If the mains water pressure is good ( between 40-80 PSI ) the RO should be able to remove 95-98% of the TDS from the mains water supply.

For example if the TDS of your tap water supply is 300 you would expect a TDS of 15-6 ppm coming out of the RO membrane, assuming the prefilters are maintained and you have 40-80 PSI mains pressure.

If the TDS coming out of the unit is consistantly higher than this there could be a few causes. Always let the RO unit run for 10-15 minutes before checking the TDS. It will generally be high when first turning the unit on due to TDS creep accross the membrane.

1. The membrane is old ( one can usually get 1-2 years from a membrane before TDS starts to creep through )

2. Prefilters are old - change every 3-6 months

3. Mains water pressure is low, check by installing a pressure gauge to the unit. If below 40 PSI you may need a booster pump

4. Mains water contains high levels of dissolved CO2 , the CO2 can make it through membranes and measured by the TDS meter , this should gas off so leave a few hours before measuring the TDs, if the TDS drops it may be CO2 that will gas off after a while.

5. TDS meter requires calibration

6. Flow restrictor doesn’t match the membrane or needs replacing, this should be done every time you change the membrane

7. The membrane is not seated correctly in the housing and water is bypassing it

8. The RO unit is set up incorrectly

It is always worth noting to store the unit between uses simply make sure that the effluent and waste pipes are either turned off with inline taps or raised above the unit so it doesn’t drain as the membrane will dry out and become damaged. When turning the unit back on remove the outlet pipe form the Di resin, if using, for the first 1/2 liter of water as when the unit is off and the membrane isn’t under pressure the membrane will suffer from 'TDS creep' and the first 500ml or so will have a higher TDS than normal and this will start to use up your TDS resin early