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Description  |
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FIELD OF THE INVENTION
The present invention relates to a method and apparatus for monitoring
biological activity in wastewater and controlling the treatment thereof,
and more particularly to a method and apparatus for real time monitoring
the metabolic activity of microorganisms in activated sludge used in a
wastewater treatment process and using the results of such monitoring to
control selected aspects of the treatment process.
BACKGROUND OF THE INVENTION
The prior art has employed many devices and systems to process and purify
water from industrial operations and municipal sources prior to
discharging the water. Activated-sludge wastewater treatment plants
(WWTP's), which are well known in the art, have been most often utilized
to address this problem. Additionally, many industrial and municipal water
treatment plants utilize biological systems to pre-treat their wastes
prior to discharging into the usual municipal treatment plant. In these
processes, the microorganisms used in the activated sludge break down or
degrade contaminants for the desired water treatment. Efficient process
performance and control requires quick and accurate assessment of
information on the activity of microorganisms. This has proven to be a
difficult task in view of the wide variety of materials and contaminants
that typically enter into treatment systems. Also, variations in the
quantity of wastewater being treated, such as daily, weekly or seasonal
changes, can dramatically change numerous important factors in the
treatment process, such as pH, temperature, dissolved oxygen, nutrients
and the like, alteration of which can be highly detrimental to proper
wastewater treatment. Of course, improperly treated wastewater poses
serious human health dangers.
Various biological nutrient removal (BNR) processes are currently being
used in wastewater treatment plants to assist in contamination
degradation. In a typical BNR process, contaminants in the wastewater,
such as carbon sources (measured as biological oxygen demand or BOD),
ammonia, nitrates, phosphates and the like are digested by the activated
sludge in anaerobic, anoxic and aerobic stages, also known in the art. In
the anaerobic stage, the wastewater, with or without passing through a
preliminary settlement process, is mixed with return activated sludge
(RAS), sometimes hereinafter referred to as "mixed liquor," discussed
hereafter.
Certain microorganisms in the RAS are capable of rapid uptake of readily
biodegradable carbon sources, such as short chain fatty acids and of
forming storage products such as poly-.beta.-hydroxbutyrate (PHB) and
poly-.beta.-hydroxyvalate (PHV). The energy for this process is provided
by the hydrolysis of intracellular polyphosphates. As a result of an
anaerobic selector, a large portion of available carbon sources are
removed by the poly-P forming microorganisms, and PO.sup.-3.sub.4 is
released into the water phase. The rapid uptake and storage of
carbonaceous substrates by poly-P forming species of microorganisms
insures proper phosphate removal in later oxic processes. It also denies
access of other competing organisms to the limited amount of substrates
available in the wastewater under anaerobic conditions.
In most wastewater treatment plants, one or several anoxic stages are
arranged in the BNR process. In the anoxic stage, denitrifiers, i.e.,
microbial species capable of denitrification, utilize nitrate and/or
nitrite as electron acceptors and consume some of the available carbon
sources during the denitrification process. NO.sub.x is stepwisely reduced
to nitrogen gas and released to the atmosphere in the following manner:
NO.sub.3.sup.- .fwdarw.NO.sub.2.sup.- .fwdarw.NO.fwdarw.N.sub.2
O.fwdarw.N.sub.2
The nitrate is usually supplied by recycling a certain volume of wastewater
at the end of the oxic stage back to the beginning of the anoxic stage.
One or several oxic stages are typically employed in BNR processes. In the
oxic stage, air containing about 20% oxygen or pure oxygen, is supplied so
that a desired dissolved oxygen level is maintained. Autotrophic
nitrifiers, i.e., microbial species capable of using ammonia as their
energy source, convert ammonia to nitrite and nitrate under aerobic
conditions. The poly-P microbial species in the wastewater uptake
phosphate from the water phase and digest their intracellular PHB and PHV
storage products converting it into polyphosphate, a compound for energy
storage. The polyphosphate pool of the poly-P microbial species is thus
replenished and phosphorous is removed from the water phase. The
phosphorous is then removed from the system by sludge wasting, which is
well known in the art. Under aerobic conditions, the remaining carbon
sources in the water phase are further digested by aerobic organisms.
Objects of the Invention
It is an object of the present invention to provide a method and apparatus
for monitoring biological activity in wastewater treatment systems during
anaerobic, anoxic and/or oxic stages to maximize the efficiency of the
treatment process.
It is a further object of the present invention to provide a method and
apparatus for real-time monitoring of the purification of wastewater to
enhance control of the anaerobic, anoxic and/or oxic stages of a
wastewater treatment process, to maximize process performance in response
to transient and other conditions.
Other objects of the present invention will be apparent to those of
ordinary skill in the art based on the following detailed description of
the preferred embodiments and the appended claims.
Summary of the Invention
In accordance with the invention, the apparatus and method monitors and
controls biological activity of mixed liquor under anaerobic, anoxic and
aerobic conditions by measuring the change of intracellular NAD(P)H of the
microorganisms. The ratio of NAD(P)H/(NAD+NAD(P)H) in the microorganisms
changes during shifts in metabolic activity of the microorganisms, changes
also. The corresponding change in NAD(P)H fluorescence (hereinafter
sometimes referred to as "NADH") is detected and then registered by a
monitoring system, such as a real time on-line computer data acquisition
system, which analyzes the changes and evaluates the biological activity
of the mixed liquor. The monitoring system then determines the changes in
operating parameters necessary for the wastewater system to maximize the
performance of the BNR processes.
A sample of the mixed liquor is pumped from a bioreactor tank to a chamber
monitored by a NADH detector in the process. The sample is agitated to
ensure uniform suspension of microorganisms in the wastewater and the
differences in NADH quantity between the aerobic, anoxic and/or anaerobic
states of the mixed liquor sample while in the chamber are registered and
analyzed by the monitoring system. The mixed liquor sample is then
returned to the bioreactor tank and the wastewater treatment system is
controlled in accordance with the results generated by the monitoring
system.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic depicting the roles of NADH in a BNR process during
anaerobic, anoxic and oxic stages of a wastewater treatment process.
FIG. 2 is a graph depicting the changes in NADH fluorescence during the
changes in metabolic activity over a period of time.
FIG. 3 shows a schematic front elevational view of a preferred embodiment
of the invention used to monitor a bioreactor tank.
FIG. 4 is a graph of an operational profile depicting changes in NADH
fluorescence over time from an anaerobic stage of treatment.
FIG. 5 is a graph of an operational profile depicting changes in NADH
fluorescence over time from an anoxic stage of treatment.
FIG. 6 is a graph of an operational profile depicting the changes in NADH
fluorescence over time from an oxic stage of treatment.
FIG. 7 is a graph of an operational profile depicting the changes in
percentage of dissolved oxygen over time from an oxic stage of treatment.
FIG. 8 is a schematic of the monitoring of a typical wastewater treatment
process utilizing embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The proper evaluation and control of a complex BNR process requires an
accurate and current assessment of the metabolic activity of the mixed
liquor in a variety of environments and under a number of conditions.
Unlike oxygen metabolism, which is only active during the aerobic stage of
the BNR process, NADH metabolism is involved in all environmental stages.
Thus, NADH is an excellent indicator of metabolic activity that can be
used to control the entire BNR process, whereas oxygen metabolism cannot.
The dominant organisms and the active biochemical pathways vary with the
environmental stages of the bioreactor. However, one common factor is the
requirement to transfer energy by the oxidization of available energy
sources. Those reactions are summarized in FIG. 1, over the anaerobic,
anoxic and oxic stages which shows the role of nicotinamide adenine
dinucleotide (NAD.sup.+ and NADH) in the energy transfer process.
It is generally believed that under anaerobic conditions the organic
materials, such as acetate, for example, are taken up by the cells and
converted to acetyl-CoA with the energy for the conversion coming from
hydrolysis of intracellular polyphosphate. Acetyl-CoA is further converted
to PHB for storage. The reducing power in the form of NADH required for
this conversion is obtained by circulating some of the acetyl-CoA through
a tricarboxylic acid (TCA) cycle. Also, there may be alternate sources of
NADH responsible for this anaerobic conversion of acetyl-CoA to PHB. The
concentration of NADH is determined by the balance between the rates of
reduction (generation) and oxidization (consumption) reactions. The
oxidizing power of the organic compounds involved in the oxidization of
NADH in an aerobic fermentation is much weaker than those of nitrate and
oxygen. For example, the reduction potential for the oxidization-reduction
pair of pyruvate/lactate is -0.19 V while those for NO.sub.3.sup.-
/N.sub.2 and 1/2 O.sub.2 /H.sub.2 O, are +0.74 V and +0.82 V,
respectively. Consequently, the rate of NADH oxidization is much slower
with anaerobic metabolism than with denitrification and respiration. The
intracellular level of NADH at the anaerobic stage is therefore higher
than those at the anoxic and oxic stages.
In order to effectively control the operation of the BNR process, it is
necessary to regulate specific process parameters based upon the
biological activity of the microorganisms in the anaerobic, anoxic and
oxic stages of the treatment. Wastewater treatment plants are often
subjected to severe transient conditions, such as during variations in
organic loads. Controlling the treatment process in response to these
conditions requires a fast and effective means of measuring biological
activity.
The present invention is directed towards an improved method and apparatus
for monitoring and controlling biological activity in wastewater treatment
systems by detecting changes in the intracellular NADH level of the
microorganisms. The apparatus includes a chamber which is opened and
closed to capture a sample of mixed liquor. The chamber contains a NADH
sensor which detects changes in the biological activity as the mixed
liquor shifts its metabolism due to changes in environmental conditions.
These real-time changes in biological activity may be monitored and can be
used as the input function for driving process and control algorithms to
ensure efficient process performance. Such algorithms are known in the art
and are not discussed further. It should be noted that the following
embodiments of the present invention are for the purpose of illustration
only and are not intended to limit the spirit or scope of the invention as
defined in the appended claims in any way.
In a preferred embodiment of the present invention, shown in FIG. 3,
bioreactor tank 1 is filled with mixed liquor 2. Pump 3, situated in tank
1, transfers mixed liquor through feed pipe 4 to return pipe 7.
Computer/monitor 13, electrically connected to solenoid valve 5, opens
solenoid valve 5 to allow a sample of mixed liquor to pass through inlet 6
to detection chamber 8. Agitator 9 mounted on both sides of both detection
chamber 8, ensures uniform suspension of microorganisms in the mixed
liquor wastewater in detection chamber 8.
Detection probe 10 is positioned in detection chamber 8 and electrically
connected to computer/monitor 13 to detect changes in fluorescence of the
NADH in the wastewater sample. A preferred detection probe 10 is disclosed
in U.S. Pat. No. 4,577,110, which is owned by the assignee hereof and is
hereby incorporated by reference. Of course, other apparatus can be
employed as a probe so long as the same detection capabilities are
available. Computer/monitor 13 may be of any suitable type such as a
personal computer or the like. Feeding device 11, also connected to
computer/monitor 13, provides nutrients to the microorganisms in the
wastewater in detection chamber 8. Outlet 12 connected to detection
chamber 8 allows the mixed liquor to be flushed from detection chamber 8
and replaced with a fresh sample.
The preferred operation of the system is as follows. Pump 3 continuously
pumps mixed liquor through feed line 4 and return line 7. At a designated
time, solenoid valve 5 is activated electronically by a computer connected
to computer/monitor 13, which opens the valve to permit a sample of mixed
liquor to pass through inlet 6 to detection chamber 8. The solenoid valve
5, which is controlled by computer/monitor 13, shuts off the flow of the
mixed liquor to detection chamber 8 after a predetermined amount of sample
has passed through feed line 4, return line 7, detection chamber 8 and
outlet 12, providing for the complete flushing of detection chamber 8 of a
prior sample of mixed liquor and refilling with a new sample of mixed
liquor. Of course, outlet 12 is equipped with a suitable blocking device
(not shown) to prevent ingress of wastewater into detection chamber 8
through outlet 12.
The mixed liquor resumes its continuous flow through return line 7 after
solenoid valve 5 closes. This flow propels agitator 9 which ensures that
microorganisms in the sample are suspended uniformly inside detection
chamber 8. Uniform suspension assists in achieving an accurate detection
of NADH by detection probe 10. Also, depending on the specific
application, certain amounts of reagents may be fed to detection chamber 8
at the moment when the chamber is filled with a fresh sample of mixed
liquor.
After filling detection chamber 8 with a fresh sample of mixed liquor, the
metabolic activity of the sample changes from an aerobic to an anoxic to
an anaerobic condition as time elapses. The time intervals that the sample
spends in the aerobic, anoxic and anaerobic states, and the changes in the
intracellular NADH corresponding to these changes in metabolic activity,
may be detected by probe 10, registered and analyzed by computer 13. The
use of computer 13 allows for the real-time, on-line monitoring of the
biological activity in detection chamber 8. Interpretation of the
information obtained by the present invention depends on its specific
application and installation location in the WWTP. The design of the
present invention may be modified to meet the specific requirements of the
wastewater treatment plant and its location.
The apparatus and method for monitoring biological activity can be used in
all stages of a WWTP or any combination thereof. Incorporation of the
apparatus and method for monitoring biological activity into a typical
WWTP is shown schematically in FIG. 8. The general application and use of
the apparatus shown in FIG. 3 in the anaerobic, anoxic and/or aerobic
stages of a typical wastewater treatment plant will now be discussed.
1. Use in the anaerobic stage
The operational profile of the biological activity monitoring apparatus
when installed in the anaerobic stage of a WWTP is illustrated in FIG. 4.
The term NFU, as shown in FIG. 4 and as used hereinafter, represents a
normalized or relative quantity or level of NADH fluorescence. Three
parameters, .DELTA.NFU.sub.1, .DELTA.NFU.sub.2, and .DELTA.t.sub.1 are
analyzed for the evaluation of the biological activity of the
microorganisms. .DELTA.NFU represents the total increase in NADH
concentration; .DELTA.NFU.sub.1 represents the first step increase of NADH
concentration; .DELTA.NFU.sub.2 represents the second step increase of
NADH concentration; and .DELTA.t.sub.1 represents the time period of the
anoxic portion during the anaerobic stage of the WWTP. The overall change
in NADH concentration through the aerobic, anoxic and anaerobic states of
the mixed liquor from the anaerobic stage of treatment can be expressed
according to the equation:
.DELTA.NFU=.DELTA.NFU.sub.1 +.DELTA.NFU.sub.2
.DELTA.NFU is proportional to the overall biomass concentration in the
sample. Although the absolute value of the biomass concentration cannot be
determined from a single measurement, it is possible to accurately and
reliably estimate the population distribution of the denitrifying and
non-denitrifying microorganisms by methods known in the art. When the
concentration of dissolved oxygen in the sample decreases to below a
critical value and is finally depleted, those microorganisms that cannot
use nitrate and/or nitrite as electron acceptors switch to an anaerobic
state, shifting the mixed liquor from an aerobic to an anoxic state. This
corresponds to the first NADH increase, .DELTA.NFU.sub.1. The majority of
microorganisms which cannot perform denitrification are autotrophic
nitrifiers, such as Nitrosomonas and Nitrobacter. Therefore, the value of
.DELTA.NFU.sub.1 /.DELTA.NFU is proportional to the percentage of
nitrifiers in the overall biomass population. Conversely, those
microorganisms that are capable of performing denitrification consume all
the nitrate in the sample before entering an anaerobic state.
The second step increase in NADH, .DELTA.NFU.sub.2, from the sample
corresponds to a shift in the sample from an anoxic to an anaerobic state.
Therefore, the value of .DELTA.NFU.sub.2 /.DELTA.NFU is proportional to
the percentage of denitrifiers in the overall biomass population.
One possible application of the biological activity monitoring apparatus in
the anaerobic stage of a WWTP is to determine the efficiency of NH.sub.3
removal. When the value of .DELTA.NFU.sub.1 /.DELTA.NFU is below a
predetermined value, the population of nitrifiers in the bioreactor tank
is lower than the required amount for proper NH.sub.3 removal. Changing
operational parameters, such as increasing hydraulic retention time or
increasing the RAS flow rate, for example, is helpful in modifying the
process to make the WWTP more efficient. If the alteration of the return
activated sludge (RAS) flow rate parameter is adopted, it should be
continued until the value of .DELTA.NFU.sub.1 reaches a set point so that
the population of nitrifiers is large enough to maintain the proper
nitrification rate.
.DELTA.t.sub.1 is the time the mixed liquor spends in the denitrification
stage before the sample shifts to the anaerobic state. When At represents
the hydraulic retention time of the mixed liquor in the anaerobic stage of
bioreactor tank 1, then the ratio of .DELTA.t.sub.1 /.DELTA.t indicates
that a fraction of the bioreactor is used for denitrification within the
whole anaerobic stage in the WWTP.
2. Use in the Anoxic Stage
The operational profile of the biological activity monitoring apparatus
when used in the anoxic stage of a WWTP is illustrated in FIG. 5. Two
parameters, .DELTA.NFU.sub.3, which represents the change in NADH
fluorescence during the anaerobic state of the sample, and .DELTA.t.sub.2,
which represents the length of time in minutes of the anoxic state of the
sample, are useful in monitoring and controlling the anoxic stage of a
WWTP.
The value of .DELTA.t.sub.2 is measured as the time period from capture of
the sample in detector chamber 8 to the moment when denitrification is
completed. The value of .DELTA.t.sub.2 can be used to evaluate whether the
hydraulic retention time in the whole anoxic stage, T.sub.den, is long
enough for the denitrification process to be completed. The ideal time is
T.sub.den =.DELTA.t.sub.2. To approach this ideal denitrification time,
the internal recycling rate can be adjusted accordingly.
3. Use in the Oxic Stage
An operation profile for the use of the apparatus in the oxic stage of a
WWTP is illustrated in FIG. 6. Since the degradation of pollutants is
almost completed, the BOD concentration is very low, and the change in
fluorescence of NADH concentration corresponding to the metabolic shift of
the captured sample from an aerobic to an anoxic state is very small, but
nevertheless detectable.
One of the applications of the present invention in the oxic stage is to
serve as a NH.sub.3 meter. This aspect preferably operates as follows: Two
sets of monitoring apparatus (not shown) may be used in the same location
in bioreactor tank 10. Both detection chambers 8 are filled with mixed
liquor samples at the same time. For the first chamber, .DELTA.t.sub.3
represents the time from capturing the sample to the start of the anoxic
state of the sample registered by computer 13. In the second chamber,
immediately after the chamber is filled with mixed liquor, a certain
amount of NH.sub.3 is added, for example 0.5 ppm, from the feeding device
11. The time, .DELTA.t.sub.4, from capturing the sample in chamber 8 to
the start of the anoxic state of the wastewater in the detection chamber 8
is then registered.
In order to determine the NH.sub.3 concentration, it is assumed that the
dissolved oxygen (D.O.) consumption in the oxic stage is mostly due to the
nitrification process. A typical operational profile for the consumption
of dissolved oxygen during the oxic stage is illustrated in FIG. 7.
Experimental results performed indicate that the oxygen consumption rate
of the mixed liquor changed negligibly when acetate and glucose (5 ppm)
were added to the system with feeding device 11, while significant change
was observed when 0.1 ppm of NH.sub.3 was added to the system.
The concentration of NH.sub.3 in the oxic state of the WWTP is expressed
as:
(NH.sub.3).sub.1 =.DELTA.NH.sub.3 .DELTA.t.sub.4 /(.DELTA.t.sub.3
-.DELTA.t.sub.4)
Where (NH.sub.3).sub.1 is the ammonia concentration in the water phase at
the end of the oxic stage, .DELTA.NH.sub.3 is the known amount of ammonia
added to the second detection chamber, respectively. The present invention
can be used in the oxic state of a WWTP to accurately monitor the NH.sub.3
concentration in the bioreactor tank. Various system parameters, such as
retention time, can then be altered to enhance the nitrification process
and, if necessary, to increase the efficiency of the waste water treatment
system.
In the method according to the invention, information about biomass
composition, efficiency of denitrification, nitrification and BOD removal
processes and NH.sub.3 concentration in the oxic stage of a WWTP can be
obtained. This information may be monitored and analyzed by computer 13
which evaluates the biological activity in the anaerobic, anoxic and
aerobic stages of a WWTP and can alter system parameters such as the RAS
flow rate, the oxygen supply rate, the internal recycling rate or the
hydraulic residence time or the like to maximize the efficiency of the
WWTP in response to transient conditions or normal operation.
Although the invention has been illustrated by use of specific embodiments
thereof, it should be noted that the present invention is not limited in
spirit or scope as defined in the appended claims. For example, the
present invention can be used to monitor various parameters of the
individual aerobic, anoxic and anaerobic stages of a wastewater treatment
plant individually, or the invention can be used to monitor and control
the entire WWTP operation in maximizing the efficiency thereof.
Additionally, individual components of the invention may utilize
equivalent substitutions. For example, the sample in detection chamber 8
may be uniformly suspended by use of any means of controllable agitation.
The filling of the detection chamber with a predetermined amount of
wastewater may be performed by a means other than a feed line, a solenoid
valve, an inlet line and a return line. The monitoring system may consist
of a personal computer with applicable software or individual electronic
meters to be analyzed separately, all of which are known in the art. Other
possible embodiments and modifications of the present invention in keeping
with the spirit and skills thereof, will be obvious to one of ordinary
skill in the art.
It should also be emphasized that although emphasis has been placed on
measurement of NADH fluorescence to determine the quantity or
concentration of NADH, this emphasis is simply the preferred manner in
which NADH quantity or concentration is determined. Other means and
methods for accomplishing this task are fully contemplated as falling
within the scope of this invention. For example, NADH quantity or
concentration may be determined by use of biochemical assays, such as
those sensitive to NADH. Such assays are known in the art and typically
employ enzymes and substrate components to assist in the assay. Still
other means known and not yet developed can be used so long as they are
capable of determining the presence of NADH.
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