RE-Filtering RODI wastewater?
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Randy Holmes-Farley
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If such systems were easy and economical, everyone would use them, They do not, and such dual systems are fairly rare. Ask yourself why and be sure you know the answer before going down the road of something most experienced reefers do not choose to use.
edd59
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i have a brs water saver rodi. it usese 2 membranes. the waste from the first goes through the second membrane, i get about 2:1 waste to product and 0 tds final product. the reason i got it is im on well water and didnt want my well pump circulating on and off too often.
Randy Holmes-Farley
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That's a fine use for it, but folks should recognize the trade off. You do not get something for nothing by adding a second membrane. The rejection rate for each membrane is poorer than one alone and hence the DI depletes faster, unless you use a booster pump to deal with the the pressure drops across the membrane.
edd59
Well-Known Member
i do use a booster. have to with a well as pressure fluctuates with cycling.
i still go through anion cause of high co2, but id rather change anion every 3 months instead of off gassing.
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On commercial RO systems we often spec recovery rates at ~50% (1:1 ratio of concentrate to permeate) or higher, and include a recycle function (x% of the concentrate is run through the membrane a second time).
High recovery rates are possible, for the most part, because we install a water softener to remove hardness before the feedwater reaches the RO. With no scale-forming hardness in the water we can reduce the amount of concentrate significantly. If you have "no" hardness in your water you can do the same thing with your RODI system.
With many (most) commercial uses of RO systems the TDS in the permeate is not as critical as it is in this hobby. For example - think of a carwash offering "spot free rinse." What they're doing is rinsing your car with permeate. But as long as the TDS in the permeate is around 30 ppm or less, it doesn't typically leave spots. So a super low permeate TDS is meaningless to them - 6 ppm? Great. 18 ppm? Great. 24 ppm? Great. Therefore if we take an adjustable percentage of the concentrate and run it through the membrane again, we effectively raise the TDS of the water entering the membrane. Higher incoming water TDS => higher permeate TDS. But as long as the permeate TDS stays less then 30 ppm or so, they're happy.
In this hobby, the idea of purposefully increasing the TDS of the feedwater (and therefore purposefully increasing the TDS of the permeate) doesn't make sense. As mentioned above, you can't make so many of these configuration changes without a tradeoff.
The demands of users of ultrapure water treatment systems in this hobby often conflict. Users want:
High recovery rates are possible, for the most part, because we install a water softener to remove hardness before the feedwater reaches the RO. With no scale-forming hardness in the water we can reduce the amount of concentrate significantly. If you have "no" hardness in your water you can do the same thing with your RODI system.
With many (most) commercial uses of RO systems the TDS in the permeate is not as critical as it is in this hobby. For example - think of a carwash offering "spot free rinse." What they're doing is rinsing your car with permeate. But as long as the TDS in the permeate is around 30 ppm or less, it doesn't typically leave spots. So a super low permeate TDS is meaningless to them - 6 ppm? Great. 18 ppm? Great. 24 ppm? Great. Therefore if we take an adjustable percentage of the concentrate and run it through the membrane again, we effectively raise the TDS of the water entering the membrane. Higher incoming water TDS => higher permeate TDS. But as long as the permeate TDS stays less then 30 ppm or so, they're happy.
In this hobby, the idea of purposefully increasing the TDS of the feedwater (and therefore purposefully increasing the TDS of the permeate) doesn't make sense. As mentioned above, you can't make so many of these configuration changes without a tradeoff.
The demands of users of ultrapure water treatment systems in this hobby often conflict. Users want:
- to run the system at low or unknown pressure
- to pay no attention to feedwater chemistry
- to run the system at very high recovery rates (very low concentrate to permeate ratios)
- to have super high membrane rejection rates
- to have almost indefinite lifespan on their DI resin
- to run their system without regard to short-cycling (e.g., a sensitive float valve in their sump).
leon.1980
Active Member
lol. Take a look at your water bill. My bill probably about 60 or so month. I’m usually 6 to 8 bucks in water cost itself. The rest is crap fees and charges!
I mean he’ll, I pay for my water then I pay to have it recycled for some other idiot to pay for to use! lol.
I mean he’ll, I pay for my water then I pay to have it recycled for some other idiot to pay for to use! lol.
EeyoreIsMySpiritAnimal
Just another girl who likes fish
But, he's in California and, despite the recent record rainfall, drought-awareness is a real thing there. I appreciate the OPs interest in conserving water if possible
leon.1980
Active Member
Is he doing it to conserve water or to save money? If it’s to conserve water I commend him (even if it’s a oz of water conserved) but if it’s for money then he’s wasting it.But, he's in California and, despite the recent record rainfall, drought-awareness is a real thing there. I appreciate the OPs interest in conserving water if possible
Contrary to what a major vendor in this space suggests, in most cases this is not a good idea. Would never recommend it for the OP with incoming TDS at 400+ as almost certainly he has high hardness. Why?
You'll shorten the useful life of the RO membrane, but if you're really dead set on this, there's a much less expensive approach - simply put a tighter flow restrictor on your existing membrane. Additionally, it will cause a decrease in your resin life as the permeate TDS will be higher.
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Double membrane user here. I didn't really notice di resin depleting any faster after adding the second membrane. That said I'm not a person that keeps track of things like how often I change my resin.
Joekovar
Active Member
You could feed your wastewater to a solar still, and have that still filling an alternate input source for your RODI unit.
If you use an input buffer tank to the RODI unit to begin with, you can feed the still output directly to that, which in theory will lower the concentration of stuff in your input water and improve overall efficiency and filter life.
If you use an input buffer tank to the RODI unit to begin with, you can feed the still output directly to that, which in theory will lower the concentration of stuff in your input water and improve overall efficiency and filter life.
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