It has been stated previously that biological filtration medias become more efficient at removing nitrogenous waste from aquarium water under conditions in which the flowrate of water through the media is reduced. While I would agree that this would seem to make sense “intuitively”, in reality, I don’t think that this is the case. First, let’s remember that a flowrate of several hundred gallons per hour is really not “fast” from the perspective of a molecule in solution. Second, aquarium water is quite warm and this imparts a tremendous amount of random movement to the ammonia, nitrites and so on. So, while the pump is forcing the water through the media bed in one direction, the molecules are actually zipping around in all three dimensions within that stream…and that does not take into account the secondary countercurrents and eddies which the media generates.
Let’s look at activated carbon adsorption to gain some insight. For most purposes, the adsorption of the solute molecule at the site on the internal carbon surface occurs almost instantaneously, so it has little effect on the overall rate at which an organic contaminant is removed from the aquarium water. The transfer of organic contaminants from the bulk liquid to the surface layer of fluid around a carbon particle can occur rather slowly, but this is ENCOURAGED by the constant movement of fluid past the surface, because this serves to maintain a concentration gradient. On the other hand, the diffusion of the solute through the porous network of the carbon may occur rather slowly (particularly if the contaminant is a large molecule and the pores are small), and this diffusion is usually the primary factor limiting the rate of the sorption. Because the rate of adsorption is usually limited by diffusion, it is influenced by the same variables which affect rates of diffusion. Specifically: 1. The concentration gradient of the solute across the surface of the adsorbent and 2. The temperature of the system.
It is quite true that bacterial uptake of nitrogenous waste molecules is different than the adsorption of an organic molecule onto the surface of activated carbon. Nonetheless, the rate- limiting step in either case is the delivery of the contaminant to the surface of the thing that is supposed to remove it. I could be mistaken, but I believe that a high flowrate should actually IMPROVE the performance of the biomedia, rather than diminishing it. Not to mention that a higher flowrate also delivers more oxygenated water to these aerobic bacteria, supercharging their metabolic rates. I would like to hear any contrary views.
