Ultraviolet light has been widely used for just over a century as a disinfectant in a number of different industries. British scientists discovered that certain wavelengths of light have germicidal properties by disrupting the reproduction of micro-organisms – viruses, bacteria, fungi and protozoa. This has obvious benefits for the fishkeeper.
Using UV light to disinfect water of such microbes is called UV sterilisation. Ultra violet light has also been demonstrated to have an algicidal effect on pelagic single-celled algae by causing them to flocculate into large clumps. These can then be filtered mechanically from pond water. The success of ultraviolet light as a germicidal and algicidal treatment lies in its simplicity of use and installation.
Wherever water is recirculating, a UV steriliser unit can be installed, rated to the volume of the system. It is very difficult to use a UVS incorrectly. Indeed, it is an environmentally friendly way of treating pathogens without affecting the livestock and water chemistry. It is impossible to overdose with a UV, which is a significant advantage when compared with the chemical alternatives.
UV treatment is odourless and tasteless and treated water is safe for human consumption being used as a disinfectant in the drinking water, pharmaceutical, food processing and beverage industries. It is even used in swimming pools as an alternative to chlorine. No more streaming eyes!
Its use as a sterilising agent in the aquatic field is advocated by MAFF in two significant areas.
1. Quarantine Facilities
Research establishments must provide adequate quarantine conditions for non-native but temperate aquatic species. As these species are potential hosts to alien disease that is capable of existing in the UK climate, all effluent and recirculation systems holding such livestock must be subjected to UVS. This also includes all water being discharged from these systems.
2. Depuration of molluscs prior to human consumption.
All bivalve molluscs prior to sale are ‘cleaned’ by being placed in UV treated water while they cleanse themselves of normal bacterial content. This process is called depuration and protects consumers from the risk of food poisoning.
What is UV?
Ultra violet light (or more correctly radiation) is found between the visible blue end of the electromagnetic spectrum and X rays. UV light fills a broad band within the spectrum from 13nm to 400nm (nanometers).
The germicidal UV light falls between 200 and 295nm with most lethal rays being at 254nm. The UV at 200nm causes ozone production which is undesirable in the aquatic environment.
Tanning lamps are effective between 280 and 330 nm (hence the need for protective eye goggles) and small UV units used to detect counterfeit bank notes use another part of the UV band.
Despite the blue appearance of UV bulbs, UV radiation is invisible and cannot penetrate glass. For this reason, glass filters are often inserted in metal halide pendant lamps over aquaria to reduce harmful UV radiation effects to humans. It is also the reason why UV sterilisers use quartz sleeves in their construction rather than glass as this permits UV radiation. Most UV bulbs are also manufactured from quartz glass although others are manufactured to remove 200nm radiation to prevent ozone production.
Germicidal UV waves are also poor at penetrating water, being absorbed in the first 2 inches (5cm). Penetration is reduced significantly by suspended solids, dissolved organic matter and dissolved salts all of which absorb and deflect UV waves.
UV light is produced artificially by passing an electrical current through mercury vapour under precisely controlled conditions. The electrical current excites the mercury molecules, emitting UV radiation.
UV lamps used in the aquatic field are usually low pressure hot cathode lamps that are started using a choke unit similar to those found in fluorescent light units.
Why is UV Germicidal?
At 254 nm, UV radiation is at its most lethal. It is an indiscriminate living tissue killer (i.e. it kills both pathogenic and non-pathogenic micro-organisms). For this reason, under no circumstances must the bulb be viewed directly or indirectly by the naked eye as it can cause tissue damage.
Within the cell of every living organism is a nucleus that contains the genetic information, coding for the structure and development of that individual. Some micro-organisms are so basic that their genetic information is not even held within a nucleus. When that cell or micro-organism divides to produce new individuals (as in asexual reproduction of bacteria and algae) then the genetic information is duplicated and a copy passed on to the offspring.
The genetic code is held within DNA (Deoxyribonucleic acid) and it is these building blocks which duplicate to produce the code for the new bacteria or algae.
UV radiation, peaking at 254nm inactivates the nucleic acids within the DNA of the micro-organism inhibiting any subsequent synthesis of DNA, preventing the cell from successfully dividing and multiplying, causing premature death. Only those organisms coming into contact with the radiation for sufficiently long will be affected, while those in the aquarium or pond will not be harmed.
It has been suggested that some strains of bacteria have the ability to repair denatured nucleic acids after UV radiation, often mutating into new and potentially UV-resistant microbes. Could the continuous use of UV select for new UV resistant bacteria? If pathogenic, these could then proliferate within the recirculation system causing extensive disease problems.
Factors affecting UV efficiency.
The effect of a UV steriliser on pelagic microbes is largely dependent on 3 factors.
Wattage of the lamp
Contact time under the lamp
Lamp area
UV output can be described as (w/sec/cm2 (microwatts of UV(millionth of a watt) per second per cm2).
Research has shown that different microbes have different minimum lethal doses depending on their size. A general trend exists to which there are exceptions where larger organisms require more radiation to be ‘killed’. I.e. the DNA with a bacterium is affected easier than that of a protozoan. (See Table)
Table showing the required UV exposure to ‘kill’ various micro-organisms.
Micro-organism Exposure of UV radiation (254nm) required to ‘kill’ each organism ((w/sec/cm2) Chlorella (algae) 22,000 Fungi 45,000 White Spot, Tomite 336,000 Paramecium (Protozoa) 88,000 Trichodina (Protozoa) 159,000 Salmonella (Bacteria) 10,000
It is inaccurate to compare UV sterilisers by Watt ratings in the same way it is inaccurate to assume the performance of cars by their engine size.
Performance of UVs of the same wattage will differ due to design and tube diameter. Some UV designs encourage turbulence of water within the unit increasing contact characteristics between water and radiation, improving lamp performance. Other units have different chamber diameters affecting radiation and disinfection rates.
It is due to design improvements such as these that different manufacturers rate UVs of identical wattage as having such vastly different disinfection rates. I.e. an 8W UV by two different manufacturers may be rated at treating 1000 gallons by one manufacturer and 2000 gallons by the other.
UV sterilisers reduce the population of free-floating (pelagic) micro-organisms and do not affect those microbes attached to decor, or fish that will continue to act as ‘reservoirs’ for further infection. In that sense they do not sterilise the water.
While being useful at controlling microbe numbers below lethal levels, during an outbreak of disease their use is no substitute for dosing with chemotherapeutic agents. However, this is not always practicable.
Typical UV sterilisers incorporate a UV bulb jacketed within a quartz sleeve which is sealed within an ABS plastic tube. Water enters one end of the tube, circulates around the quartz sleeve and exits from the opposite end. In this arrangement the bulb operates at its most efficient temperature and emits radiation to the target organisms around the entire circumference of the bulb.
Are there differences between a pond UV and an aquarium UV?
UV units are used in both aquaria and ponds for their disinfecting qualities but for different reasons and under different specifications of construction.
UVs are used in the pond environment to ‘kill’ the microscopic unicellular algae that cause green water.
Ponds contain relatively large volumes of water requiring significant volumes to be pumped through a biofilter and UV. Consequently pond UVs come with fittings up to 2″ in diameter with quite a wide diameter jacket around the tube to handle the required turnover.
The wattage of the UV must be geared so that the radiation is sufficient to denature the algae at a rate faster than it can reproduce. The radiation required to produce crystal clear water is below that of a sterilising action when the rapid flow rate is taken into account. Pond UVs can typically go up to 60W.
Consequently, to be more precise, these units are referred to as UV clarifiers rather than UV sterilisers. The distance between the quartz sleeve and the outer chamber will generally be greater than 10mm as they are designed to carry volume. This reduces the sterilising effect of the unit.
Adjacent to the pond the UV must be installed with a suitably sized biofilter with sufficient mechanical filtration capacity to remove excessive quantities of dead algae. Once installed, UVs are notorious for creating significantly more particulate waste and it is often wise to install an ‘oversized’ pond filter to remove the extra waste.
The UV radiation causes the microscopic algae to clump into ‘flocs’ of a filterable size. If these flocs are not removed then the floating detritus can often be less visually appealing than the initial green water problem. For this reason, the pump or drain should be placed where they rapidly remove the settled flocs of dead algae.
An unwelcome side effect of fitting a UV is that the nutrients (phosphates, nitrates etc.) in the water that are no longer fuelling microalgae growth can be utilised by macroalgae – particularly blanketweed. As blanketweed is not subjected to the UV radiation, then its growth may actually increase under the now improved conditions of enhanced light and freely available nutrients.
Aquarium UVs are considered to be more of a precise science as the germicidal action is required for another purpose.
UVs are most commonly installed in marine reef aquaria, where certain antibacterial and antiparasitic chemotherapeutic agents cannot be used without harming the reef invertebrates.
For this reason, UVs are installed as a prevention against infection by maintaining residual levels of pathogenic organisms below a lethal threshold. A typical aquarium UV will be rated 4 to 8W with larger systems requiring stronger sterilisation.
Consequently, marine aquarium UVs operate at higher power outputs relative to flow rates and aquarium volume compared with pond UVs. The cavity within the UV chamber is ideally less than 1cm (the narrower the better) to allow maximum radiation of the circulating water. Typically a 4W aquarium UV will have a max flow through of 120gph compared with more elevated flow rates through a similarly rated pond UV. This ensures a higher ‘kill’ rate, reducing the likelihood of disease outbreak.
Research has shown that 35,000 (w/sec/cm2 is the minimum radiation level to sterilise recirculating water, but the swarmer (tomite) cells of whitespot require almost ten times that amount of radiation to be irreversibly damaged.
Maximising UV Performance.
UV lights have a longer life when operated continuously; the expected life of most bulbs is now 12 months. UVs should be operated continuously anyway to be effective against micro-organisms.
However, for health and safety reasons, the bulb must be turned off for any maintenance and during any medication as UV can unpredictably alter the chemical nature of medications.
As UVs are not 100% efficient, the bacterial colonisation of a biofilter will not be affected if positioned before or after the filter, as some nitrifying filter bacteria will pass through the UV to colonise the filter. However, a UV must always be pump-fed to ensure full bulb coverage as maximum use of the UV radiation will not occur in a partially filled gravity-fed UV unit. The unit will also overheat if run partially filled.
UVs can also be fitted in centralised aquaria just after the filter to reduce any cross infection from other tanks before water is redistributed through the system. The filter (mechanical and chemical) removes particulate and dissolved organic matter, improving water clarity and UV efficiency.
Likewise, slime can build up on the quartz sleeve, reducing UV penetration within the unit. The sleeve must be cleaned regularly to maintain optimum radiation. Some more advanced units are available with a built-in meter to detect a drop in UV radiation. This indicates the need for a new bulb or a clean!
The speed of water flow also affects the efficiency of the UV unit. Flow rate can be reduced so that water spends longer under a lower watt bulb or increased in higher watt systems. The latter has been shown to have better germicidal performance with the added benefit of the faster flow reducing the build up of slime on the quartz sleeve.
During the natural life of a UV bulb, the mercury vapour within the bulb gradually becomes deposited on the interior of the bulb, darkening the glass and in extreme cases causing the glass to ‘silver-up’. This solarisation reduces UV output and is an indication that the bulb is at the end of its useful life and must be replaced.
UV can be a real asset to the fishkeeper by clarifying ponds and improving hygiene of aquaria. When installed correctly, they offer excellent value for money, are easy to use and pose few risks to aquarium health (unlike chemical alternatives). However, they do pose some risks to the user if safety guidelines during installation and maintenance are ignored and must be used in conjunction with rather than instead of vigilant fish husbandry.
As outlined earlier, the uses of UV through the century have diversified over time and are now considered an essential item in pond filtration and a highly desirable item in aquarium filtration. The benefits of using a UV steriliser can quite easily flatter your fishkeeping abilities by producing crystal clear pond water and reduced disease problems in aquaria. This may lead you to wonder how fishkeepers have managed so long without them!
