Prior to discharging into the environment, wastewater is disinfected to ensure pathogens are inactivated for environmental and human safety reasons. Ultraviolet (UV) disinfection of wastewater is a widely used disinfection strategy due to the challenges associated with chlorine disinfection, such as potentially harmful environmental impact. UV disinfection of wastewater eliminates the potential discharge of unwanted chemicals into the ecosystem.
UV Transmittance (UVT) is the measure of UV light at the 254nm wavelength that is transmitted through a sample of water, and is expressed as a percentage (%) transmission. The measurement provides valuable insight to the suspended solids and organics content in the water sample, and is a parameter used in the dose calculation for UV disinfection systems. Monitoring UV Transmittance provides critical data for delivering the required dose of UV light to reach target log inactivation. Dose is calculated using UVT (UVT %), UV light intensity (I mW/cm2), and contact time (t seconds):
UV Dose = I/UVT×t
While the light intensity of the UV disinfection system is important to monitor, UV dose is truly related to the amount of UV light that penetrates the water sample to achieve a log reduction of a pathogen. Suspended solids and other content present in the water that absorb or scatter UV light at the 254nm wavelength can lower the amount of UV light transmitted for pathogen inactivation.
SYSTEM DESIGN & SIZING
It’s critical to understand design requirements and constraints when designing and sizing a UV system. Generally, UV disinfection system manufacturers and/or engineering consulting firms can assist in the design to meet treatment targets. Mathematical models, such as Point Source Summation (PSS), can be used to design the system, though this method does not account for the microbiological data of the water at the site. Bioassays are an alternative method, which involves laboratory bench scale testing and a UV reactor field test using site-specific water quality data.
SYSTEM CONTROL & OPERATION
UV disinfection systems can account for up to 25% of the operational budget for a wastewater plant. With budgets tightening, there’s an opportunity to generate significant cost savings by optimizing the UV disinfection process through sensing, control, and automation.
Control of the system is commonly either flow-paced control or dose-paced control. Flow-paced is the simplest strategy, with the number of banks, lamps and channels based strictly on influent flow rate. Dose-pacing uses several parameters to ensure proper UV dosing through online monitoring of:
- Flow rate
- UV Transmittance
- Lamp power (including lamp age and on-line output intensity data)
Dose-paced control optimizes efficiency of the UV disinfection process by modulating lamp output power based on real-time data.
For instance, if UVT increases in the incoming water to the UV disinfection system, control mechanisms are in place to turn off unneeded lamps or lower lamp output power. This results in lower energy consumption while still meeting disinfection objectives.
Similar to capital costs, small changes in UVT can have a powerful impact on annual operating costs of the UV disinfection system. For example, a wastewater plant with design flow of 61 mgd (48 mgd average flow) could realize over $60k annual savings in operating costs with a just 5.0% increase in the average UVT of the water entering the UV disinfection stage.
Online UVT monitoring ensures that energy is not wasted through over-dosing, and that water is properly disinfected using the target UV dose. This data is critical to proper system design and operation to reduce capital costs and operating costs, respectively.