Three Unforeseen Challenges with Microbial Testing

Kevin Kahn, Superman

By Dr. Kevin Kahn

Principal Field Application Engineer

At some point in your design of a disinfection system, you will send it out for microbial 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 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 testing 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. The UVC intensity falls off as one over distance squared, meaning than 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 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 testing lab should answer questions like:

  • Where is the flow rate measured?
  • 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?

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.


A lot can go wrong with microbial 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.