DOSAGE
The amount of chlorine required for efficient treatment is very largely
determined by the amount required to satisfy the oxidisable matter
present in the water. Many experimenters have reported results that
would indicate that appreciable concentrations of chlorine are required
for bactericidal action but the details of the technique, as published,
show that the effect of the organic matter added with the test organism
was not thoroughly appreciated. One cubic centimetre of a culture in
ordinary peptone water, added to one litre of water, would increase the
organic content by approximately 10 parts per million, an amount that
would absorb appreciable amounts of chlorine.
Other conditions also make it very difficult to compare the results
obtained in the past: one of these is the degree of purity set as the
objective. German bacteriologists added enormous numbers of the test
organism and endeavoured to obtain the complete removal of the organism
from such quantities as one litre of water with a contact period often
as short as 10 minutes. Nissen,[1] of the Hygienic Institute of Berlin,
found that a 1: 800 dilution of bleach (420 p.p.m. of chlorine) was
required to destroy _B. typhosus_ in one minute and a 1:1600 dilution
(210 p.p.m. of chlorine) in 10 minutes. Delepine[2] obtained somewhat
similar results by means of the thread method for testing disinfectants.
Phelps,[3] using gelatine plates for enumeration of the bacteria,
obtained a 90 per cent reduction of _B. typhosus_ in twenty minutes
with 5 p.p.m. of available chlorine; over 99 per cent reduction in one
hour, and over 99.99 per cent reduction in 18 hours. Wesbrook,
Whittaker, and Mohler[4] tested bleach solutions with various strains of
_B. typhosus_ by means of the plate method and found that the most
resistant one was reduced from 20,000 per c.cm. to sterility (in 1
c.cm.) by 3 p.p.m. of available chlorine in fifty minutes and that the
least resistant one only required 1.0 p.p.m. with a thirty minutes'
contact.
Lederer and Bachmann[5] have reported the following results:
TABLE V
PERCENTAGE REDUCTION, 15 MINUTES' CONTACT
----------+-------------------------------------------------------
| NATURE OF TEST ORGANISM.
+-------+-------+-------+-------+-------+-------+-------
Available | | B. | | | | B. |
Chlorine | B. |faecalis| B. |Proteus| B. |lactis | B.
p.p.m. |cloacae.|alkali-|paraty-| mira- |enter- | aero- |choler[oe]-
| |genes. |phosus.|bilis. |itidis.| genes |suis.
----------+-------+-------+-------+-------+-------+-------+-------
0.1 | .....| 99.98| .....| 27.3 | .....| .....| .....
0.2 | 99.69| 99.99| 99.97| 45.5 | 99.83| 99.17| 95.8
0.3 | 99.75| 100.00| 100.00| 63.7 | 99.98| 99.98| 100.0
0.5 | 100.00| .....| .....| 72.7 | 100.00| 100.00| .....
0.7 | .....| .....| .....| 63.7 | .....| .....| .....
1.0 | .....| .....| .....| 63.7 | .....| .....| .....
3.0 | .....| .....| .....| 90.9 | .....| .....| .....
5.0 | .....| .....| .....| 90.0 | .....| .....| .....
Original }| | | | | | |
number of}| | | | | | |
organisms}|160,000| 9,500| 3,000| 8,000 |180,000|180,000| 500
per c.cm.}| | | | | | |
----------+-------+-------+-------+-------+-------+-------+-------
With the exception of _P. mirabilis_, which forms endospores, all the
organisms were killed (less than 1 per c.cm.) by 0.5 p.p.m. of available
chlorine in fifteen minutes.
All these observers found that _B. coli_, the organism usually employed
as an index of contamination, had approximately the same degree of
resistance to chlorine as _B. typhosus_, though Wesbrook et al. directed
attention to the varying viability of organisms derived from different
sources.
These experiments merely indicate the dosage required for exceptional
conditions such as it is inconceivable would ever occur in water-works
practice. No information is available regarding the actual _B. typhosus_
content of waters that have caused epidemics of typhoid fever, but for
the present purpose it may be assumed that the extreme condition would
be a pollution by fresh sewage giving a _B. coli_ content of 1,000 per
c.cm. or 200 times worse than the average condition that can be
satisfactorily purified without overloading a filter plant (500 _B.
coli_ per 100 c.cms.). Experiments made by the author indicate that a
suspension of 1,000 _B. coli_ per c.cm. in water, in the absence of
organic matter, can be reduced to a 2 _B. coli_ per 100 c.cms. standard
(the U.S. Treasury Standard) by 0.1 p.p.m. of available chlorine in ten
minutes at 65 deg. F. This experiment indicates the amount of chlorine that
is required for the bactericidal action only; such a dosage could never
be used in practice to meet a pollution of this degree because of the
accompanying organic matter. In actual practice the author has
experienced the above condition but once, and on that occasion the _B.
coli_ were derived from soil washings and not from fresh sewage.
The amount of chlorine required for germicidal action is small, and the
main factors that determine the dosage necessary to obtain this action
are (1) the content of readily oxidisable organic matter, (2) the
temperature of the water, (3) the method of application of the chlorine
and (4) the contact period.
=Oxidisable Matter.= The oxidisable matter may be divided into two
classes (_a_) inorganic and (_b_) organic. The inorganic constituents
naturally found in water, that are readily oxidisable, are ferrous salts
(usually carbonates), nitrites, and sulphuretted hydrogen, and these
react quantitatively with chlorine until fully oxidised. The oxygen
value of chlorine is approximately one-quarter (actually 16: 71) the
available chlorine content in accordance with the equation Cl_{2}/71 +
H_{2}O = 2HCl + O/16. One part per million of available chlorine will
oxidise 1.58 p.p.m. of ferrous iron; 0.197 p.p.m. of nitrous nitrogen;
and 0.479 p.p.m. of sulphuretted hydrogen.
TABLE VI.[A]--EFFECT OF COLOUR
TEMPERATURE 63 deg. F.
----------------+--------------------+---------------------
| Water "A" Colour 3 | Water "B" Colour 40
| Available Chlorine | Available Chlorine
Contact Period. | p.p.m. | p.p.m.
+--------------------+------+------+-------
| 0.2 | 0.2 | 0.4 | 0.5
----------------+--------------------+------+------+-------
Nil | 194 | 194 | 194 | 194
5 minutes | 121 | 165 | 129 | 66
1 hour | 7 | 95 | 20 | 1
5 hours | 0 | 4 | 0 | 0
24 hours | 0 | 1 | 1 | 0
48 hours | 0 | 0 | 0 | 0
----------------+--------------------+------+------+-------
[A] Results are _B. coli_ per 10 c.cms. of water.
The organic matter found in water may be derived from various substances
such as urea, amido compounds, and cellulose; humus bodies derived from
soil washings and swamps may also be present. The humus compounds of
swamps and muskeg are usually associated with the characteristic colour
of the water derived from these sources. The effect of this coloured
organic matter upon the chlorine dosage is well illustrated in Table VI.
In this experiment _B. coli_ was used as the test organism and the only
varying factor was the organic matter. To obtain the same result with a
contact period of one hour at 63 deg. F. it was necessary to use about two
and one-half times the amount of chlorine with a water containing 40
p.p.m. of colour as with one practically free from colour. It will be
noted that water "A," in which the colour had been reduced to 3 p.p.m.
by coagulation with aluminium sulphate, required a greater dosage of
chlorine than was necessary for bactericidal action only. This was due
to a residual organic content which produced none or but a trace of
colour, for although the colour had been reduced by 92 per cent the
organic matter, as measured by the oxygen absorbed test, had only been
reduced by 70 per cent.
The results obtained by Harrington[6] at Montreal are in the same
direction. During the greater part of the year the water is obtained
from the St. Lawrence river, which is colourless and low in organic
matter; in the spring months the flood waters of the Ottawa, a highly
coloured river, enter the intake and necessitated a much higher dosage.
CHLORINE TREATMENT AT MONTREAL
-------------+--------+-------+--------+---------+----------+---------
| | | Oxygen | Chlorine| |
Source of | Alkali-|Colour.|Absorbed| Required| Bacteria | Per Cent
Supply. | nity. | | (30 | p.p.m. | per c.cm.| Removed.
| | | mins.) | | |
-------------+--------+-------+--------+---------+----------+--------
Ottawa river | 15-20 | 50-70 | 14.0 | 1.50 | 3,000 | over 98
St. Lawrence | | | | | |
river | 90-100 | Nil. | 0.30 | 0.30 | 500 | over 99
-------------+--------+-------+--------+---------+----------+---------
Ellms[7] obtained similar results and reported "that the rate at which
sterilisation proceeds varies, in a general way, directly with the
concentration of the applied available chlorine and the temperature, and
inversely as the amount of easily oxidisable matter present."
Experience with filter plants shows the same facts, the amount of
chlorine required for the sterilisation of a filter effluent being
invariably less than that necessary to purify the raw water to the same
extent.
The effect of coloured organic matter upon the absorption of chlorine,
in the form of hypochlorite, is shown on Diagram I.
[Illustration: DIAGRAM I
EFFECT OF COLOUR ON ABSORPTION OF CHLORINE BY WATER
+-------------------------------+----------------------------+
| | Value of K calculated from |
| | |
| Absorption of Chlorine | Log(N_{1}/N_{2}) |
| by water at 63 deg. F. | K = ----------------- |
| | _t__{2} - _t__{1} |
| | |
+-------+ +-------+ When _t__{1} = 0 |
|Time of+-----------------------+Time of+--------------------+
|Contact| Colour of Water |Contact| Colour |
| in +-------+-------+-------+ in +------+------+------+
|Minutes| 3 | 25 | 40 |Minutes| 3 | 25 | 40 |
+-------+-------+-------+-------+-------+------+------+------+
| Nil | 10.00 | 10.00 | 10.00 | | | | |
| 5 | 9.62 | 7.70 | 6.50 | 5 |0.0033|0.0227|0.0374|
| 10 | 9.41 | 7.03 | 5.91 | 10 |0.0026|0.0153|0.0228|
| 20 | 9.17 | 6.40 | 5.18 | 20 |0.0018|0.0096|0.0190|
| 40 | 8.95 | 5.82 | 4.47 | 40 |0.0012|0.0057|0.0087|
| 60 | 8.85 | 5.63 | 3.90 | 60 |0.0008|0.0041|0.0068|
| 80 | 8.80 | 5.58 | 3.65 | 80 |0.0007|0.0032|0.0056|
+-------+-------+-------+-------+-------+------+------+------+]
The shape of the curve obtained with a colour of 40 p.p.m. somewhat
resembled that of a mono-molecular reaction and the results were
calculated accordingly. The mathematical expression of this law is
_dN_/_dt_ = _KN_ where _N_ is the concentration of the available
chlorine in parts per million. Integrating between _t__{1} and _t__{2}
the formula _K_ = log(_N__{1}/_N__{2})/(_t__{2} - _t__{1}) is obtained.
If the compound absorbing the chlorine were simple in character, and the
chlorine were present in large excess, the value of _K_ would be
constant. In the experiments recorded, _K_ constantly decreases, due to
the decreasing concentrations of the reacting substances and the complex
nature of the organic matter.
The results show the effect of organic matter on the reduction of the
chlorine concentration available for germicidal action and also the
importance of avoiding a local excess of chlorine (_vide_ p. 41).
An effort has been made by some observers to find a quantitative
relation between the organic matter, expressed as oxygen absorbed in
parts per million, and the chlorine required for oxidation, but without
definite result. Some of the results obtained are given in Table VII.
TABLE VII.--OXYGEN TO CHLORINE RATIO
-------------------------+-------------------------
| Oxygen Absorbed
Observer. | Ratio -----------------.
| Chlorine Absorbed
-------------------------+-------------------------
Rouquette | 1
Bonjean | 0.5
Orticoni | Less than 1
Valeski and Elmanovitsch | 0.4
Race | 0.4
Theoretical | 0.22
-------------------------+-------------------------
The value of 0.4 (0.39) obtained by the author is the average of over
one hundred determinations covering a period of two years. The
experiments of Zaleski and Elmanovitsch were made with the water of the
Neva River.
The divergence in the ratios affords additional evidence in favor of
reaction (2) mentioned on page 28 and also shows that the chlorinated
compounds are less readily oxidized than those from which they are
produced. Heise[8] has found that the amount of chlorine consumed is
usually proportional to the concentration in which it is added though
not necessarily a function of the concentration of the organic matter.
=Temperature.= The evidence regarding the effect of temperature upon the
dosage required is somewhat conflicting. Ellms (_vide supra_) found that
the velocity of the germicidal action varied directly with the
temperature and this has also been the author's experience with
laboratory experiments. Typical examples of these are given in Tables
VIII and IX.
TABLE VIII.[A]--EFFECT OF TEMPERATURE
AVAILABLE CHLORINE 0.4 PART PER MILLION
---------------+---------------------------------
|Temperature, degrees, Fahrenheit.
Contact Period.+-----------+----------+----------
| 36 | 70 | 98
---------------+-----------+----------+----------
Nil | 424 | 424 | 424
5 minutes | 320 | 280 | 240
1.5 hours | 148 | 76 | 12
4.5 hours | 38 | 14 | 3
24 hours | 2 | 0 | 0
48 hours | 2 | 0 | 0
---------------+-----------+----------+----------
[A] Results are _B. coli_ per 10 c.cms.
TABLE IX.[B]--EFFECT OF TEMPERATURE
AVAILABLE CHLORINE 0.2 PARTS PER MILLION
---------------+---------------------------------
|Temperature, degrees, Fahrenheit.
Contact Period.+-----------+----------+----------
| 36 | 70 | 98
---------------+-----------+----------+----------
Nil | 240 | 240 | 240
5 minutes | 240 | 250 | 235
1 hour | 245 | 235 | 195
4 hours | 215 | 190 | 170
24 hours | 143 | 130 | 115
48 hours | 130 | 59 | 19
72 hours | ... | 28 | ...
96 hours | ... | 16 | ...
120 hours | ... | 6 | ...
---------------+-----------+----------+----------
[B] Results are _B. coli_ per 10 c.cms.
The reaction velocity of a germicide is proportional to the
temperature[9] and the influence of temperature may be mathematically
expressed by the formula _K__{1}/_K__{2} = _[theta]_(_T__{2} - _T__{1}),
in which _K__{1} and _K__{2} are the constants of the reaction at
temperatures _T__{2} and _T__{1}, respectively, and _[theta]_ is the
temperature coefficient. From the value of _[theta]_, the velocity
constant of a germicide for any temperature may be calculated from the
equation _K__{_T_} = _K__{20} deg. x _[theta]_^{(_T_ - _T__{20} deg.)}.
_K__{1} and _K__{2} are obtained from the formula _K__{_T_} =
log(_N__{1}/_N__{2})/(_t__{2} - _t__{1}) in which _N__{1} - _N__{2} is
the number of bacteria destroyed in the interval _t__{2} - _t__{1}.
A reduction of temperature also lowers the oxidizing activity of the
chlorine so that a greater concentration is available for germicidal
action. This is shown by the results plotted in Diagram II.
[Illustration: DIAGRAM II
EFFECT OF TEMPERATURE ON ABSORPTION OF CHLORINE BY WATER
+----------------------------+----------------------------------+
| | Value of K calculated from |
| | absorption at 63 deg. F. |
| Absorption of Chlorine by | |
| water containing 40 p.p.m. | Log(N_{1}/N_{2}) |
| of colour | K = ----------------- |
| | _t__{2} - _t__{1} |
+-------+--------------------+ |
|Time of|Temperature of Water+-------+--------+--------+--------+
|Contact+--------------------+_t__{2}|_t__{1} |_t__{1} |_t__{1} |
|Minutes|32 deg. F.|46 deg. F.|63 deg. F.|minutes| = 0 | = 5 | = 10 |
+-------+------+------+------+-------+--------+--------+--------+
| Nil |10.00 |10.00 |10.00 | | | | |
| 5 | 8.00 | 7.45 | 6.50 | 5 | 0.0374 | ---- | ---- |
| 10 | 7.23 | 7.09 | 5.91 | 10 | 0.0228 | 0.0082 | ---- |
| 20 | 7.00 | 6.60 | 5.18 | 20 | 0.0190 | 0.0066 | 0.0057 |
| 40 | 6.42 | 6.05 | 4.47 | 40 | 0.0087 | 0.0043 | 0.0040 |
| 60 | 6.22 | 5.60 | 3.90 | 60 | 0.0068 | 0.0040 | 0.0036 |
| 80 | 6.13 | 5.40 | 3.65 | 80 | 0.0056 | 0.0033 | 0.0029 |
+-------+------+------+------+-------+--------+--------+--------+]
Tables VIII and IX, however, show that the temperature coefficient of
the germicidal action has a greater effect than the reduction in the
amount of chlorine absorbed and removed from the reaction.
The results obtained on the works scale with these waters are very
different to the laboratory ones and show that more chlorine is required
during the summer season than in winter. The results with bleach and
liquid chlorine are in the same direction (_vide_ Diagrams III and IV).
The bleach was regulated so as to maintain a constant purity, whilst in
the other case the dosage was constant with a varying _B. coli_ content.
In Diagram IV the _B. coli_ is plotted; this does not represent all the
factors involved as the _B. coli_ content of the treated water is also a
function of that of the raw water, but in the example given this factor
is of no moment because it was comparatively constant during the period
plotted (extreme variation 80 per cent).
The discrepancies between the laboratory and works results cannot be
easily explained. The only difference in the conditions is the nature of
the containing vessel. Glass is practically inert at all temperatures
but the iron pipes, through which the water passed before the samples
were taken, may exert an absorptive influence on the chlorine at the
higher temperatures experienced during the summer months.
Waters containing organic matter that differs much in quantity from the
examples above may yield very different results and no generalisation
can be made that will cover all cases. An increase of temperature
increases the germicidal velocity and also the rate of absorption of
chlorine by the organic matter; other factors determine which of these
competitive actions predominates.
=Method of Application (admixture).= A thorough admixture of the water
and chlorine is a _sine qua non_ for successful operation. This should,
if possible, be attained by natural means, but if there is any doubt as
to the efficiency of the mixing process, mechanical appliances should be
utilised. Pumps, especially centrifugal pumps, constitute a very
convenient and efficacious method of mixing the germicide and the
water, and the solutions should never be injected into the discharge
pipes when it is possible to make connections with the suctions.
[Illustration: DIAGRAM III
EFFECT OF TEMPERATURE]
[Illustration: DIAGRAM IV
EFFECT OF TEMPERATURE]
Inefficient admixture leads to local concentration of the chlorine, a
condition which (_vide_ p. 35), results in a wastage of the
disinfectant. Two practical examples of this effect may be cited. In one
case the water was free from colour and contained very little organic
matter. This water was chlorinated at one plant by allowing the bleach
solution to drop into one vertical limb of a syphon approximately 6,000
feet long, the other vertical limb being used as a suction well for the
pumps which discharged into the distribution mains. At the other plant
the bleach solution was injected into the discharge pipe of a
reciprocating pump through a pipe perforated with a number of small
holes. The results for two typical months are given in Table X.
TABLE X.--EFFECT OF EFFICIENT MIXING
------+-----------+-----------------------+---------------
| Available | BACTERIA PER C.CM. |
| Chlorine +-----------------------+
| Parts Per | | | B. Coli Index
Month.| Million. | Raw |Treated Water. | Per 100 c.cms.
+-----+-----+ Water.+-------+-------+-------+-------
| A. | B. | | A. | B. | A. | B.
------+-----+-----+-------+-------+-------+-------+-------
July | 0.20| 0.25| 864 | 27 | 93 | <0.2 | 8.5
August| 0.20| 0.27| 1.108 | 12 | 120 | <0.2 | 10.2
------+-----+-----+-------+-------+-------+-------+-------
A = efficient mixing. B = inefficient mixing.
The results with the "B" plant were very irregular. The hypochlorite and
water did not mix thoroughly and, as several suctions pipes were
situated in the suction shaft, there was no subsequent admixture in the
pumps; this also caused complaints regarding taste and odour but the
complaints were localised, and not general as would result from an
overdose of solution due to irregularities at the plant.
The second example deals with a water containing 40-45 p.p.m. of colour.
This supply was taken from the river by low-lift pumps and discharged
into a header which was connected with the high-lift pumps by two intake
pipes about 5,000 feet in length. During 1914 a baffled storage basin of
two hours capacity was constructed and in June the hypochlorite was
added at the inlet to this basin by means of a perforated pipe. The
object was to increase the contact period prior to the delivery of the
water into the header. The results for this month were as follows:
AVAILABLE CHLORINE 1.88 PARTS PER MILLION
------------------------+-------------------------+----------
| BACTERIA PER C.CM. AGAR.|
+------------+------------+ B. Coli.
| 3 Days at | 1 day at | Index
| 20 C. | 37 C. | Per c.cm.
------------------------+------------+------------+----------
Raw water | 410 | 104 | 0.280
Treated water | 49 | 26 | 0.036
Percentage purification | 88.2 | 75.0 | 87.5
------------------------+------------+------------+----------
During August the point of application of the hypochlorite was changed
from the inlet of the basin to the suctions of the pumps and the
solution proportioned to the amount of water pumped by the starch and
iodide test. The average of the daily tests for this month were:
AVAILABLE CHLORINE 1.55 PARTS PER MILLION
------------------------+-------------------------+------------
| BACTERIA PER C.CM. AGAR.|
+------------+------------+ B. Coli.
| 3 Days at | 1 day at | Index
| 20 C. | 37 C. | Per c.cm.
------------------------+------------+------------+----------
Raw water | 448 | 100 | 0.600
Treated water | 26 | 12 | 0.005
Percentage purification | 91.9 | 88.0 | 99.2
------------------------+------------+------------+----------
Here again thorough admixture produced better results than inefficient
admixture plus a longer contact period. Langer[10] has also noted the
effect of local concentration and found that the disinfecting action is
increased by adding the bleach solution in fractions, a cumulative
effect replacing that of concentration.
The importance of the admixture factor was not thoroughly appreciated
during the earlier periods of chlorination but later installations, and
particularly the liquid chlorine ones, have been designed to take full
advantage of it.
The point of application in American water-works practice varies
considerably (Longley[11]). In 57 per cent of those cases in which it is
employed as an adjunct to filtration, it is used in the final treatment;
in 26 per cent it is used after coagulation or sedimentation and before
filtration; in the remaining 17 per cent it is applied before
coagulation and filtration. The report of the committee adds: "The data
at hand do not give any reasons for the application before coagulation.
In general, an effective disinfection may be secured with a smaller
quantity of hypochlorite, if it is applied after rather than before
filtration. It should be noted that the storage of chlorinated water in
coagulating basins, and its passage through filters, tend to lessen
tastes and odors contributed by the treatment and this fact may in some
cases account for its use in this way."
=Contact Period.= Other things being equal, the efficiency of the
treatment will vary directly, within certain limits, with the contact
period. When a chlorinated water has to be pumped to the distribution
mains directly after treatment, the dosage must be high enough to secure
the desired standard of purity within twenty to thirty minutes. The
chlorine is sometimes not completely absorbed in this period and may
cause complaints as to tastes and odours. The examples given above show
that the lack of contact period can be largely compensated by ensuring
proper admixture. Experience has amply demonstrated that there is no
necessity to use heroic doses for water that is delivered for
consumption almost immediately after treatment, and that, with proper
supervision, complaints can be almost entirely prevented.
The general effect of the effect of contact period is shown in Tables
VIII and IX on page 37. Another example of a coloured water is given in
Table XI, whilst Table XII shows the results obtained with a colourless
water.
TABLE XI.[A]--EFFECT OF CONTACT PERIOD
-----------------+-------------------------------
| CHLORINE, PARTS PER MILLION.
Contact Period. +-------+-------+-------+-------
| 0.30 | 0.40 | 0.55 | 1.21
-----------------+-------+-------+-------+-------
Nil | 3,800 | ... | ... | ...
1 minute | 1,400 | 120 | 0 | 0
10 minutes | 720 | 5 | 0 | 0
20 minutes | 35 | 0 | 0 | 0
-----------------+-------+-------+-------+-------
[A] Results are _B. coli_ per 10 c.cms.
TABLE XII.--EFFECT OF CONTACT PERIOD AVAILABLE CHLORINE 0.27 PART PER
MILLION
-----------------+--------------------------------+-------------------
| Sampling Point. | Bacteria Per c.cm.
-----------------+--------------------------------+-------------------
Average of series| 5,000 ft. from pumping station| 300
of samples | 6,000 " " " " | 203
| 7,000 " " " " | 103
| 12,000 " " " " | 86
| 14,000 " " " " | 87
-----------------+--------------------------------+-------------------
Table XIII is taken from the work of Wesbrook et al.[4]
TABLE XIII.[B]--TREATMENT OF MISSISSIPPI RIVER WATER AUG. 8, 1910
-------------+-------------------------------------------------
| CONTACT PERIOD. (TEMP. 22 deg.-26 deg. C.).
Available Cl.+---------+---------+---------+---------+---------
P.p.m. | | 1 Hr. | | 6 Hrs. |
| 30 Mins.| 30 Mins.| 3 Hrs. | 30 Mins.| 24 Hrs.
-------------+---------+---------+---------+---------+---------
0 | 230,000 | 200,000 | 160,000 | 150,000 | 140,000
0.5 | 14,000 | 7,400 | 2,000 | 6,000 | 11,000
1.0 | 20 | 14 | 170 | 450 | 60,000
1.5 | 10 | 6 | 16 | 45 | 70,000
2.0 | 7 | 8 | 10 | 97 | 70,000
2.5 | 7 | 14 | 30 | 116 | 65,000
3.0 | 6 | 12 | 5 | 12 | 16,500
-------------+---------+---------+---------+---------+---------
[B] Results are bacteria per c.cm.
In Tables VIII, IX, XI, and XII, the bacteria decreased constantly with
increase of contact period, but the results in Table XIII show that no
advantage was to be gained by prolonging the contact beyond three hours;
after this period the bacteria commenced to increase in number and when
twenty-four hours had elapsed the number approached the original. This
increase in the bacteria is technically known as "aftergrowth" and will
be discussed more fully in Chapter IV.
The replies to queries sent out by the Committee on Water Supplies of
the American Public Health Association[11] indicate that the contact
period after treatment varies considerably in American water-works
practice. Forty per cent of the replies indicated no storage after
treatment; 18 per cent less than one hour; 9 per cent from one to three
hours; 5 per cent three to twelve hours; 11 per cent twelve to
twenty-four hours, and 17 per cent a storage of more than twenty-four
hours.
=Turbidity= is usually considered to exert an effect upon the dosage
required but no definite evidence has been adduced in support of this
hypothesis. Turbidity is generally caused by the presence of very finely
divided suspended matter, usually silt or clay, which is inert to
hypochlorites. The condition that produces turbidity, however, produces
a concomitant increase in the pollution and some of the organisms are
embedded in mineral or organic material that prevents access of the
chlorine to the organisms which consequently survive treatment. A larger
concentration is required to meet these conditions but it is not
necessitated by the turbidity _per se_.
=Effect of Light.= Light exerts a marked photo-chemical effect on the
germicidal velocity of chlorine and hypochlorites. When chlorinated
water is passed through closed conduits and basins the effect of light
is of course nil but in open conduits and reservoirs this factor is
appreciable and reduces the necessary contact period. The effect of
light on laboratory experiments made with colourless glass bottles is
so marked as to make it impossible to compare the results obtained on
different days under different actinic conditions. The following figures
illustrate the effect of sunlight:
EFFECT OF SUNLIGHT
-----------------+-------------------------------------
| AVAILABLE CHLORINE 0.35 P.P.M.
Contact Period. +-------------------+-----------------
| Exposed to Bright | Stored in Dark
| Sunlight (April) | Cupboard.
-----------------+-------------------+-----------------
Nil | 215 | 215
30 minutes | 130 | 145
1 hour | 122 | 136
2-1/2 hours | 61 | 130
3-1/2 hours | 0 | 32
-----------------+-------------------+-----------------
=Determination of Dosage Required.= The dosage required for the
treatment of a water can only be accurately determined by treating
samples with various amounts of chlorine and estimating the number of
bacteria and _B. coli_ after an interval of time equal to that available
in practice. The temperature of the water during the experiment should
be the same as that of the water at the time of sampling.
In order to limit the range covered by the experiments the approximate
dosage can be ascertained from Diagram V if the amount of oxygen
absorbed by the water is known. This diagram is calculated on the amount
of available chlorine, present as chlorine or hypochlorite, that will
reduce the _B. coli_ content to the U. S. Treasury standard (2 _B. coli_
per 100 c.cms.) in two hours. If the oxygen absorbed values are
determined by the four-hour test at 27 deg. C. they should be multiplied by
two.
Another method which has been generally adopted for military work during
the war, consists in the addition of definite volumes of a standard
chlorine solution to several samples of the water and, after a definite
interval, testing for the presence of free chlorine by the
starch-iodide reaction. The details of the method of Gascard and
Laroche, which is used by the French sanitary service, have been given
by Comte.[12] One hundred c.cms. of the water to be examined are placed
in each of 5 vessels and 1, 2, 3, 4, and 5 drops of dilute Eau de
Javelle (1:100) are added and the contents stirred. After twenty
minutes, 1 c.cm. of potassium iodide-starch reagent (1 gram each of
starch, potassium, iodide and crystallized sodium carbonate to 100
c.cms.) is added and the samples again stirred. The lowest dilution
showing a definite blue colour is regarded as the dose required, and the
number of drops is identical with that required of the undiluted Eau de
Javelle for 10 litres of water when the same dropping instrument is
used. The actual concentration represented by these dilutions depends
necessarily upon the size of the drops and the strength of the undiluted
Eau de Javelle, but one drop per 100 c.cms. usually represents
approximately 1 p.p.m.
[Illustration: DIAGRAM V
RELATION OF DOSAGE TO OXYGEN ABSORBED]
In Horrocks's method, as used in the British army, a standard bleach
solution is added and is almost immediately followed by the zinc
iodide-starch reagent. The two methods were compared by Massy,[13] who
found that the French method gave an average result of only 0.06 m.gr.
per litre (0.06 p.p.m.) higher than the English method. Water in the
Gallipoli campaign required from 0.21 to 1.06 p.p.m. as determined by
both methods.
Dienert, Director of the Paris Service for investigating drinking water,
adds 3 p.p.m. of available chlorine and allows the mixture to stand
fifteen minutes after shaking; the residual chlorine is then titrated
with thiosulphate. The amount absorbed is increased by 0.5 p.p.m. and in
the opinion of Dienert this dosage is correct for a contact period of
three hours.
For military camps where a standpipe usually provides a reasonable
contact period, it has been found good practice to add sufficient
chlorine to give a rich blue colour with the starch-iodide reagent and
subsequently reduce the dosage gradually until the water, after standing
one hour, gives but a faint reaction to the test reagent. This method
should be checked up as soon as possible by bacteriological
examinations. An example of this method is given in Table XIV.
TABLE XIV.--CONTROL OF DOSAGE BY STARCH-IODIDE REACTION
--------------------+-----------------------------------+--------------
Starch-iodide | BACTERIA ON AGAR PER C.CM. |
Reaction After One +-----------------+-----------------+ B. Coli Per
Hour. | 1 Day at 37 C. | 2 Days at 20 C. | 100 c.cms.
--------------------+-----------------+-----------------+--------------
000[也[也 | 40 | 15 | 0
0000[也 | 37 | 18 | 8
00000 | 68 | 268 | 34
00000 | 115 | 553 | 61
Raw water | 114 | 685 | 89
--------------------+-----------------+-----------------+--------------
The number of [也 signs indicates the intensity of the reaction.
BIBLIOGRAPHY
[1] Nissen. Zeit. f. Hyg., 1890, =8=, 62.
[2] Delepine, J. Soc. Chem. Ind., 1911, =29=, 1350.
[3] Phelps. Water Supply Paper No. 220, U. S. Geo. Survey.
[4] Wesbrook, Whittaker, and Mohler, J. Amer. Public Health Assoc.,
1911, =1=, 123.
[5] Lederer and Bachmann. Eng. Rec., 1912, =65=, 360.
[6] Harrington. J. Amer. Waterworks Assoc., 1914, =1=, 438.
[7] Ellms. Eng. Rec., 1911, =63=, 472.
[8] Heise. Philippine Jour. Sci., 1917, =12=, A, 17-34.
[9] Norton and Hsu, Jour. Inf. Dis., 1916, =18=, 180.
[10] Langer. Zeit. f. Hyg., 1916, =81=, 296.
[11] Longley. J. Amer. Public Health Assoc., 1915, =5=, 920.
[12] Comte. J. Pharm. Chim., 1916, =14=, 261.
[13] Massy. J. Pharm. Chim., 1917, =15=, 209.