High TDS with new RO membrane | The Aquarium Solution

High TDS with new RO membrane

Question:

Dear Sir,
I have just purchased a replacement cartridge for a DD-50g RO Unit from 1,2,3 acquatics online.
They have no knowledge of TDS and asked me to contact you instead.
I have just replaced a 3 years old membrane after discovering that TDS reading were up to 12. Once I received the new D-D membrane (new and sealed), I fitted it and let it run for about 30mins.
I took a TDS reading on this new membrane with your TDS meter and it was 25 !!
After I let it run for a few good hours it came down to 16, but this is still way higher than the "0" reading I was expecting.

Since my previous membrane was working under the same conditions (pressure,temperature,mains TDS)but performing better even after 3 years,I have reasons to believe that the readings are so high on the new membrane because it is a faulty one.
Installation was done correctly; membrane was inserted in the case with black rubber lip/seal on the same side of the screw on cap.

I am happy to give you operating conditions of the membrane even if as I said it is the same as the old one:
Temperature: 10 degree celsius
TDS : 310
Pressure: 52 psi

I very much look forward to hearing from you,
Regards,
Claudio

Answer:

Claudio
I am not familiar with Tatsfield Aquatics who apparently trade as 123 aquatics.
By doing a quick calculation on your figures it suggests that the membrane is running at 95% efficiency and at 12 was about 96%. There was probably little wrong with the original membrane even after 3 years.

The membranes are tested on a NaCl salt solution and will give between 96 and 98% rejection however certain elements in tapwater are passed more freely than the standard salt solution which will add to your TDS.

You will always find that membranes improve with time as the chemicals that the membranes are impregnated with an antibacterial solution.

The standard Filmtec Membrane is tested as follows

Filmtec TW30-1812-50 50 psi 50 >98%

* Capacity (permeate flow) and salt rejection based on the following test conditions: 250 ppm softened tap water, 77°F (25°C), 15% recovery and the specified applied pressure. Capacity for individual elements may vary +/-20%

** Minimum salt rejection is 96.0%.

In real life we are normally working well below the ideal temperature of 25C (when has your tap water ever been 25C)

You are starting with quite a high TDS at 310 as my water is only 160 in comparison which will mean that you will have an increased level of TDS in the pure water.

There is also the chance that your TDS meter requires calibration. I would recommend using a post DI resin pod to polish the water and to take the level to zero. I believe your membrane will continue to reduce slightly and them plateau.

I hope this gives some explanation for what you are seeing

Regards
Stuart