Lighting the way
Malcolm Snowball of GB Environmental looks at best practice in the configuration and operation of UV disinfection systems
Although the germicidal property of UV light has been known since the turn of the last century it is only in the last 30 years the technology has been put onto a scientific footing. This has resulted in an ever-widening use of the technology, especially in the disinfection of water and wastewater.
If applied correctly, UV disinfection can be a simple, elegant and cost-effective way of disinfecting water, with its chemical-free ability to kill the complete range of pathogenic waterborne micro-organisms by disrupting their DNA chain. There is an abundance of scientific data proving the effectiveness of the germicidal waveband of UV ‘C’ in the fight against numerous waterborne pathogens.
However, end user experiences of UV as a means to disinfect water can be found at both ends of the spectrum, with perceptions ranging from positive to those whose expectations have simply not been met.
So, why has there been a mixed experience from users of the technology? Generally speaking, the main problem has been with ensuring longevity of functionality of the various UV units.
With recurring performance-related complaints linked to inconsistent kill performance and too much manual intervention required to keep the system running, particularly when it comes to maintaining the UV lamps free from fouling, this is hardly surprising. Inconsistent kill performance is caused when an insufficient UV dose is applied to the water on an ongoing basis, which may present itself as a problem after a few weeks or months, or may fluctuate on a regular basis.
Either way, this can present a serious problem and is caused by a multitude of factors, one of which is incorrect sizing of the unit. If sized incorrectly there is little that can be done to make it work, apart from changing it to the correct size, which is both a costly and avoidable exercise. It is vital best practice procedures are followed in order to size units appropriately and imperative maximum water flow rate, degree of transmissivity at 254nm and the number and type of micro-organisms to be disinfected, are all taken into account.
A compromise on any of these areas can dramatically affect the sizing accuracy, which can result in micro-organisms passing through the chamber unharmed. In much the same way as you would not expect to buy a new car that has not been crash tested, it is crucial UV disinfection equipment should be sized according to the worst case scenario.
Each UV disinfector should be sized for dose delivered using the widely accepted averaging point source summation technique for radiation intensity, which is a complex mathematical calculation, calculated at the worst point in the reaction chamber, under maximum water flow conditions with lamps towards the end of their life. In fact, calculation with one lamp out and an additional safety margin added to the sizing calculation, will help reinforce the safety of the chosen UV technology.
There are three critical components of this calculation. Firstly, lamp power at the germicidal wavelengths (220nm to 280nm), which is dependent upon the lamp type and how the lamp drive (ballast) operates the lamp. Secondly, it is important that transmissivity (a measure of the water’s ability to transmit UV light) is measured appropriately, since certain contaminants in water can prevent UV light from effectively transmitting through the water, reducing the UV dose reaching micro-organisms. It cannot be over-stressed how important this parameter (usually measured on a spectrophotometer at 254nm), is to sizing a disinfection unit. A few percentage points difference in transmissivity can mean all the difference between success and failure.
Transmissivity tests need to be conducted long enough to get the right reading and must be conducted in the worse possible conditions. Thirdly, it is also necessary to size the chamber according to the peak liquid flow in the chamber, since this governs the retention time component of the dose formula, which is worked out using the calculation:
Obtaining the right information and calculating the correct size of UV chamber required is clearly a prerequisite for the installation of any UV germicidal barrier technology. However, there are other factors that affect consistency of performance that should not be overlooked. Systems that rely on radiation monitoring or dose monitoring to control functions within the UV disinfection system need a very stable and accurate radiation monitor. In fact, in the US, inaccurate radiation monitoring by a number of manufacturers has created a contentious issue for the Environmental Protection Agency, forcing them to distrust the results of various methods of monitoring radiation.
Radiation monitors are built around the sensing device that is usually a silicon photo diode or a vacuum photo diode tube, both of which consist of a semi-conductor doped junction, which is sensitive to the incident light radiation. The germicidal radiation is very energetic and will slowly destroy the P-N junction of the sensor causing a slow and permanent drift over time. Best practice for radiation monitors is to select the sensing device for low drift and heavily attenuate the incident radiation to the device, then boost the signal to
workable levels through the electronics. The devices should be designed to be optically blind to all but the germicidal wavelengths and the sensor window in the reaction chamber should be automatically cleaned to ensure accurate radiation readings over long periods of time.
The type of lamps used can contribute to or, in some cases, hinder the consistency of performance. There are a wide variety of lamps used in UV disinfection, although broadly speaking the most commonly used tend to fall into three categories; low-pressure mercury discharge lamps, amalgam lamps and medium-pressure mercury discharge lamps. All three effectively act to kill pathogenic waterborne micro-organisms, although some are better suited to treat different types of water flow.
If, for example, the body of water to be treated was that of a WTW, medium-pressure mercury discharge lamps would be the most suitable, since their high UV germicidal output over the entire germicidal range 220-280nm make them the most economical way to treat extremely high water flows. Low-pressure and amalgam lamps simply would not produce a high enough UV germicidal output to cope. This is why they are much more suited to treatment of water with a lower flow rate. Lasting twice as long as medium-pressure lamps, at lower skin temperatures, both lamps provide an economical way of disinfecting water.
The differences between low-pressure mercury discharge lamps and amalgam lamps are three fold. Low-pressure mercury discharge lamps are less prone to fouling caused by quartz surface heating. However, their UV germicidal output is three times lower than the amalgam lamps. Low-pressure mercury discharge lamps are susceptible to
fluctuations in water temperature, whereas amalgam lamps have a much higher resistance.
Low-pressure mercury discharge and amalgam lamps tend to be chosen for their low skin temperature. Their cool operation ensures they tend not to be prone to rapid fouling, which often results when the quartz sleeve encasing the lamp rises in temperature.
However, on a cautionary note, even low-pressure lamps are susceptible to lamp fouling, which shields the UV wavelengths from killing the micro-organisms. It cannot be stressed enough how important it is to have an integral lamp cleaning mechanism in place, irrespective of the type of lamp used. After all, a lack of sufficient means to keep UV lamps clean has played a significant role in hampering continuous functionality and has generated a certain degree of scepticism. As water passes through the UV disinfection chamber, proteins, fats, oils, dead cells, calcinations products and general debris in the water leave deposits on the quartz sleeve encasing the UV lamps.
The amount of fouling will depend on the skin temperature of the lamp and the impurities in the water, but the fact is it occurs with all lamps and must be treated seriously. Lamp fouling will limit the penetration of the UV rays through the sleeve into the water, significantly reducing the kill rate, since UV only acts to disrupt the DNA of a micro-organism it has direct contact with.
Coping with lamp fouling
The ideal action to remove this type of fouling is via a combination of scraping and raking the sleeves of the lamps both axially and radially, preferably in combination with an additional bond breaking or oxidising process. The cleaning process should be fully automatic, initiated from the radiation detector when the radiation falls below a pre-set limit or from a timer that initiates the cleaning cycle every set period of time.
Generally if the cleaning process is left to human effort sooner or later it will fail due to neglect or human error. If care is taken in sizing and applying the UV disinfector to the application correctly, it will provide a highly consistent biological kill performance with minimum
maintenance and service. Of course, it is one thing to ensure that the correct type of lamp is selected and that an appropriate method for cleaning the lamps is applied.
However, if the relationship between the ballasts (which provide power to the lamps) is mismatched or compromised in any way, the germicidal output of the lamps may not be realised, leading to inconsistent kill performance. Also, a mismatch can lead to premature failure of the lamps, which can be costly, not to mention detrimental to longevity of functionality, whereas the correct ballast will always maintain lamps in peak performance. In order to prevent this occurring, a specifically designed control system utilising high-efficiency ballasts matched exactly to the lamps, is strongly recommended.
Assuming all the lamps are kept free of fouling and the size of the unit has been correctly ascertained, it is important a turbulent flow in the reaction chamber is maintained at all times to ensure suspended particles in the water flow do not present a problem. By their nature, some particles have a tendency to float along the outer edge of the chamber, and although they will still be exposed to UV, the kill effect is greater when those particles are in closer proximity to the lamps.
Since the intensity of radiation emitted by the lamps dissipates as the distance from the lamp increases, if there is not a mechanism in place to draw these particles closer to the lamps, it is possible for some micro-organisms to pass through the chamber unaffected. Therefore, good reactor design is vital, which should create turbulent flow with minimum pressure drop, ensuring thorough mixing of the liquid as it flows through the chamber. The high turbulence will act to continuously draw the particles closer to the lamps in a controlled manner, increasing their exposure to UV and the kill rate.
A benefit of the installation of UV disinfection equipment is that it saves considerably on chemicals, electricity, maintenance and personnel. As this article demonstrates, it’s simply not the case that ‘one size fits all’. However, providing best practice procedures are followed, there is no reason why UV disinfection cannot be exactly what it purports to be – a simple, elegant and cost-effective means to disinfect water.
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