Issue 38: Ultraviolet Light Disinfection

Issue 38
January/February – 1998
Story Title: Ultraviolet Light Disinfection
Author: Steven Carruthers

Steven Carruthers reports on the latest ultraviolet sterilisation research from Holland.

Ultraviolet (UV) light was widely used as a water disinfectant around the turn of the century, but its use declined with the introduction of cheaper alternatives such as chlorination. However, over recent years, there has been a renewed interest in UV radiation as an alternative to chlorination, largely as a result of concern over toxic chemical byproducts.

UV radiation treatment is unique in its mode of action, in that it does not necessarily kill the target micro-organism. Instead, the UVradiation alters the DNAstrands so that the micro-organism is sterilised, thus inactivating the pathogen so that it cannot proliferate and cause disease. A bug that can’t reproduce simply dies.

Germicidal UV radiation is considered lethal for most micro-organisms, including bacteria, fungal spores, viruses, protozoa, nematode eggs and algae. That part of the UV light spectrum known to kill or deactivate most pathogens is between 100-280 nanometers, which is just below visible light to soft X-ray. While pathogenic bacteria are the easiest group to treat, and differ the least in the amount of UV radiation required, viruses are more resistant and variable in the amount of UV radiation needed to ‘neutralise’ them. It is worth noting that some pathogens deactivated by UV light may be reactivated when exposed to sunlight.

Muddy water is known to reduce the effectiveness of UV sterilisation, since the UV light is easily absorbed by fine, solid particles and organic matter suspended in the water. UV light can also be absorbed by deposits on the lamp sleeve. Lamps should be replaced every 12 months for optimum performance.

For hydroponic applications, especially where dam and bore water are used, UV units are often combined with other technologies such as carbon filters, membrane filtration and reverse osmosis systems, to achieve adequate water clarity.

According to recent research from Holland, the recommended UV radiation dose to control 99.9% of pathogenic bacteria and fungi is 100 mJ/, and to control viruses 250mJ/ From this research, it was also established that a minimum transparency level of 20% is needed for UV treatment to be successful. Where transparency levels are less than 20%, the control of pathogens is inadequate, and fresh water should be used to bring it up to a minimum 20% transparency level.

The transmission value of water was calculated as the percentage of spore-killing UV radiation after being passed through a 10mm layer of water. The lower the value, then the more energy is needed to achieve the correct UV radiation dosage.

Above: UV radiation sterilisers generally consist of incandescent, quartz or mercury vapour lamps housed in a prtective chamber. Water is allowed to flow through the cyclindrical chamber where it is exposed to an appropriate dose of UV light for a proper length of time to ensure a high kill rate.

The transparency of drainage water from crops grown in rockwool is usually between 20% and 40%. Where organic substrates are used, humic acid is released which lowers the transmission value of the water. The acids absorb a proportion of the UV radiation and leave less available for sterilisation. This is also the case with iron chelates. In recirculating systems used for growing vegetables, the recommended level for iron in drainage water is 25 mol per litre. It is important that this level is not exceeded.

In practice, where the drainage water has a high EC, the iron level is often too high, sometimes as high as 125 mol per litre. If the EC is increased in the feed solution, then the trace element levels should remain constant.

Other factors can also contribute to low transmission levels, including non-transparent pesticides which find their way into the drainage system, creating a situation where there is not enough UV radiation left to sterilise the water.

The research body at the Research Station for Flowers and Glasshouse Vegetables in Naaldwijk focused on drainage water from a tomato crop. The water was pre-filtered through a 40 micron filter before being inoculated with Fusarium spores and Tomato Mosaic Virus (TMV). Differing transmission values were achieved by adding varied amounts of iron chelate (DTPA), and different doses of UV radiation were achieved by altering the power of the high pressure UVlamps.

Assessment of the Fusarium trial was done by placing water samples onto culture plates, and counting the number of resulting Fusarium colonies. The TMV results were assessed by inoculating tobacco plants with water samples. These plants react by developing lesions, and the effectiveness of the treatment was determined by counting the number of lesions and comparing them to plants inoculated with untreated water.

The research concluded that a dose of 250mJ/ was inadequate to control TMV at transmission values of 12%, 8% and 5%. At 12% transmission, the UVradiation needed to be 50% greater; at 8%, three times greater; and at 5%, four times greater. In practice, these values are not realistic and the best advice is to dilute the drainage water with rainwater to bring the transmission level up to 20%.

The advised dosage of 100mJ/cm2 to control Fusarium gave adequate control at 14% transparency. However, at 8% transparency 110mJ/ was needed; and at 4% transparency, more than 174mJ/ was required. For Fusarium control, the grower has the option of diluting the water, or increasing the UV radiation.

Both trials confirm earlier work, that 20% transmission is the correct level to work from. The results from this work are also applicable to low pressure UV sterilisers, although the limitations of these sterilisers was not fully understood. Their effectiveness is influenced by the radiation pattern and the depth of the treated water layer.