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.
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.
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.
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 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 is the influent water for the membrane system; it is the water being added to the membrane system itself.
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 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 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.
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.
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, 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 is a system in which the facility reclaims some or all of the RF waste from the system.
Chemical Clean-in-Place (CIP) is a cleaning procedure that is used to restore the membrane’s capacity to something near its original capacity.
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.