To troubleshoot a RO/DI system you need three TDS measurements, tap water TDS, RO only TDS before the DI and final or RO/DI TDS. Without all three you have no idea how well the membrane is performing.
A new RO membrane should be 96-98% efficient or rejection rate. To calculate the rejection rate you need the tap water TDS and the RO only TDS. The formula is tap TDS-RO TDS/tap TDSx100.
As an example say your tap TDS is about the national average of 250 and your RO only TDS is 10. So, 250-10=240, 240/250=.96, .96x100= 96% rejection rate or removal efficiency.
96% is about the bottom of where you want to be. The reason is for every 2% your rejection rate drops, your DI resin life is cut in half. Yes, every 2% drop in efficieny cuts your DI life in half so the inverse is for every 2% you can increase the membranes efficiency you double the life of your DI. It does not take more than a few DI replacements to pay for a new more efficient RO membrane so it could be a wise purchase.
For the RO/DI your goal is 0 TDS, noting else is acceptable since you have no idea what that TDS is made up of. This is important since DI resins start to release weakly ionized substances even before it is exhausted and some of those weakly ionized substances are nitrates (actually all forms of ammonia), silicates and phosphates, none of which you want. RO membranes by themselves are not particularly effective at any of those three so you need high quality DI resin and a good membrane to remove them all to reef quality levels.
Sediment filters and csrbon blocks have very little to absolutely noting to do with TDS, their sole purpose is to protect the RO membrane from sediment, silt, particulates, colloidal materials and of course chlorine. Suspended solids in other words, TSS not TDS or Total Dissolved Solids. TSS or suspended solids can be anywhere from visible to the naked eye (you can visibly see particles down to about 40 microns with the unaided eye) down to around 0.1 microns depending on the quality of your sediment and carbon filters. TDS on the other hand is not visible, are in the 0.0001 micron range, can be measured with a TDS or conductivity meter, and are the job of the membrane and DI to remove. Using a TDS reading as a gauge of filter condition does no twork.
You change the sediment andcarbon filters every 6 months like clockwork and disinfect the system with nromal household bleach according to the vendors directions to protect the RO membrane. You replace the DI resin when you first start to see anything other than 0 TDS in the finsihed water on a consistent basis. You replace the RO membrane when it is no longer cost effective to keep replacing the DI resin, usually in the 95-96% rejection rate range.
The membrane and the DI are the only things taht effect TDS so it is important to protect them with high quality, low micron replacement filters and carbons every 6 months so they can do their job.
