(Published in Marine Habitat – July/Aug 2012)
This ‘chemical’ filtration substance is one of the more widely used materials when it comes to maintaining/improving water quality, whether for human consumption or that in freshwater and marine aquariums. Its also probably been utilized longer than any known filtration substance as the Bible has a reference of a ‘wood’ product being used to make bitter water taste sweeter.
As to the product itself, it removes dissolved organic compounds (DOC) via a process called adsorption where they become lodged/attached by both a weak physical force (Van der Waals forces) and that called molecule sieving. This differs from what is called ‘mechanical’ filtration, where absorption (matter simply trapped inside the product) occurs, similar to what a sponge accomplishes when water flows into its internal crevices and tunnels.
To ready/activate the initial product, e.g., bituminous coal, lignite, peat, bone, nutshells (Coconut nutshells) or wood, it’s heated to very high temperatures in the absence of air so as to drive off hydrogen and oxygen. The remaining char or ‘charcoal’ is again heated, sometimes to far greater temperatures in the presence of steam, air, or carbon dioxide to remove the remaining hydrocarbons and ‘activate’ the product by creating many tiny holes, passageways and crevices both inside and on the surface of the carbon particle. During this process, it can be given different adsorption characteristics by treating it with inorganic salts such as copper, zinc, phosphate, sulfate, or silicate. After the activation process completes, an acid or alkaline wash or rinse in some cases, will further change the carbon’s adsorption characteristics and reduce these soluble contaminants that could leach into the solution its finally being used in.
To help illustrate the importance of the ‘activated’ carbon particle structure think of it first as a sponge having a large outside surface area with many holes and crevices leading inward to passageways with ever decreasing smaller and smaller channels thereby actually making its internal structure much more important than its outside surface area. Then keep in mind that when discussing activated carbon, ‘surface area’ relates to its ‘internal surfaces,’ not its outer surface area. And the more internal channels there are, the more so called ‘surface area’ and the more places for molecules and matter (adsorbates) to flow into and be captured.
Even though ‘surface area is an important factor, its ‘pore size,’ i.e., the opening that leads into the particle and the volume it can hold inside these openings are even more important considerations. Very small openings allow only small molecules to enter, medium size pores allow more complex molecules to enter and large pores not only allow various size or complex molecules to enter, but also suspended solid material.
As to basic product choices, coconut shell activated carbon was highly touted during the 80’s and 90’s, but its popularity fell thereafter when aquarists began to realize its was better suited for filtering air than water because of its very small pore size/micropores (<40 angstroms). (Note - The same is true for products made from Bamboo.) That of peat also did not suffice, as its pore size was too large. What remained and has proven ideal for our purpose is bituminous coal, which has large, macrospores, i.e., 40 – 5000 angstroms that are ideally suited for removing the varying size compounds from aquaria.
Another descriptive term is called ‘pore volume,’ and pertains to the amount of open space inside the carbon particle. Within limits this simply means the more emptiness there is inside each particle the more space there is to contain those molecules. Overall, it can be thought of as the more internal surface area, the greater its capacity.
Questions are now probably coming to mind that asks why should it be used and/or if it’s really needed, and if so how do you select a good brand? As to its use, aquariums may seem stable for long periods, some with little unwanted alga growths. Nevertheless, there’s no doubt they grow more nutrient rich as they age and that increasing biological load, generally carbon based compounds (DOC’s) produced by many processes in the aquarium, need to be kept in check before real serious problems arise. And there’s no better way in my opinion to prevent that from happing than with the use of activated carbon in moderate amounts as described further on in this article.
Of course, there are still those that take the other side of the issue and say its not needed, or that it causes various problems. As with anything in this modern world, there always seems to be two sides to anything being discussed. The main complaints against its use are that it can cause Head and Lateral Line Erosion (HLLE), but there’s absolutely no proof of that! Or it will remove valuable trace elements, which is only partially true since many of those normally found in the ocean at less than 1 ppb are never used, such as cobalt, chromium, cadmium, lead, and zinc. Those that have some use, such as iodine, iron, and molybdenum for example are readily supplied by various brand salt mixes and/or additives, therefore easily renewed via water changes and regular, yet thoughtful uses of trace element additives. Therefore, trace element losses through the use of activated carbon are really a none issue!
Another product fault sometimes voiced has been that once the holding capacity of carbon becomes exhausted, its physical adsorption bonds grow very weak and desorption or leaching of the adsorbates could possibly return them back into the systems bulk water. Even though true, there would have to be a drastic pH change, e.g., something far out what would normally occur in aquariums, for this to happen, therefore, not feasible in the normal day-to-day activities in aquaria.
On the other hand, and an additional good point, activated carbon is very effective at removing substances used for treating aquariums for disease, e.g., copper, sulfa drugs, and antibiotics. Therefore, all-in-all, it’s a valuable tool in the aquarist’s arsenal for maintaining water quality.
When it comes to selecting a brand, most activated carbons are sold as Granular Activated Carbon (GAC), which refers to carbon in a granular form, usually having a particle larger than 0.1 mm. How much larger depends upon what the distributor or reseller wants to market. Thereafter, it generally comes down to its appearance, size, shape, does it float and make a hissing sound while becoming wet, and is the package labeled phosphate free.
Keep in mind there are products that look like activated carbon, but have never been activated, therefore always look for the word ‘Activated,’ as products not activated are unsuitable for aquarium use. Even this may not guarantee a truly activated product, but its at least a good starting point, and if purchasing a well-known brand, should suffice nicely. Just beware of unknown, very cheap brands! Then look for the name of the initial product, and stay with a bituminous coal product if at all possible. After that, select a dull black particle, which indicates its fairly porous over that of a shiny black particle, which is less porous. Its surface should be slightly irregular or rather rough and the particle as a whole should be more round than flat-sided, which would block water flow thereby reducing adsorption sites.
Since efficiency also relates to physical size, look for particles the approximate size of a pinhead or slightly larger. Powdered carbon may seem to have more surface area, however that only relates to its ‘outside’ surface area, and its smaller particle size actually impedes internal water flow. As to particles much larger than optimum size, they produce non-uniform flow through the bed, negatively affecting thorough penetration of many particles.
As odd as it may sound, a quality carbon product should be quite buoyant and emit a hissing sound as water initially finds its way into each carbon particle and fills its internal passageways and caverns. Any carbon that quickly sinks or emits little or no hissing sound can be considered less than ideal.
It should go with saying the product should be labeled its phosphate free or at a minimum tests have shown it not to leach phosphate! And furthermore, I’ve seen various brands labeled laboratory grade, pharmaceutical grade, research grade, or premium grade. Looks quite impressive, however, these terms have no value as to the actual efficiency of the product.
There are some technical aspects that could be used to judge its quality, however, few brands provide the information needed on their labels, but in the age of the Internet, its possible to find it if wanted. Studies have shown that better carbons have a total surface area (TSA) of 450 to 550 m2/cc and a pore volume (PV) of 0.45 to 0.60 ml/cc with a TSA/PV ratio of 700 to 1000. If the TSA is given on the basis of weight, a quality carbon should have a TSA of at least 1000 m2/gram. Therefore, when selecting an activated carbon, select one that takes up the most volume for a given weight. Keep in mind that TSA and PV, not the weight, is what should be purchased.
Finally, package price does not relate to its effectiveness/quality as some inexpensive brands have been found to out-perform more expensive brands. Therefore, the selection process can be confusing, and if so, simply stay with well-known brand names or word of mouth recommendations from fellow aquarists.
Once a good product is chosen, a decision on how much of the product will be used, and how it will be used must be made. This should begin with an honest review of the condition of the aquarium system and its bioload, as there are many thoughts on what quantity of the media should be used, how it should be used, and if it should be used full or part-time.
As to product use, one must keep in mind this is a chemical filtration product and that even water flow through it is a must if it is to be properly utilized and not wasted. Probably the most effective way to utilize it is inside a canister filter where all water flows though it’s inside media. The least effective is when the media is placed in mesh bags and placed in a convenient place in the system where water can slowly flow through or over it. The key words there are slowly through it, not over it or too quickly through it. Keep in mind high flows generate shear forces that may simply wash away contaminants before they can be adsorbed.
Then there’s the passive flow method, where carbon bags are simply placed at different non-flow areas in the aquarium system. Even some elements are physically attracted to its carbon particles, thorough penetration of the carbon particle or the entire carbon bed may never happen. No doubt the least effective way to utilize the product!
But if sacks are properly used, consider having more than one and placing them in different areas in the system. The second bed could adsorb what a single bed may miss. By having a couple of small carbon beds and changing them at different timeframes, i.e., the first bed in thirty days, the second bed thirty days later and then back to the first bed in thirty days, it will no only result in less of an impact to the loss of bacteria that colonize them, but also less initial trace element loss in the system. Also, remember to thoroughly wash the dust off the carbon particles prior to using them, as that helps reduce the initial rise/impact to pH when first introduced.
As to the quantity needed, there are many variables when it comes to individual systems, as some are better maintained than others, or simply have far more complex inhabitants requiring a higher degree of water quality. It’s a decision one must make on their own, however, it should be used in all systems, and in my opinion, it’s always better to begin with less than too much. Keep in mind the following is not rocket science, just information from many years of experience! – I always began using one heaping tablespoon of activated carbon, which is about 8 grams, per 5 gallons for fish-only aquariums. For reef aquariums, the same amount of carbon was used per 10 gallons. Again, simply what worked well for me during the past years I’ve have kept both simple and complex systems. For reef systems containing stony corals, keep in mind excessive use can induced coral bleaching within a few hours of installing a new activated carbon! Been there, done that, and sadly experienced that!
Another question often arises as to whether it should be used continuously or only part time and to that I’ve always responded that ‘chemical’ filtration is a fulltime need in aquaria, not part time! Changes in bioload are not easily seen, so why wait until a problem has arisen, then struggle to rectify it! And when it comes to replacing the media, possibly a simple ‘White Paper’ test might resolve that. Simply hold a sheet of waterproof white paper in the aquarium water where the light is very good and as far back into the aquarium water from your eye as possible. Compare it to another piece of similar paper being held outside the aquarium. If the ‘white’ on the internally held paper now appears slightly yellow compared to the one outside the aquarium, it’s time to change the carbon bed.
Nevertheless, I would recommend the bed(s), when efficiently being used, not be kept for periods longer than 45 – 60 days, although others suggest not keeping them over 30 days. Of course, many aspects come into play, with system bioload and product quality the two most important. Therefore, it’s a personal decision based on an honest assessment of the details in play, then following through with the use of a quality product and its proper utilization.
Without a doubt, GAC remains among the most economical, practical and proven chemical filtration media for use in our aquariums and highly recommend its usage!