Membrane filtration is a type of separation process that involves passing the solvent such as water through a semi-permeable membrane. Membrane filtration has extensive application in water purification treatment because it facilitates the elimination of practically all dissolved and suspended materials from the source water.
Membrane Filtration: An Alternative to Disinfection
While we have discussed in the previous pages that the conventional methods of filtration using granular media depth filters (such as sand filter beds) follow the other water treatment processes of coagulation/flocculation and sedimentation, filtration has a major drawback: it does not provide an absolute barrier, allowing pathogens to pass through along with the processed water.
The inability of filtration to remove the significant health risks brought about by the presence of pathogenic microorganisms is the primary reason why disinfection often follows the other treatment processes. It has been established that common primary disinfection methods such as chlorination and chloramination are effective in inactivating any microorganisms that remain in the water, thereby providing an additional layer of protection for the water-drinking public.
However, it has also been found out that there are microorganisms resistant to the disinfecting chemicals chlorine and chloramine, such as the protozoan Cryptosporidium which is known to cause gastro-intestinal illnesses. Further, it’s no secret that chlorination produces byproducts that are also harmful to human health. With drinking water regulations setting maximum contaminant levels for disinfection byproducts (DBPs), some treatment plants may be looking around for other treatment options that would allow them to minimize the application of disinfectants.
One such alternative technology that could address the concerns on chlorine-resistant microorganisms and formation of DBPs is membrane filtration.
The Four Membrane Processes
A membrane filtration system can either be a vacuum- or pressure-driven process that removes particulate matter larger than 1 micrometer or micron (µm). This is done by utilizing a barrier that operates primarily on a size exclusion principle. Going strictly by the regulations stipulated by the US Environmental Protection Agency (EPA), the filtration process should have a measurable removal efficiency of a targeted organism (e.g. Cryptosporidium oocysts, and Giardia cysts) and such efficiency has to be be verified using a direct integrity test.
There are basically four treatment processes that are considered to be part of membrane filtration: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). In each of these technologies, a membrane barrier is utilized that facilitates the passage of water yet blocking out contaminants.
These four processes can be further split up between microfiltration and ultrafiltration on one hand, and nanofiltration and reverse osmosis on the other. The first pair is most applicable for eliminating larger particles, while the second two processes are suitable for the removal of salts (desalination) and smaller particles.
When operating on the basis of size exclusion, there may be some overlap in the range of pathogens that a membrane filtration technology can remove. But then again, there are other factors that can affect the ability of these four processes (MF, UF, NF, RO) to remove contaminants. For one, there are marked differences in the basic principles by which they operate.
For a better understanding, let’s take a closer look at the four membrane processes.
● Microfiltration and Ultrafiltration
Microfiltration and ultrafiltration are the two process that are most-commonly associated with membrane filtration. This is because these two remove suspended solids from water primarily with the use of a sieving mechanism that clearly defines the size of the membrane pores versus the size of the matter or pathogen targeted.
Typical microfiltration membranes can have pores ranging from 0.1 to 0.2 µm in size, although some MF membranes can have pore sizes up to 10 µm. For ultrafiltration membranes, pore sizes are generally smaller than those of MF membranes, ranging from 0.01 to 0.05 µm or even less. In fact, the lower cut-off for a UF membrane can be as small as 0.005 µm.
Note that the pore size given in microns (µm in symbol) can indicate either the nominal or absolute size. The former refers to the average pore size, while the latter means the maximum pore size. The distribution of pore sizes of these membrane processes depend on the membrane material and the manufacturing process.
While both MF and UF share a common contaminant removal mechanism, there is also a marked difference between the two: UF membranes are also associated with contaminant removal based on the concept of molecular weight cutoff (MWCO) rather than just pore size.
Expressed in Daltons (Da) which is a unit of mass, the MWCO is a measure of a membrane’s ability to filter out a particulate matter basing on the atomic weight. This essentially means that the UF process can retain solutes of high molecular weights, while water and suspended solids of lower weights pass through. If MWCO is used as the standard, ultrafiltration membranes can remove solids with weights within the range of 10,000 to 500,000 Daltons. For membranes used specifically in water treatment, the ideal MWCO is about 100,000 Da.
In summary, MF membranes are generally known to reject various particles, clay, and bacteria, while UF membranes are utilized in the rejection of macro molecules, proteins, polysaccharides, and viruses.
● Nanofiltration and Reverse Osmosis
Unlike the first two membrane technology processes, nanofiltration and reverse osmosis do not work according to the concept of pores and size exclusion. As a matter of fact, NF and RO use semi-permeable membranes that have undefinable pores. Instead, in removing water contaminants, both processes operate under the principle of reverse osmosis, using diffusion through the membrane within a pressurized system.
Briefly, here’s how such a system works.
In a normal osmosis process, the solvent (for instance, water) moves through a semipermeable membrane from an area of low solute concentration to an area of high solute concentration. This process continues until such time that an equilibrium is reached, meaning, when the solute concentration on the two sides of the membrane are equal.
A reverse osmosis system however, breaks the flow of the natural osmotic process. By applying pressure on the side of the membrane barrier that has a greater solute concentration, the water will move through to the other side, leaving the solutes behind. Under this condition, the more concentrated area will have even greater concentration of solutes over time, while the other side will have a greater volume of water and less (or possibly close to zero) concentration of the solute or dissolved solids.
Because both NF and RO make use of a semipermeable membrane that serves as a barrier against particulate matter, the two are considered as membrane filtration even if they don’t actually “filter” out contaminants but achieve rejection using the principle of reverse osmosis. NF and RO membranes are ideal in water treatment plants that call for the removal of dissolved solids such as desalination and water softening.
Membranes for nanofiltration and reverse osmosis are rated by molecular weight cut off (MWCO), where the typical ranges are 200 to 1,000 Da for NF membranes, and less than 100 Da for RO membranes. With membrane pore sizes as small as <0.002 µm (hence the membranes are said to have no definable pores), NF and RO are the tightest possible membrane processes in liquid separation.
Typical NF membranes can reject organic matter of high molecular weight component (HMWC), mono-, di-, and oligosaccharides, and polyvalent negative ions. RO membranes are useful for the rejection of particulates of HMWC (such as protein molecules) and low MWC (such as sodium chloride), glucose, and amino acids.