Three Unforeseen Challenges with Microbial Disinfection Testing of UVC Devices


By

Dr. Kevin Kahn

Principal Field Application Engineer

Dr. Richard Mariita

Senior Characterization Engineer (Microbiologist)

At some point in your design of a disinfection system, you will send it out for antimicrobial validation testing. As with any experiment or test, regardless of your design or the test facility, things may not go exactly according to plan.

When preparing microbial disinfection tests and analyzing the results, it is important for project engineers to understand how various factors and steps may affect the results, and how a good procedure can mitigate the uncertainty from these challenges.

The first and foremost challenge is the human one: laboratory technicians or microbiologists performing tests, although very familiar with the microbes themselves -from growth to analysis- are often unfamiliar with UVC devices such as UVC water reactors or light modules.

Here are three challenges we’ve experienced with microbial disinfection validation testing of UVC devices that can easily be avoided.

1. Air trapped in the water reactor

Air trapped in a water reactor shortens the flow path, increases water velocity and therefore decreases the transverse time of water across the reactor. This leads to a reduced exposure time and lower UVC dosage. Depending on the reactor geometry and flow rate, air trapped in the reactor can decrease the disinfection levels significantly, by several factors.

There are two ways to minimize this risk:

  • Design the reactor to flush the air out on its own and evaluate the impact of different reactor orientations on the effectiveness of air flushing (i.e. in/out-lets axial direction with respect to the table, reactor on its long or short side, reversibility of orientation). Then, ensure the reactor is set up correctly and in one of the specified orientations during testing procedure.
  • Before starting the experiments, pass water through and gently jiggle with the reactor up and down, and sideways to make sure no air is trapped. It is typically recommended to do this for all reactors as an effective way to visually check if trapped air bubbles are present.

2. Misalignment of a petri dish

For surface disinfection of pathogens placed on petri dishes, the disinfection level will be determined by both the placement of the LED(s) and the relative placement of the petri dish. Controlling placement is therefore essential to analyze the disinfection results. In particular:

  • Tightly control the distance from the LEDs and the petri dish. Also, to ensure similar intensity distribution, petri dishes should be placed at the same spot each time during the test. The UVC intensity falls off as one over distance squared, meaning that a small change in the distance translates to a large change in intensity and therefore UVC dosage. By controlling this variable, design engineers can validate the placement of the LEDs in their product will perform as expected.
  • The relative placement and alignment of the petri dish and the LED(s) module. The irradiance or intensity per unit area at a certain point of the petri dish depends on its alignment with the axis direction of the LED. Misalignment will lead to under-performing results and provides little information to find the origin of the discrepancy between the results and expected performances.
misalignment of petri dish

3. Ambiguous testing procedures

Finally, in order to use microbial disinfection testing for validation of a design or concept, engineers should define a repeatable testing procedure that includes specific details related to the UVC elements for the laboratory.

The test procedure that accompanies a water reactor to a microbial disinfection testing lab should answer questions like:

  • Where is the flow rate measured, and what is the test flow rate?
  • How do we make sure the sample collected is not residual water from previous passage?
  • How much time is required to cycle through the whole system?
  • What should we compare the sample to?
  • When and where should a reference sample be collected?
  • Which microbial species and strain is the disinfection performance being investigated against?
  • What is the UVT required for the test? Also, a record should be kept for other test water parameters such as pH, TDS, turbidity and temperature.
  • What is the orientation of the device being tested?
  • Does water enter through the top or bottom of the device?
  • What voltage and amperage will the device be operated at?
  • Is the device being operated at the start of life or end of life? If end of life, how many hours? 
  • What type of water is being used in the disinfection test (chilled, ambient, carbonated, hot)?
  • Has the device been decontaminated and rinsed prior to use in disinfection test?
  • What is the intensity and wavelength of the device being tested?

This is not a complete list of questions, but we’ve seen that including these answers up front can improve the accuracy of test results, and more importantly to predict these issues and establish additional methods to address them.

Conclusion

A lot can go wrong with microbial disinfection testing, which is a critical step in the validation of product designs. Often these issues arise from a lack of understanding of the product being tested by a third party, or by insufficient preparation around all uncertainties from testing. Pre-empting these issues will accelerate validation and save a lot of troubleshooting efforts.