How many UVC LEDs are Needed for a Water Purification System?

Kevin Kahn, Superman

By Dr. Kevin Kahn

Principal Field Application Engineer

Designing water disinfection systems with UVC LEDs can be challenging because the type and quantity of UVC LEDs will vary based on the unique intricacies of the system. We developed the UVC LED Requirements Calculator to help answer the following question for design teams: How many LEDs are needed for your point-of-use water purification system to provide disinfection at its specified end-of-life? The resulting calculation is a function of water quality, flow rate, disinfection level, reactor’s internal material, and appliance lifetime.

Keep in mind that the dose (i.e. disinfection level) is a product of the fluence rate and the exposure time (which is equal to the transient time in a simple reactor, but the exposure time may be different from the residence time in the reactor depending on the internal structure and flow path). Here, we are only concerned with the LEDs in a simple cylinder-shaped reactor and plug flow going from the inlet (opposite the LEDs) to the outlet (directly next to the endcap containing the optical module).

How to use the UVC LED requirements calculator

The UVC LED Requirements Calculator lets you modify five factors, below, which impact the number and type of UVC LEDs recommended for your low flow water purification system.

The approximations made in this calculator mean that the results are valid for low flow systems (i.e. less than 4 liters per minute). If you are designing a water disinfection system for higher flow rates (i.e. greater than 4 liters per minute), please contact us today to discuss your requirements.

Flow Rate: Flow rate is the volume of water that passes through a reactor per unit time. The higher the flow rate, the shorter amount of time the water is in the reactor. Therefore, referring back to the dose equation, to keep a certain disinfection level, one will require a higher fluence rate. Fluence rate can be increased by the number of LEDs, their operation, and the reactor’s materials. Following this line of reasoning, a lower flow rate leads to lower power requirements. While this is generally true, one must be careful that the reactor is indeed designed for all the flow rates specified. Shorter flow rates may lead to different flow path and, potentially, microbial slippage. Another thing to be cognizant of is the reactor’s orientation at different flow rates.

Treatment Performance Goal: The target disinfection level depends on the product specification (e.g. 99.99 percent of a given pathogen), the certification with which the product wishes to comply (e.g. NSF/ANSI 55 Class B, NSF/ANSI 55 Class A, etc.), and the regional/market needs (i.e. the specific pathogen). The first question you should ask is, “What is the desired treatment performance goal at end-of-life (EoL)?” EoL will be defined, later, as the final factor in this calculator.

Water Source: The quality of the water entering a water reactor is a key factor in the LED output requirements. Beer’s law dictates that the radiant flux attenuates exponentially with distance travelled and the transmittance of the water to UVC light (i.e. UVT). Therefore, a small decrease in UVT will lead to a large increase in power requirements. Typically, filtered water’s UVT is less than 95 percent, tap water’s UVT (depending on level of chlorine) is approximately 85 percent, while certain standards require testing to be carried-out at lower UVTs. Although designing for lower UVT ensures the UVC radiant flux will be sufficient, it may sometimes be an over-design if UVT cannot be lowered (filters for instance) and lead to additional cost for no added value.

Chamber Material: Earlier, we mentioned that fluence rate can be affected by the chamber’s material. Specifically, the chamber materials’ reflective properties to UVC light will have a large impact on the number of LEDs required. The reason for this is that the percentage of UVC light being reflected by the chamber materials allows for better use of the light (especially in longer reactors), while the type of reflection (e.g. specular and diffuse) may influence the design of the reactor (answering the question, “Where is the light needed as a function of flow dynamics?”).

Appliance Lifetime: Knowing the lifetime (in years) of the appliance lets us put in to context the treatment performance goal, in order to make sure the device meets its specifications until “the last glass of water” is dispensed from the system. The calculator is based on typical water consumption per year for consumer and commercial water dispensers (except for certain applications, such as restaurants, which typically have higher water volume requirements).

For in-depth discussions on other factors such as different usage and lifetime requirements, wavelength, reactor design, and thermal management, please contact us at your convenience.


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