A "sock" is the filter element in RO. It's a polyamide spirally wound cylinder. The water that passes through it, exits via the center tube. It also produces waste water, that needs to be flushed. Higher pressure yields less waste and better filtration.
RO filter elements are purely physics. No chemistry or biology involved. They have holes so tiny, only water passes. All other "particles", from virii and bacteria to most bigger molecules don't pass the sock. Typical is a 50 micro-meter hole diameter. Smaller is better and more expensive and requires more pressure.
To work, they require a pressure of at least 600 psi. That requires a series of high pressure pumps (usually 6 to 10). Lower pressure RO exists, but will produce less output and more waste water.
A lot depends on the goal for the produced water. The ones we produced, were used with water cutters and in hospitals. Drinking water RO filters can work with less pressure, if you don't mind lots of waste brine (usually not a problem) and high energy use (not a problem in fi Saudi-Arabia as electricity is cheap and seawater abundant).
RO requires extensive pre-filtering, that's why it's impractical for household use. You need a mechanical (sand) filter if you're treating well water, to stop the largest particles. If you're treating municipal water, you need a carbon filter to stop chlorine. Etc.
The stuff they sell cheaply, uses rejected socks. These are cheap, work at lower pressure and break fast. You can't tell if they're broken, unless you have the right equipment and experience to test.
Any particle in water for a water-jet cutter can kill and ruin a machine that costs around a million...
My RO system has a flow restrictor in its output line. I encountered a failure in that output restrictor about 10 years ago and I discovered it from an elevated water bill caused by excessive water consumption.
Our machines had no restriction, as everything was under PLC control. Input pressure and flow, pressure and flow between every pump, output pressure and flow...
The smallest error stopped everything without destroying the cutter. In hospitals, output was analysed by a lab on a weekly basis.
Household RO could be cheaper, but...
The efficiency would be much lower (not economical in my mind, unless it's well water to agricultural water)
Output would be less reliable (acceptable for home use)
Socks would need to be replaced much sooner (not economical at all, that's why rejected socks are used)
At 60 psi, you get around 100 times more waste water. How do you get 600 psi pumps under your sink?
Mind you, this was 30 years ago. Tech tends to develop. But the industrial systems still are the same. Could be the industry is very conservative, but when it comes to money, they'll try anything to save a buck.
We also developed filtration for municipal water production, breweries, dry-cleaning and agriculture. None of those needed RO. Especially since the cost is high and the yield is very low. "Classical" filtration is still the way to go.
That's why I think the small/cheap RO systems are "leaky" RO. Socks leak a bit of untreated water into the output. That's OK for brewing and a lot of other stuff. It's not OK for hospitals. And it's certainly not OK for water jet cutters, that work at 50.000 to 100.000 psi. It could and would kill people. But I also consider it a bit of a scam to sell rejected socks as RO.
A combo of sand/carbon/resin will be cheaper, more efficient and simply better. It would take up more space, though. Especially sand filters tend to be big. Forget about "under the sink".
Another thing to keep in mind, is that RO water is so pure, it's corrosive. Even inox won't work. It's eaten in just a few months. All tubing needs to be special thick-walled PVC. At 600 psi, there aren't too many companies who can supply it.
Lead should not be a problem in most US markets but Flint MI is a reminder that stuff can happen.
The CDC estimated that over a thousand villages and towns had lead in the municipal water system. Most of those will be problematic one way or the other.
I have no idea about how many lead piping remains in older houses in the US.
I think it's no accident that the
one town in Belgium that still has lead piping is also the town ALL of our local terrorists seem to come from. Even the recent riots in Liège (between Albanians and Kurds, 1 death, several wounded) were between two youth gangs from Verviers. Everyone was shocked, as this kind of thing never seems to happen here.
The town of Verviers has a double problem: it's very expensive to replace all of the tubing because it's on a slope AND the local water is slightly acidic. Perfect to dissolve lead. Belgium has been paying millions a year as a fine to the EU cause nobody in politics has the balls to start the project, as it would certainly last well over four years and cost billions. Understandable, as the party that starts it, is sure to lose the next elections...
