How to Measure Reverse Osmosis Filter System Removal Effectiveness

How to Measure Reverse Osmosis Filter System Removal Effectiveness

Do you want to know how effective your reverse osmosis filter is removing contaminants?

You can use a few simple tests to check the quality of your water and how well your RO filter is working. These tests will help you understand what’s in your water and if the treatment system is doing its job.

This guide will walk you through everything you need to do in order to accurately assess your reverse osmosis filter’s performance. You will learn how to calculate contaminant removal rates as well as rejection rates for your RO system. We’ll show you how to test your RO system using pH, conductivity and ORP meters as well as a turbidity meter. We’ll also explain why these measurements matter and what they tell you about your drinking water. Finally, we’ll discuss how to collect a water sample for laboratory testing and calculate contaminant removal using rejection rate calculations.

By following this guide, you’ll be able to easily measure your RO unit’s performance, even if you have no prior experience!

Testing the Performance of Your Reverse Osmosis Filter

Reverse osmosis filters remove contaminants from water using a membrane that is permeable to water molecules but not other substances. The contaminants are left behind while clean, drinkable water passes through the filter’s membrane.

In simple terms, when we talk about the treatment effectiveness of an RO filter, we’re focusing on how much of the contaminants it removes. Therefore, the final goal of a reverse osmosis water filter is to reduce the quantity, and the concentration, of contaminants as much as possible.

Some common contaminants found in drinking water are listed below. If these impurities are in your drinking water, you should have the laboratory test for them.

  • pH
  • alkalinity
  • total dissolved solids
  • chloride
  • nitrate
  • sulfate
  • phosphate
  • fluoride
  • lead
  • iron
  • sodium
  • potassium
  • magnesium
  • calcium
  • hardness
  • cadmium
  • copper
  • zinc

1. RO water testing parameters – Laboratory testing

A very effective way to assess the performance of your reverse osmosis system is to measure the concentration of each contaminant before and after treatment. The change in concentration indicates how much of that contaminant was removed, and, therefore, how effective the filter performed.

Laboratory testing is the most accurate way to evaluate how much of each contaminant is in your water. You should know what the contaminants are – they are the reason you have the RO filter in the first place.

  1. Collect a water sample of the untreated water somewhere upstream to your RO system. This is the “Raw Water” sample.
  2. Collect a water sample of the treated water downstream to your RO system. This is the “Treated Water” sample.
  3. Submit the samples to a laboratory and have them test them for the contaminants in your water.
  4. When you receive the laboratory report, calculate the removal efficiency for each contaminant.

Calculating contaminant removal efficiency with laboratory data

([Raw Water Concentration] – [Treated Water Concentration]) / [Raw Water Concentration]

This value is between 0 and 1. You can convert it to percent by multiplying the result by 100.

Example 1: Let’s assume you have an under-sink RO filter that was installed to remove lead from your drinking water. You collect an untreated water sample from the bathroom faucet – this is your “Raw Water” sample. You also collect a sample from the treated water dispenser at your sink – this is your “Treated Water” sample.

You submit this sample to a laboratory, and they report the following values:

Raw Water lead concentration = 4.5 milligrams per liter (mg/L)

Treated Water lead concentration = 0.01 mg/L

In this example, the removal efficiency for lead is 99.8%.

  1. (4.5 – 0.01) / 4.5 = 0.998
  2. 0.998 x 100 = 99.8%

2. Reverse Osmosis – Instruments for water testing

measuring TDS
A TDS meter can be used to measure dissolved solids as a way to evaluate the performance of your RO filter.

While the laboratory testing method is the most accurate, it is also expensive and slow. Another way you can evaluate how well your RO system is working is to measure your water quality using instruments.

Unfortunately, there aren’t portable instruments that can directly measure most of the contaminants that might be in your drinking water. To overcome this limitation, we can measure other properties of your water that give us an indirect measure of the contaminants. These other properties are known as surrogates.

Reverse osmosis is a purification process which removes dissolved solids in water by forcing the contaminated water through a semi-permeable membrane and allowing pure water to diffuse. Since the RO filter is removing dissolved solids (our contaminants), we can use Total Dissolved Solids (TDS) as a surrogate to estimate the concentration of those contaminants.

  1. Total Dissolved Solids: A good field measurement for approximating the level of contaminants in water is Total Dissolved Solids (TDS). When a substance dissolves in water, such as salts and minerals, it increases the conductivity of the solution. The TDS meter measures the conductivity (flow of electricity through the solution) and estimates the TDS from that reading.
  2. Conductivity: Measuring conductivity with a portable meter is an effective way to test the quality of reverse osmosis water and determine how well the membrane is performing. Ultrapure water (which is essentially what RO treated water is) is a poor electrical conductor. The presence of ionized solids (our contaminants) dissolved in water is what gives it its conductivity.
  3. pH: Very high or low pH indicates that there is the potential for scale to form on the RO membrane. Monitoring the pH allows you to adjust your water chemistry and keep the pH in the optimal range – this protects the membrane from solids deposition and buildup.
  4. ORP: Oxidation Reduction Potential (ORP) measures the amount of oxidizer, such as chlorine, in the water. Chlorine, and other oxidizers, attach the membrane which damages it and shortens its life. The ORP value can be used to monitor the condition of the RO membrane over time.

Check out the TDS meter I use on Amazon.

Calculating contaminant removal efficiency with TDS meter data

You can calculate the overall contaminant removal efficiency of your RO filter using the same procedure and calculation method described above. Simply collect a Raw Water sample (before RO treatment) and a Treated Water sample (after RO treatment).

([Raw Water Concentration] – [Treated Water Concentration]) / [Raw Water Concentration]

Example 2: Let’s use the same example as before with an under-sink RO filter installed to remove lead. You collect an untreated water sample from the bathroom faucet – this is your “Raw Water” sample. You also collect a sample from the treated water dispenser at your sink – this is your “Treated Water” sample.

You use your TDS meter to measure the concentration of both samples, and you get the following values:

Raw Water TDS concentration = 235 parts per million (ppm)

Treated Water TDS concentration = 3 ppm

In this example, the removal efficiency for lead is 98.7%.

  1. (235 – 3) / 235 = 0.987
  2. 0.987 x 100 = 98.7%

3. RO filter evaluation using your sense of taste and smell

A very simple way to measure how well your RO filter is performing is to simply taste and smell the water. Our senses are very sensitive to many of the contaminants that we are attempting to remove. In many instances, our senses of taste and smell are able to detect lower concentrations of contaminants than laboratory testing can.

If your water has a strong chlorine odor, then your RO filter is not removing this contaminant from the water. If your water has an “off” taste or tastes salty, this is an indication that your reverse osmosis system is not functioning properly.

How to Collect a Water Sample

RO Filter sampling
Collect a treated water sample from the RO system faucet.

Collecting a water sample to evaluate your RO system is simple. You need to collect two samples – one untreated sample and the second treated sample. Collect the untreated sample upstream of your RO filter.

If you have an under-sink filter, this could be your sink faucet with the diverter valve open or a hose bib on the outside of your house. If you have a whole house filter, it should have a port on the inlet to the RO unit where you can collect a sample of your raw water.

Bacterial analysis

Water that is analyzed for bacteria requires special handling to avoid contaminating the sample with germs that might be on your hands or clothing. The laboratory will provide you with detailed instructions that you should follow. It is good practice to wash your hands thoroughly with soap and water. You should also wear nitrile gloves.

Be careful not to touch the inside of the sample jar or the lid to avoid contaminating it. Purge the faucet before collecting the sample to flush away any bacteria that might be on the plumbing fixture.

General water analysis

If you are collecting a water sample for laboratory analysis, the laboratory should have provided you with glassware. Use the jars they provide for your sample.

Before filling the jar, briefly purge the faucet to allow collection of a representative sample.

Just fill up the container and cap it tightly. Be sure to label the container with your name, date, and the type of water being sampled.

Reverse Osmosis Calculation and Evaluation

There are several properties you can calculate to evaluate how well your RO filter is performing.

1. Calculating recovery percentage

Recovery is the amount of purified water that is produced. The water that is not “recovered” is dumped down the drain as waste. The recovery percentage is the ratio of treated water made by your RO system compared to the wasted water.

The higher the recovery, the less water you dump down the drain and the more purified water you have for drinking. High recovery is good.

However, if the recovery is too high, you may have maintenance problems from scaling or fouling in your membrane.

% Recovery = Clean Water Flow Rate (gpd) / Feed Flow Rate (gpd) x 100

2. Purified water production rate

The purified water production rate is related to how well your RO filter’s membrane is performing. If you have a 50 gallon per day RO filter, it can produce about 2 gallons of clean water per hour. A properly operating unit should be able to maintain this level of production as long as necessary.

By measuring the flow rate of your RO filter periodically and comparing the result to your unit’s capacity, you can monitor how well it is performing. Over time, the production rate will decline due to fouling and other problems. This is an indication that it’s time to clean or possibly replace your RO membrane. It may also be a sign that the pre-treatment filters are fouled and need to be replaced.


Reverse osmosis systems are proven to be effective at removing contaminants, but the quality of your drinking water depends on how well the RO unit is maintained. One way you can check if your RO filter is working optimally is by calculating its removal effectiveness. This can be done using either laboratory analysis, field instrument reading, or even qualitative metrics like your sense of taste and smell.

Boch Richard

Richard Boch is a chemical engineer responsible for designing water filtration systems for industrial and residential customers. He has more than 20 years of experience with ion exchange, activated carbon, and reverse osmosis. Richard's expertise has made him a go-to source for municipalities and businesses looking to improve their water quality. When he's not working, Richard enjoys spending time with his wife and two young children. You can also follow him on LinkedIn, Twitter and Facebook.

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