UV treatment systems have become increasingly sophisticated as their usage in industry and municipal water treatment has grown, but they all contain a few critical components that are essential for keeping the whole process operating smoothly.
The UV chamber is where all the action happens. It is, literally and metaphorically, the body of the system. In fact, it’s more like an android body really – built from high quality stainless steel, its mechanical strength and corrosion resistance ensures that it usually has the longest lifetime of any of the system’s components. Chambers are installed within the pipework of the treatment plant or factory, and sized to cope with the same flow rate. As water passes through the chamber in a continuous flow, it is exposed to the UV light emitted by the lamp which mutates the DNA in any microorganisms present and prevents their reproduction.
There are a number of different chamber configurations that have been used, with recent designs optimised using computational fluid dynamics (CFD). In addition, the chamber needs to be able to cope with the pressure in the system (normally up to a maximum of 10 bar), and be available in several different designs to cover the various flanged connections that are found in treatment plants or factories.
The light source
If the chamber is the body of the system, then the UV lamp must be the heart. This ingenious device provides the ultraviolet light used to disinfect the water, and is known by various names – arc tube, gas discharge lamp – but the most familiar would be the fluorescent light, which is its well-known and almost identical twin.
It is powered by electrons released from a heated wire, which collide with gas atoms in a sealed quartz glass tube under vacuum. These collisions strip off more electrons which then collide with other atoms. Each collision raises the energy of the atom, which quickly returns to its normal energy level but releases a photon of light as it does so. Mercury vapour has just the right combination of properties for this process, and it so happens that the photons generated are in the ultraviolet region of the spectrum, which is why most UV systems currently use mercury discharge lamps as their light source.
Incidentally, this UV lamp can be transformed into a fluorescent lamp by coating the inside of the glass tube with pigments that emit visible light when struck by a UV photon. These are now used almost universally in lighting workplaces and increasingly in our homes, where compact versions are more energy-efficient replacements for the incandescent light.
Each UV lamp requires a ballast. If the lamp is the heart of the system, then the ballast could be considered as a pacemaker – a device that regulates the lamp output by controlling the electric current in the circuit. A UV lamp without a ballast will either struggle to start at all, or will rapidly consume more and more current as it heats up until it fails.
In older systems the ballast is a simple capacitor, but electronic ballasts are now widely used which actively manage the power delivered to the lamp. This ensures a controlled warm-up, a constant lamp output and therefore a longer service lifetime. And as the UV industry moves towards developing mercury-free lamps, more attention is being devoted to matching the ballast characteristics to the requirements of the lamp to ensure optimum performance.
The quartz sleeve
Within our anatomical metaphor, it’s a little tricky to think of an analogy for the quartz sleeve – the rib cage perhaps? It acts as a physical barrier to protect the lamp from the water being treated. However, the barrier itself must also be protected from surface contamination. For example, the oils and salt in fingerprints react with quartz to change its structure; this increases the brittleness of the material. Therefore, quartz sleeves (and UV lamps) should always be handled with cotton gloves, and cleaned of fingerprints using a weak solvent before being put into service.
The temperature sensor
The electrical energy consumed by UV lamps inevitably generates a certain amount of heat as well as light which can cause the water in the chamber to heat up. Air pockets and reductions in the flow rate can exacerbate this problem, and the pressure in the chamber will increase rapidly if the water approaches its boiling point of 100ºC. So it is essential to have, as a minimum, a safety thermostat attached to the outside of the chamber which cuts power to the lamp if the temperature approaches an unsafe level. This equates to the UV system’s sense of touch. More sophisticated systems may also have a temperature sensor to produce warnings so that the operator can take steps to avoid an unexpected shutdown in the process.
The UV sensor
If the temperature sensor is the system’s sense of touch, then the UV sensor must be its sight. Unlike our eyes though, the sensor is able to detect the range of wavelengths being emitted by the UV lamp, around the 300 nm region of the electromagnetic spectrum. A photodiode converts the photons into an electrical signal which is then amplified and sent to the controller, giving a measure of the UV intensity delivered by the lamp.
Sensors can be ‘wet’ (above left) or ‘dry’ (above right). A wet sensor is immersed in the water being treated, whereas a dry sensor sits in a port behind a window and so can be removed from the chamber without having to stop the process and drain the system. This makes dry sensors easier to recalibrate, which is one reason why they are commonly used in independently validated systems, where an accurate measurement of UV intensity is needed to calculate the delivered UV dose and check that it meets the benchmark for the system. Wet sensors are more often used in non-validated systems to give a relative measurement of the intensity compared to the value when the UV lamp was new, which then indicates to the operator when a lamp should be replaced.
Finally, we come to the brain of the UV system: a cabinet of electronic components that, at its simplest, allows power to be switched on to the system, controls the ballast and hence the UV lamp, monitors the temperature and UV intensity, and issues warnings or takes action to shut down the system when there are problems. However, like our brain, it can also be designed to cope with a range of “higher functions”, such as calculating the UV dose from equations derived from system testing, configuring and implementing various settings, and tracking the usage of the system’s consumable components.
As the number of functions increase, the human-machine interface must also increase in sophistication. Lights indicating specific faults are adequate for simple systems, but if more settings or options are present, liquid crystal text displays show more detailed information and allow the user to navigate menus. This technology is itself now giving way to a new generation of touchscreen interfaces with graphics generated by in-built computers. These provide more information on the screen which makes for easier navigation and a clearer and more eye-catching display.
So there we have it – the body, brain and heart of a UV system, and how its ‘senses’ of touch and sight work to control the process of disinfecting our water. And, like us, UV systems are evolving all the time…