However, before we look closer at what goes on in the biological section of a filter, it is worth considering other important aspects such as filter size, efficiency and overall design. There are many rough guides to determine required filter size and flow-rate. While a rough guide is helpful, the huge variety of different filter media and designs necessitates a more specific approach. It is the latter problem that we need to address first, by considering exactly what we expect from a filter and what are the basic parameter values needed for its optimal performance. But the first thing to be clear about is that bacteria will thrive on almost any surface and the particular choice of medium has very little influence on their growth.
Microorganisms present in the biofilm 'feed' from water that flows past.
So, as a first approximation, the amount of biological activity will be determined by the amount of available surface area for bacterial colonisation. Even this small area could support millions of microorganisms, attached in a slimy biofilm. So we can see that even a small amount of filter medium provides a potentially vast SSA for bacterial colonisation. Each square metre of biologically active surface can metabolise nearly one gram of ammonia per day, dependent on temperature, and given that most ponds will usually be producing fewer than 10g of ammonia per day, the amount of SSA required is really small - and not a lot of people know that, as Michael Caine might say. The void size or empty space within a filter medium is important in determining the right filter size and efficiency.