# ![](media/image1.wmf)Ozone

Ozone was first used for drinking water treatment in 1893 in the
Netherlands. While being used frequently in Europe for drinking water
disinfection and oxidation, it was slow to transfer to the United
States. In 1987, the Los Angeles Aqueduct Filtration Plant was placed in
service and now treats up to 600 mgd of drinking water. In 1991,
approximately 40 water treatment plants each serving more than 10,000
people in the United States utilized ozone (Langlais et al., 1991). This
number has grown significantly, with Rice (in press) reporting that as
of April 1998, 264 operating plants in the United States use ozone. Most
of these facilities are small: 149 plants are below 1 mgd.

Ozone is used in water treatment for disinfection and oxidation. Early
application of ozone in the United States was primarily for
non-disinfection purposes such as color removal or taste and odor
control. However, since the implementation of the SWTR and proposal of
the DBP rule, ozone usage for primary disinfection has increased in the
United States.

## Ozone Chemistry

Ozone exists as a gas at room temperature. The gas is colorless with a
pungent odor readily detectable at concentrations as low as 0.02 to 0.05
ppm (by volume), which is below concentrations of health concern. Ozone
gas is highly corrosive and toxic.

Ozone is a powerful oxidant, second only to the hydroxyl free radical,
among chemicals typically used in water treatment. Therefore, it is
capable of oxidizing many organic and inorganic compounds in water.
These reactions with organic and inorganic compounds cause an ozone
demand in the water treated, which should be satisfied during water
ozonation prior to developing a measurable residual.

Ozone is sparingly soluble in water. At 20^o^C, the solubility of 100
percent ozone is only 570 mg/L (Kinman, 1975). While ozone is more
soluble than oxygen, chlorine is 12 times more soluble than ozone. Ozone
concentrations used in water treatment are typically below 14 percent,
which limits the mass transfer driving force of gaseous ozone into the
water. Consequently, typical concentrations of ozone found during water
treatment range from \<0.1 to 1mg/L, although higher concentrations can
be attained under optimum conditions.

Basic chemistry research (Hoigné and Bader, 1983a and 1983b; Glaze et
al., 1987) has shown that ozone decomposes spontaneously during water
treatment by a complex mechanism that involves the generation of
hydroxyl free radicals. The hydroxyl free radicals are among the most
reactive oxidizing agents in water, with reaction rates on the order of
10^10^ - 10^13^ M^-1^ s^-1^, approaching the diffusion control rates for
solutes such as aromatic hydrocarbons, unsaturated compounds, aliphatic
alcohols, and formic acid (Hoigné and Bader, 1976). On the other hand,
the half-life of hydroxyl free radicals is on the order of microseconds,
therefore concentrations of hydroxyl free radicals can never reach
levels above 10 ^--12^ M (Glaze and Kang, 1988).

As shown in Figure 3-1 ozone can react by either or both modes in
aqueous solution (Hoigné and Bader, 1977):

-   Direct oxidation of compounds by molecular ozone (O~3(aq)~).

-   Oxidation of compounds by hydroxyl free radicals produced during the
    decomposition of ozone.

    ![](media/image2.wmf){width="6.240972222222222in"
    height="2.063888888888889in"}

[]{#__RefHeading___Toc450036688 .anchor}Figure 3-1. Oxidation Reactions
of Compounds (Substrate) During Ozonation of Water

The two oxidation pathways compete for substrate (i.e., compounds to
oxidize). The direct oxidation with aqueous ozone is relatively slow
(compared to hydroxyl free radical oxidation) but the concentration of
aqueous ozone is relatively high. On the other hand, the hydroxyl
radical reaction is fast, but the concentration of hydroxyl radicals
under normal ozonation conditions is relatively small. Hoigné and Bader
(1977) found that:

-   Under acidic conditions, the direct oxidation with molecular ozone
    is of primary importance; and

-   Under conditions favoring hydroxyl free radical production, such as
    high pH, exposure to UV, or addition of hydrogen peroxide, the
    hydroxyl oxidation starts to dominate.

    This latter mechanism is used in advanced oxidation processes such
    as discussed in Chapter 7, Peroxone, to increase the oxidation rates
    of substrates.

    The spontaneous decomposition of ozone occurs through a series of
    steps. The exact mechanism and reactions associated have not been
    established, but mechanistic models have been proposed (Hoigné and
    Bader, 1983a and 1983b; Glaze, 1987). It is believed that hydroxyl
    radicals forms as one of the intermediate products, and can react
    directly with compounds in the water. The decomposition of ozone in
    pure water proceeds with hydroxyl free radicals produced as an
    intermediate product of ozone decomposition, resulting in the net
    production of 1.5 mole hydroxyl free radicals per mole ozone.

    In the presence of many compounds commonly encountered in water
    treatment, ozone decomposition forms hydroxyl free radicals. Ozone
    demands are associated with the following:

-   Reactions with natural organic matter (NOM) in the water. The
    oxidation of NOM leads to the formation of aldehydes, organic acids,
    and aldo- and ketoacids (Singer, 1992).

-   Organic oxidation byproducts. Organic oxidation byproducts are
    generally more amenable to biological degradation and can be
    measured as assimilable organic carbon (AOC) or biodegradable
    dissolved organic carbon (BDOC).

-   Synthetic organic compounds (SOCs). Some SOCs can be oxidized and
    mineralized under favorable conditions. To achieve total
    mineralization, hydroxyl radical oxidation should usually be the
    dominant pathway, such as achieved in advanced oxidation processes.

-   Oxidation of bromide ion. Oxidation of bromide ion leads to the
    formation of hypobromous acid, hypobromite ion, bromate ion,
    brominated organics, and bromamines (see Figure 3-2).

-   Bicarbonate or carbonate ions, commonly measured as alkalinity, will
    scavenge the hydroxyl radicals and form carbonate radicals
    (Staehelin et al., 1984; Glaze and Kang, 1988). These reactions are
    of importance for advanced oxidation processes where the radical
    oxidation pathway is predominant.

![](media/image3.png){width="6.4944444444444445in"
height="3.611111111111111in"}

[]{#_Ref401041178 .anchor}Source: Gunten and Hoigné, 1996.

Figure 3-2. Reaction of Ozone and Bromide Ion Can Produce Bromate Ion
and Brominated Organics

## Ozone Generation

### Ozone Production

Because ozone is an unstable molecule, it should be generated at the
point of application for use in water treatment. It is generally formed
by, combining an oxygen atom with an oxygen molecule (O~2~):

> 3*O~2~* ⇔ 2*O~3~*

This reaction is endothermic and requires a considerable input of
energy.

Schönbein (Langlais et \< biblio \>) first discovered synthetic ozone
through the electrolysis of sulfuric acid. Ozone can be produced several
ways, although one method, corona discharge, predominates in the ozone
generation industry. Ozone can also be produced by irradiating an
oxygen-containing gas with ultraviolet light, electrolytic reaction and
other emerging technologies as described by Rice (1996).

Corona discharge, also known as silent electrical discharge, consists of
passing an oxygen-containing gas through two electrodes separated by a
dielectric and a discharge gap. Voltage is applied to the electrodes,
causing an electron flowthrough across the discharge gap. These
electrons provide the energy to disassociate the oxygen molecules,
leading to the formation of ozone. Figure 3-3 shows a basic ozone
generator.

![](media/image4.png){width="5.559722222222222in"
height="3.6902777777777778in"}

[]{#__RefHeading___Toc450036690 .anchor}Figure 3-3. Basic Ozone
Generator

### System Components

As shown in Figure 3-4, ozone water treatment systems have four basic
components: a gas feed system, an ozone generator, an ozone contactor,
and an off-gas destruction system. The gas feed system provides a clean,
dry source of oxygen to the generator. The ozone contactor transfers the
ozone-rich gas into the water to be treated, and provides contact time
for disinfection (or other reactions). The final process step, off-gas
destruction, is required as ozone is toxic in the concentrations present
in the off-gas. Some plants include an off-gas recycle system that
returns the ozone-rich off-gas to the first contact chamber to reduce
the ozone demand in the subsequent chambers. Some systems also include a
quench chamber to remove ozone residual in solution.

![](media/image5.wmf)

Figure 3-4. Simplified Ozone System Schematic

#### Gas Feed Systems

Ozone feed systems are classified as using air, high purity oxygen or
mixture of the two. High purity oxygen can be purchased and stored as a
liquid (LOX), or it can be generated on-site through either a cryogenic
process, with vacuum swing adsorption (VSA), or with pressure swing
adsorption (PSA). Cryogenic generation of oxygen is a complicated
process and is feasible only in large systems. Pressure swing adsorption
is a process whereby a special molecular sieve is used under pressure to
selectively remove nitrogen, carbon dioxide, water vapor, and
hydrocarbons from air, producing an oxygen rich (80--95 percent O~2~)
feed gas. The components used in pressure swing adsorption systems are
similar to high pressure air feed systems in that both use pressure
swing molecular absorption equipment. Low pressure air feed systems use
a heat reactivated desiccant dryer.

***Oxygen Feed Systems*** - Liquid oxygen feed systems are relatively
simple, consisting of a storage tank or tanks, evaporators to convert
the liquid to a gas, filters to remove impurities, and pressure
regulators to limit the gas pressure to the ozone generators.

***Air Feed Systems*** - Air feed systems for ozone generators are
fairly complicated as the air should be properly conditioned to prevent
damage to the generator. Air should be clean and dry, with a maximum dew
point of -60º C (-80º F) and free of contaminants. Air preparation
systems typically consist of air compressors, filters, dryers, and
pressure regulators. Figure 3-5 is a schematic of large scale air
preparation system.

Particles greater than 1 μm and oil droplets greater than 0.05 μm should
be removed by filtration (Langlais et al., 1991). If hydrocarbons are
present in the feed gas, granular activated carbon filters should follow
the particulate and oil filters. Moisture removal can be achieved by
either compression or cooling (for large-scale system), which lowers the
holding capacity of the air, and by desiccant drying, which strips the
moisture from the air with a special medium. Desiccant dryers are
required for all air preparation systems. Large or small particles and
moisture cause arcing which damages generator dielectrics.

Typically, desiccant dryers are supplied with dual towers to allow
regeneration of the saturated tower while the other is in service.
Moisture is removed from the dryer by either an external heat source or
by passing a fraction (10 to 30 percent) of the dried air through the
saturated tower at reduced pressure. Formerly, small systems that
require only intermittent use of ozone , a single desiccant tower is
sufficient, provided that it is sized for regeneration during ozone
decomposition time.

Air preparation systems can be classified by the operating pressure:
ambient, low (less than 30 psig), medium, and high (greater than 60
psig) pressure. The distinguishing feature between low and high pressure
systems is that high pressure systems can use a heatless dryer. A
heatless dryer operates normally in the 100 psig range, rather than the
60 psig range. Rotary lobe, centrifugal, rotary screw, liquid ring,
vane, and reciprocating compressors can be used in air preparation
systems. Table 3-1 lists the characteristics of many of these types of
compressors.

Reciprocating and liquid ring compressors are the most common type used
in the United States, particularly in small systems, the former because
the technology is so prevalent and the latter because liquid ring
compressors do not need aftercoolers. Air receivers are commonly used to
provide variable air flow from constant volume compressors. Oil-less
compressors are used in modern systems to avoid hydrocarbons in the feed
gas (Dimitriou, 1990).

![](media/image6.wmf)

[]{#__RefHeading___Toc450036691 .anchor}Figure 3-5. Schematic of an Air
Preparation System

[]{#_Ref401114406 .anchor}

Table 3-1. Types of Compressors Used in Air Preparation Systems

  -------------- -------------- ---------------- --------------------------
  **Compressor   **Pressure**   **Volume**       **Comments**
  Type**                                         

  Rotary lobe    Low -- 15 psi  Constant or      Common in Europe
                                variable with    
                                unloading        

  Centrifugal    30 -- 100 psi  Variable, high   Medium efficiency, cost
                 depending on   volume           effective in high volumes
                 no. of stages                   

  Rotary Screw   50 psi (single Variable with    Slightly more efficient
                 stage) to 100  unloading        than rotary lobe, draws
                 psi (2 stage)                   approximately 40% of full
                                                 load power in unloaded
                                                 state, available in
                                                 non-lubricated design for
                                                 larger capacities.

  Liquid Ring    10--80 psi     Constant volume  Does not require
                                                 lubrication or
                                                 aftercooler, relatively
                                                 inefficient, common in
                                                 United States.

  Vane           High - to 100  Constant or      Relatively inefficient,
                 psi            variable         not common in U.S.
  -------------- -------------- ---------------- --------------------------

[]{#_Ref419264800 .anchor}Table 3-2 presents a comparison of the
advantages and disadvantages of each gas feed system.

Table 3-2. Comparison of Air and High Purity Oxygen Feed Systems

+-----------+----------------------------+----------------------------+
| *         | **Advantages**             | **Disadvantages**          |
| *Source** |                            |                            |
+===========+============================+============================+
| Air       | -   Commonly used          | -   More energy consumed   |
|           |     equipment              |     per ozone volume       |
|           |                            |     produced               |
|           | -   Proven technology      |                            |
|           |                            | -   Extensive gas handling |
|           | -   Suitable for small and |     equipment required     |
|           |     large systems          |                            |
|           |                            | -   Maximum ozone          |
|           |                            |     concentration of 3-5%  |
+-----------+----------------------------+----------------------------+
| Oxygen    | -   Higher ozone           | -   Safety concerns        |
| (general) |     concentration (8-14%)  |                            |
|           |                            | -   Oxygen resistant       |
|           | -   Approximately doubles  |     materials required     |
|           |     ozone concentration    |                            |
|           |     for same generator     |                            |
|           |                            |                            |
|           | -   Suitable for small and |                            |
|           |     large systems          |                            |
+-----------+----------------------------+----------------------------+
| LOX       | -   Less equipment         | -   Variable LOX costs     |
|           |     required               |                            |
|           |                            | -   Storage of oxygen      |
|           | -   Simple to operate and  |     onsite (Fire Codes,    |
|           |     maintain               |     i.e. safety concerns)  |
|           |                            |                            |
|           | -   Suitable for small and | -   Loss of LOX in storage |
|           |     intermediate systems   |     when not in use        |
|           |                            |                            |
|           | -   Can store excess       |                            |
|           |     oxygen to meet peak    |                            |
|           |     demands                |                            |
+-----------+----------------------------+----------------------------+
| Cryogenic | -   Equipment similar to   | -   More complex than LOX  |
| Oxygen    |     air preparation        |                            |
| G         |     systems                | -   Extensive gas handling |
| eneration |                            |     equipment required     |
|           | -   Feasible for large     |                            |
|           |     systems                | -   Capital intensive      |
|           |                            |                            |
|           | -   Can store excess       | -   Complex systems to     |
|           |     oxygen to meet peak    |     operate and maintain   |
|           |     demands                |                            |
+-----------+----------------------------+----------------------------+

#### Ozone Generators

The voltage required to produce ozone by corona discharge is
proportional to the pressure of the source gas in the generator and the
width of the discharge gap. Theoretically, the highest yield (ozone
produced per unit area of dielectric) would result from a high voltage,
a high frequency, a large dielectric constant, and a thin dielectric.
However, there are practical limitations to these parameters. As the
voltage increases, the electrodes and dielectric materials are more
subject to failure. Operating at higher frequencies produces higher
concentrations of ozone and more heat requiring increased cooling to
prevent ozone decomposition. Thin dielectrics are more susceptible to
puncturing during maintenance. The design of any commercial generator
requires a balance of ozone yield with operational reliability and
reduced maintenance.

Two different geometric configurations for the electrodes are used in
commercial ozone generators: concentric cylinders and parallel plates.
The parallel plate configuration is commonly used in small generators
and can be air cooled. Figure 3-6 shows the basic arrangement for the
cylindrical configuration. The glass dielectric/high voltage electrode
in commercial generators resembles a fluorescent light bulb and is
commonly referred to as a "generator tube."

Most of the electrical energy input to an ozone generator (about 85
percent) is lost as heat (Rice, 1996). Because of the adverse impact of
temperature on the production of ozone, adequate cooling should be
provided to maintain generator efficiency. Excess heat is removed
usually by water flowing around the stainless steel ground electrodes.
The tubes are arranged in either a horizontal or vertical configuration
in a stainless steel shell, with cooling water circulating through the
shell.

![](media/image7.png){width="6.190972222222222in"
height="2.933333333333333in"}

[]{#__RefHeading___Toc450036692 .anchor}Figure 3-6. Cylindrical
Electrode Schematic

Ozone generators are classified by the frequency of the power applied to
the electrodes. Low frequency (50 or 60 Hz) and medium frequency (60 to
1,000 Hz) generators are the most common found in the water industry,
however some high frequency generators are available. Table 3-3 presents
a comparison of the three types of generators. Medium frequency
generators are efficient and can produce ozone economically at high
concentrations, but they generate more heat than low frequency
generators and require a more complicated power supply to step up the
frequency supplied by utility power. New installations tend to use
medium or high frequency generators.

#### Ozone Contactors

Once ozone gas is transferred into water, the dissolved ozone reacts
with the organic and inorganic constituents, including any pathogens.
Ozone not transferred into the process water during contacting is
released from the contactor as off-gas. Transfer efficiencies of greater
than 80 percent typically are required for efficient ozone disinfection
(DeMers and Renner, 1992).

Common ozone dissolution methods include:

-   Bubble diffuser contactors;

-   Injectors; and

-   Turbine mixers.

[]{#__RefHeading___Toc450036678 .anchor}Table 3-3. Comparison of Primary
Characteristics of Low, Medium, and High Frequency Ozone Generators

  ----------------------- --------------- --------------- ---------------
  **Characteristic**      **Low           **Medium        **High
                          Frequency\      Frequency\      Frequency\
                          (50 - 60 Hz)**  (up to 1,000    (\> 1,000 Hz)**
                                          Hz)**           

  Degree of Electronics   low             high            high
  Sophistication                                          

  Peak Voltages           19.5            11.5            10

  Turndown Ratio          5:1             10:1            10:1

  Cooling Water Required  0.5 to 1.0      0.5 to 1.5      0.25 to 1
  (gal/lb of ozone                                        
  produced)                                               

  Typical Application     \< 500 lb/day   to 2,000 lb/day to 2,000 lb/day
  Range                                                   

  Operating               0.5 to 1.5%\    1.0 to 2.5%^+^\ 1.0 to 2.5%^+^\
  Concentrations\         2.0 to 5.0%     2 to 12%        2 to 12%
  wt - % in air\                                          
  wt - % in oxygen                                        

  Optimum Ozone           60 to 75%       90 to 95%       90 to 95%
  Production (as a                                        
  proportion of total                                     
  generator capacity)                                     

  Optimum Cooling Water   8° to 10°F      5° to 8°F       5° to 8°F
  Differential                                            

  Power Required (kW-h/lb air feed: 8 to  air feed: 8 to  air feed: 8 to
  O~3~)                   12\             12\             12\
                          O~2~ feed: 4 to O~2~ feed: 4 to O~2~ feed: 4 to
                          6               6               6

  Air Feed System Power   5 to 7          5 to 7          5 to 7
  Requirements (kW-h/lb                                   
  O~3~)                                                   
  ----------------------- --------------- --------------- ---------------

Source: Adapted from Rice, 1996, with modifications.

##### **Bubble Diffuser Contactors** {#bubble-diffuser-contactors .unnumbered}

The bubble diffuser contactor is commonly used for ozone contacting in
the United States and throughout the world (Langlais et al., 1991). This
method offers the advantages of no additional energy requirements, high
ozone transfer rates, process flexibility, operational simplicity, and
no moving parts. Figure 3-7 illustrates a typical three stage ozone
bubble diffuser contactor. This illustration shows a countercurrent flow
configuration (ozone and water flowing in opposite directions), an
alternating cocurrent/countercurrent arrangement, and a cocurrent flow
configuration (ozone and water flowing in the same direction). Also, the
number of stages can vary from two to six for ozone disinfection, with
the majority of plants using two or three chambers for contacting and
reaction (Langlais et al., 1991).

Bubble diffuser contactors are typically constructed with 18 to 22 ft
water depths to achieve 85 to 95 percent ozone transfer efficiency.
Since all the ozone is not transferred into the water, the contactor
chambers are covered to contain the off-gas. Off-gas is routed to an
ozone destruct unit, usually catalysts, thermal, or thermal/catalysts.

![](media/image8.wmf){width="2.83125in" height="6.197916666666667in"}

[]{#__RefHeading___Toc450036693 .anchor}Figure 3-7. Ozone Bubble
Contactor

Bubble diffuser contactors use ceramic or stainless steel diffusers that
are either rod-type or disc-type to generate bubbles. Design
considerations for these diffusers (Renner et al., 1988) include:

-   Gas flow range of 0.5 to 4.0 scfm;

-   Maximum headloss of 0.5 psig;

-   Permeability of 2 to 15 cfm/ft^2^/in of diffuser thickness; and
    porosity of 35 to 45 percent.

The configuration of the bubble diffuser contactor structure should best
be designed to provide plug flow hydraulics. This configuration will
minimize the overall volume of the contactor while still meeting the CT
requirements for the system. Contactor volume is determined in
conjunction with the applied ozone dosage and estimated residual ozone
concentration to satisfy the disinfection CT requirement.

Table 3-4 summarizes the advantages and disadvantages of the bubble
diffuser contactor (Langlais et al., 1991). Also, diffuser pore clogging
can be a problem when ozone dosages are intermittent and/ or when iron
and manganese oxidation is required. Channeling of bubbles is dependent
on the type of diffusers used and the spacing between diffusers.

[]{#__RefHeading___Toc450036679 .anchor}Table 3-4. Bubble Diffuser
Contactor Advantages and Disadvantages

  ----------------------------------- -----------------------------------
  **Advantages**                      **Disadvantages**

  No moving parts                     Deep contact basins

  Effective ozone transfer            Vertical channeling of bubbles

  Low hydraulic headloss              Maintenance of gaskets and piping.

  Operational simplicity              
  ----------------------------------- -----------------------------------

##### **Injector Dissolution** {#injector-dissolution .unnumbered}

The injector contacting method is commonly used in Europe, Canada, and
the United States (Langlais et al., 1991). Ozone is injected into a
water stream under negative pressure, which is generated in a venturi
section, pulling the ozone into the water stream. In many cases, a
sidestream of the total flow is pumped to a higher pressure to increase
the available vacuum for ozone injection. After the ozone is injected
into this sidestream, the sidestream containing all the added ozone is
combined with the remainder of the plant flow under high turbulence to
enhance dispersion of ozone into the water. Figure 3-8 illustrates
typical in-line and sidestream ozone injection systems.

![](media/image9.wmf){width="4.889583333333333in"
height="3.1729166666666666in"}

[]{#__RefHeading___Toc450036694 .anchor}Figure 3-8. Sidestream Ozone
Injection System

The gas to liquid ratio is a key parameter used in the design of
injector contacting systems. This ratio should be less than 0.067
cfm/gpm to optimize ozone transfer efficiency (Langlais et al., 1991).
Meeting this criterion typically requires relatively low ozone dosages
and ozone gas concentrations greater than 6 percent by weight (DeMers
and Renner, 1992). High concentration ozone gas can be generated using a
medium-frequency generator and/or liquid oxygen as the feed gas.

To meet the CT disinfection requirements, additional contact time is
required after the injector, typically in a plug flow reactor. The
additional contact volume is determined in conjunction with the applied
ozone dosage and estimated residual ozone concentration to satisfy the
disinfection CT requirement.

Table 3-5 summarizes the advantages and disadvantages of injection
contacting (Langlais et al., 1991).

[]{#__RefHeading___Toc450036680 .anchor}Table 3-5. Injection Contacting
Advantages and Disadvantages

  ----------------------------------- -----------------------------------
  **Advantages**                      **Disadvantages**

  Injection and static mixing have no Additional headloss (energy usage)
  moving parts                        due to static mixers which may
                                      require pumping

  Very effective ozone transfer       Turndown capability limited by
                                      injection system

  Contactor depth less than bubble    More complex operation and high
  diffusion                           cost.
  ----------------------------------- -----------------------------------

##### **Turbine Mixer Contactors** {#turbine-mixer-contactors .unnumbered}

Turbine mixers are used to feed ozone gas into a contactor and mix the
ozone with the water in the contactor. Figure 3-9 illustrates a typical
turbine contactor. The illustrated turbine mixer design shows the motor
located outside the basin, allowing for maintenance access. Other
designs use a submerged turbine.

Ozone transfer efficiency for turbine mixers can be in excess of 90
percent. However, the power required to achieve this efficiency is 2.2
to 2.7 kW-hr of energy per lb of ozone transferred (Dimitriou, 1990).

Turbine mixing basins vary in water depth from 6 to 15 ft, and
dispersion areas vary from 5 to 15 ft (Dimitriou, 1990). Again, as with
injector contacting, sufficient contact time may not be available in the
turbine basin to meet disinfection CT requirements; consequently
additional contact volume may be required.

![](media/image10.png){width="5.061111111111111in"
height="3.790277777777778in"}

[]{#__RefHeading___Toc450036695 .anchor}Figure 3-9. Turbine Mixer Ozone
Contactor

Table 3-6 summarizes advantages and disadvantages for the turbine mixer
contactor (Langlais et al., 1991).

[]{#__RefHeading___Toc450036681 .anchor}Table 3-6. Turbine Mixer
Contactor Advantages and Disadvantages

  ----------------------------------- -----------------------------------
  **Advantages**                      **Disadvantages**

  Ozone transfer is enhanced by high  Require energy input
  turbulence resulting in small       
  bubble size                         

  Contactor depth less than bubble    Constant gas flow rate should be
  diffusion                           maintained, reducing ozone transfer
                                      efficiency

  Aspirating turbines can draw        Maintenance requirements for
  off-gas from other chambers for     turbine and motor
  reuse                               

  Eliminates diffuser clogging        
  concerns                            
  ----------------------------------- -----------------------------------

#### Off-gas Destruction Systems

The concentration of ozone in the off-gas from a contactor is usually
well above the fatal concentration. For example, at 90 percent transfer
efficiency, a 3 percent ozone feed stream will still contain 3,000 ppm
of ozone in the off-gas. Off-gas is collected and the ozone converted
back to oxygen prior to release to the atmosphere. Ozone is readily
destroyed at high temperature (\> 350° C or by a catalyst operating
above 100° C) to prevent moisture buildup. The off-gas destruct unit is
designed to reduce the concentration to 0.1 ppm of ozone by volume, the
current limit set by OSHA for worker exposure in an eight hour shift. A
blower is used on the discharge side of the destruct unit to pull the
air from the contactor, placing the contactor under a slight vacuum to
ensure that no ozone escapes.

#### Instrumentation

Instrumentation should be provided for ozone systems to protect both
personnel and the equipment. Gas phase ozone detectors should be
provided in spaces such as generator rooms where ozone gas may be and
personnel are routinely present. An ozone detector is also needed on the
outlet from the off-gas destruct unit to ensure the unit is working
properly. These units should be interlocked with the ozone generator
controls to shut down the ozone generation system should excess ozone be
detected. A dew point detector on the feed gas supply just upstream of
an ozone generator is required to protect the generator from moisture in
the feed gas (when air is the feed gas). Flow switches on the cooling
water supply are needed to protect the generator from overheating and a
pressure switch to prevent over pressurization.

Other instrumentation can be used to monitor and control the ozone
process, although manual control is adequate for small systems, but most
small systems are designed to operate automatically, particularly in
remote areas. Ozone monitors can be used in conjunction with process
flow meters to match ozone dose to process demands and control ozone
generation. Sophisticated control schemes can be implemented to minimize
the cost of dosing with ozone and reduce operator attention
requirements. Many systems include residual monitoring at various points
in the contactor to maintain a desired ozone residual and prevent
energy-wasting overdosing.

### Operation and Maintenance

Even though ozone systems are complex, using highly technical
instruments, the process is highly automated and very reliable,
requiring only a modest degree of operator skill and time to operate an
ozone system. Maintenance on generators requires skilled technicians. If
trained maintenance staff are not available at the plant, this work can
be done by the equipment manufacturer. Some facilities, such as the 600
mgd Los Angeles Aqueduct Filtration Plant, use plant mechanics to
perform generator and facilities maintenance. Therefore, backup units
are usually installed. Generators should be checked daily when in
operation. After a shutdown, dry air or oxygen should be allowed to flow
through the generator to ensure that any moisture has been purged prior
to energizing the electrodes. At initial start up and after long down
times, this process may take up to 12 hours and usually longer when air
is the feed gas. As an alternative, a small flow of dry air can be
passed through the generator continuously when it is in standby mode to
maintain the dry condition.

Filters and desiccant in air preparation systems should be changed
periodically, with the frequency depending on the quality of the inlet
air and the number of hours in operation. Compressors require periodic
service, depending on the type and operating time. LOX tanks should be
periodically pressure tested. Piping and contact chambers should be
inspected periodically to check for leaks and corrosion.

Dielectric tubes should be periodically cleaned. This operation should
be performed when the generator efficiency drops 10-15 percent. Cleaning
the tubes is a delicate operation as the tubes are fragile and
expensive. Adequate space should be provided for the cleaning operation
and for storage of spare tubes.

## Primary Uses and Points of Application of Ozone

### Primary Uses of Ozone

Ozone is used in drinking water treatment for a variety of purposes
including:

-   Disinfection;

-   Inorganic pollutant oxidation, including iron, manganese, and
    sulfide;

-   Organic micropollutant oxidation, including taste and odor
    compounds, phenolic pollutants, and some pesticides; and

-   Organic macropollutant oxidation, including color removal,
    increasing the biodegradability of organic compounds, DBP precursor
    control, and reduction of chlorine demand.

#### Disinfection

Ozone is a powerful oxidant able to achieve disinfection with less
contact time and concentration than all weaker disinfectants, such as
chlorine, chlorine dioxide, and monochloramine (Demers and Renner,
1992). However, ozone can only be used as a primary disinfectant since
it cannot maintain a residual in the distribution system. Thus, ozone
disinfection should be coupled with a secondary disinfectant, such as
chlorine, chloramine, or chlorine dioxide for a complete disinfection
system.

#### Iron and Manganese Oxidation

Ozone will oxidize iron and manganese, converting ferrous (2+) iron into
the ferric (3+) state and 2+ manganese to the 4+ state. The oxidized
forms will precipitate as ferric hydroxide and manganese hydroxide
(AWWA, 1990). The precise chemical composition of the precipitate will
depend on the nature of the water, temperature, and pH. The ozone dose
required for oxidation is 0.43 mg/mg iron and 0.88 mg/mg manganese
(Langlais et al., 1991). Iron oxidizes at a pH of 6-9 but manganese is
more effective at a pH of around 8. Also, over-ozonation has no effect
on iron, but will resolubilize manganese, which then should be reduced
to manganese dioxide downstream.

#### Oxidation of Taste and Odor Compounds

Ozone is used to oxidize/destroy taste and odor-causing compounds
because many of these compounds are very resistant to oxidation. Suffet
et al. (1986) confirmed that ozone is an effective oxidant for use in
taste and odor treatment. They found ozone doses of 2.5 to 2.7 mg/L and
10 minutes of contact time (ozone residual of 0.2 mg/L) significantly
reduced taste and odors in the specific waters they tested. Most early
U.S. water plants (i.e., 1940-1986) installed ozonation specifically for
taste and odor removal.

#### DBP Precursor Control

Early work on oxidation of DBP precursors seemed to indicate that the
effects of ozonation, prior to chlorination, were quite site-specific
and unpredictable (Umphries et al., 1979). The key variables that seem
to determine ozone's effect are dose, pH, alkalinity, and, above all,
the nature of the organic material. At low pH levels, precursor
destruction by ozone is quite effective; however, above some critical
pH, ozone actually is less effective and in fact sometimes increases the
amount of chlorination byproduct precursors. For most humic substances
this critical pH is 7.5, which is the approximate level at which
decomposition of ozone to hydroxyl free radicals increases rapidly, thus
increasing organic oxidation rates. Therefore, the implications that at
lower pH (approximately 6-7), at which molecular ozone predominates over
the hydroxyl free radical, the initial THM precursor by-products are
different in nature than those formed by the hydroxyl free radicals
oxidized at higher pH levels. This is logical in light of the greater
oxidation potential of the hydroxyl free radical over that of ozone.

As alkalinity increases, it has a beneficial effect on THM formation
potential (THMFP) (Langlais et al., 1991). This is because alkalinity
scavenges any hydroxyl free radicals formed during ozonation, leaving
molecular ozone as the sole oxidant, which is only capable of oxidizing
organic precursors to a lower oxidation sequence than does the hydroxyl
free radical. Given neutral pH and moderate levels of bicarbonate
alkalinity, THMFP level reductions of 3 to 20 percent have been shown at
ozone doses ranging from 0.2 to 1.6 mg ozone per mg carbon (Singer et
al., 1989; Georgeson and Karimi, 1988).

#### Increase Organic Biodegradation

Ozone can be effective in partially oxidizing organics in the water to
biodegradable compounds that can be removed by biological filtration
(Demers and Renner, 1992). This partial oxidation gives rise to lower
molecular weight organics that are more easily biodegradable. This
increase in the biodegradable fraction of organic carbon occurs as a
result of moderate to high levels of ozonation. These ozone levels are
typical of the doses commonly applied for disinfection.

#### Coagulation and Filtration Improvement

Ozone has been reported by some to improve coagulation and filtration
efficiency (Gurol and Pidatella, 1993; Farvardin and Collins, 1990;
Reckhow et al., 1993; Stolarik and Christie, 1997). However, others have
found no improvement in filter effluent turbidity due to ozonation
(Tobiason et al., 1992; Hiltebrand et al., 1986). Prendiville (1986)
collected data from a large water treatment plant showing that
pre-ozonation was more effective than pre-chlorination to reduce filter
effluent turbidities. The cause of the improved coagulation is not
clear, but several possibilities have been offered (Reckhow et al.,
1986), including:

-   Oxidation of organic compounds into more polar forms; and

-   Oxidation of metals ions to yield insoluble complexes, such as
    ferric iron complexes.

### Points of Application

The typical locations for feeding ozone in a water treatment plant are
at the head of the treatment plant (raw water) pre-ozonation and after
sedimentation.

Raw water quality and turbidity and ozone demand (the amount of ozone
required for all oxidation requirements of the water) can be used to
assess how to use ozone in the treatment process. Table 3-7 lists the
criteria for selecting ozone feed points based on these two parameters.
By moving the ozonation process further downstream after sedimentation,
the ozone demand and production of byproducts are reduced. The advantage
of placing ozone ahead of filtration is that biodegradable organics
produced during ozonation can be removed by subsequent biological
activity in the filters.

[]{#__RefHeading___Toc450036682 .anchor}Table 3-7. Criteria for
Selecting Ozone Feed Points for Small Systems

+---------------------+-----------------------+-----------------------+
| **Raw Water         | **Ozone Feed          | **Special             |
| Quality**           | Point(s)**            | Considerations**      |
+---------------------+-----------------------+-----------------------+
| **Category I**      | Raw Water or After    | Low ozone demand.     |
|                     | Sedimentation         |                       |
| Turbidity \< 10 NTU |                       | Low disinfection      |
|                     |                       | byproducts.           |
| Ozone Demand \<     |                       |                       |
| 1mg/L               |                       | Low biodegradable     |
|                     |                       | organics.             |
+---------------------+-----------------------+-----------------------+
| **Category II**     | After Sedimentation   | Low ozone demand.     |
|                     |                       |                       |
| Turbidity \> 10 NTU |                       | High inorganic        |
|                     |                       | particulate.          |
| Ozone Demand \<     |                       |                       |
| 1mg/L               |                       | Low biodegradable     |
|                     |                       | organics.             |
+---------------------+-----------------------+-----------------------+
| **Category III**    | Raw Water and/or      | High ozone demand     |
|                     | After Sedimentation   |                       |
| Turbidity \< 10 NTU |                       | Disinfection          |
|                     |                       | byproducts            |
| Ozone Demand \>     |                       |                       |
| 1mg/L               |                       | Biodegradable         |
|                     |                       | organics formation    |
+---------------------+-----------------------+-----------------------+
| **Category IV**     | After Sedimentation   | High ozone demand     |
|                     | and After First Stage |                       |
| Turbidity \> 10 NTU | Filtration, if        | Disinfection          |
|                     | necessary             | byproducts            |
| Ozone Demand \>     |                       |                       |
| 1mg/L               |                       | Biodegradable         |
|                     |                       | organics formation    |
+---------------------+-----------------------+-----------------------+

Source: DeMers and Renner, 1992.

For high quality water with direct filtration, the only practical ozone
feed point is the raw water.

Category II (Table 3-7) water is characterized by low ozone demand and
high turbidity. This water quality indicates the presence of inorganic
material, such as clay or silt particles. For ozone to be most effective
for Category II water disinfection, it should be added after either
pre-sedimentation or conventional sedimentation.

Raw water with low turbidity and high ozone demand (Category III, Table
3-7) contains dissolved constituents, not suspended, that contribute to
a high ozone demand. An example of this type of water is a ground water
containing bromide ion, iron, manganese, color, or organics. For this
water quality, ozone can be added to either the raw water or after
sedimentation. If the water contains organic constituents that become
more biodegradable by ozonation, a biological treatment step (see
Section 3.3.4) may be required. The presence of oxidizable organic
constituents or bromide ion will generate disinfection byproducts upon
ozonation.

Category IV (Table 3-7) water would be considered the most difficult
water to treat with ozone due to its high turbidity and high ozone
demand. An example of this water quality is surface water containing
high concentrations of organic material and inorganic particles. The
most effective use of ozone for this water quality is after
sedimentation and possibly after filtration. If the water has an
extremely high ozone demand, dual ozone feed points may be required to
achieve disinfection goals, because the presence of large amounts of
organic material may require a biological treatment step and may
generate disinfection byproducts.

### Impact on Other Treatment Processes

Ozonation does have an impact on other processes at the water treatment
facility. The impacts of ozone addition include the following:

-   The use of ozone generates biodegradable organic matter (BOM) that
    can result in biological growth which may also increase corrosion
    rates in distribution systems if not removed by biologically active
    filtration. When ozonation is placed before biological filters, it
    can impact the filters by increasing biological growth and
    increasing backwash frequency.

-   Ozone is a strong oxidant that reacts with other oxidants, such as
    chlorine, chlorine dioxide, and monochloramine.

-   Ozone oxidation of iron and manganese generates insoluble oxides
    that should be removed by sedimentation or filtration. These
    insoluble oxides also impact the filters by increasing load on the
    filters and increasing backwash frequency.

-   Using pre- and/or internal ozone on most raw waters reduces the
    subsequent chlorine, chlorine dioxide, or monochloramine demand of
    the finished water so as to allow a stable chlorine-compound
    residual to be maintained at a much lower level.

The reader is referred to EPA's *Simultaneous Compliance Guidance
Document* (expected to be available in 1999) for additional information
regarding the interaction between oxidants and other treatment
processes.

### Biologically Active Filtration

Ozonation typically increases the biodegradability of NOM in water
because many large organic molecules are converted into smaller organic
molecules that are readily biodegradable. This increase in biodegradable
dissolved organic carbon (BDOC) can lead to accelerated bacterial growth
and regrowth in the distribution system if not removed in the treatment
plant. LeChevallier et al. (1992) found that AOC levels less than 100
ppb may be necessary to control excessive bacterial regrowth in the
distribution system if not removed in the treatment plant.

When ozonation is placed upstream of filtration, and environmental
conditions such as dissolved oxygen, pH, and temperature are favorable,
microbiological activity is increased in the filter and BDOC/AOC removal
is enhanced. Ozone addition not only increases the biodegradability of
the dissolved organics, but also introduces large amounts of oxygen to
the water, thus, creating an excellent environment for biological growth
on the filter media. The advantages of biologically active filtration
(Price, 1994) include the following which are all being met in most U.S.
plants using ozone:

-   Production of a biologically stable water that does not promote
    excessive bacterial growth and regrowth in the distribution system.

-   Removal of NOM that can serve as precursors to byproduct formation
    as a result of residual disinfection with free or combined chlorine.

-   Ozone oxidation as a primary disinfectant prior to biologically
    active filtration reduces the BDOC concentration in finished water,
    thus reducing chances of regrowth.

-   Reduction of the residual disinfectant demand of the product water
    so that proposed regulations limiting the maximum disinfectant
    residual can be met.

-   Removal or control of ozonation byproducts.

Biological activity can be supported on slow sand, rapid rate, and GAC
media because these media provide a surface for bacteria to attach.
Factors such as available surface area, hydraulic loading rate, contact
time, availability of nutrients, temperature, and others will determine
the performance and BDOC removal efficiency. Biomass develops to higher
levels on GAC because of the rougher surface characteristics than on
anthracite and sand.

#### Slow Sand Filters

Ozone addition prior to slow sand filtration can increase the efficiency
of TOC removal by about 35 percent (Rachwal et al., 1988; Zabel, 1985).
Ozone addition can also increase the efficiency of BDOC removal with
slow sand filters (Eighmy et al., 1991; Malley et al., 1993).

#### Rapid Rate Filters

Research in the area of biologically active rapid rate filters has
focused on the reduction of assimilable organic carbon (AOC) instead of
BDOC. While studies have shown rapid rate filtration, employing either
sand or dual media lowers AOC levels following ozonation, AOC does not
measure all the BDOC. AOC measures only that portion of the BDOC that is
more easily assimilable or more easily biodegradable under specific
laboratory conditions by two specific microorganisms. Research data
shows that biodegradation of AOC can occur in rapid rate filters. The
data should be viewed with caution, since the more slowly biodegradable
DOC, not measured by AOC, may be passing through rapid rate filters.

#### Granular Activated Carbon

GAC is made biologically active by the deliberate introduction of
sufficient dissolved oxygen to water just before passing through GAC
columns (Katz, 1980). The high surface area and long retention time in
GAC provide an ideal environment to enhance biological growth.

Although ozone actually increases the amount of BDOC, the efficiency of
subsequent biodegradation on GAC can be such that the BDOC in GAC
effluent is lower than the BDOC in the ozone influent (Langlais et al.,
1991). The degree to which biodegradable DOC is removed by ozone/GAC
depends upon the process conditions of temperature, amount of BDOC, and
the GAC column loading rate, measured by empty bed contact time (EBCT).
For example, with an influent BDOC of 0.65 mg C/L and a 10 minute EBCT,
one would expect an effluent BDOC of 0.25 mg C/L. The effluent BDOC then
could be lowered by either:

-   Adding ozone, which would increase the GAC influent BDOC and,
    therefore, lower the effluent BDOC; or

-   Adding more GAC or decreasing the loading rate, which would extend
    the EBCT and lower the effluent BDOC (Billen et al., 1985, as cited
    in Langlais et al., 1991).

Huck et al. (1991) reported results from AOC profiles measured in a
pilot treatment plant. The plant treated Saskatchewan River water and
included coagulation, flocculation, and sedimentation prior to
ozonation. Following ozonation, the water was filtered through a dual
media (anthracite-sand) filter followed by GAC adsorption. The results
demonstrate:

-   Variable AOC removal through coagulation, flocculation, and
    sedimentation (80 percent to zero);

-   Increased AOC after ozonation;

-   AOC removal through dual media filtration improving at lower
    hydraulic loading rates and filtered effluent AOC often less than
    raw water AOC, but highly variable; and

-   AOC levels after GAC were low, almost always below raw water AOC
    concentrations and adsorption appears to contribute to some
    immediate AOC removal.

### Pathogen Inactivation and Disinfection Efficacy

Ozone has a high germicidal effectiveness against a wide range of
pathogenic organisms including bacteria, protozoa, and viruses. Because
of its high germicidal efficiency, ozone can be used to meet high
inactivation required by water treatment systems with or without
filters. However, ozone cannot be used as a secondary disinfectant
because the ozone residual decays too rapidly. The ozone disinfection
efficiency is not affected by pH (Morris, 1975), although because of
hydroxyl free radicals and rapid decay, efficiency is the same but more
ozone should be applied at high pH to maintain "C".

### Inactivation Mechanisms

Inactivation of bacteria by ozone is attributed to an oxidation reaction
(Bringmann, 1954; Chang, 1971). The first site to be attacked appears to
be the bacterial membrane (Giese and Christensen, 1954) either through
the glycoproteins or glycolipids (Scott and Lesher, 1963) or through
certain amino acids such as typtophan (Goldstein and McDonagh, 1975). In
addition, ozone disrupts enzymatic activity of bacteria by acting on the
sulfhydryl groups of certain enzymes. Beyond the cell membrane and cell
wall, ozone may act on the nuclear material within the cell. Ozone has
been found to affect both purines and pyrimidines in nucleic acids
(Giese and Christensen, 1954; Scott and Lesher, 1963).

The first site of action for virus inactivation is the virion capsid,
particularly its proteins (Cronholm et al.*,* 1976 and Riesser et al.*,*
1976). Ozone appears to modify the viral capsid sites that the virion
uses to fix on the cell surfaces. High concentrations of ozone
dissociate the capsid completely. One researcher found that the
mechanism of ozone inactivation of bacteriophage f2 ribonucleic acid
(RNA) included releasing RNA from the phage particles after the phage
coat was broken into many pieces (Kim et al.*,* 1980). This finding
suggests that ozone breaks the protein capsid, thereby liberating RNA
and disrupting adsorption to the host pili. Further, the naked RNA may
be secondarily inactivated by ozone at a rate less than that for RNA
within the intact phage. The mechanism for inactivation of
deoxyribonucleic acid (DNA) bacteriophage T4 has been found to be quite
similar to RNA inactivation: ozone attacks the protein capsid, liberates
the nucleic acid, and inactivates the DNA (Sproul et al.*,* 1982). In
contrast, more recent work on the tobacco mosaic virus (TMV) shows that
ozone has a specific effect on RNA. Ozone was found to attack both the
protein coat and RNA. The damaged RNA cross-links with amino acids of
the coat protein subunits. The authors concluded that TMV loses its
infectivity because of its loss of protein coating.

Microscopic observation of inactivation of trophozoites of *Naegleria*
and *Acanthamoeba* showed that they were rapidly destroyed and the cell
membrane was ruptured (Perrine et al.*,* 1984). Perrine and Langlais
showed that ozone affect the plugs in *Naegleria gruberi* cysts
(Langlais and Perrine, 1984). Depending on the ozonation conditions,
these plugs were completely removed or were partially destroyed. It has
been speculated that ozone initially affects the *Giardia muris* cysts
wall and makes it more permeable (Wickramanayake, 1984c). Subsequently,
aqueous ozone penetrates into the cyst and damages the plasma membranes,
additional penetration of ozone eventually affects the nucleus,
ribosomes, and other ultrastructural components.

### Disinfection Parameters

Hoigné and Bader demonstrated that the rate of decomposition of ozone is
a complex function of temperature, pH, and concentration of organic
solutes and inorganic constituents (Hoigné and Bader, 1975, and 1976).
The following sections describe the effects that pH, temperature, and
suspended matter have on the reaction rate of ozone and pathogen
inactivation.

The ability to maintain a high aqueous ozone concentration is critical
from a regulatory disinfection compliance standpoint. This means that
factors that accelerate ozone decomposition are undesirable for
inactivation because the ozone residual dissipates faster and therefore
reduces the CT credit, requiring a corresponding increase in the ozone
applied, thus increasing cost.

#### pH

Studies have indicated that pH has little effect on the ability of
dissolved ozone residuals to inactivate acid-fast bacteria, such as
*Mycobacteria* and *Actinomycetes* (Farooq, 1976). A slight decrease has
been found in the virucidal efficacy of ozone residuals as pH decreased
(Roy, 1979). However, the opposite effect was observed by Vaughn et al.
(1987) (cited in Hoff, 1986). Changes in disinfection efficacy with
variations in pH appear to be caused by the ozone decomposition rate.
Ozone decomposition occurs faster in higher-pH aqueous solutions and
forms various types of oxidants with differing reactivities (Langlais et
al., 1991). Tests carried out at constant ozone residual concentration
and different pH values showed that the degree of microorganism
inactivation remained virtually unchanged (Farooq et al.*,* 1977). More
recent studies have indicated decreased virus inactivation by ozone at
alkaline pH (pH 8 to 9) for poliovirus 1 (Harakeh and Butler, 1984) and
rotaviruses SA-11 and Wa (Vaughn et al.*,* 1987).

Inactivation of *Giardia muris* cysts was found to improve when the pH
increased from 7 to 9 (Wickramanayake, 1984a). This phenomenon was
attributed to the possible changes in cyst chemistry making it easier
for ozone to react with the cyst constituents at the higher pH levels.
However, the same study found that inactivation of *Naegleria gruberi*
cysts was slower at a pH 9 than at lower pH levels, thereby indicating
that pH effects are organism-specific.

#### Temperature

As temperature increases, ozone becomes less soluble and less stable in
water (Katzenelson et al.*,* 1974); however, the disinfection and
chemical oxidation rates remain relatively stable. Studies have shown
that although increasing the temperature from 0 to 30^o^C can
significantly reduce the solubility of ozone and increases its
decomposition rate, temperature has virtually no effect on the
disinfection rate of bacteria (Kinman, 1975). In other words, the
disinfection rate was found to be relatively independent of temperature
at typical water treatment plant operating temperatures despite the
reduction in solubility and stability at higher temperatures.

#### Suspended Matter

Ozone inactivation of viruses and bacteria contained in aluminum floc
(in the size range comparable to those that could typically escape
filtration) was not reduced at floc turbidity levels of 1 and 5 NTU
(Walsh et al.*,* 1980). This study demonstrated that the microorganisms
received no protection from the aluminum floc. Similar results have been
obtained for poliovirus 1, coxsackie virus A9, and *E. coli* associated
with bentonite clay (Boyce et al.*,* 1981). However, adsorption of the
f2 bacteriophage at 1 and 5 NTU of bentonite clay was found to retard
the rate of inactivation of ozone (Boyce et al.*,* 1981).

In some instances, river waters heavily polluted with organic matter
were ozonated, and the results indicated a degradation of large organic
molecules into fragments more easily metabolized by microorganisms. This
fragmentation coupled with the inability of ozone to maintain an active
concentration in the distribution system, has led to increased slime
growth and, consequently, water quality deterioration during
distribution (Troyan and Hanson, 1989).

### Inactivation of Microorganisms

The following sections contain a description of the disinfection
efficiency of ozone in terms of bacteria, virus, and protozoa
inactivation.

#### Bacterial Inactivation

Ozone is very effective against bacteria. Studies have shown the effect
of small concentrations of dissolved ozone (i.e., 0.6 μg/L) on *E.
coli*. (Wuhrmann and Meyrath, 1955) and *Legionella pneumophila*
(Domingue, et al.*,* 1988). *E. coli.* levels were reduced by 4 logs
(99.99 percent removal) in less than 1 minute with a ozone residual of 9
μg/L at a temperature of 12^o^C. *Legionella pneumophila* levels were
reduced by greater than 2 logs (99 percent removal) within a minimum
contact time of 5 minutes at a ozone concentration of 0.21 mg/L. Results
similar to those obtained for *E. coli.* have been found for
*Staphyloccus* sp. and *Pseudomonas fluorescens* inactivation.
*Streptococcus faecalis* required a contact time twice as long with the
same dissolved ozone concentration, and *Mycobacterium tuberculosis*
required a contact time six times as long for the same reduction level
as *E. coli*.

In regard to vegetative bacteria, *E. coli* is one of the most sensitive
types of bacteria. Furthermore, significant difference has been found
among all the Gram-negative bacillae, including *E. coli* and other
pathogens such as *Salmonella*, which are all sensitive to ozone
inactivation. whereas the Gram-positive cocci (*Staphyloccus* and
*Streptococcus*), the Gram-positive bacillae (*Bacillus*), and the
*Mycobacteria* are the most resistant forms of bacteria. Sporular
bacteria forms are always far more resistant to ozone disinfection than
vegetative forms (Bablon, et al.*,* 1991), but all are easily destroyed
by relatively low levels of ozone.

#### Protozoa Inactivation

Protozoan cysts are much more resistant to ozone and other disinfectants
than vegetative forms of bacteria and viruses. *Giardia lamblia* has a
sensitivity to ozone that is similar to the sporular forms of
*Mycobacteria*. Both *Naegleria* and *Acanthamoeba* cysts are much more
resistant to ozone (and all other disinfectants) than *Giardia* cysts.
(Bablon et al.*,* 1991). CT products for 99 percent inactivation of
*Giardia lamblia* and *N. gruberi* at 5^o^C were 0.53 and 4.23 mg •
min/L, respectively (Wickramanayake et al.*,* 1984a and 1984b). Data
available for inactivation of *Cryptosporidium* oocysts, suggest that
among protozoans, this microorganism is more resistant to ozone (Peeters
et al.*,* 1989; Langlais et al.*,* 1990). One study found that
*Cryptosporidium* oocysts are approximately 10 times more resistant to
ozone than *Giardia* (Owens et al.*,* 1994).

#### Virus Inactivation

Typically, viruses are more resistant to ozone than vegetative bacteria
but less resistant than sporular forms of *Mycobacteria* (Bablon, et
al.*,* 1991). The most sensitive forms of viruses are phages, and there
seems to be little difference between the polio- and coxsackie viruses.
The sensitivity of human rotavirus to ozone was determined to be
comparable to that of *Mycobacteria* and polio- and coxsackie viruses
(Vaughn et al.*,* 1987).

Keller et al. (1974) studied ozone inactivation of viruses by using both
batch tests and pilot plant data. Inactivation of poliovirus 2 and
coxsackie virus B3 was more than 3 logs (99.9 percent) in the batch
tests with an ozone residual of 0.8 mg/L and 1.7 mg/L and a contact time
of 5 minutes. Greater than 5 log (99.999 percent) removal of coxsackie
virus was achieved in the pilot plant with an ozone dosage of 1.45 mg/L,
which provided an ozone residual of 0.28 mg/L in lake water.

#### CT Curves for Giardia Lamblia

CT values shown in Figure 3-9 are based on disinfection studies using in
vitro excystation of *Giardia lamblia*. CT values obtained at 5^o^C and
pH 7 were used as the basis for deriving the CT values at other
temperatures. A safety factor of 2 has been applied to the values shown
in Figure 3-9.

![](media/image11.wmf){width="5.596527777777778in"
height="3.8916666666666666in"}

Figure 3-9. CT Values for Inactivation of Giardia Cysts by Ozone (pH 6
to 9)

CT values shown in Figure 3-10 for achieving 2-log inactivation of
viruses were determined by applying a safety factor of 3 to data
obtained from a previous study on poliovirus 1 (Roy et al.*,* 1982). CT
values for 3 and 4-log removal were derived by applying first order
kinetics and assuming the same safety factor of 3. Data obtained at a pH
of 7.2 was assumed to apply for the pH range of 6 to 9.

Several research groups have investigated the efficiency of ozone for
*Cryptosporidium* oocyst inactivation. Table 3-8 summarizes CT values
obtained for 99 percent inactivation of *Cryptosporidium* oocysts.
Results indicate that ozone is one of the most effective disinfectants
for controlling *Cryptosporidium* (Finch, et al.*,* 1994) and that
*Cryptosporidium* *muris* may be slightly more resistant to ozonation
than *Cryptosporidium parvum* (Owens et al.*,* 1994). A wide range of CT
values has been reported for the same inactivation level, primarily
because of the different methods of *Cryptosporidium* measurements
employed and pH, temperature, and above all, ozonation conditions.

![](media/image12.wmf){width="5.596527777777778in"
height="3.8270833333333334in"}

[]{#__RefHeading___Toc450036696 .anchor}Figure 3-10. CT Values for
Inactivation of Viruses by Ozone (pH 6 to 9)

[]{#__RefHeading___Toc450036683 .anchor}Table 3-8. Summary of Reported
Ozonation Requirements for 99 Percent Inactivation of *Cryptosporidium*
Oocycts

+--------+---------+---------+---------+---------+---------+---------+
| **Spe  | **Ozone | **Ozone | **      | *       | **CT**  | **Refe  |
| cies** | pro     | res     | Contact | *Temper |         | rence** |
|        | tocol** | idual** | time**  | ature** | **(mg.m |         |
|        |         |         |         |         | in/L)** |         |
|        |         | **(     | **      | **(     |         |         |
|        |         | mg/L)** | (min)** | ^o^C)** |         |         |
+--------+---------+---------+---------+---------+---------+---------+
| *C.    | Batch   | 0.6 &   | 4       | 25      | 2.4 -   | L       |
| ba     | liquid, | 0.8     |         |         | 3.2     | anglais |
| ileyi* | m       |         |         |         |         | et      |
|        | odified |         |         |         |         | al.*,*  |
|        | batch   |         |         |         |         | 1990    |
|        | ozone   |         |         |         |         |         |
+--------+---------+---------+---------+---------+---------+---------+
| *C.    | Flow    |         |         | 22 - 25 | 7.8     | Owens   |
| muris* | through |         |         |         |         | et      |
|        | con     |         |         |         |         | al.*,*  |
|        | tactor, |         |         |         |         | 1994    |
|        | con     |         |         |         |         |         |
|        | tinuous |         |         |         |         |         |
|        | gas     |         |         |         |         |         |
+--------+---------+---------+---------+---------+---------+---------+
| *C.    | Batch   | 0.50    | 18      | 7       | 9.0     | Finch   |
| p      | liquid, |         |         |         |         | et      |
| arvum* | batch   | 0.50    | 7.8     | 22      | 3.9     | al.*,*  |
|        | ozone   |         |         |         |         | 1993    |
+--------+---------+---------+---------+---------+---------+---------+
| *C.    | Batch   | 0.77    | 6       | Room    | 4.6     | Peeters |
| p      | liquid, |         |         |         |         | et      |
| arvum* | batch   | 0.51    | 8       |         | 4       | al.*,*  |
|        | ozone   |         |         |         |         | 1989    |
+--------+---------+---------+---------+---------+---------+---------+
| *C.    | Batch   | 1.0     | 5 & 10  | 25      | 5 - 10  | Korich  |
| p      | liquid, |         |         |         |         | et      |
| arvum* | con     |         |         |         |         | al.*,*  |
|        | tinuous |         |         |         |         | 1990    |
|        | gas     |         |         |         |         |         |
+--------+---------+---------+---------+---------+---------+---------+
| *C.    | Flow    |         |         | 22 - 25 | 5.5     | Owens   |
| p      | through |         |         |         |         | et      |
| arvum* | con     |         |         |         |         | al.*,*  |
|        | tactor, |         |         |         |         | 1994    |
|        | con     |         |         |         |         |         |
|        | tinuous |         |         |         |         |         |
|        | gas     |         |         |         |         |         |
+--------+---------+---------+---------+---------+---------+---------+

Ozone dose and contact time (CT) requirements for the inactivation of
*Cryptosporidium* oocysts in drinking water when using ozone has not
been established similar to the CT values for viruses and *Giardia* cyst
inactivation. Inactivation requirements (log removals) for
*Cryptosporidium* oocysts have not been established. In addition, as
shown in Table 3-8, the CT requirements reported in the literature vary
from study to study which adds uncertainty to design CT requirements for
specific applications or regulatory needs.

## Ozonation Disinfection Byproducts

Ozone does not form halogenated DBPs (TTHMs and HAA5s) when
participating in oxidation/reduction reactions with NOM but it does form
a variety of organic and inorganic byproducts. Table 3-9 and Figure 3-11
show the principal known byproducts associated with ozonation. However,
if bromide ion is present in the raw water halogenated DBPs may be
formed. These brominated DBPs appear to pose a greater health risk than
non-brominated DBPs.

[]{#__RefHeading___Toc450036684 .anchor}Table 3-9. Principal Known
Byproducts of Ozonation

  ----------------------------------- -----------------------------------
  **Disinfectant Byproducts**         

  **Aldehydes**                       **Aldo- and Ketoacids**

  Formaldehyde                        Pyruvic acid

  Acetaldehyde                        **Brominated Byproducts\***

  Glyoxal                             Bromate ion

  Methyl Glyoxal                      Bromoform

  **Acids**                           Brominated acetic acids

  Oxalic acid                         Bromopicrin

  Succinic acid                       Brominated acetonitriles

  Formic acid                         **Others**

  Acetic acid                         Hydrogen peroxide
  ----------------------------------- -----------------------------------

\*Brominated byproducts are produced only in waters containing bromide
ion

Source: Singer, 1992.

Although ozone is an effective oxidant and disinfectant, it should not
be relied upon as a secondary disinfectant to maintain a residual in the
distribution system. Monochloramine is attractive for this purpose
because it produces little to no halogenated DBPs. Chlorine is a
candidate for secondary disinfectant but the ozonated water may actually
produce either more or less DBPs following the addition of free chlorine
depending on the nature of the organic material following ozonation
unless biologically active filtration precedes the addition of chlorine.
The principal benefit of using ozone for controlling DBP formation is
that it allows free chlorine to be applied later in the treatment
process after precursors have been removed and at lower doses, thereby
reducing DBPFP.

![](media/image13.wmf){width="6.25in" height="3.24375in"}

[]{#__RefHeading___Toc450036697 .anchor}Figure 3-11. Principal Reactions
Producing Ozone Byproducts

Application of a secondary disinfectant following ozonation requires
special consideration for potential interaction between disinfectants.
For example, chloral hydrate formation has been observed when using
chlorine as a secondary disinfectant with ozone (McKnight and Reckhow,
1992; Logsdon et al., 1992). The ozonation byproduct of acetaldehyde is
a known precursor for chloral hydrate, a byproduct of chlorination.
Enhancement of chloral hydrate has not been observed when monochloramine
is applied as the secondary disinfectant, or if biologically active
filtration is used following ozonation and prior to chlorination
(Singer, 1992). Chloropicrin formation from free chlorine appears to be
enhanced by pre-ozonation (Hoigné and Bader, 1988) in the absence of
biologically active filtration prior to addition of chlorine.

Byproducts such as aldehydes, ketones, acids, and others will be formed
upon ozonation of water. The primary aldehydes that have been measured
are: formaldehyde, acetaldehyde, glyoxal, and methyl glyoxal (Glaze et
al., 1991). Total aldehyde concentration in drinking water disinfected
with ozone range from less than 5 μg/L to 300 μg/L, depending on the TOC
concentration and the applied ozone to organic carbon ratio (Van Hoof et
al., 1985; Yamada and Somiya, 1989; Glaze et al., 1989a; Krasner et al.,
1989; Glaze et al., 1991; LeLacheur et al., 1991). Aldehydes with higher
molecular weights have also been reported (Glaze et al., 1989b). Other
organic byproducts of ozonation are indicated in Table 3-9

Ozonation of a source water containing bromide ion can produce
brominated byproducts, the brominated analogues of the chlorinated DBPs.
Song et al. (1997) found that bromate ion formation is an important
consideration for waters containing more than 0.10 mg/L bromide ion.
These brominated byproducts include bromate ion, bromoform, the
brominated acetic acids and acetonitriles, bromopicrin, and cyanogen
bromide (if ammonia is present). An ozone dose of 2 mg/L produced 53
μg/L of bromoform and 17 μg/L of dibromoacetic acid in a water
containing 2 mg/L of bromide ion (McGuire et al., 1990). Ozonation of
the same water spiked with 2 mg/L bromide ion showed cyanogen bromide
formation of 10 μg/L (McGuire et al., 1990). Furthermore, ozone may
react with the hypobromite ion to form bromate ion (Amy and Siddiqui,
1991; Krasner et al., 1993), a probable human carcinogen (Regli et al.,
1992). Bromate ion concentrations in ozonated water up to 60 μg/L have
been reported (Amy and Siddiqui, 1991; Krasner et al.*,* 1993). Note
that the amount of bromide ion incorporated into the measured DBPs
accounts for only one-third of the total raw water bromide ion
concentration. This indicates that other brominated DBPs exist that are
not yet identified (Krasner et al.*,* 1989; MWDSC and JMM, 1992). Figure
3-12 shows the major pathways for bromate ion formation.

![](media/image14.wmf){width="6.247222222222222in"
height="4.920138888888889in"}

[]{#_Ref401031260 .anchor}Source: Song et al., 1997.

Figure 3-12. Main Pathways of Bromate Ion Formation when Ozone Reacts
with Bromide Ion

### Ozone Byproduct Control

The primary factors affecting the speciation and concentrations of
brominated byproducts are pH and the ozone-to-bromide ion and
TOC-to-bromide ion ratios (Singer, 1992). Bromate ion formation can be
controlled by ozonation at acidic pH values of which hypobromous acid
dominates over the now absent hypobromite ion (Haag and Hoigné, 1984;
Amy and Siddiqui, 1991; Krasner et al., 1993). Conversely, under
alkaline pH conditions, ozone can oxidize the hypobromous acid further
to produce bromate ion. At low pH values, the brominated organic
byproducts are favored, while at greater pH values, bromate ion
formation is favored. Therefore, the application of ozone may be limited
for source waters containing bromide ion. Bromate ion formation can be
controlled by lowering the ambient bromide ion concentration, lowering
the ozone residual, and lowering the ozonation pH. The addition of
ammonia with ozonation to form bromamines reduces the formation of both
bromate ion and organic byproducts (Amy and Siddiqui, 1991; MWDSC and
JMM, 1992). However, ammonia can act as a nutrient for nitrifying
bacteria.

The organic acid and aldehyde byproducts of ozonation discussed above
appear to be readily biodegradable and are a component of the
assimilable organic carbon (AOC) or biodegradable organic carbon (BDOC).
Ozonation increases the BOM by oxidation. Therefore, if water
disinfected with ozone is coupled with a biologically active process
(i.e., biological active carbon), removal of these biodegradable
byproducts can be reduced. The use of biologically active filters,
maintained by discontinuing the application of a disinfectant to the
filters, has been shown to successfully remove aldehydes and other
compounds representing a portion of the BDOC in a water (Bablon et al.,
1988; Rittman, 1990; Reckhow et al., 1992). See Section 3.3.4 for a
detailed discussion on biological active filters.

A recent study has shown that bromate ion and brominated organics can be
controlled during ozonation by the following techniques (Song et al.,
1997):

-   Low pH decreases bromate ion formation while increasing brominated
    organic formation;

-   Ammonia addition with short ozone contact time decreases both
    bromate ion and brominated organic formation;

-   Hydrogen peroxide decreases brominated organic formation and may
    increase or decrease bromate ion formation, depending on other water
    quality parameters; and

-   Low ozone DOC ratio leads to low bromate ion and brominated organic
    formation.

## Status of Analytical Methods

During operation of an ozonation system it is necessary to analyze for
ozone in both the liquid and gas phase to determine the applied ozone
dose, ozone transfer efficiency and (for primary disinfection) residual
ozone level. The gas stream exiting the ozone generator is monitored for
ozone content to determine the applied ozone dose. The off-gas exiting
the ozone contactor is monitored to determine the amount of ozone
transferred to the liquid phase in the contactor and to calculate the
ozone transfer efficiency. The disinfected water exiting the ozone
contactor is monitored for residual ozone to ensure that CT values are
met.

In addition, the ambient air in any ozone generating or handling room
and ozone destruct off-gas are monitored for ozone concentration to
protect workers in the event of leakages or destruct system failure.

### Monitoring of Gas Phase Ozone

Points in an ozone treatment system where gas-phase ozone is monitored
include:

-   Ozone generator output;

-   Contactor off-gas;

-   Ozone destruct off-gas; and

-   Ambient air in ozone process areas.

    The range of ozone concentrations to be measured in the gas phase
    varies from less than 0.1 ppm by volume (0.2 mg/m^3^ NTP) in ambient
    air and ozone destruct off-gas, to 1 to 2 percent (10 g/m^3^ NTP) in
    contactor off-gas, to as high as 15 percent by weight (200 g/m^3^
    NTP) in the ozone generator output.

    Analytical methods for monitoring gas-phase ozone include:

-   UV absorption;

-   Iodometric methods;

-   Chemiluminescence; and

-   Gas-Phase titration.

Table 3-10 presents the working range, expected accuracy and precision,
operator skill level required, interferences, and current status for gas
phase ozone analysis.

#### UV Absorption

Gaseous ozone absorbs light in the short UV wavelength region with a
maximum absorbance at 253.7 nm (Gordon et al., 1992). Instruments for
measuring ozone by the absorption of UV radiation are supplied by
several manufacturers for gas concentrations below 0.5 ppm by volume (1
g/m^3^ NTP). In general, these instruments measure the amount of light
absorptions when no ozone is present and the amount of light absorptions
when ozone is present. The meter output is the difference of the two
readings, which is directly related to the actual amount of ozone
present. The International Ozone Association (IOA) has accepted this
procedure (IOA, 1989).

[]{#_Ref400940475 .anchor}

Table 3-10. Characteristics and Comparisons of Gas-Phase Ozone
Analytical Methods

  ------------------- ----------- ------------ ------------ ------------ ------------------- --------- --------- -------------- -------------
  **Type of Test**    **Working   **Expected   **Expected   **Skill      **Interferences**   **pH      **Field   **Automated\   **Current
                      Range\      Accuracy\    Precision\   Level^a^**                       Range**   Test**    Test**         Status**
                      (mg/L)**    (± %)**      (± %)**                                                                          

  UV Absorption       0.5 -       2            2.5          1/2          None                NA        Yes       Yes            Recommended
                      50,000                                                                                                    

  Stripping           0.5 - 100   1 - 35       1 - 2        2            SO~2~, NO~2~        NA        Yes       No             Not
  Absorption                                                                                                                    Recommended
  Iodometry                                                                                                                     

  Chemiluminescence   0.005 - 1   7            5            1/2          None                NA        Yes       Yes            Recommended

  Gas-Phase Titration 0.005 - 30  8            8.5          2            None                NA        Yes       No             Not
                                                                                                                                Recommended
  ------------------- ----------- ------------ ------------ ------------ ------------------- --------- --------- -------------- -------------

Source: Gordon et al., 1992.

Notes: ^a^ Operator Skill Levels: 1 = minimal, 2 = good technician, 3 =
experienced chemist. NA = Not Applicable

#### Iodometric Methods

Iodometric procedures have been used for all of the ozone concentration
ranges encountered in water treatment plants (Gordon et al., 1992). This
includes measurement of ozone directly from the generator and of ozone
as stripped from aqueous solution. For the iodometric method, the
ozone-containing gas is passed into an aqueous solution containing
excess potassium iodide.

All other oxidizing materials act as interferences with iodometry
(Gordon et al., 1992). Nitrogen oxides (that may be present when ozone
is generated in air) also act as interferences to iodometric methods.
The effects of nitrogen oxides may be eliminated by passing the
ozone-containing gas through absorbents such as potassium permanganate
that are specific for nitrogen oxide gases. However, no iodometric
method is recommended for the determination of ozone in solution because
of the unreliability of the method (Gordon et al., 1989).

#### Chemiluminescence

Chemiluminescence methods can be used for the determination of low
concentrations of ozone in the gas phase (Gordon et al., 1992). One of
the most commonly used methods is ethylene chemiluminescence. Gas-phase
ozone can be measured using the chemiluminescent reaction between
ethylene and ozone. This method is specific to ozone and is suitable for
measurement of ozone in the ambient air. The ethylene chemiluminescence
procedure was adopted in 1985 by the EPA as its reference method for
determining ozone in the ambient atmosphere (McKee et al., 1975).
Chemiluminescent instruments are approved by the EPA for monitoring
ambient ozone concentrations of 0 to 0.5 or 0 to 1.0 ppm by volume. With
regular calibration, this type of instrument is capable of providing
reliable analysis of any ozone in the ambient air from an ozonation
plant.

An alternative to ethylene chemiluminescence is rhodamine B/gallic acid
chemiluminescence, which avoids the handling of ethylene (Gordon et al.,
1992). This alternative method is considerably more complex than the
more common ethylene chemiluminescence instruments. The sensitivity of
this method tends to drift and a procedure has been developed by which
corrections are made for the sensitivity on a frequent basis (Van Dijk
and Falkenberg, 1977). Given the wide availability of ethylene
chemiluminescence monitors and their approval by EPA, ethylene monitors
should be considered before rhodamine B/gallic acid monitors.

#### Gas-Phase Titration

Two gas-phase titration methods have been studied as possible
calibration methods for ambient ozone analyzers and monitors (Gordon et
al., 1992). These procedures are based on titration with nitric oxide
and back titration of excess nitric oxide (Rehme et al., 1980). These
gas phase titration procedures, evaluated by EPA, were compared with UV
absorption and iodometry as calibration methods for ethylene
chemiluminescent ambient air ozone analyzers. As a result of these
comparisons, UV absorption has been specified as the method of
calibration for ambient ozone analyzers. Therefore, gas-phase titration
methods are not recommended for use at ozonation facilities (Gordon et
al., 1992).

### Monitoring of Liquid Phase Residual Ozone

There are numerous methods for monitoring ozone in aqueous solutions.
Gordon et al. (1992) recommended the following methods for analyzing
residual ozone:

-   Indigo colorimetric method;

-   Acid chrome violet K (ACVK) method;

-   Bis-(Terpyridine) iron(II) method; and

-   Stripping into the gas-phase.

Table 3-11 shows the working range, expected accuracy and precision,
operator skill level required, interferences and current status for
liquid phase residual ozone analytical methods.

#### Indigo Colorimetric Method

The indigo colorimetric method is the only method for monitoring
residual ozone in *Standard Methods*, 1995. The indigo colorimetric
method is sensitive, precise, fast, and more selective for ozone than
other methods. There are two indigo colorimetric methods:
spectrophotometric and visual. For the spectrophotometric procedure the
lower limit of detection is 2 μg/L, while for the visual procedure the
detection limit is 10 μg/L.

Hydrogen peroxide, chlorine, manganese ions, ozone decomposition
products, and the products of organic ozonation exhibit less
interference with the indigo colorimetric method than any of the other
methods (Langlais et al., 1991). However, the masking of chlorine in the
presence of ozone can make the indigo method problematic. In the
presence of hypobromous acid, which forms during ozonation of
bromide-ion containing methods, an accurate measurement cannot be made
with this method (Standard Methods, 1995).

[]{#__RefHeading___Toc450036686 .anchor}Table 3-11. Characteristics and
Comparisons of Residual Ozone Analytical Methods

<table style="width:100%;">
<colgroup>
<col style="width: 15%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 5%" />
<col style="width: 18%" />
<col style="width: 8%" />
<col style="width: 6%" />
<col style="width: 8%" />
<col style="width: 11%" />
</colgroup>
<tbody>
<tr class="odd">
<td><strong>Type of Test</strong></td>
<td><strong>Working Range (mg/L)</strong></td>
<td><strong>Expected Accuracy (± %)</strong></td>
<td><strong>Expected Precision (± %)</strong></td>
<td><strong>Skill Level<sup>a</sup></strong></td>
<td><strong>Interferences</strong></td>
<td><strong>pH Range</strong></td>
<td><strong>Field Test</strong></td>
<td><strong>Automated Test</strong></td>
<td><strong>Current Status</strong></td>
</tr>
<tr class="even">
<td>Indigo Colorimetric, Spectrophotometric</td>
<td><p>0.01 -</p>
<p>&gt; 0.3</p></td>
<td>1</td>
<td>0.5</td>
<td>1</td>
<td>Chlorine, Manganese ions, Bromine, Iodine</td>
<td>2</td>
<td>No</td>
<td>Yes</td>
<td>Recommended</td>
</tr>
<tr class="odd">
<td>Indigo Colorimetric, Visual</td>
<td><p>0.01 -</p>
<p>&gt; 0.1</p></td>
<td>1</td>
<td>0.5</td>
<td>1</td>
<td>Chlorine, Manganese ions, Bromine, Iodine</td>
<td>2</td>
<td>No</td>
<td>Yes</td>
<td>Recommended</td>
</tr>
<tr class="even">
<td>ACVK</td>
<td>0.05 - 1</td>
<td>NR</td>
<td>NR</td>
<td>1</td>
<td>Manganese ions &gt; 1 mg/L, Chlorine &gt; 10 mg/L</td>
<td>2</td>
<td>No</td>
<td>No</td>
<td>Recommended</td>
</tr>
<tr class="odd">
<td>Bis-(Terpyridine) Iron(II)</td>
<td>0.05 - 20</td>
<td>2.7</td>
<td>2.1</td>
<td>3</td>
<td>Chlorine</td>
<td>&lt; 7</td>
<td>No</td>
<td>Yes</td>
<td>Recommended Lab Test</td>
</tr>
</tbody>
</table>

Source: Gordon et al.*,* 1992.

Notes: ^a^ Operator Skill Levels: 1 = minimal, 2 = good technician, 3 =
experienced chemist. NR = Not reported in literature cited by referenced
source.

#### ACVK Method

ACVK is readily bleached by ozone, which serves as the basis for this
spectrophotometric procedure first reported by Masschelein (1977). The
procedure has been developed further for the measurement of ozone in the
presence of chlorine (Ward and Larder, 1973).

The advantages of this method (Langlais et al., 1991) are:

-   Its ease of execution and stability of the dye reagent solution;

-   A linear calibration curve over the range of 0.05 to 1.0 mg/L of
    ozone; and

-   The apparent lack of interferences from low levels of manganese,
    chlorine or combined chlorine (up to 10 mg/L), organic peroxides,
    and other organic oxidation products.

#### Bis-(Terpyridine)Iron(II) Method

Bis-(Terpyridine)Iron(II) in dilute hydrochloric acid solution reacts
with ozone to change the absorbance spectra measured at 552 nm (Tomiyasu
and Gordon, 1984). The only known interferent is chlorine. However,
chlorine interferences can be masked by the addition of malonic acid.
Alternatively, since the reaction between chlorine and the reagent is
slower than the reaction with ozone, the spectrophotometric measurements
can be carried out immediately after reagent mixing to reduce the
chlorine interference. Chlorine dioxide does not interfere (Gordon et
al, 1992).

The primary advantages of the Bis-(Terpyridine)Iron(II) Method are its
lack of interferences, low limits of detection (4 μg/L), broad working
range (up to 20 mg/L), excellent reproducibility, and agreement with the
indigo colorimetric method.

#### Stripping into the Gas-Phase

In this indirect procedure, residual ozone is stripped from the solution
using an inert gas. The amount of ozone present in the gas phase is then
analyzed by gas-phase analytical methods such as UV absorption or
chemiluminescence described previously. This stripping technique was
developed to minimize the complications caused by the presence of other
oxidants in solution.

The success of this procedure initially depends upon the ability to
strip ozone from the treated water without any decomposition. Since
stripping conditions such as temperature, pH, and salinity can vary, the
reliability of this method is suspect (Langlais et al., 1991).

### Bromate Monitoring for Systems Using Ozone

The DBPR requires that community water systems and non-transient
non-community water systems that use ozone for disinfection or oxidation
must monitor their system for bromate. These systems are required to
take one sample per month for each treatment plant with samples
collected at the entrance to the distribution system while the ozonation
system is operating under normal conditions.

The DBPR provides reduced monitoring opportunities (i.e., quarterly
rather than monthly samples) if the system demonstrates that the average
source water bromide concentration is less than 0.05 mg/L based upon
representative monthly bromide measurements for one year. Systems can
remain on the reduced monitoring schedule until the running annual
average source water bromide concentration, computed quarterly, is equal
to or greater than 0.05 mg/L based upon representative monthly
measurements.

For compliance monitoring for bromate, systems must use the ion
chromatography analytical method as specified in *USEPA Method 300.1,
Determination of Inorganic Anions in Drinking Water by Ion
Chromatography, Revision 1.0* (USEPA, 1997).

If the average of samples covering any consecutive four-quarter period
exceeds the MCL, the system is in violation of the MCL and must notify
the public pursuant to 40 CFR §141.32. The system must also report to
the State pursuant to 40 CFR §141.134. If the system fails to complete
12 consecutive months\' monitoring, compliance with the MCL for the last
four-quarter compliance period must be based on an average of the
available data.

## Operational Considerations

### Process Considerations

Because ozone is such a strong oxidant, it will react with many organic
and inorganic compounds present in the water. Ozone is used to remove
tastes and odors by breaking down organic compounds, and to aid in the
removal of iron and manganese by oxidizing these compounds to less
soluble forms. These demands should be satisfied before any ozone is
available to satisfy primary disinfection requirements. The presence and
concentration of these compounds can dictate the location of ozone
addition, depending on the process goals.

Stolarik and Christie (1997) present the results of 10 years of
operation at the 600 mgd Los Angeles Aqueduct Filtration and Ozone
Facility. Operational experiences at this facility showed lower particle
counts (greater than one micron) with ozone use. The optimum ozone
concentration in the gas phase applied was found to be 6 percent when
using the cryogenic oxygen production facilities, and 4 to 5 percent
when using liquid oxygen (LOX).

### Space Requirements

Storage of LOX is subject to regulations in building and fire codes.
These regulations will impact the space requirements and may dictate the
construction materials of adjacent structures if the certain setback
requirements cannot be met. In general, the footprint for ozone
generated from air is smaller than that required for chloramination and
chloride dioxide applications. However, the footprint area for ozone
generated from pure oxygen is comparable to that of chlorine dioxide
because of the additional area needed for storage.

### Material Selection

Ozone-resistant materials should be used from the ozone generators
through the off-gas destruct unit. If oxygen is used for the feed gas,
oxygen resistant materials should be used up to the generators. Pure
oxygen piping should be specially cleaned after installation for oxygen
service, which increases construction cost. Materials for air
preparation systems can be those normally used for compressed air
systems. Langlais et al. (1991) recommended that piping beyond the
desiccant dryers be ozone- resistant, as some backflow and ozone
diffusion can occur. If a receiver is provided following the desiccant
dryer, the piping should be ozone-resistant, downstream of the pressure
regulator. Ozone-resistant (oxygen resistant as well if high purity
oxygen is the feed gas) check valves should be placed in the piping
ahead of the generator.

Ozone-resistant materials include the austenitic (300 series) stainless
steels, glass and other ceramics, Teflon and Hypalon, and concrete. The
304 series stainless steels can be used for "dry" ozone gas (also for
oxygen), 316 series should be used for "wet" service. Wet service
includes piping in the contactors and all off-gas piping and the off-gas
destruct unit. Teflon or Hypalon should be used for gasket materials.
Concrete should be manufactured from Type II or Type IV cement. Typical
practice in the United States is to provide 3 inches of cover for
reinforcing to prevent corrosion by either ozone gas or ozone in
solution, although Fonlupt (1979) reports that 4 cm (1.13 inches) is
adequate for protection. Hatches for access into contactors should be
fabricated from 316 series stainless steels and provided with
ozone-resistant seals.

### Ozone System Maintenance

Stolarik and Christie (1997) provide a good overview of the operational
and maintenance requirements during the 10 years of operating the 600
mgd Los Angeles Aqueduct Filtration and Ozone Plant. The ozone system
has been available 97.1 percent of the time over the 10 year period.

Fuse failure and generator cleaning comprised the major maintenance
chores on the ozone generators during the first years. Fuse failure was
caused by a malfunction when its glass dielectric tube failed. Vessels
are cleaned every three years or when exit gas temperatures rise due to
Fe~3~O~4~ deposits on the ground electrode/heat exchanger surfaces.

Rod shaped ceramic diffusers worked well as ozone diffusers for the
initial two years. These were replaced by sintered stainless steel and
ultimately a modified ceramic diffuser.

### Ozone Safety

Concern for safety even at the risk of being overcautious, would be to
follow practices that have been successfully applied to other oxidants
over the years. This would be to generally isolate the ozonation system
from the remainder of the plant. This should not be interpreted to mean
a separate building, but rather separate rooms, separate exterior
entrances, separate heating and ventilation systems, noise control, etc.
This method already is manifested in some of the European ozonation
plants, but on a lesser scale.

Ozone generators should be housed indoors for protection from the
environment and to protect personnel from leaking ozone in the case of a
malfunction. Ventilation should be provided to prevent excess
temperature rise in the generator room, and to exhaust the room in the
case of a leak. Adequate space should be provided to remove the tubes
from the generator shell and to service the generator power supplies.
Air prep systems tend to be noisy; therefore, it is desirable to
separate them from the ozone generators. Off-gas destruct units can be
located outside if the climate is not too extreme. If placed inside, an
ambient ozone detector should be provided in the enclosure. All rooms
should be properly ventilated, heated, and cooled to match the
equipment-operating environment.

Continuous monitoring instruments should be maintained to monitor levels
of ozone in the rooms. Self-contained breathing apparatuses should be
located in hallways outside the rooms liable to ozone hazards. Ambient
ozone exposure levels, which have been proposed by appropriate U.S.
organizations, are summarized below. The maximum recommended ozone
levels are as follows:

-   **Occupational Safety and Health Administration.** The maximum
    permissible exposure to airborne concentrations of ozone not in
    excess of 0.1 mg/L (by volume) averaged over an eight-hour work
    shift.

-   **American National Standards Institute/American Society for testing
    Materials (ANSI/ASTM).** Control occupational exposure such that the
    worker will not be exposed to ozone concentrations in excess of a
    time weighted average of 0.1 mg/L (by volume) for eight hours or
    more per workday, and that no worker be exposed to a ceiling
    concentration of ozone in excess of 0.3 mg/L 9by volume) for more
    than ten minutes.

-   **American Conference of Government Industrial Hygienists (ACGIH).**
    Maximum ozone level of 0.1 mg/L (by volume) for a normal eight hour
    work day or 40 hour work week, and a maximum concentration of 0.3
    mg/L (by volume) for exposure of up to 15 minutes.

-   **American Industrial Hygiene Association.** Maximum, concentration
    for eight hour exposure of 0.1 mg/L (by volume).

There is a question of whether prolonged exposure to ozone may impair a
worker's ability to smell or be aware of ozone levels at less than
critical levels. Awareness of an odor of ozone should not be relied
upon. Instrumentation and equipment should be provided to measure
ambient ozone levels and perform the following safety functions:

-   Initiate an alarm signal at an ambient ozone level of 0.1 mg/L (by
    volume). Alarms should include warning lights in the main control
    panel and at entrances to the ozonation facilities as well as
    audible alarms.

-   Initiate a second alarm signal at ambient ozone levels of 0.3 mg/L
    (by volume). This signal would immediately shut down ozone
    generation equipment and would initiate a second set of visual and
    audible alarms at the control panel and at the ozone generation
    facility entrances. An emergency ventilation system capable of
    exhausting the room within a period of 2 to 3 minutes also would be
    interconnected to the 0.3 mg/L ozone level alarm.

Ozone gas is a hazardous gas and should be handled accordingly. Ambient
ozone levels should be monitored and equipment shut- down and alarmed
when levels exceed 0.1 ppm. Emergency ventilation is typically provided
for enclosed areas. Building and fire codes will provide additional
guidance. The OSHA exposure limit for an 8-hour shift is 0.1 ppm by
volume. The pungent odor of ozone will provide warning to operators of
any possible ozone leak.

## Summary

### Advantages and Disadvantages of Ozone Use

The following list highlights selected advantages and disadvantages of
using ozone as a disinfection method for drinking water (Masschelein,
1992). Because of the wide variation of system size, water quality, and
dosages applied, some of these advantages and disadvantages may not
apply to a particular system.

**Advantages**

-   Ozone is more effective than chlorine, chloramines, and chlorine
    dioxide for inactivation of viruses, *Cryptosporidium*, and
    *Giardia*.

-   Ozone oxidizes iron, manganese, and sulfides.

-   Ozone can sometimes enhance the clarification process and turbidity
    removal.

-   Ozone controls color, taste, and odors.

-   One of the most efficient chemical disinfectants, ozone requires a
    very short contact time.

-   In the absence of bromide, halogen-substitutes DBPs are not formed.

-   Upon decomposition, the only residual is dissolved oxygen.

-   Biocidal activity is not influenced by pH.

**Disadvantages**

-   DBPs are formed, particularly by bromate and bromine-substituted
    DBPs, in the presence of bromide, aldehydes, ketones, etc.

-   The initial cost of ozonation equipment is high.

-   The generation of ozone requires high energy and should be generated
    on-site.

-   Ozone is highly corrosive and toxic.

-   Biologically activated filters are needed for removing assimilable
    organic carbon and biodegradable DBPs.

-   Ozone decays rapidly at high pH and warm temperatures.

-   Ozone provides no residual.

-   Ozone requires higher level of maintenance and operator skill.

### Summary Table

Table 3-12 presents a summary of the considerations for the use of ozone
as a disinfectant.

[]{#__RefHeading___Toc450036687 .anchor}Table 3-12. Summary of Ozone
Disinfection Considerations

  -----------------------------------------------------------------------
  **Consideration**     **Description**
  --------------------- -------------------------------------------------
  Generation            Because of its instability, ozone should be
                        generated at the point of use. Ozone can be
                        generated from oxygen present in air or high
                        purity oxygen. The feed gas source should be
                        clean and dry, with a maximum dewpoint of
                        -60^o^C. Ozone generation consumes power at a
                        rate of 8 to 17 kWhr/kg O~3~. Onsite generation
                        saves a lot of storage space.

  Primary uses          Primary uses include primary disinfection and
                        chemical oxidation. As an oxidizing agent, ozone
                        can be used to increase the biodegradability of
                        organic compounds destroys taste and odor
                        control, and reduce levels of chlorination DBP
                        precursors. Ozone should not be used for
                        secondary disinfection because it is highly
                        reactive and does not maintain an appreciable
                        residual level for the length of time desired in
                        the distribution system.

  Inactivation          Ozone is one of the most potent and effective
  efficiency            germicide used in water treatment. It is
                        effective against bacteria, viruses, and
                        protozoan cysts. Inactivation efficiency for
                        bacteria and viruses is not affected by pH; at pH
                        levels between 6 and 9. As water temperature
                        increases, ozone disinfection efficiency
                        increases.

  Byproduct formation   Ozone itself does not form halogenated DBPs;
                        however, if bromide ion is present in the raw
                        water or if chlorine is added as a secondary
                        disinfectant, halogenated DBPs, including bromate
                        ion may be formed. Other ozonation byproducts
                        include organic acids and aldehydes.

  Limitations           Ozone generation is a relatively complex process.
                        Storage of LOX (if oxygen is to be the feed gas)
                        is subject to building and fire codes.

  Points of application For primary disinfection, ozone addition should
                        be prior to biofiltration/filtration and after
                        sedimentation. For oxidation, ozone addition can
                        be prior to coagulation/sedimentation or
                        filtration depending on the constituents to be
                        oxidized.

  Safety considerations Ozone is a toxic gas and the ozone production and
                        application facilities should be designed to
                        generate, apply, and control this gas, so as to
                        protect plant personnel. Ambient ozone levels in
                        plant facilities should be monitored
                        continuously.
  -----------------------------------------------------------------------

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105. Yamada, H. and I. Somiya. 1989. "The Determination of Carbonyl
     Compounds in Ozonated Water By the PFBOA Method." *Ozone Sci.
     Engrg*. 11(2):127.

106. Zabel, T.F. 1985. "The Application of Ozone for Water Treatment in
     the United Kingdom - Current Practice and Recent Research." *Ozone
     Sci. Engrg.* 7(1):11.

Ozone [3-1](#ozone)

3.1 Ozone Chemistry [3-1](#ozone-chemistry)

3.2 Ozone Generation [3-4](#ozone-generation)

3.2.1 Ozone Production [3-4](#ozone-production)

3.2.2 System Components [3-5](#system-components)

3.2.3 Operation and Maintenance [3-15](#operation-and-maintenance)

3.3 Primary Uses and Points of Application of Ozone
[3-16](#primary-uses-and-points-of-application-of-ozone)

3.3.1 Primary Uses of Ozone [3-16](#primary-uses-of-ozone)

3.3.2 Points of Application [3-18](#points-of-application)

3.3.3 Impact on Other Treatment Processes
[3-19](#impact-on-other-treatment-processes)

3.3.4 Biologically Active Filtration
[3-19](#biologically-active-filtration)

3.3.5 Pathogen Inactivation and Disinfection Efficacy
[3-21](#pathogen-inactivation-and-disinfection-efficacy)

3.3.6 Inactivation Mechanisms [3-22](#inactivation-mechanisms)

3.3.7 Disinfection Parameters [3-22](#disinfection-parameters)

3.3.8 Inactivation of Microorganisms
[3-24](#inactivation-of-microorganisms)

3.4 Ozonation Disinfection Byproducts
[3-27](#ozonation-disinfection-byproducts)

3.4.1 Ozone Byproduct Control [3-30](#ozone-byproduct-control)

3.5 Status of Analytical Methods [3-31](#status-of-analytical-methods)

3.5.1 Monitoring of Gas Phase Ozone
[3-31](#monitoring-of-gas-phase-ozone)

3.5.2 Monitoring of Liquid Phase Residual Ozone
[3-35](#monitoring-of-liquid-phase-residual-ozone)

3.5.3 Bromate Monitoring for Systems Using Ozone
[3-37](#bromate-monitoring-for-systems-using-ozone)

3.6 Operational Considerations [3-38](#operational-considerations)

3.6.1 Process Considerations [3-38](#process-considerations)

3.6.2 Space Requirements [3-38](#space-requirements)

3.6.3 Material Selection [3-39](#material-selection)

3.6.4 Ozone System Maintenance [3-39](#ozone-system-maintenance)

3.6.5 Ozone Safety [3-39](#ozone-safety)

3.7 Summary [3-41](#summary)

3.7.1 Advantages and Disadvantages of Ozone Use
[3-41](#advantages-and-disadvantages-of-ozone-use)

3.7.2 Summary Table [3-42](#summary-table)

3.8 References [3-43](#references)

Table 3-1. Types of Compressors Used in Air Preparation Systems
[3-7](#_Ref401114406)

Table 3-2. Comparison of Air and High Purity Oxygen Feed Systems
[3-8](#_Ref419264800)

Table 3-3. Comparison of Primary Characteristics of Low, Medium, and
High Frequency Ozone Generators [3-10](#__RefHeading___Toc450036678)

Table 3-4. Bubble Diffuser Contactor Advantages and Disadvantages
[3-12](#__RefHeading___Toc450036679)

Table 3-5. Injection Contacting Advantages and Disadvantages
[3-13](#__RefHeading___Toc450036680)

Table 3-6. Turbine Mixer Contactor Advantages and Disadvantages
[3-14](#__RefHeading___Toc450036681)

Table 3-7. Criteria for Selecting Ozone Feed Points for Small Systems
[3-18](#__RefHeading___Toc450036682)

Table 3-8. Summary of Reported Ozonation Requirements for 99 Percent
Inactivation of *Cryptosporidium* Oocycts
[3-27](#__RefHeading___Toc450036683)

Table 3-9. Principal Known Byproducts of Ozonation
[3-28](#__RefHeading___Toc450036684)

Table 3-10. Characteristics and Comparisons of Gas-Phase Ozone
Analytical Methods [3-33](#_Ref400940475)

Table 3-11. Characteristics and Comparisons of Residual Ozone Analytical
Methods [3-36](#__RefHeading___Toc450036686)

Table 3-12. Summary of Ozone Disinfection Considerations
[3-42](#__RefHeading___Toc450036687)

Figure 3-1. Oxidation Reactions of Compounds (Substrate)
During Ozonation of Water [3-2](#__RefHeading___Toc450036688)

Figure 3-2. Reaction of Ozone and Bromide Ion Can Produce Bromate Ion
and Brominated Organics [3-3](#_Ref401041178)

Figure 3-3. Basic Ozone Generator [3-4](#__RefHeading___Toc450036690)

Figure 3-5. Schematic of an Air Preparation System
[3-7](#__RefHeading___Toc450036691)

Figure 3-6. Cylindrical Electrode Schematic
[3-9](#__RefHeading___Toc450036692)

Figure 3-7. Ozone Bubble Contactor [3-11](#__RefHeading___Toc450036693)

Figure 3-8. Sidestream Ozone Injection System
[3-12](#__RefHeading___Toc450036694)

Figure 3-9. Turbine Mixer Ozone Contactor
[3-14](#__RefHeading___Toc450036695)

Figure 3-10. CT Values for Inactivation of Viruses by Ozone (pH 6 to 9)
[3-26](#__RefHeading___Toc450036696)

Figure 3-11. Principal Reactions Producing Ozone Byproducts
[3-28](#__RefHeading___Toc450036697)

Figure 3-12. Main Pathways of Bromate Ion Formation when Ozone Reacts
with Bromide Ion [3-30](#_Ref401031260)