# Ultraviolet Radiation

Unlike most disinfectants, ultraviolet (UV) radiation does not
inactivate microorganisms by chemical interaction. UV radiation
inactivates organisms by absorption of the light which causes a
photochemical reaction that alters molecular components essential to
cell function. As UV rays penetrate the cell wall of the microorganism,
the energy reacts with nucleic acids and other vital cell components,
resulting in injury or death of the exposed cells. There is ample
evidence to conclude that if sufficient dosages of UV energy reach the
organisms, UV can disinfect water to whatever degree is required.
However, there has been some public health concerns with respect to the
overall efficiency of UV to disinfect potable water.

Based on the available research literature, it appears that although
exceptional for disinfection of small microorganisms such as bacteria
and viruses, UV doses required to inactivate larger protozoa such as
*Giardia* and *Cryptosporidium* are several times higher than for
bacteria and virus inactivation (White, 1992; DeMers and Renner, 1992).
As a result, UV is often considered in concert with ozone and/or
hydrogen peroxide to enhance the disinfection effectiveness of UV or for
ground water where *Giardia* and *Cryptosporidium* are not expected to
occur.

## UV Chemistry (Photochemical)

### UV Radiation

UV radiation quickly dissipates into water to be absorbed or reflected
off material within the water. As a result, no residual is produced.
This process is attractive from a DBP formation standpoint; however, a
secondary chemical disinfectant is required to maintain a residual
throughout the distribution system, which may be subject to
recontamination.

UV radiation energy waves are the range of electromagnetic waves 100 to
400 nm long (between the X-ray and visible light spectrums). The
division of UV radiation may be classified as Vacuum UV (100-200 nm),
UV-C (200-280 nm), UV-B (280-315 nm) and UV-A (315-400 nm). In terms of
germicidal effects, the optimum UV range is between 245 and 285 nm. UV
disinfection utilizes either: low-pressure lamps that emit maximum
energy output at a wavelength of 253.7 nm; medium-pressure lamps that
emit energy at wavelengths from 180 to 1370 nm; or lamps that emit at
other wavelengths in a high intensity "pulsed" manner.

### UV Disinfection Reactions

The degree to which the destruction or inactivation of microorganisms
occurs by UV radiation is directly related to the UV dose. The UV dosage
is calculated as:

*D = I* • *t*

Where: *D =* UV Dose, mW⋅s/cm^2^

*I =* Intensity, mW/cm^2^

*t =* Exposure time, s

Research indicates that when microorganisms are exposed to UV radiation,
a constant fraction of the living population is inactivated during each
progressive increment in time. This dose-response relationship for
germicidal effect indicates that high intensity UV energy over a short
period of time would provide the same kill as a lower intensity UV
energy at a proportionally longer period of time.

The UV dose required for effective inactivation is determined by
site-specific data relating to the water quality and log removal
required. Based on first order kinetics, the survival of microorganisms
can be calculated as a function of dose and contact time (White, 1992;
USEPA, 1996). For high removals, the remaining concentration of
organisms appears to be solely related to the dose and water quality,
and not dependent on the initial microorganism density. Tchobanoglous
(1997) suggested the following relationship between coliform survival
and UV dose:

*N = f • D^n^*

Where: $N$ = Effluent coliform density, /100mL

$D$ = UV dose, mW⋅s/cm^2^

$n$ = Empirical coefficient related to dose

$f$ = Empirical water quality factor

The empirical water quality factor reflects the presence of particles,
color, etc. in the water. For water treatment, the water quality factor
is expected to be a function of turbidity and transmittance (or
absorbance).

### Process Variables

Since UV radiation is energy in the form of electromagnetic waves, its
effectiveness is not limited by chemical water quality parameters. For
instance, it appears that pH, temperature, alkalinity, and total
inorganic carbon do not impact the overall effectiveness of UV
disinfection (AWWA and ASCE, 1990). However, hardness may cause problems
with keeping the lamp sleeves clean and functional. The presence, or
addition, of oxidants (e.g., ozone and/or hydrogen peroxide) enhances UV
radiation effectiveness. The presence of some dissolved or suspended
matter may shield microorganisms from the UV radiation. For instance,
iron, sulfites, nitrites and phenols all absorb UV light (DeMers and
Renner, 1992). Accordingly, the absorbance coefficient is an indication
of this demand and is unique for all waters. As a result, specific
"design" parameters vary for individual waters and should be determined
empirically for each application.

UV demand of water is measured by a spectrophotometer set at a
wavelength of 254 nm using a 1 cm thick layer of water. The resulting
measurement represents the absorption of energy per unit depth, or
absorbance. Percent transmittance is a parameter commonly used to
determine the suitability of UV radiation for disinfection. The percent
transmittance is determined from the absorbance (A) by the equation:

Percent Transmittance = 100  × 10^-A^

Table 8-1 shows corresponding absorbance measurements and percent
transmittance measurements for various water qualities.

[]{#__RefHeading___Toc450037389 .anchor}Table 8-1. Water Quality and
Associated UV Measurements

  ----------------------- ----------------------- -----------------------
  **\                     **Absorbance\           **Percent\
  Source Water Quality**  (absorbance units/cm)** Transmittance**

  Excellent               0.022                   95%

  Good                    0.071                   85

  Fair                    0.125                   75
  ----------------------- ----------------------- -----------------------

Source: DeMers and Renner, 1992.

Continuous wave UV radiation at doses and wavelengths typically employed
in drinking water applications, does not significantly change the
chemistry of water nor does it significantly interact with any of the
chemicals within the water (USEPA, 1996). Therefore, no natural
physiochemical features of the water are changed and no chemical agents
are introduced into the water. In addition, UV radiation does not
produce a residual. As a result, formation of THM or other DBPs with UV
disinfection is minimal (See Section 8.5, *Disinfection Byproducts of UV
Radiation*).

## Generation

Producing UV radiation requires electricity to power UV lamps. The lamps
typically used in UV disinfection consist of a quartz tube filled with
an inert gas, such as argon, and small quantities of mercury. Ballasts
control the power to the UV lamps.

### UV Lamps

UV lamps operate in much the same way as fluorescent lamps. UV radiation
is emitted from electron flow through ionized mercury vapor to produce
UV energy in most units. The difference between the two lamps is that
the fluorescent lamp bulb is coated with phosphorous, which converts the
UV radiation to visible light. The UV lamp is not coated, so it
transmits the UV radiation generated by the arc (White, 1992).

Both low-pressure and medium-pressure lamps are available for
disinfection applications. Low-pressure lamps emit their maximum energy
output at a wavelength of 253.7 nm, while medium pressure lamps emit
energy with wavelengths ranging from 180 to 1370 nm. The intensity of
medium-pressure lamps is much greater than low-pressure lamps. Thus,
fewer medium pressure lamps are required for an equivalent dosage. For
small systems, the medium pressure system may consist of a single lamp.
Although both types of lamps work equally well for inactivation of
organisms, low-pressure UV lamps are recommended for small systems
because of the reliability associated with multiple low-pressure lamps
(DeMers and Renner, 1992) as opposed to a single medium pressure lamp,
and for adequate operation during cleaning cycles.

Recommended specifications for low-pressure lamps (DeMers and Renner,
1992) include:

-   L-type ozone-free quartz;

-   Instant start (minimal delay on startup);

-   Designed to withstand vibration and shock; and

-   Standard nonproprietary lamp design.

Typically, low-pressure lamps are enclosed in a quartz sleeve to
separate the water from the lamp surface. This arrangement is required
to maintain the lamp surface operating temperature near its optimum of
40^o^C. Although Teflon sleeves are an alternative to quartz sleeves,
quartz sleeves absorb only 5 percent of the UV radiation, while Teflon
sleeves absorb 35 percent (Combs and McGuire, 1989). Therefore, Teflon
sleeves are not recommended.

### Ballasts

Ballasts are transformers that control the power to the UV lamps.
Ballasts should operate at temperatures below 60^o^C to prevent
premature failure. Typically, the ballasts generate enough heat to
warrant cooling fans or air conditioning (White, 1992).

Two types of transformers are commonly used with UV lamps; namely,
electronic and electromagnetic. Electronic ballasts operate at a much
higher frequency than electromagnetic ballasts, resulting in lower lamp
operating temperatures, less energy use, less heat production, and
longer ballast life (DeMers and Renner, 1992).

Typical ballast selection criteria include (DeMers and Renner, 1992):

-   Underwriter's Laboratories (UL) approval;

-   Compatibility with UV lamps; and

-   Waterproof enclosure in remote location.

### UV Reactor Design

Most conventional UV reactors are available in two types; namely, closed
vessel and open channel. For drinking water applications, the closed
vessel is generally the preferred UV reactor for the following reasons
(USEPA, 1996):

-   Smaller footprint;

-   Minimized pollution from airborne material;

-   Minimal personnel exposure to UV; and

-   Modular design for installation simplicity.

Figure 8-1 shows a conventional closed vessel UV reactor. This reactor
is capable of providing UV dosages adequate to inactivate bacteria and
viruses for flows up to 600 gallons per minute. However, it is incapable
of the higher dosages required for protozoan cysts. To increase the
dosage, either the number of UV lamps and/or the exposure time should be
increased.

Additional design features for conventional UV disinfection systems
include:

1.  UV sensors to detect any drop in UV lamp output intensity;

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1.  Alarms and shut-down systems;

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1.  Automatic or manual cleaning cycles; and

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1.  Telemetry systems for remote installations.

![](media/image1.wmf){width="4.75625in" height="5.286111111111111in"}

Source: USEPA, 1996.

[]{#_Ref396096477 .anchor}Figure 8-1. Closed Vessel Ultraviolet Reactor

In addition to conventional UV systems, two other UV processes are
currently being evaluated for drinking water disinfection:

1.  Micro-screening/UV; and

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1.  Pulsed UV

Both of these systems profess to provide sufficient UV dose to
inactivate *Giardia* cysts and *Cryptosporidium* oocysts. See Section
8.2.3.2, Emerging UV Reactor Designs, for further discussion on these
emerging technologies.

#### Hydraulic Design Considerations

The major elements that should be considered in the hydraulic design of
a UV closed vessel reactor are: dispersion, turbulence, effective
volume, residence time distribution, and flow rate (USEPA, 1996).

##### **Dispersion** {#dispersion .unnumbered}

Dispersion is the characteristic of water elements to scatter spatially.
The ideal UV reactor is plug flow, where water particles are assumed to
discharge from the reactor in the same sequence they entered and each
element of water passing through the reactor resides in the reactor for
the same period of time. An ideal plug flow reactor has no dispersion
and is approximated by a long tank with high length-to-width ratio in
which dispersion is minimal (USEPA, 1996).

##### **Turbulence** {#turbulence .unnumbered}

In addition to plug flow characteristics, the ideal UV reactor has a
flow that is turbulent radially from the direction of flow, to eliminate
dead zones. This radially turbulent flow pattern promotes uniform
application of UV radiation. A negative of having a radially turbulent
flow pattern is that some axial dispersion results, thus disrupting the
plug flow characteristics. Techniques such as misaligning the inlet and
outlet, and using perforated stilling plates, have been used to
accommodate the contradicting characteristics of plug flow and
turbulence (USEPA, 1996).

#### Emerging UV Reactor Designs

Two emerging technologies in UV reactor design are discussed below. All
testing of these two systems to date has been under controlled
laboratory or field conditions. Both of these technologies require
demonstration of efficacy and applicability in real-world treatment
operations.

##### **Micro-Screening/UV** {#micro-screeninguv .unnumbered}

This unit consists of two treatment chambers, each containing a 2 μm
nominal porosity metal screen. Each side of the screen has three 85 watt
low pressure mercury lamps, with a total of six lamps per filter. The
theoretical minimum dose is 14.6 mW⋅sec/cm^2^ at the wavelength of 254
nm. The system is designed to capture *Cryptosporidium* oocysts from the
water onto the first screen. The first cycle is set to establish the UV
dose. The flow within the first screen chamber is then reversed to
backflush to oocysts onto the second screen where the oocysts are
trapped until the preset UV dose is reached. Using valves, the pattern
of flow is designed to ensure that oocysts are temporarily captured on
both filters so that they are exposed to a total preset UV dose, which
is totally independent of treated water flowrate (Clancy et al., 1997).
Johnson (1997) stated that such a system is capable of achieving total
UV doses of 8,000 mW⋅sec/cm^2^, sufficient to inactivate *Giardia* cysts
and *Cryptosporidium* oocysts. One drawback of this type of reactor is
the headloss created (up to 65 feet) by the 2 μm openings in the screen.

##### **Pulsed UV** {#pulsed-uv .unnumbered}

In the pulsed UV reactor, capacitors build up and deliver electricity in
pulses to xenon flash tubes in the center of a 2 inch diameter flash
chamber through which water flows. The unit is designed to provide
microsecond pulses at 1 to 30 hertz (or per second). With each pulse,
the flash tubes give off high intensity, broad band radiation, including
germicidal UV radiation, which irradiates the flowing water with
irradiences of 75 mW⋅sec/cm^2^ at 2 cm from the flash tube surface
(Clancy et al., 1997). The UV dose can be adjusted by increasing or
decreasing the frequency of the pulsing.

## Primary Uses and Points of Application

The primary use of UV radiation is to inactivate pathogens to regulated
levels. UV radiation is a physical disinfectant that leaves no residual.
Therefore, it should be used only as a primary disinfectant followed by
a chemical secondary disinfectant to protect the distribution system
against coliform proliferation and biofilm formation. See Section 8.4,
Pathogen Inactivation and Disinfection Efficiency," for a detailed
discussion of disinfection efficiency and limitations.

The most common point of application for UV radiation is the last step
in the treatment process train just prior to the distribution system and
after filtration. The use of UV disinfection has no impact on other
processes at the water treatment facility.

## Pathogen Inactivation and Disinfection Efficiency

As opposed to most alternative disinfectants, UV is a physical process
that requires a contact time on the order of seconds to accomplish
pathogen inactivation (Sobotka, 1993). As with any disinfectant, UV has
its limitations. For example, because it is a physical rather than a
chemical disinfectant, it does not provide a residual to control
pathogen proliferation and biofilm formation in the distribution system.
When using UV for primary disinfection, some form of secondary chemical
disinfectant is required to maintain water quality within the
distribution system.

### Inactivation Mechanism

UV radiation is efficient at inactivating vegetative and sporous forms
of bacteria, viruses, and other pathogenic microorganisms.
Electromagnetic radiation in the wavelengths ranging from 240 to 280
nanometers (nm) effectively inactivates microorganisms by irreparably
damaging their nucleic acid. The most potent wavelength for damaging
deoxyribonucleic acid (DNA) is approximately 254 nm (Wolfe, 1990). Other
UV wavelengths, such as 200 nm, have been shown to exhibit peak
absorbance in aqueous solutions of DNA (von Sonntag and Schuchmann,
1992); however, there is no practical application for UV inactivation of
microorganisms in the wavelength range from 190 to 210 nm (USEPA, 1996).

The germicidal effects of UV light involve photochemical damage to RNA
and DNA within the microorganisms. Microorganism nucleic acids are the
most important absorbers of light energy in the wavelength of 240 to 280
nm (Jagger, 1967). DNA and RNA carry genetic information necessary for
reproduction; therefore, damage to either of these substances can
effectively sterilize the organism. Damage often results from the
dimerization of pyrimidine molecules. Cystosine (found in both DNA and
RNA), thymine (found only in DNA), and uracil (found only in RNA) are
the three primary types of pyrimidine molecules. Replication of the
nucleic acid becomes very difficult once the pyrimidine molecules are
bonded together due to the distortion of the DNA helical structure by UV
radiation (Snider et al., 1991). Moreover, if replication does occur,
mutant cells that are unable to replicate will be produced (USEPA,
1996). Figure 8-2 is a schematic of the germicidal inactivation observed
with UV radiation.

Two phenomena of key importance when using UV disinfection in water
treatment are the dark repair mechanisms and the capability of certain
organisms to photoreactivate following exposure to certain light
wavelengths. Under certain conditions, some organisms are capable of
repairing damaged DNA and reverting back to an active state in which
reproduction is again possible. Typically, photoreactivation occurs as a
consequence of the catalyzing effects of sunlight at visible wavelengths
outside of the effective disinfecting range. The extent of reactivation
varies among organisms. Coliform indicator organisms and some bacterial
pathogens such as *Shigella* have exhibited the photoreactivation
mechanism; however, viruses (except when they have infected a host cell
that is itself photoreactive) and other types of bacteria cannot
photoreactivate (USEPA, 1980; USEPA, 1986; Hazen and Sawyer, 1992).
Because DNA damage tends to become irreversible over time, there is a
critical period during which photoreactivation can occur. To minimize
the effect of photoreactivation, UV contactors should be designed to
either shield the process stream or limit the exposure of the
disinfected water to sunlight immediately following disinfection.

[]{#_Ref395322098
.anchor}![](media/image2.png){width="6.120138888888889in"
height="4.179861111111111in"}

Source: Tchobanoglous, 1997.

Figure 8-2. Germicidal Inactivation by UV Radiation

### Environmental Effects

To achieve inactivation, UV should be absorbed into the microorganism.
Therefore, anything that prevents UV from reacting with the
microorganism will decrease the disinfection efficiency. Scheible and
Bassell (1981) and Yip and Konasewich (1972) reported that pH had no
effect on UV disinfection. Several factors that are known to affect
disinfection efficiency of UV are:

-   Chemical and biological films that develop on the surface of UV
    lamps;

-   Dissolved organics and inorganics;

-   Clumping or aggregation of microorganisms;

-   Turbidity;

-   Color; and

-   Short-circuiting in water flowing through the UV contactor.

#### Chemical Films and Dissolved Organics and Inorganics

Accumulation of solids onto the surface of the UV sleeves can reduce the
applied UV intensity and, consequently, disinfection efficiency. In
addition to biofilms caused by organic material, buildup of calcium,
magnesium, and iron scales have been reported (DeMers and Renner, 1992).
Waters containing high concentrations of iron, hardness, hydrogen
sulfide, and organics are more susceptible to scaling or plating (i.e.,
the formation of a thin coat on unit surfaces), which gradually
decreases the applied UV intensity. Scaling is likely to occur if
dissolved organics are present and inorganic concentrations exceed the
following limits (DeMers and Renner, 1992):

-   Iron greater than 0.1 mg/L;

-   Hardness greater than 140 mg/L; and

-   Hydrogen sulfide greater than 0.2 mg/L.

Figure 8-3 shows the UV dose required for inactivation of MS-2 coliphage
at two pilot plants. Snicer et al. (1996) concluded that one possible
explanation for higher UV dose for the same degree of inactivation
required at pilot plant 2 could be the amount of scaling caused by
higher iron concentrations experienced at this plant. Iron
concentrations at pilot plant 2 were in the range of 0.45 to 0.65 mg/L,
which exceed the limit shown above.

A variety of chemical substances can decrease UV transmission (Yip and
Konasewich, 1972), including humic acids, phenolic compounds, and lignin
sulfonates (Snider et al., 1991), as well as chromium, cobalt, copper,
and nickel. It has been reported that coloring agents, such as Orzan S,
tea, and extract of leaves reduce intensity within a UV contactor (Huff,
1965). In addition, iron, sulfites, nitrites, and phenols can absorb UV
(DeMers and Renner, 1992).

[]{#_Ref395322125
.anchor}![](media/image3.wmf){width="6.241666666666666in"
height="4.274305555555555in"}

Source: Snicer et al., 1996.

Figure 8-3. UV Dose Required for Inactivation of MS-2 Coliphage

#### Microorganism Clumping and Turbidity

Particles can affect the disinfection efficiency of UV by harboring
bacteria and other pathogens, partially protecting them from UV
radiation, and scattering UV light (see Figure 8-4). Typically, the low
turbidity of ground water results in minimal impact on disinfection
efficiency. However, the higher turbidities of surface water can impact
disinfection efficiency.

Similar to particles that cause turbidity, microorganism aggregation can
impact disinfection efficiency by harboring pathogens within the
aggregates and shade pathogens that would otherwise be inactivated.

#### Reactor Geometry and Short Circuiting

Poor geometry within the UV contactor (which creates spacing between
lamps) can leave dead areas where inadequate disinfection occurs (Hazen
and Sawyer, 1992). A key consideration to improving disinfection is to
minimize the amount of dead spaces where limited UV exposure can occur.
Plug flow conditions should be maintained in the contactor; however,
some turbulence should be created between the lamps to provide radial
mixing of flow. In this manner, flow can be uniformly distributed
through the varying regions of UV intensity, allowing exposure to the
full range of available UV radiation (Hazen and Sawyer, 1992).

As mentioned earlier, UV systems typically provide contact times on the
order of seconds. Therefore, it is extremely important that the system
configuration limit the extent of short circuiting.

![](media/image4.png){width="4.0125in" height="4.425694444444445in"}

Source: Tchobanoglous, 1997.

[]{#__RefHeading___Toc450037395 .anchor}Figure 8-4. Particle
Interactions that Impact UV Effectiveness

### Disinfection Efficacy 

UV disinfection has been determined to be adequate for inactivating
bacteria and viruses. Most bacteria and viruses require relatively low
UV dosages for inactivation, typically in the range of 2 to 6 mW⋅s/cm^2^
for 1-log inactivation. Protozoan \[oo\]cysts, in particular *Giardia*
and *Cryptosporidium*, are considerably more resistant to UV
inactivation than other microorganisms. Results of several studies
investigating the ability of UV to inactivate bacteria, viruses, and
protozoa, are described in the following sections.

#### Bacteria and Virus Inactivation 

UV doses required for bacteria and virus inactivation are relatively
low. One study determined that UV was comparable to chlorination for
inactivation of heterotrophic plate count bacteria following treatment
using granular activated carbon (Kruithof et al., 1989).

A study of the ability of UV and free chlorine to disinfect a
virus-containing ground water showed that UV is a more potent virucide
than free chlorine, even after the chlorine residual was increased to
1.25 mg/L at a contact time of 18 minutes (Slade et al., 1986). The UV
dose used in this study was 25 mW⋅s/cm^2^.

Table 8-2 shows results from a more recent pilot plant study (Snicer et
al.*,* 1996). As indicated by the different UV doses to obtain the same
level of inactivation, water characteristics dramatically impact
disinfection efficiency. They believed that the higher concentration of
iron in pilot plant 2 water (Figure 8-3) interfered with UV or
influenced the aggregation of MS-2 viral particles.

Snicer et al. (1996) also compared the susceptibility of MS-2 coliphage
to hepatitis A virus, poliovirus, and rotavirus for 10 ground water
sources. Results indicated that MS-2 was found to be approximately 2 to
3 times more resistant to UV disinfection than the three human
pathogenic viruses.

[]{#__RefHeading___Toc450037390 .anchor}Table 8-2. Doses Required for
MS-2 Inactivation

+-----------------------+-----------------------+-----------------------+
| **Log MS-2            | **Pilot Plant 1**     | **Pilot Plant 2**     |
| Inactivation**        |                       |                       |
|                       | **(mW•s/cm^2^)**      | **(mW•s/cm^2^)**      |
+-----------------------+-----------------------+-----------------------+
| 1                     | 3.9                   | 15.3                  |
+-----------------------+-----------------------+-----------------------+
| 2                     | 25.3                  | 39.3                  |
+-----------------------+-----------------------+-----------------------+
| 3                     | 46.7                  | 63.3                  |
+-----------------------+-----------------------+-----------------------+
| 4                     | 68                    | 87.4                  |
+-----------------------+-----------------------+-----------------------+
| 5                     | 89.5                  | 111.4                 |
+-----------------------+-----------------------+-----------------------+
| 6                     | 111.0                 | 135.5                 |
+-----------------------+-----------------------+-----------------------+

Source: Snicer et al., 1996.

#### Protozoa Inactivation

Even though protozoa were once considered resistant to UV radiation,
recent studies have shown that ultraviolet light is capable of
inactivating protozoan parasites. However, results indicate that these
organisms require a much higher dose than that needed to inactivate
other pathogens. Less than 80 percent of *Giardia lamblia* cysts were
inactivated at UV dosages of 63 mW·s/cm^2^ (Rice and Hoff, 1981). A
1-log inactivation of *Giardia muris* cysts was obtained when the UV
dose was increased to 82 mW·s/cm^2^ (Carlson et al., 1982).

To achieve 2-log inactivation of *Giardia muris* cysts, a minimum
ultraviolet light dose of above 121 mW·s/cm^2^ is needed. Karanis et al.
(1992) examined the disinfection capabilities of ultraviolet light
against *Giardia lamblia* cysts extracted from both animals and humans
(Karanis et al., 1992). Both groups suffered a 2-log reduction at UV
doses of 180 mW·s/cm^2^. Two important factors to consider when
determining dose requirements for *Giardia* inactivation are the
parasite source and the growth stage of the microorganism (Karanis et
al., 1992). Figure 8-5 shows that the source of the parasites is
important in determining dose requirements. Figure 8-6 is from a study
conducted in 1992 on *Acanthamoeba rhysodes* inactivation (Karanis et
al., 1992). These data show that the age of the protozoa can
dramatically affect the dose required to achieve a desired level of
inactivation.

[]{#_Ref396096782 .anchor}![](media/image5.wmf){width="5.56875in"
height="3.5027777777777778in"}

Source: Karanis et al., 1992.

Figure 8-5. UV Doses Required to Achieve Inactivation of *Giardia
lamblia* Cysts Obtained from Two Different Sources

[]{#_Ref396096794
.anchor}![](media/image6.wmf){width="5.672916666666667in"
height="3.7555555555555555in"}

Source: Karanis et al., 1992.

Figure 8-6. Impact of Growth Stage of *A. rhysodes* on the Required UV
Dosage to Achieve Inactivation

Results from recent studies show a potential for inactivating
*Cryptosporidium parvum* oocysts using ultraviolet light disinfection. A
2 to 3-log reduction in the viability of *Cryptosporidium parvum*
oocysts was achieved using a low-pressure ultraviolet light system with
a theoretical minimum intensity of 14.58 mW/cm^2^ and a contact time of
10 minutes (ultraviolet dose of 8,748 mW·s/cm^2^) (Campbell et al.,
1995). The combination filter-UV system described by Johnson (1997) is
capable of delivering doses as high as 8,000 mW·s/cm^2^, sufficient to
achieve 2-log *Cryptosporidium* oocyst inactivation.

A pulsed UV process that delivered a minimum dose of 1,900 mW⋅s/cm^2^ to
any particle within the reactor was found to achieve *Cryptosporidium*
oocyst inactivation levels in the range of 2-log. (Clancy et al., 1997).
In this study, the reactor residence time was 4.7 seconds and the unit
was operated to deliver 46.5 pulses per volume (10 Hz). Each pulse
transfers power at the intensity rate of about 41 mW⋅s/cm^2^ .

#### UV Dose Summary

UV radiation is considered effective for inactivating bacterial and
viral pathogens. UV doses for 2 and 3-log inactivation of viruses are 21
and 36 mW⋅s/cm^2^, respectively (AWWA, 1991). These doses are based on
studies of hepatitis A virus inactivation and were derived by applying a
safety factor of 3 to the inactivation data. Results from a more recent
study on several ground water sources of hepatitis A virus, indicate
that a 6 to 15 mW⋅s/cm^2^ dose is required for a 4-log inactivation
(Snicer et al., 1996). A safety factor was not applied to the doses. A
4-log inactivation of bacteriophage MS-2 is achieved at a dose of 93
mW⋅s/cm^2^ (Snicer et al., 1996). In ground water containing high iron
levels (0.65 ppm), applying a safety factor of 1.5 to the highest
reported dose against viruses will yield a UV dose of 140 mWs/cm^2^ for
a minimum 4-log inactivation of viruses

In summary, ultraviolet radiation is effective against bacteria and
viruses at low dosages. However, much higher dosages are required for
*Cryptosporidium* and *Giardia* inactivation.

## Disinfection Byproducts of UV Radiation

Unlike other disinfectants, UV does not inactivate microorganisms by
chemical reaction. However, UV radiation causes a photochemical reaction
in the organism RNA and DNA. The literature suggests that UV radiation
of water can result in the formation of ozone or radical oxidants (Ellis
and Wells, 1941; Murov, 1973). Because of this reaction, there is
interest in determining whether UV forms similar byproducts to those
formed by ozonation or advanced oxidation processes.

### Ground Water

Malley et al. (1995) analyzed 20 ground water samples for aldehydes and
ketones before and after UV radiation. Only one ground water sample,
which contained 24 mg/L non-purgeable DOC and was highly colored,
contained DBPs after exposure to UV. Low levels of formaldehyde were
measure in duplicated experiments for this UV treated ground water
sample mentioned above. GC-ECD chromatographs before and after UV
radiation for the other 19 ground waters studied showed significant
shifts or unknown peaks after exposure to UV.

Malley et al. (1995) also determined the influence of UV on DBP
formation during subsequent chlorination. To examine these effects, the
20 ground water samples were subjected to simulated distribution system
(SDS) DBP tests with chlorine, before and after UV radiation. The data
indicate that UV radiation did not significantly alter the SDS/DBP
formation by chlorine in the ground waters studied.

To examine the effects of varying UV dosages on DBP formation, six new
ground water samples (Malley et al.*,* 1995) were subjected to UV
dosages of 60, 130, and 200 mW⋅s/cm^2^. In this case, DBPs were not
formed by UV radiation for any of the ground waters tested at any of the
UV dosages. A comparison of chromatographs for samples before and after
UV radiation, and for each UV dosage, showed no significant differences
or appearances of unknown peaks.

### Surface Water

UV radiation was found to produce low levels of formaldehyde in the
majority of surface waters studied (Malley et al., 1995). The highest
formaldehyde concentrations, ranging up to 14 μg/L, were observed in UV
treatment of raw water, whereas trace levels (1 to 2 μg/L) were found in
UV treatment of conventionally treated water. Since formaldehyde
formation was also observed for one of the ground water samples, it
appears that UV radiation of waters containing humic matter (i.e., color
producing, UV absorbing organic macromolecules) will result in low
levels of formaldehyde formation. Chromatographic examination of the
surface water samples before and after UV radiation showed no other
significant changes in the GC-ECD chromatograms.

Because of the chlorine demands of surface waters, higher chlorine
dosages were required for post disinfection following UV radiation. This
resulted in larger DBP concentrations than in the ground waters studied
(Malley et al.*,* 1995). However, the overall effect of UV radiation on
SDS/DBPs was insignificant. As in the ground water studies, UV radiation
did not significantly alter the total concentration or the speciation of
the disinfection byproducts (e.g., THMs, HAA5, HANs, or HKs).

### DBP Formation with Chlorination and Chloramination following UV Radiation

Research results suggest that UV radiation does not directly form DBPs
or alter the concentration or species of DBPs formed by
post-disinfection (Malley et al.*,* 1995). However, the question of
whether UV radiation influences the rate of DBP formation by
post-disinfection is important. Several studies have addressed this
question. Two surface waters that produced significant concentrations of
a wide variety of DBPs in previous tests were chosen as samples. With
the chlorine residuals carefully monitored to ensure they were
consistent for pre-UV and post-UV samples, the results of the
experiments suggested that UV radiation did not significantly affect the
rate of DBP formation.

Studies were only performed to determine the pentane extractable DBP
formation rate of a surface water sample for varying pH conditions. The
results showed that UV radiation did not affect the rate of chloroform
formation at pH 8.0 (Malley et al.*,* 1995). Similarly, UV did not
affect the DBP formation rate at pH 5.0. At pH 8.0, chloroform was the
only pentane extractable detected, whereas at pH 5.0, chloroform,
bromodichloromethane, chlorodibromomethane, and 1,1,1-trichloroacetone
were formed.

The effects of UV radiation on the DBP formation rate following
chloramination were also tested in this study using a surface water
sample (Malley et al.*,* 1995). Chloroform, trichloroacetic acid,
dichloroacetonitrile (at low levels), and cyanogen chloride (at low
levels) were the only detectable DBPs. Chloroform was the only compound
formed at pH 8.0, and its rate of formation was not affected by UV
radiation. At pH 5.0, chloroform and dichloroacetonitrile were formed,
but their rate of formation was unaffected by UV radiation. Data showed
that the effects of UV radiation on cyanogen chloride formation at pH
8.0 and pH 5.0 had no significant trends.

In summary, the DBP formation rate studies indicated that UV radiation
did not significantly affect DBP formation rates when chlorine or
chloramines were used as the post-disinfectant.

## Status of Analytical Methods

Ultraviolet radiation intensity meter readings are taken to monitor the
output of the UV disinfection system. These readings, coupled with the
flow through the UV reactor, are used to determine the UV dosage
applied. UV radiation leaves no residual disinfectant behind to monitor.
Therefore, some form of secondary chemical disinfectant should be added
to protect the distribution system against coliform proliferation and
biofilm formation. Analytical methods for these chemical disinfectants
are discussed elsewhere in this report.

### Monitoring of Generated Ultraviolet Radiation

Ultraviolet intensity at 253.7 nm (the predominant wavelength emitted by
low pressure mercury vapor lamps) is the water quality parameter used to
monitor UV disinfection system output (Snider et al., 1991). The rate of
disinfection is directly related to the average intensity of the UV
light. Since UV intensity probes can only indicate UV intensity at a
single point, there is no practical way to measure the average intensity
of a UV system in the field by the operator.

The average intensity is dependent upon the three dimensional lamp
geometry. Scheible (1983) developed a mathematical model that calculates
the intensity at any point within the UV reactor. This Point Source
Summation (PSS) method is used to estimate the average intensity emitted
by any specific unit. UV disinfection system manufacturers use this
method to design the system. The single point UV intensity probe reading
is typically used for routine monitoring of the UV system only.

UV intensity sensors are typically photodiode sensors properly filtered
to monitor the lamp intensity in the germicidal range only (DeMers and
Renner, 1992). A minimum of two sensors (mounted near separate lamps)
located at the center of each lamp for each reactor is recommended as
part of the system controls and instrumentation. White (1992) recommends
installing the sensors in the wall of the disinfection chamber at the
point of greatest distance from the tube or tubes.

The UV sensors should continuously sense the UV intensity produced in
the bank of lamps. The sensor should be field calibrated to account for
lamp geometry. Each UV sensor installed should provide a "low UV output"
warning and a "low-low UV output" alarm indication that is field
adjustable.

#### Ultraviolet Sensor Performance

To test electronic sensor performance, Snicer et al. (1996) placed a
single electronic UV sensor at the center of a UV reactor. The UV sensor
converted the UV energy into an electronic signal, which was used to
indicate system performance. Initial results indicated that the sensor
originally installed with the two UV reactors tended to wear or degrade
in performance over time. The original sensor readings in the pilot
facility were erratic and had a general downward trend over 6 months of
operation. This loss in performance could not be attributed to UV lamp
aging. In addition, these readings did not correlate well to actual
system performance. After operation of these original sensors for 6
months, the manufacturer was consulted and a new type of sensor was
installed into both pilot facilities. The new sensor design specifically
addressed the problems encountered during the initial 6 months of the
study. These new proprietary sensors performed consistently and sensor
readings were shown to correlate well with actual MS-2 coliphage
inactivation.

### Disinfectant Interferences

Suspended solids may be the most important water quality parameter
impacting UV intensity measurements. Particles can harbor bacteria and
at least partially protect them from UV light. Particles can be
completely penetrated, partially penetrated, or scatter UV light (Figure
8-4). All particles in water may not absorb UV light. Qualls et al.
(1983) suggested that clays merely scatter UV light and, therefore, do
little to inhibit performance.

Yip and Konasewich (1972) listed many chemical substances that interfere
with UV transmission at 253.7 nm including phenolic compounds, humic
acids, and ferric iron.

## Operational Considerations

Onsite pilot plant testing is recommended to determine the efficiency
and adequacy of UV disinfection for a specific quality of water. The
efficiency test involves injecting select microorganisms into influent
water and sampling effluent water to determine survival rates. The
National Science Foundation's Standard 55 for ultraviolet water
treatment systems recommends that UV disinfection systems not be used if
the UV transmittance is less than 75 percent (NSF, 1991). If the raw
water UV transmittance is less that 75 percent, the UV system should be
proceeded by other treatment processes (to increase UV transmittance) or
a different disinfectant should be used.

As previously discussed, some constituents that adversely interfere with
UV disinfection performance by either scattering and/or absorbing
radiation are iron, chromium, copper, cobalt, sulfites, and nitrites.
Care should be taken with chemical processes upstream of UV disinfection
process to minimize increasing concentrations of these constituents
since disinfection efficiency may be adversely affected.

### Equipment Operation

UV disinfection facilities should be designed to provide flexibility in
handling varying flow rates. For lower flow rates, a single reactor
vessel should be capable of handling the entire flow rate. A second
reactor vessel with equal capacity of the first reactor vessel should be
provided for redundancy should the first reactor vessel be taken out of
service. For higher flow rates, multiple reactor vessels should be
provided with lead/lag operation and flow split capacity to balance run
time for each reactor vessel, if desired, and to avoid hydraulic
overloading. Valves should be provided within the interconnecting piping
to isolate one reactor vessel from another. There should also be a
positive drainage system to remove water from within a reactor vessel
when it is taken out of service.

#### UV Lamp Aging

The output of UV lamps diminishes with time. Two factors that affect
their performance are: solarization which is the effect UV radiation has
on the UV lamp that causes it to become opaque; and, electrode failure
which occurs when electrodes deteriorate progressively each time the UV
lamp is cycled on and off. Frequent lamp cycling will lead to premature
lamp aging. When determining the requirement for UV disinfection, a 30
percent reduction of UV output should be used to estimate end of lamp.
Average life expectancy for low pressure UV lamps is approximately 8,800
hours.

#### Quartz Sleeve Fouling

Fouling of the quartz sleeve reduces the amount of UV radiation reaching
the water. The quartz sleeve has a transmissibility of over 90 percent
when new and clean. Over time, the surface of the quartz sleeve that is
in contact with the water starts collecting organic and inorganic debris
(e.g., iron, calcium, silt) causing a reduction in transmissibility
(USEPA, 1996). When determining the requirements for UV disinfection, a
30 percent reduction of UV transmission should be used to reflect the
effect of quartz sleeve fouling.

### Equipment Maintenance

#### UV Lamp Replacement

Adequate space should be provided around the perimeter of the reactor
vessels to allow access for maintenance and replacement of UV lamps.
With modular electrical fittings, lamp replacement consists of
unplugging the pronged connection of the old lamp and plugging in the
new.

#### Quartz Sleeve Cleaning

Quartz sleeve cleaning may be accomplished by physical or chemical
means. Physical alternatives include:

-   Automatic mechanical wiper;

-   Ultrasonic devices;

-   High water pressure wash; and

-   Air scour.

Chemical cleaning agents include sulfuric or hydrochloric acid. A UV
reactor vessel may contain one or more physical cleaning system with
provision for an occasional chemical cleaning.

#### Miscellaneous

Effective maintenance of a UV system will involve:

-   Periodic checks for proper operation;

-   Calibration of intensity meter for proper sensitivity; and

-   Inspect and/or clean reactor vessel interior.

### Standby Power

Producing UV radiation requires electricity to power the electronic
ballasts, which in turn power the UV lamps. Since disinfection is of
utmost importance in producing potable water, the UV system should
remain in service during periods of primary power failure. A dual power
feed system or essential circuitry powered by a standby generator are
typical ways to achieve the desired reliability. Each low pressure UV
lamp requires approximately 100 Watts of standby power. A second
precaution that should be considered is not powering the UV system from
the same motor control center (MCC) that powers variable frequency
drives (VFDs). The electronic ballasts produce harmonics that may
require mitigation (active harmonic filters) for the VFDs.

## Summary Table

[]{#__RefHeading___Toc450037391 .anchor}Table 8-3. Summary of UV
Disinfection

  ----------------------- -----------------------------------------------
  **Consideration**       **Description**

  Generation              Low pressure and medium pressure UV lamps
                          available.

  Primary uses            Primary physical disinfectant; requires
                          secondary chemical disinfectant for residual in
                          distribution system.

  Inactivation efficiency Very effective against bacteria and viruses at
                          low dosages (5-25 mW•s/cm^2^ for 2-log removal
                          and 90-140 mW•s/cm^2^ for 4-log removal). Much
                          higher dosage required for *Cryptosporidium*
                          and *Giardia* (100-8,000 mW•s/cm^2^ for 2-log
                          removal).

  Byproduct formation     Minimal disinfection byproducts produced.

  Limitations             Limited experience and data with flows greater
                          than 200 GPM. Water with high concentrations of
                          iron, calcium, turbidity, and phenols may not
                          be applicable to UV disinfection.

  Point of application    Prior to distribution system.

  Special considerations  Extremely high UV dosages for *Cryptosporidium*
                          and *Giardia* may make surface water treatment
                          impractical.
  ----------------------- -----------------------------------------------

## References

1.  AWWA (American Water Works Association). 1991. *Guidance Manual for
    Compliance with the Filtration and Disinfection Requirements for
    Public Water Systems Using Surface Water Sources*.

2.  AWWA and ASCE (American Society of Civil Engineers). 1990. *Water
    Treatment Plant Design.* Second edition, McGraw-Hill, Inc., New
    York, NY.

3.  Campbell, A.T., et al. 1995. "Inactivation of Oocysts of
    Cryptosporidium parvum by Ultraviolet Radiation." *Water Res.*
    29(11):2583.

4.  Carlson, D. A., et al. 1982. *Project Summary: Ultraviolet
    Disinfection of Water for Small Water Supplies.* Office of Research
    and Development, U.S. Environmental Protection Agency; Cincinnati,
    OH, EPA/600/S2-85/092.

5.  Clancy, J.L., T.M. Hargy, M.M. Marshall, and J.E. Dyksen. 1997.
    "Inactivation of *Cryptosporidium parvum* Oocysts in Water Using
    Ultraviolet Light." Conference proceedings, AWWA International
    Symposium on *Cryptosporidium* and Cryptosporidiosis, Newport Beach,
    CA.

6.  Combs, R., and P. McGuire. 1989. "Back to Basics - The Use of
    Ultraviolet Light for Microbial Control." *Ultrapure Water Journal*.
    6(4):62-68.

7.  DeMers, L.D. and R.C. Renner. 1992. *Alternative Disinfection
    Technologies For Small Drinking Water Systems.* AWWARF.

8.  Ellis, C., and A.A. Wells. 1941. *The Chemical Action of Ultraviolet
    Rays*. Reinhold Publishing Co., New York, NY.

9.  Hazen and Sawyer. 1992. *Disinfection Alternatives for Safe Drinking
    Water.* Van Nostrand Reinhold, New York, NY.

10. Huff, C. B. 1965. "Study of Ultraviolet Disinfection of Water and
    Factors in Treatment Efficiency." *Public Health Reports*.
    80(8):695-705.

11. Jagger, J. 1967. *Introduction to Research in Ultraviolet
    Photobiology.* Prentice-Hall Inc., Englewood Cliffs, NJ.

12. Johnson, R.C. 1997. "Getting the Jump on Cryptosporidium with UV."
    *Opflow*. 23(10):1.

13. Karanis, P., et al. 1992. "UV Sensitivity of Protozoan Parasites."
    *J. Water Supply Res. Technol. Aqua*. 41(2):95.

14. Kruithof, J.C., et al. 1989. Summaries, *WASSER BERLIN '89*;
    International Ozone Association, European Committee, Paris.

15. Malley Jr., J.P, J.P. Shaw, and J.R. Ropp. 1995. " Evaluations of
    Byproducts by Treatment of Groundwaters With Ultraviolet
    Irradiation*.*" AWWARF and AWWA, Denver, CO.

16. Murov, S.L. 1973. *Handbook of Photochemistry*. Marcel Dekker, New
    York, NY.

17. NSF (National Science Foundation). 1991. *NSF Standard 55:
    Ultraviolet Water Treatment Systems*. National Sanitation
    Foundation, Ann Arbor, MI.

18. Qualls, R., Flynn, M., and Johnson, J. 1983. "The Role of Suspended
    Particles in Ultraviolet Disinfection." *J. Water Pollution Control
    Fed.* 55(10):1280-1285.

19. Rice, E.W. and J.C. Hoff. 1981. "Inactivation of *Giardia lamblia*
    Cysts by Ultraviolet Irradiation." *Appl. Environ. Microbiol.*
    42:546-547.

20. Scheible, O.K. and C.D. Bassell. 1981. *Ultraviolet Disinfection Of
    A Secondary Wastewater Treatment Plant Effluent.* EPA-600/2-81-152,
    PB81-242125, U.S. Environmental Protection Agency; Cincinnati, OH.

21. Scheible, O.K. 1983. "Design and Operation of UV Systems." Presented
    at Water Pollution Control Federation Annual Conference, Cincinnati,
    OH.

22. Slade, J. S., N.R. Harris, and R.G. Chisholm. 1986. "Disinfection of
    Chlorine Resistant Enteroviruses in Ground Water by Ultraviolet
    Radiation." *Water Sci. Technol.* 189(10):115-123.

23. Snicer, G.A., J.P. Malley, A.B. Margolin, and S.P. Hogan. 1996.
    "Evaluation of Ultraviolet Technology in Drinking Water Treatment."
    Presented at AWWA Water Quality Technology Conference, Boston, MA.

24. Snider, K.E., J.L. Darby, and G. Tchobanoglous. 1991. *Evaluation of
    Ultraviolet Disinfection For Wastewater Reuse Applications In
    California.* Department of Civil Engineering, University of
    California, Davis.

25. Sobotka, J. 1993. "The Efficiency of Water Treatment and
    Disinfection by Means of Ultraviolet Radiation." *Water Sci.
    Technol.* 27(3-4):343-346.

26. Tchobanoglous, G.T. 1997. "UV Disinfection: An Update." Presented at
    Sacramento Municipal Utilities District Electrotechnology Seminar
    Series. Sacramento, CA.

27. USEPA (U.S. Environmental Protection Agency). 1996. *Ultraviolet
    Light Disinfection Technology in Drinking Water Application - An
    Overview.* EPA 811-R-96-002, Office of Ground Water and Drinking
    Water.

28. USEPA. 1986. *Design Manual: Municipal Wastewater Disinfection.*
    EPA/625/1-86/021, Office of Research and Development, Water
    Engineering Research Laboratory, Center for Environmental Research
    Information, Cincinnati, OH

29. USEPA. 1980. *Technologies for Upgrading Existing and Designing New
    Drinking Water Treatment Facilities.* EPA/625/4-89/023, Office
    Drinking Water.

30. Von Sonntag, C. and H. Schuchmann. 1992. "UV Disinfection of
    Drinking Water and By-Product Formation - Some Basic
    Considerations." *J. Water SRT-Aqua.* 41(2):67-74.

31. White, G.C. 1992. *Handbook of Chlorination and Alternative
    Disinfectants.* Van Nostrand Reinhold, New York, NY.

32. Wolfe, R.L. 1990. "Ultraviolet Disinfection of Potable Water."
    *Environ. Sci. Tech.* 24(6):768-773.

33. Yip, R.W. and D.E. Konasewich. 1972. "Ultraviolet Sterilization Of
    Water - Its Potential And Limitations." *Water Pollut. Control*
    (*Canada*). 14:14-18.

THIS PAGE INTENTIONALLY LEFT BLANK

8\. Ultraviolet Radiation [8-1](#ultraviolet-radiation)

8.1 UV Chemistry (Photochemical) [8-1](#uv-chemistry-photochemical)

8.1.1 UV Radiation [8-1](#uv-radiation)

8.1.2 UV Disinfection Reactions [8-2](#uv-disinfection-reactions)

8.1.3 Process Variables [8-2](#process-variables)

8.2 Generation [8-3](#generation)

8.2.1 UV Lamps [8-4](#uv-lamps)

8.2.2 Ballasts [8-4](#ballasts)

8.2.3 UV Reactor Design [8-5](#uv-reactor-design)

8.3 Primary Uses and Points of Application
[8-8](#primary-uses-and-points-of-application)

8.4 Pathogen Inactivation and Disinfection Efficiency
[8-8](#pathogen-inactivation-and-disinfection-efficiency)

8.4.1 Inactivation Mechanism [8-8](#inactivation-mechanism)

8.4.2 Environmental Effects [8-10](#environmental-effects)

8.4.3 Disinfection Efficacy [8-12](#disinfection-efficacy)

8.5 Disinfection Byproducts of UV Radiation
[8-15](#disinfection-byproducts-of-uv-radiation)

8.5.1 Ground Water [8-15](#ground-water)

8.5.2 Surface Water [8-16](#surface-water)

8.5.3 DBP Formation with Chlorination and Chloramination following UV
Radiation
[8-16](#dbp-formation-with-chlorination-and-chloramination-following-uv-radiation)

8.6 Status of Analytical Methods [8-17](#status-of-analytical-methods)

8.6.1 Monitoring of Generated Ultraviolet Radiation
[8-17](#monitoring-of-generated-ultraviolet-radiation)

8.6.2 Disinfectant Interferences [8-18](#disinfectant-interferences)

8.7 Operational Considerations [8-18](#operational-considerations)

8.7.1 Equipment Operation [8-19](#equipment-operation)

8.7.2 Equipment Maintenance [8-19](#equipment-maintenance)

8.7.3 Standby Power [8-20](#standby-power)

8.8 Summary Table [8-21](#summary-table)

8.9 References [8-21](#references)

Table 8-1. Water Quality and Associated UV Measurements
[8-3](#__RefHeading___Toc450037389)

Table 8-2. Doses Required for MS-2 Inactivation
[8-13](#__RefHeading___Toc450037390)

Table 8-3. Summary of UV Disinfection
[8-21](#__RefHeading___Toc450037391)

Figure 8-1. Closed Vessel Ultraviolet Reactor [8-6](#_Ref396096477)

Figure 8-2. Germicidal Inactivation by UV Radiation
[8-9](#_Ref395322098)

Figure 8-3. UV Dose Required for Inactivation of MS-2 Coliphage
[8-11](#_Ref395322125)

Figure 8-4. Particle Interactions that Impact UV Effectiveness
[8-12](#__RefHeading___Toc450037395)

Figure 8-5. UV Doses Required to Achieve Inactivation of *Giardia
lamblia* Cysts Obtained from Two Different Sources
[8-14](#_Ref396096782)

Figure 8-6. Impact of Growth Stage of *A. rhysodes* on the Required UV
Dosage to Achieve Inactivation [8-14](#_Ref396096794)