that the aerobic metabolic reactions inside detection
chamber
8 is the same or close to the nutrient removal process in the main body of
wastewater. Similarly, pH probe 12 detects changes in pH. Additionally,
propeller 48 may be periodically or constantly rotated to maintain the sample
in
well-mixed and suspended condition.
Aeration in the apparatus 11 is interrupted for the measurement interval
after the maximum DO concentration is attained. During this period, residual
DO concentration and pH, both unaffected by aeration of the wastewater batch
at large, are monitored through the probes. The pH and residual DO signals
from the respective probes 12 and 10 are sent to controllers which convert
changes in DO over time to BOCR and changes in pH over time to pHPR by
numerical differentiation according to the equations (6) and (7) described
above.
In most wastewater treatment plants, concentrations of BOD and
ammonium in the final effluent are below the 2KS values of BOD and NH4+
When the concentration of BOD and NH4+ in the detection chamber decrease to
below 2Ks values, the aerobic metabolic reactions for nutrients removal is
considered complete with significant changes in BOCR and pHPR values. The
completion of aerobic metabolic reactions for nutrients removal can be
detected
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through BOCR and pHPR analysis according to the criteria listed in TABLE I.
For other biological processes the concentrations of substrates in the medium
is
usually considerably higher than 2K5 values to maintain maximum rate of
microbial growth and production of target substance. Thus, the detection of
completion of metabolic reactions will signal the requirement of nutrient and
substrate addition, or the time to stop the biological process, or the time to
harvest the target substance produced during the process.
Information about the aerobic metabolic reactions for nutrients removal,
such as the nitrification completion time (NT), denitrification time (DNT),
etc.
can be used for adjusting and controlling the wastewater purification process
and
other aerobic metabolic processes. For example, the measured NT can be
compared with the average hydraulic retention time of wastewater in aeration
basins in a wastewater treatment plant. If NT is significantly shorter than
HRT,
the aerobic nutrient removal is finished in a section in the middle of the
aeration
basin. The rest of the aeration basins after this section where nutrients
removal
is finished is, in fact, in an idle condition and does not contribute to the
wastewater purification process. In this case, the plant can take proper
actions
to: (1) remove certain sections of aeration basins from service to save
operation
costs, and/or (2) accept more volume of wastewater and effectively increase
the
treatment capacity of the plant, and/or (3) reduce the amount of air supplied
to
the aeration basins to reduce the rate of aerobic metabolic reactions so that
NT
will closely match HRT in the aeration basins and reduce the power consumption
from air blowers.
EXAMPLE 1
A mixed liquor sample recovered from the aerobic basin of an advanced
biological wastewater treatment plant located in Oaks, Pennsylvania, was
isolated
in a vessel equipped with devices to measure sample pH and dissolved oxygen
saturation levels, as well as devices to aerate and maintain the sample in a
well
mixed condition. The data from the devices measuring sample pH and dissolved
oxygen saturation levels was recorded and analyzed by a computer to calculate
BOCR and pHPR. The sample was exposed to fixed, alternating periods of
aerated and non-aerated conditions. Aeration began and continued until a level
= of dissolved oxygen was reached that was compatible with that in the main
body
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of the wastewater plus a margin when the sample was isolated. Once this level
was reached, aeration was stopped and only began once the level of dissolved
oxygen within the sample fell to the level lower by a margin than the DO of
the
main body of the wastewater when the sample was isolated. Concentrations of
NH4+ and soluble carbonaceous organic substrates were measured and reported
as COD. Linear correlation existed between COD and BOD. Therefore, COD
analysis was used to represent BOD concentration. During the periods of non-
aeration, examples of which are marked with arrows on Figs. 5 and 6, both
BOCR and pHPR was evaluated and calculated by numerical differentiation as
described above.
Fig. 5 shows dissolved oxygen saturation and BOCR during a period of
the test where the measured COD concentration was consistently greater than
150 mg COD/L, which was well above the 2Ks value for COD, but where the
ammonia concentration varied from a concentration above the 2Ks value to a
concentration below the 2KS value. Fig. 5 reveals the relationship between the
raw dissolved oxygen data, that is the rate of oxygen change between the
cessation and onset of aeration as indicated, and BOCR. Fig. 5 also
illustrates
the transition in the level of BOCR from a high to a moderate level during the
metabolically significant transition when ammonia concentration dropped below
its 2K, value. BOCR is expressed as % change in oxygen saturation per minute.
Fig. 6 shows the sample pH and pHPR for the same period as depicted
in Fig. 5. During this period the measured COD concentration was consistently
greater than 150 mg COD/L, which was well above the 2K5 value for COD, but
the ammonia concentration varied from above its 2KS value to below the 2KS
value. Fig. 6 illustrates the relationship between the raw pH data, i.e, the
pH
change between the cessation and onset of aeration as indicated, and pHPR.
Fig.
6 also illustrates the transition in pHPR from a moderate to a near zero level
during the metabolically significant transition when the ammonia concentration
dropped below its 2K, value. pHPR is expressed as change in pH per minute.
Fig. 7 shows the changes in measured ammonia levels and calculated
pHPR for the same period as depicted in Fig. 6. Fig. 7 illustrates the
transition
in pHPR from a moderate to a near zero level during the ammonia concentration
transition from about 2Ks to below 2K,. pHPR is expressed as change in pH per -
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minute.
Fig. 8 shows the changes in measured ammonia levels and calculated
BOCR for the same period as depicted in Fig. 5. Fig. 8 illustrates the
transition
in BOCR from a high to a moderate level during the ammonia concentration
transition from above 2KS to below 2K,. BOCR is expressed as % change in
oxygen saturation per minute.
Fig. 9 graphically depicts the consistency of the response of pHPR to
ammonia concentrations. This was accomplished by the addition of an ammonia
solution to the mixed liquor sample at points where the ammonia contained
within the sample was depleted, i.e., at T=120 and T=170 minutes. From the
period between T = 0 and T = 195 minutes, COD concentration was well above
its 2Ks value. After T=195 minutes, COD concentration dropped below its 2K,
value. At about T=90 minutes, a significant transition can be observed in pHPR
as ammonia concentration is depleted below its 2Ks value.
Subsequent additions of ammonia were made at T=120 and T=170
minutes when pHPR was at a near zero level. Fig. 9 shows that pHPR jumped
from the near zero level immediately before each subsequent addition to a
comparably moderate level like that observed between T = 0 and T = 90 minutes.
After the subsequent ammonia additions, pHPR returned to a near zero or low
level upon depletion of ammonia below its 2K5 value. COD was plentiful and
ammonia depletion resulted in a pHPR decrease to a near zero level in the case
of the first ammonia addition at T=120 minutes. Ammonia depletion occurred
at a time when COD concentration was also just depleted to below its 2K, value
in the second case of ammonia addition at T=170 minutes. Consequently, the
pHPR decreases to a low, but not zero, level as shown in Period C of Figs. 2
and 3.
Fig. 10 provides a more complete picture of the data shown in Fig. 9 and
includes calculated pHPR, calculated BOCR, ammonia and COD concentrations.
Fig. 10 best illustrates the transitions in pHPR and BOCR between the
different
relative levels as significant metabolic events occur.
It is possible to quickly and accurately ascertain the moment when the
concentration of organic substrates and/or inorganic substrates fall below
their
respective 2Ks levels as evidenced by this Example, by monitoring relative
levels
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of BOCR and pHPR, in accordance with the invention. Detecting the depletion
of a particular substrate below its respective and metabolically significant
2Ks
concentration value frequently signals a significant change in the condition
of a
microbial population or its environment, or a change in the metabolic pattern
and/or behavior of a sample containing active microbes.
Various control steps may then be taken in response depending on the
particular process. For example, the depletion of a particular substrate in a
microbial population might signal a change in metabolism whereby the
production of a desirable secondary metabolite may ensue, thus indicating that
the process should proceed to a separation, collection and/or purification
phase.
Similarly, in biological processes where it is the object to maintain a
particular
step-feeding protocol of substrate to a microbial population, the capability
to
detect the depletion of this substrate below its 2K, concentration and/or when
substrate addition increase the substrate concentration above 2Ks can be used
to
indicate that increased or decreased feeding of substrate is desirable.
Example 1 involved aerobic biological wastewater purification, where it
is often the object to reduce through biological mechanisms particular
inorganic
and organic substrates such as the reduction of soluble ammonia and
carbonaceous organics. Thus, various control steps may be taken in response to
the depletion of one or more of these substrates below its 2K5 concentration,
as
a concentration of 2KS is often below the low concentration level targeted for
many substrates. For example, if both organic and inorganic (ammonia)
substrates are found to be below their respective 2K, concentration values,
the
flow rate through the wastewater treatment process can be increased, thereby
increasing the capacity of the treatment facility. If both organic and ammonia
substrates are found to be above their respective 2Ks concentration values,
the
flow rate through the wastewater treatment process can be decreased. When
ammonia substrate is below 2Ks but organic substrate is above 2Ks, aeration of
the batch can be decreased due to the reduced desire for nitrification.
Finally,
if the organic substrate is below 2Ks but ammonia substrate is above 2Ks,
aeration can be increased to create a more favorable condition for
nitrification.
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EXAMPLE 2
In Example 2, a mixed liquor sample was isolated in the same manner as
described in Example 1. Continuous aeration to the mixed liquor sample was
maintained throughout the period in which the sample was kept in isolation.
The
aeration rate was selected so that the dissolved oxygen concentration level in
the
sample was higher than the critical value required for biological carbonaceous
nutrient and ammonia removal. The oxygen concentration and pH changes were
monitored by a dissolved oxygen probe and a pH probe as shown in Fig. 11.
Then, a small volume of mixed liquor was periodically withdrawn from
the isolated sample and the ammonia concentration analyzed. Fig. 12 shows the
changes in pH and ammonia concentrations during the entire aeration period in
which the sample was isolated. The end of nitrification (ammonia concentration
was lower than detection level, i.e. 0.1 ppm) was accompanied by a slow
increase in the pH value.
A derivative of pH against time, d(pH)/dt was plotted and is shown in
Fig. 12. When the ammonia concentration approached zero, the value of
d(pH)/dt passed the second zero point. The characterization of d(pH)/dt as
being
at the second zero point can also be referred to as the point where d(pH)/dt
changes from a negative value to zero. The time corresponding to this point is
defined as the nitrification completion time of the mixed liquor or NT. In
Example 2 and as shown in Fig. 12, NT is measured at about 75 minutes. The
d(pH)/dt measurement in Example 2 is different from that in Example 1. In
Example 1, the d(pH)/dt was measured during the non-aeration period, while in
Example 2, d(pH)/dt was measured with continuous aeration. Due to continuous
strip off of CO2 from the mixed liquor, it is possible to see a pH decrease in
the
pH measurement. Thus, pHPR is sometimes negative.
Fig. 13 shows the dissolved oxygen profile and its derivative, d(DO)/dt
for the same sample. As ammonia was consumed, the value of the first
derivative of DO, d(DO)/dt, started to increase significantly. The value of
nitrif~ication time (NT) measured from DO was also at about 75 minutes.
One practical application of NT measurement in the control of biological
nitrification process will now be described here. In a bioreactor or a series
of
bioreactors where the biological nitrification process is taking place, one
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sampling device is installed at the very beginning of the bioreactor or the
front
of the first bioreactor in the series. The measured NT indicates that at
current
biomass concentration and ammonia loading, it will take time NT to complete
the nitrification.
The hydraulic retention time (HRT) of the mixed liquor in the bioreactor
or the series of bioreactors is calculated by considering the flow rate and
flow
pattern of the mixed liquor and the geometry of the bioreactor or the series
of
bioreactors. NT is then compared with the hydraulic retention time of the
mixed
liquor. A proper nitrification process will have comparable values of NT and
HRT in daily operations. When NT is considerably smaller than HRT,
nitrification is finished in the bioreactor or the series of bioreactors
earlier than
the given HRT, which means the process has extra nitrification capacity. In
the
case where other contaminants are removed before ammonia is fully nitrified,
NT detection signals the end of the wastewater treatment process. This
indicates
that the process can treat more wastewater with the given tankage volume under
the same operation conditions or the process can reduce the volume of tankage
in the operation and realize some saving in operation costs.
On the other hand, if NT is longer than HRT, the concentration of
ammonia will be higher than zero but not necessarily higher than the discharge
permit. To ensure the quality of the plant discharge, the aeration rate to the
bioreactor(s) and/or mixed liquor concentrations is increased. When the
condition that NT is longer than HRT for a prolonged period, this tends to
indicate that the process is overloaded regarding ammonia removal and the
treatment facility will likely have to be expanded to treat the given volume
of
wastewater.
In general, by comparing NT and HRT, information such as the
nitrification capacity of the process, the required aeration rate to the
bioreactor
or the series of bioreactors, and the quality of the effluent from the
bioreactor,
can be determined and sent to the plant operator for adjustment of the
nitrification process.
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The invention may be applied to any kind of microbial process including,
but not limited to, wastewater purification (municipal, industrial and the
like),
pharmaceutical/biotechnology production, brewing, fermentation or any other
process involving pure or mixed populations of microorganisms.
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