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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a method for the detection of toxic
substances in the waste water supplied to a biological treatment plant.
In such plants where waste waters are biologically treated, the effluents
are brought into contact with micro-organisms which metabolize the
dissolved organic materials in the presence of oxygen.
The activity of these micro-organisms may be partly or completely
inhibited, when the waste waters contain a concentration of toxic
materials such that the purifying biomass becomes poisoned, thus reducing
or stopping the purifying process.
When these toxic materials are not detected soon enough, the entire biomass
of the plant may become poisoned and it then takes several weeks before
the purifying plant can resume its activity. The advantage of detecting
the presence of toxic substances in waste waters before they can affect
the purifying plant, is therefore imperative.
Many methods of detecting toxic substances are already known and used, such
as for example methods based on respirometric measurements: wherein toxic
substances modify the metabolism of the micro-organisms with which they
come into contact, and the variation of metabolic activity can be detected
with great sensitivity by studying one essential function which is,
respiration.
For example, according to the method described in French Patent Application
No. 2,428,842, the liquid for testing is oxygen-saturated and introduced
into a reactor where it is mixed with a fraction of bacterial culture and
with a nutrient. The culture is so prepared as to have a bacterial aqueous
suspension with stable characteristics. The liquid is introduced at the
inlet to a reactor formed by a thin long tube and the oxygen content is
measured at the reactor outlet, situated at the other end of the tube; the
peaks detected in the recordings of the oxygen content measured value
indicate the presence of toxic substances. A similar method is described
in French Pat. No. 2 266 885 consisting in oxygen-saturating part of the
effluent to be tested and causing it to flow through a bacterial filter,
at the outlet of which the dissolved oxygen is measured. Where the
consumption of oxygen is too low a toxicity alarm installation is
activated. Seeding of the reactor is periodically renewed and dilution of
the effluent with a nutrient solution becomes necessary when effluent load
variations are too great. With such methods, the micro-organisms used in
the reactor can react differently from the micro-organisms present in the
purifying plant. Moreover, the dilution of the effluent with a nutrient
solution entails a dilution of the toxic substances which delays its
detection.
In the method according to U.S. Pat. No. 4,260,490, the respiration
measured on an activated sludge sample taken from the treatment plant is
compared with the respiration measured on an activated sludge sample from
the plant having the effluent to be analyzed. If the oxygen consumption
speed difference between the samples is below a certain value, determined
with a non-toxic effluent used as reference, the monitored effluent is
deemed toxic. If on the contrary, said difference is higher than the value
determined with an effluent of average oxygen biological intake, the
oxygen intake is greater. A computer works out the results of the
respirometric tests and if necessary releases a signal in the case of
toxicity. This particular method is therefore a discontinuous method
which, in addition to computer means, requires two oxygen sensing means as
well as two analyzers.
According to yet another method of the type described in French Pat. Nos.
1,567,181, 1,567,182 and 2,067,184, a reactor is used, which reactor
receives a certain quantity of activated sludge from the plant oxygenated
by injection of compressed air. When these sludge samples reach a constant
respiration, a sample of effluent to be analyzed is injected. The oxygen
intake from that particular moment is continuously measured and a computer
gives the metabolizing curve. Then, if during subsequent measurements the
respiration falls below a preset value, the presence of toxic substances
is highly probable. This is therefore a discontinuous monitoring method.
Finally, European patent application EP No. 0,009,580 discloses a system
permitting control of a biological purifying installation using a
measuring device which can receive a mixture or suspension of waste water
and recycled sludge collected before its introduction into the basin of
the installation. The measuring device comprises a reactor, into which the
suspension is introduced and oxygen-saturated, and a measuring cell set
apart from the reactor and designed to measure oxygen consumption. The
distance between the reactor and the measuring cell is so defined as to
correspond to a pre-set reaction period. If the oxygen consumption
measurement goes beyond or below first pre-set maximum and minimum
thresholds, the flow of recycled sludges to the purifying plant is
adjusted. If a second minimum threshold, lower than the first is crossed,
this is a sign of poisoning.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method and device
specifically designed to continuously monitor the toxicity of waste
waters, in conditions as near as possible to the purifying plant operating
conditions, with immediate signalling upon the appearance of toxins.
More specifically, the invention relates to a method wherein the waste
water to be tested is brought into contact with activated sludge samples
taken from the biological treatment plant, inside a reactor which is
constantly supplied with oxygen. The quantity of oxygen dissolved in the
reactor is measured with a view to releasing a toxicity alarm when an
abnormal reduction of respiration occurs. In accordance with the
invention, the reactor is continuously supplied with activated sludge
samples from the plant. The quantity of oxygen dissolved in the reactor is
compared to permanently preset maximum and minimum values. The flow of
waste water supplied to the reactor is automatically regulated by being
reduced or increased whenever the measured content value is below the
preset minimum value or above the preset maximum value, in order to keep
substantially constant the load applied to the reactor. The toxicity alarm
is released whenever the measured content exceeds the maximum value when
the waste water supplied to the reactor is at a maximum.
Continual supply of the reactor with sludge samples from the treatment
plant and with waste water intended for said plant results in
approximating the normal running conditions of the plant. Also, by
automatically controlling the flow of waste water the applied polluting
load is kept substantially constant and any abnormal reduction of
respiration may be attributed with certainty to the presence of toxic
substances in the waste water. The alarm is released if it becomes
impossible to stabilize the quantity of dissolved oxygen to a given level
just by altering the reactor load. Thus, the fact of increasing the waste
water flow rate whenever the quantity of dissolved oxygen decreases,
enables to speed up the detection of toxicity.
A further object of the present invention is to provide an apparatus for
carrying out said method.
This object is achieved by an apparatus comprising a reactor designed to
contain activated sludge samples taken from the biological treatment
plant, means of supplying the reactor with oxygen, means of supplying the
reactor with waste water to be analyzed, means of measuring the quantity
of oxygen dissolved in the reactor and an alarm device connected to the
measuring means to release a toxicity alarm signal in case of abnormal
reduction of the respiration. Accordingly, the apparatus of the present
invention provides a means of permanently supplying the reactor with
activated sludge samples taken from the plant; a regulating means
comprising a comparator circuit designed to compare the measured value of
the dissolved oxygen content to maximum and minimum values, a control
circuit for controlling the pump supplying waste water to the reactor; and
a selector circuit connected to the comparator circuit and designed to
apply a variable signal to the control circuit, depending on whether the
measured oxygen content is less than said minimum value, or more than said
maximum value or between the two.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be more readily understood on reading the
following description with reference to the accompanying drawings.
FIG. 1 is a schematic diagram of an embodiment of the apparatus according
to the present invention.
FIG. 2 is a diagram of the circuits utilized for regulating the waste water
flow and of the alarm device shown in FIG. 1.
FIGS. 3 to 6 are curves illustrating the variations in the dissolved oxygen
content of the reactor in the apparatus in response to load variations or
to the appearance of toxic substances in the waste water to be monitored.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The apparatus schematically illustrated in FIG. 1 comprises a reactor 1
supplied with waste water to be analyzed for toxic substances, and
activated sludge samples taken from a biological treatment plant (not
shown) used for purifying the monitored waste water.
The biological sludge samples can be taken from the activated sludge basins
or, preferably, from the circuit in which the sludge is recycled after
being concentrated in the plant decanter. Samples of the waste water to be
monitored are taken upstream of the plant or of the homogenization basin,
if any. The apparatus for detecting toxic substances is preferably
installed proximate to where the samples of sludge and waste water are
taken so that the time elapsing between the collection of the samples and
their arrival to the apparatus is no more than a few minutes. To shorten
this time as much as possible, and to avoid any deposit in the pipes
supplying the waste water and sludge, fast supply loops, 2 and 3
respectively, are created on the circuit. Hydrostatic pressure or the
delivery pressure from pumps already installed in the plant, may be used:
for example a sludge recycling pump. Branch-connections are made on these
loops at the level of the apparatus, to supply the reactor 1 with waste
water via peristaltic pump 4, and with sludge, via peristaltic pump 5. Any
excess of waste water or sludge or any overflow from reactor 1 are
recycled at the head of the plant via respective pipes 6, 7, 8. The
reactor is supplied continuously with activated sludge and waste water to
be monitored.
Oxygen is also constantly fed in the reactor 1 by bubbling of compressed
air via a porous or drawn out tube 9 traversing the bottom of the reactor.
The pressure and flow rate of the compressed air brought into tube 9 are
regulated and controlled by regulating means 10. Dissolving of the oxygen
is helped by agitator 11 rotating at as constant a speed as possible.
An oxygen sensor 12, (polarographic type) supplies a signal which
represents the amount of oxygen dissolved in the reactor. Sensor 12 is
located near the output of the reactor 1 at the upper part thereof. The
output signal of the sensor 12 is applied to a regulation and alarm device
13 described in more detail with reference to FIG. 2.
The variation of dissolved oxygen concentration in the reactor determines
whether the applied polluting charge is normal, high, low or shows any
toxicity, depending on whether said dissolved oxygen concentration remains
stable, decreases or increases.
According to a characteristic of the present invention, the waste water
supply flow to the reactor 1 is controlled in relation to the measured
value of the dissolved oxygen concentration. If the concentration
decreases and becomes less than a preset minimum threshold value, which
indicates an increase of the applied charge, the flow of waste water is
reduced. If on the contrary, the concentration increases and exceeds a
preset maximum threshold value, which would indicate a reduction in the
applied charge or in the toxicity, the flow of waste water is increased.
In the case of a reduction of the charge, the dissolved oxygen
concentration should return to a normal level. If toxic substances appear,
the oxygen concentration will increase abnormally, which in turn releases
an alarm, increasing the flow of waste water speeds up the detection
process, which is one of the advantages brought by the invention.
To regulate the load applied to the reactor, the flow of waste water may be
varied continuously or in stages. In this last case, several waste water
flow values may be predetermined, the passage from one value to another
being controlled in response to the minimum or maximum threshold values of
dissolved oxygen concentration being crossed.
A comparison between the measured value of the concentration and the
predetermined thresholds is made in the regulation and alarm device 13
which delivers signals designed in particular to control the pump 4 to fix
the flow of waste water being brought to the reactor and when the need
arises, to release a toxicity alarm.
This signal issued by the sensor 12 is amplified by an amplifier circuit 14
having an adjustable gain. After amplification, the signal is applied to
the input 15a of a discriminator 15. Said circuit comprises three outputs
15s, 15s', 15s" which are activated when the amplitude of the signal at
input 15e is respectively within a predetermined range [C-.DELTA.C,
C+.DELTA.C], less than C-.DELTA.C or more than C+.DELTA.C. The value C is
the middle of the range, and is adjustable by way of a potentiometer 16
forming a voltage divider between ground and a terminal of potential +V of
the supply voltage and applying an adjustable voltage on a first adjusting
input 15r of the discriminator 15. Likewise, the width 2.DELTA.C of the
range is adjustable by way of another potentiometer 17 forming voltage
divider and applying an adjustable voltage on a second adjusting input
15r' of the discriminator. Said latter is a circuit known per se, found on
the market as an integrated circuit, such as for example the circuit
manufactured under reference TCA 965 by The West German Company SIEMENS.
The signal applied to the input of discriminator 15, is transmitted, after
a further amplification by means of an amplifier circuit 18, to a recorder
19.
Outputs 15s, 15s', 15s" of the discriminator are respectively connected to
the magnet coils of three Reed type relays 21, 22, 23, the movable
contacts of which act as switches between ground and first respective NOR
gates inputs 24, 25, 26. Diodes 27, 28, 29 in parallel on the coils of
relays 21, 22, 23 protect outputs 15s, 15s', 15s".
The second inputs of gates 24, 25, 26 receive the signals appearing
respectively on outputs Q of RS-type flip-flops 31, 32, 33. Said signals
are in the form of pulses having the same frequency but of different
durations. Thus, duration D2 of the pulses outputted by flip-flop 32 is
more than duration D1 of the pulses outputted by flip-flop 31 but it is
less than duration D3 of the pulses outputted by flip-flop 33, all these
durations having adjustable preset values, as described hereinafter.
The outputs of gates 24, 25, 26 are connected via diodes 34, 35, 36 to the
input of an amplifier circuit 37 of which the output energizes the
magnetic coil of a relay 38. Said relay, when closed, enables to supply
the motor 40 of pump 4. A hand-operated switch IP is connected in series
with a diode 39 between a terminal of potential +V and the input of
amplifier 37, in order to allow the manual control of pump 4.
Depending on whether relay 21, 22 or 23 is closed, the pump 4 is turned on
at regular intervals for a respective period D1, D2 or D3 such as
D1<D2<D3. Waste water supply flows, controlled respectively to "low",
"normal" and "high", are thus directed into the reactor 1.
The movable contacts or relays 21, 22, 23 also act as switches, connected
between ground and the first inputs of respective NOR gates 41 42, 43.
Said first inputs are connected, on the one hand, to the first inputs of
respective gates 44, 45, 46, also of the NOR type, and on the other hand,
to a terminal of potential +V via respective resistors 47, 48, 49. The
second inputs of gates 41, 42, 43 receive the signal applied to the input
of amplifier 37 and the second inputs of gates 44, 45, 46 receive the same
signal reversed by a gate 50.
The outputs of gates 41 to 46 are connected via resistors to the bases of
respective transistors 51 to 56. The collector of each of said transistors
is grounded via a resistor. An electroluminescent double diode 61 is
connected between the transmitters of transistors 51 and 54 so that a red
luminescence is transmitted when transistor 54 is conducting and
transistor 51 is blocked. Conversely, a green luminescence will appear
when transistor 51 is conducting and transistor 54 is blocked. The diode
will not transmit when the two transistors 51 and 54 are in the same
state.
Likewise, an electroluminescent double diode 62 is connected between the
transmitters of transistors 53 and 56.
If the relay 21, 22, or 23 is closed, the gates 41, 44 or 42, 45 or 43, 46
are validated.
The control signal applied at the input to the amplifier 37 controls the
red-lighting up of diodes 61, 62 or 63, whereas the absence of said signal
controls the green-lighting up said diodes. Thus, depending on whether the
lit diode is diode 61, 62 or 63, the real waste water supply flow to
reactor 1 is "low", "normal" or "high". The red-lighting up of said diode
indicates that the motor of the pump 4 is powered whereas its
green-lighting up indicates that the motor of the pump 4 is not powered.
The signals applied to inputs R and S of flip-flops 31, 32, 33 are worked
out as follows.
A signal having the frequency of the sector (50 Hz) is taken for example
from the supply transformer and sent through a diode 71 to a shaping
circuit constituted by two NOR gates 72-73 with hysteresis and connected
in series. The square signal of 50 Hz outputted from gate 73 is applied to
a frequency divider 74 producing a signal of period equal for example to
1/100th of a minute. This signal is applied to the counting input of a
decimal counter-decoder 75 of which the carry-over output CO is connected
to the counting input of a second decimal counter-decoder 76. Three coding
wheels 81, 82, 83 each enable selection of any one of outputs 0 to 9 of
the counter 75. Likewise, three coding wheels 81', 82', 83' each enable to
select any one of outputs 0 to 9 of counter 76.
The outputs selected by coding wheels 81 and 81' are interconnected by an
AND-type gate 84 formed with diodes, the output of which is connected to
the input S of flip-flop 31. Likewise, the outputs selected by coding
wheels 82 and 82' are interconnected via an AND gate 85, the output of
which is connected to the input S of flip-flop 32, and the outputs
selected by coding wheels 83 and 83' are interconnected by an AND gate 86
the output of which is connected to the input S of flip-flop 33. Another
AND gate 87 has its inputs connected to outputs 0 of counters 75, 76 and
its output connected to inputs R of flip-flops 31, 32 and 33.
Thus the pulses produced by flip-flops 31, 32, 33 have, in the illustrated
example, a period of 1 min. and durations D1, D2, D3 expressed in 1/100th
of a minute and selected by the pairs of coding wheels 81-81', 82-82' and
83-84'. A one-minute cycle is thus produced for controlling the motor of
the waste water pump, which cycle has a WORK-STOP cyclic ratio varying
between 0 and 99% of the time.
The one-minute signal outputted by the carry-over output of counter 76 is
applied to the counting input of a decimal counter-decoder 91, the
carry-over output of which is connected to another counter-decoder formed,
for example, simply by a D-type flip-flop 92. The resetting inputs R of
counter 91 and of flip-flop 92 are grounded via the switch formed by the
movable contact of relay 23. A coding wheel 93 enables selection to any
one of outputs 0 to 9 of counter 91 and a second coding wheel 94 enables
selection of the value 1 or 0 by connection to the output Q of flip-flop
92 or with a terminal 99 of potential +V. The terminals thus selected are
connected to the inputs of an AND-type gate 95 formed with diodes, the
output of which is connected to the base of a transistor 96. The
transmitter of said transistor is grounded and its collector is connected
to a terminal of potential +V via the magnetic coil of a Reed-type relay
97. A protection diode 98 is connected in parallel on said coil.
The movable contact of relay 97 acts as a switch connected between a
terminal of potential +V and, respectively, the counting-inhibiting input
CI of counter 75, an electroluminescent diode 100 and the input of an
amplifier circuit 101. Said latter has its output connected to the
energization coil of a relay 102 whose closure controls the operation of a
toxicity alarm circuit 103.
When relay 21 closes, inputs R of counter 91 and flip-flop 92 pass to zero
and the output signals of counter 76 are counted by counter 91. If relay
21 remains closed for a period at least equal to the period displayed in
minutes by the coding wheels 94 and 93, the toxicity alarm is released and
the diode 100 lights up. The inhibition of counter 92 keeps the alarm on
as long as said counter is not reset. If, on the contrary, the measured
concentration returns to normal before the end of timing, relay 21 opens.
Counter 91 is reset as well as flip-flop 92 and the output signals of
counter 76 are no longer counted.
Obviously, the way in which the circuits described with reference to FIG. 2
work, is a direct result of the foregoing.
The set value C of the concentration of oxygen dissolved in the reactor and
the acceptable difference .DELTA.C above or below that value are
controlled by the operator by means of potentiometers 16 and 17. With the
coding wheels 81-81', 82-82' and 83-83', the operator selects also the
values of the working periods of pump 4 for each WORK-STOP cycle thereof,
i.e. the values of the "low" flow rate, "normal" flow rate, and "high"
flow rate of waste water supply to the reactor. These controls being set,
the reactor is filled by actuating the hand-operated switch IP of pump 4.
Then, the output rate of pump 4 is controlled automatically by the output
signal of discriminator 15, thus permitting regulation of the charge
applied to the reactor. Diodes 61 to 66 inform the operator whether the
waste rate is "low", "normal" or "high", meaning that the load is high,
normal or low. It is thus possible, on the basis of that information, to
regulate for example the oxygen flow into the treatment plant.
The operator also predetermines with coding wheels 93 and 94, the maximum
time period after which the dissolved oxygen concentration, having
exceeded the maximum value C+.DELTA.C and caused the switching over of the
pump output to its "high" value, should return within the tolerable
limits, failing what the fact of maintaining the measured concentration to
a level above C+.DELTA.C is imputed to the appearance of toxic substances.
In the illustrated example, said period may be set to between 0 and 19
minutes. With the method according to the invention, toxic substances are
detected in a very short time because of the acceleration effect due to
switching over to the "high" flow rate. Owing to this rapid detection, the
necessary measures can be taken at the opportune moment to prevent any
degradation of the biomass in the basins of the treatment plant, for
example, the feeding of the basins with the waste water is interrupted.
FIGS. 3 to 6 illustrate the measured variations of dissolved oxygen
concentration measured as a function of time, in the different tests
conducted with the afore-described apparatus.
The set value for said concentration was selected to be equal to 3 ppm,
plus or minus 0.5 ppm, as an acceptable possible difference. The reactor,
having a capacity of 2.4 liters, was permanently supplied with samples of
sludge taken at the input to the plant with a flowing rate of 5.7 l/hour
and was ventilated by a constant airflow equal to about 1 l/hour. Selected
"low", "normal" or "high" flow rate values were 110 ml/hr, 220 ml/hr, and
440 ml/hr. Timing of the toxicity alarm was fixed at 10 minutes.
FIG. 3 shows that an increase of the polluting load at time t.sub.1
resulted in a reduction of the measured concentration. The reduction was
temporary, and of relatively short duration due to compensation through
regulation of the waste water flow as soon as the concentration dropped
below the threshold of 2.5 ppm.
FIG. 4 shows that a very strong increase of load at time t.sub.2 is
compensated for in a longer period of time.
FIGS. 5 and 6 show the curves of concentration variation in the case of
deliberate introduction of toxic substances, respectively sodium
hypochlorite in concentration of 2.3 g of active chlorine per liter of
waste water and tertiobutylcatechol at the rate of 1 g introduced in the
reactor. In the first case, the alarm was released 40 mins. after the
introduction. In the second case, there was first a rapid consumption of
oxygen due, very likely, to the oxidation of tertiobutylcatechol in very
toxic quinones; after what the dissolved oxygen concentration increased
very rapidly, causing the release of the alarm 20 minutes after the
introduction of tertiobutylcatechol into the reactor.
According to the embodiment of the apparatus according to the invention
described hereinabove, only three levels of waste water flows are set.
Obviously, the number of these levels can be increased, especially if
great load variations have to be compensated. For example, a "very low"
and a "very high" flow rate can be selected. It could also be possible to
make finer adjustments of the load by varying the waste water flow,
between a minimum and a maximum, with very small increases, continuously
if necessary, as a function of the dissolved oxygen measured
concentration. In all cases, the toxicity alarm is released when it is
impossible to keep the oxygen content equal to or below a preset maximum
value and when the waste water flow is at a maximum.
The invention is in no way limited to the description given hereinabove and
on the contrary covers any modifications that can be brought thereto
without departing from its scope or its spirit.
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