05 Chemical Filtration
Description
This article is from the Aquaria: Filters FAQ, by Bruce Hallman.
05 Chemical Filtration
Chemical filtration, in short, is the removal of dissolved wastes from
aquarium water. Dissolved wastes exist in the water at a molecular
level, and fall into two general categories, polar and nonpolar. The
most common chemical filtration method involves filtering the water
through gas activated carbon which works best on the nonpolar wastes
(but also removes polar wastes). Another effective method is protein
skimming, which removes polar wastes such as dissolved organics.
Granular activated carbon (GAC) is manufactured from carbon, typically
coal, heated in the presence of steam at very high heat. This process
causes the carbon to develop huge numbers of tiny pores, which trap
nonpolar wastes at the molecular levels by means of adsorption and ion
exchange, and removes heavy metals and organic molecules, which are
the source of undesirable colors and odors, through a process known as
molecular sieving.
The best GAC for filtering water is made from coal and is macroporous
(having larger pores). A good macroporous activated carbon feels light
(not dense) and fizzes and floats when initially wetted. GAC intended
for removing wastes from air (such as odors) are commonly made from
coconut shell and are microporous. Carbons for filtering air feel more
dense.
Some people (especially those with reef aquaria) are concerned about
phosphate leaching from activated carbons. As a rule, buy only carbons
made by reputable aquarium supply companies which have been acid
washed during manufacture to minimize ash content. Carbons low in ash
also help reduce the chance of undesirable pH shifts. Low ash carbons
typically have lower phosphate leaching levels too.
The phosphate in GAC stems from the fact that activated carbon is
manufactured from coal, which was once living plant matter. All living
matter is high in phosphates. The leaching of phosphate from GAC is
known to be high initially and to decrease over time. This problem can
be mitigated significantly by presoaking your activated carbon for a
few weeks before use.
Some people are concerned about GAC removing trace elements required
by plants and invertebrates for healthy growth. Trace element
depletion is a problem in planted aquaria and minireefs, with or
without activated carbon. The potential benefits of activated carbon
are great enough that on whole you will be better off using it. If
trace element depletion is a worry, use a trace element supplement in
conjunction with the activated carbon.
GAC cannot be rejuvenated outside a laboratory, but fortunately, it is
cheap enough to use liberally. Always wash your carbon before use to
remove the dust that accumulates during shipment. Advice on how much
to use vary, but smaller amounts changed more frequently seem to work
best. You probably want to experiment, but 1/2 cup per 20 gallons
water, changed monthly is a good starting point. In summary, activated
carbon is an excellent, cheap and effective filtration method which is
highly recommended for all aquaria.
A variety of special chemical filtration media have been developed to
remove specific chemicals. A common one is made from the zeolite clay
(also used as some cat litters), and is marketed under such brand
names as ``Ammo-Carb''. This media removes ammonia from water, and is
good for short term use. The aquarist should be aware that if zeolite
is used, especially when cycling a new aquarium, then the
establishment of natural biological filtration will be delayed or
disrupted.
Protein skimmers are primarily used in saltwater aquaria, especially
reefs. They have the remarkable ability to remove dissolved organic
wastes before they decompose. The process involves taking advantage of
the polar nature of the organic molecules, which are attracted to the
surface of air bubbles injected into a column of water. The resultant
foam is skimmed off and discarded.
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