a major source of pollution
Written by: Frank Prince-Iles
Dealing with pollution
In an active koi pond we have two types of pollution; dissolved and
solid. If we could remove the solid wastes from the system before they had
chance to dissolve and pollute the water we would have better water
quality, less dissolved pollutants and fewer health problems.
Solid wastes and koi health
If we summarize the situation so far, we can see that if we are to
maintain the status quo as far as water quality is concerned, we need to
remove the pollutants at approximately the same rate as they are produced.
We have also seen that the pollution is basically in three forms:
dissolved compounds, such as ammonia, inorganic pollutants such as
phosphate and DOC, and solid particulate waste.
Unseen but there!
Solid waste will ultimately be broken down by decomposer microorganisms
into a wide range of dissolved pollutants, adding to those already in the
system. It makes no difference where in the system these solids decompose
- the end result will always be the same, that is, further pollution. This
is an important point as many koi-keepers think that once solid waste is
out of sight (in the filter) it is no longer a problem.
With the rapid throughput of most filters, the dissolved pollutants
produced as these solids break down are quickly pumped back into the pond.
What we really need is two filtration systems - one that enables us to
remove the solid wastes from the system before it has time to pollute the
water and the other to deal with the dissolved pollutants. After all, if
we could remove waste solids from the system, we would prevent most of the
sources of pollution.
matter whether the solid wastes decompose in the pond or the
filter - the result is the same - polluted water!
So perhaps, we should look on our filter as a system of two parts, one
part dedicated to removing solid waste matter from the system (not just
the pond) and the other removing dissolved pollutants.
Remember, too, that the pond is also part of the filtration system, as
a significant amount of mineralisation and nitrification will take place
on surfaces within the pond. The pond will also act as a settlement
chamber for solid wastes, which will need regular removal to prevent them
polluting the water. It is my opinion that the regular removal of
accumulating solid wastes presents the koi-keeper with his or her biggest
challenge and a great many problems will be avoided if this can be done
effectively. For regular disposal of solid waste there are essentially two
practical options: settlement and entrapment.
Settlement areas or chambers have to be fed by gravity-flow systems,
ideally via a bottom drain. This way, solids are moved gently to a
collection area, ready to be flushed out of the system. The traditional
method required a large settlement chamber and these can be very effective
provided that the chamber is large enough and the flow rate is low enough
to give lighter solids time to settle. The retention time for water in the
chamber is important. The retention time is simply the filter volume
divided by the flow rate, thus:
Retention time = filter
For instance, if a
filter has a flow rate of 2,000 gallons per hour and the
settlement chamber holds 200 gallons, then the retention time will
200 gal / 2000 gal per
hour = 0.1 hour = 6 minutes.
It has to be said that, in the above example, the short retention time
of 6 minutes is unlikely to be satisfactory, whereas a chamber volume or
capacity of 300 gallons would give a retention time of 9 minutes, allowing
much better settlement.
Having collected solid wastes, it is important that they are flushed
out of the system regularly, before they have time to decompose. During
summer this could be as often as twice a day, and obviously less
frequently in winter. This means that the settlement chamber will need to
have a drain to facilitate easy flushing to waste.
||To maintain good
water quality It is essential that solids are removed from the
pond and filter before they
have time to pollute the water
A better way of collecting solids
An increasingly popular settlement option nowadays is the cylindrical
chamber with conical base that promotes a slow swirl of water. The
cylindro-conical shape encourages settlement of wastes into the bottom of
the cone, where they collect together, making removal to waste simple and
efficient. Again, retention time is important and a slow throughput will
be more successful than a chamber that resembles a vigorous whirlpool. One
should be guided by the manufacturer as to the right size for your systems
but, if in doubt, err on the large side.
When set up correctly these chambers work well and are probably
superior in practice to rectangular settlement chambers of similar
dimensions. Both types need additional cleaning if solids are not readily
flushed to waste since solids may cling to the sides and there may be
areas of poor water flow or 'dead spots'.
Last but not least is the pond itself. Even the best-designed pond
seems to have dead spots, where mulm and fish waste collect. Any waste
that isn't drawn through the bottom drain will need to be removed from the
pond before it pollutes the water and there are several options, depending
on the pond design. It could be gently pushed towards the drain with a
soft broom; the waste could be carefully removed with a fine net; or the
pond could be vacuumed. In most cases it is probably a question of
combining all three actions, with most ponds benefiting from a regular
vacuum during summer.
The most common entrapment systems consist of filter brushes or sheets
of foam. We should consider what type of bacteria likely to be attached to
the brushes or foam, which are heavily loaded with trapped solids? Common
sense tells us that it is going to be heterotrophic bacteria, and do we
really want to encourage high levels of these bacteria in any part of the
system? You will recall that many heterotrophs are also opportunistic
pathogens and are quite happy to lunch on our koi - given half a chance!
Obviously the answer is No. So the important thing with entrapment is that
the entrapment media are kept clean, otherwise they themselves become a
source of pond pollution.
solids must be removed from the system on a regular basis,
otherwise they will simply decompose and pollute the pond. They
will also encourage high levels of opportunistic bacteria.
This reminds me of a case last year, when several fish in a quarantine
tank became ill. The tank was spotless yet the fish had parasites and were
suffering from the onset of bacterial problems. Further investigation
showed that while the tank was exceptionally clean, the filter wasn't.
When we took the media out for cleaning, the smell was overpowering. The
media were covered in a yellow slime which, of course, was all solid fish
waste slowly rotting down. In this case, the filters were slowly poisoning
the fish. Following a good clean-out of the filters, the fish were soon
back to normal
By using a combination of settlement and entrapment it is possible to
remove a lot of solids from your pond before they rot down provided, of
course, that these areas are cleaned regularly. If we are successful in
removing solids from the pond before they pollute the water we are part
way to achieving the ideal of unpolluted water.
Read further in Part 4
Article placed here with permission from the author,