UV treatment has been accepted as an effective and safe means of disinfecting water. However, in order to work efficiently to deliver the required UV dose, a system must be accurately designed. Good chamber design can help achieve this, but the properties of the process liquid itself are also critical.
UV water treatment, which inactivates microorganisms and breaks down chemical molecules in water, has a number of advantages over chemical treatment, but with the critical requirement that the liquid receives the required UV dose while it is inside the UV chamber. Unlike the case of chemical treatment, there is no residual effect after the liquid has left the chamber.
Designing the chambers to ensure reasonable residence times and good mixing of the liquid flow is clearly important, and this aspect has been the focus of recent research efforts in the industry using CFD modelling and validation. More powerful UV lamps can also be used to increase the delivered dose, but there is a limit on the lamp power output that can be applied to a given flow rate of liquid before the system overheats or becomes uneconomical to operate.
Another important factor that controls the dose that the microorganisms receive is the transmission and penetration of UV light from the lamp into the liquid. How can this be maximised in a system, and what might hinder it?
A tortuous path
There are a number of phenomena that can affect UV light on its journey from the lamp where it is generated through the quartz sleeve to the liquid stream that is to be treated. These phenomena are illustrated by items 1–5 in the diagram below.
UV light interacts with the liquid in a chamber in a number of different ways
Refraction is the change in direction of light as it passes through the interface between one material and another. In the example shown in the diagram (1), the light changes direction when it passes both into and out of the quartz sleeve surrounding the lamp. Refraction in most cases does not reduce the intensity of the light but will affect the angle at which it strikes other surfaces, which may affect scattering and reflection, described below.
Once the UV light has entered the liquid, it will gradually be absorbed due to interactions with the molecules in the liquid. The percentage of UV light transmitted through a specific distance in the liquid (usually 1 cm) is called the UV transmittance. All molecules absorb UV light to some extent, but certain chemical species are more strongly absorbing than others. For example, adding a mere 18 mg of coffee to a litre of deionised water can reduce the UV transmittance from 100% to 84%! If a liquid is strongly absorbing then the UV light will not penetrate into the entirety of the process stream, as shown in 2.
All but the cleanest liquids will contain some solid particles, and these can scatter the UV light in all directions (3). The smaller the particle, the larger the scattering angle, and the greater the proportion of the light that will be scattered back in the direction from where it came, which again can hinder progress of the light into the bulk of the liquid.
Finally, all surfaces will reflect light to some degree, even if, as in the case of the quartz sleeve, they are also transparent (4). Consider the windows in your house or workplace: although most of the light passes through them, a faint reflection is still visible. Even an opaque surface such as the wall of a UV chamber will usually reflect a proportion of the light striking it (5).
All of these phenomena can act to modify and often impede the progression of light through the liquid being treated. Understanding them is one thing, but can they be mitigated to improve the efficiency of the UV system?
One obvious way to reduce the scattering effect (3) is to filter the liquid to remove particles. However, this may present practical difficulties especially for systems with high throughputs, as a filtration unit will cause a pressure drop and restrict the flow. A coarse filtration of larger particles will be easier and gives other benefits such as preventing blockages, but it won’t necessarily remove the very small particles that cause the greatest degree of light scattering.
More difficult still are the dissolved substances that strongly absorb UV light (2), which can only be removed by chemical reaction. The liquid’s transmittance measurement is of great importance. It is used by UV treatment systems to control the lamp output and ensure that the required UV dose is delivered to the system, but clearly a lower transmittance requires a higher lamp power output and there may come a point when the lamp cannot deliver the required dose.
Accurate measurements of the UV transmittance of a process liquid are critical in optimising the disinfection provided by a UV system
If the UV transmission properties of the liquid are basically fixed by the process, then what can be done to optimise the system’s delivery of UV light to the liquid? The trick is to tackle the reflection and transmission properties of the other system components.
Making a clean sweep
One way to improve efficiency is to keep the system as clean as possible, in particular the quartz sleeve surrounding the lamp. The surface of this component becomes very hot during operation and this can cause substances dissolved in the liquid to be deposited on the sleeve, significantly reducing the UV light transmitted to the liquid. One ubiquitous example is hard water which contains dissolved calcium and magnesium salts. Just as in your kettle at home, scale can form a tenacious layer on the outer surface and must be removed if the system is to continue to operate properly.
A mechanical wiper is often installed in a UV chamber which slides a rubber ring up and down the sleeve at regular intervals and physically rubs the deposits off the surface. It should be used frequently to prevent deposits becoming baked on to the sleeve, which can make them almost impossible to remove mechanically. More sophisticated systems can include a chemical cleaner such as citric acid which helps to dissolve the scale, although this may not be effective against other types of deposits.
Mechanical wipers used frequently keep quartz sleeves clean and maintain process efficiency
UV light from the sun can change the chemical composition of many materials, despite the protection of the earth’s atmosphere which filters out the most harmful wavelengths. Plastics can become brittle and pigment colours can fade. So imagine how much more damage can occur when a material is bathed in the full glow of a UV lamp.
Even a sleeve that is regularly cleaned will gradually transmit less and less UV light over time. This is caused by a phenomenon known as solarisation, where the UV photons interact with the quartz and form defects in its structure. These defects absorb visible light which leads to a gradual darkening of the quartz, and although this may only be visible when compared side-by-side with a new sleeve, the effect on UV transmittance can be significant. The solarisation effect is irreversible and so it is necessary to replace quartz sleeves every couple of years, depending on usage, to ensure that the system continues to deliver its optimum performance.
Know your enemy
It is important to understand as much as possible about the process stream being treated when designing a UV treatment system. Not only should the target microorganisms be identified to set the required UV dose, but the properties of the liquid being treated such as UV transmittance and particulate content should also be known. Only then can a system and service schedule be properly specified to ensure that the maximum benefit is obtained from each photon of UV light.