AQUARIUM FILTRATION Part 3

In Part 1 all the main filter types were explained with advantages / disadvantages and relative costs. Part 2 discussed biological media, - the different types with advantages / disadvantages and relative costs. We now move on to the last segment on chemical filtration and types of media.

Chemical filtration is the direct removal of dissolved compounds by adsorbtion. The most important function of chemical filtration is the removal of nitrogenous organic waste. This is vital, because such waste is both inhibitory to the biological filter and increases the load on the biological filter. It has a secondary function of polishing the water to give it a sparkle.

Adsorption

Please bear with this section, - it is a little technical, but gives a good insight into how chemical media works in general.
Adsorption, the binding of molecules or particles to a surface, must be distinguished from absorption, the filling of pores in a solid. The binding to the surface is usually weak and reversible. Just about anything including the fluid that dissolves or suspends the material of interest is bound, but compounds with color and those that have taste or odor tend to bind strongly. Compounds that contain chromogenic groups (atomic arrangements that vibrate at frequencies in the visible spectrum) very often are strongly adsorbed on activated carbon. Decolourisation can be wonderfully efficient by adsorption and with negligible loss of other materials.
The most common industrial adsorbents are activated carbon, silica gel, and alumina, because they present enormous surface areas per unit weight. Activated carbon is produced by roasting organic material to decompose it to granules of carbon - coconut shell, wood, and bone are common sources. Silica gel is a matrix of hydrated silicon dioxide. Alumina is mined or precipitated aluminum oxide and hydroxide. Although activated carbon is a magnificent material for adsorption, its black color persists and adds a grey tinge if even trace amounts are left after treatment; however filter materials with fine pores remove carbon quite well.
A surface already heavily contaminated by adsorbates is not likely to have much capacity for additional binding. Freshly prepared activated carbon has a clean surface. Charcoal made from roasting wood differs from activated carbon in that its surface is contaminated by other products, but further heating will drive off these compounds to produce a surface with high adsorptive capacity. Although the carbon atoms and linked carbons are most important for adsorption, the mineral structure contributes to shape and to mechanical strength. Spent activated carbon is regenerated by roasting, but the thermal expansion and contraction eventually disintegrate the structure so some carbon is lost or oxidized.
Temperature effects on adsorption are profound, and measurements are usually at a constant temperature.

Types of Media

Activated Carbon

The most familiar chemical adsorbent is activated carbon. Activated carbon should be a little larger than pinhead in size. When washed and dry, it should be dull and not shiny. When placed in water, it should hiss. It should also tend to float at first. Be careful of charcoal, however, because it is dull and floats, but does not hiss. Charcoal is usually very soft, crumbling easily between the fingers and is usually available only in pea-size. Good activated carbon is hard but fragile, feels hard and does not crumble, but fractures under finger pressure. Not all true activated carbons are equivalent. The most common available carbons are economical water purification grades, usually derived from wood or nutshells. These are not bad carbons, but you may wish to seek out some better grades. The best carbons are usually produced from bituminous coal and have high porosity and low density. They should also have low ash content to minimize impact on pH. Most activated carbons need to be thoroughly washed prior to use. Because it is soft it has a tendency to crush a little during shipping and is therefore covered it carbon dust. Rinse in clean befor use. All activated carbons release phosphate, despite claims to the contrary, and only those that release the least should be selected for reef aquaria.
Activated carbon adsorbs a small quantity of ammonia, nitrite and nitrate, but the quantity is quite small. In most aquariums it would take a large bucket of activated carbon to remove enough nitrate to be effective. Its main use is to adsorb organic compounds. These compounds give the water its aged look (yellow). It adsorbs dissolved food, fat, and minute dirt particles.
Advantages: Easy to use, Relatively Cheap, quickly polishes the aquarium water.
Disadvantages: Must be cleaned before use, has a short life (approx. 1 month), many carbons tend to release the waste products back into the water once saturated, does not specifically target unwanted compounds (often removes fertilisers from planted tanks), releases phosphates into the tanks water when first introduced.

Zeolite

Zeolites are white, dusty clays, usually sold for removing ammonia from freshwater. They are ineffective in seawater or even freshwater that contains modest amounts of salt. Zeolites are ineffective for removing nitrates. It is often sold in boxes or bags specifically designed for use in aquariums. However, it is quite expensive considering how long it is effective for.
Zeolite really only needs to be used when setting up a new aquarium, and then only as a precautionary measure. It is also useful to have a box of it handy just in case you get an ammonia spike in an established tank. It must also be washed prior to use to remove the white dust covering it. Failure to do so will result in a rather cloudy aquarium for a while.
Some brands of Kitty litter are zeolite based. The granule size is not perfect for use in aquarium filters however, and it is also very dusty.
Advantages: Quickly removes ammonia from tanks with an ammonia spike, usefull when setting up a new tank.
Disadvantages: Quite expensive, does not last long (about 1 month), must be thoroughly washed prior to use.

Aluminum Oxide

Aluminum oxide is used to remove phosphates and silicates from the aquarium water. Excess phosphate in an aquarium is the primary cause of most algae outbreaks (combined with nitrates). Can be used in both freshwater and marine aquariums. It helps solve many of the red and brown silica algae problems in marine aquariums. It is relatively expensive, but lasts a long time. Get it in bulk as it is around one third of the price by weight.
Most food has high levels of phosphate in it. Uneaten food, decomposing plants and fish waste also contribute strongly to the build-up of phosphates. The build-up usually happens faster than the aquatic plants can remove it. Also, many tap water sources are rich in phosphates. The ideal phosphate level in aquariums should be below 0.1ppm (below the measurement level of most test kits). This will slow algae growth to a level that is easily controllable. In large cichlid aquariums huge amounts of waste are produce, - all rich in phosphate. The eating habits of large fish are also such that they munch their food up leaving small uneaten particles in the water. This is style of aquarium presents the hardest algae free challenge, either requiring very regular large water changes with phosphate free water, or large quantities of phosphate removing media.
Advantages: Quickly removes phosphates down to trace level, lasts a long time, does not leach back into the aquarium.
Disadvantages: A little pricey, must be added slowly to a large volume of water as a lot of heat is produced initially. Also requires rinsing to remove the white powder deposits from shipping.

Other Products

There are some products that mix two or more types of media together. Some examples are the Ammo-Carb type products. These have zeolite and activated carbon mixed together. They are excellent for new aquariums as a precautionary measure. They need to be washed prior to use.

Nitrazorb

An aquarium pharmaceuticals product, it specifically targets nitrate. It is also the fisrt of the filter media listed that can be regenerated. Many of the other filter media previously listed can actually be regenerated, but the method to do it is usually well beyond the capability of the equipment we have around our homes. (E.g. To regenerate activated carbon, it must be baked at over 1000’C in the absence of air). Nitrozorb can be easily regenerated using a slat solution.
Advantages: Can be regenerated, is available in a convenient size that suits most aquariums, is relatively well priced considering it can be used multiple times.
Disadvantages: Must be regenerated, - however it is an easy task.

Purigen

This is a form of synthetic activated carbon made be Seachem. It is said to have all the advantages of activated carbon, but none of the disadvantages. It adsorbs approximately 3 times as much by volume as activated carbon, taking up much less space in the filter. It can be regenerated up to 30 times using a medium strength solution of household bleach (sodium hypochlorite). Experience with the product however has shown a tendency to adsorb some of the compound in plant fertilisers, - something Seachem claims it does not do. The effect is not a problem as purigen only needs to be used for 3-4 days to clean up a tank. It can then be removed or moved to another tank. It starts off white, and turns dark brown when full. Soaking in bleach for 24 hours makes it good as new again.
It is quite expensive, but very cheap if you look at price comparisons. A 250g bottle of purigen has about equivalent filtering capacity of a 700-800g activated carbon (depending on the brand). Purigen costs approximately $52.00 for 250g. Activated carbon is about $18.00. However, you can regenerate the purigen 25-30 times at about $0.50 each time, so:
Purigen $52.00 + 30x$0.50+ 27.00 (bag) = $94.00 for about 2 years supply.
Carbon $18.00 x 30 = $540.00 for about 2 years supply.
Purigen is clean, and can be used straight away with only a light rinse. It does not have a container however, and you must purchase a bag from Seachem to house the purigen.
Advantages: Is cleaner the activated carbon, can be regenerated many times, required less space than activated carbon.
Disadvantages: Requires regeneration, a bag to contain the purigen must also be purchased. The initial purchase is a bit pricey, but look at how much it saves long term.

Ion Exchangers

Synthetic ion exchangers are useful in freshwater to control ionic balance, remove ammonia, nitrite, and nitrate. In marine water, ion exchangers can remove some nitrite and nitrate, but have no significant effect on ammonia. They can also help to retard ionic imbalance. But, generally, the most useful function of ion exchangers, in both fresh and marine water, is organic removal, and in this they excel.
Although not an ion exchange process, this ability of ion exchangers to remove organics is phenomenal and works in both marine and fresh water alike.
Ion exchange is best used to treat the incoming tap water if it is on insufficient quality to use unprocessed. It would be a very expensive process to continually use ion exchange to filter aquarium water.
Ion exchange resins can be regenerated, but it is a time consuming process due to the large quantity of resin required. The resins are also very expensive.

References

Dr Bernhard Spang – Chemical Engineer
Seachem
Eheim
Aquarium Pharmaceuticals
Hagen