A Primer on Aquarium Filtration
Leo G. Morin, Ph.D
Filtration is fundamental to maintaining a healthyaquarium environment and, unfortunately, it is an
aspect of the hobby that is, at best, poorly understoodand, 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 theaquarium: 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 removalof particles simply because they are too big to passthrough 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 tortuouschannels and cavities. With most methods of mechanical filtration, such as with polyester pads, sand, gravel,
etc., the principal mechanism is the depth effect. Somepressurized 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-termclosed system aquarium. The biological filter is easily
established by seeding a mechanical filter bed with anitrifying 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 orother ammonium salt.

I have had good experience establishing the biologicalfilter in a reverse mode by the addition of sodium nitrite to establish the Nitrobacter population first andthen the addition of ammonia to establish the Nitrosomonas. The bacteria are not fussy about their sourceof 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 confusedwith reverse flow) is the avoidance of the Nitrobacter
inhibition by excess ammonia that ordinarily takes
place in establishing a cycle. This ammonia inhibitionof Nitrobacter is the main cause of cycle delays andfailures.
Chemical filtration is the removal of solutes (dissolved
substances) from solution by retention on a medium
through physical-chemical interaction of solute andadsorbent. There are three basic properties of solutes
that can be utilized to remove them from solution:
molecular size, net charge, and polarity. Separation bymolecular 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 nonet charge. Examples of small solutes include all thecations 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 internallybalanced. 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 apositive 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 the1000 Seachem Drive • Madison, GA 30650 • 706.343.6060 • FAX 706.343.6070 • 888-SEACHEM • www.seachem.com

poles of opposite charge. Another important distinction is that polar are always polar, but charged canbecome 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 ofphysical-chemical
properties that permit
the separation of solutes, so, also there
are three types offiltrants or filtration
media: (I) molecularsieves that separate
on the basis of size:(2) ion-exchangers
that separate on thebasis of charge; and(3) adsorbents that
separate on the basisof polarity. While agiven filtrant may
belong to one of thesethree types, it is not
unusual for two or all
three processes to be
going on.

pushed and held against non-polar surfaces by surrounding polar substances (water). Usually, polar andcharged 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 molecularsieving and opposite
charge effects, including ion-exchange. Of
the three processes,
the most important
for aquarium filtration is adsorption,
followed by ionexchange. Molecularsieve action is usually
an integral aspect ofboth adsorption andion-exchange, and is alimiting factor forboth types of filtrants.
Because the action ofmost chemical filters
tends to be mixed
rather than excluFig. 1. Fundamental structures of aquarium biologicals
sively one process, it
associated with ionic, hydrophyllic, hydrophobic, and pois best to look at spelar attractions useful in filtrationcific filtrants rather
than process types.

Molecular sieves can be visualized as multiple mazeswith openings of varying sizes. As solutes pass bythese mazes, those that are small enough to enter doso and become trapped. Occasionally, some solutes
find their way out, but more enter than leave. Eventually, however, the mazes fill up and it becomes easierto. 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

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 outhydrocarbons, 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 arealso 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 thepolar solvent water. Charged solutes such as ionizedsalts are repelled by carbon and not adsorbed. Water

is strongly polar and is poorly retained by carbon. The
more non-polar or hydrophobic, or the less soluble inwater, the solute is, the more strongly is it retained oncarbon. Many metabolites, such as amino acids, areretained only at a certain pH, called the isoelectric
point, where the solute has no charge. If the pHchanges, the substance acquires a charge and is released by the carbon. Proteins and peptides tend to bestrongly retained because they have many non-polarside chains. If the carbon is rich in zinc or copper itwill retain ammonia and other amines through complex formation. Although ionizing salts are repelled bycarbon, 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 acidwashed have more of these polar groups than acidwashed carbons, but also contain more soluble contaminants, such as metals and carbonates, which cancause toxicity and pH problems, particularly in freshwater aquaria.

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 specificuse of aquarium water purification. The best measureof 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, withfew exceptions, sources of aquarium carbon do notprovide these valuable figures. While there are only
five manufacturers of carbon in this country and allcommercial aquarium carbons come from one ofthese, all these sources supply numerous grades ofcarbon 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.

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 thatlead successively to smaller and smaller channels withsmaller and smaller openings. The capacity and, tosome 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

My study of carbons for the purification 9f aquarium
water indicates that the better carbons have a TSA ofrefers 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 theefficiency. 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 thesurface 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, isa valuable guide to the mean pore size: the greater theratio, the smaller the pore size.

450 to 550 m2/cc and a PY of 0.45 to 0.60 mI(cc witha TSA/PV ratio of 700 to 1000. Carbons that have notbeen acid-washed have better buffering ability for marine aquaria and greater retention of polar andcharged solutes. Acid-washed carbons, however, aresafer for fresh water and, with poor grades of carbon,
the acid-washed versions are safer for marine aquaria
as well. Given a choice, the acid-washed version of aparticular carbon is usually preferable. Generally, thebetter carbons are prepared from bituminous coal.
Acceptable carbons can be prepared from nut shells,
wood and bone. Paper mills waste and other organic
waste carbons 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 isreported 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 thebasis of weight and the dry density is not given, thendirectly comparable figures are not possible, but, gen-erally, 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 notgoverned by surface area considerations. The particle
size of choice should permit unimpeded and uniform
flow through the carbon and allow rapid penetrationof solute into the inner network of the carbon particle.
The optimum size that satisfies these requirements isabout the size of a pinhead, 0.5-1.5 mm or 1/32-1/16
in. (10-40 mesh). Smaller sizes impede flow whilelarger 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 thesize of a small pea (5-8 mm), only about 40% of theavailable surface area will be utilized before exhaustion, due to the inability of solutes to penetrate thecarbon 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 heavoided. 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 thatfor a given volume, they weigh less. Carbon's action isa consequence of its surface area and volume, not its
weight. It is inconsistent to buy or sell carbons byweight alone.
POLYMERIC ADSORBENTS. These adsorbents aresynthetic porous molecular sieves based on styrene or
acrylic polymers with controlled non-polar to polar
surface properties. They function in essentially thesame 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 consideredpolymeric adsorbents. Several synthetic adsorbents
available for aquarium use are not uncharged, but arein fact ion-exchangers and they will be consideredseparately. By comparison to carbons, polymeric adsorbents generally have a less efficient porous structure, but more effective surface properties and morepredictable 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 andinorganic 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, andhave 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 exchange 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 ionexchange property is primarily due to surface oxygenof the silicate, making this material primarily a cationexchanger, usually exchanging ammonium ions fromwater 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 renderthem 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 physicalsimilarity between your ammonia absorbent and kitty
litter, the similarity is not accidental. These exchangers also have limited adsorptive capacity for polarcharged 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 todeionize water or soften water for household use. In
the aquarium, however, proteins, bacteria, colloids,
and other large solutes quickly plug up or "foul" themicropores 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, themain difference of importance for aquarium use between strong and weak exchangers is that only thestrong 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 anionand 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 acontrolled 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. Weakexchangers have limited but useful applications. Theseexchangers can be used to effectively remove heavy
metals and to remove acids. Acid removal with weakexchangers 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 tothe exchanger.
Most ion-exchangers available commercially foraquarium 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 ion-exchangers in salt water are atbest confused. The combined use of carbons, polymeric adsorbents, and judicious selection of ionexchangers 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 todark brown. Microporous exchangers are usuallytranslucent and shiny, while macroporous exchangers
are opaque and dull. Anion exchangers are usually offwhite to tan, while cation exchangers tend to be grayor 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 another, 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 ofheavy metals, including trace elements such as copperand vanadium. Many organics, including proteins,amino acids, vitamins, andmedications 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 ofmagnesium 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 longterm route to ammonia removal is biological filtration.
There are, however, numerous products available toaquarists 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. Theinterconversion ofammonia and ammonium is a reaction atequilibrium, and, withincreasing pH, ammonia is favored,
while, with decreasing
pH, ammonium isfavored. If an absorber removes one of
the forms, then theequilibrium is shiftedin that direction andall the ammonia is
eventually consumed.

place. Ammonium ions, however, are not strongly heldby ion-exchangers and the addition ofeven relatively small quantities of salts, as is frequently done in fresh-water aquaria, dramatically decreases the ability of ion-exchangers to remove ammonium. Under ideal conditions, the best synthetic
ammonia absorber has a capacity of about 60 mg ofammonia 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 bymore than 50%. Zeolite absorbers haveabout 1/4 to 1/5 thecapacity of synthetic
absorbers. Zeolites areeasily recognizable as
dusty, white to tan
granules, similar to
kitty litter. Synthetic
absorbers are dustless, tan to brownbeads or fibers.

DYNAMICS OF
Removal of ammoniaA Q U A R I U M F I LFig. 2. Inter-relationship of factors influencing theas an uncharged polar
TRATION. The mosteﬃciency of aquarium filtration
compound requires auseful vantage fromvery strong polar adwhich to examine
sorbent, since the environment is water and, likewise,
aquarium filtration is efficiency, which is defined asstrongly polar. Some polymeric adsorbents are able topercent impurity removal per unit of time. A complex
do this, but only to a very limited extent, and more sointeraction of interdependent factors, depicted in Figin salt water than in fresh water. The polar solventure 2, govern the efficiency of aquarium filtration.
water competes very effectively with ammonia for poAny factor which results in an increase in the volume
lar adsorbent sites, and the polar interactions between
that must pass through the filter decreases efficiency,
ammonia and water are effective in eluting ammonia
while any factor that increases throughput increases
from the adsorbent.
efficiency. However, since many factors do both. theRemoval of ammonia as the ammonium ion calls for
net result of changing any filtration variable is not
an ion-exchange process and ion-exchangers can dousually as obvious as might be supposed.
this effectively in fresh water. Even these exchangers,
The most obvious, but also the most overlooked, factor
however, have limited capacity and once the available
is recirculation, which causes clean or filtered watersites are saturated no further adsorption can take

issuing from the filter to be continuously mixed withthe relatively less clean or unfiltered in the aquarium.
The mathematics of recirculation are similar to compound interest, except the percent change isnegative. Assuming a filter totally removes all of agiven impurity, that is, it retains 100% of an impurityof the water passing through the filter, no mixing at all
would require that 100% of the total volume of waterpass through the filter to remove all available impurity; a 50% mix would require that 332% of the waterpass 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 withaquarium 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 isretention, the percentage of impurity concentration
retained on the filter medium or removed from thepassing water. Figure 3 shows the percentage of total
volume, or number of cycles, that must pass through afilter 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
Figure 3

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 ofabout 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 ofseparate 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 areawith limited flexible bed depth. Disposable "cartridges" for box filters impose, in addition, a fixed andshallow 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. BothFigure 4

Figure 5

cartridge and canister filters have insignificant by-passor 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 asvolumes of the filter medium, at which solutes leakthrough the medium. It is evident that canister filters,with their deep beds, have remarkably more retention
than either box or cartridge filters. The
Figure 6
break-through flow can be determined
empirically with dye solutions. It canalso 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 canistervolume (cc) = dπr2
where I is length, w is width, h is height,
d is depth, r is radius. For box and canis-

ter filters, d = h; for cartridge filters,
d is equal to distance between external and internal walls. All dimensions should be in cm. Typically. thebreak-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; andfor 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 passthrough the filter for effective removal of impurities. For this to happen in a timely manner, it is usuallynecessary to sacrifice some absolute retention for
overall timely removal. Figure 5 shows the effect on
retention of increasing the flow rate beyond the breakthrough 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 breakthrough 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 flowrate can be increased up to about 400-500% of the
break-through 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 andhave already been discussed, but, in general, hydrophilic solutes are adsorbed at hydrophilic sites andhydrophobic solutes at hydrophobic sites. Carbons
and polymeric adsorbents are more hydrophobic thanhydrophilic. Synthetic ion-exchangers are more hydrophilic than hydrophobic. Some gel-type adsorbents
are more hydrophilic than hydrophobic.
Concentration of solutes affects solute adsorption in amanner which is predictable from mass action considerations. The greater the concentration of solute, up toa 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 oflittle importance and external surface area becomesparamount. For mechanical filtration by sieving action, cartridge filters are unexcelled. For all othertypes of filtration, including mechanical depth filtration, biological filtration. and chemical filtration, thecanister filter is clearly superior. Box filters, as they
currently exist, with their poorly placed tubing, small
volumes, and shallow bed depths, are remarkably inefficient.