8 replies to this topic

[Bee5]

Hello

I have been informed by an ion exchange resin supplier that ion exchange can lower TDS levels in waste water. This does not make sense to me from a mass balance point of view, since the ions are simply 'being exchanged'; unless adsorption is also taking place onto the resin. I would appreciate any advice from anyone with experience with treatment using ion exchange. The waste water has a TDS level of approximately 50 000 mg/l.

Regards,
Bjanka

[thorium90]

Firstly, 50000mg/L is more like sludge values. Even raw wastewater has TDS of <1000mg/L. Do confirm your numbers are correct.

I dont know what kinda ion exchange resin your supplier gives you that can treat raw wastewater with 50000mg/L of TDS but I can describe a bit of mine that produces demin water. Mine consists of a strong acid cation to remove cation. A weak base acid to remove anion. And a strong base acid to remove Si. Na+ and Ca2+ are taken up first and H+ is replaced into the water. Then HNO3-, Cl- and SO42- are taken up and OH- is replaced into the water. And so H+ and OH- makes water. Its like you give me one dollar and I exchange one dollar with you. You and I still have one dollar after exchange, so its still the same, just that we swapped dollars. Therefore nothing is wrong with the mass balance.

If you like to learn more, perhaps you can read the following book.
Reynolds, T.D, and Richards, P.A., “Unit Operations and Processes in Environmental Engineering”, PWS Publishing Company

Edited by thorium90, 21 January 2013 - 10:14 AM.

[markymaark]

There are beds that will trade, ion for ion such as a WAC bed in Sodium form. But these beds can also, like thorium90 said, exchange H+ for ions in your wastewater. But again, with 50 000 mg/L this would have to have an enormous amount of resin and the regeneration would also take a very very long time. Seawater is about 35 g/L and you're at 50 g/L.

Also, when considering a H+ bed, check your alkalinity. The carbonate/bicarbonate will act as a buffer to changing the wastewater pH as the H+ falls off the resin and into solution. This will create a very large amount of CO2 gas.

[Bee5]

Thorium90, my mistake, one of the streams feeding into the water that needs to be treated has 50 000 mg/l, but in the final tank it dilutes down to 8500 mg/l. Do you think this is too high for ion exchange? Do you think that RO would be a better solution?

Markymaark, the alkalinity is negligible, but thanks for the tip, I will remember it in future.

[thorium90]

Hi, 8500mg/L is equivalent to 8500ppm, which appears to be on the high side for simple regular domestic wastewater or industrial wastewater. What kinda water is that? Perhaps you could enlighten us as to whether this water is produced from the process or is it some effluent from an upstream water treatment unit or are you sucking in seawater and what is its intended final purpose after treatment. Is it for discharge into sewer, waterway etc? The requirements for the treated water determine what is the most suitable treatments needed. It could be as simple as sand filtration if the requirements are not that strict or ultrafiltration or electrodialysis to reach industrial water quality. Do provide other relevant parameters of the wastewater too if you have them, as it also helps in deciding what suitable process to use. Btw, are you sure it is TDS? Or is it TS or TSS or BOD or COD or TOC etc?

Edited by thorium90, 23 January 2013 - 12:34 PM.

[Bee5]

It is an industrial wastewater, that needs to be discharged to sewer. I checked the quality again and all streams considered I am dealing with approximately:
TDS 2000 mg/l
Cl- 1500 mg/l
Ca 200 mg/l
Zn 400 mg/l

Which is lower that what I initially thought. My instinct is to go for ion exchange, your thoughts?

[thorium90]

Ion exchange is expensive if the inlet TDS is high since the chemicals have to be bought and more TDS means more regeneration and thus more chemicals needed. It would be better to use RO instead and the ion exchange for polishing up. I'm not sure what are the regulations for discharge in your municipal sewer but I'm guessing RO alone would be enough to meet permissible discharge levels. RO would most likely be alot cheaper than ion exchange for TDS in the thousands.

[cleanwater7]

Bjanka if you still have an interest in the removal of those metal and TDS respond back. One question that was not asked as it is wastewater, is there any oil and grease present?

[Bee5]

There is negligible oil and grease present.