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Using the Principles of Biofiltration to Improve Any Goldfish or Koi Pond ... Instant Chemical Engineering

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. However everything I discuss here is included in the latest Hozelock biofilters called the Trinamic series. The Trinamic surely is a major step forward in effective and efficient biofiltration for all small to medium sized ponds and these pond filter and UV combinations don't take up much room either

We'll borrow an example from The Pond Professor ... let's enjoy a cup of instant coffee with sugar of course.

You first put coffee granules in a cup and then you boil the water and pour the hot water into the coffee cup. You then stir the water granule mixture. After that you add sugar and then you stir the mixture again ... Ever stopped to think why you stir? Probably not but instinctively you know that if you don't the sugar will sit in a pile at the bottom of the cup and your coffee will be on the bitter side.

Have you ever stopped to think why you add boiling water then allow the coffee to cool before drinking it? Have you noticed that if you use lumps of sugar or coarse brown unrefined sugar then you have to stir longer to get the sugar to dissolve? Well if you think about the answers to these questions you will be on top of the design principles for any biofilter and you could easily go out and make your own highly effective pond filter unit.

Stirring Anything Makes Contact Between 2 or More Substances More Intimate and Therefore A Better Reaction or Result Is Obtained

This is what, in the world of chemical engineering, we refer to as Mass Transfer Processes. These are processes that also take place in a biofilter where ammonia is converted to nitrates by preventing poisonous build-ups of ammonia. Lets look at our need to stir the cup of coffee 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. 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 with every second we were stirring the concentration of dissolved sugar in water or coffee 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 10 teaspoons of sugar into your cup of coffee not all would dissolve no matter how long you stirred. This is because the water becomes saturated to the point it cannot dissolve any more sugar.

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.

Here's What We've Learned About Biofilter Design For A Fish Pond So Admirably Converted To A Working Trinamic Biofilter By Hozelock

Turbulence improves the rate of mass transfer - this can be achieved by blowing air into water pond filter. If you think the point of lumps of sugar or unrefined sugar you would conclude that size of sugar particle effects the rate of dissolution or mass transfer. Quite correctly you would have concluded that 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. Smaller particles always have more surface area than large particles in terms of surface area per unit of volume. This is why Alfagrog as we discussed earlier in site is available in different sizes.

We can conclude we can optimise the biofiltration process which is essentially the conversion of ammonia in contact with bacteria and oxygen at a solid interface (refresh your memory of this by clicking the link) by:

  1. Increasing surface area for contact between the two media and

  2. Maximising concentration differences at the interface where the transfer is taking place.

There is one other factor to consider and the coffee example used to illustrate this is the use of boiling water. Mass transfer almost always increases as temperature increases ... aroma creation is just another example of mass transfer ... at high temperature the coffee aroma molecules in contact with hot water are vapourised and so you can smell them which makes coffee so inviting.

You can see therefore that other variables can also influence the rate of transfer and temperature is one of the most common. Generally speaking higher temperatures speed up mass transfer processes.

 

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