10

REPORT OF THE KING INSTITUTE, GU1NDY

      The Disinfectants examined have yielded coefficients varying from 2—11
In those cases where the manufacturers or agents have claimed a definite figure
we have almost invariably obtained a much lower value for disinfectants derived
from coal tar, There is no doubt that the claims made by the best manufacturers
are bona fide and based on their own examinations and as, further, other
laboratories in India find that they are unable to get the results obtained in
England, it would appear that there is some factor, at present undetermined which
affects the stability of coal tar disinfectants in this country. The matter is under
investigation.

(D) WORK FOR THE WATER AND SEWAGE PURIFICATION COMMITTEE.

      I.  Water.—The experiments described in page 7 of the report for 1925-26
on the value of a percolating non-submerged filter as compared with an ordinary
submerged slow sand filter was continued throughout the year at the Experimen–
tal filters at the Institute. These are controlled for the Committee by Mr.
Westerdale, B.Sc., Sanitary Engineer to Government, with whom the idea of this
experiment originated.

         Period IV —May to October 1926.—The percolating filter was supplied with
water from a revolving sprinkler above it. The filtering rates (4 inches vertical
per hour) for the percolating and the control submerged filter were the same.
The percolating filter gave much better results both bacteriologically and chemi–
cally than the submerged filter. Thus in the twenty-six analyses made during
this period the ratio of the number of .occasions on which the percolating filter
was the better to the number on which the control was the better is as follows:—

         Period V—November to December 1926.—The percolating filter working at 6
inches vertical flow per hour with the control at the ordinary 4 inches rate.
Despite the quickness of filtration, the percolating filter still gave decidedly better
results bacteriologically and with regard to the albuminoid ammonia present, than
did the control The figures for oxygen absorbed, however, were now almost the
same as for the control. With this quicker rate it was noticed that there was a
tendency to pond. With the ordinary equal rates of filtration, the percolating
filter was better than the control in clogging less frequently.

         Period VI—January to March 1927.—Percolating filter re-made with 3 feet
of coarse sand. Control submerged filter relaid with fine sand. Filtering rates
4 inches in both filters at first, later increased to 6 inches for the percolating
filter and still later to 8 inches. Throughout the whole period even during the
double rate of filtration the percolating filter gave much better result bacteriolo–
gically than did the control. At first the percolating filter did not give as
good results chemically as the control, probably because its coarse sand took
longer to " ripen," but in the latter half of the period despite the quicker rate of
filtration the water passed through the percolating filter was decidedly superior
to that from the control. This became more noticeable as the organic content of
the raw water increased; finally the control began to show the presence of
sulphuretted hydrogen, which never appeared in the percolating filter. This is
interesting in point of view of the trouble that this gas causes in the slow
filtration of Madras City water.

         The experiment is not yet at an end and will continue with a coarse sand
submerged filter as the control.

         II.     Sewage.—At the beginning of the year the Engineering College, Guindy,
completed its sewage disposal plant, a closed septic tank, an open septic tank, and
an Imhoff tank, all three discharging on to an aerating contact bed. During this
year of the experiment the plant was under the control of the Sanitary Engineer.
Rooms near the plant were converted into a laboratory supplied from this