The Science behind your Pond UV light

Every Koi keeper is aware that in the first year of establishing a new pond, the chance that the dreaded green soup occurs is high. The advice from Koi dealers is to employ a UV clarifier unit to help address this issue. These UVC units are nowadays seen as essential equipment in keeping our ponds healthy and clear but so that we can all enjoy our Koi carp.

How can we get the best out of our UVC units?

To understand how these UVC units work we need to explore the basic principles to apply them in ways that our Koi and pondwater will gain optimal benefit from. We cannot naturally create UV-C on earth, but we can do this with a low-pressure mercury lamp in a hermetically sealed tube of silica or quartz with electrodes at both ends. The electrodes are made from tungsten, and the tube is filled with an inert gas (typically, argon) and a small amount of mercury.

A gas discharge is struck by applying a high voltage across the electrodes. UV light is emitted from the lamp when mercury vapours return to a lower energy state; a state which is now far better controlled by the latest generation of electronic ballasts.

The presence of Argon serves to facilitate the lamp’s starting, as well as help to extend electrode life and reduce thermal losses. Crucially, it does not contribute to the spectral output of the lamp. In our application, the lamp is inserted into a quartz sleeve to help maintain a stable temperature of 40°C. As a result, the lamp can generate especially high levels of UV-C.

Not all UV-C lamps are made equal

There are some important differences between lamps that are worth noting.

The first is that better quality lamps are manufactured with a shield around the filament. This shield creates a stable filament temperature and thereby removes any hot spots that you can experience with inferior UVC lamps. Second, the shield promotes a more efficient and higher output level.

In the top two images:

• You can see a hot spot in the filament that would reduce the UVC output.

In the bottom two images:

• They have a shield around the filament to get an equal heat that increases the UVC output.

Please do not try to make these images by yourself because it is harmful to your eyes and these were carried out under controlled conditions.

What other components are used in a UVC unit?

One of the most underrated aspects of a UVC unit is the ballast which fires up the UV lamp. Recent developments in areas such as the slow start-up process and the automatic regulation of voltage to meet the lamp’s particular requirement have enabled us to both reduce energy costs but also significantly extend the lifetime of the lamp. To secure optimal UVC output, one ought to seek out the optimum ballast-lamp combination.

In addition to the above, our research has demonstrated the benefits of placing the lamp in a quartz sleeve to maintain it at a stable temperature. However, not any old quartz sleeve will do.

To illustrate: If the quality/purity of the quartz sleeve is poor, UV-C levels can be reduced by 6% or more.

Finally, there are various materials such as ABS, High-Density Polyethylene, PVC, stainless steel, titanium and others that, when combined with a quartz sleeve and lamp, can result in either increased absorption or reflection of UVC.

The reflective materials are to be preferred in this context as they increase UVC efficiency in the chamber, and so are a great benefit for the pond.

Clarity and reflectivity

The clarity/turbidity of our pond is measured in NTU and is dependent on the size and distribution of organic debris present in pond water. For example, the tap water we drink ranges between 0.4 and 0.8 NTU, whereas a well-maintained pond will average between 0.4-1. In contrast, a green pond will average around 5-10 NTU.

Simply put, the less clear the water is when it passes through the UV unit, the less effective the UV dose will be as a consequence of the increased turbidity and its consequent reduced reflectivity. From the table below, you can see that with an NTU level of 10 the UV dose is reduced by 35%, and with a well-maintained pond the level is between 2-3%. This reduction can be mitigated by utilising optimally reflective materials.

The correlation between UV dosage an NTU

Turbidity NTU

(Image above: source, Journal American Water Works Association. How particles affect UV light in the UV Disinfection of Unfiltered Drinking Water, Jason Christensen, April 2003).

Let’s imagine the following scenario in which a Koi keeper has chosen to place a UV-C lamp directly into their pond. If the UV lamp is not enclosed in a suitably reflective pipe, and the nearest wall surface is more than 20cm away from the quartz tube, then the positive contribution made by any process of UV-C reflectivity can be dismissed as being as close to zero as makes no difference.

In the table below you can see the efficacy of various materials in comparison with a non-reflected value (termed a baseline UVC dose value). In short, you can see the same tube but with different reflective materials being compared with a zero-reflection scenario. For reference, the table also includes details of the results for materials such as aluminium which would undoubtedly improve the UV dose dramatically.

However, as we know, aluminium is entirely unsuitable for pond contexts. Similarly, ABS (with its admittedly cheap but brittle nature) is also best avoided.

Material and reflectivity table

(The relationship between material and reflectivity demonstrates a clear area where UV's can become far more effective).

UV dose, flowrate, and organism size

In general terms, when it comes to photo-chemical filtration, the size of water-borne organic debris is of key importance: the larger particulate matter is, the more difficult it becomes for UV-C to penetrate it/be absorbed by it. For example, to reduce the algae levels of a species such as Chlorella Vulgaris you would need a minimum UV dose of 22mWs/m2, whereas for E-coli you would need a UV dose of 3mWs/ m2. Alternatively, for a parasite such as Icthyopthirius sp. (aka. ‘white spot’) you would need a dose of the order of 336mWs/m2.

In our industry, one often hears the common confusion resulting in the description of a UVC unit as a clarifier or a steriliser whose function is to clear ‘the green’ from a pond in short order.

Micro organism kill rates table

The confusion stems not from the essential nature of the units being wholly misunderstood. Rather, such confusion is based upon a misunderstanding of, or a downplaying of the critical role played by time in this process; or, to be more specific, the movement of water volume in a given time period. In simple terms, a UV-C lamp produces a set amount of light of a certain wavelength. Its level of production is utterly indifferent to the speed at which water does or does not flow past it as it does so. However, if we as Koi keepers are in the business of optimising the effectiveness of the UV-C produced by our lamps then we need to give some thought to flow rate. After all, at the end of the day, sterilisation in this context is essentially all about contact time.

Contact time

This can be controlled by the volume, the turbulent design, and the reflectivity of the reactor. So, if the flow is the same but you have a reactor volume of 8L instead of 1.6L the contact times become 5x larger. This is more complex than perhaps generally imagined as there are a lot of parameters involved. To simplify this further, we have calculated the flow through a 55-Watt lamp for several species of micro-organisms. If you wish to remove common green algae quickly then you need to reduce the flow to 1960lph. If you have a 110- Watt unit then you can increase the flow to 3920lph and for a 220-Watt ladder, this would be 7840lph. Most Koi keepers would not have a 220-Watt UV, and those that do will, in all likelihood, have a pump flow that is far too high to optimize the functionality of their UV unit; causing it to take longer to reduce algae levels. However, this remarkably common practice can be addressed by creating a separate circuit with a bypass.

Installation set-up and bypass

For many of us, the installation of a UV unit can be something of an afterthought, typically purchased once we have fallen victim to what seems to be, and despite our best efforts, an immutably green pond. Typically, we reason that we do not have enough space in our filter room or that maintaining the UV would be problematic, especially when you need to replace the UV bulb.

However, all is far from lost. If you employ a ball valve for both the inlet and outlet of the UV, you can then easily remove the entire unit. This means that performing your yearly bulb replacement and your recommended bi-annual quartz tube clean becomes a relative breeze. Additionally, if you make a bypass between the inlet and outlet and include a third ball valve you then have full control of the flow that goes through the UV, and this can be especially beneficial when you have Trichodina sp. (fish parasite). If an outbreak does occur in the pond, you will need a very low flow through the UV. Furthermore, we always recommend that you install your UV unit after the filter because the water is already that much clearer.

Having the latest Tempest filter (which removes even the very smallest particulate debris from the water) before your UV unit will secure an impressively low NTU level; and represents the most effective UV treatment set-up. If the water returns to your pond through a flexible hose you can perform a simple bucket test by timing how long it takes to fill a 10L bucket. This way you can calculate how many litres are being cycled per hour. For example, if it takes 30 seconds to fill the bucket then you can 3600/30 times 10L equalling 1200L/hr.

If you have two separate 55-Watt UV units we would recommend you install them in parallel configuration instead of a serial arrangement (where you put them behind one another). The principal reason for this is if you have a system flow of 10,000L/hr, and you have run 5000L/hr through each 55-Watt unit in a parallel configuration instead of 10,000L/hr through a serial set-up which is less efficient because of the higher concentrations of water borne debris per litre which accompany (and unhelpfully obscure) the various organisms that we are targeting with our UV-C, and which thereby serve to reduce and diminish the efficacy of the contact time.

Summary

With exciting new material developments that do not absorb the UVC wavelength but instead act as a perfect mirror (Why not Google “isotropic luminance and Lambertian reflectance” if you fancy reading more) we can significantly increase the UVC dose of our UV. This in turn will help Koi keepers by reducing their electricity costs since a 55-Watt has the same UV-C output as a 110-Watt unit.

Additionally, and we cannot commend this to you all enough, if you create a separate dedicated circuit for your UV, maintenance will become simple and hassle-free. Plus, creating a bypass will enable you to keep your filtration online and uninterrupted (something your Koi will certainly appreciate).

Furthermore, if a bacterial outbreak such as a white spot occurs in your pond, then with the bypass you can effectively control and manage the outbreak swiftly. The bypass allows you to control contact time and increase this further through opening the ball valve, resulting in the outbreak disappearing quickly.

In essence, modern UVs have become vital preventative equipment that benefits the pond and its inhabitants. I hope that the above has addressed (and enabled you to dispel) any doubts or concerns that you may have entertained regarding the benefits of employing a UV-C unit as part of your Koi pond setup.

Happy Koi Keeping!

Author: Jasper Kuijper (Technical Director at Evolution Aqua Ltd)

Evolution Aqua range of Ultraviolet Clarifiers For Ponds

Our range of evoUV pond clarifiers have been manufactured to deliver optimum performance at all times. evoUVs have been designed to clear green water, which is caused by single celled green algae, and prevent it from returning. 

Our Titanium Enhanced Ultraviolet Clarifiers for Ponds

Our New range of evoUV Ti pond clarifiers, Titanium Enhanced UV Clarifier is up to 1.5 times more effective than a standard UV Unit. The evoUV Ti is made using the latest photocatalytic technology and is lined with a thick and polished titanium tube, to clear green pond water fast. 

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Programmed cell death
Loosely, this is where a cell ‘chooses’ at a particular time (as determined by its DNA; hence, “programmed”) to commit suicide for the greater benefit of the organism of which is a part. Think Rutger Hauer as Roy Batty in the original ‘Blade Runner’: “Time to die”

Photoproducts
These are products or outcomes produced by photo-chemical reactions. That is, products of chemical reactions that require visible light (or non-visible forms of electromagnetic radiation) to increase the energy of a system.

DNA photoproducts
When DNA is exposed to UV light, it too leads to the production of photoproducts. These particular photoproducts are called ‘pyrimidine dimers’. It is the cell’s inability to deal with these photoproducts, and so repair its ‘wounded’ DNA, that ultimately results in a cell’s loss of ability to divide & grow.

Cyclobutane pyrimidine dimers
Pyrimidine dimers are molecular lesions or ‘wounds’ formed in DNA as a consequence of certain kinds of photochemical reactions (i.e.) chemical reactions utilising UV light.

Cellular lethality
The capacity to cause death, serious harm or damage to a cell

Senescence
Deterioration with age which occurs as result of loss of an organ, tissue or cell’s power to efficiently replace worn-out aspects of itself (e.g.) the diminished elasticity of human skin as we get older.

Mutagenesis
The process of generating a genetic mutation (= pyrimidine dimers, for example) in the DNA of a cell which will have the effect of preventing it from successfully dividing & growing in the future.

Oxidative stress
Living systems need to maintain a specific balance of free radicals* and antioxidants**. When free radicals outnumber antioxidants, it can lead to a state we call oxidative stress. In simple terms, many of things which can kill bacteria, algae etc. do so by manipulating this free radical – antioxidant balance in their favour. The better they are at this manipulation, the more effective they will be as biocides.