# Combined Disinfectants

Multiple disinfectants, the sequential or simultaneous use of two or
more disinfectants, have been used with increasing frequency in recent
years. This trend is attributed to the fact that:

-   Less reactive disinfectants, such as chloramines, have proven to be
    quite effective in reducing DBPs formed during disinfection and are
    more effective for controlling biofilms in the distribution system.

-   Regulatory and consumer pressure to produce water that has been
    disinfected to achieve high inactivation for various pathogens, has
    pushed the industry towards more effective disinfectants. Sometimes
    more effective disinfection meant using higher disinfectant doses
    which also produces more DBPs.

-   Recent research has shown that the application of sequential
    disinfectants is more effective than the added effect of the
    individual disinfectants. This process where two (or more)
    disinfectants produce a synergistic effect by either simultaneous or
    sequential application to achieve more effective pathogen
    inactivation, is referred to as ***interactive disinfection*** in
    this manual.

This chapter discusses recent industry applications of multiple
disinfectants to meet the varied requirements for inactivation and
reduction in DBPs. The initial discussion focuses on traditional
disinfectants used in primary and secondary application. This is
followed by a discussion of interactive disinfectants where two
disinfectants are applied together specifically to achieve primary
disinfection. Note that the IESWTR does not have any provision for
additional credits for interactive disinfection or taking additional
credit for the synergistic effects from interactive disinfection. Until
such credit is established, interactive disinfection is considered an
emerging technology. This chapter does not discuss mixed oxidant
systems, which are designed to generate mixed oxidants on-site for
drinking water disinfection, and are also considered an emerging
technology.

## Primary and Secondary Disinfectants

By separating the inactivation function and residual disinfection
function in water treatment, each can be optimized independently. Thus,
the combination of disinfectants currently used in disinfection is
typically identified as a primary or secondary disinfectant, as follows:

-   Primary disinfection refers to the inactivation of microorganisms to
    meet the regulatory bacteriological requirements. This requirement
    typically is met by achieving certain CT requirements to assure a
    target log inactivation of target organisms as set forth in the
    Surface Water Treatment Rule (SWTR) (AWWA, 1991).

-   ![](media/image1.wmf)Secondary disinfection refers to application of
    a disinfectant to meet regulatory requirements for distribution
    system bacteriological quality as set forth in the Total Coliform
    Rule (TCR). The SWTR requires that a residual disinfectant be
    measured in the distribution system, or that the bacteriological
    quality meet certain standards (heterotrophic plate count (HPC) less
    than 500/100mL).

To be an effective primary disinfectant, the disinfectant should
effectively inactivate the target organism. Table 9-1 lists potential
primary disinfectants as discussed in Chapters 2 through 8.

[]{#__RefHeading___Toc450037472 .anchor}Table 9-1. Potential Primary
Disinfectants

+--------------------------+---------------------+---------------------+
| Target Organism          | Potential Primary   |                     |
|                          | Disinfectants       |                     |
+==========================+=====================+=====================+
|                          | With Filtration^1^  | Without Filtration  |
+--------------------------+---------------------+---------------------+
| Coliform Bacteria        | Chlorine\           | Chlorine\           |
|                          | Chloramines\        | Chlorine dioxide\   |
|                          | Chlorine dioxide\   | Interactive         |
|                          | Ozone\              | disinfection^3^     |
|                          | UV\                 |                     |
|                          | Interactive         |                     |
|                          | disinfection        |                     |
+--------------------------+---------------------+---------------------+
| Giardia cysts            | Chlorine^2^\        | Chlorine^2^\        |
|                          | Chlorine            | Chlorine            |
|                          | dioxide^2^\         | dioxide^2^\         |
|                          | Ozone^2^\           | Interactive         |
|                          | Interactive         | disinfection^3^     |
|                          | disinfection        |                     |
+--------------------------+---------------------+---------------------+
| Viruses                  | Chlorine^2^\        | Chlorine^2^\        |
|                          | Chlorine            | Chlorine            |
|                          | dioxide^2^\         | dioxide^2^\         |
|                          | Ozone^2^\           | UV^2^               |
|                          | UV^2^\              |                     |
|                          | Interactive         | Interactive         |
|                          | disinfection        | disinfection^3^     |
+--------------------------+---------------------+---------------------+
| Cryptosporidium oocysts  | Chlorine dioxide\   | Chlorine dioxide\   |
|                          | Ozone\              | Interactive         |
|                          | Interactive         | disinfection^3^     |
|                          | disinfection        |                     |
+--------------------------+---------------------+---------------------+

^1^ Natural or treatment filtration reduces disinfection inactivation
requirements.

^2^ Inactivation credit established in SWTR.

^3^ Any interactive disinfection that uses ozone or peroxone without
filtration is strongly discouraged.

As discussed in earlier chapters, certain disinfectants (e.g., ozone,
UV, peroxone, and in some cases chlorine dioxide), while being effective
disinfectants, do not leave a long-lasting residual disinfectant.
Therefore, secondary disinfection is limited to those disinfectants that
remain stable in the distribution system. In order of decreasing
stability, these secondary disinfectants are chloramines, chlorine, and
chlorine dioxide.

Based on the above, the combinations of disinfectants that are viable
options to meet the disinfection requirements can be determined for
various treatment trains. These combinations are shown for various
treatment objectives. Note that the treatment objectives are dependent
on the treatment currently in place.

To meet DBP, and specifically, THM limits, several studies have
evaluated the application of various primary/secondary disinfectants.
Table 9-2 presents the typical application of these combined
disinfectants.

[]{#__RefHeading___Toc450037473 .anchor}Table 9-2. Primary/Secondary
Disinfectant Combinations and Typical Applications in Water Treatment

  -----------------------------------------------------------------------
  **Primary /           **Typical                **Comment**
  Secondary**           application**\*          
  --------------------- ------------------------ ------------------------
  Chlorine/Chlorine     Low THMFP raw water, low Most commonly used
                        TOC, conventional        disinfection scheme.
                        treatment with optimal   Effective system.
                        coagulation.             

  Chlorine/Chloramine   Moderate THM production  Chlorine to provide
                        situation, typically     disinfection and
                        with conventional        monochloramine to limit
                        treatment.               DBP formation.

  Chlorine              High DBP production,     Primary and secondary
  dioxide/Chlorine      require filter process   usage requires a limit
  dioxide               to remove                on chlorine dioxide dose
                        *Cryptosporidium*, low   to reduce residual
                        chlorine dioxide demand  chlorate/chlorite.
                        in treated water.        

  Chlorine              High DBP production,     Primary chlorine dioxide
  dioxide/Chloramine    require filtration to    dose limited to residual
                        remove                   chlorate/chlorite.
                        *Cryptosporidium*.       Stable, low reactive
                                                 secondary disinfectant.

  Ozone/Chlorine        Moderate DBP formation,  Highly effective
                        direct or no filtration, disinfection to achieve
                        low THMFP.               high log inactivation;
                                                 low THMFP to accept free
                                                 chlorine.

  Ozone/Chloramine      Moderate DBP formation,  Highly effective
                        direct or no filtration, disinfection to achieve
                        higher THMFP.            high log inactivation,
                                                 low THMFP to require
                                                 combined chlorine
                                                 residual.

  UV/Chlorine           Requires membrane        Rare application but
                        treatment to provide     feasible in special
                        effective *Giardia* and  circumstances. Little
                        *Cryptosporidium*        *Giardia* and no
                        removal. UV only for     *Cryptosporidium*
                        virus inactivation;      inactivation.
                        ground water             
                        disinfection; low THMFP. 

  UV/Chloramine         Requires membrane        Rare application but
                        treatment to provide     feasible in special
                        effective *Giardia* and  circumstances. No
                        *Cryptosporidium*        *Giardia* or
                        removal. UV only for     *Cryptosporidium*
                        virus inactivation;      inactivation.
                        ground water             
                        disinfection, moderate   
                        THMFP.                   
  -----------------------------------------------------------------------

Notes:

\* Low DBP formation is defined as producing less than the Stage 2 D/DBP
Proposed Standard (less than 0.040 mg/L TTHM; less than 0.030 mg/L
HAA5). Moderate DBP formation is defined as producing less than the
Stage 1 D/DBP Standard and more than the Stage 2 D/DBP Proposed
Standard. High DBP formation is defined as producing more than the Stage
1 D/DBP Standard (greater than 0.080 mg/L TTHM; greater than 0.060 mg/L
HAA5).

### DBP Formation with Various Primary and Secondary Disinfectant Combinations

The concentrations and types of DBPs formed depend on, among other
things, the combination of disinfectants used to achieve primary and
secondary disinfection and the water quality. For example, under certain
water quality conditions, ozone/chloramine disinfection is known to
produce lower THM concentrations than chlorine/chloramine disinfection.
However, the ozone/chloramine alternative can increase the formation of
other DBPs such as aldehydes and BOM. No single combination of
disinfectants is applicable to all situations. Table 9-3 summarizes the
potential DBPs formed by various combinations of disinfectants. The
disinfection byproducts referenced here are discussed in greater detail
in earlier chapters of this manual.

[]{#__RefHeading___Toc450037474 .anchor}Table 9-3. DBPs Associated with
Various Combined Oxidation/Disinfection Processes

+---------+-------+-------+--------------------+--------------------+
| *       |       |       |                    |                    |
| *Altern |       |       |                    |                    |
| ative** |       |       |                    |                    |
+=========+=======+=======+====================+====================+
| **      | **Pr  | *     | **Potential DBPs** | **Remarks**        |
| Preoxid | imary | *Seco |                    |                    |
| ation** | Disi  | ndary |                    |                    |
|         | nfect | Disi  |                    |                    |
|         | ion** | nfect |                    |                    |
|         |       | ion** |                    |                    |
+---------+-------+-------+--------------------+--------------------+
| C       | Chl   | Chl   | XDBPs\*            | Maximum XDBP       |
| hlorine | orine | orine |                    | formation compared |
|         |       |       |                    | to all other       |
|         |       |       |                    | strategies.        |
|         |       |       |                    | Principal          |
|         |       |       |                    | components are     |
|         |       |       |                    | TTHMs and HAAs.    |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Aldehydes          | Formed at          |
|         |       |       |                    | relatively low     |
|         |       |       |                    | levels.            |
+---------+-------+-------+--------------------+--------------------+
| C       | Chl   | Chlor | XDBPs              | Formation of XDBPs |
| hlorine | orine | amine |                    | (specifically      |
|         |       |       | Cyanogen chloride  | TTHMs and HAA5s)   |
|         |       |       | Cyanogen bromide   | significantly      |
|         |       |       |                    | reduced compared   |
|         |       |       |                    | to chlorine/       |
|         |       |       |                    | chlorine/          |
|         |       |       |                    | chlorine.          |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Aldehydes          | Formed at          |
|         |       |       |                    | relatively low     |
|         |       |       |                    | levels.            |
+---------+-------+-------+--------------------+--------------------+
| C       | Chl   | Chl   | XDBPs              | Formation of XDBPs |
| hlorine | orine | orine |                    | may be decreased   |
| dioxide | di    |       |                    | by delaying the    |
|         | oxide |       |                    | point of chlorine  |
|         |       |       |                    | addition.          |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Aldehydes,         | Formed at          |
|         |       |       | carboxylic acids,  | relatively low     |
|         |       |       | maleic acids       | levels.            |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Chlorate           | Chlorite is a      |
|         |       |       |                    | major breakdown    |
|         |       |       | Chlorite           | product of         |
|         |       |       |                    | chlorine dioxide.  |
+---------+-------+-------+--------------------+--------------------+
| C       | Chl   | Chlor | XDBPs              | Formation of XDBPs |
| hlorine | orine | amine |                    | (especially TTHMs  |
| dioxide | di    |       |                    | and HAA5s)         |
|         | oxide |       |                    | minimized by       |
|         |       |       |                    | avoiding use of    |
|         |       |       |                    | free chlorine.     |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Aldehydes,         | Formed at          |
|         |       |       | carboxylic acids,  | relatively low     |
|         |       |       | maleic acid        | levels.            |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Chlorate           | Chlorite is a      |
|         |       |       |                    | major breakdown    |
|         |       |       | Chlorite           | product of         |
|         |       |       |                    | chlorine dioxide.  |
+---------+-------+-------+--------------------+--------------------+
| Po      | Chl   | Chl   | XDBPs              | Formation of XDBPs |
| tassium | orine | orine |                    | may be decreased   |
| perma   |       |       |                    | by delaying the    |
| nganate |       |       |                    | point of chlorine  |
|         |       |       |                    | addition.          |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Aldehydes          | Formed at          |
|         |       |       |                    | relatively low     |
|         |       |       |                    | levels.            |
+---------+-------+-------+--------------------+--------------------+
| Po      | Chl   | Chlor | XDBPs              | Formation of XDBPs |
| tassium | orine | amine |                    | may further be     |
| perma   |       |       | Cyanogen chloride  | decreased compared |
| nganate |       |       | Cyanogen bromide   | to potassium       |
|         |       |       |                    | permanganate/      |
|         |       |       |                    | chlorine/          |
|         |       |       |                    | chlorine.          |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Aldehydes          | Formed at          |
|         |       |       |                    | relatively low     |
|         |       |       |                    | levels.            |
+---------+-------+-------+--------------------+--------------------+
| Ozone   | Ozone | Chl   | XDBPs              | Formation of       |
|         |       | orine |                    | certain XDBPs may  |
|         |       |       |                    | increase or        |
|         |       |       |                    | decrease compared  |
|         |       |       |                    | to chlorine/       |
|         |       |       |                    | chlorine/          |
|         |       |       |                    | chlorine.          |
|         |       |       |                    | Brominated         |
|         |       |       |                    | byproducts may be  |
|         |       |       |                    | of concern when    |
|         |       |       |                    | bromides are       |
|         |       |       |                    | present in the raw |
|         |       |       |                    | water.             |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Bromate,           | Although formed at |
|         |       |       | Aldehydes,         | relatively high    |
|         |       |       | carboxylic acids   | levels significant |
|         |       |       |                    | amounts of this    |
|         |       |       |                    | BOM can be removed |
|         |       |       |                    | through biological |
|         |       |       |                    | filtration.        |
+---------+-------+-------+--------------------+--------------------+
| Ozone   | Ozone | Chlor | XDBPs              | Formation of XDBPs |
|         |       | amine |                    | (especially TTHMs) |
|         |       |       | Cyanogen chloride  | minimized by       |
|         |       |       |                    | avoiding use of    |
|         |       |       | Cyanogen bromide   | free chlorine.     |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Bromate,           | Although formed at |
|         |       |       | Aldehydes,         | relatively high    |
|         |       |       | carboxylic acids   | levels significant |
|         |       |       |                    | amounts of this    |
|         |       |       |                    | BOM can be removed |
|         |       |       |                    | through biological |
|         |       |       |                    | filtration.        |
+---------+-------+-------+--------------------+--------------------+
| P       | Chl   | Chl   | XDBPs              | Formation of       |
| eroxone | orine | orine |                    | certain XDBPs may  |
|         | or    |       |                    | increase or        |
|         | Ozone |       |                    | decrease compared  |
|         |       |       |                    | to chlorine/       |
|         |       |       |                    | chlorine/          |
|         |       |       |                    | chlorine.          |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Bromate,           | Although formed at |
|         |       |       | Aldehydes,         | relatively high    |
|         |       |       | carboxylic acids   | levels significant |
|         |       |       |                    | amounts of this    |
|         |       |       |                    | BOM can be removed |
|         |       |       |                    | through biological |
|         |       |       |                    | filtration. Also,  |
|         |       |       |                    | the formation of   |
|         |       |       |                    | bromate will       |
|         |       |       |                    | increase if        |
|         |       |       |                    | peroxone is used.  |
+---------+-------+-------+--------------------+--------------------+
| P       | Chl   | Chlor | XDBPs              | Formation of XDBPs |
| eroxone | orine | amine |                    | may decrease       |
|         | or    |       | Cyanogen chloride  | compared to        |
|         | Ozone |       | Cyanogen bromide   | peroxone/          |
|         |       |       |                    | chlorine/          |
|         |       |       |                    | chlorine.          |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Bromate,           | Although formed at |
|         |       |       | Aldehydes,         | relatively high    |
|         |       |       | carboxylic acids   | levels significant |
|         |       |       |                    | amounts of this    |
|         |       |       |                    | BOM can be removed |
|         |       |       |                    | through biological |
|         |       |       |                    | filtration. Also,  |
|         |       |       |                    | the formation of   |
|         |       |       |                    | bromate will       |
|         |       |       |                    | increase if        |
|         |       |       |                    | peroxone is used.  |
+---------+-------+-------+--------------------+--------------------+
| C       | U     | Chlor | XDBPs              | May form XDBP from |
| hlorine | V\*\* | amine |                    | pre-oxidation.     |
|         |       |       | Cyanogen chloride  |                    |
|         |       |       | Cyanogen bromide   |                    |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Aldehydes          | Low levels.        |
+---------+-------+-------+--------------------+--------------------+
| Po      | U     | Chlor | XDBPs              | Very low due to    |
| tassium | V\*\* | amine |                    | less reactive      |
| Perma   |       |       |                    | oxidants.          |
| nganate |       |       |                    |                    |
+---------+-------+-------+--------------------+--------------------+
|         |       |       | Aldehydes,         | Very low, if any,  |
|         |       |       | carboxylic acids   | due to less        |
|         |       |       |                    | reactive oxidants. |
+---------+-------+-------+--------------------+--------------------+

\* XDBPs - Halogenated Disinfection Byproducts.\
\* \* Although "conventional" UV use as primary disinfectant is limited
to virus inactivation (may require membrane filtration), pulsed UV may
be able to inactivate *Giardia* and *Cryptosporidium*.\
Source: Adopted in part from USEPA, 1992; Richardson et al., 1994.

Raw water chlorination, applied prior to natural organic matter (NOM)
removal processes, combined with chlorination for residual disinfection
produces the greatest concentrations of halogenated DBPs. Studies
indicate that pre-oxidation of raw water with ozone or chlorine dioxide
can reduce the formation of halogenated DBPs because it shifts the point
of chlorine application from raw water to settled or filtered water
which has lower DBP precursor concentrations (MWDSC and JMM, 1989).

The use of ozone can reduce the formation of halogenated byproducts in
waters containing low concentrations of bromide. However, ozone
increases BOM and may encourage bacterial growth in the distribution
system. Removal of AOC with biological filtration (e.g., biological
activated carbon) reduces the potential for bacterial growth in the
distribution system. The use of chloramines as a secondary disinfectant
instead of chlorine shortens the chlorine contact time and thus reduces
the formation of chlorinated byproducts. However, chloramine produces
by-products of its own (cyanogen chloride and cyanogen bromide). In
addition, a short chlorine contact time prior to ammonia addition will
help inactivate heterotrophic plate count bacteria that are found in the
effluent of a biologically active filter. Bench or pilot studies will be
required to evaluate the trade-offs in DBP formation for various
disinfectant combinations for a specific application.

The application of ozone should be carefully considered because it
produces aldehydes, aldoketoacids, and carboxylic acids. However, these
can be removed in a biologically active filter. In bromide-containing
waters, ozonation can increase the formation of brominated organic DBPs
and form bromate.

In pilot plant studies for water containing low concentrations of
bromide, Lykins et al. *(*1991) determined that ozonation followed by
chloramination produced the lowest levels of halogenated disinfection
byproducts. However, this is not applicable to source waters containing
significant bromide concentrations due to the potential for bromate
formation and brominated THMs and HAAs. The addition of chlorine dioxide
will produce chlorite and chlorate and may form some oxygenated DBPs
(e.g., maleic acids).

### Impact of Modifying Disinfection Practices

EPA and the Association of Metropolitan Water Agencies funded a 2-year
study of 35 water treatment facilities to evaluate DBP production. Among
four of the facilities, eleven alternative disinfection strategies were
instigated to evaluate the difference in DBP production from the plants'
previously existing disinfection strategies. Three reports (MWDSC and
JMM, 1989; Jacangelo, 1989; USEPA, 1992) analyzed and documented
different aspects of the study. Table 9-4 shows the eleven potential
strategies used for primary and secondary disinfection. Table 9-5 shows
the changes in DBP production observed in the four plants after eight of
these new strategies were implemented. Following are overviews of the
potential implications of these strategies, as detailed by the three
reports.

[]{#__RefHeading___Toc450037475 .anchor}Table 9-4. Strategies for
Primary and Secondary Disinfectants

  ------------------------------------- ---------------------------------
  ***Base Disinfection Condition***     ***Modified Disinfection
                                        Practice***

  Chlorine/Chlorine                     Chlorine/Chloramine

  Chlorine/Chlorine                     Chloramine/Chloramine

  Chlorine/Chlorine                     Chlorine Dioxide/Chloramine

  Chlorine/Chlorine                     Ozone/Chlorine

  Chlorine/Chlorine                     Ozone/Chloramine

  Chlorine/Chlorine                     Chlorine Dioxide/Chlorine

  Chlorine/Chlorine                     Chlorine Dioxide/Chlorine Dioxide

  Chlorine/Chloramine                   Ozone/Chloramine

  Chlorine/Chloramine                   Chlorine Dioxide/Chloramine

  Ozone/Chlorine                        Ozone/Chloramine

  Chloramine/Chloramine                 Ozone/Chloramine
  ------------------------------------- ---------------------------------

> Note: Disinfectants are listed as primary disinfectant/secondary
> disinfectant

[]{#_Ref396120594 .anchor}

Table 9-5. Impacts of Disinfection Practice on DBP Formation

+------------+----------+--------+---------+-----------+------------+
| **Di       | **Change |        |         |           |            |
| sinfection | in       |        |         |           |            |
| B          | Disi     |        |         |           |            |
| yproduct** | nfection |        |         |           |            |
|            | Practice |        |         |           |            |
|            | (Primary |        |         |           |            |
|            | Disinf   |        |         |           |            |
|            | ectant/S |        |         |           |            |
|            | econdary |        |         |           |            |
|            | Disinfe  |        |         |           |            |
|            | ctant)** |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
|            | **Chl    | **C    |         | **Chl     | *          |
|            | orine/Ch | hlorin |         | orine/Chl | *Chlorine/ |
|            | lorine** | e/Chlo |         | oramine** | Chlorine** |
|            |          | rine** |         |           |            |
|            | **to**   |        |         | **to**    | **to**     |
|            |          | **to** |         |           |            |
|            | **Chlor  |        |         | **        | **Chlor    |
|            | ine/Chlo | **Ozon |         | Ozone/Chl | amines/Chl |
|            | ramine** | e/Chlo |         | oramine** | oramines** |
|            |          | rine** |         |           |            |
+------------+----------+--------+---------+-----------+------------+
|            | *        | **U    | **      | **Ut      | **Utility  |
|            | *Utility | tility | Utility | ility#7** | #36**      |
|            | #7**     | #19**  | #36**   |           |            |
+------------+----------+--------+---------+-----------+------------+
| Total      | decrease | de     | no      | decrease  | decrease   |
| Triha      |          | crease | change  |           |            |
| lomethanes |          |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| Total      | decrease | de     | no      | decrease  | decrease   |
| Haloacetic |          | crease | change  |           |            |
| Acids      |          |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| Total      | decrease | de     | no      | decrease  | decrease   |
| Haloace    |          | crease | change  |           |            |
| tonitriles |          |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| Total      | decrease | no     | i       | increase  | decrease   |
| H          |          | change | ncrease |           |            |
| aloketones |          |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| Total      | not      | not    | i       | not       | decrease   |
| Aldehydes  | analyzed | an     | ncrease | analyzed  |            |
|            |          | alyzed |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| Ch         | no       | in     | i       | decrease  | no change  |
| loropicrin | change   | crease | ncrease |           |            |
+------------+----------+--------+---------+-----------+------------+
| Chloral    | decrease | in     | i       | decrease  | decrease   |
| Hydrate    |          | crease | ncrease |           |            |
+------------+----------+--------+---------+-----------+------------+
| Cyanogen   | no       | not    | no      | no change | increase   |
| Chloride   | change   | an     | change  |           |            |
|            |          | alyzed |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| **Di       | **Change |        |         |           |            |
| sinfection | in       |        |         |           |            |
| By         | Disi     |        |         |           |            |
| -Product** | nfection |        |         |           |            |
|            | Practice |        |         |           |            |
|            | (Primary |        |         |           |            |
|            | Disinf   |        |         |           |            |
|            | ectant/S |        |         |           |            |
|            | econdary |        |         |           |            |
|            | Disinfe  |        |         |           |            |
|            | ctant)** |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
|            | **       | **C    |         | **C       |            |
|            | Ozone/Ch | hloram |         | hlorine/C |            |
|            | lorine** | ines/C |         | hlorine** |            |
|            |          | hloram |         |           |            |
|            | **to**   | ines** |         | **to**    |            |
|            |          |        |         |           |            |
|            | **Oz     | **to** |         | **        |            |
|            | one/Chlo |        |         | Ozone/Chl |            |
|            | ramine** | **O    |         | oramine** |            |
|            |          | zone/C |         |           |            |
|            |          | hloram |         |           |            |
|            |          | ines** |         |           |            |
+------------+----------+--------+---------+-----------+------------+
|            | *        | **U    | **      | **Utility | **Utility  |
|            | *Utility | tility | Utility | #7**      | #36**      |
|            | #36**    | #25**  | #36**   |           |            |
+------------+----------+--------+---------+-----------+------------+
| Total      | decrease | de     | no      | decrease  | decrease   |
| Triha      |          | crease | change  |           |            |
| lomethanes |          |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| Total      | decrease | de     | no      | decrease  | decrease   |
| Haloacetic |          | crease | change  |           |            |
| Acids      |          |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| Total      | decrease | no     | no      | decrease  | decrease   |
| Haloace    |          | change | change  |           |            |
| tonitriles |          |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| Total      | decrease | no     | i       | decrease  | decrease   |
| H          |          | change | ncrease |           |            |
| aloketones |          |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| Total      | decrease | in     | i       | not       | increase   |
| Aldehydes  |          | crease | ncrease | analyzed  |            |
+------------+----------+--------+---------+-----------+------------+
| Ch         | increase | in     | i       | decrease  | increase   |
| loropicrin |          | crease | ncrease |           |            |
+------------+----------+--------+---------+-----------+------------+
| Chloral    | decrease | de     | i       | decrease  | decrease   |
| Hydrate    |          | crease | ncrease |           |            |
+------------+----------+--------+---------+-----------+------------+
| Cyanogen   | increase | in     | i       | no change | increase   |
| Chloride   |          | crease | ncrease |           |            |
+------------+----------+--------+---------+-----------+------------+
| Notes:     |          |        |         |           |            |
| Results    |          |        |         |           |            |
| based on   |          |        |         |           |            |
| full scale |          |        |         |           |            |
| evaluation |          |        |         |           |            |
| at         |          |        |         |           |            |
| Utilities  |          |        |         |           |            |
| #19 and    |          |        |         |           |            |
| #25 and on |          |        |         |           |            |
| pilot      |          |        |         |           |            |
| scale      |          |        |         |           |            |
| e          |          |        |         |           |            |
| valuations |          |        |         |           |            |
| at         |          |        |         |           |            |
| Utilities  |          |        |         |           |            |
| #7 and     |          |        |         |           |            |
| #36.       |          |        |         |           |            |
|            |          |        |         |           |            |
| Free       |          |        |         |           |            |
| chlorine   |          |        |         |           |            |
| contact    |          |        |         |           |            |
| time was 4 |          |        |         |           |            |
| hours for  |          |        |         |           |            |
| Utility #7 |          |        |         |           |            |
| during use |          |        |         |           |            |
| of         |          |        |         |           |            |
| chlorine/  |          |        |         |           |            |
| chloramine |          |        |         |           |            |
| strategy.  |          |        |         |           |            |
|            |          |        |         |           |            |
| Systems    |          |        |         |           |            |
| must       |          |        |         |           |            |
| d          |          |        |         |           |            |
| emonstrate |          |        |         |           |            |
| efficacy   |          |        |         |           |            |
| of         |          |        |         |           |            |
| c          |          |        |         |           |            |
| hloramines |          |        |         |           |            |
| as a       |          |        |         |           |            |
| primary    |          |        |         |           |            |
| di         |          |        |         |           |            |
| sinfectant |          |        |         |           |            |
| if they    |          |        |         |           |            |
| are to be  |          |        |         |           |            |
| used as    |          |        |         |           |            |
| such.      |          |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+
| Source:    |          |        |         |           |            |
| USEPA,     |          |        |         |           |            |
| 1992;      |          |        |         |           |            |
| Jacangelo, |          |        |         |           |            |
| et al.,    |          |        |         |           |            |
| 1989.      |          |        |         |           |            |
+------------+----------+--------+---------+-----------+------------+

### Chlorine/Chlorine to Chlorine/Chloramine

Chloramines as a secondary disinfectant shortens the free chlorine
contact time and, therefore, reduce the formation of most halogenated
DBPs, including TTHM. However, the consumer is exposed to a chloramine
residual that has a different health concern (i.e., can cause
methemoglobinemia and adversely affect the health of kidney dialysis
patients) than chlorine residual. For maximum benefit in reducing
halogenated byproducts, the ammonia/chlorine mixing conditions should be
optimized to quickly remove free chlorine from the system, with the
optimal pH for the chlorine/ammonia reaction considered to be pH 8.3. In
one case, changing the secondary disinfectant from chlorine to
chloramines resulted in a reduction in average TTHM concentrations from
105 μg/L to 45 μg/L with most (80 percent) of the TTHM formation in the
chloraminated system occurring prior to the addition of ammonia (Singer,
1988).

### Chlorine/Chlorine to Ozone/Chlorine

A change from chlorine to ozone for primary disinfection is attractive
because ozone generally produces fewer regulated DBPs than chlorine.
However, as mentioned in previous chapters, ozone will react with
organic compounds to produce BOM, aldehydes, organic acids, and ketones,
many of which are biodegradable. In addition, for water sources
containing significant bromide ion concentrations, site-specific
engineering evaluations should be completed prior to using ozone as a
primary disinfectant due to the potential formation of bromate ion or
brominated organic byproducts. At low pH values, the brominated organic
byproducts are favored, while at higher pH values, bromate ion formation
is favored. Therefore, changing from chlorine to ozone may be limited in
cases where the source water contains bromide ion. In the ozone/chlorine
strategy, bromine has the opportunity to react with NOM prior to
chlorine application, potentially increasing the extent of bromine
incorporation into DBPs. Also, in this situation, switching to
biological filtration can increase regrowth and corrosion rates in the
distribution system.

In systems containing negligible concentrations of bromide ion,
replacement of pre-chlorination with ozonation may allow for the point
of chlorine addition to be moved beyond clarification or filtration. The
principal benefit derived from using ozone for controlling THM formation
is that it allows free chlorine to be applied later in the treatment
process after precursors have been removed. This would be expected to
result in reduced formation of chlorinated DBPs due to the reduced
chlorine dose requirement and the reduced NOM concentrations following
treatment. Replacement of chlorine with ozone appears to be most
effective in decreasing chlorinated DBPs when it eliminates the need for
pre-chlorination. However, replacing chlorine with ozone while
maintaining free chlorine as a secondary disinfectant has produced mixed
results, with some byproduct concentrations being reduced at the expense
of increasing the concentrations of others. *The human health
implications of this trade-off are largely unknown*. It has been shown
that substituting ozone for chlorine as a primary disinfectant, while
maintaining free chlorine as a secondary disinfectant when not
practicing pre-chlorination, resulted in an increased THM formation in
raw waters for the Cities of Phoenix and San Diego (Malcolm Pirnie,
1989; Malcolm Pirnie, 1990).

An operational consideration of the ozone/chlorine system is the
application point for chlorine. The ozone should be completely
decomposed or chemically quenched prior to chlorine addition. If ozone
is present when chlorine is added, the ozone will react with the
chlorine and NOM present to form chlorinated DBPs.

Ozonation converts NOM into low molecular weight humic NOM and may
increase the concentrations of precursors to some DBPs. For instance,
ozonation followed by chlorination as a secondary disinfectant may yield
high concentrations of chloral hydrate (Logsdon et al., 1992; McKnight
and Reckhow, 1992). This may occur because the byproduct of ozonation,
acetaldehyde, is a known precursor for chloral hydrate, a byproduct of
chlorination. Enhancement of chloral hydrate has not been observed when
biologically active filtration is used following ozonation and prior to
chlorination (Singer, 1992). In addition to chloral hydrate, ozonation
followed by chlorination can produce greater THM and haloketones levels
than chlorination alone, particularly when chlorine is applied at high
pH levels (Jacangelo et al., 1989; Reckhow et al., 1986). Ozonation
followed by chlorination or chloramination can increase chloropicrin and
cyanogen chloride concentrations above those observed with chlorination
or chloramination alone (Jacangelo et al., 1989). The most promising
treatment strategy for preventing the enhancement of these biodegradable
ozonation byproducts and BOM is to locate ozonation after sedimentation
and follow it by biologically active GAC.

### Chlorine/Chlorine to Ozone/Chloramine

In addition to the concerns addressed in Sections 9.1.3 and 9.1.4,
switching from chlorine to chloramine residual exposes the consumer to a
residual that may be a more significant health concern (particularly for
kidney dialysis patients). The impact of switching from
chlorine/chlorine to ozone/chloramines on the production of byproducts
was investigated in a 5 gpm pilot study (MWDSC and JMM, 1989; Jacangelo
et al., 1989). That switch produced greater concentrations of
chloropicrin, cyanogen chloride, formaldehyde and total aldehydes than
in the original chlorine/chlorine strategy. Concentrations of TTHMs,
total haloacetic acids, total haloacetonitriles, total haloketones and
chloral hydrate were lower with ozone/chloramine. Brominated DBPs were
not reported. Ozonation followed by chloramination has been observed to
increase cyanogen chloride concentrations beyond those observed with
chlorination only (Jacangelo et al., 1989). Increased chloral hydrate
has not been observed when monochloramine is applied as the secondary
disinfectant (Singer, 1992).

### Chlorine/Chlorine to Chlorine Dioxide/Chlorine

Use of chlorine dioxide as a pre-oxidant to replace chlorine may allow
moving the point of chlorination downstream in the process train for
application to water with lower NOM concentrations. The reduced
precursor concentration and the reduced chlorine dose should result in a
reduction of chlorinated DBPs. However, if excess chlorine is present in
the chlorine dioxide feed stream, it would react with NOM prior to
removal in sedimentation and filtration if pre-oxidation is practiced.
Switching from chlorine/chlorine to chlorine dioxide/chlorine produces
mixed results. Like ozone, chlorine dioxide alters the nature of NOM
molecules, potentially forming greater precursor concentrations for some
DBPs while reducing the precursor concentrations for other DBPs. *The
human health implications of these trade-offs are largely unknown.*
Chlorine dioxide/chlorine appears to be most effective in decreasing
chlorinated DBPs when it can replace the need for pre-chlorination.
However, for facilities that use pre-chlorination but do not require it,
continuing to use chlorine/chlorine while eliminating pre-chlorination
may be as effective in decreasing chlorinated DBPs.

### Chlorine/Chlorine to Chlorine Dioxide/Chlorine Dioxide

The potential to apply chlorine dioxide as both a primary and secondary
disinfectant is limited because:

-   Chlorine dioxide is a strong oxidant and dissipates rapidly in both
    raw and treated waters; and

-   Approximately 50 to 70 percent of chlorine dioxide is converted to
    the inorganic byproducts chlorite and chlorate.

On the positive side, chlorine dioxide/chlorine dioxide application will
significantly lower the formation of organic DBPs.

### Chlorine/Chloramine to Ozone/Chloramine

In addition to the concerns raised in Sections 9.1.4 and 9.1.5,
switching from chlorine/chloramine to ozone/chloramine resulted in
reduced formation of most of the halogenated DBPs (MWDSC and JMM, 1989).
Other studies also indicate reduction in the formation of most
halogenated DBPs but increased formation of 1,1-dichloropropanone (MWDSC
and JMM, 1989). The primary difference in chlorinated DBP formation when
switching from chlorine/chloramine to ozone/chloramine could be
attributed to the shorter contact time with free chlorine.

### Chlorine/Chloramine to Chlorine Dioxide/Chloramine

The Louisville Water Company evaluated the feasibility of switching from
a chlorine/chloramine to chlorine dioxide followed by chloramine to
control THM formation (Hubbs et al., 1981). The treatment plant includes
lime soda-ash for softening. Disinfection occurs prior to the lime
treatment step. Ammonia is added to form chloramine before the water
enters the softening phase. There is a 10 minute lag period between the
first disinfectant (chlorine or chlorine dioxide) and second
disinfectant (chloramine) addition. The study showed a significant
decrease in THM formation from 25 μg/L with chlorine to 5 μg/L using
chlorine dioxide as the initial disinfectant. At the same time, treated
water coliform densities were essentially unchanged; however, results
showed slightly more scattered data during the chlorine dioxide test
period. Based on these results, the Water Company decided to use
chlorine/chloramine to meet disinfection and THM targets. No other DBPs
were measured during the test period.

### Ozone/Chlorine to Ozone/Chloramine

In addition to concerns raised in Sections 9.1.4 and 9.1.5, when
compared with ozone/chlorine, ozone/chloramine produced greater
concentrations of cyanogen chloride. Concentrations of TTHM, total
haloacetic acids, total haloacetonitriles, total haloketones, chloral
hydrate, total aldehydes and formaldehyde were lower with
ozone/chloramine than with ozone/chlorine. Ozone/chloramine produces
some chlorinated DBPs at greater concentrations than ozone/chlorine;
however, ozone/chloramine significantly reduces TTHMs compared to
ozone/chlorine (LeLacheur et al., 1991).

### Summary

EPA is not encouraging systems to switch to different disinfectants due
to unknown risks to public health. When needed for compliance with
regulations or increasing *Cryptosporidium* inactivation, careful
selection of alternative disinfectants as primary and secondary
disinfectants, can produce less DBPs and increase inactivation. In
general, the results followed the characteristic DBPs associated with
the primary disinfectant (halogenated DBPs with chlorine compounds or
ozone in the presence of bromide oxidized organics, AOC with ozone or
peroxone). However, by carefully selecting the primary and secondary
disinfectant and avoiding long contact times and high dosages of
halogens (chlorine, bromine), the total DBP formation declined. The
quantity and types of DBPs that form are site-specific, depending on the
water quality, disinfectant dose and type, and are best determined by
bench testing. Note that any system changing disinfectants is subject to
the profiling and benchmarking requirements as described in Section
1.3.1 and specified in 40 CFR § 141.172.

## Pathogen Inactivation with Interactive Disinfectants

In 1988, several reports appeared on the combined efficiency of some
disinfectants on pathogen inactivation. Worley and Williams (1988)
reported that a mixture of free chlorine and organic *N‑*halamine
produced higher levels of inactivation of a variety of bacteria. The
combination of free chlorine and sodium bromide was also investigated
and found to be more effective than using free chlorine alone (Alleman
et al., 1988). In a study at the University of Arizona, the synergistic
inactivation of *E. coli* and MS-2 coliphage was demonstrated by the
combined application of chloramine and cupric chloride (Straub et al.,
1994).

Recently there has been a great deal of interest in the potential of
interactive disinfectants because reports showed that some of these
combinations are more effective for inactivating *Cryptosporidium*
(Finch et al., 1994). Research on interactive disinfectants for primary
pathogen inactivation is under way for several combinations of
disinfectants:

-   Chlorine followed by chloramine;

-   Chlorine dioxide followed by chlorine;

-   Chlorine dioxide followed by chlorine dioxide;

-   Chlorine dioxide followed by chloramine;

-   Ozone followed by chlorine;

-   Ozone followed by chlorine dioxide; and

-   Ozone followed by chloramine.

### Inactivation Mechanism

Bernbaum (1981 and 1985) developed a testing method for determining the
kind of interaction that can be expected when agents are combined to
produce a given observation. Synergism can be tested using the
mathematical model developed by Bernbaum and modified for disinfection
kinetics by Kouame and Haas (1991). The principle is that, if the agents
in a given combination do not interact in producing the effect observed,
then regardless of the effect relations, the following equation is
satisfied:

$$\sum_{i = 1}^{n}\frac{x_{i}}{y_{i}} = 1$$

where:

  -------- ---- -----------------------------------------------------------
  *x~i~*   =    Concentration of the individual agent in the combination

  *y~i~*   =    Concentration of the agents that individually would produce
                the same magnitude of effect as that of the combination

  *i*      =    Individual agent

  *n*      =    Total number of agents
  -------- ---- -----------------------------------------------------------

The sum calculated from this equation for a set of data is interpreted
as follows:

-   The sum is less than 1 in the case of synergistic interaction;

-   The sum is greater than 1 in the case of antagonistic interaction;
    and

-   The sum is equal to 1 in the case of additivity (zero interaction).

Using this approach, Kouame and Haas (1991) showed that a synergistic
interaction exists in the inactivation of *E. coli* when exposed
simultaneously to free chlorine and monochloramine.

The authors described a possible mechanism in which both of the
disinfectants work together to inactivate bacteria. The researchers
hypothesized that bacterial inactivation is caused by monochloramine
penetrating the cell and oxidizing thiol groups, which in turn causes
structural changes in the cell membrane. Once these changes have been
made, copper is allowed to pass into the cell and binds either to
sulfhydryl groups of respiratory enzymes or nucleic acids. More
recently, the researchers investigating *E. coli* inactivation
hypothesized that a potential synergistic mechanism consisting of
sub-lethal injury caused by free chlorine resulted in enhanced
sensitivity to monochloramine (Kouame and Haas, 1991).

Another hypothesis for the increased effectiveness of interactive
disinfectants is that the first oxidant (i.e., chlorine, chlorine
dioxide, or ozone) conditions the outer membrane of *Cryptosporidium*
oocysts so that the secondary oxidant (i.e., chlorine, chlorine dioxide,
and monochloramine) can penetrate the oocyst more easily (Liyanage et
al., 1996). For example, preliminary work on the disinfection of
*Cryptosporidium parvum* using free chlorine followed by monochloramine
suggested that there may be a synergism involving two chlorine species.
Sequential treatment of these chlorine species was found to provide
greater inactivation than expected from the additive effects of the two
disinfectants used alone (Gyurek et al., 1996).

Recent studies have utilized a straight forward method to determine if
synergism has occurred based on measured inactivation (Finch, 1997;
Gyurek et al., 1996; and Liyanage et al., 1996). According to this
approach, synergism is demonstrated if the sequential application of
disinfectants provides more inactivation than is expected from the
additive effects of the individual, separate disinfectants. In addition,
the magnitude of the synergistic effects is equal to the difference in
the level of inactivation achieved from multiple disinfectants and the
additive inactivations achieved from the single disinfectants.

### Environmental Effects

Similar to most chemical disinfectants, the preliminary results from an
AWWARF ongoing study suggest that pH and temperature affect the amount
of synergistic inactivation achieved by sequential applications of
disinfectants (Finch, 1997). The following sections briefly describe the
effects these parameters have on pathogen inactivation.

#### pH

The level of inactivation due to the sequential application of chemical
disinfectants is believed to be pH dependent (Finch, 1997). Figure 9-1,
Figure 9-2, and Figure 9-3 illustrate the impact of pH on the log
inactivation of *Cryptosporidium parvum* attributed to synergistic
effects for three sequential combinations of ozone-chlorine dioxide,
chlorine dioxide-free chlorine, and chlorine dioxide-chloramine,
respectively. As shown in these figures, the amount of log inactivation
due to synergistic effects is lower at high pH (e.g., pH = 11). These
results show that neutral pH is more effective than low pH except for
ozone-chlorine dioxide.

#### Combination of Low Temperature and pH

The combined effect of low temperature and high pH is believed to
significantly reduce the amount of *Cryptosporidium* inactivation
attributed to synergism (Finch, 1997). One possible explanation for this
reduction is that the oocysts contract under these conditions and become
harder to penetrate. However, significant reduction in *Cryptosporidium*
oocysts inactivation is true under reduced water temperature and high pH
whether interactive disinfection is practiced or not. Therefore, reduced
inactivation may not be necessarily due to synergism between combined
disinfectants.

![](media/image2.wmf){width="5.406944444444444in"
height="3.6319444444444446in"}

Source: Finch, 1997.

[]{#__RefHeading___Toc450037490 .anchor}Figure 9-1. Inactivation of *C.
parvum* Attributed to Synergistic Effects. Application of Ozone Followed
by Chlorine Dioxide

[]{#_Ref396209375 .anchor}![](media/image3.wmf){width="5.3125in"
height="3.567361111111111in"}

Source: Finch, 1997.

Figure 9-2. Inactivation of *C. parvum* Attributed to Synergistic
Effects. Application of Chlorine Dioxide Followed by Free Chlorine

[]{#_Ref396209422
.anchor}![](media/image4.wmf){width="5.406944444444444in"
height="3.6972222222222224in"}

Source: Finch, 1997.

Figure 9-3. Inactivation of *C. parvum* Attributed to Synergistic
Effects. Application of Chlorine Dioxide Followed by Monochloramine

#### Pathogen Susceptibility

*Cryptosporidium* oocysts are more susceptible to inactivation by
combinations of disinfectants than by individual disinfectants.
*Giardia* cysts were also found to have a similar response to:

-   Ozone followed by free chlorine;

-   Ozone followed by monochloramine;

-   Chlorine dioxide followed by free chlorine;

-   Chlorine dioxide followed by monochloramine; and

-   Free chlorine followed by monochloramine.

However, no synergism was observed with bacterial spores, specifically
*Bacillus cereus* spores (Finch, 1997). These results suggest that
encysted parasites might show more susceptibility to synergistic effects
than bacterial spores. Masking effects caused by turbidity for
interactive disinfectants are expected to be similar to those of the
individual disinfectants.

### Pathogen Inactivation Efficiency Using Interactive Disinfectants

Within the last few years there have been several studies to investigate
interactive disinfectants. These studies were conducted under various
conditions of pH, bench or laboratory scale, and using various
organisms:

-   Battigelli and Sobsey (1993) studied viral inactivation under lime
    softening conditions using sequential addition of chlorine and
    monochloramine;

-   Kouame and Haas (1991) determined *E. coli* inactivation during
    simultaneous addition of free chlorine and monochloramine;

-   Finch (1997) is studying various combinations of chlorine, chlorine
    dioxide, ozone, and monochloramine on inactivation of
    *Cryptosporidium parvum* oocysts, *Giardia muris* cysts, and
    *Bacillus cereus* spores under laboratory conditions; and

-   Oppenheimer (1997) is developing CT requirements for
    *Cryptosporidium parvum* inactivation in a variety of natural waters
    using ozone followed by chlorine.

The following is a summary of the findings of these studies to date.

#### Virus Inactivation Using Chlorine and Monochloramine Under High pH

One of the primary objectives of the Battigelli and Sobsey study (1993)
was to evaluate the disinfection efficiency under high pH conditions
encountered in conventional lime softening treatment with and without
the addition of chlorine and monochloramine. The three microorganisms
selected for evaluation were hepatitis A virus, poliovirus 1, and MS-2
coliphage.

During the study, the inactivation kinetics of the three test viruses
were determined when 2.0 mg/L monochloramine were formed dynamically
after the viruses had been exposed to lime solution and free chlorine
for 60 seconds. The authors believed that this approach simulates the
conditions typically encountered in a water softening facility where
ammonia is applied post-chlorination.

Results indicated that a high degree of inactivation occurs during the
first 60 seconds of chlorine addition at approximately 2.4 mg/L free
chlorine.

Table 9-6 shows the amount of inactivation attributed to the lime
solution, free chlorine, and monochloramine for the three viruses. The
table also contains the amount of inactivation attributed to the
sequential application of lime solution, free chlorine and
monochloramine previously described (simultaneous chloramination).
Results shown in Table 9-6 are based on a pH of 11.0 and a total contact
time of 360 minutes.

Except for poliovirus 1, the summation of the individual disinfectants
was greater than the level of inactivation achieved from the
simultaneous chloramination. These results imply that the sequential
addition of free chlorine and monochloramine after lime addition to
raise the pH to 11, form an antagonistic (negative) interaction for
inactivation of hepatitis A virus and MS-2 coliphage. For poliovirus 1,
under similar conditions, an enhancement of 1.4 log inactivation was
achieved suggesting positive synergism for poliovirus 1 inactivation.

[]{#__RefHeading___Toc450037477 .anchor}Table 9-6. Virus Inactivation By
Individual Disinfectants and Simultaneous Chloramination

  ------------------------- --------------- ------------ ----------- -------------------
  **Disinfectant(s)**       **Log                                    **Condition**
                            Survival^1^**                            

                            Hepatitis A     Poliovirus   MS-2        
                            Virus                        Coliphage   

  Lime solution only        -3.0            -0.5         -4.0        360 min contact
                                                                     time

  Free chlorine             -1.8            -1.2         -1.6        60 second contact
                                                                     time, 2.5 mg/L
                                                                     chlorine

  Monochloramine            -3.7            -1.9         -3.8        2.0 mg/L
                                                                     monochloramine

  Summation                 -5.5            -3.1         -5.4        Additive
  free+monochloramine                                                

  Simultaneous              -4.5            -4.5         -4.5        2.4 mg/L chlorine
  chloramination                                                     60 second contact
                                                                     time, 2.0 mg/L
                                                                     monochloramine, 359
                                                                     minutes.
  ------------------------- --------------- ------------ ----------- -------------------

^1^ All data at pH 11 after lime addition

Source: Battigelli and Sobsey, 1993.

#### Inactivation of E. coli Under Simultaneous Free and Combined Chlorination

The inactivation of *E. coli* bacteria by the simultaneous application
of free chlorine and monochloramine was investigated at the Illinois
Institute of Technology (Kouame and Haas, 1991). Figure 9-4 shows the
level of *E. coli* inactivation by free chlorine and monochloramine,
separately and combined. The level of inactivation by monochloramine
alone after a contact time of 300 seconds was found to be significantly
less than that of free chlorine. Therefore, the sum of the individual
inactivation by free chlorine and monochloramine was assumed to be equal
to that of free chlorine alone. Note that in this case, the residual
disinfectant rapidly disappeared due to the breakpoint reactions that
occur when monochloramine and free chlorine are combined.

The surviving fraction of bacteria following the simultaneous
application of free chlorine and monochloramine is substantially less
than what would be expected by adding the individual levels of
inactivation. In other words, at similar doses and contact times, the
amount of inactivation from the combined disinfectants was greater than
the sum of the inactivation due to free chlorine alone and
monochloramine alone.

In summary the Kouame and Haas (1991) study showed that high levels of
bacteria inactivation can be achieved when free chlorine and
monochloramine exist simultaneously in a continuous flow system and that
the combined action of both chemicals on the bacteria is synergistic.

#### Cryptosporidium, Giardia, and Bacillus Inactivation in Laboratory Grade Water 

The preliminary results of an AWWARF study that investigated the
application of multiple disinfectants was presented at a American Water
Works Association Technology Transfer Conference in Portland, OR, in
August 1997. The objectives of this study were to screen interactive
chemical disinfectants (ozone, chlorine, chlorine dioxide, and
monochloramine) for inactivation of *Cryptosporidium parvum*, *Giardia
muris*, and *Bacillus cereus* and develop design criteria for
*Cryptosporidium parvum* inactivation using the best combinations.

##### **Ozone Followed By Chlorine Dioxide** {#ozone-followed-by-chlorine-dioxide .unnumbered}

Table 9-7 shows the results obtained from ozone and chlorine dioxide
application for the inactivation of *Cryptosporidium parvum*.

Based on the data shown in Table 9-7, ozone followed by chlorine dioxide
was the most effective disinfectant combination for *Cryptosporidium*
inactivation. A total contact time of 124 minutes was required to
achieve 3 to 4-log inactivation with ozone and chlorine dioxide
residuals of 0.9 and 1.3 mg/L, respectively.

![](media/image5.wmf){width="6.165972222222222in"
height="4.215972222222222in"}

Source: Kouame and Haas, 1991.

[]{#__RefHeading___Toc450037493 .anchor}Figure 9-4. Inactivation of *E.
coli* Using Free Chlorine and Monochloramine

[]{#__RefHeading___Toc450037478 .anchor}Table 9-7. *C. parvum*
Inactivation Using Ozone Followed by Chlorine Dioxide

  ------------------------ --------------- --------------- ---------------
  **Disinfectant**         **Level of                      
                           Inactivation                    
                           (log-units)**                   

                           **pH 6.0**      **pH 8.0**      **pH 11.0**

  Ozone                    1.6             0.8             0

  Chlorine dioxide         0.9             1.4             2.4

  Ozone followed by        4.0             3.6             2.9
  chlorine dioxide                                         

  Inactivation attributed  1.5             1.4             0.5
  to synergism                                             
  ------------------------ --------------- --------------- ---------------

Source: Finch, 1997.\
Ozone: 0.9 mg/L for 4 minutes, chlorine dioxide 1.3 mg/L for 120
minutes.

##### **Chlorine Dioxide Followed by Free Chlorine** {#chlorine-dioxide-followed-by-free-chlorine .unnumbered}

Table 9-8 through Table 9-10 summarize of the results obtained for
chlorine dioxide followed by free chlorine for *Cryptosporidium parvum*,
*Giardia muris*, and *Bacillus cereus*, respectively.

[]{#__RefHeading___Toc450037479 .anchor}Table 9-8. *C. parvum*
Inactivation Using Chlorine Dioxide Followed by Free Chlorine

  --------------------------- --------------- -------------- --------------
  **Disinfectant**            **Level of                     
                              Inactivation                   
                              (log-units)**                  

                              **pH 6.0**      **pH 8.0**     **pH 11.0**

  Chlorine dioxide            1.0             1.4            1.6

  Free chlorine               0               0              0

  Chlorine dioxide followed   2.2             3.0            2.3
  by free chlorine                                           

  Inactivation attributed to  1.2             1.6            0.7
  synergism                                                  
  --------------------------- --------------- -------------- --------------

Source: Finch, 1997.\
Chlorine dioxide 1.3 mg/L for 120 minutes, free chlorine 2.0 mg/L for
120 minutes.

[]{#__RefHeading___Toc450037480 .anchor}Table 9-9. *G. muris*
Inactivation Using Chlorine Dioxide Followed by Free Chlorine

  ---------------------------- -------------------- ---------------------
  **Disinfectant**             **Level of           
                               Inactivation         
                               (log-units)**        

                               **pH 6.0**           **pH 8.0**

  Chlorine dioxide             0.8                  0.8

  Free chlorine                0.8                  0.6

  Chlorine dioxide followed by 2.2                  2.0
  free chlorine                                     

  Inactivation attributed to   0.6                  0.6
  synergism                                         
  ---------------------------- -------------------- ---------------------

Source: Finch, 1997.\
Chlorine dioxide: 1.0 mg/L for 10 minutes, free chlorine 2.0 mg/L for 30
minutes.

[]{#__RefHeading___Toc450037481 .anchor}Table 9-10. *B. cereus*
Inactivation Using Chlorine Dioxide Followed by Free Chlorine

  ---------------------------- ------------------------------------------
  **Disinfectant**             **Level of Inactivation (log-units)**

  Chlorine dioxide             1.8

  Free chlorine                1.2

  Chlorine dioxide followed by 2.9
  free chlorine                

  Inactivation attributed to   0
  synergism                    
  ---------------------------- ------------------------------------------

Source: Finch, 1997.\
Chlorine dioxide: 2.3 mg/L for 20 minutes, free chlorine for 30 minutes.

Chlorine dioxide followed by free chlorine was capable of achieving
between 2 and 3 logs of *Cryptosporidium* inactivation following a total
contact time of 240 minutes and approximately 2 logs of inactivation of
*Giardia* following only 40 minutes of contact time. No synergism was
observed with *Bacillus cereus*. However, approximately 3 logs of
inactivation after a contact time of 50 minutes were achieved by the
additive effects of chlorine dioxide and free chlorine.

##### **Chlorine Dioxide Followed by Chloramine** {#chlorine-dioxide-followed-by-chloramine .unnumbered}

Table 9-11 and Table 9-12 show the results of the inactivation of
*Cryptosporidium parvum* and *Giardia muris* when using chlorine dioxide
followed by monochloramine.

[]{#__RefHeading___Toc450037482 .anchor}Table 9-11. *C. parvum*
Inactivation Using Chlorine Dioxide Followed by Chloramine

  ------------------------------- --------------- ------------- -------------
  **Disinfectant**                **Level of                    
                                  Inactivation                  
                                  (log-units)**                 

                                  **pH 6.0**      **pH 8.0**    **pH 11.0**

  Chlorine dioxide                1.0             1.5           1.6

  Monochloramine                  0               0             0

  Chlorine dioxide followed by    2.2             2.8           2.1
  monochloramine                                                

  Inactivation attributed to      1.2             1.3           0.5
  synergism                                                     
  ------------------------------- --------------- ------------- -------------

Source: Finch, 1997.\
Chlorine dioxide: pH 6, 8, and 11: 1.3 mg/L for 120 minutes.
Monochloramine: pH 6 and 11: 2.0 mg/L for 120 minutes, pH 8: 2.8 mg/L
for 180 minutes.

[]{#__RefHeading___Toc450037483 .anchor}Table 9-12. *G. muris*
Inactivation Using Chlorine Dioxide Followed by Chloramine

  ------------------------------- ------------------- -------------------
  **Disinfectant**                **Level of          
                                  Inactivation        
                                  (log-units)**       

                                  **pH 8.0**          **pH 11.0**

  Chlorine dioxide                0.8                 0.8

  Monochloramine                  0.5                 0.7

  Chlorine dioxide followed by    1.7                 1.5
  monochloramine                                      

  Inactivation attributed to      0.4                 0
  synergism                                           
  ------------------------------- ------------------- -------------------

Source: Finch, 1997.\
pH 8.0: Chlorine dioxide 1.0 mg/L for 5 minutes, monochloramine 2.0 mg/L
for 150 minutes.\
pH 11.0: Chlorine dioxide 1.0 mg/L for 5 minutes, monochloramine 2.0
mg/L for 5 minutes.

At similar disinfect residuals and contact times, chlorine dioxide
followed by monochloramine was found to achieve the same levels of
*Cryptosporidium* inactivation as chlorine dioxide followed by free
chlorine at pH values of 6 and 11. However, at pH 8, a higher
monochloramine residual and contact time were required to achieve
inactivation levels comparable to chlorine dioxide and free chlorine. No
synergism was found for *Giardia* inactivation at pH 11.0 with only a
minimal increase in effectiveness at pH 8.0.

##### **Ozone Followed by Free Chlorine** {#ozone-followed-by-free-chlorine .unnumbered}

Table 9-13 and Table 9-14 show the levels of inactivation of *Giardia
muris* and *Bacillus cereus* obtained by using ozone followed by free
chlorine.

[]{#__RefHeading___Toc450037484 .anchor}Table 9-13. *G. muris*
Inactivation Using Ozone Followed by Free Chlorine

  ------------------------------ --------------- ------------- -------------
  **Disinfectant**               **Level of                    
                                 Inactivation                  
                                 (log-units)**                 

                                 **pH 6.0**      **pH 8.0**    **pH 11.0**

  Ozone                          0.5             0.8           0.4

  Free chlorine                  0.8             0.6           0

  Ozone followed by free         2.3             2.2           0.4
  chlorine                                                     

  Inactivation attributed to     1.0             0.8           0
  synergism                                                    
  ------------------------------ --------------- ------------- -------------

Source: Finch, 1997.\
pH 6.0: ozone 0.1 mg/L for 60 seconds; free chlorine 2.0 mg/L for 30
minutes.\
pH 8.0: ozone 0.1 mg/L for 17 seconds; free chlorine 2.0 mg/L for 60
minutes.\
pH 11.0: ozone 0.1 mg/L for 5 seconds; free chlorine 2.0 mg/L for 60
minutes.

[]{#__RefHeading___Toc450037485 .anchor}Table 9-14. *B. cereus*
Inactivation Using Chlorine Dioxide Followed by Free Chlorine

  ------------------------------ ----------------------------------------
  **Disinfectant**               **Level of Inactivation (log-units)**

  Chlorine dioxide               1.4

  Free chlorine                  2.0

  Chlorine dioxide followed by   3.4
  free chlorine                  

  Inactivation attributed to     0
  synergism                      
  ------------------------------ ----------------------------------------

Source: Finch, 1997.

Ozone followed by free chlorine was capable of achieving approximately 2
logs of *Cryptosporidium* inactivation at pH 6.0 and 8.0; however, only
a 0.4 log inactivation was achieved at pH 11.0. The difference in
inactivations was primarily caused by the inability of free chlorine to
inactivate *Cryptosporidium* at pH 11.0. Similar to the disinfectant
combination of chlorine dioxide and free chlorine, no synergism was
observed for *Bacillus cereus* inactivation; however, the additive
effects of ozone and free chlorine achieved greater than 3 logs of
inactivation.

##### **Ozone Followed by Monochloramine** {#ozone-followed-by-monochloramine .unnumbered}

Table 9-15 shows the results obtained for *Giardia muris* inactivation
by ozone followed by monochloramine.

[]{#__RefHeading___Toc450037486 .anchor}Table 9-15. *G. muris*
Inactivation Using Ozone Followed by Chloramine

  -------------------------------- ------------------- -------------------
  **Disinfectant**                 **Level of          
                                   Inactivation        
                                   (log-units)**       

                                   **pH 8.0**          **pH 11.0**

  Ozone                            0.8                 0.4

  Monochloramine                   0.5                 0.7

  Ozone followed by monochloramine 2.1                 1.8

  Inactivation attributed to       0.8                 0.7
  synergism                                            
  -------------------------------- ------------------- -------------------

Source: Finch, 1997.\
pH 8.0: ozone 0.1 mg/L for 17 seconds; monochloramine 2.0 mg/L for 150
minutes.\
pH 11.0: ozone 0.1 mg/L for 5 seconds; monochloramine 2.0 mg/L for 5
minutes.

Because of the different residuals and contact times, inactivation
efficiencies of ozone followed by chloramine and ozone followed by free
chlorine could not be compared. However, for similar monochloramine
residuals, a very short monochloramine contact time of 5 minutes at pH
11 was found to achieve a greater inactivation than a contact time of
150 minutes at pH 8.

##### **Free Chlorine Followed by Monochloramine** {#free-chlorine-followed-by-monochloramine .unnumbered}

Table 9-16 shows the results obtained for *Giardia muris* inactivation
by free chlorine followed by monochloramine.

Similar to the results obtained for ozone followed by monochloramine, a
very short monochloramine contact time of 5 minutes at pH 11 was found
to achieve a greater inactivation than a contact time of 150 minutes at
pH 8. However, free chlorine did not achieve any inactivation at pH 11.

[]{#__RefHeading___Toc450037487 .anchor}Table 9-16. *G. muris*
Inactivation by Free Chlorine Followed by Monochloramine

  ------------------------------- ------------------- -------------------
  **Disinfectant**                **Level of          
                                  Inactivation        
                                  (log-units)**       

                                  **pH 8.0**          **pH 11.0**

  Free chlorine                   0.6                 0

  Monochloramine                  0.5                 0.7

  Free chlorine followed by       2.4                 0.7
  monochloramine                                      

  Inactivation attributed to      1.3                 0
  synergism                                           
  ------------------------------- ------------------- -------------------

Source : Finch, 1997.\
pH 8.0: free chlorine 2.0 mg/L for 60 minutes; monochloramine 2.0 mg/L
for 150 minutes.\
pH 11.0: free chlorine 2.0 mg/L for 60 minutes, monochloramine 2.0 mg/L
for 5 minutes.

#### Bench-Scale Tests Using Natural Waters

In another AWWARF study, Oppenheimer (1997) is developing CT
requirements for *Cryptosporidium parvum* inactivation in a variety of
natural waters, developing design criteria for full-scale contacting
systems from bench scale CT values, and investigating the impact of
selected variables on CT requirements. To date, samples have been
collected and analyzed from 13 geographically disperse locations.
Although a significant amount of data were not available, results from
the California State Water Project and Ohio River appear to show that
the sequential application of ozone and chloramines resulted in an
enhanced inactivation of *C. parvum* as shown in Table 9-17. The
sequential application of free chlorine and monochloramine appears to
enhance *C. parvum* inactivation by providing some synergistic effects.
To obtain the log reduction, however, very high ozone residuals were
required which appear to be impractical. In addition, bromate formation
was also a problem.

[]{#__RefHeading___Toc450037488 .anchor}Table 9-17. *C. parvum*
Inactivation by Sequential Application of Ozone and Chloramine

  ------------------ ------------- ------------ -------------- ------------- ------------ ---------------- ------------- ------------ ---------------
                     **Ozone**                  **Chlorine**                              **Chloramine**                              

  **Water Source**   **Residual\   **Contact\                  **Residual\   **Contact\                    **Residual\   **Contact\   **Log
                     mg/L**        min**                       mg/L**        min**                         mg/L**        min**        Inactivation
                                                                                                                                      Enhancement**

  California State   0.8           12                          1.5           \~0                           2.5           30           0.3 to \> 1.4
  Water Project                                                                                                                       

  Ohio River         4             15                          1.5           120                           0.5           120          0.9 to 1.4
  ------------------ ------------- ------------ -------------- ------------- ------------ ---------------- ------------- ------------ ---------------

Source: Oppenheimer, 1997.

### Summary: Pathogen Inactivation with Interactive Disinfectants

Various studies have shown the synergistic effects of interactive
disinfectants: either simultaneous application or sequential
application. The improved disinfection efficiency due to interactive
disinfection is variable, ranging from negative (antagonistic) effects
(in two studies) to positive enhancement of disinfection efficiency.
Many of the studies show definite improvement in inactivation for
interactive disinfectants.

Several research projects on the effects of combined disinfectants are
underway at the time this manual is being prepared. These projects
should provide insight on the mechanisms and applicability of multiple
disinfectants. Based on current information, EPA believes that under
appropriate situations a positive improvement in disinfection efficiency
exists. This enhanced inactivation varies from organism to organism, and
with different disinfectant combinations. For the key organisms of
interest under normal pH conditions:

-   Coliform bacteria inactivation appears to increase with combined
    disinfectants;

-   *Giardia* cyst inactivation appears to increase with combined
    disinfectants;

-   Hepatitis A virus and MS^-2^ coliphage inactivation using combined
    disinfectants appears to be less efficient than the individual
    disinfectants;

-   Poliovirus 1 inactivation appears to increase with combined
    disinfectants;

-   *Cryptosporidium* oocyst inactivation appears to increase with
    combined disinfectants; and

-   Inactivation of spores appears neutral.

Interactive disinfection is still however considered an emerging
technology. As such, CT credits for interactive disinfectants have not
yet been established.

## Analytical Methods

In general, most of the analytical methods for residual disinfectants
are impacted negatively by the presence of other disinfectants.
Fortunately, for most of the disinfectants and oxidants listed below, at
least one method exists that can be used successfully in the presence of
other oxidizing agents. For analytical method details, see the
individual disinfectant chapters.

### Ozone

Residual ozone analysis cannot be performed in the presence of other
oxidizing agents including chlorine, chloramine, and potassium
permanganate. Typically, the ozone analytical methods exhibit
interferences from chlorine, bromine, iodine, and manganese ions. The
ACVK is the least susceptible to interference and can be used when
manganese concentrations are less than 1 mg/L and free or combined
chlorine concentrations are less than 10 mg/L (Gordon et al., 1992).

### Chlorine Dioxide

Some of the analytical methods for chlorine dioxide, chlorate, and
chlorite cannot be performed in the presence of oxidizing agents.
Amperometric and iodometric methods cannot be used in the presence of
metal ions such as manganese. Analytical methods that can be used in the
presence of other disinfectants and oxidants include UV
spectrophotometric methods and ion chromatography (Gordon et al., 1992).

### Potassium Permanganate

The atomic adsorption method for permanganate analysis can be performed
in the presence of any of the other disinfectants (Standard Methods,
1995).

### Chloramine

None of the colorimetric analytical methods for chloramine can be
performed in the presence of oxidizing agents such as ozone or hydrogen
peroxide. Analytical methods that can be used in the presence of other
disinfectants and oxidants include the UV spectrophotometric method and
the amperometric titration methods (Gordon et al., 1992).

### Hydrogen Peroxide

The analytical procedures for hydrogen peroxide in drinking water are
all impacted by other oxidizing species such as ozone and chlorine
(Gordon et al., 1992).

### UV Radiation

There are no known interferences from other disinfectants with the
measurement of UV radiation (DeMers and Renner, 1992).

### Summary of Analytical Methods

Ozone analysis in the presence of chlorine is limited. However, these
disinfectants are not commonly present simultaneously, especially with
the rapid decomposition of ozone.

Hydrogen peroxide analysis is difficult in the presence of ozone and
other oxidizing agents. However, when using peroxone, the ozone residual
is the analyte used to meet disinfection requirements.

All of the other disinfectants and oxidizing agents can be selectively
monitored in the presence of other disinfectants.

## Summary

Table 9-18 summarizes, in general, the factors and uses of combined
disinfectants. Specific considerations depend on the actual combination
of disinfectants used.

[]{#__RefHeading___Toc450037489 .anchor}Table 9-18. Summary of Combined
Disinfectants

+----------------------+-----------------------------------------------+
| **Consideration**    | **Description**                               |
+----------------------+-----------------------------------------------+
| Generation           | Generation depends on the type of chemicals   |
|                      | used. Ozone, chlorine dioxide, and            |
|                      | chloramines require on-site generation.       |
+----------------------+-----------------------------------------------+
| Primary uses         | Two separate disinfectants can be used to     |
|                      | provide primary and secondary disinfection.   |
|                      | By separating the primary and secondary       |
|                      | disinfection functions, the processes can be  |
|                      | optimized for maximum inactivation and        |
|                      | minimum DBP formation.                        |
|                      |                                               |
|                      | Interactive disinfection (using synergism     |
|                      | between two disinfectants to enhance          |
|                      | inactivation) can serve as a primary          |
|                      | disinfectant.                                 |
+----------------------+-----------------------------------------------+
| Inactivation         | The use of interactive disinfection as        |
| efficiency           | primary disinfectant for inactivation of      |
|                      | *Giardia, Cryptosporidium,* and viruses are   |
|                      | feasible. Interactive disinfection is         |
|                      | typically more effective than the individual  |
|                      | disinfectants.                                |
+----------------------+-----------------------------------------------+
| Byproduct formation  | DBP formation is in general reduced by using  |
|                      | combined disinfectants. Specifically,         |
|                      | continued use of chlorine in combination with |
|                      | other disinfectants can reduce DBP formation. |
+----------------------+-----------------------------------------------+
| Limitations          | Data on the inactivation efficiency of        |
|                      | combined disinfectants are still being        |
|                      | generated with much information coming from   |
|                      | controlled laboratory studies. Additional     |
|                      | information is still needed, specifically on  |
|                      | full-scale implementation. Dual               |
|                      | (primary/secondary) disinfection for DBP      |
|                      | control is well established as a preferred    |
|                      | treatment option.                             |
+----------------------+-----------------------------------------------+
| Point of application | Applied for primary and secondary             |
|                      | disinfection. Ozonation should occur after    |
|                      | settling and prior to biofiltration.          |
+----------------------+-----------------------------------------------+
| Special              | The efficiency and application of combined    |
| considerations       | disinfectants follow to a large extent the    |
|                      | limitations and features of the individual    |
|                      | disinfectant. The combined disinfectant is    |
|                      | often a more effective disinfectant.          |
+----------------------+-----------------------------------------------+

## References

1.  Alleman, J. E., et al. 1988. "Comparative evaluation of alternative
    halogen-based disinfection strategies." Conference proceedings,
    Industrial Waste Conference, forty-second edition.

2.  Battigelli, D.A. and M.D. Sobsey. 1993. "The Inactivation of
    Hepatitis A Virus, Poliovirus and Coliphage MS2 By Lime Softening
    and Chlorine/Monochloramine Disinfection." Conference proceedings,
    AWWA Water Quality Technology Conference.

3.  Bernbaum, C.M. 1981. "Criteria for Analyzing Interactions Between
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4.  Bernbaum, C.M. 1985. "The expected effect of a combination of
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5.  DeMers, L.D. and R.C. Renner. 1992. *Alternative Disinfection
    Technologies for Small Drinking Water Systems*. AWWA and AWWARF,
    Denver, CO.

6.  Finch, G.R. 1997. "Control of *Cryptosporidium* Through Chemical
    Disinfection: Current State-of-the-Art." AWWARF Technology Transfer
    Conference, Portland, Oregon.

7.  Finch, G.R., E.K. Black, and L.L. Gyurek. 1994. "Ozone and Chlorine
    Inactivation of *Cryptosporidium*." Conference proceedings, Water
    Quality Technology Conference; San Francisco, CA.

8.  Gordon, G., W.J. Cooper, R.G. Rice, and G.E. Pacey. 1992.
    *Disinfectant Residual Measurement Methods.* Second Edition, AWWARF
    and AWWA, Denver, CO.

9.  Gyurek, L., L. Liyanage, M. Belosevic, and G. Finch. 1996.
    "Disinfection of *Cryptosporidium Parvum* Using Single and
    Sequential Application of Ozone and Chlorine Species." Conference
    proceedings, AWWA Water Quality Technology Conference, Boston, MA.

10. Hubbs, S., D. Amundsen, and P. Olthius. 1981. "Use of Chlorine
    Dioxide, Chloramines, and Short-Term Free Chlorination as
    Alternative Disinfectants." *J. AWWA.* 73(2):97-101.

11. Jacangelo, J.G., N.L. Patania, K.M. Reagan, E.M. Aieta, S.W.
    Krasner, and M.J. McGuire. 1989. "Impact of Ozonation on the
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    *J. AWWA.* 81(8):74.

12. Kouame, Y. and C.N. Haas. 1991. "Inactivation of E coli. by Combined
    Action of Free Chlorine and Monochloramine." *Water Res*.
    25(9):1027.

13. LeLacheur, R.M., P.C. Singer, and M.J. Charles. 1991. "Disinfection
    Byproducts in New Jersey Drinking Waters." Conference proceedings,
    AWWA Annual Conference, Philadelphia, PA.

14. Liyanage, L., L. Gyurek, M. Belosevic, and G. Finch. 1996. "Effect
    of Chlorine Dioxide Preconditioning on Inactivation of
    *Cryptosporidium* by Free Chlorine and Monochloramine." Conference
    proceedings, AWWA Water Quality Technology Conference, Boston, MA.

15. Logsdon, G.S., S. Foellmi, and B. Long. 1992. Filtration Pilot Plant
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    proceedings, AWWA Annual Conference, Toronto, Ontario.

16. Lykins, B.W., J.A. Goodrich, W.E. Koffskey, and M.H. Griese. 1991.
    "Controlling Disinfection Byproducts with Alternative
    Disinfectants." Conference proceedings, AWWA Annual Conference,
    Philadelphia, PA.

17. Malcolm Pirnie, Inc. 1990. "Task 1.3 - Water Quality Report."
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18. Malcolm Pirnie, Inc. 1989. "Water Quality Master Plan." City of
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19. McKnight, A. and D. Reckhow. 1992. "Reactions of Ozonation
    Byproducts with Chlorine and Chloramines." Conference proceedings,
    AWWA Annual Conference, Vancouver, British Columbia.

20. MWDSC AND JMM (Metropolitan Water District of Southern California
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    Byproducts in United States Drinking Waters*. Volume I. EPA and
    Association of Metropolitan Water Agencies. Cincinnati, OH and
    Washington, D.C.

21. Oppenheimer, J.A. 1997. "*Cryptosporidium* Inactivation in Natural
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22. Reckhow, D., B. Legube, and P. Singer. 1986. "The Ozonation of
    Organic Halide Precursors: Effect of Bicarbonate." *Water Res.*
    20(8):987-998.

23. Richardson, Susan D., Alfred D. Thurston, Timothy W.Collette,
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24. Singer, P.C. 1988. *Alternative Oxidant and Disinfectant Treatment
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9\. Combined Disinfectants [9-1](#combined-disinfectants)

9.1 Primary and Secondary Disinfectants
[9-1](#primary-and-secondary-disinfectants)

9.1.1 DBP Formation with Various Primary and Secondary Disinfectant
Combinations
[9-3](#dbp-formation-with-various-primary-and-secondary-disinfectant-combinations)

9.1.2 Impact of Modifying Disinfection Practices
[9-6](#impact-of-modifying-disinfection-practices)

9.1.3 Chlorine/Chlorine to Chlorine/Chloramine
[9-8](#chlorinechlorine-to-chlorinechloramine)

9.1.4 Chlorine/Chlorine to Ozone/Chlorine
[9-8](#chlorinechlorine-to-ozonechlorine)

9.1.5 Chlorine/Chlorine to Ozone/Chloramine
[9-9](#chlorinechlorine-to-ozonechloramine)

9.1.6 Chlorine/Chlorine to Chlorine Dioxide/Chlorine
[9-9](#chlorinechlorine-to-chlorine-dioxidechlorine)

9.1.7 Chlorine/Chlorine to Chlorine Dioxide/Chlorine Dioxide
[9-10](#chlorinechlorine-to-chlorine-dioxidechlorine-dioxide)

9.1.8 Chlorine/Chloramine to Ozone/Chloramine
[9-10](#chlorinechloramine-to-ozonechloramine)

9.1.9 Chlorine/Chloramine to Chlorine Dioxide/Chloramine
[9-10](#chlorinechloramine-to-chlorine-dioxidechloramine)

9.1.10 Ozone/Chlorine to Ozone/Chloramine
[9-11](#ozonechlorine-to-ozonechloramine)

9.1.11 Summary [9-11](#summary)

9.2 Pathogen Inactivation with Interactive Disinfectants
[9-11](#pathogen-inactivation-with-interactive-disinfectants)

9.2.1 Inactivation Mechanism [9-12](#inactivation-mechanism)

9.2.2 Environmental Effects [9-13](#environmental-effects)

9.2.3 Pathogen Inactivation Efficiency Using Interactive Disinfectants
[9-16](#pathogen-inactivation-efficiency-using-interactive-disinfectants)

9.2.4 Summary: Pathogen Inactivation with Interactive Disinfectants
[9-23](#summary-pathogen-inactivation-with-interactive-disinfectants)

9.3 Analytical Methods [9-23](#analytical-methods)

9.3.1 Ozone [9-23](#ozone)

9.3.2 Chlorine Dioxide [9-24](#chlorine-dioxide)

9.3.3 Potassium Permanganate [9-24](#potassium-permanganate)

9.3.4 Chloramine [9-24](#chloramine)

9.3.5 Hydrogen Peroxide [9-24](#hydrogen-peroxide)

9.3.6 UV Radiation [9-24](#uv-radiation)

9.3.7 Summary of Analytical Methods
[9-24](#summary-of-analytical-methods)

9.4 Summary [9-25](#summary-1)

9.5 References [9-26](#references)

Table 9-1. Potential Primary Disinfectants
[9-2](#__RefHeading___Toc450037472)

Table 9-2. Primary/Secondary Disinfectant Combinations and Typical
Applications in Water Treatment [9-3](#__RefHeading___Toc450037473)

Table 9-3. DBPs Associated with Various Combined Oxidation/Disinfection
Processes [9-4](#__RefHeading___Toc450037474)

Table 9-4. Strategies for Primary and Secondary Disinfectants
[9-6](#__RefHeading___Toc450037475)

Table 9-5. Impacts of Disinfection Practice on DBP Formation
[9-7](#_Ref396120594)

Table 9-6. Virus Inactivation By Individual Disinfectants and
Simultaneous Chloramination [9-17](#__RefHeading___Toc450037477)

Table 9-7. *C. parvum* Inactivation Using Ozone Followed by Chlorine
Dioxide [9-19](#__RefHeading___Toc450037478)

Table 9-8. *C. parvum* Inactivation Using Chlorine Dioxide Followed by
Free Chlorine [9-19](#__RefHeading___Toc450037479)

Table 9-9. *G. muris* Inactivation Using Chlorine Dioxide Followed by
Free Chlorine [9-19](#__RefHeading___Toc450037480)

Table 9-10. *B. cereus* Inactivation Using Chlorine Dioxide Followed by
Free Chlorine [9-19](#__RefHeading___Toc450037481)

Table 9-11. *C. parvum* Inactivation Using Chlorine Dioxide Followed by
Chloramine [9-20](#__RefHeading___Toc450037482)

Table 9-12. *G. muris* Inactivation Using Chlorine Dioxide Followed by
Chloramine [9-20](#__RefHeading___Toc450037483)

Table 9-13. *G. muris* Inactivation Using Ozone Followed by Free
Chlorine [9-21](#__RefHeading___Toc450037484)

Table 9-14. *B. cereus* Inactivation Using Chlorine Dioxide Followed by
Free Chlorine [9-21](#__RefHeading___Toc450037485)

Table 9-15. *G. muris* Inactivation Using Ozone Followed by Chloramine
[9-21](#__RefHeading___Toc450037486)

Table 9-16. *G. muris* Inactivation by Free Chlorine Followed by
Monochloramine [9-22](#__RefHeading___Toc450037487)

Table 9-17. *C. parvum* Inactivation by Sequential Application of Ozone
and Chloramine [9-22](#__RefHeading___Toc450037488)

Table 9-18. Summary of Combined Disinfectants
[9-25](#__RefHeading___Toc450037489)

Figure 9-1. Inactivation of *C. parvum* Attributed to Synergistic
Effects. Application of Ozone Followed by Chlorine Dioxide
[9-14](#__RefHeading___Toc450037490)

Figure 9-2. Inactivation of *C. parvum* Attributed to Synergistic
Effects. Application of Chlorine Dioxide Followed by Free Chlorine
[9-14](#_Ref396209375)

Figure 9-3. Inactivation of *C. parvum* Attributed to Synergistic
Effects. Application of Chlorine Dioxide Followed by Monochloramine
[9-15](#_Ref396209422)

Figure 9-4. Inactivation of *E. coli* Using Free Chlorine and
Monochloramine [9-18](#__RefHeading___Toc450037493)