Monitoring solids content of liquid sludges

The solids content of a thick sludge is monitored by means of a solids content monitor located in a dilution chamber into which a predetermined quantity of sludge is deposited from a sampling chamber to be diluted by a known factor (e.g. 10) by the addition of water to a predetermined dilution level. Water is sprayed into the sampling and dilution chambers to clean their surfaces and the filling and discharge of the chambers is controlled by solenoid valves operated in an automatic sequence.

This invention relates to the monitoring of liquid sludges, and in 
particular the measurement of the solids content of sludges, that is, the 
percentage by weight of solid particle content in liquid sludges. 
An object of the invention is to provide a method and apparatus for 
measuring solids content in liquid sludges over a large range of solids 
content, up to 15% (corresponding to 150,000 mg dry weight of solid matter 
per liter of water). At the upper end of this solids content range the 
sludge would have the consistency of thick soup. Methods used for the 
measurement of the solids content of such sludges, for example ultrasonic 
beam techniques, are impeded by the inevitable presence in such sludges of 
air bubbles and occlusions. Furthermore, the high density and solids 
content of such thick sludges causes fouling of the measuring apparatus, 
and precludes the practical use of optical apparatus for solids content 
measurement. 
The present invention avoids this difficulty by providing in an automatic 
cycle for the dilution of a sludge to be monitored to a predetermined 
controlled extent before making a solids content measurement. 
According to the invention in one aspect there is provided a method of 
monitoring the solids content of liquid sludge comprising the following 
steps in an automatically controlled sequence: 
(a) filling a sampling chamber with the liquid sludge; 
(b) discharging the sampling chamber into a dilution chamber when a 
predetermined volume of sludge is contained in the sampling chamber; 
(c) filling the dilution chamber with water to a level which corresponds to 
a predetermined dilution of the sludge; 
(d) measuring the solids content of the diluted sludge by measuring 
apparatus in the dilution chamber, and 
(e) cleaning both the sampling chamber and the dilution chamber by spraying 
clean water into them. 
The invention also provides an apparatus for monitoring the solids content 
of a liquid sludge, comprising a sludge flow duct, a first valve operable 
to direct sludge from said duct into a sampling chamber having a drain 
valve, control means for opening the drain valve to drain the sampling 
chamber into an underlying dilution chamber and to close the first valve 
when the sludge in the sampling chamber reaches a predetermined level, 
means for delivering water to the dilution chamber to fill the latter to a 
level corresponding to a predetermined dilution of the sludge, 
solids-content measuring apparatus located in the dilution chamber for 
measuring the solids content of the diluted sludge, a third valve operable 
to drain the dilution chamber upon completion of the measurement, and 
means for spraying clean water into both the sampling chamber and the 
dilution chamber to clean the walls thereof. 
The filling of the sampling chamber, the subsequent filling of the dilution 
chamber, the measurement of the solids content, and the cleaning out of 
the apparatus, occurs in an automatic sequence, enabling effective 
measurements of solids content to be made even where the solids content of 
the sludge is very high. In a typical embodiment of the invention the 
dilution effected in the dilution chamber would be tenfold--that is, the 
water added to the sludge in the dilution chamber would be ten times the 
volume of the sludge obtained from the sampling chamber. Thus if a sludge 
having a solids content of 15% is to be monitored the dilution chamber 
will result in a diluted sludge having a solids content of 1.5%, which is 
capable of measurement using currently available optical instruments for 
solids content measurement. The measurements made would be multiplied by 
the dilution factor to obtain an indication of the solids content of the 
original sludge, such multiplication being made on the output of the 
measuring instrument, or in the instrument itself. 
By sampling sludge from the sludge flow duct at intervals it is possible to 
monitor the solids content of a liquid sludge, at any desired frequency. 
The invention finds particular application in monitoring the solids 
content of sludge in sewage treatment installations. For example, where 
sewage sludge is to be transported to sea for discharge at sea the solids 
content of the sludge may be monitored in a collection tank until it 
reaches a sufficiently high level, when a warning indication may be given 
by the measuring apparatus. 
Preferably the sampling chamber is surrounded by an overflow collecting 
chamber which drains directly into the sludge flow duct. 
The valves in the apparatus may be solenoid-operated and controlled by 
electrical signals derived from liquid level sensors. For example, the 
level of sludge in the sampling tank may be monitored by an electrical 
conduction probe having two contacts which are bridged by the liquid 
sludge when this reaches the predetermined level in the sampling chamber 
to provide an output signal controlling operation of the drain valve, 
draining the sampling chamber into the dilution chamber, and at the same 
time returning the diverter valve to its non-diverting position. 
The means for spraying water into the sampling chamber and the dilution 
chamber are preferably used both for the dilution of the sludge and, 
subsequently to the solids content measurement, for the flushing of the 
apparatus with clean water to clean the internal surfaces of the sampling 
and dilution chambers prior to the next measurement cycle. The 
solids-content measuring apparatus may have an associated water spray 
device for cleaning the surfaces of the measuring apparatus between 
measuring cycles.

Referring first to FIG. 1, a sludge inlet pipe 1 is shown which includes a 
first solenoid valve V1. The inlet pipe 1 feeds into a vertical sludge 
flow pipe 2 which in turn leads to a drain pipe 3. In a closed loop sludge 
monitoring system the drain pipe 3 may lead into a sludge tank the 
contents of which are to be monitored. 
The solenoid V1 is a three-way valve which is normally in the position 
shown in FIG. 1 allowing sludge to flow from the pipe 1 directly into the 
sludge flow pipe 2. When enlarged, the solenoid valve V1 is effective to 
divert liquid sludge from the inlet pipe 1 into a sampling pipe 4 which 
delivers liquid sludge into a sampling chamber 5. The sampling chamber 5 
has a funnel-shaped base terminating at its lower end in a 
solenoid-operated valve V2 which is normally closed. The sampling chamber 
5 is open at its upper end and surrounded by an annular overflow tank 6 
which drains through an inclined overflow pipe 7 directly into the sludge 
flow pipe 2. 
The sampling chamber 5 is housed coaxially within a dilution chamber 8, 
which also has a funnel-shaped base terminating in a normally closed 
outlet valve V3, also solenoid-operated. 
An electrical liquid level sensor 9 is mounted in the sampling chamber 5 at 
a position clear of contamination by liquid sludge. The sensor 9 has a 
pair of electrodes 9a which project into the top of the sampling chamber 5 
and terminate at free ends which are separated horizontally and disposed 
at a level corresponding to a predetermined filling level of the sampling 
chamber 5. When liquid sludge fills the sampling chamber 5 to the 
predetermined level the sludge bridges the two electrodes 9a, the change 
in electrical resistance between the electrodes 9a being detected by the 
sensor 9 which provides an output signal controlling the cyclic operation 
of the apparatus, as later described. 
A second liquid level sensor 10 is located externally of the side wall of 
the dilution chamber 8 and has a pair of conductive electrodes 10a 
projecting into the dilution chamber 8 at a level below the level of the 
outlet valve V2, the sensor 10 providing an output signal when the 
dilution chamber 8 is filled to a level L indicated in broken outline 
below the level of the sampling chamber outlet valve V2. 
A photoelectric solids content measuring instrument 11 is mounted in a wall 
of the dilution chamber 8 below the level L and is connected to a 
monitoring circuit 12. A direct reading instrument 13 and/or a pen 
recorder 14 may be associated with the monitoring circuit 12. 
A supply of clean water, for example a water main, is connected to the 
sampling pipe 4 through a water supply pipe 15 including a solenoid valve 
V4. A further water supply pipe 16 is connected to the dilution chamber 8 
through a solenoid valve V5 and communicates with spray nozzle manifolds 
17 within the dilution chamber 8, the sampling chamber 5 and the overflow 
chamber 6, as illustrated diagrammatically. 
The illustrated apparatus operates in an automatic cycle controlled by a 
control unit 18. The automatic cycle which has electrical input 
connections to the level sensors 9 and 10 and electrical output 
connections to the solenoid valves V1-V5, as shown schematically, would 
typically be completed at predetermined timed intervals, typically of the 
order of 5 minutes, controlled by a time switch incorporated in the 
control unit 18. At the end of each timed interval the following cycle of 
sequential operations would be initiated by the control unit 18: 
(i) the solenoid valve V1 is energised into its diverting position, 
diverting the liquid sludge from the inlet pipe 1 into the sampling pipe 4 
and filling the sampling chamber 5; 
(ii) when the level of sludge in the sampling chamber 5 reaches the 
predetermined level set by the sensor electrodes 9a the sensor 9 provides 
a trip signal which de-energises the solenoid valve V1, which returns to 
its non-diverting position (FIG. 1); 
(iii) after a predetermined interval (for example, five seconds) the 
solenoid valve V2 is opened and at the same time the solenoid valves V4 
and V5 are opened. The liquid sludge in the sampling chamber 5 is 
thereupon drained through the open valve V2 into the dilution chamber 8, 
and the sampling chamber 5 is rinsed by water entering through the inlet 
pipe 15 and through the spray manifold 17; 
(iv) the water fills the dilution chamber 8, diluting the liquid sludge 
therein, until the level L is reached. Before this level is reached, 
however, the solenoid valve V4 is closed, under control of a timer, the 
"topping up" of the dilution chamber 8 being effected by the water sprayed 
into the chambers through the spray manifold 17; 
(v) when the predetermined level L is reached in the dilution chamber 8 the 
sensor 10 provides an output signal which closes the solenoid valves V2 
and V5 and initiates the measurement of solids content by the instrument 
11 connected to the circuit 12; 
(vi) when the solids content measurement has been completed the solenoid 
valve V3 is opened, allowing the contents of the dilution chamber 8 to 
discharge into the drain pipe 3; 
(vii) the solenoid valves V4 and V5 are then opened for a period of about 
20 seconds to flush the dilution chamber 8 with clean water. During this 
flushing operation the solenoid valve V2 is open to allow further flushing 
of the sampling chamber 5 while the valve V4 is open; 
(viii) the solenoid valves V2 and V3 are closed and the apparatus is then 
ready for a further monitoring cycle. 
In a typical installation the operating cycle of the apparatus would occupy 
a time of about 1-2 minutes. 
FIGS. 2 and 3 illustrate a typical practical embodiment, in which the same 
reference numerals are used to indicate corresponding component parts. It 
will be seen that the sampling chamber 5 is mounted on one side of the 
dilution chamber 8, above the liquid level therein, and the level sensors 
9 and 10 are grouped together in a common housing. 
The solids content measuring instrument 11 may have an associated water 
spray device 19 (FIG. 1) for cleaning its measuring surfaces. A "hold" 
circuit may be associated with the measuring apparatus to "memorise" the 
measurements made by the instrument 11 in each measuring cycle and to 
initiate alarms or warnings in the event of predetermined threshold 
measurements being exceeded.