How to size a UV system

Introduction :

UV (Ultraviolet) disinfection systems are a safe, chemical-free, and highly effective method to purify water by inactivating harmful microorganisms, making the water safer for consumption and use. They utilize UV-C light, a type of ultraviolet light that damages the DNA or RNA of bacteria, viruses, and other pathogens, preventing them from replicating and causing infection.

These systems play a crucial role in various applications, ranging from municipal drinking water and wastewater treatment to industries such as food and beverage, pharmaceuticals, and aquaculture. 

One of the key benefits of UV disinfection systems is that they do not introduce any chemicals into the water, thereby eliminating the risk of chemical residues or the formation of harmful disinfection byproducts. UV treatment also does not alter the taste, odor, or pH of the water, which is particularly beneficial in applications where water quality is paramount. Furthermore, UV systems are effective against a broad range of microorganisms, including those resistant to chlorine, making it a reliable choice for water disinfection.

However, it is essential to design a UV system correctly for it to function effectively. Incorrect designing can decrease the system’s efficacy and impact performance. 

In this article, we will discuss the various factors that can influence the performance of UV systems. We will also briefly cover the key components of a UV system.

Parameters affecting UV disinfection system performance

The quality of water along with its physical and chemical characteristics will vary with the source. For instance, wastewater is likely to contain more organic load along with high contamination levels, while industrial effluents will have a higher COD levels and a higher TDS concentration. Similarly, depending on the geographical location, ground water can contain high levels of iron, manganese and other particles that can affect UV transmission and reduce the efficacy of the disinfection system. The following factors are important to consider when sizing a UV system-

1. Requirements :

Ultraviolet (UV) dose, also referred to as UV fluence, is the product of UV intensity (irradiance) and time. It is a critical factor in UV disinfection systems, including those that disinfect water and wastewater. This dose is usually expressed in units of millijoules per square centimeter (mJ/cm²) or microwatt-seconds per square centimeter (µWs/cm²).

UV dose is a measure of the UV-C energy to which microorganisms are exposed in a disinfection system. It is an important determinant of the system’s effectiveness in inactivating these microorganisms. The more UV-C energy microorganisms are exposed to, the greater is the extent of damage to their genetic material, thus preventing them from replicating and causing infections.

Different microorganisms require different UV doses to be effectively inactivated. Therefore, it’s crucial to ensure that the UV dose is sufficient to inactivate the specific pathogens that are present in the water being treated. For example, public health and regulatory agencies worldwide recommend a minimum UV dose for drinking water disinfection of around 40 mJ/cm² (equivalent to 40,000 µWs/cm²). This recommendation is based on research showing that this dose is generally sufficient to inactivate most waterborne pathogens, including bacteria, viruses, and certain protozoa, like Cryptosporidium and Giardia, which are resistant to traditional chlorine disinfection.

Also, the World Health Organization (WHO) issued guidelines for drinking-water quality, which includes guidance on UV disinfection. They do not specify a UV dose but recommend that systems deliver at least an inactivation of 99.99% (4-log inactivation) of known pathogenic bacteria, virus, and protozoa.

2. Flow Rate :

The flow rate of water through a UV disinfection system is a crucial factor that can significantly impact the UV dose that the water receives. Here’s why-

As explained above, the UV dose is essentially a function of the intensity of UV light and the exposure time, which defines how long the microorganisms in the water are exposed to the UV light. The flow rate of water through the UV disinfection system directly influences this exposure time.

If the flow rate is too high, water moves through the system quickly, reducing the exposure time. This can result in insufficient UV dose, allowing some microorganisms to pass through the system without being fully inactivated. Conversely, if the flow rate is too low, water may reside in the system for a longer time than necessary, it could potentially lead to inefficiencies (meaning higher energy consumption and lamp replacement costs).

So, a properly calibrated flow rate is crucial to ensure that all the water gets an effective UV dose for sufficient disinfection. Alfaa UV will always provide detailed technical specification sheets and guidelines for optimal flow rates to achieve the desired UV dose for every product and model.

3. UVT :

Ultraviolet transmission (UVT) is a measure of how much UVC energy can pass through a specific path length of water. It is usually expressed as a percentage, where a higher UV transmission indicates that more UVC light can transmit.

UV transmission is an important factor in UV disinfection because it affects the UV dose that the water receives. It is important to accurately consider the UVT of the water when sizing a UV system for a particular target dose. 

If a unit/model designed for a high UVT is operated with water/wastewater having low UVT, less UV light can pass through the water. This can result in a lower UV dose and less effective disinfection. 

Low UV transmission can be caused by various factors, such as:

• High turbidity: Water that is cloudy or filled with suspended particles can scatter and absorb UV light, reducing its transmission.
• Dissolved organics or inorganics: These substances can absorb UV light, again lowering the UV transmission.
• Colour: Coloured substances in the water can also absorb UV light, reducing its transmission.

It is important to note that UV disinfection as a technology is equally effective for both high quality water sources (eg. surface water sources) as well as low quality effluents (eg. secondary or tertiary wastewater sources). However, the system must be specifically sized and designed around this. At Alfaa UV, we have models suitable for water streams with UVT as low as 35% and high as 99%!

Specifications based on application

The challenges faced and target disinfection levels required will vary based on the application of the disinfected water. As a result, the size and capacity of the UV system will also change depending on the application. For instance, Alfaa UV’s WWR series for treating municipal sewage water is designed to reduce the large concentration of eliminate potential contamination by E.coli and fecal coliforms in water having high organic and biological load. On the other hand, Alfaa UV’s HF Series is designed for disinfecting water to be utilized in the food, beverage, and pharmaceutical sector.

Components of a UV system

A typical UV system includes a UV reactor chamber, control panel, UV monitoring system, and automatic wiper system. 

These key components are briefly described as follows.

• UV Reactor Chamber:

The UV chamber, or UV reactor, is an integral part of a UV water disinfection system and holds the water to be disinfected. It also houses the UV light source, usually in the form of UV-C lamps. The chamber is made of stainless steel which is both strong and resistant to UVC. The design of the UV chamber also plays a role in controlling the flow rate of the water. Its integrated baffles and flow diverters are designed to allow water to flow at a rate that provides sufficient exposure time to the UV light for effective disinfection.

• Control Panel and Monitoring System:

The control panel in the Alfaa UV water disinfection system serves as the central hub for monitoring and controlling the system’s operation. It’s an essential component that helps to ensure that the system is functioning effectively and safely. Here’s a list of some specific functions it may perform:

• • System Monitoring: The control panel can display various system parameters, such as the UV intensity and dose, flow rate, and operating temperature. This allows operators to monitor the system’s performance and make adjustments as necessary.
  • UV Intensity Monitoring: Many Alfaa UV models have a UV sensor that measures the real-time intensity and UV dose imparted to the water stream. This is crucial for ensuring that the UV dose is sufficient for effective disinfection. If the UV dose level drops below a certain level, the control panel can alert the operator or automatically shut down the plant.
  • Lamp Life Tracking: UV lamps have a limited lifespan, after which they need to be replaced. The control panel can keep track of the lamp’s operating hours and alert the operator when it’s time to replace the lamps.
  • System Control: The control panel often has controls for turning the system on and off, adjusting the target UV dose rate (dose pacing), and performing other functions. In some systems, it may also be able to automatically adjust these parameters to maintain optimal performance.
  • Alarm System: The control panel can alert operators to any problems with the system, such as a drop in UV dose, a lamp failure, or an abnormal flow rate. This can help to prevent ineffective disinfection and potential damage to the system.
  • Data Logging: Some control panels can log data on the system’s performance, such as UV intensity and flow rate over time. This can be useful for maintaining records, tracking system performance, and identifying any trends or issues.
  • UV Monitoring System: This system checks the UV dose administered and ensures that the right dose is delivered.

• Automatic Wiper System:

An automatic wiper system in a UV disinfection system is designed to clean the surface of the quartz tubes which in turn house the UV lamps. This is an important feature because, over time, the quartz surface can become coated with scale, biofilm, or other substances present in the water or wastewater. When this happens, it can reduce the passage of UV light, which can lead to a decrease in the UV dose and a reduction in the system’s disinfection effectiveness.

The automatic wiper system helps to prevent this issue by periodically wiping the surface of the UV lamps to remove any accumulated substances. This ensures that the system can deliver a sufficient UV dose for effective disinfection.
The need for an automatic wiper system will depend on the quality of the water being treated. If the water has a high level of hardness, turbidity, or other substances that can coat the lamps, an automatic wiper system can be particularly beneficial. On the other hand, if the water is relatively clean, an automatic wiper system may be less necessary.

Conclusion

Parameters such as water source, water quality, the specific biological challenge, UVT, required dose and flow rate help design a UV system with precision. These factors need adjustments as per the intended application of water. It is crucial to select the right UV disinfection technology for administering germicidal activity.

About Alfaa UV

Alfaa UV (Ace Hygiene Products Pvt. Ltd.) is an ISO 9001 certified company. With two decades of experience, Alfaa UV is the undisputed leader and exporter of UV purification systems for Water and Air treatment. Based in Mumbai, Alfaa UV has its own fully integrated manufacturing facility in Talegaon, Maharashtra and a pan India presence with sales and service offices across the country. Alfaa UV systems are also exported to over 15 countries around the world. Behind the company’s products are some of the most experienced and knowledgeable professionals in the industry from engineering, chemistry and microbiological backgrounds. Alfaa’s R&D team creates many of today’s successful UV technology innovations.