High TDS and new RO membrane | D-D The Aquarium Solution
Question: 

Hi there,

I have recently changed my Reverse Osmosis membrane and after a few days of it running, the TDS (which was initially very high) has dropped to 47.  Is the issue a faulty membrane, or could it be something else? It is a 50g unit and both of its cartridges are new. The TDS of my tap water is roughly 330. The flow restrictor valve is set at about 45 degrees.

Kind Regards, Giovanni

Answer: 
Dear Giovanni
 
There are a few things that may upset the TDS ratio of an RO unit, I will list below:
 
  1. The incoming water pressure is low and not putting enough pressure on the membrane, typical we are looking at a mains water pressure of 50 psi and up to around 80 PSI for best performance. Low water pressure can lead to TDS creep across the membrane and lack lustre efficacy. We would expect a TDS of your product water to be between 17.5 – 8ppm when tested on a freshly calibrated TDS meter if the unit is operating efficiently. This would be pre an Di resin stage if installed.
2. Incorrect or broken flow restrictor – The flow restrictor needs to be specced for the correct membrane, if the flow restrictor model is incorrect this will effect performance. 50 Gallon per day RO unit  = 300 ml/min flow restrictor
 
3. Prefilters – If the pre filters were installed at the same time and the unit wasn’t flushed before use carbon dust liberated from the carbon filter may have been forced into the membrane upsetting TDS readings.
 
4. The rubber gasket on the body of the membrane or one of the o-rings has been pinched or damaged during installation
 
5. Allow the unit to run for at least 10 minutes before testing TDS, if the unit has been turned off and not in use TDS will creep across the membrane during this period of low pressure, once the unit has been pressurised again and run for 10 minute this initial spike will drop again 
 
I hope this helps Tony