Iron and manganese and their inhibitory effect on UV performance

While it is common sense that a light based disinfection technology like UV will have reduced effectiveness as water quality visibly declines, there are some contaminants that can cause a dramatic reduction in UV’s effectiveness when they are present in even small amounts. Iron and manganese are two such contaminants worth noting as they can inhibit UV light from passing through the quartz sleeve and into the water, which results in a reduction in UV’s microbial effectiveness – even though the UV lamp may be in perfect working condition.

Over time, iron and manganese will bond to the quartz sleeve that surrounds the UV lamp. As these elements accumulate and attach to the sleeve, they can eventually result in a visible staining effect on the quartz (brown for iron and purple for manganese). This staining effect prevents the UV light from penetrating the quartz sleeve and therefore reduces the UV light transmitted through the water, resulting in inadequate microbial inactivation.

Iron and manganese are commonly found in well water and groundwater systems. It is therefore important to understand the concentrations of these contaminants in the water supply before implementing UV, especially in applications where UV is used as a ‘firewall’ and where no pretreatment technologies are present prior to the UV system.

As a rule of thumb, concentrations greater than 0.3 ppm of iron or 0.05 ppm of manganese are enough to create a concern when it comes to ensuring reliable UV disinfection. If iron or manganese levels are greater than these recommended limits, facilities can consider a few different options to ensure optimal UV performance:

1. Implement a pretreatment technology such as sequestration or cation exchange, or use an oxidation and filtration technique which can reduce iron and manganese levels considerably.

2. Ensure the UV equipment includes a UV intensity monitor that can monitor the quartz sleeve fouling issue. When reductions in UV intensity occur, conduct a CIP of the UV equipment to clean the fouling from the quartz sleeve. The required CIP frequency will depend on the severity of the fouling which in turn depends on the concentrations of iron and manganese in the water.

3. Consider installing UV equipment that has a chemical wiping system built into the chamber. Although a physical wiper will help with some quartz sleeve fouling issues such as total suspended solids (TSS), minerals and hardness, physical removal of the iron and manganese staining is not highly effective due to the chemical bond that forms with the quartz sleeve. Using a chemical wiper such as Aquionics’ UltraWipe can effectively clean the iron/manganese staining without requiring the downtime associated with a full CIP.

4. If possible, consider implementing UV further downstream in the process, where iron/manganese will have already been removed through filtration or other processes.

Issues like iron and manganese fouling are an important reminder of why food and beverage facilities should always implement UV systems equipped with an intensity monitor that can track sleeve fouling issues. UV systems without an intensity monitor are only capable of displaying that the lamp is “on” and operating correctly. Proper monitoring using an intensity monitor is a requirement to ensure that the system is delivering the necessary dose for adequate disinfection. Without this monitoring, the first indication of a fouling problem will be microbial contamination downstream in the process or product.

For more information on pretreatment options for iron and manganese please review the article linked below from Greg Gilles at AdEdge.

https://www.watertechonline.com/an-in-depth-look-into-iron-and-manganese-treatment/