Chlorination of Water Part 4

[4] Griffen and Hedallen. Jour. Soc. Chem. Ind., 1915, 34, 530.

[5] Norton and Hsu. Jour. Inf. Dis., 1916, 18, 180.

[6] Rideal, S. Jour. Roy. San. Inst., 1910, 31, 33.

[7] Dakin, Cohen, Duafresne, and Kenyon. Proc. Roy. Soc., 1916, 89B, 232.

[8] Noyes and Lyon. Jour. Amer. Chem. Soc., 1901, 23, 460.

[9] Bray and Dowell. Jour. Amer. Chem. Soc., 1917, 39, 905.

[10] Jakowkin. Zeit. f. Phys. Chim., 1899, 19, 613.

[11] Cross and Bevan. Jour. Soc. Chem. Ind., 1898, 28, 260.

[12] Breteau. Jour. Pharm. Chim., 1915, 12, 248.

CHAPTER III

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 quant.i.ties as one litre of water with a contact period often as short as 10 minutes. Nissen,[1] of the Hygienic Inst.i.tute 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 a.s.sumed 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 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 cla.s.ses (_a_) inorganic and (_b_) organic. The inorganic const.i.tuents naturally found in water, that are readily oxidisable, are ferrous salts (usually carbonates), nitrites, and sulphuretted hydrogen, and these react quant.i.tatively 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 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 a.s.sociated with the characteristic colour of the water derived from these sources. The effect of this coloured organic matter upon the chlorine dosage is well ill.u.s.trated 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 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.

[Ill.u.s.tration: 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 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.