|
In the first of this series on filtration we explored some of the fundamentals of bioconversion (biofiltration) and
mechanical filtration. We looked at the whole pond system as if it were a living organism.
We even equated it to the human
body demonstrating how the pond must work.
One of the fundamental functions ensuring a healthy existence of life is the
circulation of fluids or air. In the body circulation brings each and every cell into contact with "fresh" blood containing
oxygen and nutrients.
The circulation carries the metabolic waste products of the cells to specific organs (filters) that
will eventually be remove from the body. Clogged arteries or veins result in poor health. A weak heart results in a weak body.
During exercise the requirements of the system increase. The cells need more oxygen, more nutrients and must have the
metabolic wastes removed at a faster rate. The breathing increases to get more oxygen from the atmosphere and the heart rate
increases to pump the oxygen and the nutrients in the body to the areas where they are needed during exercise.
In other
words the flow rate of the circulation system increases as the needs of the living organism increase. Exactly the same occurs
in our fish ponds. As the fish load increases so do the requirements of the pond change. More fish (and/or larger fish) =
more is removed from the system and more impurities (excretion) is added to the system. There is a higher oxygen demand on
the pond water, there are greater loads of ammonia being excreted in the water.
So as the demands on the system increase
we have to compensate for this -and this is achieved through flow rates (and in some cases combined with an increase in filter
size).
FLOW RATES
Flow rates are one of the critical factors influencing the water quality and life in the pond.
The flow rates of the pond water (turnover rates and flow rates through filter systems) are significant as this determines
the rate at which oxygen and other essential resources (e.g. calcium carbonate which will buffer a system's pH) are brought
into the system, and also the rate at which excretory products such as faeces, carbon dioxide and ammonia as well as solids
and organics are removed from the system.
Facts: Flow rates are very important because the nitrifying bacterial adhere
to all surfaces in the pond, therefore their food source (ammonia and oxygen) has to be brought to them. The movement of the
water -circulation, speed of flow and turnover rates determines the speed at which the bacteria get their food.
Stocking
densities and flow rates are intimately connected. In general, the higher the stocking density the faster the turnover rate
should be.
There are two fundamental concepts we should have about flow rates -firstly there is the flow rate of the whole
pond volume this is called the turnover rate and secondly there is the flow rate or speed at which the
water moves through
the various filters and bioconverters.
Basic Principles of flow rates:
1. All the water should pass through
the fiIter system. Any water by-passing the filter will dilute the effect and efficiency of the fiIter system.
2. The
entire pond volume should pass through the fiIter system as quickly as is practical and be returned to the fish, i.e. the
turn-over rate of the pond volume should be as fast as possible. We are led to believe that the pond volume should move through
the fiIter system in under two hours. However, there are enormous benefits from faster turnover rates.
3. Once in the
mechanical or settlement part of the fiIter, the water should slow down. Slow moving water encourages settlement however,
it is very important that flow rates of the entire system must be maintained. Increasing or decreasing the size of the fiIter
chambers can regulate the speed of the water flow in the fiIter system without affecting the turnover rates.
4. In the
bioconverter part of the system, the water can actually flow fairly quickly as the conversion of ammonia to nitrite to nitrate
takes place immediately on contact with the nitrifying bacteria.
5. Flow rates are greatly affected by the piping (restrictions),
additions such as sand filters and the design of fiIters and bioconverters.
In general, the faster the turnover rate,of
the whole system, the faster the impurities will be removed and the more oxygen will be dissolved into the water.
The
faster the impurities such as ammonia are removed, the healthier the pond water will be. The faster the organic matter such
as algae is removed the clearer the water will be.
Myth: Flow rates must not be too fast or the nitrifying bacteria
will wash off the media.
Facts: As the bacterial colonies mature they become very resilient. It would take a really
powerful pressurized spray to remove the biofilm. If the nitrifying bacteria are capable of growing inside the pipe work then
they can with stand quite a strong flow. At very fast flow rates (speed of water moving through a chamber or pipe) there is
a shear effect on the biofilm. In other words the biofilm of nitrifying bacteria can be skimmed or shaved to some extent.
Myth: There must be a contact time inside the bioconverter for complete nitrification to take place. Some say 15 minutes
inside the bioconverter.
Facts: If we take a close look at what is actualIy happening inside a bioconverter then we
realize that as long as the pump is pumping the water is moving! On a microscopic level the water is flowing past the bacteria
on the media quite fast. Wonderfully nature has made provision for this -the conversion by the nitrifying bacteria of ammonia
to nitrite and further to nitrate takes place immediately on contact. The speed of the passing water has little to do with
the speed at which the conversion takes place. Faster flow rates through bioconverters work better than slower ones.
For
water to be in contact with the media for 15 minutes or so in the chambers, the pumps would have to be turned off.
As
an example of flow rates consider the trickle filter. This is considered to be the most efficient method of bioconversion.
Why? Because the media is exposed to the atmosphere and has a thin film of water flow over it, therefore, the water has
high levels of oxygen. And secondly the speed at which the water flows downwards through an open media bed is very fast indeed.
Myth: The filter media must be submerged in water.
Facts: The media does not have to be submerged. The bacterial
colonies growing on the filter media need a film of water running over them. The bacterial colonies are microscopic.
Trickle
filters shatter the above misconceptions. In the trickle system of biofiltration, there is no actual water level within the
chamber surrounding the filter material; it just passes through the filter box and filter material by trickling down from
above. Each piece of fiIter material is covered with a thin fiIm of water (all that is really needed as bacteria are microscopic
creatures). The speed at which the water is pulled through the trickle fiIter by gravity is faster than most recommended flow
rates through water filled filters! The contact time is, therefore, relatively very brief. One would think that the bacteria
would not have time to purify the water as it passes through the filter material so quickly. It is also difficult to imagine
how the bacteria can cling to the filter material beneath the torrent of water, yet it does. In fact, this design of biofilter
is very efficient.
Try it -take a sieve and fill it with media (stone, plastic -anything) now pour a glass of water
onto the top. Watch how fast the water exits at the bottom.
Flow Rates Through Bioconverters and Filters
Basic
Principles
Oxygen must be introduced into the pond water before the fish can be introduced and before nitrifying bacteria
will develop.
Introducing oxygen into the water activates the system.
Water passing through the bioconverter chamber
and media should not be so fast that conversion of impurities is minimal. Very fast flow rates through bioconverters can scrub
off the bacteria growing on the media.
Conversely the flow rate should not be so slow that water that has been purified
is sitting in the filter chamber taking up space that could be used more efficiently. It is remarkable that both fast and
slow moving flow rates work to a degree. However, filters that flow too fast and filters that flow too slowly are inefficient.
Mature filters with large masses of nitrifying bacteria will be able to convert almost all ammonia to nitrite and nitrite
to nitrate on a single pass through the filter medium. This should be the goal -small, cheap to run, easy to maintain and
efficient.
The turnover rate of the whole pond should be as fast as possible, but the flow rate through the filter/bioconverter
system should be set so the solids can be removed and efficient nitrification can take place.
Effects of Flow Rates
on Ambient Ammonia
The flow rate of the pond water through the filter chamber or chambers has a bearing on the levels
of oxygen and ammonia in the system. This is a very important consideration because it directly effects the ambient or daily
background ammonia levels as well as oxygen levels in your pond.
Myth: A pond must have zero ammonia, nitrite and
nitrate.
Facts: The fact is, the actual ammonia level in a fully recirculating pond can never be zero -even if the
filter design is so efficient in design as to remove 100% of the ammonia, nitrites and nitrates that passes through it.
There
will always be some quantity of ammonia in the pond water by virtue of the fact that the fish are continuously adding ammonia
to the water. The filter can only remove ammonia from that portion of the pond water that is moving through it at any given
time. However, as one portion of the pond water has ammonia removed, the fish in the pond are polluting the balance.
The
greater the fish load, the higher the ambient ammonia level will be {background ammonia). The filter design should also take
this into account. The bioconverter should be designed in such a way that the water that is purified is returned as quickly
to the pond as is practical.
From this explanation, it should be obvious that the greater the number of pond volumes moved
through the filter each day, the lower the ambient (average daily background) ammonia level will be.
Pond volume, bioconverter
volume and flow rates are intimately connected.
In a properly designed system, the ambient ammonia level, while never
actually zero, it is simply too low to be measured with most hobbyist test kits.
Low Ambient Ammonia
In order
to ensure an acceptably low ambient ammonia level, the filter should be designed to process at least 12 pond turnovers per
day. Thus, one-half of the pond's volume will be pumped through the bioconverter every hour. Increasing the turnover rate
to one full pond volume per hour (24 turnovers per day) would be even better. However, a close examination of the costs for
the larger pump and higher electricity running costs should be made.
To determine a rate of one-half pond turnover
per hour, divide the volume of the entire system, the pond, the fiIter and the bioconverters volume by two. This will give
you the flow rate entering and exiting the filter in gallons or litres per hour.
For example if a pond has a total capacity
of 19,000 litres, then at one-half turn over per hour we would need a pump that would be capable of pumping a minimum of 9,500
litres per hour, or 160 litres per minute.
This would be the water actually delivered after various restrictions are taken
into account.
Other Advantages of Higher/Faster flow Rates:
1 .It is an undisputed fact that koi have better growth,
live longer and their colours are brighter and more intense in oxygen rich environments.
2. It is also an undisputed fact
that biological fiItration is more efficient and flourishes in a more oxygen rich environment.
3. Therefore, your pond
system should include a method of aerating the water at ALL times to replace the lost oxygen. In other words, the pump must
run continuously and provide a high turnover rate.
Dissolving Oxygen into The Water.
The only way oxygen can dissolve
in water is by contacting the water. This is typically accomplished by diffusion between molecules of water in contact with
the oxygen in the atmosphere.
Some interesting deductions can be made from this fact:
a. all the water's molecules
should be brought into contact with the atmosphere at some time during a complete circulation cycle.
b. the water should
ideally be brought into contact with the atmosphere as much as possible (surface area and turnover rates) to assist with the
diffusion of oxygen into the water and the release of unwanted gasses into the atmosphere.
If the water was drawn
exclusively from the surface of the pond and returned to its surface, the oxygen levels in the lower levels of the pond water
may become critically low (unless supplemental circulation within the pond is provided like air stones). The water will take
on a "dead" look. Anaerobic bacteria will flourish on the floor. The settled solids will decompose and produce the deadly
Hydrogen Sulphide, gradually poisoning the system and fish.
Flow Rates and Removing Solids
Higher turnover rates through
filters will remove solids at a faster rate. In many cases this is the difference between murky water and clear water. However,
if the water moves too fast in the settlement chamber nothing settles out.
If the turnover rate is faster or equals that
of the production and accumulation of solids, then the water will clear.
FACTORS AFFECTING FLOW RATES
No matter
how powerful the pump, the final output will be restricted by several factors. Restrictions will reduce flow rates and ultimately
affect the stocking density allowable in the pond.
Myth: The diameter of the piping is not relevant because the pump
is pushing the water under pressure.
Facts: The diameter of the piping in the whole system has a dramatic effect on
the flow rates. Smaller diameter piping will reduce the flow. Longer lengths of piping will reduce the flow. Bends and elbows
reduce the flow. Using larger diameter piping will improve the flow rates. Reducing the number of 90 deg bends will improve
flow rate. A single 90 deg bend has the equivalent frictional loss of about 25m of piping.
Piping on a koi pond can be
equated to the veins and arteries in the body. Restricted piping will negatively affect flow rates and water delivery. The
size and length of the pipe work in combination with the pump
will dictate the flow rate irrespective of the size of the
filters and bioconverters. Pipe work will alter the flow rate calculations because of frictional losses. Additions with restrictions,
such as sand filters and venturis, will greatly reduce the flow rates.
Koi keepers can get increased flow rates simply
by increasing the diameter of the piping.
Other design features affecting flow rates are sand filters (used incorrectly),
mechanical filtration that is restrictive, the height from the pump to the return to the pond. If the waterfall is 3 meters
high expect some water loss.
Claimed water delivery vs. actual water delivery of pumps
There is often a disparity
between claimed flow rates and actual delivery of water of pumps. Different makes of pumps may have the same power motor driving
them but often they deliver different amounts of water.
Article by,
Chris Neaves
|
|
