Aerated Moving Bed Filtration (Part I)

   There are many different approached to koi pond biological filtration, more common approaches are:
  • Static bed using crushed coral, pumice rocks, filter mat, etc. as media. These media are fixed in location with respect to water and filter chamber
  • Trickle or shower type where water is pumped above, then falls through media which are above water surface.
  • Moving bed where media are not static, but moving/flowing with the water.
   Basic Concepts of Nitrification Nitrification is the conversion of ammonia to nitrite(and other byproducts such as nitrous oxide, etc.) then to nitrate by different kinds of bacteria in our filter. Let's call the bacterias that do the conversion "nitrifiers". Basic concepts are:
  • Nitrifiers need surfaces to adhere to, the amount of free-floating nitrifiers is very small compared to those grown on surfaces.
  • The conversion is "reactant-limited", meaning the rate of conversion depends mainly on rate at which ammonia or nitrite come in contact with the nitrifiers.
  • The conversion is almost instantaneous.
  • The conversion requires oxygen.
   Biological Filtration Requirements For all filteration approaches,
  • We want to minimize the amount of ammonia in our pond, so we need to turn our pond over every two hours or less.
  • Large surface area of media are needed for nitrifiers to adhere to
   Limitations of static bed Approach:
   In this approach, water flows through filtration chamber while the media is stationary. While the water is flowing through the media, it has only a fixed amount of time in which it has a chance to come in contact with the nitrifiers which reside on the media before exiting the chamber. This time is normally called "resident time". Because of the water turn over requirement and commonly accepted resident time of approximately 15 minutes, the size of filter is required to be about one third that of the pond, which is fairly large. The use of filter mat with large void space and specific surface area(surface area per unit volume) increases the effectiveness of static bed approach, but the volume of the filter itself is still very large.

   Limitations of Trickle or shower Approach:
   Although trickle/shower appraoch has many advantages compared to other approaches, trickle or air shower uses a lot of (electrical)energy because water has to be pumped above the pond surface before falling through the media.

   Moving Bed Concept
   Unlike two approahes mentioned previously, the media is no longer stationary. By allowing the media to move along with water that is flowing through filtration chamber, we dramatically increase the chance that the water come into contact with the media; thus improving the efficiency of conversion. This is a very important concept.

   Older Moving Bed Approach
   Older moving bed systems use fine sand as media because it has extremely large specific surface area. The media is kept moving by the upflow of water. Major limitation of this approach is low water flowrate. Water flowrate through the media has to be kept low otherwise sand would flow out through the chamber's exit.

   Aerated Moving Bed Approach
   The media itself is normally made of small piece of plastics whose specific density is slightly lower than 1, so it mostly submerges in the water. The most commonly used media is called "K1", but other items such as diced plastic drinking straws can also be used. The media should be light and small enough such that it easily flows with the water, and it should have large specific surface area. In this approach, air is injected at the bottom of the filter chamber; rising air bubbles causes the media and water to circulate together in the filter chamber.
Air injection also increases oxygen concentration in the water as well as assisting in degassing of CO2/ammonia/nitric oxide, etc. from the water. Because water and the media are churned constantly, old/dead bacterial films easily sloughed off the media exposing new/fresh bacterial layer to the water, further improving conversion efficiency. The automatic removal of old/dead bacterial film is called "self cleaning" which is another strong point for this approach because less maintenance is required.

   Advantages of Aerated Moving Bed Approach
  • Extremely high nitrification efficiency
  • Due to high nitrification efficiency, filter chamber could be kept small even with high flow rates.
  • Self cleaning, almost no maintenance is needed
  • Automatic air stripping/degassing of unwanted nitrification byproducts.
 


    Disadvantages of Aerated Moving Bed Approach
  • Water need to be prefiltered to removed organics such as feces and uneaten food. The presence of organics in the chamber would encourage the growth of heterotropics which could easily outcompete nitrifiers for available surfaces on the media.
  • Dead bacterial films are constantly sloughed off, reducing water clarity.
  • Susceptible to power failure because bacterial films could die due to lack of oxygen
  • Easily impacted by chemicals or medications because the bacterial film is relatively thin.
  • Occasional residual nitrite problem
   Common Filter Configuration using Aerated Moving Bed Approach
  • Mechanical prefilter - remove feces, uneaten food, etc.
  • Aerated Moving Bed Chamber
  • Last chamber utilizing static media such as filter mat. Main purposes of this chamber are:
    • Traps dead bacterial films etc. that came through aerated moving bed chamber, improving water clarity
    • Removes residual nitrite from the previous chamber.

Written by Dr. Bancherd De Long    June 10 th, 2005