1.1.3 RO Filters
Description
This article is from the ReefKeepers FAQ, by several authors (see the Credits section).
1.1.3 RO Filters
Reverse osmosis (RO) units are normally based upon one of two
membrane technologies: cellulose triacetate (CTA) and thin film
composite (TFC). CTA based systems are typically cheaper and do
not filter as well (90-95% rejection rates). TFC based systems
cost more but have higher pollution rejection rates (95%-98%).
CTA membranes break down over time due to bacterial attack whereas
TFC membranes are more or less impervious to this. CTA units are
not recommended for reef tank purposes.
RO filters work by forcing water under pressure against the
membrane. The membranes allow the small water molecules to pass
through while rejecting most of the larger contaminates.
RO units waste a lot of water. The membrane usually has 4-6 times
as much water passing by it as it allows though. Unfortunately,
the more water wasted, the better the membrane usually is at
rejecting pollutants. Also, higher waste water flows are usually
associated with longer membrane life. What this means in practice
is that 300 gallons of total water may be required to produce 50
gallons of purified water.
Like any filter, RO membranes will eventually clog and need to be
replaced. Replacement membranes cost around $50-$100. Prefilters
are often placed in front of the membrane to help lengthen the
lifetime. These filters commonly consist of a micron sediment
filter and a carbon block filter. The micron filter removes large
particles and the carbon filter removes chlorine, large organic
molecules and some heavy metals. Of course, the use of prefilters
makes initial unit cost more expensive but they should pay for
themselves in longer membrane life.
RO units are rated in terms of gallons per day of output with
10-50 gallon/day units typically available. Note that the waste
water produced by a RO unit is fine for hard water loving
freshwater fish such as Rift Lake cichlids. Some route the reject
water to the family garden.
The Spectapure brand of RO units has a good reputation.
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