Water Purification Question

[Anonymous]

Hello all. Here's a quote from Tullock's "Corals":

"Deionization can be used as the sole means of water purification, dispensing with RO altogether, but this is a more expensive option as the resins must be replaced periodically. The advantages of using deionization alone are that water is produced on demand, rathr than drop by drop, and there is no wastewater production."

I'm having a discussion with some friends about this; they think that RO performs a different function than DI, I think it performs the same function differently.

Any opinions?

 

[AnotherGoldenTeapot]

Well their similar.

RO works by forcing water through microscopic holes in a membrane. The idea is that only the water molecules are small enough to pass through the holes and so all of the pollutants get washed away with the waste water.

The problem is that with the low cost membranes that any of us can afford, that the microscopic holes are far too big and so some pollutants make it through into the "clean" water.

Rejection rates are in the order of 95% in perfect conditions, so the "clean" water still has 5% pollution in it. This may or may not be good enough for reef purposes - it all depends on how good (and consistent) the input water is. Many people follow with a DI chamber to remove most of the residual contaminants.

DI filters use a different approach. There are two resins, one of which binds to anions and the other to cations. Water passes slowly over each resin in turn and the resin binds to the contaminents in the water.

DI is much more efficent (in terms of how clean the output water is) than RO - removing almost all of the contaminants in the water.

It's always possible to just use DI and this is not necessarily expensive. The key is to have the two resins separate so that they can be recharged. The chemicals used to recharge the resins are very cheap (almost free if bought in bulk - 5lb costs much the same as 50lb), but also very caustic (so care is needed - safety glasses, gloves, and a face mask).

If driven by a RO pre-filter then the DI resin can last a very long time before it needs recharging (20 times longer than without the RO filter in ideal conditions). On cheaper filters the DI unit is designed to be thrown out once it has exhausted. However, even those can be recharged - its just more effort.

Both approaches require a sediment pre-filter and carbon pre-filter to remove silt, chlorine and other such compounds.

So my opinion is that they both perform the same fundimental function but in a different way.

 

[Anonymous]

Thanks AGT. I appreciate the response. If I understand you correctly, I would be better off to stick with DI than to buy a low-end RO unit?

How can you regenerate the resins in a mixed unit?

Thanks!
djp

 

[Anonymous]

hi.
You can't. You need to separate the resin in a mixed unit before you can regenerate they. They have different density, so that's how we separate them first. Luckily you only have to do it once. You need HCl and NaOH to regenerate.