Hydrogen Tech World October 2023 | Page 32

[ water treatment ]

Feed Feed

Feed

Cation Exchange Resin

Anion Exchange Resin

CEM

AEM

Na +

Na +

CEM

Na + Na +

Na +

Na +

H +

H +

OH –

OH –

CI –

OH –

AEM

OH –

CI –

H + OH – CI – OH –

CI –

H +

Na +

H + Na + H + OH –

Product Reject Product

H +

During operation , a transverse DC electrical field is applied to the electrodes . Ions entering the dilute cell in the feedwater stream , under the influence of this electric field , migrate from the dilute cell towards their respective electrode and become trapped in the concentrate cells .

Fig . 2 . Ion removal in an EDI cell

This continuous electrochemical regeneration of the ion exchange resin enables EDI systems to operate continuously without the need for chemical regeneration and associated downtime .

Commercially available EDI systems can handle feed water conductivity levels of up to 100 µ S / cm and hardness levels up to 4 ppm as CaCO 3

, achieving up to 99.9 % salt removal . The levels of individual ionic species are reduced to parts-per-billion levels . In terms of conductivity , typically , a range of 0.1 to 0.055 µ S / cm is obtained , equating to a resistivity of 10 to 18 megohm-cm .

EDI modules operate with a typical recovery rate of 95 %. The concentrate ( reject ) stream contains the feedwater contaminants at concentrations 5 – 20 times higher than the feed water and can be recovered by returning this stream to the inlet of the reverse osmosis ( RO )

32 Hydrogen Tech World | Issue 12 | October 2023