Chlorination of Water

by Joseph Race

BACTERIA SURVIVING CHLORINATION

A disinfectant is usually described as a substance capable of destroying bacteria and other micro-organisms, and an antiseptic as one that restrains or retards their growth or reproduction. This distinction is entirely arbitrary as the ability of a substance to kill organisms or merely inhibit their growth depends upon the concentration employed.

Chlorine and hypochlorites, even in minute doses, exert a toxic effect that is sufficient to produce death in organisms but when still smaller concentrations are employed the toxic effect is transient and the reproductive faculty may be entirely regained.

The enumeration of bacteria by means of solid media depends upon the ability of the organism to reproduce at such a rate as to produce a visible colony within the period of incubation and any substance that prevents the growth of a visible colony is classified as a disinfectant; if on further incubation the bacterial count approximates that of the untreated sample the added substance has acted mainly as an antiseptic. In practice no substance acts entirely as an antiseptic as the organisms present have varying degrees of resistance and the less viable ones are killed by doses that are only antiseptic to the more resistant ones. An example of an antiseptic effect followed by a mild disinfectant action, caused by small doses of bleach is shown in [Table XV]. In this experiment the water designated as control was from the same source as the treated water. In order to make the bacterial count in this water approximately the same as in the treated water, the original count was reduced by diluting the sample with water from the same source, sterilised by boiling, and afterwards reaërated with sterile air.

TABLE XV.[A]—ANTISEPTIC EFFECT OF CHLORINE

Sample treated with 0.1 part per million of available chlorine.
Plated.			Incubation Period, Days.					Ratio of Bacterial Counts.
Time.		Day.	2	3	4	5	6	2 : 4 Days.		2 : 5 Days.		2 : 6 Days.
11	a.m.	1	520	940	1,350	2,360	2,780	1 :	2.6	1 :	4.5	1 :	5.3
12	noon	1	390	770	1,080	2,040	2,320		2.8		5.2		5.8
2	p.m.	1	187	260	690	1,840	2,080		3.7		9.9		16.4
4	p.m.	1	91	130	280	760	840		3.1		8.3		9.2
10	a.m.	2	42	120	670	920	...		15.9		22.		...
10	a.m.	3	320	1,210	3,500	...	...		10.9		...		...
10	a.m.	4	8,700	14,200	26,000	...	...		2.9		...		...
Control. No Chlorine Added
Plated.			Incubation Period, Days.					Ratio of Bacterial Counts.
Time.		Day.	2	3	4	5	6	2 : 4 Days.		2 : 5 Days.		2 : 6 Days.
11	a.m.	1	121	184	285	liquid	...	1 :	2.4	1 :	...		...
12	noon	1	115	171	223	380	392		1.9	1 :	3.2	1 :	3.2
2	p.m.	1	109	152	221	362	375		2.0		3.3		3.4
4	p.m.	1	121	175	251	410	415		2.1		3.4		3.4
10	a.m.	2	6,200	8,500	8,800	8,900	liquid		1.4		1.4		...
10	a.m.	3	425,000	650,000	670,000	liquid	...		1.5		...		...
Original Sample. Untreated and Undiluted
11	a.m.	1	915	1,410	1,630	2,150	3,200	1 :	2.2	1 :	2.8	1 :	3.5
[A]Results are bacteria per c.cm

[Table XVI] shows the effect of a concentration of 1.0 p.p.m. of chlorine; the hypochlorite at this concentration acted almost entirely as a germicide or disinfectant.

TABLE XVI.[B]—EFFECT OF CHLORINE AS A DISINFECTANT

Available Chlorine 1.0 p.p.m.
Plated.			Incubation Period, Days.
Time.		Day.	2	3	4	5	6
11	a.m.	1	2	5	7	8	10
12	noon	1	1	1	2	2	4
2	p.m.	1	0	0	0	2	2
4	p.m.	1	1	2	2	6	6
10	a.m.	2	0	0	0	1	..
10	a.m.	3	0	0	0	..	..
10	a.m.	4	5	13	16	..	..
10	a.m.	5	79	166	..	..	..
Untreated water		..	915	1,410	1,680	2,150	3,200
[B]Results are bacteria per c.cm.