Automated method and apparatus for determining total suspended solids in liquids

The determination of total suspended solids (non-filterable residue) in water and waste water is automated to operate without operator intervention. Volatile total suspended solids can also be determined.

BACKGROUND OF THE INVENTION 
The present invention relates to robotics systems for use in automated 
laboratory applications, and more particularly to a method and apparatus 
for the automated determination of total suspended solids (non-filterable 
residue) in water and waste water. 
Automation in analytical laboratories is not, of itself, a new concept, but 
instead has been widely practiced for many years. More recently, it has 
appeared mainly in the form of microprocessor controlled analytical 
instrumentation with dedicated hardware, such as auto samplers, continuous 
flow systems, and computerized data collection, calculation, and report 
generation facilities. The very recent past has seen important 
improvements wherein laboratory automation has been extended by the use of 
robotics, combined with programmable computers, to new tasks which include 
sample preparation, and even entire analytical determinations. The first 
such robotic system was introduced in 1982 by Zymark Corporation 
(Hopkinton, Mass.). As experience has been gained with these systems, they 
have been successfully applied to ever more sophisticated laboratory 
operations. 
An example has to do with the filtration of liquids, and in particular the 
filtration of a sample through an extremely fine filter for measuring 
suspended solids. Environmental Protection Agency requirements, such as in 
the EPA approved protocol specified in the manual procedure US EPA method 
160.2 for the determination of total suspended solids (TSS) in water and 
waste water is routinely performed at many manufacturing locations 
throughout the country, and in support of waste water treatment research 
and development studies. It is a routine EPA test procedure that is highly 
repetitive, usually involves a large number of samples to be analyzed, and 
requires the committed attention of the human analyst--a prime example of 
a procedure wherein robotic automation would be highly desirable. 
Although the repetitive nature of the procedure and the large number of 
sample analyses typically performed made the total suspended solids 
analysis a good candidate for automation, it was discovered that such a 
robotic procedure involved a much more complex system than commercially 
available. Although the procedure involved sample preparation steps which 
had already been successfully performed by other laboratory robotic 
systems, it also included operations that required robot-friendly modules 
and sensors, and procedures for their exploitation, which were not yet 
commercially available. In other words, while the procedure was well 
established for manual execution by a human operator, its automation in a 
robotics environment was found to be beyond the state of the robotics art. 
A need therefore remains for an automated method and apparatus for 
determining total suspended solids in liquids, and particularly for such a 
method which can be implemented on a robotics system in a robot-friendly 
manner in order to substantially eliminate the performance of the 
repetitive steps by hand. Such a method should be highly accurate, 
efficient, reliable, repeatable, non-intrusive, non-invasive (to prevent 
contamination), and sufficiently economical to lend itself to widespread 
utilization in such analyses. 
SUMMARY OF THE INVENTION 
Briefly, the present invention meets the above needs and purposes with a 
new and improved automated method and apparatus for determining total 
suspended solids in liquids. The liquid samples are contained in sample 
flasks and are filtered through crucibles which support filters therein on 
their perforated bases. 
In operation, the robotics system robot arm and manipulator first gets a 
fresh, dry crucible with a filter therein from the system's desiccator and 
moves the crucible to the system's balance where the initial or tare 
weight is obtained. Next, the robot takes the crucible and filter to the 
system's rinse station where a small quantity (2-ml) of water is placed on 
the filter. This helps seal the filter to the crucible. The robot then 
places them into the system's filter station, and backs away. Vacuum in 
the filter station is applied to the base of the crucible, and a 
capacitance electronics system in the filter station takes a reading of 
the initial or base capacitance of the crucible and filter. 
Next, the robot gets a sample flask from the system's storage rack and 
inverts the flask into the crucible. The robot leaves the flask in a 
holder at the top of the filter station, with the neck of the flask below 
the top of the crucible. This allows up to 100 m/l of sample to be 
filtered using a crucible of only about 40 m/l capacity. The crucible will 
not overflow since the neck of the flask is sealed below the liquid 
surface. The robot backs away and a second capacitance measurement is 
made. If the filtration is complete, this second reading will approach the 
base reading, and the vacuum will be shut off for that filter station. If 
not, the robot proceeds with loading the next sample and rechecks all 
loaded stations until filtration is complete. The rinse procedure operates 
in exactly the same way. The samples can sit for some time without 
affecting the performance as long as the vacuum is off. Preferably, the 
filters are not allowed to dry. Similarly, the filters should not be too 
wet prior to placing them in the oven. For this reason, just before the 
filter-containing crucibles are loaded into the oven the vacuum is again 
briefly applied to remove any accumulated droplets. 
Next, the crucibles are heated in the system's oven at 105.degree. C. for 
two hours to remove moisture, then cooled in the system's desiccator and 
subsequently re-weighed. This provides a final weight, which is verified 
by again heating, cooling, and weighing the crucible as before. The amount 
of suspended solids then removed by the filter is determined by 
calculating the difference between the initial and final weights. 
It is therefore an object of the present invention to provide a new and 
improved method and apparatus for the automated determination of total 
suspended solids in liquids; such a method and apparatus which can be 
directly and readily implemented by means of an automated robotic 
manipulator; in which the automated procedure is commenced by weighing a 
fresh, dry, filter-containing vessel to determine its initial weight; in 
which the automated robotic manipulator then places the vessel in the 
system's filtration station; in which the manipulator then inverts a 
sample-containing sample vessel in the filtration station over the 
filter-containing vessel; which continues by detecting conclusion of the 
filtration of the liquid sample through the filter-containing vessel in 
the filtration station; which next adds clean rinse liquid to the drained 
sample vessel; which then again inverts the sample vessel in the 
filtration station over the filter-containing vessel to pour residual 
sample solids from the sample vessel into the filter-containing vessel; 
which then again detects conclusion of the filtration of the liquid 
through the filter-containing vessel in the filtration station; in which 
the filter-containing vessel is then heated to dry it; in which the 
filter-containing vessel is then cooled in the system's desiccator; in 
which the filter-containing vessel is then re-weighed; in which the 
difference between the initial and final weights is then calculated to 
determine the weight of the suspended solids removed by the filter from 
the liquid sample; in which the filter-containing vessel may be a 
crucible; in which the fresh, dry, filter-containing vessel may be stored 
in the system's desiccator prior to weighing the dry vessel to determine 
it's initial weight; in which the filter in the filter-containing vessel 
may be moistened prior to inverting the sample-containing vessel over the 
filter-containing vessel in the filtration station; in which the 
sample-containing sample vessel may be a flask; in which the 
sample-containing sample vessel may be initially stored in a storage rack 
and finally returned thereto when empty; in which the rinsing steps may be 
repeated several times to improve removal of residual sample solids from 
the sample vessel and to wash the filter cake; in which a vacuum may be 
applied to the filter-containing vessel to expedite filtration of the 
liquid sample through the filter; in which such a vacuum may be re-applied 
to the filter-containing vessel just prior to the removal thereof from the 
system's filtration station; in which the heating, cooling, and 
re-weighing steps may be repeated to verify the final weight; in which 
volatile suspended solids may also be determined by heating the 
filter-containing vessel in a furnace to vaporize the volatile suspended 
solids in the filtrate, cooling the filter-containing vessel, and again 
weighing the vessel, and based on the weights determined in the several 
weighing and re-weighing steps, calculating volatile total suspended 
solids as well as the total of all suspended solids removed by the filter 
from the liquid sample; and to accomplish the above objects and purposes 
in an inexpensive, uncomplicated, durable, versatile, and reliable method 
and apparatus, inexpensive to implement, and widely suited to the widest 
possible utilization in robot-friendly applications for determining total 
suspended solids in liquid samples. 
These and other objects and advantages of the invention will be apparent 
from the following description, the accompanying drawings, and the 
appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to the drawings, the new and improved automated method and 
apparatus for determining total suspended solids in liquids will be 
described. FIG. 1 shows a robotics system 10 for carrying out the method 
according to the present invention. System 10 includes a robot controller 
13 and, in the center of system 10, a robot arm and manipulator 15 
controlled by controller 13. Controller 13 and arm/manipulator 15, in the 
preferred embodiment, are the Zymate II system available from Zymark 
Corporation (Hopkinton, Mass.). Suitable computer support equipment 18 may 
also be provided for processing, storing, printing, graphing, and other 
desired functions in support of the robotics system 13,15. System 10 
further includes an electronic balance 21 having a remote controlled door, 
and an oven 23 (shown with the lid closed) and desiccator 24 (shown with 
the lid raised), also equipped with remote controlled doors. 
Eight filtration stations 25, in the preferred embodiment, are of a special 
and unique design capable of detecting when filtration of the liquid has 
concluded, independently of the time required for the filtration. A 
suitable filtration station 25 and capacitance measuring configuration 
(FIGS. 3 and 4) is described in detail in copending U.S. patent 
application Ser. No. 149,254, filed contemporaneously herewith, and 
entitled "Liquid Sensor for Robotic Filtration Station", the disclosure of 
which is entirely incorporated herein by reference. 
Other major components of system 10, as shown in the drawing figures, 
include rinse dispenser station 28 and a storage rack 29. Rack 29 is 
designed to hold sample-containing flasks 30 from which the liquid samples 
32 therein will eventually be poured through crucibles 35 (FIG. 3) which 
contain filters 40 (FIG. 3). 
The automated total suspended solids determination procedure then proceeds 
as follows. First, the operator preconditions the system. Water is loaded 
into the rinse dispenser station 28. The crucibles 35 with their filters 
40 are heated, such as in a furnace, to drive off all organic material. 
The clean crucibles with clean filter are then placed (using tweezers) in 
the desiccator 24, and the sample-containing flasks 30 are loaded into the 
storage rack 29. Initial heating of the crucibles may be, for example, for 
fifteen minutes at 550.degree. C., followed by cooling at ambient for 
thirty minutes, and then equilibration for one hour in the desiccator, 
before the analysis run is started. Suitable crucibles are readily 
available, one type being a Gooch crucible (Coors part No. 60151). 
The robot 15 then removes a crucible 35 from the desiccator 24 and places 
it in the balance 21 to determine the initial or tare weight. Weights are 
automatically transmitted from the balance 21 to the memory in the robot 
controller 13 for subsequent calculation and results. 
The robot then moves the tared crucible 35 from the balance 21 to the rinse 
station 28, where the filter 40 is dampened with water. The crucible is 
then placed in the filtration station 25. 
Next, the robot 15 gets a flask 30 containing a sample 32 from the storage 
rack 29. Just before the robot begins to pour the sample 32 into the 
crucible 35, the vacuum in station 25 is applied to the bottom of the 
crucible 35 to assist in the filtration. Simultaneously, the capacitance 
sensor (not shown) in the filtration station 25 measures the base 
capacitance for the crucible and the dampened filter 40. The sample is 
then poured into the crucible and filtered. No overflow of the crucible 
occurs because the pouring is self regulated hydrostatically. During the 
filtration, the capacitance sensor in filtration station 25 repeatedly 
monitors the liquid level in the crucible. Filtration is finished when the 
capacitance reaches the original base capacitance condition. At that time 
the vacuum is also discontinued. 
The robot 15 then moves the flask 30 from the filtration station to the 
rinse station 28 where rinse water is added. The flask is then moved back 
to filtration station 25 to pour any residual sample from the flask into 
the crucible 35. This also washes the filter cake which has been captured 
by filter 40. During this operation, the vacuum and liquid level sensing 
are again operated as described above. 
In the preferred embodiment, the rinsing operations are repeated twice 
more. 
The robot 15 then returns the empty flask 30 from the filtration station 25 
to the storage rack 29, and then moves the crucible 35 from the filtration 
station to the oven 23 where the filter cake is dried at 105.degree. C. 
for two hours. 
After drying, the robot 15 moves the crucible 35 from the oven to the 
desiccator 24 for thirty minutes to cool before being weighed. The robot 
then moves the crucible 35 from the desiccator 24 to the balance 21 where 
the crucible 35, filter 40, and filter cake thereon are weighed. 
To verify the final weight, the robot 15 returns the weighed crucible to 
the oven 23 to dry an additional fifteen minutes. After heating for this 
period of time, the robot then moves the crucible to the desiccator 24 
where it is allowed to cool for thirty minutes before being re-weighed 
again. Finally, the robot moves the crucible to the balance where it is 
re-weighed, ordinarily verifying a constant final weight. Finally, the 
robot returns the crucible to the desiccator 24 where it is stored until 
the operator removes it at the conclusion of the measurement of all the 
original liquid samples 32. The difference between the initial and final 
weights of each individual crucible then indicate the amount of suspended 
solids which were captured by the respective filters 40. 
If volatile total suspended solids are to be determined, in addition to 
just the total amount of suspended solids, then after the drying and 
re-weighing steps are concluded, the crucibles may be placed in a high 
temperature oven or furnace to vaporize the volatile suspended solids. In 
the preferred embodiment, this is performed at 550.degree. C. After 
forty-five minutes in the furnace at this temperature, the crucibles are 
allowed to cool thirty minutes at ambient conditions and then returned to 
the desiccator for an additional sixty minutes before being re-weighed. 
This additional data then allows the total amount of volatile suspended 
solids in the filter cake to be determined as well. 
As may be seen, therefore, the present invention has numerous advantages. 
It is extremely precise and accurate, and can be operated very 
successfully while unattended. Results show precisions at least as good as 
those with manual methods. The capital investment is modest, so that labor 
savings quickly justify the cost of the system. Operator time required for 
the procedure has been found to be less than one-third that for the manual 
procedure. Additionally, the invention provides advantages for the 
operator, who finds the robotic approach more interesting than the manual 
procedure and also gains additional time for performing other tasks, 
therefore experiencing a greater sense of productivity. The invention, in 
this context, is not only economical, but actually can be considered 
inexpensive. It is also uncomplicated, durable, very versatile, and highly 
reliable. It thus lends itself to wide application in automated analytical 
and laboratory procedures directed to the analysis of total suspended and 
volatile total suspended solids measurements. 
Based upon the teachings herein, other improvements will suggest themselves 
to practitioners in this art. For example, the robot could fetch a rinse 
hose after the final flask rinse, and use the hose to wash down the inside 
of the crucible, to wash away dried solubles, such as salt, although this 
has not been found to be necessary. 
Therefore, while the methods and forms of apparatus herein described 
constitute preferred embodiments of this invention, it is to be understood 
that the invention is not limited to these precise methods and forms of 
apparatus, and that changes may be made therein without departing from the 
scope of the invention.