Performance of reverse osmosis can be measured by an instrument called a TDS meter. This process uses a conductivity meter immersed in water to measure the solids dissolved in the water. The meter converts the conductivity to a measure parts per million (ppm).
What is TDS?
TDS stands for Total Dissolved Solids—tiny particles like minerals, salts, and metals in your water. Your reverse osmosis (RO) system is built to reduce most of these solids, giving you cleaner, better water. You can test how well your system is working by following the steps below. A reverse osmosis unit, when functioning correctly, will reduce 75-90% of the TDS on the water. The performance of a reverse osmosis system will vary based on the feed water:
- Temperature
- Pressure
- TDS (total dissolved solids)
What You Need to Know:
- The membrane (the middle cartridge in your system) is the part that reduces TDS in your water.
- Membranes usually last 1–3 years, depending on your water usage and quality.
- When you install new filters, you might see higher TDS levels for a few days. This is normal as carbon fines are rinsed out. Wait a few days of regular use before testing for accurate results.
- TDS includes both positive and negative minerals, such as:
- Positive ions (+): Calcium, Magnesium, Potassium, Sodium
- Negative ions (–): Carbonates, Nitrates, Chlorides
How to Test TDS Levels:
- When testing the performance of a reverse osmosis system, test both the feed water to the RO system and the product water out of the unit.
- You can check your system’s performance by calculating the TDS Rejection Percentage Rate using a TDS meter. Here’s how:
1. Test the feed water - this meter has a hold button that holds the TDS reading:
This test yielded a TDS reading of 356 ppm.
2. Test Reverse Osmosis product water:
This test yielded a TDS reading of 32 ppm.
Reverse Osmosis Performance Formula
The rejection rate is the percentage of the TDS (total dissolved solids) removed by the RO membrane on your water purification system. For example, a 90% rejection rate means that the RO product water from your RO system contains 90% less TDS than the feed water coming into the system. The formula below is used to calculate the TDS rejection between feed water and product water.
Example: