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No Better RO/DI Anywhere!!!
RO/DI stands for Reverse Osmosis and Deionization. The product is a
multi-stage water filter, which takes ordinary water in and produces
highly purified water.
Why do I need it?
Water often contains impurities that can cause problems when added to an
aquarium. These may include phosphates, nitrates, chlorine, and various
heavy metals. Phosphates and nitrates can cause algae blooms. Copper is
often present in tap water due to leaching from pipes and is highly
toxic to invertebrates. A RO/DI filter removes practically all of these
How does it work?
There are typically four stages in an RO/DI filter: sediment filter,
carbon block, reverse osmosis membrane, and deionization resin. If there
are less than four stages, something was left out (typically the DI
stage). If there are more, something was duplicated.
The sediment filter, typically a foam block, removes particles from the
water. Its purpose is to prevent clogging of the carbon block and RO
membrane. Good sediment filters will remove particles down to one micron
The carbon, typically a block of powdered activated carbon, filters out
smaller particles (often down to 1/2 micron or smaller), adsorbs some
dissolved compounds, and deactivates chlorine. The latter is the most
important part: free chlorine in the water will destroy the RO membrane.
The RO membrane is a semi-permeable thin film. Water is forced through
it under pressure. Molecules which are larger/heavier than water (which
is very small/light) penetrate the membrane less easily and tend to be
The DI resin exchanges the remaining ions, removing them from the
What are CTA, TFC, and PVC?
There are three types of RO membranes on the market: Cellulose
Triacetate (CTA), Thin Film Composite (TFC), and Poly-Vinyl Chloride
(PVC). Almost all of the membranes sold for aquarium use in the USA are
TFC. PVC membranes are currently available only outside the US. The
notable difference between these types is how they are affected by
chlorine: CTA membranes require chlorine in the water to prevent them
from rotting. TFC membranes are damaged by chlorine and must be
protected from it. PVC membranes are impervious to both chlorine and
bacteria. This FAQ assumes you're buying a TFC membrane.
Do I need a DI stage?
You can save some money by purchasing a three-stage filter lacking the
DI stage. Reverse osmosis typically removes 90-98% of all the impurities
of note. If that is good enough for your purposes, then the DI stage is
not necessary. RO filtration by itself is certainly better than plain
tap water and in many cases is perfectly adequate.
RO filtration by itself is not adequate if your tap water contains
undesirable elements that need to be reduced by more than 90-98%. For
example, if there is 10 PPM of phosphates in your tap water, reducing it
by 90% takes it to 1 PPM, which is still too high.
To save money up front, a DI stage can be easily added to the system at
a later date.
Can I use just DI?
A DI stage by itself (without the other RO filter stages) will produce
water that is pretty much free of dissolved solids. However, DI resin is
fairly expensive and will last only about 1/20th as long when used by
itself. If you're only going to buy RO or DI, go for the RO unless only
small amounts of purified water are needed.
Do I need a ½ micron sediment filter?
Opinions vary on whether filtering the input water finer than 5 microns
has any value. Finer filtration presumably helps prevent clogging of the
RO membrane, but there is a cost in terms of dollars and pressure loss.
The pressure loss is particularly a problem if you have low water
Each sediment and carbon stage should be finer than the one before it.
For example, a 5-micron sediment filter in front of a 1-micron carbon
block will work fine, but using a 1-micron sediment filter in front of a
5-micron carbon block is not advisable.
Where's the value in a 7-stage filter?
Duplicating stages can extend their life or improve their efficiency.
For example, if there are two DI stages in series, one can be replaced
when it's exhausted without producing any impure water. If both a 5-
micron sediment filter and a 1-micron filter are used, they will take
longer to clog up. If there are two carbon stages, there will be less
chlorine attacking the TFC membrane. Whether the extra stages are worth
the extra money is largely a matter of circumstance and opinion (they're
more useful if you use a lot of water).
Do I care about GPD?
RO/DI capacities are measured in gallons per day (GPD), typically in the
25 -100 GPD range. The main difference between them is the size or
permeability of the RO membrane. Other differences a
(a) The flow restrictor that determines how much waste water is
produced, which must match the membrane, and
(b) The water gets less contact time in the carbon and DI stages in
high-GPD units than low-GPD units.
As the GPD rating increases, the purity of the water produced by the RO
membrane declines. Membranes above 35 GPD are typically constructed by
welding two smaller membranes, meaning there's a seam. 100 GPD membranes
are typically more permeable, with a lower rejection rate. The DI stage
will make up the difference by removing the remaining impurities but
that affects the life of the DI resin.
Most aquarists won't use more than 25 GPD averaged over time. If a
decent size storage container is used, that size should be adequate. A
higher GPD rating comes in handy, however, when filling a large tank for
the first time or in emergencies when a lot of water is necessary in a
The advertised GPD values assume ideal conditions, notably optimum water
pressure (65 PSI) and temperature (70°F). The purity of your tap water
also affects it. In other words, your mileage will vary.
What if I have chloramine in my water?
Some water agencies add chloramine (a mix of ammonia and chlorine) to
disinfect drinking water. That's fine, except some carbon blocks are
inadequate to neutralize chloramine, so it damages your TFC membrane. It
can also pass right through an RO membrane and DI resin, yielding
ammonia in the resultant "pure" water. This is particularly a problem
with high-GPD units.
To find out if you have chloramine in the water, check with your local
water company. Chloramine use is particularly common in large
If chloramine is present in your water supply, this should be discussed
with the vendor prior to purchasing a system. The vendor may recommend a
second carbon stage, a "catalytic" type of carbon filter, or a lower-GPD
unit. At the time of this writing, the single best solution is not yet
clear and a combination may be required. In any case, don't trust a
vendor who isn't familiar with the problem.
What if I have well water?
Well water is free of chlorine so there's no need to worry about it
attacking the RO membrane. Do not buy a CTA membrane if you have well
water, as bacteria will destroy it. A carbon block is usually not needed
for well water, but well water often contains higher levels of
particulate matter than treated water. Consider adding a second
particulate filter in place of the carbon. If your well is prone to
"red" water problems due to iron bacteria, a back-flush option will help
reduce membrane fouling.
Why are there multiple outputs?
An RO filter has two outputs: purified water and wastewater. A well-
designed unit will have about four times as much wastewater as purified
water. The idea is that the impurities that don't go through the
membrane are flushed out with the wastewater.
There is nothing wrong with the wastewater except for a slightly
elevated dissolved solid content. It may be cleaner than your tap water
because of the sediment and carbon filters. Feel free to water your
plants with it.
An RO/DI filter may have separate outputs for the RO stage and the DI
stage. The RO output can be used for drinking or other purposes that
don't need highly purified water.
How do I hook it up?
The input line is connected to a cold water line. It can either be
hooked up with a saddle valve that pierces your existing copper pipe,
attached to your sink faucet with a special adapter (which is good for
those in rental properties or apartments), or to a hose bib.
The wastewater goes down the drain. If you have a PVC drainpipe, the
waste line can be connected to it. You just need a drill and a saddle.
The purified outputs go where you want them. A common approach is to
feed the DI output through a float valve to a reservoir. When using a
float valve system, an auto-shutoff valve is required to shut off the
incoming water to the RO unit. Otherwise, water will continue to pass
through the wastewater outlet, consuming water un-necessarily. Check
with the vendor at the time of purchase to discuss this option.
To make sure you have the correct adapters, installation problems should
be discussed with thereefkeeper when purchasing the system.
What is a TDS meter and do I need one?
A TDS (Total Dissolved Solids) meter measures the conductivity of the
water, which is an indication of water purity. Without one, it's
difficult to tell how well the RO/DI unit is working.
Read your tap water first. Readings in the 50-500 PPM range are typical
for most households. The RO output should be less than 10% of the tap
water. The DI reading should be 0 or 1. For example, if your tap water
reads 200, the RO output should be less than 20 and the DI output should
be 0 or 1.
Always let the unit run for a few minutes before measuring TDS on the
output. The first half-gallon or so will normally have an elevated
reading. That's because impurities will equalize across the RO membrane
over time when the unit is idle.
New RO/DI units may need to be thoroughly flushed out before reading the
TDS values. Let the manufacturer's instructions be your guide.
Note that TDS is not a good measure of water quality. It's entirely
possible to have perfectly good water with a reading of 500 and toxic
water with a reading of 50. Also note that some impurities don't
register. The purpose of the TDS meter is to measure the efficiency of
the RO/DI unit, not cast judgment on your water.
How do I know when the filter needs servicing?
Sediment and carbon stages: If you have city water (with chlorine) the
sediment and carbon stages should be replaced regularly. The rule of
thumb is every six months. Alternately, a swimming pool chlorine test
can be used: the carbon is OK if the test reads as zero. This is less
critical if you have well water. If you have a pressure gauge you can
tell when the sediment & carbon filters are clogged: the pressure will
start to drop.
RO membrane: There are two ways the RO membrane can fail. It can develop
holes, allowing impurities through, or it can get clogged up. If the
input pressure is OK but you're not getting the expected output, the
membrane is probably clogged. If the TDS meter shows RO output above 10%
of your tap water, it's developing holes. A RO membrane typically lasts
DI resin: The TDS reading on your DI output should read 0 or 1. The DI
resin is exhausted when the reading starts to climb. Some DI resins
change color as they are exhausted. Note that the color will probably
change well before the DI resin really needs to be replaced - use the
TDS reading to decide when to replace the resin.
Do I need a pressure gauge?
The gauge that comes with some RO units measures the pressure on the
input side of the membrane (or on the waste side, before the flow
restrictor, which will give the same reading). This allows you to tell
if there is adequate line pressure and if the sediment & carbon stages
are getting clogged. Optimum input pressure is in the 60-80 PSI range.
Below about 40 PSI the unit will operate less efficiently. The units are
typically not rated to operate above 80-90 PSI.
Do I care about temperature?
The GPD ratings are for room temperature (~70 ° F). Colder water travels
more slowly through the membrane, which reduces the output. If a high-
GPD unit is connected to a cold water line, that can be a problem. You
want approximately 25' or 30' feet of tubing from the connection at the
cold water running to the RO/DI unit.
Fill a 5-gallon bucket with water, and coil the excess tubing in the
bucket so it is submerged. Immerse a small aquarium heater set it to 78
degrees F. As the RO/DI unit kicks on, water in the tubing will be
warmed up to 78 as well, since it processes rather slowly, and the
membrane will be able to produce maximum output in the dead of winter.
Do I need a flush kit?
A flush kit allows periodic flushing of some water across the RO
membrane, thereby removing some of the gunk that sticks to it. Regular
flushing will extend the life of the membrane.
Do I need a booster pump?
The RO membrane works best when the input pressure is in the 60-80 PSI
range. Lower pressure reduces the output and increases the ratio of
waste to purified water. Below 40 PSI it will be greatly reduced. Below
30 PSI it won't work well at all. If your input pressure is less than
about 40 PSI, it's advisable to consider getting a booster pump.
Make sure there is a pressure cutoff switch for the booster pump
(connected to the RO output). Otherwise, it will run continuously.
Can I drink the purified water?
The RO output water is excellent for drinking. Most vendors offer a
drinking water kit that includes a pressure tank, a small faucet which
can be attached to your sink, and a post-filter for the drinking water.
The post-filter supposedly improves the taste.
A DI stage is not recommended for drinking water.
Do I need an UV sterilizer?
Some RO drinking water systems include an UV (ultraviolet) sterilizer.
This is appropriate if you are concerned about biological contamination
of your drinking water. It is a waste of money if you're just using the
water for an aquarium.
Should I buy a premium RO membrane?
Some of the name brands (notably Kent Marine and Spectrapure) offer
premium RO membranes that are claimed to remove a higher percentage of
impurities. Assuming it's true, these will extend the life of the DI
resin. For instance, if a "normal" RO membrane removes an average of 95%
and a premium membrane removes 98%, the DI resin should last over twice
as long. Whether that's worth the extra up-front cost, you can decide
What about silicates?
There's a lot of advertising hype about how some RO membranes are better
than others at removing silicates. Before spending extra money for that
feature read Silica in Reef Aquariums by Randy Holmes-Farley in the
January 2003 issue of Advanced Aquarist.
|Thread||Thread Starter||Forum||Replies||Last Post|
|"Recipe" for RO/DI water||DocBob||Tech||2||January 20th 04 01:41 PM|
|RO/DI||Pat Hogan||General||0||November 11th 03 02:16 AM|
|Best RO/DI 100GPD Anywhere!!||Pat Hogan||Tech||0||October 13th 03 06:04 PM|
|Best RO/DI 100GPD Anywhere!!||Pat Hogan||General||0||October 13th 03 06:02 PM|