Microfiltration vs. Ultrafiltration vs. Nanofiltration vs. Reverse Osmosis

Choosing the right filtration system for your home can be an overwhelming process, especially considering how many options are available on the market. Four popular mechanical filtration processes include microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. All four processes use pressure and a semipermeable membrane that allows water to pass through, but blocks certain particles based upon their size. The main differences between the filtration types are the pore size in the membranes and what particles they block.

Microfiltration, also known as MF, employs the use of usually a plastic, polymeric, ceramic, glass, or metal membrane that has the largest pore size of the filtration processes, but requires the least amount of pressure to operate. MF pore sizes tend to be between 0.1-10.0 µm (microns), blocking particles such as bacteria, yeast cells, fat, small colloids, and other suspended solids. Some of the popular applications for microfiltration include:

• Pretreatment for other filtration processes, such as nanofiltration and reverse osmosis
• Water and wastewater treatment
• Food and beverage industry, such as juice, wine, dairy, and beer
• Textile industry
• Effluent treatment

Microfiltration has the advantages of a low-operating pressure, generally only requiring the water pressure of your home's plumbing system, as well as low energy consumption, almost no manual operations required, and relatively inexpensive compared to other systems. However, microfiltration has some disadvantages, including sensitivity to oxidizing chemicals and hard and sharp particles, and only filtering out bacteria and suspended particles and not viruses or dissolved solids.

Ultrafiltration, or UF, is very comparable to microfiltration, filtering out the same particles but also filtering viruses. Ultrafiltration tends to be the preferred method for separating oils from water over microfiltration as well, being the more effective solution. Ultrafiltration gets its increased filtration from a smaller pore size, generally between 0.01 to 0.1µm. While ultrafiltration requires more pressure to properly operate than microfiltration, the pressure from your home's plumbing system is usually still sufficient, meaning you do not need to install an outside pressure source like a pump. Typically, this filtration method is the best choice for those who desire microscopic contaminants reduced from water, but want the healthy minerals to remain. Some of the other typical applications for ultrafiltration include:

• Pretreatment for other filtration processes, such as nanofiltration and reverse osmosis
• Water and wastewater treatment
• Food and beverage industry, such as juice and dairy
• Biotechnology and chemical processing
• Concentrating proteins
• Separating oil/water emulsions

As mentioned before, a big advantage of ultrafiltration is the ability to operate at a lower pressure as well as the fact that while it removes bacteria and viruses, healthy minerals remain intact. It also installs quickly and easily, requires a lower investment cost and reduced operating costs, does not produce wastewater, does not require coagulants, and requires little chemical usage. However, ultrafiltration membranes are susceptible to fouling (membrane pore blockages) that do require some cleaning measures to maintain. UF does not remove salts, fluoride, or TDS dissolved in water.

Nanofiltration, or NF, uses membranes that have a pore size of 0.1 to 10 nm (nanometers). Often referred to as “coarse” or “leaky reverse osmosis,” nanofiltration allows a greater selectivity of what is filtered while requiring a lower pressure rate than reverse osmosis. In general, it is used to greatly reduce nitrates, sulfates, tannins, turbidity, color, TDS, and moderate levels of salt. Nanofiltration is also a popular solution for water softening. NF common applications include:

• Pretreatment for reverse osmosis
• Water and wastewater treatment
• Water softening
• Salt reduction
• Removal of specific heavy metals from process streams for water reuse
• Pharmaceuticals
• Textile industry
• Food and beverage industry, such as dairy and bakeries

The required operating pressure, investment and maintenance costs, and maintenance in general are comparatively lower for nanofiltration than they are for reverse osmosis. NF does not require special treatments or mechanical stirring. However, compared to microfiltration and ultrafiltration, nanofiltration energy consumption and filter replacement costs are more expensive. NF membranes also tend to cost more than reverse osmosis membranes. Pretreatment is required for nanofiltration to operate correctly.

Of the four filtration methods, reverse osmosis is the most complete. With a pore size of 0.0001 micron, reverse osmosis, or RO, removes organic molecules, viruses, monovalent ions, turbidity, and even minerals, essentially resulting in pure water. The resulting water is referred to as permeate, and the water and contaminants that are left over are called the brine.

While RO is still a mechanical filtration method like microfiltration, ultrafiltration, and nanofiltration, it requires a little more process and pressure to work. First, you need to understand the process of osmosis. For example, if you have two sections of water separated by a filter membrane, and one side of water is pure water and the other is heavily salted, the pure water side will migrate through the membrane to the salted size to try to equal out the amount of water to salt ratio. Reverse osmosis, like the name implies, reverses this process, using pressure created by a pump to force the salted water into the pure water side, while having the membrane filter out the salt, or whatever other contaminants may be present in the water.

Because RO membranes filter our so many contaminants, they are highly susceptible to fouling and do require pretreatment filters to prevent damage. As a result, most home RO systems consist of 3 to 5 stages of filtration, combining sediment, carbon, and RO membrane filters.

Because of the way reverse osmosis works, it can be used for desalination, recycling, wastewater treatment, and can even produce energy. Some parts of the world use RO to turn saltwater into drinkable water. For example, 17% of the drinking water in Perth, Australia, is desalinated sea water from a reverse osmosis plant. Examples of the common applications for reverse osmosis include:

• Water and wastewater treatment
• Food and beverage processing, such as maple syrup, dairy, and wine
• Water softening
• Cosmetics
• Pharmaceutical production
• Chemical recovery
• Desalination

So reverse osmosis is a highly effective option to reduce unwanted contaminants and achieve purified water, as well as a very versatile filtration method. It also can be used at relatively lower temperatures than other methods. However, RO also requires more pressure, and thus energy, to operate. In addition to higher energy consumption, it also tends to have higher maintenance costs requiring pretreatment to properly function. RO also produces a high amount of wastewater as it sends contaminants down the drain. As it removes the unwanted contaminants, reverse osmosis also removes healthy minerals like calcium, magnesium, and potassium, and results in a highly acidic water that is harmless to consume, but can be corrosive to plumbing over time.

Overall, installing a filtration system in your home can help ensure your family has access to cleaner, healthier water. What filtration system is best for your home between microfiltration, ultrafiltration, nanofiltration, and reverse osmosis can come down to pressure requirements, costs, maintenance, and what contaminants you are looking to filter.