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Filtration Processes
Introduction
Filtration is a process that
removes particles from suspension in water.
Removal takes place by a number
of mechanisms that include straining, flocculation, sedimentation and surface
capture.
Filters can be categorised by the
main method of capture, i.e. exclusion of particles at the surface of the
filter media i.e. straining, or deposition within the media i.e. in-depth
filtration.
Strainers generally consist of a
simple thin physical barrier made from metal or plastic.
In water treatment they tend to
be used at the inlet to the treatment system to exclude large objects (e.g.
leaves, fish, and coarse detritus).
These may be manually or
mechanically scraped bar screens.
The spacing between the bars
ranges from 1 to 10 cm. Intake screens can have much smaller spacing created by
closely spaced plates or even fine metal fabric.
The latter are usually intended
to remove fine silt and especially algae and are referred to as microstrainers.
Filters, as commonly understood
in water treatment generally consist of a medium within which it is intended
most of the particles in the water will be captured.
Such filters might be
manufactured as disposable cartridge filters, which can be suitable for domestic
(i.e. point-of-use treatment) and small-scale industrial applications.
Larger forms of cartridge filters
exist which can be cleaned.
One version is precoat filtration
in which a porous support surface is given a sacrificial coating of
diatomaceous earth, or other suitable material, each time the filter has been
cleaned.
Additionally, a small amount of
the diatomaceous earth is applied continuously during filtration.
However, in most cases, filters
used in municipal water treatment contain sand or another appropriate granular
material (e.g. anthracite, crushed glass or other ceramic material, or another
relatively inert mineral) as the filter medium.
Filtration using such filters is
often referred to as in-depth granular media filtration.
Granular media filters are used
in either of two distinct ways which are commonly called slow-sand filtration
and rapid gravity or pressure filtration.
When the filters are used as the
final means of particle removal from the water, then the filters may need to be
preceded by another stage of solid-liquid separation (clarification) such as
sedimentation (Sedimentation Processes), dissolved-air flotation (Flotation
Processes) or possibly a preliminary stage of filtration.
Other processes take place in
vessels similar to those used for granular media filtration, and in some
respects the processes do have similarities with filtration but filtration is
not their sole or primary purpose.
Therefore, such processes are not
considered further in this article. Examples include vessels filled with
granular activated carbon for removal of dissolved organic substances, and
vessels filled with ion exchange resin for removal of inorganic and organic
ions.
There are applications of filters
that whilst filtration (removal of particles) does take place a secondary
process is intended to also occur, e.g. iron and manganese removal, and arsenic
removal.
Strainers
There is a vast variety of
strainers with respect to how the straining is carried out, with and by what
(Purchas, 1971).
The straining part might be made
of metal or other inert material e.g. plastic, cotton or a ceramic.
If metal, it could be simply a
perforated sheet, a grid of rods, a stack of discs or woven wire. If plastic,
it could be a grid, woven or simply a fused felt.
In cartridge filters the usually
disposable cartridge might simply consist of a porous and non-compressible
material or be cord wound on a cylindrical support.
Cartridge filters find
application generally in small scale applications such as for domestic point-of-use
water treatment.
Only a few types of strainers are
likely to find application in municipal water treatment.
Some require manual cleaning
others are cleaned mechanically and even automatically when the pressure drop
across them reaches a specific value.
A water treatment works might
have a simple bar strainer at its inlet to keep out logs, large fish and
swimming animals.
Next there might be a fine
strainer with its aperture small enough to exclude all but the smallest of
fish, leaves, clumps of algae etc.
Generally, this strainer would
have to be automatically cleaned.
Where algae might be a distinct
problem then the bar strainer might have closely spaced bars and be
automatically cleaned followed by a microstrainer.
One particular type of mechanical
strainer has found limited application in smaller municipal water treatment
works.
The straining medium is a bundle
of fibres. In filtration mode the bundle is twisted tight. In the wash mode the
bundle is untwisted and the trapped detritus removed by reversing the flow of
water.
Precoat Filters
In precoat filtration a thin
layer of an inert medium is laid down on a support structure to provide a
porous straining surface.
The precoat layer might be
created with loose fibres or powders (Purchas, 1971).
A small quantity of the precoat
or other similar material might be added continuously during filtration such
that some in-depth filtration also then takes place.
When resistance to flow becomes
too great then the accumulated detritus and inert medium are discharged and the
cycle repeated. In most instances the precoat material is used just once and is
not recovered and recycled.
Precoat filtration is unlikely to
be used in conjunction with coagulation and therefore its application in
municipal water treatment is very limited.
Slow Sand Filters
In slow sand filtration the rate
of filtration is intentionally slow with use of sand that is smaller than sand
used in rapid sand filters, so that particles are not driven far into the bed
of sand held within the filter shell.
The principal mechanisms taking
place in slow sand filters is accumulation of a layer of debris on the surface
of the filter (straining) and capture within about the top 20 cm of the sand.
This debris is allowed to develop
biological activity which contributes to the treatment of the water passing
through it.
This biologically active layer is
often called the ‘schmutzdecke’.
Because the filtration rate is
relatively slow the resistance to flow through slow sand filters develops
slowly and may take up to 3 months before it becomes unacceptable.
Because filtration rate is slow a
large area for filtration is needed.
Consequently, the large filters
are cleaned by removing the schmutzdecke with about 5 cm of sand usually by
mechanical means.
Eventually the depth of sand
remaining becomes too shallow and the remaining sand is removed, cleaned and
replaced with additional clean sand back to the original starting depth.
Slow sand filtration was the main
method of filtration of potable water before rapid sand filtration was
developed.
Although it has a large
footprint, many slow sand filters are still used.
Developments to make them more
cost effective have included:
· Sand removal, washing and
replacement have been mechanised as much as possible.
· The need for sand removal has
been made as predictable as possible so that the equipment and labour is
efficiently utilised.
· Filtration rates have been
increased as much as possible to improve the economics and contribute to
predictability of need for sand removal.
· Pre-treatment, including raw
water storage and management, is applied to reduce the impact of solids in
suspension and contribute to predictability.
· Granular activated carbon has
been used in some filters to replace the lower part of the sand to help with
removal of pesticides, taste and odour and other trace organic substances that
the biological mechanism does not deal with effectively.
There are two important
requirements for slow sand filters to function properly.
Firstly, the water entering the
filters must not contain any disinfectant or other chemical that might
interrupt the biological activity of the schmutzdecke.
Secondly, if pre-treatment is
carried out with coagulation then most of the resulting floc particles must be
removed as part of the pre-treatment, otherwise the floc will accelerate the
rate at which resistance to flow through the filter develops.
Rapid Gravity and Pressure Filters
In-depth granular media
filtration can be carried out under gravity (rapid gravity filtration) or under
pressure (pressure filtration).
The basic mechanisms of particle
removal are fundamentally the same in both gravity and pressure modes.
The principal differences between
the two modes are likely to be hydraulic, notably distribution of flow between
filters and control of flow through individual filters.
The filter media is usually sand,
but other relatively inert material can be used, but the choice depends on
costs and what other objectives there might be.
In some cases, part of the sand
might be replaced with anthracite.
The lower density of the
anthracite allows a larger grain size to be used such that after backwash the
larger anthracite sits on top of the smaller sand.
In this way filtration takes
place through first a larger and then a smaller media to help make better use
of filter bed depth.
The principal mechanism of
in-depth filtration is surface capture. The area of media available for surface
capture depends on both media depth and size.
Depth and size also govern the
space available for storage of captured detritus.
Grain shape of the filter media
also affects capture and storage, in that angular particles are preferable to
rounded particles.
The choice of size has to take
account of how quickly the medium might become blocked by captured detritus and
the ease with which it can be backwashed.
Regardless of the choice of media
material, size tends to be limited to the range 0.5 to 2.0 mm.
The greatest application of
in-depth filtration in municipal water treatment is after coagulation, perhaps
also with prior clarification.
The choice of coagulation
chemistry, its application and any clarification, govern the nature and
quantity of the particles to be removed by the filtration, which in turn affect
the choice of filter media, depth and filtration rate.
In potable water treatment,
in-depth filtration is often the last, and sometimes the only, physical barrier
to particles.
Therefore the performance
reliability of the filters is important in ensuring the quality of the water on
completion of treatment complies with the standards.
The standards defined by the
relevant regulations have become substantially more rigorous as they have
developed over the past 50 years.
Reliability of exclusion of
Cryptosporodium oocysts has been of particular concern.
The bed of granular filter media
is cleaned by applying backwash.
This generally involves: draining
down the water until its upper surface is at about the same level as the top of
the media, loosening the bed with air (air scour), applying water upwash at a
rate great enough to just fluidise the functional part of the bed of filter
media, allow a short interval for the media to settle, and starting to refill
the filter with water from above the bed whilst opening the outlet so that
filtration starts slowly.
A more rigorous backwash can be
achieved if the water upwash is started at a reduced rate whilst the air scour
is occurring (combined air-water wash).
Older filter installations
sometimes have other features like mechanical rakes or surface flush that
operate during upwash.
The viscosity of water depends on
water temperature. Therefore, it is important that the rate of upwash takes
account of water temperature to ensure the filter media is fluidised.
It is usual to have at least four
filters, so that the filtration can continue whilst one filter is backwashed.
Large treatment works have many
more than four in a group, and possibly two or more independent groups of
filters.
Problems with operating in-depth
filters include:
· Loss of media during backwash,
· Ineffective backwashing resulting
in mud-binding of the media and its associated symptoms.
· Short filter runs due to either
rapid rate of headloss or early breakthrough of particles.
These are usually indicators of
the likes of incorrect upwash rate, problems with the underdrain system,
excessive dosing of polyelectrolyte, presence of filter-blocking algae,
inappropriate choice of either or both filter media size and depth, or simply
either or both inadequate prior coagulation and clarification.
Trouble-shooting should also
check to what extent distribution of flow between filters in a bank or group is
equitable or not.
Novel Forms of Granular Media Filters
There are a number of relatively
novel forms of granular media filters.
Each is a 'horse for a course'
having its specific set of advantages and disadvantages and therefore relative
appropriateness for certan applications.
Upflow filters
In normal in-depth granular media
filtration the flow of water is down through the filter bed, except during
backwashing.
Upflow of water during filtration
is possible; it offers an advantage but also poses problems.
With backwashing of the filter
media, normally the media is encouraged to stratify with the largest and
densest material towards the bottom of the filter bed and the smallest and
lightest towards the top.
This means that in downward
filtration, the filtration is progressively through increasingly larger media,
unless the media is tightly graded before installation.
This contradicts the ideal bed
geometry of filtration through progressively smaller media. It follows that
that one way of avoiding this situation is to filter upwards.
Upward filtration allows the
capacity of the media to collect and store solids to be exploited better.
However, as the filter bed
accumulates deposit and the resistance to flow through it increases the bed
progressively becomes more likely to be hydraulically disrupted.
Two approaches have been used to
restrict this hydraulic disruption.
The Immedium filter uses a simple
metal grid about 15 cm below the top of the bed to help keep the bed compacted.
The Biflow filter applies
downflow filtration to the top of the bed to keep the lower part with upflow
filtration compacted.
A reservation for the use of
upflow filters as the final stage of solids removal in potable water treatment
is that backwash flow is in the same direction of filtration. Another
reservation is that filter breakthrough can happen suddenly.
Consequently, upflow filters are
more likely to be found in applications where protection of treated water
quality does not have to be as rigorous as required for potable water
treatment, although they might be appropriate to use as a clarification stage
prior to normal in-depth filtration.
a. Immedium filters
The Immedium filter was developed
in the Netherlands in the 1960’s.
The key feature is the use of a
simple metal grid across the filter bed about 15 cm below the top of the sand.
The grid delays the onset of
breakthrough of particles in the water.
The grid helps to maintain
compaction of the sand and delays the start of localised penetration of flow as
the water finds paths of least resistance through the sand.
A point is reached when the flow
through such a low resistance path is too great for particles to be removed and
is great enough to fluidise the sand in the upper part of the flow path.
This can be observed at the upper
surface of the bed by the appearance of ‘blow holes’.
b. Biflow filters
The Biflow filter was developed
as an alternative to the Immedium filter.
As the name implies, flow for
filtration is in two directions.
The larger proportion of flow is
upwards from the base of the filter bed, whilst the smaller proportion is
downwards from the top of the filter bed.
The two flows meet a short way
down the bed where there is an outlet grid across the bed.
When the filter needs washing
both flows are stopped and air scour applied for a few minutes before water
upwash is carried out to wash out the detritus.
Combined air and water upwash can
be carried out only if the filter has been designed for this.
c. Buoyant media filters
Whilst in Immedium and Biflow
filters the filter sand is kept compacted, in buoyant media filters the media
is chosen to be buoyant and is retained in the filter by a straining mesh above
the media.
The media is selected to have a
low density and accordingly is usually a plastic.
During the filtration mode the
media is in a compacted state under the retaining mesh.
When the media needs to be washed
to clean out the captured detritus, the upflow rate is reduced to release the
compaction and air is bubbled up through the bed.
Buoyant media filters have been
used in water treatment as a clarification stage prior to normal filtration
d. Moving bed filters
All the granular media filters
described above have flow through for filtration stopped whilst they are
backwashed.
In a moving bed filter, the
filter media is constantly moving so that filtration is not interrupted for the
sand to be backwashed.
The sand in the filtration zone
slowly moves downward due to its own weight against the upflow of the water
being filtered.
In the conical base of the filter
the sand is hydraulically carried into a vertical tube up through the centre of
the filter bed.
As the sand is carried up through
the tube the filtered deposits are released.
At the top of the tube above the
filter bed the sand settles out from the wash water and feeds back to the top
of the filter bed whilst the dirty wash water is kept separate from the
filtered water emerging from the top of the filter.
In order that the proportion of
water lost in the wash stream is kept small, moving bed units should be
operated close to design capacity.
Cell filters
There is a maximum size to which
a normal filter can be built if the whole of the filter bed is to be backwashed
at the same time.
If the filter bed can be backwashed
in sections then the filter shell can be larger.
A bed can be backwashed in
sections by having the filter bed divided by walls from the filter floor to
just above the bed so that a hood can be placed over the section to be
backwashed.
The hood is mounted on a gantry
that runs on rails along the top of the main side walls of the filter.
This approach results in reduced
civil engineering costs but greater mechanical engineering costs, compared to a
larger number of filters of equivalent total filtration area.
The operational reliability of a
cell filter depends much on the functioning of the gantry and hood system and
how effectively the hood seals with the walls of a cell.
Automatic backwash filters
As deposits accumulate in a
filter bed the resistance to flow through the bed increases.
Flow can be kept constant by
having an outlet valve that is progressively opened and provide less resistance
to flow through it to compensate for the increased resistance to flow through
the bed.
In this way the level (head) of
water above the media remains relatively constant.
Alternatively, the flow to the
filter is kept constant and the flow through the filter remains relatively
constant with the level of water above the bed increasing.
If the filter is contained in a
deep shell then the increasing level of water can be used to prime a siphon.
When the level reaches a
predetermined level the siphon is activated and is used to draw water up
through the filter to cause backwash.
A risk is that the upwash rate of
water may be inadequate for effective backwashing.
However, the design lends itself
to package plant and situations where quality and quantity of particles to be
removed remains relatively constant.
The design is unlikely to be
suitable for potable water treatment.
Horizontal and radial filters
a. Horizontal filters
Instead of the flow of water
being up or down through a filter bed, it can be horizontally across through
the bed.
If the filter bed is contained in
a rectangular tank then the filtration rate remains constant along the length
(inlet to outlet) of the filter.
The filter can be backwashed
hydraulically as required.
It would be necessary for the
main filter material to be as uniform in size as possible so that there is not
a distinct bias through the depth due to stratification of the media by size by
the backwashing, or the backwash is arranged to keep the media mixed.
A horizontal filter could be
split into two or more sections each with a different size media, with a
vertical mesh between each to keep the different size media separated.
The backwash of each section
would need to take account of this.
Horizontal filters have been used
filled with gravel (pebbles) of selected sizes in third world situations for
use as clarifiers.
Because the size of the gravel
precludes normal backwashing, they filters are routinely cleaned by draining
and hosing and occasionally by removing the gravel for washing.
b. Radial filters
A radial filter is a horizontal
filter but with increasing width of filter bed in the direction of flow.
The ultimate shape of the filter
bed is annular in cross-section with flow from the centre to the periphery.
The rate of filtration decreases
as the water progresses through the filter media so allowing progressively more
efficient removal of particles.
Membrane Filters
Historically cloth has been used
to filter water.
In microstraining the water is
filtered through fabric made from finely woven wire. In both these cases the
cloth or fabric is a kind of membrane, albeit a coarse one.
Modern technology allows
manufacture of membranes from synthetic materials, to be less than about 1mm
thick and be semi-permeable.
Being semi-permeable means that
the membrane is selective in what submicron-size particles can and cannot pass
through it that is in the feed stream.
During operation, permeable
components in the water pass through the membrane with the water whilst
impermeable submicron-size components are retained on the feed side.
Consequently, the product stream
is relatively free of the impermeable components and the waste stream is rich
in impermeable components.
Flow of water through such a
semi-permeable membrane is achieved by pressure, usually produced by pumping.
There are four categories of
membranes loosely defined by the types of materials rejected, operating
pressure and nominal pore size.
The categorisation of pore size
is approximate since, for example a high-end UF membrane can have similar
permeability to a low-end NF membrane:
· Microfiltration (MF) -
approx 0.1 µm pores: impermeable to particles, algae, animalcules and bacteria
· Ultrafiltration (UF) – approx
0.01 µm pores: impermeable to small colloids and viruses
· Nanofiltration (NF) – approx
0.001 µm pores: impermeable to dissolved organic matter (DOM) and divalent ions
· Reverse osmosis (RO) –
effectively non-porous: impermeable to monovalent ions
The predominant mechanism in MF
and UF is straining, or simple size exclusion.
In NF and RO separation of
dissolved species involves mass transfer, a process of diffusion that depends
on concentration, pressure and rate of flow through the membrane (flux).
Consequently, membrane filtration
usually refers to MF and UF but not NF and RO, whilst NF is usually considered
to be a form of RO.
The thickness of membranes means
that they have to be formatted in a way that provides structural strength, so
they will not collapse because of the pressure difference across them, provide
a large area for filtration but are compact and can be cleaned effectively.
They are generally structured as thin
tubes (hollow fibres) or as a coiled sheet.
A coil is a sandwich of the
semi-permeable membrane, a separating mesh, a thin sheet of impermeable
material and a second layer of thin mesh.
The layers of mesh provide the
channels for flow to the inlet and from the outlet side of the membrane.
It is usual to include a
preliminary stage of treatment before membrane filtration to protect the
membrane from being fouled too rapidly by excluded material, although there are
also ways to operate membrane filters to slow the rate of fouling of the
membrane before having to apply a cleaning process.
The routine, and frequent,
cleaning process is flushing to remove the accumulated detritus on the feed
side.
However, over time there is a
slow loss in membrane performance that can only be recovered by chemical
cleaning.
Membrane filtration (MF, UF and
low end NF) have become relatively common in potable water treatment, such as
for removal of colour from otherwise relatively good quality water so avoiding
complexities associated with coagulation, and for reliable exclusion of
Cryptosporidium.
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