What is the correct use of activated carbon?
1 Elimination of oxidising substances from the water
The elimination of oxidising substances does not concern the above mentioned ability to absorb carbon, but instead it involves a different property: chemical absorption. In this process the substances that are able to oxidise, such as chlorine and ozone, are catalysed and destroyed; they accumulate on the surface just briefly and soon after they break up into their main components. To eliminate chlorine from water, for example, just a few seconds of contact are enough. Carbon filters can therefore be small; while a low quality product is usable. Together with modern osmosis systems, a carbon pre-filter should be used, in order to eliminate chlorine and ozone. In this way, the membrane is protected against early oxidation and the resulting deterioration (Sellner & Ramsch 1996). Even when protein skimmers with high ozone doses are used, post-filtering with carbon should always be considered, especially if the osmosis system is installed downstream from de-nitrification filters (anaerobic). In such an environment Redox values increase even up to 700 mV, and the reflux water, rich in ozone, can endanger more delicate animals. In cases where the use of ozone in the system is moderate, carbon filtering is not necessary. During use as we described it, the material does not wear out; however the particles in suspension gradually obstruct the surface, reducing its power as a catalyst. The same can be said for osmosis systems’ pre-filters; when they are dirty, they need to be replaced. The activated carbon that is used for cleaning the protein skimmer’s reflux water should always be replaced in case you identify sediment, excessively high Redox values, or free ozone is measured.
2 Air de-ozonation
From the protein skimmers in which ozone is used, air containing this gas comes out. The air should always be channelled through a carbon filter to avoid the diffusion of ozone into the environment. Even in low concentrations, (100ng/m3; 1ng=1 thousandth of a milligram) it can cause headaches and discomfort. If you can smell it, the concentration is definitely too high! In large protein skimmers it is therefore advisable to use ozone measuring and alarm equipment. The filters for the protein skimmer’s air should always be checked, because excessively damp or even wet carbon significantly compromises the effectiveness, and the time needed for the ozone to break down increases substantially.
3 Elimination of organic substances from tap water
In some areas, drinking water from the the water main is not suitable for aquariological purposes. Even when the values measurable by the aquarist, such as hardness, nitrates, phosphates and silicate minerals are in the norm, pesticides, residues of disinfectants and hormones can be a danger for more sensitive marine organisms. In this case too, activated carbon can be of help. The above-mentioned substances are, in tap water, only present in micro or nano-grams, but even in these minimal concentrations they constitute a potential risk. To eliminate these substances in such concentrations, for example 100ng/l (1ng = 1 millionth of a milligram), from tap water through activated carbon filtering, the carbon should stay in contact with the water for at least 30 minutes. Unfortunately there are some filtering systems on the market that, according to the maker, are able to purify several litres per minute. These claims are usually followed by rather questionable data. An evident decrease in pesticides, on the milligram range, can definitely be true. In the range of micrograms, however, in these levels of water flow, the provided data are surely unreliable. Therefore, to use activated carbon to filter tap water, the time of contact between carbon and water needs to be very long. If, for example, we use 1 litre of activated carbon, not more that 2 litres of water per hour or 50 litres per day should flow through it in order to get a filtering effect. As for the duration, instead, we should use the provided data as a reference, which unfortunately does not always happen. To be sure, it is advisable to not use the activated carbon for more that 5000 litres of tap water. Beyond this we cannot be sure it has an effective absorption ability. Because of the above-mentioned reasons, it would be appropriate to ask whether it would not be more suitable to use a reverse osmosis filter, which gives better and especially more easily verifiable results, even by the amateur. (Selner & Ramsch 1996).