Biological filtration


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A brief description of the working biological filter
Biological filtration relies on the activity of specific bacteria cultured in the filter to break down toxic waste products into less harmful substances. The bacteria occur naturally and are used to break down domestic sewage in special treatment plants. Water treated at these plants is so clean that it is often recycled for domestic use.

In effect, a biological filter on a Koi pond is really a small sewage treatment unit, exploiting the natural sequence of biochemical reactions that occurs in the nitrogen cycle.

There are two stages in the breakdown of ammonia in a biological filter system, each stage involving different types of bacteria to 'power' the detoxification process. In the first stage, ammonia is broken down to nitrite by a number of different nitrifying bacteria, the most important of which is NITROSOMONAS. A second group of nitrifying bacteria, principally NITROBACTER, converts the nitrite to nitrate. (These two stages, and how they relate to other steps in the nitrogen cycle, are clearly shown in the diagrams). Both these groups of bacteria are aerobic, i.e. they need oxygen to thrive and thus purify the water.

The nitrifying bacteria require an oxygen level of at least 1mg/litre in the water continuously flowing through the medium. Sediment building up in the bottom of a filter can deplete oxygen levels in the water and encourage the growth of anaerobic bacteria (those that thrive in the absence of oxygen, rendering the filter ineffective. It is vital, therefore, to keep sediment to a minimum in the filter. The second chamber in a filter system is usually the biological filter and contains a medium that will provide a large surface area on which the nitrifying bacteria can grow. Again a variety of different, and equally effective, media is available. It really does not matter what type of filter medium you use, provided that the system is capable of breaking down the poisonous waste products into harmless ones.

Installing a biological filter system to a pond does not guarantee an immediate improvement to the water quality, ammonia and nitrite levels are likely to fluctuate in a new filter and it may take from six months to a year before the filter has finally matured. You may notice that the concentration of ammonia in the water increases dramically, sometimes to dangerous levels, once you introduce koi into a new pond. These high levels of ammonia may take only a matter of days to subside in summer, but up to several weeks during the winter months. This is because these filters are biologically active and thus their efficiency is affected by external parameters, such as the weather, water temperature and pH value, and the number and size of fish in the pond.

The growth and reproduction of bacteria are inhibited by cold, for example; both will cease at temperatures below 5c .With these fluctuations in mind, it is important to check pond water quality regularly, after installing a new filter system. Get into the habit of taking ammonia and nitrite readings every week and recording these analyses on paper. If you run into difficulties with koi health, it is useful to have a record of any preceding water quality problems. Both freeze dried and live bacteria cultures of nitrifying bacteria are available; adding one of these preparations to the biological filter on a new pond, or to an established pond in the spring, will boost the filtration process.