-----------------+--------------------------------+------------------- | 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°-26° 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.