Why chemical engineers pretend to know so much (or so little!)
Koi Pond particle dynamics - William's Soapbox!
I wanted to touch on particle dynamics as means of demonstration. Chemical engineers spend considerable time in developing models for a subject known as transport phenomena. This includes diffusion, solid particle / liquid / gas interactions, slurries, settling tanks, froth film adhesions and so forth and so on. In a Koi pond example, it can get extraordinarily complicated to model what happens to solid wastes. But it is absolutely vital to know what happens with them.
Solid wastes are removed from the pond by the bottom drains. Or are they? Have we now just assumed that this is what happens? The truth is that it is possible that they are. It is also possible that they're not.
These are my own viewpoints. Don't take them as gospel.
Firstly, it stands to reason that if Koi are 90% water themselves, then the solid wastes that they excrete are likely to contain water in them. However, I would expect that the nutrients taken out of the food by the Koi would be done so in at least a semi efficient process (from a chemistry point of view) - fish are energy efficient animals. Hence the nutrients extracted from the food would result in mass being lost from the food and being transferred into the Koi - in other words, the Koi gets heavier (grows). I would also expect that some mass is added to the solid wastes by the Koi in an attempt to get rid of substances that it have entered its body from the environment.
So from the above, all that I can conclude is that I would expect the solid wastes to contain water, and not be of the same composition as the food that entered the Koi via its mouth.
I might expect these solids to be denser than water. This is based on common sense and the fact that I already know that Koi excrete their most dangerous toxic wastes in the form of ammonia, dissolved directly into the water surrounding them.
I also know that Koi take this step to save energy - we humans and other air breathing animals convert our waste ammonia into urea, which is much less toxic than ammonia, but it requires more energy to do the conversion.
Now the question becomes - how much more dense are the solids than water? If Koi excreted lead pellets, the problem would become very simple - these pellets would sink like a stone and collect on the bottom of the pond and nothing short of a watery equivalent of a tornado would be able to disturb (move) them.
But we know that this doesn't happen - we've all seen that solid Koi waste at best sinks much more slowly than a lead weight. This tells me that the densities between the solid waste and the Koi pond water are not that different. They are barely sufficient to cause a settling out of the solids.
Now we have to consider the effects of turbulence and moving water. No Koi pond comprises of water that is stagnant - by that I mean non moving. There is considerable water movement in a Koi pond created by the water returns from the filter system and also by the fish themselves.
If we are lucky the solid waste will slowly sink without breaking up and enter the bottom drain within a reasonable time period. It will not break up because of the increased water velocity through the drain pipe and our mechanical filter system will not break it up before trapping it, collecting it with the rest of the solids and where it can be removed from the system without being further broken up and re-entering the system.