RO units again

Barry Meyers-Rice (
Thu, 7 Apr 94 13:21:12 MST

The following posting is just about RO units and water purity, without
even a single CP species noted. Just a warning....

You may recall I posted the other day with a comment about Reverse Osmosis
(RO) units. I expressed concern that my unit and an associate's unit are
both producing waste and product water at a 1:1 ratio, and not a 3:1 ratio
or something like that. Well, I had a Culligan repairman come by, and had
an enlightening conversation. (For those unsure, Culligan is the name of
a business that supplies services to people and companies that require water
of various standards of purity. They do a lot of business with hospitals
and are generally considered to be a very reputable company.)

It turns out that in most home installations of RO units, the product
water is fed into a metal container which has a small, air-filled bladder
in it. As water is produced it first fills the tank. Additional water
produced by the unit enters the tank and acts to compress this bladder.
When someone wants a glass of water, they open a valve, the bladder expands
and forces water through the spigot.

As water is being produced by the unit to fill a nearly full metal
container, the product water must force its way into the chamber
which is progressively under higher and higher pressure. This results in
a greater rate of waste water production. So when you hear about a 3:1
ratio, that is the ratio of water production when the unit is being
operated in this way. But when I and my associate had our units installed,
we had no interest in using the metal container, and instead just let
the product water accumulate in 60 gallon drums. In this set-up, water
is produced at approximately a 1:1 ratio of product:waste which he and I
both measured. And an interesting thing is that the water produced this
way *is just as pure* as water produced with the back-pressure from the
metal container. I guess water conservation is not considered too important.

The serviceman had a nifty device for measuring total dissolved solids
(TDS) and found that my waste water had a TDS of about 200, while the
product water had a TDS=4.

One complication about this. When you shut off a unit for a few days,
the first water it produces is impure, and the unit must run for perhaps
30 minutes before the water produced is up to full purity. I did not know
this. Each week when I would water my plants and deplete my 60 gallon tank,
a switch in the tank would detect the water level drop and activate the
RO unit. This impure water would enter my tank. After a 30 minutes, the
unit began producing purer water which would enter the tank and dilute
the water produced during the first half hour. Then, after a few days,
the water was replenished and the unit shut off. But this effect is not
important, since the impure water at start-up is still only TDS=15 or so,
which is still very pure for CP purposes! I had the guy measure the water
quality in my 60g tank, and that had a TDS=6, so the effect is minimal
(there are a whole lot of dead bugs in that tank!)

I must say I was impressed with this guy from Culligan. This guy knew his
equipment, knew its capabilities, and was pretty informative once he sized
me up and decided I knew what I was talking about and that my questions
were intelligent. (I'm reminded of once getting some dental X-rays, and
seeing that the technician was frittering time by carelessly twirling
one of the knobs on the X-ray equipment. The knob was labelled in volts.
I asked the tech what the knob controlled, and he shrugged and said he
didn't know. I called for the dentist and had the knob reset to what it
was supposed to be at).

So I am relieved. The unit works well, is more efficient than I thought,
and all is placid in the world.