Joining forces for de-bugging

Until the end of the last century, i.e. up to three to four years ago, the majority of specialists were questioning the efficiency of ultraviolet (UV) light for inactivating protozoa such as Cryptosporidium parvum oocycsts and Giardia lamblia. Recent data from lab and full-scale applications shows this premise was based upon improper assay techniques. In actual fact, both Crypto. oocycsts and Giardia lamblia are very easily inactivated by UV light, even with much lower UV dose rates than those typically used for bacteria inactivation.

As a consequence, the US Environmental Protection Agency (USEPA) has classified UV disinfection as a “critical compliance technology” in the treatment of drinking water, to protect the public against these micro-organisms.

Figure 1 shows a graphical comparison between UV dose rates tabled by the USEPA for various Crypto. oocycst and Giardia lamblia inactivation rates and the ‘standard’ UV dose rate for a > 99.99% (or > 4 log) inactivation of typical human pathogens potentially present in drinking water.

This comparison shows the inherent safety of all UV systems sized with 40mJ/cm2, as the dose applied is approximately three-fold of that required for a 99.9% inactivation of both Crypto. oocycsts and Giardia lamblia.

It should be noted that UV disinfection is not a removal process such as membrane filtration. When water flows through a UV reactor the micro-organisms are inactivated by the UV irradiation. Micro-organisms can no longer reproduce and thus are no longer able to affect human health.
The immediate advantage of UV is the absence of any highly polluted and concentrated filtrate to be disposed of. Secondly, there is a drastic cost difference between these two treatment options.

Inactivation by means of UV disinfection dramatically reduces both capital and operating costs when compared to membrane filtration. As a consequence, UV disinfection is defined as a ‘low-cost barrier’ for Crypto. and Giardia lamblia.

How does a user ensure the design of the UV system meets the disinfection requirement and that the claimed performance is achieved during the operation?

The challenging question of accurate performance monitoring is identical for all disinfection methods because the direct approach via the measurement of the actual ‘Crypto. kill’ is neither practical nor safe. Any breach in performance would be discovered several days after the event, i.e. several days after the water has been used by the consumers, which is obviously too late.

In direct analogy to the performance monitoring of chemical disinfection with chlorine, UV systems use monitors measuring the concentration of the specific disinfection agent. In the case of chlorine disinfection, it is the chemical itself via residual chlorine concentration monitors. In the case of UV disinfection, it is the UV light.  UV light intensity monitors are continuously measuring the irradiance of the UV lamps within the reactor thus providing a suitable means of performance measurement.

But how is the system operator to understand the relationship between UV intensity and the performance of a specific UV reactor?

Degrees of intensity
Figures 2 and 3 show two UV systems which are entirely different in appearance. One is an in-line reactor with several UV lamp rows perpendicular to the water flow, the other has cylindrical vessels with lamps parallel to the water flow. The measured UVintensity values will obviously not be identical.

Imagine also, that the UV lamps themselves can be entirely different in length, specific output and efficiency. The consequence of this multitude of variables in the design of UV systems for drinking water disinfection is that each individual UV reactor needs to undergo a proper performance validation test in which the performance is linked to reactor-specific UV intensity values.

Independent performance validations of UV systems according to international standards can currently be performed at third-party test sites in Austria, Germany and the USA. A UV reactor validation process covers the following areas:

• proof of the UV reactor’s capability to deliver the required performance, i.e. the required UV dose for a certain inactivation rate, via a microbiological challenge,
  
• definition of the UV reactor construction,
  
• definition of the UV intensity sensor performance and construction,
  
• definition of the water quality with respect to the ability to be disinfected with the UV reactor.
  

The validation process provides, as a huge benefit for the customer, a basis for comparing UV systems’ performance independently of the manufacturers’ claims. The process is also a good means of linking the measured UV intensity to the verification of the performance on-site.
In comparison to other disinfection technologies, UV systems are undergoing the most thorough tests prior to installation and operation in WTWs. UV is the only technology for which each individual system is tested to guage performance.
With today’s threats and protection requirements, one should no longer look at disinfection as a single treatment step within a series of processes to deliver drinking water to consumers. Instead, a holistic disinfection strategy needs to be put in place so risks are minimised and even sudden microbiological threats, contamination with bacillus anthracis or with the severe acute respiratory syndrome (SARS) virus for example, can to some extent be dealt with.
Relying solely on a chemical disinfection step with chlorine or chlorine-related products leaves the WTW vulnerable in several ways:

• what happens if the supply of the disinfectant is endangered?
  
• what happens if a ‘new’ microbiological contamination, for example with Crypto. oocycsts, is not effectively treated with the chemical disinfection step?
  
• what happens if the chemical concentrations need to be drastically lowered as the treated water may contain unwanted by-products due to the addition of the disinfection chemical?
  

UV provides one ideal solution to these questions as it can usually be integrated into existing WTWs at an acceptable cost, providing an additional and – very importantly – different type of barrier against microbiological contamination of source water. In combination with UV, chlorine concentrations can even be lowered by 50-65% of a sole chlorine disinfection step while maintaining an identical quality of the treated water. Such an approach has been used in Finland, where two large UV systems were installed at Pitkaekoski and Vanhakaupunki in 1997. This approach of combining UV with other disinfection technologies is called multi-barrier. A logical third barrier, upstream of UV and chlorine, is an ozone treatment step. besides providing additional disinfection capacity, ozone improves the water quality by oxidizing polluting constituents. This has a very
beneficial impact on the UV system as both capital and operating costs can be substantially lowered by improving the water quality. The quantity of large WTWs adopting this kind of strategy is rapidly growing, especially in Europe and North America.
Safe drinking water can be achieved at affordable costs using UV, without the production of contaminated filtrate generated by membranes and dangerous disifection by-products which can result from the use of chlorine in high concentrations.
With validated UV reactors, both system performance and the performance monitoring regime have been thoroughly tested by a third-party.
This makes the UV disinfection step among the safest means of providing a barrier against Crypto. oocycsts and Giardia lamblia.