Canreef Aquatics Ltd.

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RO/DI Guide

by Dave Visovatti ReefPure Inc.

Benefits of Using Pure Water in Reef Tanks

The benefits of pure water to the reef hobbyist are well documented:
elimination of nuisance algae and enhanced health of corals, invertebrates
and other marine wildlife. Currently the best technology for producing this
ultra-pure water for reef tank use is the RO/DI system. The RO/DI system is
a set of discreet technologies, which must be properly integrated to
optimize pure water production and minimize operation costs.

Basic System Design Considerations

In a properly designed RO/DI system the most vulnerable and expensive
components are placed nearest the point of use and are protected by upstream
water treatment components, which are less vulnerable and expensive. For
example, the DI cartridge is the last component in the chain of purification
and is one of the most vulnerable and expensive. It is protected by the
preceding reverse osmosis membrane, which in turn is protected by the carbon
block filter, which is preceded by the sediment filter. If any one of these
critical components is not properly selected or is of sub-standard quality
premature failure of downstream components can result.

Sediment Filters

First, let's talk about sediment pre-filters and the difference between
absolute and nominal micron ratings. In an absolute rated filter the filter
removes all particles larger than the stated micron rating. In a nominal
rated filter a certain percentage of particles larger than the nominal
rating will actually pass through the filter. The % efficiency rating of the
nominal rated filter tells you how many of the larger particles will pass
through. Most sediment filters are of the nominal rating variety. String
wound and spun filters have a low % efficiency rating and offer little
protection for downstream components. Resin bonded sediment filters offer a
very high nominal % efficiency rating and offer good protection. New
micro-sediment filters are absolute filters and provide superior protection
of downstream carbon filters and membranes.

Carbon Block Filters

Next in line are carbon block filters. The primary function of these filters
is to remove chlorine and further filter out sediment. They are available in
three ratings: 10 micron (removes chlorine for up to 3000 gallons), 5 micron
(6000 gallons) and 0.5 micron (20000 gallons) which is also effective in
removing organic chemicals such as pesticides, herbicides and
trihalomethanes. Since carbon block filters are absolute filters and are
prone to plugging with sediment it is essential that they are preceded by a
very high efficiency sediment filter. The best means of determining when to
replace the filter is to test the wastewater stream for presence of
chlorine. Any chlorine level above 0.1 ppm will cause damage to the membrane
and indicates that the carbon block filter must be changed.

Reverse Osmosis Membranes

The RO membrane is the workhorse of the water purification system. It uses a
thin film membrane technology to "electronically" reject impurities. RO
membranes are especially good at rejecting large atoms and molecules and
those species, which exhibit relatively strong electronic charge. It is less
effective in rejecting small atoms and molecules (smaller than water) and
those with relatively weak electronic charge. RO membranes will allow all
dissolved gases (such as ammonia and carbon dioxide) to pass through. The %
rejection rating for a membrane is an indication of how much of these
impurities will pass through the membrane and on to the DI cartridge. A
membrane, which has a 98% rejection rating, will pass only half as many of
these impurities as one with a rating of 96%. The direct result of using a
98% rejection membrane as opposed to a 96% membrane is that you will double
your DI cartridge lifetime. It is important to note that all membrane
manufacturers adhere to a lower % rejection limit for small membranes (2" x
10") of 96% at 60 psi and that the only way to ensure that you are using a
higher % rejection membrane is to purchase it from a source which screens
for higher % rejection membranes.

De-ionization Cartridges

The DI cartridge serves as a final polish of the water before it sees the
actual application. The cartridge contains a mixed bed of anion and cation
resins which uses an ion exchange mechanism to remove impurities that pass
through the RO membrane such as silica, phosphates, nitrates, ammonium and
other trace impuities. Like a filter, the DI cartridge must be monitored for
performance and replaced when it becomes exhausted. The best way to
accomplish this is through use of a resistivity monitor. Another way is
through the use of color change resins, which give a visual indication of
when the resins are exhausted. It is difficult to determine the exact
replacement point of color change cartridges, which often results in dumping
of silica and other contaminants back into the water.

Applications for Reef Tank Enthusiasts

So how does a reef tank enthusiast go about selecting the right water
purification equipment and components? Selection starts with understanding
the feed water you will be using and the water purity requirements of the
wildlife you intend to care for. Water is a very complex medium. In fact, it
is technically a solvent and readily dissolves many materials, which we
refer to as contaminants. The method of removal for these contaminants
determines what equipment and components must be employed in the design of
the water purification system. An aquarist who lives in an area which has
relatively problem free water and whose wildlife water purity requirements
are minimal may only need a stand alone RO system. More sensitive wildlife
such as corals and invertebrates will require the added water purification
performance of a DI cartridge. An aquarist in an area with problem water may
have to resort to difficult, relatively expensive solutions, especially if
the wildlife water purity requirements are high. Some of the more serious
problems encountered with feed water include:

Chloramine

Chloramine is used by some municipalities to control bacteria growth.
Chloramine is extremely detrimental to many wildlife species and readily
passes through RO membranes. Employing a DI cartridge will effectively
remove the chloramine but the high concentrations typically seen will
rapidly exhaust the cartridge making this a very expensive solution. An
alternative method often recommended is injection of acid into the feed
water prior to purification so that the chloramine is effectively removed by
the membrane.

Carbon Dioxide

Some areas have water which contains high levels of carbon
dioxide. While this is often not a big problem for wildlife it is a very big
problem for RO/DI systems. Carbon dioxide is a gas and readily passes
through the RO membrane and on to the DI cartridge where it is removed, once
again at great cost. The solution we most often recommend is direct removal
of the carbon dioxide through degassification of the RO product water prior
to introduction