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Placing the thru-hull discharge 'higher' than the level of the RO membrane insures that when the unit is shut down the membrane is kept fully wetted when/if the discharge valve is left opened for a short time.
Once the typical polyamide RO membranes are fully wetted out, they should be left fully wetted ... or they can lose their ability to be later 'wetted out' (and they become 'hydrophobic'). Letting a RO / watermaker membrane drain and 'dry out' quickly reduces its efficacy and service life.

My preference is to run an unvented loop to the discharge valve/through-hull, the top of the loop being several feet above the discharge (at the water line). When operating and the loop filled with fluid, there will be no 'static head' differences other than minuscule friction losses as the loop will be operating as a 'siphon'. If when shutting down and I forget to close the discharge through-hull valve, the membrane will stay 'wet' (for a long time) - because of the loop. I prefer also to install a loop at the fresh water/ retentate side, too; and, for the very same reason - keep that membrane WETTED.

With 800 psi potential pressure 'driving' the discharge brine it really makes no difference of a few static height of discharge. Each 2.31 feet of static height of water is equivalent to 1 psi pressure .... pretty minuscule when the high pressure pump is 'driving' at up to 800 psi.
 

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........ is that it's good to be able to see the discharge and IIRC, it's to confirm that it's running properly.
and you dont have to read a gage to SEE the water coming out !!!! ;-)

If you forget to open the discharge through-hull valve, you can/will get ~800 psi or more 'just before' that discharge exit. Such are the 'features' of 'constant displacement' pumps. :-o
 
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