Membrane filtration is a rapidly
expanding field in water treatment. There are many different types of filters
available in a wide range of pore sizes and configurations. In addition, there
are numerous possible applications for membrane filtration ranging from the
removal of relatively large particulate material to the removal of dissolved
compounds.
Definition
A membrane is a
semi-permeable thin layer of material capable of separating contaminants as a
function of their physical/chemical characteristics. A more common way to express this is: A
membrane is a thin layer of material that will only allow certain compounds to
pass through it. Which material will pass through the membrane is determined by
the size and the chemical characteristics of the membrane and the material
being filtered.
Comparison to
Conventional Treatment
Conventional
Filtration Design
·
Conventional
filtration relies on a number of mechanisms to remove particulate and dissolved
material from the filter influent; these mechanisms are adsorption, settling,
and straining. These mechanisms are
illustrated in the following graphic.
·
Although
conventional filtration typically produces a high quality of finished water,
the probability of capture is something less than 100%. Regardless of how well
the filter is performing, some particulate material will not be captured in the
filter.
Membrane
Filtration Design
·
Membrane
filtration is a mechanical barrier that uses a straining mechanism only to
remove material from the water.
·
If
the barrier is intact, no particles larger than the membranes pore size can
pass through the filter. This is illustrated in Figure 1.2.
·
The
process of particulate removal n the microfiltration membrane process is
through size exclusion. An exception to this is found in larger molecular
substances such as humic and fulvic
acids. Adsorptive affinity to the membrane can influence removal of these
larger molecules.
Membrane
Filtration Uses in Water Treatment.
Regulatory
Perspective
The primary use of membranes from
a regulatory perspective is in compliance with the Surface Water Treatment Rule
(SWTR) and its children, the Enhanced SWTR, the Long Term 1 and Long Term 2
SWTRs.
System
Configurations
There are two basic
configurations that could be used for a membrane filtration system:
Principles
of Membrane Filtration
Membrane filtration
is a mechanical filtration technique which comes as close to offering an
absolute barrier to the passage of particulate material as any technology
currently available in water treatment. In order to understand the concept of
membrane treatment, the concept of osmosis must be discussed.
Osmosis
is a naturally occurring phenomenon that describes the tendency of
clean water to dilute dirty water when they are placed across a permeable
membrane from each other.
Osmotic pressure
is
the pressure created by the difference in concentration of the constituents on
either side of the membrane, and this pressure drives the osmosis process.
Osmosis is not desirable from a
water treatment standpoint since the goal of treatment is to produce fresh
water and not to dilute dirty water with fresh water.
Reverse osmosis
(RO) is
the process of forcing water from the dirty side through the membrane into the
clean water side, while leaving the undesirable constituents behind on the
membrane itself.
Considerations
for RO
Head loss is pressure
drop. It is the difference between the pressure on the upstream side of the
filter and the pressure on the downstream side of the filter.
Membrane Filter
Applications
Membranes can be used for many
different types of filtration applications; most of them are not related to
potable water production. For example, they are used in industry to produce
high purity process water or to remove contaminants from waste streams prior to
discharge. In addition, membranes have applications in wastewater treatment.
Membranes are used to remove
undesirable constituents from the water. If these constituents are dissolved in
the water, very tight membranes are required; if the constituents are
particulate, then a looser membrane is appropriate.
Membrane
Filtration Summary
There are four levels of membrane
filtration. These levels are (from largest to smallest pore size):
microfiltration, ultrafiltration, nanofiltration,
and reverse osmosis. Each level has a pore size range associated with it and is
used to remove certain sized contaminants.
Figure 1.6 below displays
possible contaminants and possible filtration processes linearly aligned with
the corresponding μm and Molecular Weight
Cut-Off (MWCO), which is expressed in Daltons. This figure illustrates which
process (or processes) could be used for separate a particular contaminant.
TERMINOLOGY AND
DEFINITIONS
Flux
Flux is the flow rate
through an individual membrane filter module expressed in terms of gallons of
flow per square foot of membrane filter surface area per day.
When an operator discusses flow
rate, he or she will typically say that the plant produces so many million
gallons per day or so many gallons per minute. But, when discussing filter
operations it is common to discuss flow in terms of gallons per minute per
square foot of filter surface area (gpm/sq. ft.). In
membrane filtration, however, flow through a membrane filter is discussed in
terms of gallons per square foot of membrane filter surface area per day (GFD).
Temperature
corrected flux is
used to discuss flux in terms of a standard feed water temperature. The
standard temperature for this is 20°C (68°F). This term is useful for comparing
performance between different manufacturers’ membranes. It is usually expressed
as gfd @ 20°C.
To calculate flux, first figure
the flow rate in gallons per day and then divide by the square footage of
filter area.
Feed Water
Feed water is the influent
water for the membrane system; it is the water being added to the membrane
system itself.
Transmembrane Pressure
Transmembrane pressure is the change in
the pressure of the water as it passes through the membrane. Transmembrane pressure is referred to as TMP by most
manufacturers.
Specific Flux
Specific flux is the flux of
the membrane divided by the TMP of the membrane itself. The lower the specific
flux, the more pressure loss through the system and the more expensive it is to
operate the system.
Temperature
corrected specific flux for a membrane system is calculated by dividing a
system’s temperature corrected flux by the membranes’ TMP.
To calculate specific flux, first
calculate the flux, and then divide the flux by the TMP.
Permeate
Permeate is the filtrate
from a membrane filter. It is called permeate due to the way that the feed
water permeates through the membrane.
Cross Flow vs.
Dead End Flow
Cross flow means that a
small portion of the feed water is allowed to flow across the surface of the
membrane (rather than through the membrane).
Dead end flow means that all
of the feed water entering the membrane is passing through the filter.
Reverse Flow and
Back Pulse
Reverse flow is the process
of reversing the direction of water flow through the filter using permeate
(filtered water). The permeate removes the material deposited on the surface of
the membrane and the waste stream is collected and removed from the module.
Back pulse is a similar
process of reversing the flow direction, but is used on submersible types of
membrane filter systems.
Some membrane manufacturer
actually uses air pressure to pressurize the inside of each membrane fiber. The
air is then released, causing the membrane to quickly collapse back to its
original shape. As the membrane collapses, the material deposited on the
surface is released. A water stream then carries this material out of the
module.
Frequency and
Duration
The frequency of the RF process is
determined by the membrane filtration level selected, the characteristics of
the membrane itself, and the quality of the feed water.
Volume
As with the frequency and
duration of the RF process, the volume used to complete an RF is also
determined by the membrane filtration level selected, the characteristics of
the membrane itself, and the quality of the feed water.
Percent Recovery
It is common in membrane
treatment to refer to the efficiency of the treatment process by referring to
the system's percent recovery.
Concentrate or
Reject
Concentrate, or reject, is
the waste created from an RF of a membrane system.
Air Scrub
Air scrub, or AS, is
a process by which some membrane systems introduce air into the feed side of
the filter module in conjunction with the RF process in order to enhance the
RF’s efficiency. The air agitates the surface of the membrane and enhances the
removal of the material from the membrane’s surface.
Backwash
Recovery
Backwash
recovery is
a system in which the facility reclaims some or all of the RF waste from the
system.
Chemical
Clean-In-Place
Chemical
Clean-in-Place (CIP) is
a cleaning procedure that is used to restore the membrane’s capacity to
something near its original capacity.
New Methodology
A recently developed procedure
has been gaining favor in the world of membrane treatment. Since it is new, there has not yet been a
consensus on the terminology for referring to this procedure. Each manufacturer
refers to it in different ways, but it is ultimately a mini chemical cleaning
procedure. A solution of chemicals used to conduct the normal cleaning
procedure are mixed, perhaps at the same concentration used for a normal
chemical cleaning or perhaps at a lower concentration. The solution may or may
not be heated. The cleaning solution is added to the membrane module and it is
either circulated around the feed side of the
membrane or the membrane is just allowed to soak in the solution. The purpose
of the CIP is to return the membrane to near its original condition, but the
purpose of this mini-cleaning is to maintain the membrane in its current
condition and increase the interval between CIPs. For this reason this
procedure is referred to as a maintenance chemical cleaning.
MEMBRANE
CONSTRUCTION AND SYSTEM CONFIGURATIONS
Membrane System
Components
There are four components in a
membrane filtration system: membranes; modules; racks; and piping.
Membranes
Modules
Racks