TABLE XII.—EFFECT OF CONTACT PERIOD
| Available Chlorine 0.27 Part Per Million | |||||||
| Sampling Point. | Bacteria Per c.cm. | ||||||
| Average of series of samples | 5,000 | ft. | from | pumping | station | 300 | |
| 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.[E]—TREATMENT OF MISSISSIPPI RIVER WATER
| Aug. 8, 1910 | ||||||
| Available Cl. P.p.m. | Contact Period. (Temp. 22°‑26° C.). | |||||
| 30 Mins. | 1 Hr. 30 Mins. | 3 Hrs. | 6 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 |
| [E] Resultsare 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: