Apparatus and method for automatically controlling the permeability of a traveling bridge filter system

The present invention comprises apparatus and a method of automatically controlling the permeability of filter beds. In particular, this invention directs itself to filter beds or tanks that are divided into a plurality of contiguous compartments or cells, each compartment containing a filtering media substrate, which substrate is cleansed or backwashed, periodically, when necessary, by a traveling bridge backwash mechanism. The duration and frequency of backwashing is automatically controlled to maintain a desired overall permeability and throughput performance of the filter bed. Permeability control is dynamic to maintain optimum performance under all conditions, i.e. changing in response to fluctuating filter bed hydraulic or solids loading rates.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns improvements in traveling bridge filtering 
systems. More particularly, it concerns methods and improved apparatus for 
automatically controlling the backwash cycle and permeability of the 
filter bed in such systems. 
2. Description of Related Art 
The traveling bridge filter system has proven to be quite efficient, cost 
effective and durable in certain applications. The art is exemplified in 
U.S. Pat. Nos. 3,239,061 and 4,152,265 and further exemplified in 
assignee's brochure, Clean Water for Less, Copyright 1991. 
A principal use of a traveling bridge filter system is the removal of 
suspended material or solids from a liquid influent in order to meet 
governmental regulatory requirements. In recent Years, regulatory 
requirements have become more stringent, making filtering system 
efficiency and reliability a paramount concern. 
Presently, filters of this type initiate a backwash whenever the influent 
level in the filter bed reaches a predetermined and fixed elevation. 
During the backwash cycle, a traveling bridge backwash mechanism is 
commenced to move, cell by cell, from one end of the filter bed to the 
opposite end. Once the backwash mechanism reaches the opposite end of the 
bed, if the water level in the bed has fallen below a given elevation, 
also predetermined and fixed, the backwash cycle terminates. If the water 
level in the filter bed has not fallen below the predetermined fixed 
elevation, the backwash mechanism will continue to operate, traveling cell 
by cell from one end of the filter bed to the other, until an operator 
intervenes, or until, when the backwash mechanism is at one end of the 
bed, the water level has fallen below the predetermined elevation. This is 
a very limited method of backwash control. 
It is known that in traveling bridge filter systems fluctuating flow rates 
and solids loading are a commonplace occurrence. Higher hydraulic loading 
alone typically results in higher water levels in the filter bed even when 
the filtration media is clean. With their limited method of backwash 
control, current filters are unable to distinguish between conditions that 
develop as a result of solids versus hydraulic loading. Thus, current 
filters initiate a backwash cycle even when insufficient solids have been 
deposited to warrant media cleansing. This is an undesirable and limited 
method of backwash control that adversely effects the efficiency, 
cost-effectiveness, permeability and throughput performance of the filter 
bed system. 
It is widely known by those familiar with the art that over time a mat of 
solids develops on the surface of the filter media schmutzdecke resulting 
in beneficial and substantial solids removal as well as detrimental and 
substantial headloss. As the quantity of material forming the mat 
increases, the permeability of the filter bed decreases. Removal of the 
entire mat by backwashing, however, is not desired since within limits, 
the solids removal efficiency and effluent or filtrate quality are 
directly related to the quantity of material forming the mat. 
Current methods of backwashing filter beds result in excessive and 
unnecessary backwash usage and impair filter performance by removing 
excessive solids thereby damaging the mat of solids which is so 
influential in optimizing filter performance. 
A principal object of the present invention is the improvement of traveling 
bridge filter systems vis-a-vis new apparatus and methods for 
automatically controlling the backwash cycle and the permeability of the 
filter bed by evaluating dynamic hydraulic and solids loading conditions, 
initiating and controlling the backwash cycle and executing automatic 
adjustments to maintain the highest filtering efficiency possible, without 
operator intervention. 
A further object is the automatic optimization of filter throughput. 
The detailed description discloses the preferred embodiments of the 
invention however, various changes and modifications to the preferred 
embodiments are within the spirit and scope of the invention. 
SUMMARY OF THE INVENTION 
The apparatus and method of the invention include improvements to traveling 
bridge filter systems such that the filter bed for such a system is 
cleansed automatically by way of a backwash cycle only when the filter 
contains sufficient solids to warrant cleansing. 
The objects of the invention are accomplished by various embodiments of the 
invention including the addition of structures which automatically (a) 
calculate the filter bed headloss under changing hydraulic and solids 
loading conditions, (b) calculate the filter permeability under such 
changing conditions, (c) control the initiation and operation of a 
backwash cycle, and (d) modify and/or correct permeability initialization 
values based on changing filter bed applications and/or operational 
conditions. 
The objects are also accomplished by embodiments of the invention which 
utilize steps including (a) loading permeability initialization values 
into the backwash controller, (b) monitoring fluid levels in the filter, 
(c) determining the initialization, duration and frequency of the backwash 
cycle, and (d) determining whether modifications and/or corrections should 
be made to the permeability initialization values based on the filter bed 
application and the changing hydraulic and solids loading conditions. 
The invention provides an artificial intelligence feature that allows the 
filter to adapt and change to dynamic hydraulic and solids loading 
conditions, without operator intervention. The invention also provides the 
ability to display current filter system operating conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention relates to improvements in traveling bridge filter 
bed systems. Such systems include rectangular or circular filter beds. A 
typical rectangular traveling bridge filter bed is depicted in FIG. 1, 
wherein the filter bed (10) is comprised of a plurality of compartments or 
cells (12) defined by partitions (14). In the system depicted, each filter 
contains porous filter media (16), such as sand, through which the 
influent (18) flows to a filtrate or effluent compartment (20). Systems of 
this type currently perform both a filtering operation and a periodic 
backwash operation of individual cells during the filtering operation of 
the remaining cells. 
During filtering operation, solids (22) are deposited on and within the 
surface of the filter media (16). By reference to FIG. 2, it is seen that 
as the quantity of solids (22) stored within the filter increases filter 
permeability decreases. This will cause an increase in filter headloss, as 
well. By maintaining a dense mat of solids (22) on the filter surface, 
however, good filtrate quality can be achieved. This will result in higher 
headloss than would be experienced with a mat containing a lesser amount 
of solids. 
In general, it is desirable to operate a filter at the lowest possible 
headloss condition consistent with the filter application and performance 
requirements. Thus, two identical filters may exhibit different operating 
characteristics depending on their particular application. 
FIG. 3 illustrates that hydraulic loading also influences headloss. As the 
flow rate of influent (18) into the filter (10) (typically expressed as 
gallons per minute per square foot o filter area) increases, filter 
headloss increases. This is true even when the filter bed (10) is clean. 
The present invention utilizes these relationships between solids loading 
and permeability, and hydraulic loading and headloss and causes 
backwashing to be initiated only when the overall filter permeability 
required for the desired effluent quality decreases below a predetermined 
value. The present invention thus solves the problem of unnecessary and 
excessive backwashing since initiation of backwash may occur at various 
influent levels in the filter bed (10), depending on the filter throughput 
parameters established for the particular application of each individual 
filter, but only when the filter media (16) contains sufficient solids 
(22) to warrant cleansing. 
In order to evaluate the need to backwash, in the preferred embodiment of 
the present invention it is necessary to first obtain a measurement of the 
hydraulic flow passing through the filter (10) and convert this to a 
hydraulic loading value by factoring in the size of the filter bed (10) in 
square feet. This is accomplished by sampling the head level of the 
effluent (24) over the filter weir every 10 milliseconds. This gives a 
good approximation of the influent flow rate. Based on the known area of 
the filter bed in feet squared, the hydraulic loading for the particular 
filter is determined. Headloss is influenced by the filter media type 
used, the particle size of suspended solids, water temperature and other 
factors. 
The actual headloss through the filter bed (10) is then determined by 
subtracting the height of the effluent (24) in the filtrate compartment 
(20) from the height of influent level in the filter bed (10). Only then 
may a comparison be made between the calculated headloss and the desired 
or expected headloss value for an individual filter in its particular 
application. 
The prior art uses a probe type backwash control method illustrated in FIG. 
4. Points along the clean filter curve (34) illustrate filter headloss for 
changing values of hydraulic loading in the filter. Backwash is initiated 
at all points along the backwash initiate line (32). Wasted backwash flow 
is depicted as the large shaded area (33) of FIG. 4 under the clean filter 
curve (34) but above the backwash initiate line (32). Thus, in the prior 
art, backwash is initiated at a predetermined and fixed value for filter 
headloss illustrated in FIG. 4 by the small shaded area (31) under the 
backwash initiate line (32) but above the clean filter curve (34). Fixed 
backwash initiation allows the filter (10) to be in filtering operation, 
without backwash operation, only in the narrow hydraulic range (31), which 
limits filter efficiency and optimization. 
As illustrated in FIG. 5, the present invention initiates a backwash cycle 
at points along the dynamic backwash initiate curve (38). Dynamic backwash 
initiation (38) allows the filter (10) to be performing a filtering 
operation, without a backwash operation, for a full range of hydraulic 
loading, as shown by the shaded area (40) in FIG. 5. The present invention 
modifies the dynamic backwash initiate curve (38) for each individual 
filter application. This solves the problem of excessive and unnecessary 
backwashing that exists in the prior art, wherein backwash initiation is 
predetermined and fixed (32). 
FIG. 6 schematically illustrates the input values received by the 
programmable logic controller or PLC, the PLC internal parameters used to 
determine initiation and control of the backwash operation and the PLC 
output values which control the backwash operation. Input values and 
internal parameters of the PLC can be input as initialization values by 
the operator manually, or electronically. 
The preferred method of determining headloss is by utilizing at least one 
ultrasonic level detector in the filter bed (10) and filtrate compartment, 
respectively, to provide a continuous 4-20 milliamp analog output signal 
that is proportional to and thereby indicating the influent level in the 
filter bed and filtrate compartment, respectively. Ultrasonic detectors 
utilizing digital outputs may also be used. Ultrasonic level detectors are 
desirable because they do not require physical contact with the fluid 
being measured and thus, are unaffected by pH, corrosivity, temperature or 
other attributes of the influent being filtered. Other means for 
determining headloss, such as pressure head measurement, differential 
pressure measurement, or use of various types of mechanical floats may be 
utilized, but are problematic and not desired since they require physical 
contact with the fluid being measured. 
In the preferred embodiment of the present invention, the decision to 
initiate a backwash cycle in order to control filter bed permeability is 
made by a programmable logic controller (PLC) contained within a control 
panel of the filter system. An example of such a programmable controller 
is Model No. 311 manufactured by General Electric Corporation or any other 
General Electric Series 90-30 PLC. Other comparable programmable 
controllers or other electronic computing systems could also be used. 
In the preferred embodiment, the PLC uses outputs received from the 
ultrasonic filter bed and filtrate compartment fluid level sensors and a 
central processing unit (CPU) of the PLC with a control algorithm 
utilizing a mathematical function that determines the anticipated headloss 
under expected operating conditions, initiates the backwash cycle only 
when the headloss exceeds the value anticipated for the hydraulic and/or 
solids loading conditions present at that time. Solids loading is 
determined in the present invention inferentially. 
Only when solids loading causes the anticipated or expected headloss value 
to be exceeded, thereby indicating that suspended solids are decreasing 
permeability by restricting filter flow, will the PLC activate the 
backwash cycle. Once the PLC determines that a backwash cycle is to begin, 
it will cause the washwater and backwash pumps to be turned on, and the 
backwash mechanism drive unit to be activated allowing the backwash 
mechanism to approach the first cell to be cleansed. 
When the dwell limit switch attached to the traveling bridge mechanism 
encounters a positioning peg located over a particular cell, the PLC will 
cause the bridge drive unit to cease operation. The backwash mechanism, 
attached to the traveling bridge and suspended therefrom will remain 
stationary over and hydraulically sealed with the cell being cleaned for a 
predetermined period of time, i.e. dwell time, which is related to the 
desired bed permeability. During this operation, the washwater and 
backwash pumps remain operating. 
The dwell time is initially set based on the goal of effecting a 
restoration of headloss (i.e. reduced water level in the filter 
compartment) after backwashing only a minimum number of cells in the bed. 
This minimum number of cells is the cell target for the controller. 
Backwashing, in this manner, for example, will achieve backwashing of only 
20 to 30 percent of the cells leaving 70 to 80 percent of the filter bed 
with an undisturbed mat of solids. This will maintain filter permeability 
and high levels of effluent quality and filter efficacy. 
If during backwashing operation on any individual cell the dwell time has 
elapsed, the PLC will once again calculate the headloss and determine 
whether the backwash cycle will continue to the next cell or whether the 
backwash cycle should terminate. If headloss has not been reduced to the 
desired clean bed level, the controller will activate the bridge drive to 
cause the backwash mechanism to position itself stationary over the next 
cell to be cleansed and repeat the backwash process. 
If, after cleansing the desired number of cells (cell target) the headloss 
has not been restored, the PLC will increase the dwell time value for 
cleansing each cell, thereby attempting to minimize the number of cells 
above the cell target that must be cleansed in a backwash cycle to achieve 
the desired permeability of the filter bed. 
If headloss is restored after cleansing less than the cell target, the PLC 
will decrease the dwell time for cleansing each cell so that during the 
next subsequent backwash cycle the actual number of cells cleansed should 
increase to approach the cell target. The cell target value is set to 
achieve the goal of maintaining a percentage of the filter bed in a 
ripened state. The degree of ripening effects both filtrate quality and 
the headloss. The percentage of the ripened bed to be maintained depends 
on specific filtrate quality objectives that are established for each 
individual application. 
When headloss is restored to the desired and expected level, the PLC will 
cause the backwash cycle to terminate by suspending operation of the pumps 
and drive unit on the bridge. The backwash mechanism will remain 
stationary in position over, but not sealed with, the last cell that was 
backwashed, until a next subsequent backwash is initiated 
The electronic output from at least one turbidimeter, immersed in the 
backwash flow from the cell being backwashed, is monitored by the PLC. If 
during the backwash cycle, the dwell time on a cell is exceeded, or if all 
the cells in the filter bed are backwashed (indicating the cell target was 
exceeded) then the controller will cause the backwash cycle to operate 
such that cells will be backwashed, one by one, until the turbidimeter 
output reaches a value indicating the cell is olean. Dwell time is not a 
controlling consideration during this modified backwash operation. 
The amount of solids, i.e., wash water suspended solids or WWSS (See FIG. 
6), contained in the backwash waste water for a cell provides a signal to 
the PLC, allowing the PLC to calculate the precise amount of solids to be 
removed individually from each cell for each filter bed application. In 
the event solids loading is determined to be in excess of the filter bed 
design values, headloss might not be restored even after all cells within 
the filter bed have been backwashed. In this case, the PLC will modify the 
backwash operation and cause the backwash mechanism to dwell on an 
individual cell for whatever length of time is necessary to remove 
substantially all of the suspended solids from the filter media of that 
cell. The backwash mechanism will proceed in this manner, cell by cell, 
until headloss and permeability is restored to the desired level, at which 
time the PLC will terminate backwash operation by deactivating the bridge 
drive and the pumps. 
In an alternative embodiment of the present invention, the cell target is 
decreased if the permeability of the filter reaches the desired value 
prior to reaching the cell target during backwash operation. Likewise, the 
cell target is increased if the permeability of the filter fails to 
decrease to the desired value by the time the cell target is reached 
during backwash operation. 
The invention also provides the ability to display current filter system 
operating conditions by an electronic display panel, or otherwise.