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Aimed primarily at beginners to the hobby, this series of articles will take you step by step through the process of understanding how a good koi pond works.
Part 5: Improving water clarity
The previous four parts in this series have been about developing a koi pond for the benefit of the koi that will live in it. This part is for the benefit of those koi-keepers who prefer clear water. So far, the reason for removing silt, floating debris or fish waste has been partly in order to keep the biological media as clean as possible and partly to prevent silt building up and becoming a breeding ground for heterotrophic bacteria which will either break down organic waste into ammonia and other pollutants or possibly infect the fish. This has been achieved by mechanical filtration. Mechanical filters also help improve water clarity but there is another problem that affects water clarity and which cannot easily be filtered out mechanically and that is floating algae.
There are many different species of algae that cause problems in koi ponds but, these can be regarded as falling into two main types; filamentous algae and planktonic algae. Filamentous algae are the type that attach themselves to solid objects such as pond walls or floors and look like green cotton wool. These types are normally referred to as blanket weed. Planktonic algae are microscopic individual cells that float freely in water giving it a green colour. These types are known as green water algae.
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Figure 1: Dependent on species, 100 to 200 algae cells will fit between zero and the first division
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A typical species of planktonic algae is usually between 5 and 10 microns in diameter. The scientifically minded will know that one micron is defined as one millionth of a metre but, for the non-scientifically minded, to understand what 10 microns means in terms of size, take a look at the ruler in figure 1. One hundred algae cells of 10 microns in diameter could fit, side by side, between zero and the first 1 mm mark. One thousand would fit between the zero and the 10, and for algae cells of 5 microns in diameter, twice as many would fit into those spaces. Their small size makes them almost impossible to filter out by mechanical means as it would involve using filter media such as diatomaceous earth which is a powder that is so fine that it would literally be clogging after a few minutes.
Clarifying the clarifier
Another way to remove planktonic algae from water is by using either an ultraviolet clarifier or steriliser. The ways in which these two devices work are almost identical but the effects are different. There are many mistaken ideas about the way in which they work, so let us begin by clarifying the clarifier.
The general principle is simple and may be understood in this way: Most people will agree that a short exposure to sunshine is beneficial but that a long exposure will cause very bad sunburn. This is because, apart from the light that we can see coming from the sun, there are three extra wavelengths of light that we can’t see. These are called ultraviolet light because, if our eyes could actually see them, they would appear to be even more violet than the deepest and most violet light that we can see, so to speak. Since naming them with a colour that we cannot see would make no sense, they are simply called UVA, UVB and UVC. It is the UVA and UVB that, in short doses, give us a healthy tan. It is very fortunate that almost all of the sun’s UVC light is filtered out by the atmosphere before it can reach us because it has a far greater energy than either UVA or UVB. Even in short doses it burns, and in longer doses the burning can be fatal.
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Figure 2: Simplified UV clarifier schematic
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Figure 2 shows a simplified diagram of an ultraviolet clarifier. It’s principle of operation is to pass water very close to a high power UV lamp, usually the nasty UVC variety. The UV lamp is inside a quartz glass tube which is sealed to the body of the unit by rubber “O” rings to ensure that the lamp and its electrical connections are kept dry. Quartz glass is used instead of ordinary glass because it allows UV light to pass through it more easily than through ordinary glass. As water passes though the unit, any algae cells are exposed to intense UV light. The algae won’t necessarily have been killed but will have been exposed to the light for long enough to have received severe damage by the time it leaves. Its cell walls will become ruptured and sticky. At the very least it will be unable to reproduce. More likely is that it will soon die and the sticky cell walls will cause dead or dying algae cells to clump together making the mass large enough for them to be removable by the mechanical filtration methods described earlier.
Flow rate is important
The manufacturer’s recommended flow rate through a clarifier will have been calculated taking into account the power of the lamp inside and how close to it the algae cells are travelling. At this flow rate, each cell will have spent a sufficient length of time close enough to the lamp to be sufficiently damaged so that it cannot survive. If the manufacturer’s recommendation is ignored and water is pumped through the clarifier too quickly, the clarifier won’t be damaged but each algae cell will only be exposed to the ultraviolet light for a short time. Just as a short time in the sun won’t harm us, a short time in a UV clarifier will not be long enough for algae cells to receive sufficient damage for the unit to be effective. Pumping water through the clarifier much more slowly than the recommended rate also won’t damage it and although each cell will receive more than enough damage to stop it from reproducing, a clarifier that has far less than the recommended rate may not actually be effective at keeping the pond clear.
With good summer sunlight and water temperatures, algae can reproduce very quickly. A clarifier may be fatally damaging all the algae that passes through it but, unless the whole of the pond volume is pumped through it quickly enough, the rate at which algae is being killed in the clarifier may not keep up with the reproduction rate in the pond.
Clarifiers come in different power ratings; 6, 8, 11, 15 watts etc. This refers to the power of the lamp inside and how much ultraviolet light it emits in a similar way to the power rating of a modern low energy domestic light bulb being a measure of how bright it will be. An approximate rule of thumb for clarifiers is that with typical UK weather, average pond conditions and assuming the flow rate through it is near the recommended rate, ten watts per thousand gallons will keep the pond clear. Those fortunate enough to be reading this in sunnier climes will have more sunlight on their ponds and will need a higher power clarifier. Those who have a pergola with a solid roof or whose ponds are built in the shade will have less sunlight hitting the water and could use lower power clarifiers without sacrificing water clarity.
Lamp life
It is very important not to attempt to look directly at the light from a UV lamp in a clarifier. Remember, its sole purpose in life is to cause severe damage to living cells and it will cause possibly irreparable damage to the eye in just a few seconds of close exposure. Manufacturers are aware that we need to be able to see if the unit is working and so they include in their designs a way to do this such as semi transparent end caps that allow us to see the glow inside. This glow is usually too faint to be seen in daylight but is easily visible at night. Looking at the glow through the end caps, even staring at it, is not harmful but on no account should any other method be tried, especially peering into the water inlet or outlet of a UV unit with it switched on.
With that warning in mind, what can be said about the blue light that can be seen (through the end caps) coming from a UV lamp? It certainly doesn’t indicate that the lamp is emitting any ultraviolet light whatsoever because the human eye cannot see ultraviolet light. The blue light emitted by a UV lamp is just visible blue light, nothing more. It is a by-product of the way ultraviolet light is made in a fluorescent tube and has no harmful effect on algae. The strength of the ultraviolet light emitted from the lamp falls slowly from its highest value when the lamp is new and is usually considered to be too weak to be of any use after about 12 months of continuous use. Although the strength of the ultraviolet falls as the lamp ages, the strength of the blue light doesn’t fall to any noticeable extent, so the blue light from a UV clarifier can only be regarded as an indication that it is on, not that it is producing any useful amount of ultraviolet light. Since the UV output decreases with age, it is usually recommended that the lamp is changed every year. Whilst changing the lamp, it is a good idea to clean the quartz sleeve as any build-up of scale on it will reduce the amount of ultraviolet light transmitted into the water. Cleaning must be done carefully and in accordance to manufacturer’s instructions because the quartz sleeve is very fragile and expensive to replace.
A UV steriliser is very similar to a clarifier, except that the power of the lamp is much higher and that the water is confined into a smaller space so that it is forced to flow very much closer to the lamp. This ensures that anything that is alive when it enters receives such an intense exposure to UV light that it is killed before it can leave. They are called sterilisers because that is exactly what they do – sterilise the water flowing through them. They not only kill any algae cells that pass through but also bacteria and parasites too.
Clarifiers might encourage blanket weed!
There are some common misunderstandings about clarifiers and sterilisers. They do nothing to the chemistry of the water flowing through them. UV light can break down some pond treatments such as Chloramine T so it is best to turn off UV units when medicating the pond but, apart from that, water chemistry isn’t affected.
They don’t poison the water or make it in any way unsuitable for algae. This means that they can only kill any planktonic algae cells that actually flow through them. Similarly only any free floating strands of blanket weed that pass through them will be killed, they won’t kill blanket weed growing in the pond. Perversely, using a clarifier might even encourage blanket weed. Different types of algae compete against each other for nutrients and light. By
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Figure 3: Filter system including UV
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removing planktonic algae from a pond there will be more nutrients available for the blanket weed, also more light will be able to penetrate to give it further encouragement, as if that were needed!
UV clarifiers or sterilizers are best fitted after the main filtration system as the water returns to the pond as shown in figure 3. This will ensure that the water entering it is as clean as possible and help prevent any build-up of deposits inside it.
Obviously UV units are optional, some koi-keepers prefer green water, some more fortunate souls never have an algae problem anyway! And so with the optional addition of a UV, the evolving pond design is almost ready for koi.
More improvements could be made, but for outdoor ponds, one essential feature is missing before it can be stocked – a heater. Yes, all ponds should now have a source of heat available and it may breach the Animal Welfare Act not to have one. Next month I will explain why.
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When fitting an ultraviolet clarifier or steriliser, if there are any manufacturer’s instructions regarding the way in which it should be plumbed in, they should be followed.
The unit shown in figure 2 in the text above is a typical design and was shown in the normal position for operation. In that position, with the outlet at the top, when the pump is turned on the flow of water will push any air inside it along the tube and out of the unit as it leaves.
There are two ways in which it could be mounted incorrectly. If it is vertical, as on the left, air will collect at the top. Most of the quartz sleeve will be cooled by the water but the top of the sleeve will become hot and, as water splashes over it, there is the possibility it might crack. Another incorrect way is horizontally, but upside down as shown above. Air will collect at the top and, at best, this will reduce the flow rate through it or, at worst, part of the quartz sleeve may not be under water, and again there is the risk that cold water splashing onto it may cause it to crack. Similar problems will occur if other designs are not mounted correctly.
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