4
32 per cent. This contrast is even more striking when we note that the rate of
filtration in the covered filter was usually higher. In no case was the rate of
filtration in the uncovered ever increased beyond four vertical inches per hour
which is the usual standard rate employed and on many occasions it was only 2.6
vertical inches per hour. On the other hand, rates up to 6.6 vertical inches per
hour were tried with the covered filter and a rate of 77 per cent of good results
was obtained. In comparison III the rate was pushed to 5.2 vertical inches per
hour and 90 per cent of good results got. From this it may be argued that
faster rate of filtration can safely be employed, if other conditions are satisfactory.
Finally in comparison VI the filtering head in the covered filter was raised to
36 inches, 24 inches being the standard maximum. This resulted, however, in a
drop in efficiency, only 38 per cent of good results being obtained ; so this practice
is not to be recommended. It was during comparisons IV and V that the above
mentioned trouble was experienced with both filters, the percentage of good results
with the covered filter falling to 43 per cent and 40 per cent and that of the
uncovered filter to nil.
The Madras Corporation filters.—Although a large number of samples were
examined, no satisfactory conclusions were arrived at. The results were on the
whole much inferior to those obtained with the experimental filters, and no
improvement was noticed as between the covered and uncovered portions of a
filter. Owing to pressure of other work in connection with the Kilpauk installa-
tion, the experiments were abandoned from the 1st of October but it is hoped to
recommence them.
TABLE II.
Nature of examination.
Number of samples.
1912.
1913.
1914.
1915.
1916.
Chemical analyses . . .
416
498
277
498
321
Bacteriological examinations . . .
620
634
617
1,103
1,360
Table II shows the number of samples of water that have been examined
chemically and bacteriologically in each of the last five years.
Of the 1,360 samples examined bacteriologically, 685 were from the Experi
- m e n t a l f i l t e r s a t G u i n d y , 6 2 5 f r o m t h e M a d r a s C o r p o r a t i o n f i l t e r b e d s a t K i l p a u k
and 50 from mufassal water-works and wells.
Experiments were also carried out to ascertain the cause of a smell of
sulphuretted hydrogen occurring in the filtered water from the Red Hills supply.
Its appearance in the water seems to be associated with insufficient aeration and
possibly light. Thus it was not found in the lake water or on the open filters,
but was found in situations where the water had been moving very slowly and
without access to air. Madras water has a very large quantity of organic matter
and it seems likely that in such situations all the oxygen dissolved in the water is
absorbed thus giving rise to conditions suitable to the putrefaction of the organic
matter.
The white filamentous growth sometimes found in the filtered water seems
to be derived from the filters themselves. It is not found in the Red Hills Lake
or in the conduit or on the filters. It makes its first appearance in the filtering
chambers and is often associated with, but does not cause the smell of sulphu
-retted hydrogen.
Specimens sent for examinations.— There was a further drop of 4.2 per cent in
the number of specimens sent to the laboratory for examination. Table III gives
a classification of these and the nature of the examinations required.