Forums > Specific Focus > Home, Garden & Construction in Mexico

Rolly / Moderator

Feb 13, 2012, 2:33 PM

Post #1 of 6 (5528 views)

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Questions about Reverse Osmosis systems

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If you have an RO system, i hope you will share your experience.
> What was the installed cost?
> What maintenance is required? How often? By whom?
> Does it remove enough minerals to soften the water?
> I have read that RO systems need at least 40psi water pressure to operate properly. Since 40 is higher than is common in México, have you had a problem using it with a lower pressure?
> I also read that chlorine in the water can harm the membrane in the unit. Is this true?
> Where/how do you get rid of the removed stuff?

Thanks for your help.

Rolly Pirate

YucaLandia

Feb 14, 2012, 9:29 AM

Post #2 of 6 (5480 views)

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Re: [Rolly] Questions about Reverse Osmosis systems

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Since no one else has answered from Mexico - with the Mexico specific answers Rolly has requested, I'll hesitatingly offer my US experience with R.O. Here are answers to your questions (in order):

We had an RO system for 12 years and have talked with 2 friends who have them here. Installed cost in the US was about $500 USD. Installed cost for my friends here was about $650 -$700 USD with a pump for a home system.

Maintenance varies with how hard your water is and how many gallons a day you use. Heavy users here with hard Yucatecan water describe having to replace membranes every 9 months or so. Light users replace membranes every 2 - 3 years.

The water comes out very soft.

40 psi - 60 psi is a good target range - which usually requires a small pump here.

The chlorination issue depends on what type of membrane you have and also on what prefilters you use: Cellulose triacetate (CTA) membranes are protected from rotting by chlorinated water. Thin-film composite (TFC) membranes are damaged by chlorinated water. CTA membranes are generally considered inferior to TFC membranes, because they do not last as long (typically only 18-30 months) and they do not purify the water as well as TFC membranes.

If you have non-chlorinated water: you can always replace the CTA membrane with a TFC membrane. If you have chlorinated water, a carbon pre-filter is needed to remove the chlorine which would otherwise ruin the TFC membrane. If your R.O. system only runs the water through a sediment filter before going to the membrane, and you use a TFC membrane then you would also need to add carbon filter into the sediment filter housing (so that water flows through the carbon filter before going to the membrane). Typical carbon filters treat about 2500 gallons of chlorinated water.

The RO systems tend to send 80%-90% of the input water to waste so if you use 8 gallons per day (including waste), to get 2 gal of drinking water, then a carbon filter it will last for about a year. ~ So, your maintenance would be a carbon filter every year, and a membrane every 2 years.

The waste water (now (more salty and harder than tap water) can be routed to... plants that tolerate hard water... since the waste water has 25% more salt and minerals than your normal tap water.
steve
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Read-on MacDuff
E-visit at http://yucalandia.com

sfmacaws

Feb 14, 2012, 10:19 AM

Post #3 of 6 (5470 views)

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Re: [Rolly] Questions about Reverse Osmosis systems

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Rolly, I looked at this and realized that I don't know the answer to a lot of those questions. What Steve said sounds about right. We had the RO system installed when the house was renovated, more than 4 years ago, and I don't know the cost now.

Our system is in the kitchen and the water that goes to it has already been through the water softener and the pressure pump on the roof. Even so, there is a small pump next to the RO tank that adds additional pressure when it is working. If you remove a couple liters of water you can hear the pump start and the tank refill. Ours has 2 outlets, one to a faucet on the island and another to the ice maker in the refrigerator. The waste goes into the nearby sink drain.

We change one filter every 6 months and the others every 2 years. Mimi handles that so I don't really know which is which, there are at least 3 filters maybe 4. In 4 years we have had one problem, we had to replace the tank. It's a small pressure tank that should have part air and part water but it was filling with water. I've no idea why but the replacement was not expensive. We could tell something was wrong because the pump went on every time you got any water.

The small pump that is next to the filters looks like the kind of water pump we had in the RV. Quite small and not loud. I can hear it because I'm usually standing right over it when it goes on. However, remember that we have a whole house pressure system so the water coming to the RO is already close to 40lbs pressure. The whole shebang is in a cabinet under the island bar sink with room to spare.

I can have Mimi look at this if you need more precise answers. Also, since a picture may be worth more than most of my words, here are a couple of pictures of our setup. The bar sink above has a regular water faucet and the small faucet to the right is the RO water.

Jonna - Mérida, Yucatán

Rolly / Moderator

Feb 14, 2012, 2:45 PM

Post #4 of 6 (5444 views)

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Re: [YucaLandia] Questions about Reverse Osmosis systems

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With so much waste water, is a whole house RO system practical?

Rolly Pirate

sfmacaws

Feb 14, 2012, 3:37 PM

Post #5 of 6 (5430 views)

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Re: [Rolly] Questions about Reverse Osmosis systems

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I wouldn't think so, why flush the toilet with RO water? It seems wasteful.

Jonna - Mérida, Yucatán

YucaLandia

Feb 14, 2012, 8:05 PM

Post #6 of 6 (5411 views)

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Re: [Rolly] Questions about Reverse Osmosis systems

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"With so much waste water, is a whole house RO system practical? "

Small household systems only make about 2-3 gal of water per day, due to small membranes and lower pressure pumps.
Water is cheaper than electricity, and the cost of large membranes and extremely high pressure pumps is out of most homeowner's expectations.**

Background: R.O. systems depend on semi-permeable membranes that in theory allow only water to pass through the pores of the membrane. Since osmosis is the natural movement from a high concentration zone to a lower concentration zone, the water molecules naturally want to flow from the pure water side - over to the salty side to equalize the concentration. Since we instead want pure fresh water, we have to REVERSE the natural osmosis process, and use pressure to force the water molecules backwards from the salty side over to the fresh side. Hence, it takes serious pressure on the inlet side to force the water molecules out of the salty side through the pores over to the pure water side of the membrane. The more salt or mineral ions next to the membrane, the more pressure that is needed to get the water molecules backwards (in reverse) through the membrane. The more water you want per day means higher and higher pressure, and larger and more expensive membranes, and more waste water to keep the salt concentrations at the membrane as low as possible.

Because we are forcing the natural process to run in reverse, the more salt on the inlet side, the higher the pressure is needed, so salt water takes very high pressures (500 - 1000 psi), while home systems with just hard water need only 40 - 60 psi to make 2-3 gal per day.

**Said another way, to get more water for a whole house, bathroom sinks etc, you would need to run the pump a longer time and at higher pressures with more expensive plumbing and expensive fittings, than the costs for a lower output RO under-the-kitchen-sink system. If you have a fat enough wallet, you could buy a whole house system.
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