Membrane Efficiency

A friend of mine who is also engaged in water business, asked my help about membrane efficiency.

He used the Filmtec LE 4040 to a feed water that has a TDS of 900. As per test protocol, said membrane has the following data:

NACL – 2000 ppm
Production- 2500 GPD
Applied pressure- 150 psi
permeate flux- +/- 20%
recovery- 15%
efficiency- 99%

He told his client that once the membrane is used, product TDS will be 5 to 10 ppm.However when the membrane was installed,it yielded a product TDS of 25 ppm.

His client reacted and commented that there is something wrong with the membrane because it did not give the expected result.

The membrane was bought to a supplier here in Manila and when he discussed the issue,the supplier told him that maybe there is a problem with the system that is why it has a high product TDS.

As I have reiterated before,most of the times, we are dealing with the membranes blindly.

The test protocol of membranes will not confirm it’s general performance. Results obtained from the protocol were established on a certain test conditions,so it does not mean that if we use it here,we can obtain same results as that of the protocol.

As a general rule for membranes,we have to assume the 95% efficiency,so if raw TDS is 900,then product TDS will be 45.

Basing on the TDS results obtained w/c is 25 ppm,it is assumed that membrane is at more than 95% efficiency,97.22% efficiency to be exact.

So now,where is the problem? Actually there is no problem but we are the one’s creating our own problem,to assume that LE membrane will perform 99% efficiency as per test protocol without considering the test conditions will be a no!! no!!.

LE was able to achieve the 99% efficiency because recovery is just 15%,so that means, 85% is reject water,now in our application, where in we usually apply 40% recovery and 60% reject,do you think we can have the same results?

15/85 ratio would require a higher feed water volume than 40/60 ratio based on the same production volume in a given day.

One of the principles of reverse osmosis is, “the higher the concentrate flow (reject) the higher the salt rejection(efficiency)”.

Just a basic tip,the next time you sell the membrane,consider this concept and you will not be having problems with your clients.

- Orly Shanghai