What Pump To Feed Rodi System

Andy S

This is not strictly a question about RO/DI but I don't know who else to ask, hopefully somebody out there has the knowledge I'm lacking.

I have a 4 stage RO/DI system but I cannot run it off my mains water due to the original TDS reading being 4,800. No, that's not a misprint, it really is 4,800, it comes from a (not remotely efficient) desalination plant.

I have a huge rainwater storage system under the house which contains about 40,000 litres of rain water which has a TDS reading of 110. There is a pressure operated pump above the tank which feeds this water to a tap in my utility room. The pump only operates when the tap is opened and the pressure between the tap and the pump drops. As soon as you close the tap the pump takes a few seconds to re-pressurise before turning off.
The problem I have is that this pump is obviously only intended to run for a matter of minutes and the longer it runs the hotter it gets. Because the actual flow through the pump is only about 40 litres an hour when attached to the RO/DI unit there is nothing to cool it down apart from the fan on the end of the impeller shaft. I ran the system yesterday but after 2 hours the entire RO/DI unit was warm to the touch and the water coming from it was up to 28 degrees. I did not check the actual temperature of the pump, my best guess is that it was pretty hot.

The solution to this is to not use this external pump but to purchase a submersible pump specifically for the RO unit. Problem I have is that I have no idea what size or type I need. 'Technical Specification' on every pump I have checked only gives a flow rate and a 'maximum head', I do not know how these figures relate to PSI. I need to produce 65 PSI going into the unit and the water level is at most 1.5 metres below ground level where the RO/DI unit is situated. It also needs to rated for continual use rather than just for a few minutes at a time. It needs to be able to run for about 6 hours at a time to produce the amount of RO/DI water I need.

Is there anybody out there with any suggestions? I don't want to be spending a fortune on a pump which is way too powerful but equally I don't want to buy one and find it does not produce enough pressure. Any suggestions? Is there a correlation between flow rate, maximum head and water pressure? I know there must be but I have no idea how you do that calculation.

Yes, I am aware that you can buy booster pumps for RO/DI systems, they are designed for people whose tap water runs at low pressure, below 65PSI. It boosts the incoming tap water supply to a suitable level. My problem is I do not have an incoming water supply. My water is maybe 1.5 metres underground, I need a pump to get the water up to ground level at 65 PSI.
 

Thunder_o_b

Being where you are these may not be an option.

shallow well pump
 

coralbandit

I would think you need two pumps ?
A supply pump that does not have to be real high pressure ,just get water in the line[without other pump that heats up] and then the pump for the ro system made for it .
I have a booster pump like your water pump that turns on and off with pressure that runs continuously when needed..Used 90g of RO yesterday !
 

Andy S

I would think you need two pumps ?
A supply pump that does not have to be real high pressure ,just get water in the line[without other pump that heats up] and then the pump for the ro system made for it .
I have a booster pump like your water pump that turns on and off with pressure that runs continuously when needed..Used 90g of RO yesterday !
Yes, I'm coming to that conclusion. I tried running it straight off my 'hippo' sump pump but although it has a great flow rate as far as litres per hour and it will pump up to 7 metres head it's at very low pressure and it wasn't enough to push through the RO membrane.
I'm now looking at a second pump, the ones specifically supplied to run RO systems but I can't make out if there is a minimum pressure required Most that I have seen give a maximum inlet pressure of 2 or 3 bar, I don't know if they will work with a much lower pressure. The hippo pump will supply plenty of water to it but at very low pressure.
 

coralbandit

Most RO/DI units only do better with pressure up to 80psi.
Over 100GPD may require even higher .
The only indication of pressure I have is after my second stage[built in on my BRS unit] that shows if the pressure is effected by first two cartridges.
when running proper it has between 80-90 psI but this is a 6 stage dual DI with the 150gpd upgrade and one extra cartridge thrown on the front end..
This way I have 5 micron and 1 micron sediment filters that are cheaper to keep up on then the carbons that come next.
I use this pump ;
on this set up basically with the extra canister in front with 5 micron sediment ;

I needed the pump from the start due to pressure around 40psI but waited a couple years until I wanted to upgrade for more GPD and got the second TFC membrane. Then I added the pump and it has been GREAT since.
 

richiep

sorry to but in, but as usual when talking about ro dI systems coralbandit is at the top of his game as I found out when I needed advise from him not so long ago, anyway don't know if this will help, all ro units advertised in the UK recommend a minimum of 60psI but this dosn't give you 100% output, I was running a 50gal pd and boy was it slow, I upgraded to a 150gl pd and a pump pressure of 135psI that was giving me 12ltr in 25 minutes, now the temp is dropping here my psI is still 135 but the time to do 12ltr today as gone up to 31 minutes I expect this to go up even further as the winter chill comes in,
hope you solve your problem Andy
 

Andy S

sorry to but in, but as usual when talking about ro dI systems coralbandit is at the top of his game as I found out when I needed advise from him not so long ago, anyway don't know if this will help, all ro units advertised in the UK recommend a minimum of 60psI but this dosn't give you 100% output, I was running a 50gal pd and boy was it slow, I upgraded to a 150gl pd and a pump pressure of 135psI that was giving me 12ltr in 25 minutes, now the temp is dropping here my psI is still 135 but the time to do 12ltr today as gone up to 31 minutes I expect this to go up even further as the winter chill comes in,
hope you solve your problem Andy
Thanks, I think (hope) that I have solved it, see my reply to coralbandit

Most RO/DI units only do better with pressure up to 80psi.
Over 100GPD may require even higher .
The only indication of pressure I have is after my second stage[built in on my BRS unit] that shows if the pressure is effected by first two cartridges.
when running proper it has between 80-90 psI but this is a 6 stage dual DI with the 150gpd upgrade and one extra cartridge thrown on the front end..
This way I have 5 micron and 1 micron sediment filters that are cheaper to keep up on then the carbons that come next.
I use this pump ;
on this set up basically with the extra canister in front with 5 micron sediment ;

I needed the pump from the start due to pressure around 40psI but waited a couple years until I wanted to upgrade for more GPD and got the second TFC membrane. Then I added the pump and it has been GREAT since.
Thanks for that, I believe that I am well on the way to solving this one.

I spoke to a friend of a friend who is MD of a pump manufacturing company.

He explained that in order to achieve 65psI you effectively need a pump that will produce 45.7 metre head of water. Since most submersible 'hippo' sump pumps will only pump to around 4-8 metres there is no way this type of pump will ever give the amount of pressure required. Basically it is a centrifugal pump which supplies a huge volume of water but at very low pressure. The R.O. booster pumps are diaphragm pumps which do not produce anywhere near the flow rate of a centrifugal pump but they will produce huge water pressure.
He compared the two types to the difference between a fan and a set of bellows - a fan will move a huge amount of air but it's only the equivalent of a strong breeze whereas a set of bellows only moves a small amount of air but at huge pressure.

My RO system has a 75GPD membrane and when I did have it working it was producing 13 litres an hour product water and 25 litres an hour waste water at an input pressure of 65 psi. In effect I only need to supply water to this pump at a rate of 38 litres an hour. That can very easily be achieved merely by gravity feed from a storage vat just a few inches higher than the RO unit input through a small diameter hose connection. So - rather than have the hippo pump running for hours at a time I only need to run it for a few minutes to fill a 100 or 200 litre storage vat and let gravity do the rest. As long as there is water flowing to the RO pump, even at very low pressure it should work.

This is all theoretical because my RO pump has not yet arrived but it does seem to be fairly logical. I'll repost when I have it set up and running but I'm quietly confident this should be the answer.

Just one question - the RO booster pump will produce 120 psi. My RO unit has a pressure limiting valve to keep it at 65psi. Would I do better to leave the pressure reducing valve in place and run at 65 psI or is it better leaving the valve out and run the system at 120?
 

richiep

When I up graded from 75 to 250 I also upgraded the flow restrictor but coralbandit may advise on a different setup for your USA systems, I've put a link to the page of the restrictor upgrade I used,

150 GPD Flow Restrictor
 

Andy S

When I up graded from 75 to 250 I also upgraded the flow restrictor but coralbandit may advise on a different setup for your USA systems, I've put a link to the page of the restrictor upgrade I used,

150 GPD Flow Restrictor
This appears to be the same flow restrictor that I have fitted to my unit. The pressure limiting valve is something else, it is connected between the incoming source and the first sediment filter chamber so that the pressure flowing into the system does not exceed 65psI regardless of how high the incoming supply is.
 

richiep

I see what you mean, can't see why you can't remove it the unit will take the pressure mine runs at 135psi
hopefully coralbandid can advise better
I hope you get it sorted you seem to be on the right road, I'll still follow with interest to the outcome
 

Andy S

I see what you mean, can't see why you can't remove it the unit will take the pressure mine runs at 135psi
hopefully coralbandid can advise better
I hope you get it sorted you seem to be on the right road, I'll still follow with interest to the outcome
Sorry for the delay, had to wait for the booster pump to arrive. Booster pump is rated at 80psI which is lower than I was expecting but, whatever. Plumbed it into the unit and positioned it so that it is gravity fed from a storage container. I took the pressure limiting valve out figuring that 80psI is not that much greater than 65psI and since everybody seems to run at higher pressure than that I figured it was probably OK.
Started with source water form the tank under my house TDS 179. Producing water at a little under 12 litres an hour with TDS reading of 4. Product to waste is 1 : 2.39 which I thought was OK, unlikely to need more than 50 litres at a time so that's just over 4 hours.

I do have a second RO membrane and housing so plumbed that in just to see what results would be. The membrane is old and has been somewhat abused by me trying to force water with TDS of 4,800 through it. I let it flush for over half an hour before attempting to produce RO water.
It took a bit longer to produce decent water, flow of product water was 8 1/2 litres an hour but the TDS reading was 6, I was expecting it to be much worse than that.

I then connected the two membranes in series, the waste from the first feeding the second. I did not have the fittings to rejoin the two product lines together so only one of them went through the DI resin.
Not surprisingly the flow rate from both individually was reduced however the two lines combined produced nearly 18 litres an hour with a combined TDS reading of 5. The product to waste water ratio is now down to 1 : 1.57. I am only wasting just over one and a half litres for every litre of product water and the time needed to produce 50 litres has dropped from 4 hours using just the one membrane to 2 hours 45 minutes by using two..

Maybe if my pump produced a little more pressure than 80psI I could get even better results but overall I'm pretty pleased with how this has turned out. I'm fairly confident that joining the two product lines together so that both go through the DI chamber, suitably repacked with fresh resin, replacing the old membrane with a new one should see the TDS drop from 5 to virtually nothing but even if it doesn't, I can live with TDS reading of 5.
 

richiep

You've worked hard to get where I'm pleased you've had success, who know something may come up to increase your pressure even more
 

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