Watermaker

MrUnix · Mar 17, 2012

The 'technology' behind reverse osmosis is simply pushing water through a semi-permeable membrane to remove contaminants.. so in that respect, they the same technology. But there is more to a RO filter than just the RO membrane. Pre/post filtering needs to take into account the source of water being filtered, so I imagine that marine RO water makers may have some significant differences there compared to home units.

And all RO filters require pressure to push that water through. Typical home water pressure is somewhere in the 30-50psi range more or less. We have a RO filter under our kitchen sink because we are on well water. Our pressure switch cycles between 40-60psi at the collection tank. Just because you are on a boat doesn't change anything.. you still need pressure to push that water through the filter.

Cheers,
Brad

RichH · Mar 17, 2012

It all depends on the size of the statistical pores in the membrane!

Membranes used in 'reverse osmosis' process are offered in varying retention sizes and you have to select the correct 'removal rating' for each application, just like all 'filters'. "RO" membranes can have a pore size of 0,05 nanometers (0,00005 µM up to 5,000 nanometres (5µM)

Desalination, to remove 'salt' ions, requires a pore size of ~0,1 nanometres (3.9×10−9 in or 0,0001µM) and will typically require 800-1000 psi to operate.

The typical 'under the sink RO membrane' will be in the range of 0,05 µM to 0,1 µM and will require only ~10 to 20 psi to operate; domestic water is between 40-60 psi. In that application, the RO membrane is just a 'fancy filter'.
That 0,05 to 0,1µM size will remove 'most' bacteria, and 'many' viruses to a 'high degree' of efficiency; but, arent 'perfect' due to normal manufacturing defects in the RO membrane. The reason that 'under the sink' RO membranes are used is because of the exceptionally large surface area of the very thin membrane requires less work getting the fluid though the pores ... in comparison to a small surface area 'micro-filter' at ~0,1µM. The downside for "under the sink RO" is that there no flow tangential to the surface of the membranes which keeps a normal high pressure RO application from fouling ... so that 'under the sink RO' is *just a filter* using a LOT of very thin membrane; and, your going to have to change it as often as any other 'filter'!

Reverse Osmosis doesnt always imply 'salt removal' ... you have to specify the size of the 'particle' that you want to 'retain' ... just like any other 'filter'.

Dantodd · Mar 17, 2012

The only RO systems I am familiar with in home use do, in fact, have tangential flow but only "waste" about 80 or 90% of the source water. I am not sure what the brine rejection is on a marine desalinization RO system is but I suspect it is a lot higher to get proper brine flushing.

RichH · Mar 17, 2012

Nope, the under the counter single housing ROs sold at HDepot, etc. are all 'dead end' usage, no retentate, just 100% permeate.

Dantodd · Mar 17, 2012

RichH said:
Nope, the under the counter single housing ROs sold at HDepot, etc. are all 'dead end' usage, no retentate, just 100% permeate.

There must be a few different home versions available. Mine had effluent and did not "dead end" at the membrane. While I appreciate your experience may be different it would be incorrect to say that they all work this way.

BuckeyeFS · Mar 18, 2012

All residential RO's on the market produce flow to two outlets:
1. "waste water (a.k.a concentrate)
2. purified water (a.k.a. permeate).

Given factory spec pressure and temperature, most systems are configured to produce apx. 4 parts waste water to 1 part purified water. To our knowledge there is no RO technology that does not produce "waste water."

Residential scale RO membranes are speced at standard pressures that vary a bit by the manufacturer. Most manufacturers spec the membranes at 60 psi and 77 degree F water. One manufacturer (Filmtec) specs their membranes at 50 psi and 77 F water. Most residential scale RO systems are run at pressures below 80 psi. Here are some data re the various membranes we are most familiar with.

When you jump up to membranes that produce more than about 150 gpd, they are often used in systems run at high pressures. Very high TDS feedwater (e.g., salt water), require higher feedwater pressures.

Russ

BuckeyeFS · Mar 18, 2012

RichH said:
Nope, the under the counter single housing ROs sold at HDepot, etc. are all 'dead end' usage, no retentate, just 100% permeate.

The RO's sold at HDepot and the like produce about 4x as much waste water as purified water.

BuckeyeFS · Mar 18, 2012

To answer the original post, some residential systems do require a pump. For RO systems that use a Filmtec membrane, we recommend pumps for anyone with line pressure below 40 psi. Some owners will add a pump to systems with 45 psi or higher to take advantage of the benefits of higher feedwater pressure.

Additionally, high feedwater pressure can compensate for the effects of low feedwater temperature.

Boat/ship mounted systems likewise require minimum feedwater pressures which are provided by a pump.

Marc Bergmueller · Mar 18, 2012

I have a Buckeye Field Supply RO DI for my saltwater aquarium. It is the best of the 5 or so I have owned. After trying the HD ones and others that would last for 50 gals of 0.0 ppm water then creep up and clog. Buckeye know water. But still on a sailboat you would need a way to pressure the water with a pump. My marinas water comes out green so I fill 5 gals a trip from the house.

AtlanticAl · Mar 18, 2012

I had a hand pump, salt watermaker, they use them for emergencies, you would get a glass of water, with an awfull lot of effort, they take so much effort, that it tells you in the instructions, to use it at night, when its cool, as on a hot day, you can dehydrated more water using it, than the thing can produce. Fortunately, i never had to use it in an emergency.