Biological filtration and filter media

  The Fish Environment

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 time.

Biological filtration

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 harmful substances.

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 mulm,
  • reducing the level of dissolved organic compounds by effective settlement/entrapment, together with regular cleaning of the settlement area.

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 bacteria

  • 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 function
  • 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:

Medium SSA (m2/m3)
Gravel 100 -200
Plastic 250 – 300
Matting 300 – 400
Foam 400-500

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 timeor 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 cleaningIn 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 

  • cleanability