enough media capacity?

Discussion in 'Filters, Filtration and Pumps' started by fourmations, 10 Feb 2010.

  1. fourmations

    fourmations Member

    30 Aug 2008
    hi all

    i have a converted eheim 2327
    which does 900lph (tested) but its media capacity is quite small (4.9l)

    i this a concern? tbh I only clean it once a month max
    i dont particularly want to replace it

    its going sidemounted on a 150l fully planted tank with a koralia


  2. dw1305

    dw1305 Expert

    7 Apr 2008
    nr Bath
    Hi all,
    In terms of the biological filtration 4.9 litres is a potentially immense volume. How efficiently your filter functions depends upon 2 factors, the degree of oxygenation of the water in the filter, (and how quickly this is depleted), and the internal area that can be colonised by bacteria. The important question is "when you clean the filter is it very full of "sludge"? with a much lower water flow than normal?" If the answer is yes, you need to clean it more often and possibly get a larger capacity filter, the other question is to you have a mechanical pre-filter? If the filter is still fairly clean when you service the filter? once a month is plenty, and the filter is big enough.

    The oxygenation level of the water is important because a filter media may offer a huge potential area for colonisation, but these may not be utilised if the retention time of the water in the filter is too long and the water is de-oxygenated before it reaches all the media in the filter. Sintered glass (like "Siporax") is very good as a media, but this has as much to do with the good water flow through the rings, as the high surface area of the substrate. This is why a mechanical pre-filter (ppi 10 sponge for example) is useful in stopping clogging and retaining flow rate.

    I'll ignore active aeration but as for passive aeration from the atmosphere much flow speed as possible is your aim, as this is the critical measure for determining the rate of gas exchange. (Which is why reef keeper like Koralias).

    cheers Darrel

    I used to spend a lot of my time working with the biological amelioration of landfill leachate, so here is:

    Biological filtration - the important bits.
    In the filter (and aquarium substrate, biofilm etc) organic matter is broken down by fungi and aerobic decomposing bacteria, which utilise oxygen and eventually producing ammonia. These ordinary “heterotrophic” bacteria are opportunistic scavengers, reproducing rapidly every 15-20 minutes. The more waste is in the aquarium, the larger the heterotrophic bacterial colonies will grow, and potentially the more oxygen they will consume. The contribution of organic matter to the Biochemical Oxygen Demand (BOD) depends upon its composition, for example a food source consisting of structural carbohydrates such as the cellulose and lignin (wood for example) will be decomposed very slowly and contribute little to the BOD, whilst soluble protein and/or sugar rich substance like sweet potato or carnivore pellets will contribute greatly.

    The other source of ammonia (NH3) is as a by-product of all aerobic metabolisms, excreted through the fish’s gills and by snails, copepods, fungi, bacteria etc. Ammonia is converted to nitrite and nitrate in the nitrogen cycle, and these nitrifying bacteria require much more oxygen than the "ordinary" metabolism of aerobic bacteria, the ones that break down organic matter and metabolize organic carbon. The important factor to remember is that if BOD is reduced, nitrogen conversion is enhanced. This is because the nitrifiers compete poorly for oxygen with the community of bacteria that are breaking down the organic matter, the ones responsible for much of the BOD. Simply stated, a heavy load of organic materials being degraded in your system inhibits the nitrifiers by competing with them for oxygen. Dissolved oxygen concentrations above 1 mg/l are essential for nitrification to occur. If DO levels in the filter drop below this level nitrifications slows, or ceases altogether, with potentially catastrophic effects.

    So when you bear all these factors in mind, it becomes apparent that the volume or capacity of a filter may not actually be that important. For example an external power filter containing a large volume of ceramic media or sintered glass, and with high water turnover volume (x 10 or more), may be working at a fraction of its capacity, if the water is rapidly de-oxygenated during its initial contact with the filter media. A larger volume filter will add more potential sites for biological filtration, but if the factor that is limiting nitrification is the oxygen supply, they will remain as potential, rather than actual sites. A filter which is extracting a large amount of faeces and saw-dust may become partially clogged, reducing flow and also essentially oxygen, with a “double whammy” as the bacteria degrading the organic materials now inhibit the nitrifying bacteria in the filter by competing with them for oxygen.

    These factors are much less of a problem for planted tank keepers as aquatic plants will be net contributors of oxygenation when they are photosynthesising, but when they are not they will be part of the bio-load. However in any aquarium, with actively growing plants, their oxygen production will massively exceed their oxygen usage, and realistically oxygen levels will be similar, or higher, at night in the planted tank when compared to an un-planted tank. This seems a nonsensical statement, but particularly for emergent plants (with access to atmospheric oxygen and CO2), the plant may be able to use atmospheric oxygen for respiration, even in roots deep in the substrate. This is a quote from the abstract of Li & Jones 1995 paper "CO2 and O2 transport in the aerenchyma of Cyperus papyrus L."

    “…..While the water surrounding the rhizomes remained strongly hypoxic, the O2 concentration in the submerged rhizomes was 15.1% during the day and 10.3% at night.….”

    Another advantage is that ammonia, nitrite and nitrate will be preferentially taken up by the plants, and the ammonia they utilise will not enter the bacterial nitrogen cycle, and therefore will not deplete the tank oxygen in the same way that it would if plants were absent.

    Another factor is that as well as the filter, plants, wood and other surfaces in the aquarium offer a potential home to the community of aerobic bacterial that metabolize ammonia to nitrite and then nitrate. The uppermost surfaces of the substrate are a good location for these bacteria, because the nitrification process uses a lot of oxygen. However only a few centimetres below the substrates’ surface, the diffusion of oxygen can't supply enough oxygen, and as oxygen levels fall anaerobic bacteria become more frequent. Many of these bacteria are in fact “facultative anaerobes”; when oxygen is in short supply, they are able to switch to a metabolism that doesn't require oxygen, instead, they use nitrate, stripping the oxygen and leaving nitrogen (N2) gas. The nitrifying bacteria provide the nitrate, and their high oxygen demands also tend to exhaust the limited supply of oxygen. These two types of bacteria will occur across a fluctuating boundary lying not far beneath the surface of the substrate. The same processes will also occur in the “rhizosphere” the aerated zone lying around aquatic plants roots. These processes are both a good reason for:

    · having a substrate, and for
    · leaving it relatively undisturbed.

    Li M & Jones MB. (1995) "CO2 and O2 transport in the aerenchyma of Cyperus papyrus L." Aquatic Bot. 52. 93-106.

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