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Not only do these organisms demand oxygen, they must receive it
insufficiently high concentrations so that the function and health
of the koi pond is maintained. Diverse populations of
micro-organisms such as autotrophic and heterotrophic bacteria,
fungi and microscopic invertebrates rely on a supply of oxygen, just
as a burning fire does to keep it functioning effectively.
Deny them oxygen, and the water quality will deteriorate; add
extra and the pond will roar into life, just as a bonfire does in a
gust of wind.
Oxygen is everything in a pond
Without oxygen, filter bacteria cannot oxidise (and detoxify)
ammonia into nitrite and then into nitrate. If a pond or filter
are allowed to become oxygen deficient for any period of time,
anaerobic conditions will prevail causing unstable water conditions
and the tell-tale bad-egg smell.
So how do we best use those devices that are available to us as koi
keepers to add extra oxygen into our demanding ponds? How do they
work, and how can we measure how efficient and effective we are at
adding extra aeration?
Dissolving oxygen into pond water.
The physical nature of water puts limitations on how much oxygen
we can expect our pond water to hold. It also sets our goals for
what we should expect to achieve under different pond conditions. In
the air we breathe, there is approximately 21% oxygen. In any
natural water body, we will typically only find 1% dissolved oxygen,
and usually less than that. So minute are the concentrations of
dissolved oxygen in pond water, that their levels are measured in
milligrams per litre or parts per million (ppm). For example 3
milligrams of oxygen dissolved in a litre of water equates to 3 ppm.
A number of physical factors affect the levels of DO that can be
achieved in a given pond.
Temperature: Oxygen behaves in a surprising way (compared to other
solutes, such as salt) in that as the water temperature rises, its
ability to dissolve oxygen reduces. Compare this to salt (which
dissolves better in warm water) and you can start to appreciate the
unusual relationship water has with oxygen.
Other solutes.
The presence of other solutes in the pond will also
have a negative effect on achievable D O levels. This would include
salt, as well as liquid pond treatments. All the more reason to
aerate vigorously under treatment conditions.
Saturation levels.
These physical factors will determine how much DO
your pond water body will be able to hold, setting the levels at
which pond water will become saturated with oxygen.
At lower temperatures
Water will dissolve greater levels of
oxygen, making it more difficult for your aeration systems to
completely saturate your water. Conversely, at high water
temperatures, saturation can be achieved more easily as water can
only hold reduced levels of DO. By taking into account the
saturation levels of pond water under different conditions, it is
then possible for you to measure the effectiveness of your aeration
system.
- For example, at 10 degrees C, the water's approximate saturation
level will be 11.5 milligrams per litre where at 20 degrees C it
will be 9 milligrams per litre. You can quickly measure the
saturation level of your own pond by using the graph below (see
figure 1), and substituting in your pond's temperature and oxygen
readings.
- Draw a straight line between your two readings on the two
horizontal lines and the saturation level is the value where the
line intercepts the saturation scale. The closer your saturation
figure is to 100%, the better your system is performing. Could your
aeration system be performing better?
How much DO do I need?
Using the illustration of bellows on a fire, it safe to say that
the higher the DO the better the function and quality of life in the
pond. Fortunately, koi being members of the Carp family, compared to
some other fish species, require relatively low levels of DO (6
milligrams per litre). From Figure 1, you can see that even at the
highest UK temperatures we are looking to achieve approximately 70
per cent saturation of oxygen. But if you can improve on that , the
better all the other pond organisms will function. You can also see
from Figure 1 why trout (which generally require a high DO of 11 ppm)
must be kept in cool flowing water.
How do aeration systems increase the DO?
How an air pump works.
The most common form of air pump found around a koi pond is one that
uses a rubber diaphragm. The pump takes in atmospheric air,
compresses it and delivers that air under pressure to an aeration
device (usually a diffuser). The pump itself consists of an
electromagnet and a permanent magnet, attached in some way to a
rubber diaphragm. As the charge of the electro magnet changes many
times each second, the permanent magnet (which is free to move) is
attracted and repelled the same number of times each second. This
causes the diaphragm to move rapidly to and fro, causing the air to
flow into and out of the diaphragm. The suction, compression and
discharge is achieved through the use of one way valves that only
open on either suction or discharge.
Pumps are rated on three measurable features. Their power
consumption (measured in Watts), their flow rate (or volume,
expressed as litres per minute) and the pressure to which the air
can be pumped. An air pump will have a performance curve (Fig 2)
that shows the relationship between the pump's output (in litres per
minute) and the pressure (or depth of pond) to which the air can be
delivered. You will notice that the deeper the air has to be
delivered, the lower the flow rate becomes. This will also show
whether a particular pump is strong enough to deliver air to the
bottom of your pond. The air is usually delivered through a manifold
to allow numerous diffusers to be used.
Different methods of aeration.
- Moving/flowing water
- Venturi
- Diffused aeration
Moving/ flowing water.
Just as sugar dissolves far better in a
cup of coffee when it is stirred, so too oxygen dissolves better in
water that is more highly energised (eg flowing, falling and
cascading). In a natural environment, more energetic water courses
such as Highland streams are higher in DO than the lowland rivers,
and besides the oxygen that is released by aquatic plants, water
movement is the only method by which water can naturally become
oxygenated. In a koi pond, a waterfall can be an effective means of
aerating the water but in an intensively stocked pond, can lead to
limitations. Oxygenated water that enters the pond at the waterfall
will rarely sink to lower layers of water, being effective only at
the surface. Furthermore, the physical work required to move water
to oxygenate is significantly less efficient than moving air,
especially when it only affects the upper layers of a pond. What
more efficient methods can be used to oxygenate pond water?
Venturi.
This method is most easily installed through the wall of a pond from
a final filter chamber. A venturi is a cylindrical hollow tube with
a unique shape through which water is pumped. The unique shape is
often referred to as an hour glass by which the diameter of the tube
entrance tapers down to a smaller diameter, and finally returns to
its original diameter. The effect that the internal taper has,
causes the velocity of the pumped water to increase. At the minimum
diameter (or throat) of the venturi, the pressure drops only to
eventually recover by the end of the venturi tube. If the pressure
loss is sufficient at the throat, then a small hole at the throat
connected to atmospheric air to be sucked into the returning pumped
water, literally injecting air into the pumped returned water.
A venturi can look quite impressive as an aeration device, but in
fact, compared to air pumped into the pond water directly, are
relatively inefficient aeration devices. They are energy inefficient
in that they involve the pumping of water (rather than air) and the
bubble size is very large. This means (as we will see later) that
the rates of oxygen diffusion will be reduced. The large bubble size
also means that only the upper layers of water receive the full
benefit of the freshly oxygenated water.
Diffused aeration.
There are many different types of diffusers available to the koi
keeper, ranging from the basic (and disposable) airstone to other
larger (and permanent) self-cleaning diffusers. Typical diffused
aeration provides a constant stream of air bubbles, rising through
the water column. A fine bubble size is essential to provide the
maximum surface area for efficient oxygen transfer.
For example, if a bubble size of 1/4" is reduced to a fine bubble
a quarter of that size (1/16") the surface area is increased by a
huge factor of 16! Even if some air diffusers look impressive by the
sheer volume of large bubbles they are relatively inefficient as
they provide a relatively small surface area for gas exchange and
most of the pump energy is used to blow air back into the
atmosphere. Contact time between air and water as bubbles rise
through the water column is also important. As water depth
increases, contact time also increases combined with the increased
head of pressure providing better physical conditions for dissolving
oxygen into water.
Furthermore, if fine bubbles are generated by a suitable diffuser
from depth, their large surface area creates a drag against the
water, reducing vertical velocity increasing contact time further.
An added benefit of this "drag" experienced by a rising column of
fine bubbles is the mixing and circulation of significant volumes of
water. This mixing action enhances water quality by making the pond
a well-aerated, homogenous and stable environment. Fine bubble
aeration also reduces surface agitation, permitting continued and
undisturbed viewing of your koi.
Traditional air diffusers (airstones) are perhaps now regarded as
inefficient because as the volume of air supplied is increased to
increase aeration, the bubble size increases reducing gas exchange
rates. They are also prone to clogging and blocking up, reducing
airflow considerably.
A relatively new and unique flexible rubber diffuser (called an
airdome) is now available which has been specifically developed to
satisfy all the necessary criteria for optimum gas exchange. In
addition the flexible nature of the unit has self-cleaning
properties that ensure optimum efficiency during long-term use. This
can be fitted onto a bottom drain where it aerates the pond from the
bottom, giving enhanced aeration in addition to mixing of the whole
water column, enhancing the removal of waste towards the bottom
drain.
In conclusion, the levels of DO achievable in a koi pond are
determined by natural physical conditions, particularly water
temperature. We have various options when choosing an aeration
device, with some methods proving more efficient and effective than
others. The most efficient method by far is using diffused aeration,
but the overall performance is still determined by the careful
selection of air pump and diffuser.
7 things you should know about koi pond aeration.
-
Unlike other substances that dissolve in water, oxygen is
LESS soluble in warm water.
-
Even though koi only require 6mg/l DO, both they andthe
other pond organisms will benefit from higher DO levels.
-
Diffused oxygen is the most efficient method of aeration
-
Small bubbles are far more effective at aerating than larger
bubbles.
-
Venturis and water falls may look impressive as aerating
devices, butin fact they generally only aerate upper layers of
pond water.
-
The addition of airstones to a biochamber will enhance its
performance. Do not aerate a mechanical chamber as this will
adversely affect settlement.
-
Koi require more aeration in warmer weather and during pond
treatments as both these features reduce your pond water's
ability to hold oxygen.
water pond oxygenate aeration air koi diffusers bubble dissolve
saturation pump temperature litre venturi "aeration device"
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