Nearly all membrane systems,
no matter what their raw
water source, must utilize
some form of pretreatment
to remove unwanted organic and inorganic
suspended solids. Bag and cartridge
filters are efficient at removing
suspended solids but must be replaced
on a frequent and often costly basis.
Some form of suspended solids
removal system that cleans itself,
maintains high flow rates and stays
online at all times with a very low
pressure drop would be ideal.
That piece of equipment exists. A
versatile fully automatic self-cleaning
screen filter can remove up to 99% of
all suspended solids from membrane
influent, allowing fine cartridge filters,
if needed, to polish with very infrequent
replacement. New screen filter
technology makes possible the removal
of all particles down to 10 microns
without depending upon filter aids
such as diatomaceous earth or self-forming
filter cakes.
Terms and Definitions
The smallest particle size requiring
removal from the fluid stream in a specific
application is called the filtration
degree. Two conventions are used to
define filtration degree. One is taken
from the textile industry, referring to
the density of threads expressed as the
number of threads per linear inch. This
definition uses the term "mesh" to
describe this density measurement. In
the field of filtration the term has
come to mean the number of pores or
openings per linear inch in a woven
media. Although still in common use,
the term "mesh" is not a true parameter
of measurement since the actual
opening or pore size of such a
described medium depends on the
diameter of the threads or wires and
the type of weave used in the manufacturing
process.
The second convention used to
describe nominal filtration degree,
preferred in the municipal and industrial
arenas, is an actual linear dimension
of the shortest straight-line distance
(length or width) across an individual
opening or pore of the filter
medium. This is most often given in
microns; i.e. 1/1000 of a millimeter or
0.00004 of an inch. The absolute filtration
degree is the length of the longest
straight-line distance across an individual
opening of the filter medium.
Another important definition needed
when comparing filters and filtration
methods is the filtration open
area. This is the pore area or sum of all
the areas of all the holes in the filter
medium through which the fluid can
pass. Filtration open area is expressed
as a percentage of the effective filtration
area. Basic physics says that the
pressure drop across a porous medium
is proportional to the square of the
velocity. For a given flow rate, less
open area means higher velocity, thus,
a higher pressure drop.
Figure 1. Time vs Differential Pressure (DP).
Screen filters, when clean, have
enough open area to cause insignificant
pressure drops across the screen.
However, as dirt and debris begin to
plug up openings in the screen, the
open area that is available for the fixed
flow rate to pass through is decreased,
leading to an ever increasing velocity
through the screen. Since the pressure
drop is proportional to the square of
this velocity, the differential pressure
across the screen will increase over
time as an exponential function. This
phenomenon is clearly shown in
Figure 1. Less open area also means
less dirt required to increase pressure
drop across the screen element. The
type of weave used to construct a filter
screen can affect the open area greatly
as shown in Table 1. Notice the relative
consistency in open areas of weave-wire
screens regardless of the filtration
degree while wedge-wire screens show
a sharp decrease in open areas as the
filtration degree diminishes.
Table 1. Filtration Open Area.
Figure 2. Filter components.
Technology
Algae have traditionally been one of
the biggest contaminants in raw surface
waters going to membrane systems.
Because surface water sources
such as lakes, rivers, reservoirs and
canals are dynamic, water quality can
change dramatically. Future changes to
the watershed such as land developments
or changing farming practices
can significantly alter the water quality
in both still and moving water bodies.
These watershed changes, more
often than not, increase sediment
runoff due to accelerated erosion. Not
only does the inorganic TSS increase
in the water body but nutrient runoff
also accelerates, adding to the organic
TSS load and eventually leading to the
overgrowth of organic matter causing
the condition of eutrophication.
Since the pretreatment system must
handle present water quality conditions
and anticipate possible future
degraded conditions, the controls,
along with the inlet and outlet manifolds,
are designed for the future addition
of more filtration capacity. The
filtration system is provided with a
programmable logic controller (PLC)
for system operation and monitoring
functions
Each filter is made up of the components
shown in Figure 2. Dirty water
enters the inlet flange at the bottom of
the filter housing. The water passes
into the cylindrical screen element
made of multiple 316L stainless steel layers, through the screen and out the
side outlet flange. Suspended solid
particles such as algae or sand are captured
on the inside surface of the
screen and build a filter cake. The
open area of the screen decreases as
this cake thickens, causing the water
velocity through the screen to increase,
thus increasing the differential pressure
across the screen element. A differential
pressure switch (DPS) constantly
compares the pressure inside
and outside of the screen element.
When a preset differential pressure
threshold is reached (0.5 bars or 7 psi),
the DPS signals the PLC to first open
the exhaust valve to atmospheric pressure.
This valve is connected to the
hollow 316 stainless steel suction
scanner that has nozzles that end with
a small opening (12 – 14 mm in diameter)
within a few millimeters of the
screen surface. The differential pressure
at each nozzle opening caused by
the difference between the working
gauge pressure (2.4 – 10 bars or 35 –
150 psi) and atmospheric gauge pressure
(0 bars or psi) results in a lowpressure
area in the vicinity of each
nozzle opening. This pressure differential
causes water to flow backward
through the screen in this small area at
a velocity of 9 – 15 m/sec (30-50
ft/sec), violently pulling the filter cake
off the screen and sucking it into the
suction scanner and out the exhaust
valve to waste.
While this is taking place, the PLC
starts the electric drive unit that slowly
rotates the suction scanner at 24 rpm.
This slow rotation does not disturb the
filter cake except where it is being
sucked into the scanner at the nozzles.
At the same time, the suction scanner
is moved linearly by a threaded shaft
passing through a fixed threaded bearing.
This gives each suction scanner
nozzle a spiral motion.
Figure 3. Typical installation.
When the upper limit switch is
reached by an actuator on the drive
shaft, signaling that every square inch
of the screen has been covered by nozzles
and that all debris has been
cleaned from the screen surface, the
PLC closes the exhaust valve and the
drive unit reverses to move the scanner
down to its starting position at the
lower limit switch. The second filter
will then go through the same cleaning
cycle if the system has multiple filters,
then the third, fourth and so on until all
the filters in the system have been
cleaned. Each filter takes from 15 to
40 seconds, depending upon filter
model, to complete its cleaning cycle.
At this point the system waits for
the next threshold pressure differential
across the filtration system to occur.
The filtration process is never interrupted;
therefore clean water is always
being delivered downstream even if the
filtration system is made up of only
one filter. A time backup system is
standard in the PLC control to initiate
a cleaning cycle periodically even if a
threshold pressure differential does not
occur. If one or more filters should be
off-line in a multiple system for repairs
or any other reason, the PLC will skip
those filters during the cleaning cycle
and go to the next operating filter. A
typical multiple filter installation is
shown in Figure 3.
Summary
Membrane technology has come a
long way in the past few years. System
dependability as well as durability are
increasing just as operating pressures
are decreasing.
New applications
are appearing
around the world.
With each
application comes
the need for pretreatment to
remove organic
and inorganic particles
that can
damage or at least
compromise the
membrane structure.
Membranes
can only perform
to the degree that
the pretreatment system performs. Therefore, the pretreatment
system must function adequately
and be reliable and robust.
Automatic self-cleaning screen filters
have proven their reliability and functionality
as companions to membrane
systems. With the ability to remove all
or nearly all particles greater than 10
microns in size, these filters can stand
alone as pretreatment for all but the
finest R.O. membrane systems. And
even R.O. systems need only add a
fine polishing cartridge between the
automatic self-cleaning screen filter
and the membranes to form a complete
functional and reliable water
treatment system.
Dr. Marcus N. Allhands, PE, is Senior
Application Engineer with Amiad
Filtration Systems.
Amiad is represented
in Canada by Metcon Sales and
Engineering.
Contact e-mail: metcon@metconeng.com.
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