UV Disinfection: Drinking Water Applications & Reuse.
UV Disinfection is a tried-and-true disinfection process for wastewater effluent, especially where effluent is discharged to a receiving stream or lake. UV has been studied since 2002 for use in drinking water disinfection, but its use has not been approved by all regulatory agencies. Chlorine is the choice for residual disinfection; however, when combined with organics, chlorination by-products can result.
From an operations standpoint, UV simplifies the effluent disinfection discharge process: it does not require de-chlorination for wastewater, eliminating the associated taste and odor problems; and it doesn’t cause formation of chlorinated by-products. One of the major differences between chlorine and UV disinfection is that UV’s chemical application damages the cellular structure of the microbe. UV targets the DNA so that the microbe is unable to replicate itself. UV eradicates the majority of the pathogens targeted by chlorine. It is ineffective to adenoviruses, however.
How UV Disinfection Works
UV disinfection’s primary use in wastewater uses light to eradicate microbes and disable the microbe’s DNA. The light spectrum used is in the range of 100-400 nanometers between x-ray and visible light:
- UVA at 315-400 nanometers
- UVB at 280-315 nanometers
- UVC at 200-280 nanometers
- UV at 100-200 nanometers
The most effective sub-regions of light for water disinfection are UVC and UVB ranges.
The metric of measurement for chemical disinfection is mg/L where UV is measured in mJ/cm2 (millijoules per square centimeter or mW-s/cm2 (milliwatt-seconds/square centimeter). UV lights are varied in application. The UV light or tubes are rated based on typical pressure of the lamp that is then related to flow rate. The three typical UV light types are:
- Low Pressure
- Low Pressure High Output
- Medium Pressure
Pressure of the lamp is the mercury vapor pressure on the inside of the lamp. The first two types—Low Pressure and Low Pressure High Output—are germicidal UV light that is monochromatic. UV monochromatic light is light of a single color or wavelength. Low Pressure and Low Pressure High Output are recommended for flow rates up to 3 million gallons per day. Medium pressure is typically used for flows more than 3 million gallons per day and emits polychromatic light—light of multiple colors, hence various wavelengths.
General design for UV disinfection systems says the greater the lamp pressures, the greater the UV’s ability to disinfect water at a specific dosage/exposure. Power input is directly proportional to the number of lamps used for disinfection. Power consumption is a major downside to UV Disinfection. The design criteria for UV Disinfection systems are shown below, as is a general listing of the data needed.
- Flow rate (gpm)
- Number of UV light banks (3 in series is the minimum recommended)
- Redundancy either an additional channel with lights are a standby by bank of lights.
- Backup power (generator)
- Flow through the channel is designed as a “plug flow” system. Geometry of the channel is particularly important.
- Inlet approach and outlet are critical to provide adequate exposure of light. Uniform flow throughout the channel is necessary:
- 2 x (channel water depth) or 4 feet minimum approach prior to first UV light bank
- Same for outlet after passing the last UV light bank
- Spacing of banks is determined based on maintenance and access to units
- The channels should be covered to reduce the growth of algae
- Plug Flow is a known volume of water passing through a control section of pipe or channel
- Detention time for UV disinfection is instantaneous when the water is exposed to the UV light, hence a plug flow reactor design allowing for uniform flow through the channel or pipe section
Water clarity is extremely important where UV is planned. The ideal water turbidity or clarity of the water to be treated must be low. Particle sizes of 7um or larger decrease the inactivation of UV disinfection. Filtering of the water stream prior to UV treatment is a must to provide low suspended solids entering the UV chamber.
Cleaning of the lights or tubes is chemically/mechanically done with most systems. Trojan uses the ActiClean process that eliminates handling of the tubes and reduces damage and breakage of the UV tubes.
UV disinfection has been successfully used in wastewater effluent disinfection. UV is preferred over chlorination where the effluent is discharged to lakes, rivers or streams where aquatic life exists. Where chlorine disinfection is used, a de-chlorination step is required prior to discharge; otherwise, the chlorine residuals deplete oxygen and cause fish kills.
UV Disinfection as an Option for Drinking Water
UV is an up-and-coming disinfection method. The upsides include the destruction of the DNA of microbes which prevents them from reforming, the lack of taste and odor problems, and the prevention of the formation of trihalomethanes and other byproducts found in chlorine-added systems. The downside is that it leaves no residual disinfectant in the distribution system.
EPA is allowing use of UV disinfection systems for small public water systems. Massachusetts is a leading state in UV disinfection for drinking water, as is Texas on a case-by-case basis; however, other residual methods must still be incorporated to ensure a safe public drinking water.
UV disinfection is a viable disinfection method for wastewater and drinking water, but more research is required in providing adequate and alternative residual disinfection. Technology is changing quickly and UV disinfection has improved ten-fold in the last five years.
UV disinfection is here to stay with promising results for all water disinfection applications.