A Primer on Aquarium Filtration Leo G. Morin, Ph.D

Filtration is fundamental to maintaining a healthy aquarium
environment and, unfortunately, it is an aspect of
the hobby that is, at best, poorly understood and, at
worse, fraught with unmitigated nonsense. Filtration is
the removal, or separation, of one or more substances
from one or more other substances There are three
basic types of filtration that apply to the aquarium:
mechanical, biological, and chemical.
Mechanical filtration is the removal of solid or undissolved
particulates from suspension in water by passing
the water through some type of mesh or porous mass.
The particles are removed from the water either by a
simple sieving effect or impaction on the filter medium.
The size of the particles removed can range from larger
than a grain of sand to smaller than bacteria or viruses.
Two mechanisms are involved in mechanical
filtration: the screening effect, or the
removal of particles simply because
they are too big to pass
through the pores of a filter, and
the depth effect, or removal or
entrapment of particles smaller than
the pores of the filter by impaction through
tortuous channels and cavities. With most methods of
mechanical filtration, such as with polyester pads, sand,
gravel, etc., the principal mechanism is the depth effect.
Some pressurized systems use pleated cartridges with
membranes that utilize primarily the screening effect. I
will wait to compare the relative merits of these systems
until we have considered the dynamics of aquarium
filtration or recirculation.
Biological filtration is the removal of both undissolved
and dissolved substances (the latter are called "solutes")
by biological consumption or biological conversion
of a toxic solute to a less toxic or harmless solute.
Biological filtration
(primarily nitrification) is absolutely essential to the
maintenance of any long-term closed system aquarium.
The biological filter is easily established by seeding a
mechanical filter bed with a nitrifying bacterial source,
such as ocean or pond water, gravel from an established
aquarium, or a commercial product. The required
ammonia can be supplied with a hardy fish or simply the
metered regular addition of ammonia as ammonium
hydroxide or other ammonium salt.
I have had good experience establishing the biological
filter in a reverse mode by the addition of sodium nitrite
to establish the Nitrobacter population first and then the
addition of ammonia to establish the Nitrosomonas. The
bacteria are not fussy about their source of ammonia or
nitrite, so that it is really not necessary to jeopardize a
fish to establish the cycle. The main advantage of this
reverse cycling (not to be confused with reverse flow) is
the avoidance of the Nitrobacter inhibition by excess
ammonia that ordinarily takes place in establishing a
cycle. This ammonia inhibition of Nitrobacter is the main
cause of cycle delays and failures.
Chemical filtration is the removal of
solutes (dissolved substances) from
solution by retention on a medium
through physical-chemical interaction
of solute and adsorbent. There
are three basic properties of solutes that can
be utilized to remove them from solution: molecular
size, net charge, and polarity. Separation by molecular
size should not be confused with mechanical filtration:
mechanical filtration involves undissolved substances,
while chemical filtration involves dissolved substances
(solutes). In the aquarium, solutes are dissolved in one
common substance, the solvent water. Water is a small
polar molecule with no net charge. Examples of small
solutes include all the cations and anions of dissolved
salts, small metabolites such amino acids, sugars, and
fatty acids. Examples of large solutes are proteins, peptides,
starches, some fats, and added polymeric conditioners.
Any ionizing substance is a good example of charged
solutes, such as salts, acids, and bases. Uncharged
solutes are covalent compounds with no ionizing
groups, for example, water, sugars, starches, alcohols,
ketones, and aldehydes. Charge and polarity are easily
confused, but they are not equivalent. Charge refers to
a net negative or positive charge that is not internally
balanced. Polarity refers to the possession of balanced
charged zones. Sodium is a positively charged ion, but
it is not polar; water has no net charge, but it has a
positive and negative zone and is, therefore, polar. In an
electrical field, charged solutes migrate to, and collect
at, the pole of opposite charge; polar substances do
not migrate, but only orient their zones towards the
poles of opposite
charge. Another important
distinction is
that polar are always
polar, but charged can
become uncharged,
as for example, with a
change in pH. A good
example of this in the
aquarium is ammonia.
Free ammonia is an
uncharged polar substance,
but, with
decreasing pH, more
and more free ammonia
ionizes to the ammonium
ion, a still
polar but now also
positively charged
substance: NH3 <—>
NH4. See Fig. 1 for
some illustrations of
these concepts.
Just as there are three
general types of physical-chemical properties that permit
the separation of solutes, so, also there are three
types of filtrants or filtration media: (I) molecular sieves
that separate on the basis of size: (2) ion-exchangers
that separate on the basis of charge; and (3) adsorbents
that separate on the basis of polarity. While a given
filtrant may belong to one of these three types, it is not
unusual for two or all three processes to be going on.
Molecular sieves can be visualized as multiple mazes
with openings of varying sizes. As solutes pass by these
mazes, those that are small enough to enter do so and
become trapped. Occasionally, some solutes find their
way out, but more enter than leave. Eventually, however,
the mazes fill up and it becomes easier to. find a way
out until just as many leave as enter.
Ion-exchange is a process where an ionized solute
present in large numbers or with a strong charge takes
the place of another ion that is attached to a matrix.
Adsorption is a process where polar solutes become
attached to polar surfaces and non-polar solutes are

pushed and held against non-polar surfaces by surrounding
polar substances (water). Usually, polar and charged substances
are hydrophilic (attracted to water) and non-polar and
uncharged substances are hydrophobic (repelled
by water). Since water is the solvent in
all aquarium filtration, hydrophobic solutes are removed more
easily by adsorption. As solutes become progressively hydrophilic,
removal becomes more and more dependent on molecular sieving and
opposite charge effects, including ionexchange.

Of the three processes, the most important
for aquarium filtration is adsorption, followed by ionexchange.
Molecular sieve action is usually an integral
aspect of both adsorption and ion-exchange, and is a
limiting factor for both types of filtrants. Because the
action of most chemical filters tends to be mixed rather
than exclusively one process, it is best to look at specific
filtrants rather than process types.
CARBON. Activated carbon is prepared by carbonizing
coal, wood, bone, nut shells, or other organic material
at 900°C, then activating with steam, air, or carbon dioxide
at 800°-900°C. This treatment drives out hydrocarbons,
increases surface area, and develops porosity.
Differences in adsorption characteristics are due to
these treatments and the addition of inorganic salts
such as zinc, copper, phosphate, sulfate, and silicate
before activation. Caustic and acid washes are also
frequently used to both change adsorption characteristics
and to remove soluble materials.
In the aquarium, activated carbon removes relatively
non-polar or hydrophobic organic solutes from the polar
solvent water. Charged solutes such as ionized salts are
repelled by carbon and not adsorbed. Water is strongly
polar and is poorly retained by carbon. The more nonpolar
or hydrophobic, or the less soluble in water, the
solute is, the more strongly is it retained on carbon.
Many metabolites, such as amino acids, are retained
only at a certain pH, called the isoelectric point, where
the solute has no charge. If the pH changes, the substance
acquires a charge and is released by the carbon.
Proteins and peptides tend to be strongly retained because
they have many non-polar side chains. If the
carbon is rich in zinc or copper it will retain ammonia
and other amines through complex formation. Although
ionizing salts are repelled by carbon, some heavy metals
such as copper, mercury, and zinc are retained by
carbon under alkaline conditions. Some carbons are
rich in insoluble phosphates, carbonates, silicates, or
oxides, and these carbons have a relatively high capacity
for polar and positively charged groups. Carbons that
have not been acid-washed have more of these polar
groups than acid-washed carbons, but also contain
more soluble contaminants, such as metals and carbonates,
which can cause toxicity and pH problems,
particularly in fresh-water aquaria.
Aside from the chemical nature of the carbon surface
(non-polar), the major factor in carbon filtration is actually
molecular sieving. Carbons can be looked upon as
sponges or mazes with large openings that lead successively
to smaller and smaller channels with smaller
and smaller openings. The capacity and, to some extent,
the adsorptive characteristics of a given carbon
depend on its surface area and pore volume. Surface
area refers to the internal surface of the carbon particles.
The more channels inside the carbon, the greater
the surface area. Pore volume
refers to the amount of emptiness inside the carbon.
The greater the surface area, the greater is the capacity;
and the greater the pore volume, the greater is the
efficiency. There is a working limit of about 0.7 ml/cc for
pore volume, since increasing pore volume also increases
the fragility of the carbon. Increasing the surface
area without increasing the pore volume results in
diminished mean pore size (fewer large channels and
more small channels), which, in turn, limits entrance to
the carbon to progressively smaller solutes. The ratio of
surface area to pore volume, then, is a valuable guide to
the mean pore size: the greater the ratio, the smaller the
pore size.
Several ways of grading carbon include surface area,
iodine number, molasses index, and carbon tetrachloride
activity, but none of these are, in themselves,
meaningful for evaluating adsorbents for the specific
use of aquarium water purification. The best measure of
an adsorbent is the ratio of total surface area (TSA) to
pore volume (PV). To facilitate interpretation, total surface
area should be expressed in square meters per
cubic centimeters (m2/cc) and pore volume in milliliters
per cubic centimeters (mi/cc). If these are reported on
the basis of weight (grams) instead of volume (cc), then
the density in grams per cubic centimeters (g/cc) must
also be reported. Unfortunately, with few exceptions,
sources of aquarium carbon do not provide these valuable
figures. While there are only five manufacturers of
carbon in this country and all commercial aquarium
carbons come from one of these, all these sources
supply numerous grades of carbon from very economical
water treatment grades to expensive pharmaceutical
grades. Not all aquarium carbon vendors provide the
best carbon for the application; many provide the carbon
with the best profit margin.
My study of carbons for the purification 9f aquarium
water indicates that the better carbons have a TSA of
450 to 550 m2/cc and a PY of 0.45 to 0.60 mI(cc with a
TSA/PV ratio of 700 to 1000. Carbons that have not
been acid-washed have better buffering ability for marine
aquaria and greater retention of polar and charged
solutes. Acid-washed carbons, however, are safer for
fresh water and, with poor grades of carbon, the acidwashed
versions are safer for marine aquaria as well.
Given a choice, the acid-washed version of a particular
carbon is usually preferable. Generally, the better carbons
are prepared from bituminous coal. Acceptable
carbons can be prepared from nut shells, wood and
bone. Paper mills waste and other organic waste car-
bons are not acceptable. Some carbons are not truly
activated carbons, but mere charcoal or, worse, just
ground-up coal. If the TSA of a carbon is reported in
units other than those used here, it is possible to convert
the units for comparison by recognizing that:
1 m2 = 1.2 yd2 = 10.8 ft2; 1 cc = 0.06 in3 = 0.0338'
fluid oz; and 1 g = 0.035 oz. If the TSA is given on the
basis of weight and the dry density is not given, then
directly comparable figures are not possible, but, generally,
a good carbon should have a TSA of at least 1000
m2/gram and the better carbons have a TSA of about
1500 m2/gram. The PY should be at least 0.4 mI/cc.
Since an important consideration with carbon adsorption
is surface area, it might be surmised that powdered
carbon is better than granular carbon. This, however, is
not the case, since the surface area that is important is
the internal surface area, not the external Diminished
particle size only increases external surface area, and,
by comparison to the total surface area, the gain in
surface area from smaller particles is relatively slight.
The choice of particle size, then, is not governed by
surface area considerations. The particle size of choice
should permit unimpeded and uniform flow through the
carbon and allow rapid penetration of solute into the
inner network of the carbon particle. The optimum size
that satisfies these requirements is about the size of a
pinhead, 0.5-1.5 mm or 1/32-1/16 in. (10-40 mesh).
Smaller sizes impede flow while larger sizes produce
non-uniform flow and retard penetration of solute into
the carbon
matrix. With a carbon about the size of a pinhead,
nearly 90% of the available surface area will be utilized
before exhaustion; but with a carbon about the size of a
small pea (5-8 mm), only about 40% of the available
surface area will be utilized before exhaustion, due to
the inability of solutes to penetrate the carbon particle.
If specifications of a carbon are not available or if you do
not want to bother with all those numbers and calculations,
what should you look for in a carbon? First, is it
the right size? That should be pinhead size. Avoid the
more common, convenient, and prevalent larger sizes.
What is the appearance of a rinsed, but dry, particle? If
it is dull, flat black, this indicates a fairly porous particle.
If it is relatively shiny or glossy black, the carbon is relatively
non-porous and should he avoided. What happens
when you put it in water? If it floats or is buoyant
and takes several hours to fully wet, making a hissing
sound as it does, that indicates a porous, air-filled,
hydrophobic carbon. If it sinks relatively quickly and
emits little or no air or hissing sound, avoid it. What
about on the shelf? Compare weights and volume.
Select the carbon that takes up the most volume for a
given weight. The better carbons are more porous (less
dense), which means that for a given volume, they
weigh less. Carbon's action is a consequence of its
surface area and volume, not its weight. It is inconsistent
to buy or sell carbons by weight alone.
POLYMERIC ADSORBENTS. These adsorbents are
synthetic porous molecular sieves based on styrene or
acrylic polymers with controlled non-polar to polar surface
properties. They function in essentially the same
way as carbons and the operating optimum TSA, PV,
ratio and particle size are the same as for carbons. Their
TSA range from about 300 m2/cc to 500 m2/cc with PY
ranging from about 0.4 to 0.6 ni/Icc. The TSAIPV ratio
ranges from 700 to 850. Strictly speaking, only uncharged
adsorbents should be considered polymeric
adsorbents. Several synthetic adsorbents available for
aquarium use are not uncharged, but are in fact ionexchangers
and they will be considered separately. By
comparison to carbons, polymeric adsorbents generally
have a less efficient porous structure, but more effective
surface properties and more predictable adsorption of
polar as well as non-polar solutes. Although the overall
capacity of these adsorbents is less than that of carbon,
they have strong affinity for some solutes of importance
that are not retained by carbon. Organic acids and both
organic and inorganic nitrogen compounds are good
examples. Overall, carbon is superior, but there is a
sound basis for using both polymers and 'carbons together.
Polymeric adsorbents are usually white to tan,
dull, and have the shape of small beads about the size
of a pinhead. It is also possible to manufacture them as
fibers.
ION-EXCHANGERS. There are several types of ionexchangers.
There are mineral or natural exchangers
and synthetic exchangers. The mineral exchangers are,
like carbon, molecular sieves, but have much less TSA,
only about 5-50 m2/cc. These exchangers are zeolites,
kaolins, or other type of clays. They have limited ex-
change capacity and are poorly defined; consequently,
they have limited application. They probably would not
be used at all, were it not for being very economical.
Chemically, they are mixtures of aluminum, magnesium,
zinc, and other metal silicates. The ion-exchange property
is primarily due to surface oxygen of the silicate,
making this material primarily a cation exchanger, usually
exchanging ammonium ions from water for sodium
or calcium ions on the exchanger. Some also have very
limited anion exchange capacity. These mineral exchangers
are not suitable for salt water use, because
the high salt content would render them ineffective and
would tend to release toxic metals into the waler. These
exchangers are promoted commercially mainly for removal
of ammonia from fresh-water aquaria. If you have
noticed a physical similarity between your ammonia
absorbent and kitty litter, the similarity is not accidental.
These exchangers also have limited adsorptive capacity
for polar charged groups.
Synthetic exchangers are defined as either anion or
cation exchangers and are available as either microporous
or macroporous types. The microporous types have
only very small pores that admit only small inorganic
ions. These have been used for many years to deionize
water or soften water for household use. In the aquarium,
however, proteins, bacteria, colloids, and other
large solutes quickly plug up or "foul" the micropores of
this type of exchanger and render it useless. The macroporous
types are molecular sieves with TSA ranging
from 25 to 506 m2/cc with PV ranging from 0.2 to 0.6
mi/cc. These macroporous exchangers are much more
resistant to fouling than microporous types. There are
four types of macroporous exchangers: strong anion,
weak anion, strong cation, weak cation. Without getting
overly technical, the main difference of importance for
aquarium use between strong and weak exchangers is
that only the strong exchanger is a true exchanger in
that it will split off its counterions and generate and
adsorb corresponding counterions from solution. Weak
exchangers are not true exchangers in that they only
adsorb already existing free ions without actually generating
any ions themselves.
Ion-exchangers have valuable uses in the aquarium,
particularly for fresh water. A mixture of strong anion
and strong cation exchangers will effectively produce
soft water of slightly acid pH as well as remove ammonia
(ammonium) and other ionic metabolites. Weak exchangers
will also produce soft water and at a controlled
pH. Customized stable fresh water can easily be
attained by the intelligent use of ion-exchangers.
With salt water, the high sodium, chloride, calcium,
magnesium, and sulfate content quickly equilibrates
with strong exchangers and renders them virtually useless
as ion-exchangers. Macroporus types, however,
retain their usefulness for organic removal. Weak exchangers
have limited but useful applications. These
exchangers can be used to effectively remove heavy
metals and to remove acids. Acid removal with weak
exchangers significantly promotes good pH control.
Weak anion exchangers are amines and are very effective
in removing copper, including many types of chelated
copper, turning blue as copper Is complexed to
the exchanger.
Most ion-exchangers available commercially for aquarium
use are clays for fresh water use, usually, but not
always, restricted to ammonia removal. There are a few
sources of microporous strong exchangers for softening
water. The possibilities for ion-exchange in fresh-water
aquaria of aquarists have not been adequately or intelligently
explored. The use and recommendations for ionexchangers
in salt water are at best confused. The
combined use of carbons, polymeric adsorbents, and
judicious selection of ion-exchangers results in improved
water quality, which, in turn, leads to more colorful,
healthier, more active fish and invertebrates than is
otherwise possible. Appetites are more aggressive and
one mixed blessing is the comparatively unimpeded or
uninhibited growth.
Synthetic ion-exchangers are usually beads about pinhead
in size ranging in color from tan or off-white to
dark brown. Microporous exchangers are usually translucent
and shiny, while macroporous exchangers are
opaque and dull. Anion exchangers are usually off-white
to tan, while cation exchangers tend to be gray or
brown to dark brown.
BOTTOM FILTRATION. Although bottom filtration is
primarily biological, considerable chemical activity is
also involved, at least in the marine aquarium. The principal
component of bottom filtration in the marine
aquarium is magnesium carbonate in one form or an-
other, either as dolomite, crushed oyster shell, or
crushed coral. This material behaves as a cation exchanger
and polar adsorbent, not unlike the zeolites
used in fresh water. The principal recognizable chemical
action of magnesium carbonate is the removal of heavy
metals, including trace elements such as copper and
vanadium. Many organics, including proteins, amino
acids, vitamins, and
medications are also adsorbed to, magnesium carbonate,
although the capacity is very limited. The adsorbed
material is eventually attacked biologically. Carefully
controlled experiments show that a surprising amount
of ammonia is retained on the surface of magnesium
carbonate. This is also eventually attacked biologically.
AMMONIA ABSORBERS.
Ammonia removal is so important to aquarium maintenance
that it warrants separate attention.
Unquestionably, the best long-term route to ammonia removal is
biological filtration.
There are, however, numerous products available to aquarists
to supplement the action of biological filtration. To look at
these products intelligently, it is necessary to understand ammonia,
the solute. Ammonia, as already indicated in Figure
1, is a covalent polar compound, not unlike water,
which ionizes in aqueous solution to the positively
charged ammonium ion. The interconversion of ammonia
and ammonium is a reaction at equilibrium, and,
with increasing pH, ammonia is favored, while, with
decreasing pH, ammonium is favored. If an absorber
removes one of the forms, then the equilibrium is shifted
in that direction and all the ammonia is eventually consumed.
Removal of ammonia as an uncharged polar compound
requires a very strong polar adsorbent, since the environment
is water and, likewise, strongly polar. Some
polymeric adsorbents are able to do this, but only to a
very limited extent, and more so in salt water than in
fresh water. The polar solvent water competes very
effectively with ammonia for polar adsorbent sites, and
the polar interactions between ammonia and water are
effective in eluting ammonia from the adsorbent.
Removal of ammonia as the ammonium ion calls for an
ion-exchange process and ion-exchangers can do this
effectively in fresh water. Even these exchangers, however,
have limited capacity and once the available sites
are saturated no further adsorption can take place.
Ammonium ions, however, are not strongly held by ionexchangers
and the addition of
even relatively small
quantities of salts, as
is frequently done in
fresh-water aquaria,
dramatically decreases
the ability of ionexchangers
to remove
ammonium. Under
ideal conditions, the
best synthetic ammonia
absorber has a
capacity of about 60
mg of ammonia per ml
of absorber. For a 10
gallon aquarium, this
translates into a capacity
of 1.5 mg/Liter
(ppm) for each ml of
absorber. For a 10
gallon aquarium, about 50-200 ml of absorber would
generally be used, giving a total capacity of 75-300
ppm of ammonia. This is a cumulative capacity and
once attained the absorber will be saturated and no
longer function, unless regenerated. The addition of as
little salt as 5 teaspoons per gallon will cut capacity by
more than 50%. Zeolite absorbers have about 1/4 to 1/
5 the capacity of synthetic absorbers. Zeolites are easily
recognizable as dusty, white to tan granules, similar to
kitty litter. Synthetic absorbers are dustless, tan to
brown beads or fibers.
DYNAMICS OF AQUARIUM FILTRATION. The most
useful vantage from which to examine aquarium filtration
is efficiency, which is defined as percent impurity removal
per unit of time. A complex interaction of interdependent
factors, depicted in Figure 2, govern the efficiency
of aquarium filtration.
Any factor which results in an increase in the volume
that must pass through the filter decreases efficiency,
while any factor that increases throughput increases
efficiency. However, since many factors do both. the net
result of changing any filtration variable is not usually as
obvious as might be supposed.
The most obvious, but also the most overlooked, factor
is recirculation, which causes clean or filtered water
issuing from the filter to be continuously mixed with the
relatively less clean or unfiltered in the aquarium. The
mathematics of recirculation are similar to compound
interest, except the percent change is
negative. Assuming a filter totally removes all of a given
impurity, that is, it retains 100% of an impurity of the
water passing through the filter, no mixing at all would
require that 100% of the total volume of water pass
through the filter to remove all available impurity; a 50%
mix would require that 332% of the water pass through
to remove all of the impurity; a 90% mix would require
437%; a 99% mix would require 458%; and continuous
mixing, the actual situation with aquarium filtration,
would require 460%. Put differently, recirculation is a
constant that decreases aquarium filtration efficiency by
increasing by a factor of 4.6 the cycle frequency, or the
number of aquarium volumes, that must pass through
the filter to effect 99% impurity removal.
The factor that has the most influence on efficiency is
retention, the percentage of impurity concentration
retained on the filter medium or removed from the
passing water. Figure 3 shows the percentage of total
volume, or number of cycles, that must pass through a
filter to achieve 99% impurity removal at different %
retentions. Since 100% retention is a rare exception. it
is clear that the combination of low retention and recirculation
requires very large volumes of water pass
through a filter for effective removal of impurities.
Four factors directly effect retention: geometry. flow rate,
solute-adsorbent effects. and concentration effects. Poor
geometry is without a doubt the principle cause of low retention
and consequent low filtration efficiency characteristics
of too many aquarium filters. The two most common
geometry defects are tubing locations that allow leakage
around the filter medium, and low filter bed heights. Filtration
requires a minimum bed height of about I cm, and the
deeper the bed the better. Deeper beds are more retentive
because they minimize leakage, they increase contact time,
and each progressive layer behaves as a series of separate
filters rather than a parallel of separate filters.
There are three basic filter geometries: the box filter, the
cartridge filter, and the canister filter. The box filter is
characterized by a relatively small surface area with
limited flexible bed depth. Disposable "cartridges" for
box filters impose, in addition, a fixed and shallow bed
depth. Box filters also generally have several locations
for by-passing (leakage) the filter medium around flow
tubes or cartridges. Cartridge filters are characterized
by relatively large surface areas with fixed shallow bed
depths, and, therefore, are more retentive as sieve filters
than as depth filters. Canister filters have small surface
areas, but deep beds. Both cartridge and canister filters
have insignificant by-pass or leakage.
Retention of solutes on a filter medium is
directly proportional to the volume or
quantity of that medium, and, for a given
amount of medium, is proportional also to
the bed depth. Figure 4 shows the effect
of bed depth on the break-through flow
rate for representative volumes of filter
medium.
Break-through flow is the minimum flow,
expressed as volumes of the filter medium,
at which solutes leak through the
medium. It is evident that canister filters,
with their deep beds, have remarkably
more retention than either box or cartridge
filters. The break-through flow can
be determined empirically with dye solutions.
It can also be approximated by
calculation:
Break-through flow (gal/hr) =
dcm(0.009)cc where d is the depth of
the filter bed in cm and cc is the
volume of filter medium. The volume
of various filters can be calculated as
follows: box volume
(cc) = lcm x wcm x dcm cartridge
volume (cc) = [hπr2]e – [hπr2]i canister
volume (cc) = dπr2
where I is length, w is width, h is
height, d is depth, r is radius. For box
and canister filters, d = h; for cartridge
filters, d is equal to distance between
external and internal walls. All dimensions should be in
cm. Typically. the break-through flow rate for a small
box filter equipped with a disposable cartridge is about
2 gal/hr; for a cartridge filter, about 10-12 gal/hr; and for
a canister filter, about 100-140 gal/hr.
The break-through flow rate is not usually the optimum
operating flow rate. As is evident from Figure 3, large
volumes of water must pass through the filter for effective
removal of impurities. For this to happen in a timely
manner, it is usually necessary to sacrifice some absolute
retention for overall timely removal. Figure 5 shows
the effect on retention of increasing the flow rate be-
yond the break-through flow rate. Figure 6 re-draws the
data from Figure 3 against unit time where unity is defined
as the time required to clear 99% impurity at
break-through flow rate. This type of plot shows the
effect of the interaction of retention and flow rate on
filtering efficiency.
For example, the plot shows that filtration at 100%
break-through rate is no more effective than filtration at
340% that rate. The overall optimum is about 200% of
the break-through flow. The plot shows that flow rate
can be increased up to about 400-500% of the breakthrough
rate without seriously sacrificing efficiency. Beyond
that, however, increased flow rate begins to have
consequential negative effects on filtration efficiency.
Typically, this limit to about 10 gal/hr for box filters, 60
gal/hr for cartridge filters, and 700 gal/hr for canister
filters. Both box and cartridge filters generally are operated
well beyond this maximum, while canister filters are
operated well below this maximum. Canister filters, in
fact, are operated very closely to their optimum, generally
around 150 gal/hr.
Solute-adsorbent effects are relatively complex and
have already been discussed, but, in general, hydrophilic
solutes are adsorbed at hydrophilic sites and
hydrophobic solutes at hydrophobic sites. Carbons and
polymeric adsorbents are more hydrophobic than hydrophilic.
Synthetic ion-exchangers are more hydrophilic
than hydrophobic. Some gel-type adsorbents are more
hydrophilic than hydrophobic.
Concentration of solutes affects solute adsorption in a
manner which is predictable from mass action considerations.
The greater the concentration of solute, up to
a limit characteristic of the capacity of the adsorbent,
the more readily is it adsorbed. As the solute concentration
drops, the rate of removal drops proportionately.
These principles of filtration dynamics have been
worked out primarily for chemical filtration, but they
apply, generally, to any type of aquarium filtration, with
the exception of mechanical filtration by sieving action.
In that case, the depth of the filter bed is of little importance
and external surface area becomes paramount.
For mechanical filtration by sieving action, cartridge
filters are unexcelled. For all other types of filtration,
including mechanical depth filtration, biological filtration.
and chemical filtration, the canister filter is clearly superior.
Box filters, as they currently exist, with their poorly
placed tubing, small volumes, and shallow bed depths,
are remarkably inefficient.