Note: Descriptions are shown in the official language in which they were submitted.
2~0~9~3
APPARATUS FOR MONITORING SETTLEMENT OF A SOLID
IN A LIQUID, AND A SYSTEM INCORPORATING SAME
Th- invention relates to apparatus for monitoring
settlement of a solid in a liquid, and a system
ncorporating same.
It is often necessary to monitor settlement rates of
solids in liquids, for example in treatment of
effluent such as sewage in an activated sludge process.
An apparatus is known for obtaining a parameter known
as the Stirred Specific Volume Index (SSVI) of the
sludge. The SSVI is a function of the settlement
volume and the suspended solids concentration of the
sludge and can produce useful control data for the
process.
The parameter obtained from settlement of activated
sludge over a 30 minute period using the apparatus is
the Stirred Specific Volume (SSVI). By interpolation,
the SSVI at an intermediate solids concentration of
3.5 g/1 may be obtained. A relationship between the
SSVI at 3.5 g/l and FST performance have been
established such that maximum FST solids loading and
maximum FST underflow rates may be estimated from data
which includes the SSVI.
Although the SSVI is determined routinely at many
sewage works, the procedure is labour-intensive and
in any case is impractical outside normal working
hours or as a routine at an unmanned works.
It is accordingly an object of the invention to seek
to mitigate these disadvantages.
- 2 - ~ 3 ~ ~
According to the invention there is provided apparatus for monitoring the rate
of settlement of solids in a liquid, including a settlement vessel comprising
means for containing the liquid. Automatic electronic means is also provided forsensing the height of settled solids in the liquid in the vessel and comprises a5 plurality of infra-red LEDs at one side of the vessel, a plurality of infra-red
sensors horizontally opposed to said LEDs at an opposite side of the vessel, andelectronic circuit means for reading the sensors. The LEDs and the sensors are
arranged so as to define an upper set and a lower set of opposed LED-sensor pairs
with a vertical spacing between successive LED-sensor pairs of the upper set being
10 greater than that of the lower set. Rotatable eccentric stirrer means is disposed in
the vessel for rotating between the LEDs and the sensors for effecting uniform
settlement of the solids in the liquid in the vessel. Electronic control circuitmeans is provided for ceasing rotation of the stirrer means when the stirrer
means is in an angle to an optical path between the LEDs and the sensors so as
15 not to block the optical path. The apparatus also includes means shielding the
vessel from ambient infra-red light.
It is to be unde stood that the term "settlement" used
herein includes wlthin its scope coagulation, floccula-
tion and other processes whereby solids settle or
20 become concentrated in liquids.
Using the invention it is possible to provide data
about the settlement characteristics of activated
sludge. Such data is useful for control purposes,
since it may be used to predict safe flow and solids
25 loadings to final sedimentation tanks (FST's).
There may be thirty pairs of LEDs and sensors each
spaced vertically over the length of the vessel.
30 The spacing in the one set of LED sensor pairs may be 2cm, and in the other set
may be lcm.
., .
- 3 -
The electronic control circuit means for the stirrer
may comprise Hall Effect means.
The Hall Effect means may comprise a fixed Hall Effect
switch and a magnet mounted on the stirrer.
The switch may be mounted on a housing for a motor for
rotating the stirrer.
The invention may extend to a system for monitoring
settlement of activated sludge, incorporating apparatus
as hereinbefore defined.
Apparatus and a system incorporating same, embodying
the invention are hereinafter described, by way of
exampie, with reference to the accompanying drawings.
Figure l is a schematic side elevational view of
apparatus for monitoring settlement of a solid
in a liquid, specifically sewage;
Figure 2 is a schematic side elevational view of part
of automatic sensing means of the apparatus of
Figure l;
Figure 3 is a schematic representation of a system
incorporating apparatus as shown in Figures l
and 2;
Figures 4-6 are circuit diagrams respectively for
operation of the monitoring means trigger circuit
and stirrer switch;
Figure 7 shows an enlarged diagrammatic perspective
~iew of the apparatus of Figure l, showing a Hall
200~913
Effect switch detail; and
Figure 8 shows graphically results obtained using the
apparatus of Figures 1-7.
Referring to the drawings, there is shown apparatus 1
for monitoring settlement of a solid in a liquid,
comprising a s e t t 1 e m e n t v e s s e 1 2 for
containing the liquid, specifically sewage containing
solids in the embodiment, and automatic means 4, 4'
for sensing solids' height in the liquid. The apparatus
in this embodiment also includes a stirrer 3 adapted to
provide uniform settlement in the vessel 2. The vessel
2 is substantially upright.
The vessel 2 is a transparent lOOcm diameter cylinder
of nominal 50cm settlement depth, and the stirrer 3 is
an eccentric stirrer comprising two substantially
parallel bars 5 each suspended from a respective arm 6
and 7 which arms in turn project from a drive spindle
8 of an electric motor which is in a housing 9 on top
(as viewed) of the apparatus 1. The arm 7 is longer
than the arm 6, to achieve the eccentric stirring
effect to and uniform settlement of solids in the
sewage. The vessel 10 has 20mm i.d. inlet and outlet
ports, a 20mm overflow port and logic control of the
eccentric stirrer.
The automatic sensing means 4,4' measures the depth of
solids' settlement in the vessel 2 and comprises a
plurality of infra-red LEDs (light emitting diodes)
10 arranged in a vertical array on one side of the
vessel 2 and a plurality of infra-red sensors 11
arranged in a vertical array on the opposite side of
the vessel 2, the LEDs 10 and sensors 11 being
horizontally opposed in pairs. The LEDs 10 and
~Q~319~3
se~sors ll are eacn arranged in two sets A and B
vertlcally, adjacen~ LEDs/sensors of one, upper set A
be1ng spaced apart vertically by 2cm while the vertical
spacing of adjacent BEDs/sensors in the other lower,
set B is lcm. The closer spacing at B, near the bottom
of the housing, is to provide a better response at
lower settling rates in this region, There are ten
L~Ds/sensors in the one, upper set A and twenty in
the other, lower, set B, making thirty in all. The
l~ sensors 10,11 determine the position of the sludge
effluent interface.
Infra-red light is supplied to the sensors ll by the
complementary array of LEDs lO. These are supplied
with power by a 9v dc supply; the current being
limited by a 220 R resistor between each anode and
the supply. The cathodes are commoned, as are the
cathodes of the photodiodes.
Both LEDs and photodiodes are fixed into sheet
aluminium housings with epoxy resin the Al shielding
the photodiodes from stray and ambient infra-red
radiation.
The sensing means 4,4' is controlled electronically
and automatically by suitable circuitry, for example
as shown in Figures 4 and 5.
The eccentric stirrer 5 revolves at l rpm and in
order to ensure an optical path between the sensors
ll and lO and LEDs uninterrupted by anything other
than sludge solids during measurement, electronic
means (Figure 6) is provided to switch the stirrer
off when it is 90~ away from the light beam, thus
ensuring reliable readings. The electronic means
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-- 6
includès a Hall Effect device in the form of a Hall
Effect switch 12 mounted flush with the bottom (as
viewed, Figure 1) face of a housing 9 for the motor
of the stirrer 5. A suitable magnet 13 is mounted on
top of the longer arm 7 of the stirrer 5 so that it
passes under the switch 12 with a vertical separation
of about 2mm every revolution of the stirrer. The
action of the magnet 13 is to pull the switch 12
output low whilst in close proximity. Referring to
6, the output from ICl is fed to one of the pair of
inputs to one of the gates of IC2, a CMOS 4071B Quad
2 input OR gate.
The other input to the gate is supplied by a digital
output line from the computer interface board.
Resistors Rl and P~2 serve to pull the inputs high to
provide a default condition.
In order to switch off the stirrer, input 2 of IC2 is
taken low by the computer. The output of the gate
will remain high, however, as input 1 is held high
by the output from ICl. When the magnet reaches ICl
the output from ICl goes low and thus the output from
IC2 also goes low. This switches off the gate of TRl
which then prevents any current from reaching the
stirrer motor.
Once the sensors have been read, input 2 of IC2 is
taken high under software control. This in turn
takes the output of the OR gate high which excites
the gate of TRl, switching it on and thereby ICl,
input 2 of the OR gate may be taken low in anticipation
of the next revolution of the stirrer when the process
repeats itself.
~13
Additionally, the output from the OP~ gate is monitored
by the computer in order to check that the Hall Effect
switch has in fact turned off and that readings may
commence.
The circuit itself is mounted on a board inside the
motor housing.
A system 14 for monitoring settlements of solids in
sewage sludge to obtain a parameter for settlement of
activated sludge over a 30 minute period, the Stirred
Specific Volume (SSV), is shown in Figure 3. The
system includes the apparatus 1, and a pump 15, a
'Mono' type MS, which is kept running continuously
with a soienoid valve SV4 in the 'waste' position.
Flowstream switching is achieved vla pneumatically
actuated valves Vl-V5, which are themselves controlled
by pilot solenoid valves. These are in turn controlled
by solid state relays connected to TTL logic outputs
on the computer. The positive displacement pump 15,
which is also controlled by the computer, provides a
flowrate sufficient to fill the settlement cylinder in
10 to 15 seconds.
Suspended solids are measured using a pre-calibrated
suspended solids monitor built 18 into a flow cell 17
above the pump output port. The output from the
monitor 18 is fed to a 12 bit analog to digital
converter (ADC) to provide a resolution of 1 part in
4096.
A solid state level sensor in the wall of the settlement
vessel or cylinder 10 is used to control switching of
the valves when the cylinder is filled to the 50cm
mark. Additionally, a safety overflow is provided
20(~1913
to prevent any overfill.
Actual settlement height of sludge is determined by
scanning the array of infra-red photodiodes arranged
vertically down the settlement cylinder. These are
supplied with infra-red light by the complementary
array of LEDs on the other side of the cylinder. The
computer then utilizes a 'lookup' table to calculate
the height of the sludge/effluent interface.
In addition to suspended solids and settlement data,
details of settled sewage flowrate, recycle flowrate
and total FST surface area are required in order to
perform tank loading calculations. Linking of the
system to flow metering e q u i p m e n t i s
feasible on works where such signals are readlly
accessible, thus presenting the possibility of on
line measurement and control of FST loading.
Final data output is in the form of a settlement
curve and a printed report to a small printer/
plotter (Figure 8).
A total of five pneumatically actuated valves Vl-V5
are used within the system 14, both for selection of
input flowstream and also for re-direction of the
selected flow. The valve actuators are supplied with
compressed air at 8 bar via GEMU type 322 pilot
solenoid valves. These pilot valves are actuated via
solid state 240v relays, which are themselves switched
by TTL logic levels from a digital I/O port on the
computer.
Compressed air for the valves is supplied by, for
example, a JUN-AIR Minor compressor with pressure
vessel.
The pump 15 is a Mono pump used to pump the sludge to
the vessel 10. This pump is capable of filling the
settlement cylinder 10 within 10 to 15 seconds and is
self priming, provided the stator/rotor assembly has
been wetted. (It will also run 'dry' for extended
periods without damage, should the input flowstream
be lost.)
The mixed liquor and recycle sludge suspended solids
(MLSS and RSSS) are measured by the suspended solids
monitor 18 mounted in a flowcell 17 above the pump 15
outlet. This provides linearity over the working
range of 1000 to 8000 mg/litre solids, regression
analysis being used accurately to determine the slope
and intercept of the calibration line.
A solid state liquid level switch is used to detect
the point at which the vessel 10 is full. The sensor
output is connected to a digital input on the computer
which generates a software interrupt when the vessel
10 is full. An overflow, accurately aligned at the
50cm fill mark, ensures reproducible filling.
The photodiodes 11 are scanned in turn using the
circuit shown in Figure 4. The top 15 diodes have
their anodes connected to input lines 0 to 14 of ICl,
in the embodiment a CMOS 4067 16 way analog multiplexer
(MUX). These connections are duplicated with the
lower 15 diodes and IC2. All of the cathodes are
commoned to the 5v supply, derived from the computer
I/O port. The 4067 MUX is a tri-state device, having
an enable/inhibit input. If this line is taken low
(Ov), the chip is 'enabled', and by placing the
appropriate binary code on the address lines A-D, the
complementary input line will be connected to the
;~1[)01913
-- 10 --
output line. A truth table for the multiplexer is
utilized. Only one of the MUX's is 'enabled' at any
time, which means that the outputs can be commoned
together, effectively producing a 30-way MUX. Resistors
Rl and R2 serve to pull the inhibit lines up to 5v in
order to provide a default condition where the chips
are disabled.
Multiplexing in this way allows the anode of each
photodiode in turn to be connected to the trigger
circuit shown in Figure 5. ICl is a CA 3140 operational
amplifier with its inputs across a bridge circuit. The
resistance of the selected photodiode and consequently
the balance of the bridge itself may be offset vla VRl.
The circuit is balanced in this way whilst the photodiodes
11 are illuminated by the LEDs 10 with the vessel 2
filled with typical effluent. Once this has been
achieved, blocking of the light beam to the selected
diode by sludge solids causes the bridge to go out of
balance which then causes the output of ICl to go fully
positive. The output from ICl is taken to IC2, in this
case a 7414 TTL hex inverting Schmitt trigger. This
ensures that the signal 'latches' cleanly at either
logic statem thus producing a TTL output suitable for
connection to the computer I/O port. By scanning the
logic state of this signal after multiplexing each
diode in turn, the position of the sludge/effluent
interface may be calculated.
As any optical method of settlement level measurement
required an uninterrupted beam (other than interruption
by sludge solids), computer control of the stirrer and
the ability to sense the stirrer position is incorporated.
The circuit used is shown in Figure 6, whilst the
2~0~913
physical arrangement is shown in Figure 7. A Hall
Effect switch 20 is mounted flush with the bottom
face of the motor housing. A suitable magnet 21 is
mounted vertically on top of the longer arm 7 of the
stirrer 5 so that it passes under the switch 20, with
a vertical separation of about 2mm, once every
revolution. The action of the magnet 21 is to pull
the switch 20 output low whilst in close proximity.
The output is red to one of the pair of inputs to one
of the gates of IC2, a CMOS 4071 quad 2-input OR gate.
The other input to the gate is supplied by a digital
output line from the computer I/O board. Resistors
Rl and R2 serve to pull the inputs high to provide a
default condition. In order to switch off the
stirrer 5,input 2 of IC2 is taken low (Ov) by the
computer. The output of the gate will remain high,
however, as input 1 is held high by the output from
ICl. When the magnet 21 reaches ICl the output from
ICl goes low and thus the output from IC2 goes low
also. This switches off the gate of TRl which then
prevents any current from reaching the stirrer motor.
To switch the motor on again, all that is necessary
is to take input 2 of IC2 high again under software
control. This causes the output of IC2 to go high,
which in turn excites the gate of TRl, which switches
on to allow power to the stirrer motor. In addition,
the output from IC2 is connected to one of the
computer's digital inputs. This allows the status of
the OR gate to be monitored, and to cause a software
interrupt as soon as the magnet 21 reaches 20.
At the start of an operation of the system, once
20mm id hose has been connected to all inlets,
outlets and to the overflow, and the inlet or inlets
2001913
- 12 -
have been immersed in a suitable source of sludge,
power is applied to the system. The monitor screen
displays a multi-choice set of options, for example:-
(1) Run SSVI on Mixed Liquor only.
(2) Run Mixed Liquor & Recycle Alternately.
(3) Add or Amend Process Values.
(4) Reset System Clock.
Assuming that no process variables (settled sewage
flow, recycle flow, number of FST's and surface area
of an FST) have been entered, and that no options are
selected from the keyboard within 30 seconds,
programme operation will continue automatically using
option 1 as a default. The 30 second delay also
allows the compressor's pressure vessel to fill in
preparation. Option 3 allows values for the flowrates
in litres/sec and FST data to be entered or edited,
whilst option 4 allows review and reset of the real
time clock. Both the process values and the clock
are retained by battery backup. Once a valid run
option has been selected, this will remain the
default on subsequent programme runs until changed.
Once a run option has been selected, the system
checks the status of the level sensor and drains the
vessel 10 if necessary (a power failure may have
occurred when it was full). The settlement curve
graph axes and titles are then displayed on the
monitor. Valves 1 and 4 are then opened and the
pump 15 is started. The software then arranges for
the stirrer to be stopped under the Hall Effect
switch 20 ready for the first scan of the photodiodes
11. A 3 minute flush cycle follows, at the end of
which 50 samples are taken from the output of the
solids monitor. The integerized mean of these
2001913
samples represents the value of suspended solids in
mg/litre. Valve v3 is then opened, whilst valve v4
is closed and the vessel 10 begins to fill. At this
point, a software interrupt is enabled which
automatically invokes a Multi-BASIC background 'task',
the purpose of which is to stop the pump and close all
valves as soon as the level sensor indicates that the
vessel 10 is full.
The settlement routine then commences. The 30 sensors
11 are scanned via the 2 multiplexers, and the 'on' or
'off' status of each is stored in an array. Once a
scan is complete, the array is checked to ensure that
there is a complete series of 'on' sensors; i.e.
sensors 11 which did not receive any light and are
therefore below the level of the sludge/effluent
interface. This enables the system to detect risen
sludge and hence reject that particular run. If this
is not the case, a 'lookup' table is consulted in
order to correlate the position of the lowest 'off'
sensor with the height of the interface. The stirrer
5 itself is only stopped momentarily in order to
establish its position, and is immediately restarted.
As the motor drives the stirrer 5 at 1 rpm, the scan
is repeated 30 seconds later, when the stirrer is 180~
away from its start position and then the Hall Effect
switch 20 interrupt is enabled once more to await the
return of the stirrer to its start position. This
process is repeated 30 times, to give 60 scans in the
standard 30 minute settlement time. The sludge
height after each scan is represented on the monitor
as a vertical bar, so that as the settlement progresses,
the rate of settlement is indicated by the reduction
in height of the bars.
~aols~3
- 14 -
The Rate of Hindered Settlement (RHS) is determined by
counting the number of scans between the first sensor
becoming exposed and the next sensor becoming exposed.
As each scan takes place at 30 second intervals, a
calculation of RHS in metres/hour is possible. In the
event that the final settlement height is less than
8cm, which is the height of the lowest sensor, the
SSV is reported as being less than the SSV corresponding
to a sludge whose final settlement height was actually
8cm. At the ena of the 30 minute settlement period,
valve v5 is opened and the cylinder is allowed to
drain for 20 seconds. A copy of the settlement curve
is produced on the plotter, together with a printout
of the results. The number of results calculated
will vary, depending upon the run option selected or
other factors. Once the mixed liquor results have
been printed, the whole cycle is repeated. However,
if alternate mixed liquor and recycle runs are being
made, the sludge is pumped in via valve v2. In
addition, under these conditions, data relating to
FST loadings is calculated and reported at the end
of the recycle settlement.
If mixed liquor alone is being examined, the suspended
solids, SSVI and RHS are reported. If mixed liquor
and recycle sludge are being settled alternately, the
SSVI at 3.5 g/litre is interpolated, provided that
the MLSS are less than 3.5 g/litre or that the RSSS
are greater than 3.5 g/litre. If either of these
conditions is met, a message is printed, stating that
interpolation is impossible. Should interpolation be
possible, the following results are output :
2001913
(i) The sewage flowrate, as entered ......... . lit~s~ssor~
(ii) The recycie flowrate, as entered ........ .li~/ssxr~
(iii) ~.e lnterpolated S~ at 3.5 gramsilitre... ~S~m
(iv) The underflow rate per unit area of FST....~ r
(v) The total flowrate per unit area of FST.... m~ey~r
(vi) The applied solids loading .............. .k~ /~r
(vii) The calculated ~xi.~um solids loading.. .~m2~r
In all cases, the settlement curve is also produced on
tne plotter. An e~ample of the settlement curves for
mixed liquor and recycle sludge, together with the
full report, is given in Figure 8.
The apparatus 14 may be installed in a small towable
trailer.
The detection of infra-red from the LEDs 10 is prone
to interference from ambient IR. In a trailer of
glass-reinforced plastic (GRP) construction,
there is virtual transparency to the IR component of
sunlight which can swamp the IR from the LEDs 10. The
problem can be obviated by the addition of IR shield
means, for example two half cylindrical PVC pipe
sections, with aluminium foil bonded to them, which
shield the vessel 10.
The pump 15 and valves are carried on an L-shaped
bracket or carrier 16 of for example sheet plastic.
The LEDs 10, sensors 11 and associated circuitry 18,19
are carried by a suitable frame or jig 20, for example
of aluminium, which is complementary to and fits over
the vessel, the sensors and LEDs being connected by
straps 21 or the jig, and so that they are in use
situated externally of the vessel 2.
In a modification, in order to seek to overcome
interference of the IR light sensing at high levels
of ambient IR :
20~1913
- 16 -
1) ~nstead of the present 2 x ~067 CMOS multipiexers,
may be used. The purpose is to allow both lines
to each sensor 11 to be switchea into the trigger
circuit in isolation from the other sensors 11,
,hus preventing induced signals from any other
sensors li from reaching the trigger circuit via
the otherwise common cathodes.
(2) The IR LEDs 10 will be pulsed at a nominal 4 kHz,
-o provide a square wave output. An astable 555
timer chip, driving a 2N3055 power transistor is
used to achieve this. The existing 12v power
supply is also used, although changing the square
wave mark/space ratio (on/off period) would allow
a smaller supply to be used.
(3) The trigger circuit may be modified to comprise
a 486 IR pre-amp chip as its front end. Pulses
detected by the sensor 11 under scrutiny may be
amplified by this device to provide a signal
level suitable for driving a 567 phase locked
loop device. This chip is configured, by use ~y
an externalresistor and capacitor, to drive its
internal oscillator at a 'free running' frequency
of (in our case) 4 kHz. Circuits within the chip
allow this frequency to be adjusted automatically
when a pulse train sufficiently close to the free
running frequency is detected at the input. When
this occurs, the circuit is said to be 'locked'
and will follow any small changes in frequency at
the input. Under these conditions, a TTL
compatible logic output is produced, which may
~e used to replace the logic output in the trigger
clrcuit of the first embodiment.
20(~913
Using the apparatus described herein and shown in the
drawings an automatic SSVI procedure under computer
control is achieved, so that the SSVI is obtainable on
an approximately hourly basis 24 hours a day, if
necessary with minimal intervention other than routine
cleaning and maintenance. The apparatus could also be
used to measure the Rate of Hindered Settlement (RHS).
RHS is produced by obtaining the slope of the straight
portion of the curve produced by plotting height of
settlement against time over the 30 minute period.
RHS of the sludge at the mixed liquor concentration
gives an indication of the maximum possible overflow
rate for a final tank.
All calculations of SSVI are provided as follows :
Stirred Sludge Density :
SSD = initial height x initial concentration of SS (%)
hei~t of interface after 30 minutes
Stirred Specific Volume Index :
SSVI = 100
SSD
Underflow (recycle) rate per unit area of FST :
u = Underflow rate m3/h m3/m2 h
Total FST area sq m
Total flow rate per unit area of FST = (underflow + sewage) m3/h
Total FST area sq. m
predicted maxim~m permissible solids loading on an FST:
F L = 307 (SSVI3 5) 0'77 (u)0 68 k / 2
where u is the rate of sludge recycle per unit area (m/h)
Applied solids loading ='.'SLSS g/l x Tot. flowrate per unit area
N.B. the predicted solids loH~ing is accurate to +/- 20%
