2
increase, in the amount of alum could bring about the desired result. This was
found to be a much more promising line. With 3 grains of alum per gallon and
at 50 gallons per square foot per hour there was a notable improvement in the
quality of the yield from all three filters, the reduction in the total count being
specially marked. The improvement was even more marked when 4 grains of
alum per gallon were added, the rate of 50 gallons per hour being maintained.
With five grains of alum per gallon the good quality of the yield was maintained
when the flow was gradually raised from 50 to 100 gallons per square foot per
hour and with 5 grains of alum finally the full rate of 100 gallons per square foot
per hour was obtained ab initio with results quite as good as when the flow had
been gradually raised from 50 to 100 gallons per square foot per hour.
In the case of Adyar river water therefore it would seem that the amount of
alum added is the controlling factor and that this amount has to be a high one.
These results would need to be compared with those got elsewhere and notably in
localities such as Bangalore where there is a large amount of suspended matter in
the water. There, however, the raw water is comparatively good from the bacte
-rial point of view and an elimination of the suspended matter is mainly what is
aimed at and attained. Whether sufficient bacterial improvement can be got in a
badly contaminated water with a reasonable addition of alum has got to be
proved.
The sand filters .—A good deal of trouble was experienced with both the sand
filters. During the spring and summer months the results got were very unreli
- a b l e . T h i s w a s h e l d t o b e d u e t o t h e f a c t t h a t t h e t i m e h a d a r r i v e d w h e n t h e
whole of the sand had to be removed and renewed or washed. There are several
theories as to the reason for this necessity but none of them seem to have any
satisfactory basis. One theory is that there is something in the sand which gets
exhausted in the course of time. An attempt to prove this was made by removing
the sand from the filter, putting one foot of fresh sand at the bottom and replacing
the old sand on the top. Had the exhaustion theory any foundation the layer of
new sand at the bottom would have supplied the deficiency, but no improvement
followed. No reason indeed was found to reject the belief that the function of .the
sand is a purely mechanical one, namely, to support the filtering skin of organic
matter and microscopic growth deposited from the water itself. It was suggested
that some physical alteration in the grains of sand, rendering them less able to
support the filtering skin, had taken place. This suggestion was negatived by the
result of an experiment in which one foot of new sand on the top of the old sand
was used. Again no essential improvement followed. Finally all the sand was
removed, washed and replaced with sufficient new sand to make up for the amount
lost in the process. The reason for this loss of efficiency in sand filters still
remains obscure, only the fact is corroborated that, for some reason or other, once
in three years or so, a sand filter requires a much more thorough cleaning process
than that given in routine practice.
The breakdown in the sand filters came after a run of really surprisingly
good results. These results were obtained by carrying out the examination of the
raw and filtered water to a limit extent. Normally the minimum volume of raw
water tested for lactose fermenters was 1/100 of a c.c. and the maximum volume
of filtered water tested for their absence was 360 c.c. These limits were purely
arbitrary and only fixed upon from practical considerations, such as time, consump
- t i o n o f m e d i a , e t c . I n a s e r i e s o f e x p e r i m e n t s , i t w a s d e c i d e d t o t e s t t h e r a w w a t e r
down to 1/1000 c.c. or one milligramme of water and to carry the tests for absence
of lactose fermenters up to so large a volume of water as one litre. Even these wide
limits were not found sufficient to completely estimate the purifying power of a
properly worked sand filter. For days together the covered sand filter showed
no lactose fermenters in a litre during which time the raw water often showed
them in 1/1000 c.c. This corresponds to over a million-fold reduction in the
lactose fermenters, far beyond anything, I think, that has yet been recorded from
sand filtration. A large number of comparisons were carried out between the
respective efficiencies of the covered and the uncovered filters.