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Re: ro vs deionized water



Attached is a summary of the differences among RO, DI, and distilled water
from my reef book. - Bob Goldstein

Robert J. Goldstein & Associates, Inc.
Environmental Consultants
8480 Garvey Drive
Raleigh, NC 27616 USA
tel  (919) 872-1174
fax (919) 872-9214
URL   www.rjgaCarolina.com
                   RO, DI, and Distilled Water
     Evaporation can be made up with distilled, deionized (DI), or
reverse osmosis (RO) water. Distilled water has no salts or
nutrients, but does pick up atmospheric carbon dioxide during
condensation. Stills are laboratory instruments far too expensive
for aquarists. You can buy distilled water at supermarkets for
about a dollar a gallon. Tap water is usually unsuitable for make-
up water because its dissolved chemicals will alter the chemistry
of the aquarium over time, its nutrients may promote growth of
noxious algae, and municipal additives (chlorine, chloramine) may
require the addition of even more chemicals such as sodium
thiosulfate for neutralization. 
                 Differences between DI and RO 
     Commercial DI and RO units for the aquarium hobby produce good
water at a reasonable cost. Both require maintenance. RO membranes
must be replaced when flow rates drop. Deionizer units must be
recharged or have their cartridges replaced. RO water takes many
hours to produce in multi-gallon quantities, whereas running tap
through a DI unit will produce all the water you need in a short
time. RO units waste 75 percent or more of the water coming through
the unit, while deionziers waste none. RO units are expensive to
operate if your water/sewer service charges are based on water use.
RO units must be run for long periods, and accidental flooding due
to failure to turn off the unit is quite common. Both RO and DI
product water ideally should be tested with a total dissolved
solids meter to monitor effectiveness, but a hardness kit is a good
substitute.
     The combination of DI and RO units in series produces
excellent quality water, provides backup in case one unit fails,
and increases the lifetime of the resins and the membranes. It may
be worth the cost in parts of the country with very hard water, or
where house plumbing contains lead pipes or copper pipes with lead
solder. Where lead in tap water is high due to plumbing, let the
tap run ten minutes before taking water for drinking or aquarium
use. The effectiveness of an RO or DI unit can be increased by
combining it with a mechanical prefilter and activated carbon
adsorption filter. The best quality water combines all of the above
in the treatment train.

                         Deionized Water
     Deionization removes non-ionic gases, and is purer than
distilled water. A deionizer canister contains charged synthetic
resin beads or granules that attract ions of opposite charge. Newer
units use synthetic resins with differing attractiveness for
calcium and magnesium and for the anions nitrate and phosphate (NO3-
, PO4---). Units in series remove a broad spectrum of charged
chemicals. Deionizers require replacement of the resins or recharge
in a laboratory with strong acids or bases. As with distilled
water, you can purchase DI water in supermarkets for about a dollar
a gallon.

                        Reverse Osmosis 
     Reverse osmosis removes a high percentage (not all) of
minerals, nitrates, and phosphates, but not silicates. Tap water is
forced through a semipermeable membrane using the pressure provided
at the tap. Tap water is under pressure sufficient to squeeze the
water through the membrane while not allowing passage of minerals
and salts. An RO unit requires several hours of breaking in, but
afterward might produce 25 percent by volume of oxygen depleted,
cation-poor and anion-poor water and 75 percent by volume
wastewater containing 85-95 percent of the ions and other
impurities of the tap water.    
     RO efficiency depends on the impurity of incoming water, water
pressure, water temperature (room temperature is better than cold
or hot water), and the type and age of the semipermeable membrane.
Cellulose triacetate (CTA) membranes are readily degraded by
bacteria, and must be used in chlorinated water or they will break
down. More expensive thin film composite (TFC) membranes are
damaged by chlorine but work better on nitrates, yet in most areas
nitrates are not a problem. Vendors will advise you on the type
(CTA, TFC) and size unit (gallons/day) for your applications.  All
membranes have limited life expectancy, so product water should be
tested every couple of months with an expensive total dissolved
solids (TDS) meter or an inexpensive marine hardness test kit. 
     You can divert RO product water and wastewater to remote
locations in your fish room with tubing and connectors from a
hardware or appliance store stocking parts for refrigerator
icemakers.