Before we dive into how to calculate the expected TDS reading that we should be getting, let’s first get a better understanding of what TDS actually is. Firstly, TDS stands for Total Dissolved Solids. The solids being referred to are minerals, salts, metals, cations or anions dissolved in our tap water, of which there is an abundance.
Total dissolved solids (TDS) comprise inorganic salts. So when we take a reading of a sample of water using a TDS meter, we are essentially measuring all negative (anions) and positively charged ions (cations) that are present in the water. These ions include salts such as calcium, sodium, magnesium, phosphate, nitrate and silicate to name just a few. The more of these ions found in our water, the higher the TDS value and therefore the more impure our water is.
The sole purpose of reverse osmosis is to filter out and remove as many of these ions as possible before giving the water a final polish with DI resin. The results that reverse osmosis can achieve alone will differ dramatically from system to system and from household to household. There are many reasons for this, however, the two main reason being the home’s tap water TDS and water pressure.
For example, if you took two identically built reverse osmosis systems, with the same capabilities and features and used them in two different homes separated by distance, you would get two very different TDS results from their resulting product water. This is because both homes will have a different starting TDS, with different contaminants present and a different water pressure.
So to calculate the TDS we can expect from our RO systems, we need to do this on an individual level and it all starts with confirming your home’s tap water TDS. The next part that will play a big role in your RO membrane’s ability to filter out contaminants effectively, is water pressure. All RO membranes require a minimum water pressure to perform well and all manufacturers will report results based on an optimal water pressure.
The leading brands suggest that 50 psi is an optimal pressure and suggest expected salt rejection rates at these pressures of 99%.
So if we assume that we have a home with a beginning tapwater TDS of 250 ppm and the feed in water pressure to the RO membrane is at the required/optimal 50 psi, then we should expect a resulting TDS of 2.5 ppm.
This resulting TDS will vary as the TDS of your tap water varies from time to time but gives you a very good indication of what you can expect from your system. Increasing the water pressure with the use of a booster pump to somewhere between 80-100 psi can also improve the salt rejection rate that your system can achieve.
Achieving the lowest possible TDS value out of your RO membrane is very important. The lower the TDS value achieved by your RO membrane, the fewer contaminants that your DI (deionisation) resin needs to contend with. This could save you a small fortune in DI resin replacement costs in the long run. The best way to do this is to make sure that your system is receiving the recommended 50 psi minimum. If your homes tap water is well below the recommended 50 psi, you should consider upgrading your system with a booster pump, which will dramatically improve the results.