Giving up on my local water

[bionut]

But i will need to check every hour or two the water pressure. Anyways i would have to collect the water at least a day in advance, but at least, with a bigger outpud, i would only have to check the pressure for a day, not two or more.
A booster pump it's another expensive solution that i may consider in the future.

 

[ajdelange]

But i will need to check every hour or two the water pressure.

Not really. When the water pressure goes down the throughput goes down but does not stop until the feed side pressure is less than the collecting vessel pressure plus the osmotic pressure. Collect to an atmospheric tank and you will
a)Never have 0 throughput
b)Have a higher throughput at any pressure

So all you have to do is check once or twice a day to see if you have enough water collected.

Anyways i would have to collect the water at least a day in advance,

Yes, that's the idea. Use a smaller system but over a longer period of time. If you have a vessel big enough to store a whole day's water this works very well.

...but at least, with a bigger outpud, i would only have to check the pressure for a day, not two or more.

You woudn't have to monitor the pressure with a larger system either. With pressure fluctuations there will be times of greater and lesser flow. But all you really care about is the total amount collected.

Lots of people here use float switches to automatically turn the RO system off when the collector is full.

A booster pump it's another expensive solution that i may consider in the future.

Yes, they work too.

 

[bionut]

You are saying that the water pressure won't affect the quality of the RO water? That's what i tought, that without big pressure you will get less efficiency in term of mineral reductio...
If that is not the case it is great, i already keep the water in some plastic containers, so atmospheric pressure only.
Another question, it's safe to store the water in hdpe containers? Will the water pull some nastys from the plastic? I guess not, because are the same containers i use for no-chill, but you can never be sure.

 

[ajdelange]

You are saying that the water pressure won't affect the quality of the RO water?

There will be some effect but it should be minor.

That's what i tought, that without big pressure you will get less efficiency in term of mineral reductio...

Water flow through the membrane does depend on pressure but salt flow through it does not depending only on the concentration difference on the two sides. Pushing a few numbers around should be helpful. Suppose you are treating water with 100mg/L sodium in a new system which rejects sodium at the 99% level and suppose it delivers 1 lpm at 400 kPa. This means 1 liter of water and 1 mg of Na+ pass through the membrane each minute. Now suppose the pressure falls to 200 kPa. Water flow is now 0.5 lpm but the sodium still goes through a 1 mg/min so the permeate now contains 2 mg/L rather than 1 at 400 kPa. Rejection is now 98%. Suppose you start collecting water without paying attention to pressure and that you collect for some period of time over half of which the water pressure is at 400 kPa and it is at 200 kPa the rest of the time. In time T (minutes) you will collect
(T/2)*1 + (T/2)*(1/2) = (3/4)*T liters of water which contains (T/2)*1 + (T/2)*1 mg of sodium. The sodium concentration is T*1/(3*T/4) = 4/3 mg/L so your rejection is (100 - 4/3)/100 = 98.7% rather than 99% it would be if the pressure stayed at 400 kPa.

Now suppose the system is older and only rejects sodium at the 95% level at 400 kPa so that 5 mg/min passes through the membrane and the sodium concentration is 5 mg/L in the permeate. Again assume 1 mg/L permeate at 400 kpa so total sodium in T minutes would be 5*T and the water would still be 3*T/4 for a sodium concentration of 5/(3/4) = 6.7 mg/L and rejection is 93.3% rather than 95%.

What is happening is that low water pressure hurts you in the sense that the rejection becomes worse but it doesn't hurt you that much because less water is collected when the pressure is low.

Another question, it's safe to store the water in hdpe containers? Will the water pull some nastys from the plastic? I guess not, because are the same containers i use for no-chill, but you can never be sure.

AFAIK that should be OK but I would only use containers that were certified as being suitable for the storage of potable water or food.

 

[bionut]

Thanks, very nice explanation!

The containers have the food grade stamp on them.