How a pond bio filter works

Pond bio filter principles ... In writing this section I would ask the more technically inclined to bear with me since I am writing to help the average pond-keeper and not the expert pond-keeper. The performance of a pond bio filter no matter what its design is greatly influenced by the principles described below. Yes, it is so important for everyone to understand the principles. Lets have a cup of coffee to start off and use this analogy to make pond bio filters easy to understand. I hope you take sugar with your coffee because this will help you to understand what is important in assessing and improving performance of any pond bio filter.

  • I like coffee with 2 sugars. So I pour the coffee and put two spoons of white granulated sugar in it. I leave it for a while to cool down and upon taking my first sip it does not taste sweet. I think I have forgotten to put sugar in so I put another 2 teaspoons in and then stir the coffee. I take a sip and say heck, or something like that . I only forgot to stir the coffee.
  • On another occasion I go to a fancy restaurant and have a cup of coffee and put 2 teaspoons of that fancy brown crystal sugar in my coffee - looks like small lumps of light brown coal. This time I stir normally. The coffee tastes less sweet than normally. So I stir a bit more and again for good measure. Eventually it tastes sweet but my arm aches from all the stirring.
  • In another instance I let my coffee go cool before I put my 2 teaspoons of normal granulated sugar in it. I find that I have to stir it much longer to get the sweetness I am used to.

In the above 3 distinct cases we had the same coffee and we had the same amount of the same chemical (sucrose which is a fancy name for sugar) but we only achieved the same sweetness after different actions.

In case 1 we did not stir so the sugar sat on the bottom of the cup and it could only slowly dissolve. In case 2 it took much longer to get the sugar to dissolve because it was much lumpier and in the third case it took longer to dissolve because the temperature of the coffee was cooler than normal before we stirred the coffee.

We can learn something from these situations that can be applied to many processes taking place around this and us includes those processes occurring in pond bio filter.

This is what, in the world of chemical engineering, we refer to as Mass Transfer Processes.

These are processes that also take place in pond bio filter where ammonia is converted to nitrates thereby preventing poisonous build-ups of ammonia.

Lets look again at case 1:

We wanted to transfer a quantity (mass) of sugar from its solid state to become dissolved in coffee (or hot water). We noticed that the rate of transfer (ie the time it took for the sugar to dissolve) was slow until we stirred the coffee - the mistake was even made to put more sugar in.

Immediately we stirred vigorously the sugar dissolved. So what did we do?

  • In simple terms we mixed the sugar with the coffee better. In technical terms we exposed each granule of sugar to a new bit of liquid containing only a little dissolved sugar and we did this repeatedly until the stirring stopped.
  • Although the average amount of dissolved sugar in the cup was increasing the concentration was still low and this allowed further dissolution of sugar to take place.
  • By the way the harder we stir the quicker the sugar dissolves. If you were to put about 20 teaspoons of sugar into your cup of coffee not all would dissolve no matter how long you stirred.
  • Before I started stirring my cup of coffee very little sugar was in contact with the hot liquid because it was sitting in a heap at the bottom of the cup.
  • This meant only a small amount of sugar was dissolving and the liquid immediately next to the sugar was becoming saturated with sugar - once the coffee in immediate contact with the sugar becomes saturated it cannot dissolve any more until this saturated liquid has been replaced by a new bit of coffee with no sugar yet dissolved in it.

This principle is important:

AT THE INTERFACE BETWEEN TWO COMPONENTS (sugar and water in this case) TRANSFER BETWEEN THE TWO IS LIMITED BY THE CONCENTRATION DIFFERENCE. THE LARGER THE CONCENTRATION DIFFERENCE THEN THE FASTER THE TRANSFER WILL TAKE PLACE.

In our example of the stirred coffee we created a situation whereby the sugar was exposed to water in which sugar had not yet become dissolved or had only a little bit dissolved and thus had a low sugar concentration.

Compare this with the heap of sugar at the bottom of the cup. This sugar was surrounded by liquid having a high concentration of dissolved sugar which meant further dissolution took longer and longer.

This is what the above teaches us:

In general and in real practical situations by creating a turbulent situation we create quicker mass transfer - ie by stirring vigorously for example or by shaking if this was possible or by blowing air into water inside the filter.

If you think about case two where we had larger particles of brown sugar it took longer to dissolve the sugar. The reason was that the larger particles of brown sugar had a much lower surface area in contact with the hot liquid. The principle here is that:

THE MORE SURFACE AREA WE CREATE FOR MASS TRANSFER TO TAKE PLACE THEN THE MASS WILL BE TRANSFERRED MORE QUICKLY. DOUBLE THE AREA AND YOU WILL HALF THE TIME TAKEN TO TRANSFER THE MASS.

In the above case because brown sugar lumps are bigger than white sugar particles then the total brown sugar surface area in contact with the hot coffee was lower than would be the case for normal white sugar.

This is why as you have no doubt experienced it takes much longer to dissolve lumps of sugar then small granules. Smaller particles always have more surface area than large particles in terms of surface area per unit of volume. This is why Alfagrog or Supra we discussed earlier is available in different sizes.

We can conclude so far that where a transfer of mass or a substance is required to take place we can optimise the rate of this mass transfer by

  • increasing surface area for contact between the two media and
  • maximising concentration differences at the interface where the transfer is taking place.

This is the secret to good biofiltration and understanding good water quality that suits your fish.

Do reconsider your intention to build a pond if you are not prepared to spend money on a pond bio filter. You will be wasting money if you go ahead without a bio filter and I promise you will do it sooner or later or you will turn the pond into a sand pit.

These principles are best incorporated into modern pressure filters made by Hozelock and Fishmate for small to medium sized fish ponds (NOT SAND FILTERS) and bead filters for large ponds. You can see range of pond bio filters to suit your pond here . Don't forget the turbulent action is created by the pond pump you choose

Alfagrog is called Supra in USA Alfagrog is called Supra in USA and can be bought here ... it is best all purpose media for small to medium ponds . Alfagrog looks a bit like cinders, it is lightweight, comes in different sizes and you can literally blow through it because it is so porous.

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