Municipal wastewater generally requires disinfection to meet specific bacterial limits before discharge to surface waters. The main objective of disinfection is to reduce the number of waterborne pathogens to safe levels, thereby lowering the risk of exposing the public to infectious disease. The persistence of some pathogens in receiving waters and soils indicates that disinfection of wastewater effluents provide the first line of defense for drinking water from surface or ground sources. To meet this objective, the disinfectant must inactivate a wide range of bacteria and viruses in a variety of water and wastewater qualities. Disinfection may be accomplished by chemical or physical methods. However, an increased awareness of the disadvantages of chemical disinfectants, specifically chlorine, has resulted in the selection of UV as an alternative with many attractive features and benefits.
Ultraviolet (UV) light is the portion of the electromagnetic spectrum with wavelengths between 100 and 400 nanometers (nm). Germicidal wavelengths are located in the spectral region of 200nm to 300nm (Figure 1). The low pressure mercury lamp radiation is essentially monochromatic with >90% of its output at 254 nm.
How Does UV Work?
Unlike chemical approaches to wastewater disinfection, UV light provides rapid inactivation of potentially harmful microbes through a physical process. Microorganisms are inactivated by UV light as a result of photochemical damage to nucleic acids. The high energy associated with short wavelength UV radiation is absorbed by cellular RNA and DNA. This absorption of UV energy creates dimers. The formation of numerous dimers in the DNA of bacteria and viruses prevents replication and results in cell death. The germicidal effectiveness of low pressure lamps at wavelength 254 nm correlates with the absorption spectrum for nucleic acid which peaks at 260nm.
Factors Affecting UV Disinfection
The germicidal effects of UV are directly related to the “dose” of UV energy absorbed by a microorganism. UV dose in a disinfection system is related to flow rate/retention time and UV intensity in the reactor. UV intensity is a function of wastewater quality combined with UV equipment design optimization. Exposure time is directly related to flow rate and retention time which are controlled by optimizing reactor design and lamp spacing to control the head loss (see Table 1). UV disinfection is not affected by water temperature or pH.
Table 1. Equipment Design and Water Quality Directly Affect UV Dose Delivery
|UV Light Intensity||Time|
|UV Equipment Parameters |
• UV Lamp spacing
• Age of UV lamps
• Quartz Sleeve fouling (affected by iron, calcium and magnesium ions, algae and biofilms)
|Water Quality Parameters - Related to Upstream Processes |
• UV Transmittance (affected by dissolved organics, dyes, iron)
• Suspended solids (TSS)
• Particle size distribution (PSD)
• Total hardness (can affect rate of sleeve fouling)
(Note: UV Dose = UV Light Intensity x Time)
UV Equipment Design Parameters
The UV intensity in a UV disinfection reactor is directly related to specific design parameters such as lamp type, total number of lamps and lamp spacing. A proper UV reactor is designed to optimize the number of UV lamps required for disinfection with the necessary hydraulic capacity. The ideal hydraulic characteristic is turbulent plug flow for mixing and minimal head loss. Lamp spacing is designed to control the water layer around the quartz sleeve and to provide the maximum average intensity in the reactor. A water level controller is included in the design to maintain the appropriate water depth in the channel and to keep UV lamps submerged.
Equipment maintenance factors affecting UV intensity include lamp age and sleeve fouling. The UV intensity gradually decreases with use (as the lamps age) and this is factored into the design to ensure the required UV dose is delivered at the end of lamp life. The recommended low-pressure (amalgam) lamp replacement varies between 9,000 to 12,000 hours depending on the manufacturer.
An accumulation of inorganic and organic matter (fouling) on the quartz sleeves that encase the UV lamps decreases the intensity of UV light penetrating the water. The fouling rate varies with process and effluent types and may be more rapid in the presence of high concentrations of iron, calcium and magnesium. UV systems without fully-automatic cleaning devices will require manual cleaning and maintenance by plant operators on a regular basis. However, through product innovation by Trojan Technologies, UV reactors like the TrojanUV3000Plus™ include an automatic quartz sleeve wiping system combining chemical and mechanical cleaning so does not require operator maintenance or equipment downtime for sleeve cleaning.
Comparison of UV Disinfection and Chemical Disinfection
The disadvantages associated with chemical (chlorine) disinfection have resulted in the increased selection of UV for wastewater disinfection in many regions of the world today. Table 2 summarizes the advantages and disadvantages of UV and chlorine disinfection.
The UV disinfection process takes place entirely within the irradiation chamber (concrete channel) and has no negative effect on downstream water sources, environments or aquatic life. When high doses of chlorine are used, and depending on the quality of the effluent, carcinogenic and mutagenic chloro-organics such as trihalomethanes (THM, e.g. chloroform) and the less volatile higher molecular weight byproducts can be produced. These by-products persist in water and can affect downstream environments.
Concerns for the safety of the environment by the public and plant operators result in additional expenses for those who opt for chlorine instead of UV disinfection. Dechlorination to remove toxic chlorine residuals can add at least 30% to the cost of chlorination. UV disinfection does not require buildings, whereas some of the new regulations require specially designed buildings for chlorine.
Further concerns are associated with the transportation and storage of the toxic chemicals used for chlorination/dechlorination disinfection. The risk of gas leaks at the WWTP or chemical storage sites continue to receive high public visibility – especially in densely populated areas.
Table 2. Advantages and Disadvantages of UV Disinfection vs. Chlorination
|Chlorination (Gas & Hypochlorite) |
• Well-established technology
• Effective disinfectant against bacteria
• Residual can be maintained and monitored
• Relatively inexpensive though can be subject to local safety codes which can significantly increase costs
|• Increased safety regulations combined with need for dechlorination and special scrubbing facilities
• Gas is toxic gas to humans and requires special handling and training
• Residual toxicity of treated effluent should be reduced by dechlorination
• Forms trihalomethanes (THMs) and other chlorinated hydrocarbons
• Release of volatile organic compounds (VOCs) to air from chlorine contact basins
• Increases concentration of Total Dissolved Solids (TDS), chloride and may alter pH of final effluent
|Ultraviolet Disinfection |
• Effective against bacteria and viruses and protozoa
• Very short contact time required
• No residual toxicity
• No chemicals to handle
• Relatively inexpensive
• Outdoor installation (buildings not required)
• Small physical footprint required
• Lower carbon footprint
|• No immediate measure of effectiveness of disinfection (overcome by validation and UV dose monitoring)
• No residual effect
UV has several advantages that result in UV disinfection being the right choice for new treatment plants constructed. The simple concrete gravity flow-through channel, small overall footprint and outdoor installation significantly reduce construction costs relative to a chlorination system with large contact tanks.
The widespread adoption of UV for wastewater disinfection is the result of the technology’s many benefits, including ensuring community and operator safety, providing public health protection, highly effective treatment performance, minimal environmental impact, and ability to address increasingly stringent regulations. For these reasons, UV has been successfully applied over a wide range of applications from low quality wastewaters (primary effluent, blended effluent), typical wastewater (secondary effluent discharging to streams and rivers) as well as advanced wastewater (filtered effluents being reused or discharging to environmentally sensitive areas).