Watermaker : maximum recovery rate ?

[Fredrikx]

Hello,


I am now doing some calculations prior to buy parts for a watermaker that I will assemble myself.
In all cases, the watermaker will have a single 2540 membrane.


I have read some statements about the maximum recovery rate that is acceptable for the membrane.
As SV Third Day explains it here :

https://www.cruisersforum.com/forums...ml#post1754916


I understand that the 8% recovery rate mentionned in the Filmtec is not a limit in the case of a boat watermaker and that it can be close to 20% without problems.




Nowadays, we can find watermakers with a single 2540 that are close to 50% recovery rate. For example, 60 L/h of fresh water with a 2L/min high pressure pump, so exactly 50% recovery rate. In that case, the watermaker consumes only about 30A for 60L/h (about 16 US gal/min).





In assembling myself a watermaker I intend to minimise the energy consumption but on the other hand I don't want to use the membrane in conditions that can shorten its life span.
Is a 50% recovery rate acceptable without the need of frequent membrane cleaning with chemicals ?


If some of you have high recovery rate watermakers, are you happy with it ?


Thank you in advance

[Lepke]

I have a self made watermaker using 2 2540 membranes in parallel. I set mine by incoming salt water temp and maximum pressure drop. I usually get about 10 years out of the membranes. It's a good idea on a home system to plumb in another pressure gauge on the waste side. Maximum pressure difference is feed psi minus waste psi.
Membranes are rated at 77°F. Colder water produces less fresh water and warmer water produces more. See ratio table.
If you exceed the flow rate or the maximum pressure drop, you shorten the membrane life. I set mine by flow rate based on the Output Temp Factor. The psi is below 900. I have never measured waste water volume.
At 50% recovery rate you'll damage the membrane, how fast I don't know, but it will eventually tear and your TDS will shoot up. Also at 50% you'll get less salt rejection. And you may be running above the 1000 psi maximum.
I filter my incoming water because I make water year round, sometimes in rivers and bays, in small batches every 2-3 days so I don't have to pickle. I also have low TDS of about 100. Where I go, water temp is rarely above 55°F so I heat the water before the hp pump.

[Fredrikx]

Thank for your answer Lepke,
Could you tell me the flowrate of your HP pump and freshwater flowrate ?


The 50% recovery rate is not achieved by increasing the pressure but by using a small HP pump. For example, a watermaker producing 16 gph freshwater with a 0,5 gpm HP pump with one SW2540.



I understand that 50% recovery rate is very high (compared to industrial standards) an offers a very poor rinsing effect of the membrane but, in the case of the use on a boat (few hours per year, frequent freshwater flushing, storage period with Sodium metabisulfite or PG, ...), is it still suitable regarding the balance between benefit on energy saving and the supposed cost of more frequent replacement of the membrane ? I know that a few companies make watermaker with these specs, so it works, but I'm looking for users feedback.


For people that have a watermaker that consumes around 26 Watt of electricity per USGal/h of produced freshwater, your watermaker runs with a recovery rate of approx 50%. Are you happy with it ? Do you have to regularly replace or clean your membrane ?

[Lepke]

I have a power boat. I make water year round. In the winter in fresh water.
My pump is from a pressure washer. It's run by a 240v motor rated in amps and could supply 4+ membranes. It's rated at 2.7gpm @ 2850 psi. Running at about 900 psi it pumps a lot more water, but never measured. Fresh water is about 1 gallon and 3 gallons waste. In Alaska and 50° water I make about 28 gallons an hour. I've never been in water above 55°. In theory with 2 membranes at 77° I should make 58 gph. Sometimes I run a preheater before the membrane and get more water. Still working on that. I prefilter the water and usually have a TDS around 100. Almost zero in fresh water.

My pressure pump doubles as a pressure washer. That's why I picked it.

In the summer I spend a lot of time anchored. I make water about every 2-3 days when I run a generator to heat water, do laundry, and run the dishwasher. My inverter bank is 48v and I can go several days before charging. I almost never pickle. Membranes last about 10 years. I once plugged them with mud making water in a river and didn't check my mud filters. I reversed the flow and flushed them out and they work as good as before. I clean the membranes when production falls off. Not often.
I'm not an expert on RO, but with a 50% rate I'd have to way over pressure my membranes. Ant they would tear.

[Fredrikx]

Thank you answer but I don't understand everything.
If you have measured the freshwater flowrate and also the waste/brine flowrate, then indirectly you have measured the pump flowrate which is freshwater + brine.


The speed of the motor and of the pump depends on the the frequency of the AC and will vary very little with the pressure. If your pump is a 2.7 gpm, then the flowrate should be about 2.7 gpm whatever the pressure is.


Assuming the seawater flowrate is 2.7 gpm and the frshwater flowrate is 58 gph, if you have the membranes in parallel, the recovery rate of the watermaker and of each membrane is 36%.


Assuming the seawater flowrate is 4 gpm (3 + 1 that you measured) and freshwater flowrate is 58 gph, if you have the membranes in parallel, the recovery rate of the watermaker and of each membrane is 24%.




I will go with a 4 Liter/min pump with which, based on WAVE simulations and other users feedback, I expect about 30% recovery.