Utilization of ultraviolet (UV) light for drinking water disinfection has been around for many years. After chlorinated disinfection byproducts (DBPs) were discovered, UV disinfection became popular in Europe, and later, the United States. Because of the susceptibility of Cryptosporidium and giardia to UV disinfection and the emphasis in the Surface Water Treatment Rule regulations on controlling Cryptosporidium, the number of public water systems (PWSs) using UV disinfection has grown significantly.

Ultraviolet Systems

UV light is the region of the electromagnetic spectrum that lies between X-rays and visible light (Figure 11.1). The UV spectrum is divided into four regions: vacuum UV [100 to 200 nanometers (nm)]; UV-C (200 to 280 nm); UV-B (280 to 315 nm); and UV-A (315 to 400 nm). UV disinfection primarily occurs due to the germicidal action of UV-B and UV-C light on microorganisms. The EPA defines the practical germicidal wavelength range for UV light between 200 and 300nm.

Figure 4.1 EPA-Defined Germicidal Wavelength is 200nm – 300 nm.

UV light in this range will inactivate pathogens by disrupting their DNA strands, making them non-viable and non-infectious.

Ultraviolet (UV) light can also be used in combination with hydrogen peroxide or ozone in the oxidation of micropollutants. These are typically referred to as “advanced oxidation processes”. This technology has been shown to be effective in destroying many micropollutants present in the groundwater (e.g., MTBE, perchlorate, pesticides, 1,4-Dioxane, etc.) and surface water (pharmaceutical and personal care products, taste, and odor compounds such as Geosmin) through direct chemical oxidation.

The use of UV light to disinfect drinking water involves (1) generating UV light with the desired germicidal properties and (2) delivering (or transmitting) that light to pathogens. As UV light propagates from its source, it interacts with the materials it encounters through absorption, reflection, refraction, and scattering. In disinfection applications, these phenomena result from interactions between the emitted UV light, UV reactor components and the quality of the water being treated. UV Transmittance (UVT) is the percentage of light passing through material (e.g., a water sample or quartz) over a specified distance. The UVT is usually reported for a wavelength of 254 nm and a pathlength of 1-cm.  The UVT of the water directly influences UV dose delivery from the reactor and is the most important water quality parameter for determining the proper UV dose.

UVT incorporates the effect of absorption and scattering.  Water with low turbidity and color is preferred in order to promote high UV transmittance. As such, UV light is applied after filtration in a conventional surface water treatment plant as shown in Figure 11.2. A free chlorine residual will still need to before water enters the distribution system.

Figure 4.2 Conventional treatment with UV disinfection.

UV Dose – Response

The formula for UV dose is:  [latex]UV dose=UV intensity x Retention time[/latex]

Intensity is in units of power per surface area (W/m2) and retention time is in seconds.

Most manufacturers provide a lamp intensity of 30,000 – 50,000 μWatt·sec/cm2. A lamp with a low intensity means the UV dose is too low to provide adequate disinfection. Treatment operators working with UV disinfection will spend a lot of time electronically monitoring UV related parameters.

Microbial response is a measure of the sensitivity of the microorganism to UV light and is unique to each microorganism. UV dose-response is determined by irradiating water samples containing the microorganism with various UV doses and measuring the concentration of infectious microorganisms before and after exposure.  Microbial response is expressed as log inactivation. The EPA has UV dose tables and inactivation log credit for Cryptosporidium, Giardia and viruses and will be discussed in a later section. UV dose units are given in (mJ/cm2). Here is a quick unit review.

• J = Joule, which is a (Newton-meter) and is a unit of energy.
• W = Watt, is unit of power and is a (J/sec)
• 1 mJ = 0.001 J
• 1mW = 1 x 10-6 W = 1 x 10-6 J/sec

UV Reactors (Disinfection Equipment)

The goal in designing UV reactors for drinking water disinfection is to efficiently deliver the dose necessary to inactivate pathogenic microorganisms.  Commercial UV reactors for drinking water treatment consist primarily of closed-channel vessels, containing UV lamps, lamp sleeves, UV sensors, and temperature sensors. Water flows under pressure (i.e., no free surface) in closed-channel reactors (see Figure 11.3). Notice the multiple UV lamps housed in the reactor example shown in Figure 11.3 a). UV lamps typically are housed within quartz lamp envelopes, which protect and insulate the lamps. The figure shows the lamps arranged in a parallel configuration in the same direction as the flow of water, which flows between the spaces of the sleeves.  Lamps arranged perpendicular to the flow are also used.

Some reactors include automatic cleaning mechanisms to keep lamp sleeves free of deposits. UV sensors, flow meters, and, in some cases, UVT analyzers, are used to monitor dose delivery by the reactor.

a)

b)

Figure 4.3 a)  b) Large Closed Channel UV Reactor in treatment plant.

UV Lamps

UV light is typically produced by the following variety of lamps:

• LP mercury vapor lamps.
• Low-pressure high-output (LPHO) mercury vapor lamps.
• MP mercury vapor lamps.

The typical design life of a low-pressure UV lamp is on the order 12000 hrs with a guaranteed life of 8000 hrs. The lamp configuration in a reactor is designed to optimize dose delivery. In a reactor with a circular cross-section, lamps typically are evenly spaced on one or more concentric circles parallel to flow. However, UV lamps may be oriented parallel, perpendicular, or diagonal to the flow direction. A typical UV lamp is shown Figure 11.4.

Figure 4.4 Lamps can be concentrically spaced parallel to flow direction. Water flows between the lamps.

Lamp Envelope

A UV lamp envelope is the outer enclosure of a light source, usually made of glass or quartz. It contains electrodes and the envelope of the lamp is designed to transmit germicidal UV light, act as an electrical insulator, and not react with the lamp’s fill gases. A noncrystalline form of quartz, vitreous silica, is often used for the lamp envelope because of its high UVT and its resistance to high temperatures. The UVT of the envelope affects the spectral output of lamps, especially with MP lamps at lower wavelengths. Because of this, lamp envelopes can be made from doped quartz (quartz that is altered to absorb specific wavelengths) to prevent undesirable non-germicidal photochemical reactions. Envelopes are approximately 1 – 2 millimeters (mm) thick, and the diameter is selected to optimize the UV output and lamp life.

Note that a lamp sleeve is a glass tube with end caps that holds a UV lamp and protects it from moisture.

Electrodes

Electrode design and operation are critical for reliable long-term operation of lamps. Electrodes promote heat transfer so that lamps can operate at an appropriate temperature. The electrodes in LP and LPHO lamps are made of a coil of tungsten wire embedded with oxides of calcium, barium, or strontium. In MP lamps, electrodes consist of a tungsten rod wrapped in a coil of tungsten wire.

Mercury Fill

The mercury fill present in UV lamps can be in the solid, liquid, or vapor phase. Amalgams (alloys of mercury and other metals such as indium or gallium in the solid phase) are typically used in LPHO lamps, while LP and MP lamps contain liquid elemental mercury. As the lamps heat, the vapor pressure of mercury increases. LP and LPHO lamps operate at lower temperatures and have lower mercury vapor pressures than MP lamps. In MP lamps, the concentration of mercury in the vapor phase is controlled by the amount of mercury in the lamp. In LPHO lamps, an excess of mercury is placed in the lamp, and the amount of mercury entering the vapor phase is limited by either a mercury amalgam attached to the lamp envelope, a cold spot on the lamp wall, or a mercury condensation chamber located behind each electrode.

Inert Gas Fill

In addition to mercury, lamps are filled with an inert gas (typically argon). The inert gas aids in starting the gas discharge and reduces deterioration of the electrode. The vapor pressure of the inert gas is typically 0.02 – 1 psi.

Lamp Sleeves

UV lamps are housed within lamp sleeves to help keep the lamp at optimal operating temperature and to protect the lamp from breaking. Lamp sleeves are tubes of quartz (vitreous silica) that are open at one or both ends. The sleeve length is sufficient to include the lamp and associated electrical connections. The sleeve diameter is typically 2.5 – 5.0 cm for LP and LPHO lamps and 3.5 – 10.0 cm for MP lamps. The distance between the exterior of the lamp and interior of the lamp sleeve is approximately 1 cm. The positioning of the UV lamp along the length of the sleeve can vary, depending on reactor configuration.

Lamp Output

The light that LP and LPHO lamps emit is essentially monochromatic at 253.7 nm in the ultraviolet range and is near the maximum of the microbial action spectrum.

Deposition of compounds in the water on the lamp sleeve surface cause fouling on external surfaces. UV reactor manufacturers have developed different approaches for cleaning lamp sleeves, depending on the application. These approaches include off-line chemical cleaning, or on-line mechanical cleaning.

UV lamps degrade as they age, resulting in a reduction in output that causes a drop in UV dose delivery over time. Lamp degradation occurs with both LP and MP lamps and is a function of the number of lamp hours in operation, number of on/off cycles, power level, and water temperature.

UV lamps are sensitive to small fluctuations in power. Ballasts are used to regulate the incoming power supply at the level needed to energize and operate the UV lamps. Ballasts can be magnetic or electronic.  They each have advantages and disadvantages.

UV Reactor Hydraulics

The flow through UV reactors is turbulent. Residence times are on the order of tenths of a second for MP lamps and seconds for LP lamps. In theory, optimal dose delivery is obtained with plug flow hydraulics through a UV reactor. In practice, however, UV reactors do not have such ideal hydrodynamics. For example, turbulence and eddies form in the wake behind lamp sleeves oriented perpendicularly to flow. Some manufacturers insert baffles to improve hydrodynamics in the reactor. Improvements to the hydraulic behavior of a reactor are often obtained at the expense of head loss.

The thickness of the water layer between lamps and between the lamps and the reactor wall influences dose delivery. If the water layer is too thin, the reactor wall and adjacent lamps will absorb UV light. If the water layer is too thick, water will pass through regions of lower UV intensity and experience a lower UV dose. The optimal spacing between lamps depends on the UVT of the water, the output of the lamp, and the hydraulic mixing within the reactor.

Inlet and outlet conditions can significantly affect reactor hydrodynamics and UV dose delivery. For example, changes in flow direction of 90 degrees at inlets and outlets promote short-circuiting, eddies, and dead zones within the reactor. Straight inlet configurations with gradual changes in cross-sectional area will help create flow conditions for optimal dose delivery.

Surface Water Treatment Rule Requirements for UV Disinfection

The Long Term 2 Enhanced Surface Water Treatment Rule has several requirements related to the use of UV disinfection.  As noted earlier, EPA developed UV dose requirements for Public Water Systems to receive credit for inactivation of Cryptosporidium, Giardia, and viruses. These dose values are listed in Table 11.1 and are applicable only to post-filter applications of UV disinfection in filtered systems and to unfiltered systems. The EPA feels that UV disinfection is much less effective at economical doses for viruses as reflected in Table 11.1. Unlike chemical disinfectants, UV leaves no residual that can be monitored to determine UV dose and inactivation credit. The UV dose depends on the UV intensity (measured by UV sensors), the flow rate, and the UV transmittance (UVT).

Table 11.1 UV Dose Requirements – millijoules per centimeter squared (mJ/cm2)1

While the UV dose requirements listed in the Table account for uncertainty in the UV dose-response relationships of the target pathogens, they do not address other significant sources of uncertainty in full-scale UV disinfection applications. These other sources of uncertainty are due to the hydraulic effects, UV reactor equipment (e.g., UV sensors), and the monitoring approach. Consequently, Public Water Systems are required to use UV reactors that have undergone full scale validation testing for the anticipated operating conditions, and must include operation flow rates, UV intensity as measured by a UV sensor, and UV lamp status. Further, validation testing must account for:

• UV absorbance of the water (i.e., UV Transmittance).
• Lamp fouling and aging.
• Measurement uncertainty of online sensors.
• UV dose distributions specific to velocity profiles within the closed reactor.
• Failure of UV lamps or other critical components.
• Inlet and outlet reactor piping.
• Power quality.

Validation testing must also demonstrate inactivation of a test microorganism whose dose-response characteristics have been quantified with a low-pressure mercury vapor lamp [40 CFR 141.720(d)(2)(ii)]. If the design parameters are not sufficiently conservative, the UV reactors may often operate off-specification and be out of compliance.

Operation of UV Disinfection System

Treatment Operators will work with a significant amount of monitoring equipment and instrumentation. Much of is incorporated by reactor manufacturers with electronic sensors and software which includes alerts and alarms. Signals from the monitors and sensors will be displayed on a control panel.

The UV equipment is controlled by a central programmable logic controller (PLC). The central PLC uses flow rate and direction data from treatment train to control the overall operation of the UV equipment and to sequence the operation of individual UV units. Input for controlling the UV equipment is provided by a flow meter on each UV treatment train, two on-line UVT analyzers in the piping header, and UV sensors in each UV unit. The individual control panel for each UV unit adjusts lamp power and the calculated dose of each UV unit in response to the flow rate, UVT, and UV intensity, to ensure an appropriate level of disinfection.

Generally, UV facilities use PLCs to monitor operating parameters, control the UV reactor, and generate alarms. Increased automation (e.g., remote monitoring capability) may be incorporated to further reduce operator requirements. Typically, each UV reactor has a dedicated control panel, and the plant’s SCADA system receives control signals from each control panel to control the entire UV system.

UV Sensors

UV sensors measure the UV intensity at a point within the UV reactor (Figure 11.3) and are used with measurements of flow rate and, potentially, UVT to indicate UV dose delivery. The measurement responds to changes in lamp output due to lamp power setting, lamp aging, lamp sleeve aging, and lamp sleeve fouling. Depending on sensor position, UV sensors may also respond to changes in UVT of the water being treated.

UV sensors can be classified as dry or wet. Dry sensors monitor UV light through a monitoring window, whereas wet UV sensors directly contact the water flowing through the reactor.

Temperature Sensors

The energy input to UV reactors that is not converted to light (approximately 60 – 90 percent, depending on lamp and ballast assembly) is wasted as heat. As it passes through a reactor, water can absorb the heat, keeping the reactor from overheating. Nevertheless, temperatures can increase when either of the following events occurs:

• Water level in the reactor drops and lamps are exposed to air.
• Water stops flowing in the reactor.

UV reactors can be equipped with temperature sensors that monitor the water temperature within the reactor. If the temperature is above the recommended operating range, the reactor will shut off to minimize the potential for the lamps to overheat. Because of the high operating temperature of MP lamps, dissipating heat can be more difficult than in reactors that use LP or LPHO lamps. As such, UV reactors with MP lamps typically have temperature sensors; however, reactors with LP or LPHO lamps may not because of the lower lamp operating temperature.

UVT Analyzers

As stated previously, UVT is an important parameter in determining UV dose delivery and/or monitoring. UVT analyzers are essential. Several commercial UV reactors use the measurement of UVT to calculate UV dose in the reactor and, if necessary, change lamp output or the number of energized lamps to maintain appropriate UV dose delivery.

Two types of commercial on-line UVT analyzers are available. One analyzer calculates UVT by measuring the UV intensity at various distances from a lamp.  The other type of on-line UVT analyzer is a flow-through spectrophotometer that uses a monochromatic UV light source at 253.7 nm.

UV Reactor Dose-Monitoring Strategy

The dose-monitoring strategy establishes the operating parameters used to confirm UV dose delivery.  There are two basic approaches and other approaches may be developed. Any dose monitoring strategy must be evaluated during full scale reactor validation.

1. UV Intensity Setpoint Approach

This approach relies on one or more “setpoints” for UV intensity that are established during validation testing to determine UV dose. In the UV Intensity Setpoint Approach, UVT does not need to be monitored separately. Instead, the intensity readings by the sensors account for changes in UVT.

2. Calculated Dose Approach

This approach uses a dose monitoring equation to estimate the UV dose based on the flow rate, UV intensity, and UVT, as measured during reactor operations. The dose monitoring equation may be developed by the UV manufacturers using numerical methods; however, EPA recommends that water systems use an empirical dose monitoring equation developed through validation testing.

Alarms and Control Systems Interlocks

Many UV reactor signals and alarms are specific to the UV facility and the level of automation used. Alarms may be designated as minor, major, or critical, depending on the severity of the condition being indicated. Typical alarm conditions would include:

• Lamp Age (Run-time).
• UV sensor calibration check.
• Low UV Validated Dose.
• UV intensity.
• UVT.
• High Flow rate.
• Mechanical wiper failure (fouling potential).
• Lamp/Ballast failure.
• High temperature.

System interlocks are employed protect the equipment and maintain water quality. For example, a UV reactor will shut down if the temperature rises above a setpoint.

Safety Issues

This section provides some recommended safety precautions for UV reactor operations. The recommended precautions in this section should be considered in addition to manufacturer’s recommended safety precautions and procedures, Occupational Safety and Health Administration (OSHA) regulations, and state guidance and regulations for UV reactor operations.

In addition to the standards and procedures established for WTP operations, the following safety issues pertain specifically to UV reactors:

• UV light exposure.
• Electrical safety.
• Burns from hot lamps or equipment.
• Abrasions or cuts from broken lamps or sleeves.
• Potential exposure to mercury from broken lamps.

Threshold limit values (TLVs) for UV light apply to occupational exposure to UV incident on the skin or eyes. The recommended TLVs depend on the lamp wavelengths emitted and the UV intensity (mW/cm2).  To limit or prevent operator exposure to the UV light, UV reactors should have interlocks that deactivate the lamps when reactors are accessed. Viewing ports, if provided, should be fitted with UV filtering windows, or operators should wear a UV-resistant face shield when working in the UV reactor. To minimize the danger of exposure, warning signs also should be posted.

To reduce the risk of electrical shock, the main electrical supply to the UV reactors should be disconnected and the operator should wait at least 5 minutes for the lamps to cool and the energy to dissipate before maintenance is performed in areas where electric shock may be a risk. All safety and operational precautions required by the National Electric Code (NEC), OSHA, local electric codes, and the UV manufacturer should be followed.

Review Questions

1. Explain UV disinfection theory.
2. Describe UV disinfection applications.

Test Questions

1. Which part of a microorganism is damaged by UV light?

1. cell wall.
2. organelles.
3. plasma membrane.
4. genetic material.

1. Which is the usual source of the UV radiation for disinfection systems?

1. low-pressure mercury vapor UV lamps.
2. low-pressure, low-intensity lamps.
3. low-pressure, high-intensity lamps.
4. medium-pressure, high-intensity lamps.

1. What must be maintained in the UV reactor to ensure acceptable disinfection levels over the entire range of design flows?

1. proper chlorine residual.
2. proper flow rate.
3. proper TDS removal.
4. none of these are correct.

1. Which describes the UV light necessary to disinfect the water?

1. at a very high wavelength to penetrate the cell walls.
2. at a very low wavelength to penetrate the cell walls.
3. intense enough to penetrate the cell walls.
4. of short duration in order to penetrate the cell walls.

1. The design life of a typical low pressure UV lamp is

1. 5,000 hours of use.
2. 8,000 hours of use.
3. 12,000 hours of use.
4. 14,000 hours of use.

1. What can shield bacteria and reduce the effectiveness of the UV disinfection process?

1. excessive water.
2. turbidity.
3. excessive alkalinity.
4. low chlorine residuals.

References

• Detection and Absorption of Ultraviolet Light (Experiment) by Libretexts
• Ultraviolet Irradiation: Treatment Process Navigation, Overview by USEPA
• Ultraviolet Disinfectio  Guidance Manual for the Final Long Term 2 Enhanced Surface Water Treatment Rule, Office of Water (4601) EPA 815-R-06-007 November 2006 (courtesy Calgon Corporation. See page 2-15 and Severn Trent Services)
• Ultraviolet (UV) Treatment Toolkit: Technical Resource for States using EPA’s Ultraviolet Disinfection Guidance Manual to Evaluate UV Technology
• Office of Water (MS-140) EPA 815-B-21-007 May 2022