'Recycling' blackwater

LordJoshaeus · Nov 5, 2019

Hi everyone! This is not directly related to reef tanks, but I still thought this would be the best place to get this question answered. Anyhow...I have had a blackwater tank set up for a little over a week. The tank has very soft (TDS less than 25 ppm) water, and I am planning on acquiring that water in the long run by passing dechlorinated tap water (with a TDS of 40 ppm) and rain water (depending on availability) through DI resin.

What I have been wondering is, 'Would it make more sense to make new blackwater by passing old tank water through the resin, instead of the tap water? Or would the tannins rapidly wear out the resin even though they have little impact on TDS?' Thanks

lpsouth1978 · Nov 5, 2019

Either way, I think you will burn through resin in a HURRY. Even @ 40 TDS, your resin will become exhausted after very few gallons have passed through it. For my blackwater system, I just used my RO system without passing the water through the DI. This gave me a TDS of <10. I would then add ROright to add the necessary minerals back in.

There really is no reason to run water through DI resin if the TDS is only 40 to begin with. Just run the water through sediment and carbon filters to remove any chlorine and sediment. I only used RO because I have a LARGE RO system, so I figured I might as well put it to use.

Hermie · Nov 5, 2019

DI resin absorbs anions and cations. Whether or not blackwater particles affect TDS, if they contain any ions, they will affect the Resin. so I can't answer your question directly, only add some info. I wouldn't advise doing it though.

Red_Beard · Nov 5, 2019

I would think the tannins would very quicky wear out the DI. Im sure it would work but would probably be expensive. If you have rain water, it should already be low PH, or if you have RO water it would work too. They make buffering compounds specifically for your application, when i was doing it i was using ro right by kent, it was decent. A cheaper way to drop ph and buffer would be to use peat moss though

LordJoshaeus · Nov 5, 2019

The tank needs very low TDS, so just passing it through peat moss (which is not really sustainable anyway) or adding a buffer will not do the job. I wasn't assuming the DI resin would last as long as it would at the end of an RO unit...I was just hoping it would last about 100-150 gallons with the amount of dissolved solids in my tap water. More expensive than an RO unit, but I was hesitant to get a machine that could take 15 to 30 minutes just to produce a single gallon of water, and store bought water would be FAR more expensive for me.

Randy Holmes-Farley · Nov 5, 2019

Im not sure what organic levels you expect in the water, but I think it unlikely the organics will have nearly as much tendency to deplete the di as the ordinary salts like sodium.

LordJoshaeus · Nov 5, 2019

Ah...the tank water has a lot of tannins, but the TDS is quite low. The tank's TDS is currently 15 ppm, and my target level is no more than 25 ppm.

Randy Holmes-Farley · Nov 5, 2019

LordJoshaeus said:
Ah...the tank water has a lot of tannins, but the TDS is quite low. The tank's TDS is currently 15 ppm, and my target level is no more than 25 ppm.

yes, but do you imagine that you might have 100 ppm of tannins? That seems unlikely to me but I do not know what you are doing. The tannin concentration would have to be way, way higher than all other ions for it to deplete a di faster than the other ions, because tannins have a very high molecular weight.