Comparison of UV Low Pressure (LP) and UV Medium Pressure (MP) for UV Disinfection
UV disinfection is a process that uses UV-C light to destroy the DNA of harmful bacteria (including Legionella), viruses, and other microorganisms, preventing their reproduction. UV-C is especially effective against chlorine-resistant organisms like cryptosporidium and giardia.
Low Pressure (LP) and Medium Pressure (MP) lamps are the two main types of UV lamp technologies. LP lamps emit a single wavelength and consume less energy, while MP lamps emit multiple wavelengths and provide higher output, allowing for a smaller footprint and fewer lamps.
Key factors for UV selection include flow type and water system type. MP lamps are generally better for constant flow, disinfection, water quality, maintenance, footprint, consumable usage, and temperature extremes. LP lamps are better suited for intermittent flow, lamp life, and energy usage.
| Low Pressure Lamps | Medium Pressure Lamps | |
| Wavelength | Single Wavelength | Multiple Wavelengths |
| Energy Usage | Less Energy | Higher Energy |
| Key Factors |
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UV dose, measured in mJ/cm2, determines the energy required to inactivate a microorganism or destroy a substance. UV system performance is affected by factors like UV transmittance, turbidity, mineral content, and suspended solids. Intensity is only one variable in the equation, and monitoring it must be meaningful and actionable. UV Legionella pathogen control systems require maintenance, with MP lamps having a runtime of 8,000 hours and LP lamps 12,000-16,000 hours. Some microorganisms require higher UV doses than the industry standard of 30mJ/cm2, which non-validated systems may not provide. UV technology is also used for Total Organic Carbon (TOC) destruction, dechlorination, endotoxin removal, and microbiological control. Higher UV doses can inactivate additional organisms, reduce biofouling, and prolong the life of RO membranes.
What is UV Disinfection?
- Mercury vapor lamps produce UV-C light at a wavelength of 253.7nm
- DNA of many common bacteria & viruses have a peak absorption of around 260-265nm
- Exposure to UV-C destructs part of the DNA, preventing reproduction
- UV-C is the only effective treatment for many chlorine-resistant organisms such as cryptosporidium and giardia
UV Lamp Technology Comparison
Pressure, in this context, refers to the pressure of gases inside the UV light bulb.
Low Pressure (LP) or Low Pressure Amalgam (LPHO) lamps are monochromatic in that they emit only one wavelength. LP lamps are usually longer, and more are required to treat the same amount of flow as an MP lamp, however, they consume less energy and typically last longer. Low Pressure UV Lamps produce a single monochromatic wavelength of 254 nanometers. Low-pressure UV does not have the wavelength of UV light that destroys free chlorine. While it can destroy monochloramine, low pressure does not destroy dichloramine or trichloramine. Typical output levels for LP UV are 35 mJ/cm2.
Medium Pressure (MP) lamps are polychromatic lamps in that they emit multiple wavelengths. MP lamps provide a higher output but for usually a shorter amount of time, allowing for a smaller footprint and fewer lamps. Medium Pressure lamps produce multiple types of wavelengths between 200 and 400 nanometers, a broader spectrum of UV than low pressure lamps. Output levels for MP UV are typically much higher than LP by an order of magnitude. For example, MP UV may be used for TOC destruction at 600 mJ/cm2, or chlorine destruction at >1000 mJ/cm2.
Maintenance Expectations for UV Lamps
- Works best with no more than 4 on/off cycles in 24 hours of operation
- Medium Pressure Lamps – 8,000 hours of run time
- Low Pressure Lamps – 12,000 – 16,000 hours of run time
UV Chamber Hydraulics and Dose Delivery
Contact Legionella Control Systems for Expert Analysis and Engineering of UV Legionella pathogen control systems. The use of UV low pressure (LP) and UV medium pressure (MP) systems for UV disinfection, UV dechlorination, and UV TOC Destruction is a complex area with major cost and safety considerations. Given that UV systems can last more than a decade, it is important to contact Legionella Control Systems for a UV engineering consultation early in your planning process to develop the most effective and cost efficiency approach for your facility.
