United States
                    Environmental Protection
                    Agency
 Water Engineering
 Research Laboratory
 Cincinnati OH 45268
                    Research and Development
 EPA/600/S2-86/067 Sept. 1986
<>EPA          Project Summary
                    Inactivation  of  Microbial
                    Agents  by  Chemical
                    Disinfectants
                                                        RECEIVED
                    John C. Hoff
       NOV181986

ENVIRONMENTAL PROTECTION AGENCY
       LIBRARY, REGION V
                      Drinking water disinfection kinetics
                    are used to evaluate Escherichia coli,
                    poliovirus, and Giardia lamblia cysts
                    with regard to their relative resistance
                    to inactivation under a variety of physi-
                    cal and chemical conditions. The report
                    explains the concept of C-t product (the
                    product of residual disinfectant, C, in
                    mg/L and contact time, t, in minutes)
                    and reviews the effects of temperature
                    and pH on C-t values. The limitations
                    and dangers of extrapolating C-t values
                    beyond the range of experimental data
                    are also discussed.
                      This Project Summary was devel-
                    oped by  EPA's Water Engineering Re-
                    search Laboratory, Cincinnati, OH, to
                    announce key findings of the research
                    project that is fully documented in a
                    separate  report of the same title (see
                    Project Report ordering information at
                    back).

                    Introduction
                      The primary purpose  of drinking
                    water disinfection is to control water-
                    borne diseases by inactivating  the
                    pathogenic microorganisms in  the
                    water. Disinfection is the final (and
                    sometimes only) engineering process
                    barrier to the entry of viable pathogens
                    into the water distribution system.
                      After chlorine began to be used as a
                    drinking water disinfectant around the
                    turn of the century, interest in its bioci-
                    dal effectiveness brought  about disin-
                    fection research. Information  on  the
                    kinetics of disinfection was soon devel-
                    oped. Since the early 1970's, concern
                    about chemical by-products of chlorina-
                    tion has resulted in a higher level of re-
                    search activity involving alternative dis-
infectants, including chloramine, ozone,
and chlorine dioxide. The early disinfec-
tion research was focused on inactiva-
tion of bacteria.  Viruses were studied
later. Mostrecently, inactivation of Giar-
dia cysts has been the topic of much
research work.
  This report presents a comprehensive
review of disinfection research. The
concepts of disinfection  kinetics that
were developed  by early researchers
and later modified are used in this re-
port to evaluate  Escherichia coli, po-
liovirus, and Giardia cysts with  regard
to their  relative resistance to inactiva-
tion under a variety of physical and
chemical conditions. The document ex-
plains the concept of C-t product (the
product of residual disinfectant, C, in
mg/L and contact time, t, in minutes)
and reviews the effects of temperature
and pH  on C-t values. The  limitations
and dangers of extrapolating C-t values
beyond the range of experimental data
are also discussed.

Disinfection Kinetics
  Inactivation of microorganisms can
be considered to have the characteris-
tics of a first-order chemical reaction,
with the microorganism and the disin-
fectants constituting the reactants. This
concept was expressed as Chick's Law
and is written as
        logN/N0=-K-t        (1)
where N0 = the original number of or-
          ganisms
       N = the number of  organisms
          remaining at time t
        t = the contact time
       K = a proportionality constant

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Ideally, plots of log N/N0 versus t for
various contact times should provide a
straight line (first-order kinetics). In ac-
tual experiments, first-order kinetics are
often not observed throughout the en-
tire range of experimental conditions,
but rather during only a portion of the
experiment. Thus  survival  curves  may
depart from the ideal (Figure 1 a) and
show (1) an initial lag  period before
first-order kinetics are observed (Figure
1 b), (2) a rapid initial decline in popula-
tion (Figure 1 c), or (3) multiple kinetics
sometimes referred to as "tailing off"
(Figure 1 d). Experimental disinfection
data commonly fail to follow first-order
kinetics strictly (Figure 1 a). Other disin-
fection  kinetic models have been pro-
posed, but they are not reviewed in this
report.
  When the biocidal efficacy of disinfec-
tants are compared, the major consider-
ations are disinfectant  concentration
and time needed to inactivate a certain
proportion of the population of exposed
organisms. The  C-t concept can be ex-
pressed as
             k = Cn • t
(2)
where C = disinfectant concentration,
           mg/L

       n = a constant, also called the
           coefficient of dilution

       t = the contact time (minutes)
           required to inactivate  a
           specified percentage of mi-
           croorganisms

       k = a constant for a specific mi-
           croorganism exposed under
           specific conditions

To apply Equation 2 to disinfection data,
the results are used from a number of
individual experiments performed with
different disinfectant concentrations
under identical  experimental condi-
tions. Disinfectant concentrations (C)
and times (t) needed to attain the speci-
fied degree  of inactivation (e.g.,  99%)
are plotted on double  logarithmic pa-
per.  Such plots should  produce a
straight  line with a slope of n. When
n = 1, the C-t value remains constant re-
gardless  of  disinfectant concentration,
and disinfectant concentration and ex-
posure time are of equal importance. If
n exceeds 1, disinfectant concentration
is more important than contact time,
and C-t values required for a specified
kill decline as C increases. On the other
hand, when n is less than 1, contact time
is more important than disinfectant con-
      centration, and C-t values for a specified
      kill increase as C increases and  t de-
      creases.
        The value of n is an important factor
      in determining the degree to which ex-
      trapolation may  be  valid beyond the
range of experimental observations. In
addition, evaluating n is valid only if the
experimental data follow Chick's Law
(Equation 1), which is often not the case.
Values of n have been evaluated, and
results generally fall in the range of 0.5
      I
I
                        Time

                  Exponential Kinetics
                   Time

           Concave Upward Kinetics
           (Initial Shoulder Curve)
       I
                                              1
                        Time

               Concave Downward Kinetics
                 (Initial Rapid Rate Curve)
                  Time

            Multiple Kinetics
            (Tailing Off Curve)
       Figure 1.    Typical survival curves for disinfection experiments. Adapted from: Prokop. A., and
                  A. E. Humphrey, 1970. Kinetics of disinfection. In: Disinfection, M. A. Bernarde, ed..
                  MarcelDekker. Inc.. N.Y. pp. 61-83.

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to 2. Because of uncertainties about n,
extrapolation of data from  specific C
and t conditions to other values under
the assumption that n = 1 would be of
questionable validity. Tables of n values
for free chlorine, chloramine, chlorine
dioxide, and  ozone are given in the full
report.

Temperature Effects
  Effects of temperature change on dis-
infection efficacy have been evaluated
by a number of investigators. Disinfec-
tion rates are generally  increased by a
factor of 2 to 3 as temperature increases
by 10°C. This coefficient is referred to as
the QIQ value. Reported Q10  values for
viruses are usually in the range of 2 to 3.
A slightly wider range of QIO values is
found for disinfection studies in  which
ozone  and chlorine dioxide have been
used. Some  concerns have been ex-
pressed  that  as temperature ap-
proaches 0°C, disinfection rates  might
decrease by a much greater factor than
would  be indicated by Q10 values. How-
ever, no aspect  of physical laws that
govern chemical diffusion and reaction
rates in aqueous media  would support
such a concept. Thus the common rule
of a 2- to 3-fold increase in inactivation
rates per 10°C increase in temperature
seems fairly well substantiated.

Characteristics  of Disinfectants
and Microorganisms

Disinfectants
  The disinfectants reviewed in this re-
port (free chlorine, chloramines, chlo-
rine dioxide, and ozone) have individual
characteristics that influence both the
results of laboratory tests and their per-
formance in the field. These characteris-
tics are reviewed here briefly.
  Free chlorine exists in aqueous solu-
tion as HOCI and OCI~.  HOCI is a much
more effective biocide than OCI~, so the
efficacy of free chlorine  is pH-
dependent.
  Chloramines are formed when chlo-
rine and  ammonia react. Chloramines
are generally much less effective than
free chlorine, with equivalent inactiva-
tion times about 25- to 100-fold higher
for monochloramine than for equivalent
concentrations of  free chlorine. Chlo-
ramine efficacy is  also  pH-dependent.
Application of chloramine laboratory re-
sults to field  conditions  is fraught with
uncertainty.  Most laboratory studies
have been done with  preformed chlo-
ramine, whereas treatment plant prac-
tice would result in at least some con-
tact  with  free  chlorine before
chloramines are formed, even when the
order of addition is ammonia first and
chlorine second. Chloramine disinfec-
tion as practiced in the field may  be
more effective than laboratory  results
would suggest, but the extent of this im-
provement would be site-specific and
would need to be evaluated on  a plant
scale at each site.
  Chlorine dioxide efficacy is less sub-
ject to the influence of  pH than either
free chlorine or chloramine. Chlorine
dioxide is a more effective disinfectant
at pH 9 than at pH 6. This is a reversal of
the behavior shown by free chlorine and
chloramine.  Because it is present in
water as  an undissociated dissolved
gas, chlorine dioxide is more easily lost
through volatilization than free chlorine
or chloramine. This behavior could  af-
fect the kinetics  of  disinfection  experi-
ments with long  exposure times, espe-
cially at higher water temperatures.
  Ozone, like  chlorine dioxide, is
present in water as a dissolved gas,
must be prepared onsite, and cannot be
stored.  Ozone is subject to losses  by
volatilization during disinfection experi-
ments. The volatility and high reactivity
of ozone make it very difficult to main-
tain a stable concentration during ex-
periments and in actual practice.  For
these reasons, C-t values for ozone tend
to be less precise than C-t values for the
other disinfectants.

Microorganisms
  Waterborne pathogens of concern
can be divided into three groups: bacte-
ria, viruses, and  protozoan cysts. They
encompass a wide diversity of sizes, life
cycles, and other biological characteris-
tics, including resistance to chemical
disinfectants. Note that even within dif-
ferent isolates of the same species,  re-
sistance to disinfection  can vary. Fur-
thermore, differences in disinfection
resistance have been observed between
organisms that were cultured in the lab-
oratory and those found  naturally in the
environment. Finally, differences exist
in relative resistance to various  chemi-
cal  disinfectants. Whereas organism A
might be more resistant to chlorine than
organism  B, the  opposite might be ob-
served for chloramine or chlorine diox-
ide.

Application of the C-t Concept
to  Disinfection Practice
  In 1962, Watson's Law (k = Cn-t) was
used as a basis for a procedure for mak-
ing recommendations on disinfection
practice. The C-t value recommenda-
tions were based on constant C-t values,
making an  implicit assumption that
n = 1. This use of the C-t concept may
have been the first to relate disinfection
laboratory data to recommended field
practice.
  C-t values were used to compare bio-
cidal efficiencies in 1980, but the back-
ground of the concept was  not ex-
plained, and no attempt was made to
extrapolate to other values for either C
or t  from those calculated from avail-
able data.
  The use of C-t values to interpret dis-
infection data has become more preva-
lent in the 1980's. The 99% inactivation
level has been used for calculating C-t
values in most studies, probably be-
cause it is the level at which exponential
kinetics (N/N0 = K-t) are usually best ap-
proximated. If exponential kinetics were
followed, and if C-t values for 99% inac-
tivation were known, C-t values for
other levels of inactivation could easily
be calculated. The ideal is not often ob-
served, though. Problems associated
with initial lags (Figure 1 b) and tailing
off (Figure 1 d) make it difficult to calcu-
late C-t values for conditions not directly
observed in experiments. These diffi-
culties should be noted when applying
data from the following section of this
report.

Inactivation of Microorganisms

Bacteria
  Though  pathogenic bacteria are
among the target organisms for disin-
fection, little information is available on
their inactivation. Most of the research
related to bacteria has focused on indi-
cator organisms. Studies in the 1940's
did not reveal substantial differences in
disinfection  resistance between bacte-
rial pathogens and members of the col-
iform group. Thus data for E. coll should
indicate the degree of disinfection
needed for the pathogenic bacteria.
  Two  factors that can influence disin-
fection  results  are the  relative  resis-
tance of laboratory-grown cultures ver-
sus  that of natural  organisms and
protection  of bacteria  by particulate
matter. Cell cultures grown in the labo-
ratory are more easily inactivated. Bac-
teria that are within particles of feces or
other organic matter or that are at-
tached to activated carbon particles are
not inactivated as readily as  bacteria
that are not associated with such partic-
ulate matter.

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  In the full report, data show ranges of
C-t values for 99% inactivation of E. coli
by free chlorine, chloramine, chlorine
dioxide, and ozone. With free chlorine,
the range of experimental conditions
for which data are  available is some-
what reduced for E. coli. The reason is
that at low pH and high temperature (pH
6,  25°C), inactivation proceeds so  fast
that C-t measurements  are difficult to
attain with confidence.  C-t values for
free chlorine are given for pH 6 and 10,
and for 5° and  15°C. The mean C-t for
99% inactivation at 5°C  and pH 6 was
0.045 mg/L • minutes. For chloramine at
5°C and pH 7, the mean C-t was 22 mg/L
• minutes. Chloramine data are given for
pH 7 and 9, and for 5°, 15°, and 25°C. The
mean C-t for chlorine dioxide at 5°C and
pH 6.5 was 0.6 mg/L • minutes, a higher
value than that observed for free chlo-
rine. This level  contrasts with the rela-
tive efficacy of free chlorine and chlo-
rine dioxide for poliovirus and Giardia
cysts. In both of the latter cases, chlo-
rine dioxide is the more  powerful disin-
fectant. Chlorine dioxide data span  a
temperature range of 5° to 25°C and a
pH range of 6.5 to 7. A mean C-t value of
0.2 mg/L • minutes  was obtained for
99% inactivation of E. coli by ozone at
pH 7.2  and 1°C. Ozone data are also
available at 12°C and pH 7.0.
  For the four disinfectants, the n values
were generally  near 1 when E. coli was
the target organism. The ranges of C-t
values were relatively narrow for exper-
iments conducted at the same pH  and
temperature using different disinfectant
concentrations. This result suggests
that C-t values for E. coli are relatively
reliable.
Viruses
  The most extensive research on virus
inactivation has been done with mem-
bers of the enterovirus  group because
the viral agents responsible for water-
borne  disease (Hepatitis A virus, ro-
ta virus, Norwalk virus, etc.) were identi-
fied only recently.  Methods for
laboratory growth and enumeration of
the pathogenic viruses  are difficult or
not yet available. Most  of the disinfec-
tion data presented in this report are for
poliovirus.
  Factors involved in viral resistance to
disinfection are their natural or innate
resistance,  aggregation  into virus
clumps, and  association with particu-
late materials. Research results suggest
that viral aggregation or clumping can
cause deviation from exponential inacti-
vation  kinetics, particularly the tailing
off curves (Figure 1 d). The protective
effects of paniculate matter are similar
for viruses and bacteria. The best pro-
tection is offered by virus-particle com-
plexes associated with human fecal ma-
terial. Viral clumps in fecal particles are
most likely to be highly protected from
inactivation.
  The viral C-t data base is largest for
poliovirus 1. For 99% inactivation with
free  chlorine, C-t values  averaged 1.1
and 2.0 mg/L • minutes for two different
researchers. For 5°C and pH 10, C-t aver-
aged 10.5 mg/L • minutes. Data  are
available for 5° and  15°C, and for pH 6
and 10.
  In  contrast to these values  for free
chlorine, the 99% inactivation value for
chloramine at 5°C and pH 9 averaged
1420  mg/L •  minutes, indicating that
chloramine is a very weak viral disinfec-
tant. Chloramine data are available at
pH9and5°, 15°, and 25°C.
  Chlorine dioxide was as effective as
free chlorine, with a mean C-t of 3.6 mg/
L • minutes for 99% inactivation at 5°C
and pH 7. Data are available for pH 7 and
9, and for 5° to 25°C. Ozone was the
most effective agent. A 99% inactivation
was  attained at 5°C and pH 7.2 with a
mean C-t of 0.2 mg/L • minutes. Ozone
data are available for 5°, 10°, and 20°C,
and for pH 7.0 or 7.2.
  A  limited number of other data are
also presented in the full report. Overall,
the C-t values for poliovirus 1, rotavirus,
and bacteriophage f2 are similar. Labo-
ratory studies done  with preformed
chloramine indicate that all of these
viruses  are extremely resistant to chlo-
ramine. The apparent biocidal efficiency
of chloramine as it is used in water-
works practice would be higher because
of the free chlorine that is present for a
short time.
Protozoan Cysts
  The inactivation of Endomoeba his-
tolytica cysts by chlorine and other dis-
infectants was studied extensively dur-
ing the 1940's and 50's, mainly because
of concerns about waterborne transmis-
sion of amoebic dysentery in  military
forces operating in areas where this dis-
ease was prevalent. These studies  es-
tablished conclusively that the cysts of
E. histolyticawere very resistant to inac-
tivation. The appearance of giardiasis
as an important waterborne disease in
the United States stimulated  disinfec-
tion research on the inactivation of cysts
of the etiologic agent Giardia lamblia. A
method for determining cyst viability by
in vitro excystation was developed, but
problems developed in obtaining G.
lamblia cysts, and deficiencies occurred
in the excystation procedure. Thus most
disinfection  research is currently con-
ducted using G. muris  cysts (a species
infective for mice) as  a model for G.
lamblia cysts. This approach seems to
work well. A comparative study of ex-
cystation and  mouse infectivity for
measurement of chlorine-exposed
G. muris cysts indicated that the results
were similar for both methods.  Giardia
lamblia has a complex life cycle.  The
conversion from the  active trophozoite
to the inactive,  resistant cyst occurs in
the lower portion of the intestinal tract.
The cysts do not multiply and are rela-
tively inert in the environment,  excyst-
ing to form the trophozoite stage only
after ingestion by the host.
  Because the cysts are relatively large
(ovoid bodies 8 to 12 by 7 to 10 urn in
diameter), protection from  inactivation
by  association  with  particulate matter
may be less important for them than for
smaller,  more  easily occluded patho-
gens. Little information has been devel-
oped on this subject.
  The largest data base available for Gi-
ardia is from disinfection research with
G.  muris cysts. The  available data for
cysts of the human pathogen G.  lamblia
also are included in the report.
  Mean C-t values for  99% inactivation
of  G. lamblia  cysts by free chlorine
range from 65 to greater than 150 mg/L
•  minutes for 5°C and  pH 6. Data are
available for 5°, 15°, and 25°C,  and for
pH 6, 7, and 8.  Data  on G.  muris cover
the range of 3° to 25°C,  and pH 5 to 9. At
5°C and pH 6, a C-t of greater than 150
mg/L - minutes  has been reported. How-
ever,  researchers have also obtained a
C-t of 68 mg/L • minutes for 3°C, pH 6.5,
and 99% inactivation.
  Chloramine data are available for G.
muris in a temperature range of 3° to
18°C and a pH of 6.5 to 8.5. A mean C-t
of 463 mg/L • minutes was obtained for
99% inactivation at 3°C, pH 6.5, but the
chloramine was not preformed. At  18°C
and pH 7, a C-t of 184 mg/L • minutes
was obtained when chloramine was not
preformed. In contrast,  at 15°C and pH 7,
a mean C-t of  848 mg/L •  minutes re-
sulted from use of preformed chlo-
ramine.
  Data for 99% inactivation of G. muris
by chlorine dioxide are available for 5°
and 25°C at pH  7 and 9. At 5°C and pH 7,
the mean  C-t is 11.2  mg/L • minutes.
This value is about one order of magni-
tude lower than those  for free chlorine

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and chloramine,  and it suggests that
chlorine dioxide is a powerful cysticidal
agent.
  Ozone disinfection data are available
for G. muris and G. lamblia at pH 7 from
5° to 25°C. At pH 7 and 5°C, the mean C-t
value was 1.9 mg/L • minutes for 99%
inactivation of G. muris and 0.6 mg/L •
minutes for G.  lamblia. Ozone appears
to be somewhat more effective against
cysts than chlorine dioxide.
  A summary of the  comparative effi-
ciency of free chlorine, chloramine,
chlorine dioxide, and ozone for inactiva-
tion of specific bacteria, viruses, and
protozoan cysts  appears in Table 1.
Ozone shows the highest efficiency, in-
activating 99%  of all types of  microor-
ganisms  at very low C-t values. Chlo-
ramine shows the lowest  efficiency.
Chloramine C-t values for viral agents
are particularly  high. Free chlorine at pH
6 to 7 and chlorine dioxide at  pH 7  are
approximately equivalent for poliovirus
1 inactivation. Free chlorine appears to
be  considerably more effective than
chlorine dioxide for inactivation of  ro-
tavirus, bacteriophage f2, and E. coli,
whereas chlorine dioxide appears to be
much more effective than free chlorine
for G. muris cysts. The data in Table 1
also show the relative variability in  re-
sistance among and within the groups
of microorganisms. The general pattern
of greater resistance of cysts compared
with viruses, and of viruses compared
with bacteria is evident for free chlorine,
chlorine dioxide,  and ozone. Although
cyst C-t  values for preformed chlo-
ramines  at 5°C are not yet available,
the available values at 15°C suggest that
cysts may be  more sensitive to pre-
formed chloramine than the viruses.
  The bacteriophage f2 C-t values sug-
gest that the use of this virus to indicate
virus inactivation is questionable.  On
the other hand, poliovirus appears to be
a relatively good indicator, since it is
substantially more resistant to free
chlorine and  chlorine dioxide than  ro-
ta virus and bacteriophage f2. Rotavirus,
however, appears to be  somewhat
more resistant to preformed chloramine
than poliovirus 1.
  Finally,  G. muris cysts appear to  be
somewhat more resistant than G. lam-
blia cysts to  free chlorine  and ozone.
Also, considerable uncertainty exists
with regard to C-t values for 99% inacti-
vation of cysts  by free chlorine. Values
derived from studies by different inves-
tigators show substantial variation.
Table 1.    Summary of C-t Value Ranges for 99% Inactivation of Various
          Microorganisms by Disinfectants at 5°C

                                          Disinfectant
Micro-
organism
E. coli
Polio 1
Rotavirus
Bacterio-
phage f2
G. lamblia
cysts
G. muris
cysts
Free
Chlorine,
pH6to7
0.034-0.05
1.1-2.5
0.01-0.05
0.08-0.18
47- > 150
30-630
Preformed
Chloramine,
pH8to9
95-180
768-3740
3806-6476
—
—
—
Chlorine
Dioxide,
pH6to7
0.4-0.75
0.2-6.7
0.2-2. 1
—
—
7.2-78.5
Ozone,
pH6to7
0.02
0. 1-0.2
0.006-0.06
—
0.5-0.6
7.8-2.0
  All of the C-t values discussed above
are based on 99% inactivation of the mi-
croorganisms. As indicated, the nature
of inactivation curves prevents extrapo-
lation from the 99% inactivation C-t val-
ues to obtain reliable  C-t values for
other levels of inactivation (e.g., 50%,
90%,  95%, 99.9%, etc.). The curves
nearly always show either an initial
shoulder, tailing off, or other more com-
plex configurations (see Figure 1). Ex-
trapolation from initial shoulder curves
on the 99% inactivation level (assuming
exponential  inactivation rates*) will
underestimate C-t  values for less than
99% inactivation and overestimate C-t
values for greater than 99%  inactiva-
tion. Extrapolation from initial rapid rate
and tailing off curves will usually have
the opposite  effect, depending on the
point at which the inactivation rate be-
gins to decrease. Thus extreme caution
must be used if any attempt is made to
extrapolate C-t data to  other  inactiva-
tion percentages.

Conclusions
1. The C-t values  compiled provide a
   basis  for comparing the effective-
   ness of different disinfectants for in-
   activation of specific microorgan-
   isms and for comparing the relative
   resistance of different microorgan-
   isms to specific disinfectants. In
   some  cases, the C-t values derived
   from exposure to different concen-
   trations of the same disinfectant
   under specific pH and temperature
3.
*A straight line extended from 100% survival at
 time 0 through the 99% inactivation time point.
conditions show little variation, and
in other cases, a wide range of C-t
values occurs. Discerning the rea-
sons for widely differing values is al-
most always difficult, whether con-
sidering the results from only one in-
vestigation or from  several. These
factors make it difficult to pinpoint
disinfection requirements. C-t values
must be used cautiously to evaluate
disinfection  practice  or to establish
disinfection criteria  for use in  the
field,  and appropriate safety factors
must be incorporated  into the C-t
values.
Some major problems in applying
the results of C-t values to develop-
ing disinfection requirements are as
follows: a) the failure of disinfection
data to follow the exponential rates
described by the empirical C-t  equa-
tion, b) differences in disinfection re-
sistance between different isolates of
the same species and between differ-
ent species within groups (bacteria,
viruses, cysts), c)  state-of-the-
microorganism effects  such as  ag-
gregation, prior growth conditions,
and protective effects that cannot be
factored into the values, d) influence
of experimental conditions  (mixing
intensity, disinfectant concentration
variations, etc.) on inactivation rates,
e) problems relating to the relevance
of laboratory data to field conditions.
Because of the limited data available,
uncertainties still remain regarding
disinfection requirements for Giardia
lamblia cysts. Most of the data avail-
able for G. lamblia cysts indicate
lower requirements than  do  the

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   more extensive data available for the
   model G. muris cysts. These uncer-
   tainties are very important because
   this pathogen is the most resistant of
   all waterborne pathogens of con-
   cern. Additional research is in prog-
   ress using G. muris cysts and chlo-
   ramines  and free chlorine  at low
   temperatures. Other research indi-
   cates that an alternative to the excys-
   tation method for determing G. lam-
   blia and G. muris viability may soon
   be available. Such an alternative
   would facilitate disinfection research
   on this species. The method would
   still involve microscopic observation
   and therefore would not result in im-
   proved detection of viability at low
   cyst concentrations (greater than
   99% inactivation).
4.  For some disinfectants (mainly chlo-
   ramines), utilities should perhaps be
   required to demonstrate the efficacy
   of their disinfection practices for con-
   trolling pathogens of concern. This
   alternative  approach  may well be
   warranted because of the extreme
   dependence of chloramine disinfec-
   tion efficiency  on field  conditions
   that cannot all be taken into account
   in developing overall C-t values.
5.  For inactivation by free chlorine, pH
   is a very important factor because of
   the rapid decrease in the more effec-
   tive disinfectant chemical species
   (HOCI) that occurs over a pH range of
   7 to 8. Many natural waters fall into
   this pH range. Monitoring of pH and
   subsequent pH modification may be
   advisable in some cases to enhance
   disinfection efficiency, particularly at
   low temperatures.

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      TheEPA author JohnC. Hoff (see below) is withthe Water Engineering Research
       Laboratory. Cincinnati, OH 45268.
      The complete report,  entitled "Inactivation  of Microbial Agents by Chemical
       Disinfectants, "(Order No. PB 86-232 568/AS; Cost $11.95, subject to change)
       will be available only from:
             National Technical Information Service
             5285 Port Royal Road
             Springfield, VA 22161
             Telephone: 703-487-4650
      John C. Hoff can be contacted at:
             Water Engineering Research Laboratory
             U.S. Environmental Protection Agency
             Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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