In meno di 24 un tecnico di DOW mi ha contattato con una lunga E-mail di consigli su come poter verificare il mio sistema e alcune verifiche da fare, ve lo dico perchè ritengo vivamente che sia da un servizio clienti di questo tipo (eccellente) che si capisce subito l'affidabilità e la qualità di un brand del livello di DOW.

Detto questo, oggi ho finalmente trovato l'inghippo in tutta la faccenda: dopo un'altra serata passata a fianco dell'impianto a fare prove di flusso, pressione e rimontaggio.... mi sono accorto che il primo carattere dello schermo del mio condutivimetro è parzialmente oscurato dalla plastica che sta a fianco... con il risultato che sono mesi che leggo in ingresso 350 anzichè 850 e con un valore del genere non posso certo lamentarmi se in uscita ho 40uS.

E qui direi che si chiude lo strano caso della membrana pazza, mi sa che prima o poi dovrò prendere un secondo posiltro a resine per perfezionare l'impianto visto l'alto valore iniziale. Un saluto e grazie a tutti!

Dimmi che verifiche ti ha fatto fare .per cui hai circa un ingresso in tds di 425 e uscita 20 .

copiaci l'email qui che può essere utile.....

Nessun problema, volevo già farlo ma non sapevo se potevo.
Metto qui sotto la mia mail e la risposta

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Dear all,
I bought a TW30-1812-50 membrane from an Italian e-commerce store to replace my older CSM membrane to improve my RO system.
I chose your membrane instead of the CSM because i trust in your brand and because your data-sheets are better than the other.

Now, i keep my system at a pressure of 4.1 bar and 350uS of conducibility and a after 100 litres of permeate i measure a salt rejection of only 89% instead of 96% like your data-sheet.

I contacted mine supplier and he told me that didn't exists FILMTEC membranes with defects (sorry but I little doubt it) and that the rejection problem are only my fault. I'm graduated in material science and i know how to manage those kind of RO systems, so i would like to receive a better explanation from you.

Your sincerely
Alesssandro

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Dear Alessandro Tinozzi,

Thank you for your trust in our FILMTEC(TM) Membranes.

Our FILMTEC elements undergo many quality checks during manufacturing, which makes sure that FILMTEC RO elements are of the highest possible quality. As such new defective elements are rare.
Based on the description of low rejection (or high salt passage), unfortunately there are many possible causes, which can vary from a defect, to feed water conditions or operating issues.

Possible issues, which can cause high salt passage are:
- leakage around the connections of the permeate and concentrate streams. Leakage of the brine seal (the black rubber seal all around the element on the feed side of the element). Please make sure that the element is properly installed in the pressure vessel
- leakage in the element, either caused by a manufacturing defect or by exposure of the element to high sudden pressures changes (water hammer), oxidation of the element due to chlorine, or abrasion of the membrane due to sharp suspended particles in the water.
- fouling or scaling of the element, although after just 100l this is not a very likely cause
- other operating conditions than specified in our data sheet. The feed pressure as well as the pH, temperature and composition of the feed water can all have an influence on the salt passage. Also the recovery rate will have a very big impact on the salt passage of the element. In our data sheets the rejection of the elements is 15%.
- The exact measurements used to measure the salt rejection. Conductivity for example is not a good measurement to measure salt passage as for many ions the conductivity is not linear with the actual concentration of the ions. Also hydrogen ions (and by default) pH has a very big impact on conductivity. This is particular important in reverse osmosis systems as the pH of the product water will always be lower than the pH of the feed water.

As you can see a high salt passage can have many possible causes or a combination of causes. Without detailed information on the performance of the system it is nearly impossible to determine the cause(s).
To be able to troubleshoot the system, maybe you can answer the following questions:
- is the salt passage stable at 89%, or does it change?
- What is the recovery of the system?
- How do you measure the salt passage? Are you allowing for pH changes and carbonate chemistry? Dissolved gaseous carbon dioxide will not be rejected by RO membranes, but carbonate is. Due to the disruption of the carbondioxide/carbonate equilibrium, some carbon dioxide will be changed into carbonate in the product water, leading to an increase in TDS in the product water.Also the pH and temperature of the feedwater have a big effect on the salt rejection of the element. Salt rejection will be lower when the pH is below 7.
- Are there any leakages (even small ones) on the outside of the pressure vessel?
- If you remove the element for the pressure vessel, do you see any traces of leakages on the element, typical would be traces of salts around the brine seal or the connections of the element.

I hope this information is already a bit helpful. Please update your question with additional information so we can continue the discussion and see if we can find a solution for your issue.

Best regards,
The Dow Chemical Company