With more than 30 years’ experience in the residential and commercial water treatment space, Mark Nelson is a Class 1 Drinking-Water Operator and a CBWA (Canadian Bottled Water Association) Certified Plant Operator. As founder and president of Nelson Water in Ottawa, Mark focuses on dealing with challenging water treatment system designs for problem water. He also heads the largest water bottling plant in the city of Ottawa with a delivery network throughout the Valley.

Reverse osmosis (RO) water is virtually pure, the RO filtration process removes up to 99% of contaminants, including microorganisms, sodium, chloride, copper, lead, and many others. At the heart of this system, there is a semi-permeable membrane filter with tiny pores that prevent contaminants from passing through. But, this is only a brief explanation, and the way that RO filter membranes work is very interesting. In this article, we will take a close look at the RO filtration system and the important role that the filter membrane plays in it.

What is a RO Filter Membrane?

Reverse osmosis uses the RO membrane as a separation filter for water treatment. The basic premise is simple; the water passes through the RO filter membrane, which has tiny pores. These pores intercept the microorganisms and other impurities. The molecules will penetrate the membrane filter surface for efficient filtering. The reason why this process is called reverse osmosis is because that’s the principle that’s being used here.

Reverse osmosis is a pressure differential that is used to drive apart a solvent from a given solution and then apply that material on the outer surface of the membrane. When the pressure is higher than the osmotic pressure, there is a reversal in the direction of the liquid. This is how the permeated solvent is left behind on the lower pressure side of the membrane. This is known as the permeate, and on the high pressure side, there is a concentrated solution.

As an example: when seawater is filtered with reverse osmosis, fresh water is created on the low-pressure side, and the salty brine is obtained on the high-pressure surface. This may seem complex, but all you need to know is that the contaminants are left behind, and virtually pure water passes through.

How Does the Reverse Osmosis Process Work?

As you can see from the explanation above, a certain amount of external force must be applied to the incoming solution on the outer side of the filter membrane. The pressure must be higher than the osmotic pressure to reverse the natural flow of the water. The action of this process will force the water through the tiny pores of the semi-permeable membrane filter. These pores are extremely small, they are only 0.0001 microns in diameter, and considerable pressure is required to move a solution through them. To put this into some perspective, each pore in a RO membrane filter is 39 millionths of an inch in diameter, and a human hair cross section is 50-70 microns wide! So, the filter membrane pores are invisible to the naked eye, but microorganisms such as bacteria and viruses are much larger. This is important because only water molecules and certain trace minerals can pass through the membrane filter. This removes a wide variety of contaminants, including organic compounds, bacteria, colloids, inorganic salts, viruses, heavy metal ions, and more.

What Happens to the Source Water Contaminants?

The source water contaminants are left behind on the outer surface of the filter membrane. These impurities include bacteria, chemical ions, virions, fungi, and more. They cannot pass through the tiny filter membrane pores and they remain on the side of the filter when the thicker liquid is effectively dissolved. This process is widely used in the desalination of sea water and certain other industrial processes. But, in a home setting, it leaves the homeowner with a membrane filter coated with waste contaminants. To deal with this problem, the RO filtration system will periodically flush the semi-permeable membrane surface to clean it. The contaminants are then flushed away into the drain, where they cause no problems.

Protecting the RO Filter Membrane

The RO filter membrane works on a simple principle. It’s a purely mechanical filtration system, and no chemicals or other additives are added to the water to purify it. But, this is a sensitive component in the system, and it is vulnerable to damage from chlorine and sediments. For this reason, the filter membrane is only one of 3-4 or more filters in the RO filtration system, depending on the exact make and model. The other filters are designed to protect the filter membrane and to add some extra character to the final product. In a typical system, the filter configuration would follow the pattern below, with each filter installed in series with the next:

  • The Sediment Pre-Filter: This initial filter removes the dirt, dust, sand, and small rock particulates that could cause physical damage to the RO filter membrane.
  • The Granular Activated Carbon (GAC) Filter: This filter removes the chlorine and the chlorine in chloramine (chlorine mixed with ammonia) from the water to prevent chemical damage to the RO filter.
  • The Semi-Permeable RO Filter Membrane: As we’ve shown, this is the main filter that removes the contaminants from the incoming water supply.
  • A Second GAC Filter: This filter adds some character and a final polish to the purified RO water to make it more palatable.

RO Filter Membrane Maintenance

Like any system, the RO filtration system requires a certain level of maintenance to maintain normal purification processes. The membrane element can lose efficacy and efficiency over time, and a filter replacement may be required. Let’s take a look at three RO filter maintenance tips:

1.   Cleaning the RO Filter Membrane

After an extended period of normal operations, a RO filter membrane element may be polluted with insoluble or suspended matter that isn’t flushed away. This may exist in the feed water too, and under these conditions, the performance can drop by 10% or more. The membrane filter can be washed and dosed with a chemical cleaner to restore the lost filtering efficiency. There are many products on the market, and cleaning may even prolong the useful lifespan of the RO filter membrane.

2.   Out of Service Issues

If the RO filter system is out of service for more than 30 days, it can become polluted. Adding a 1% sodium bisulfite solution to the filter membrane surface will prevent bacterial reproduction, and this can be followed up with regular cleaning.

3.   Short-Term Shutdowns

If the RO filtrations system is not in use for a shorter period of time, it should be flushed. As a rule of thumb, flush the system for every five days of inactivity. During the flush, close the value to limit the potential for blockages and fouling.

If you’re interested in a RO filtration system for your home or your existing systems need some essential maintenance, contact your local water treatment specialist.