filtration and filter media
what goes on inside the filter?
Written by: Frank Prince-Iles
At the heart of the koi pond
At the heart of a koi pond is the filtration system. To encourage a
vigorous growth of nitrifying bacteria in the filter we need to keep the
filter media fairly clean, which is often easier said than done! Efficient
biological filtration also depends on the media of choice having an
adequate specific surface area (SSA), adequate voiding and water retention
For good filtration and good water quality very little solid waste
should enter the 'biological' section of the filter. Although, for
simplification, I make a clear distinction between the settlement /
entrapment areas of the filtration system and the biological section, it
is worth remembering that the two are linked, so some nitrification occurs
in the settlement area, and some settlement and mineralisation occurs in
the biological section.
However, the aim is to ensure that conditions in the biological section
are such that vigorous growth of important nitrifying bacteria is
encouraged, while conditions that would promote growth of heterotrophic
bacteria are avoided. Most likely, heterotrophic bacteria will predominate
at the start of the biological section, where the smaller particles of
remaining solid organic matter become trapped. So, in general, when we
refer to the biological section we mean the area where dissolved
pollutants and particulate matter are converted by microbes into less
To re-state the point made on other pages; the more effective the
settlement area of the filter at removing solid waste, the lighter the
load on the following biological section - provided, of course, that
trapped waste is removed before it decomposes.
More than nitrification
I made the point that a filter does more than simply convert ammonia to
nitrate. Although ammonia is often the most toxic pollutant in a pond, we
should not forget that normal biological, metabolic and chemical activity
produces a wide range of pollutants. Even with the most effective
settlement and entrapment system many organic wastes, particularly fish
faeces, will start to decompose, producing various dissolved organic
carbon compounds (DOC). We expect the filtration system to deal with these
pollutants also, not just with metabolic ammonia.
The diverse range of biochemical processes occurring in the filter are
due to many species of bacteria, fungi, protozoa and various worms and
possibly snails present- not just nitrifying bacteria. However, our pond
husbandry routine should be designed to maintain conditions which
encourage nitrifying bacteria, and this is achieved by:
- regular maintenance to keep the biological area clean and free of
- reducing the level of dissolved organic compounds by effective
settlement/entrapment, together with regular cleaning of the
||If we can remove
solids from the system before they decompose and at the same time
keep the biological section of the filter fairly clean we will;
- encourage a vigorous growth of nitrifying
- reduce the load on the biological section
Filtration how & why
Understanding the fundamentals of biological filtration is helpful in
diagnosing how common fish health problems occur. 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.
One such suggests that the filter surface area should be approximately
one tenth that of the pond; another, that there should be a pond turnover
rate of once every 2 to 3 hours. While a rough guide is helpful, the huge
variety of different filter media and designs necessitates a more specific
approach. When it comes to water quality there are three main reasons for
why things go wrong:
- indiscriminate use of chemical treatments, which can damage filter
- poor maintenance of pond or filter
- poor filter design
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.
Probably one of the most discussed subjects in the hobby of koi-keeping
is the merits of various filter media - and there are sometimes quite
incredible claims made for the various types. 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.
How much surface area?
Nearly all types of filtration system rely on attached-growth processes
in which a bacterial slime layer or biofilm -comprising bacteria, algae
and often larger invertebrates - forms on the media. 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. However, in
practice this available specific surface area (SSA), as it's called, is
rarely a limiting factor since most filtration systems are large.
Obviously, if you had just a square piece of material measuring say I m
x 1 m this would give a total area of two square metres (with both sides
being available for bacterial colonisation and assuming almost zero
thickness). Even this small area could support millions of microorganisms,
attached in a slimy biofilm. But typical filter media have a far greater
SSA. For instance, gravel has an available surface area of about 100 to
200 square metres per cubic metre (100-200 m2/m3). And other, more
specialist media can have significantly more surface area; for example:
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. If we based filter sizing on the basis of SSA alone, filters could be
incredibly small -perhaps the size of a shoebox! However, there are other
factors to consider ....
Is void important?
The void size or empty space within a filter medium is important in
determining the right filter size and efficiency. Void size is a measure
of how much of the medium consists of empty space. If we consider sand,
for instance, each particle has a large surface area in relation to its
volume and the total SSA per cubic metre of sand works out at thousands of
square metres. Despite this enormous SSA, sand would make a poor filter
medium because the small particle size would soon lead to blockages and
subsequent 'tracking' as water found the 'easy routes' round the medium.
And, of course, because of the dense packing, any flow through the sand
would be very slow. So, despite its massive surface area, once compacted
and blocked the amount of surface area exposed and the volume of water
that could be treated per hour, would actually be quite small.
There is another important disadvantage of a medium like sand -
retention time, or the amount of time the water spends in contact with the
biofilm. It is obvious that if we wish to avoid blockages and tracking,
some void space in the filter medium or media is desirable. If we consider
a medium such as gravel, although its larger size yields less SSA it is
less prone to tracking and blocking. And specialist media such as filter
matting, plastic or sintered glass, have both a large SSA and a generous
void space. In fact, many of them are more than 90% void or empty space!
This makes tracking and blockage almost impossible.
What about cleaning?
Another important consideration - which becomes more important the
longer you keep koi!- is ease of cleaning. In the early days of the hobby,
part of the novelty lies in spending weekends cleaning and vacuuming. But
after a while, strangely, it seems that there are more pleasurable ways to
spend a sunny Sunday. And with gravel and other granular media, it really
isn't much fun trying to clean several tons of the stuff! Compared to
gravel, cleaning light-weight media is a delight. Obviously, regular
maintenance is somewhat easier if each filter chamber has its own bottom
drain but, even so, ease of maintenance has to be a major consideration in
the choice of filter medium.
||the three major
factors affecting our choice of filter media are
- specific surface area
- void space
Read further in Part 5
Article placed here with permission from the author,