52

REPORT ON TEE WORKING OF THE KING INSTITUTE, GUINDY

     When the filters were not being used for the experiments on rapid filtration, certain other
experiments which were awaiting solution were carried out.

     The Sanitary Engineer had suggested that a few experiments on the question of "inter-
mittent " and " continuous " filtration might be carried out as he found that at certain
places intermittent filtration with slow sand filters was being practised. Both the sand filters
were utilized for this experiment. It was soon shown that the practice of running a slow
sand filter intermittently was most unsatisfactory. Continuous filtration yielded uniformly
good results while intermittent filtration resulted in frequent breaks in the film through which
raw water passed into the filtered water chamber. Intermittent filtration is therefore a
wasteful and unsafe method of purifying water.

     At the same time the mechanical filters were used for preliminary tests on the correct
technique for matting sand beds with alum. These experiments were to be carried out on
lines similar to those which had already been carried out in America with good results.

     Both the Jewel and Paterson filters were used and it was found that the method was, as
far as the experiments went, promising. It was decided by the Committee that this method
should be given a thorough trial under varying conditions of the raw water. These experiments
will therefore be reported upon, when they are completed.

     The last two experiments of the series consisted of a repetition of the 12-inch rate
under fine weather conditions. These were commenced in April 1923 and completed by the
end of May. Only one of the filters, viz., the filter with tiles, was now available for experi-
mentation as the other had been altered in design for a different experiment of an urgent
nature, which was then in progress. The filter ran for 18 days on the first occasion and 17
days on the second. The raw water was hazy and contained the maximum amount of solids
and organic matter. Tho bacteriological results were also very bad. The filtrate was perfectly
clear and the reduction in the organic matter and bacteria was very satisfactory. The organic
matter was reduced by about 50 per cent; bacteria by 94 per cent; and lactose fermenters
reduced 2,000-fold on an average.

     The following conclusions from these experiments appear to be justifiable:—
        (1) Rapid filtration through a shallow thickness of sand clarified Adyar River water,
even when at its greatest turbidity, to a satisfactory extent.
        (2) The degree of clarification reached, is much the same in both types of filter experi-
mented with, and there is practically no advantage gained by the use of tiles in one of them.
        (3) The filters yield satisfactory results with regard to clarification after the third day.
        (4) The tiles present in the one filter served to fix the limit for scraping off the sand
during the cleanings and thus constituted a safety factor for practical work.
        (5) Breaks in the film occurred during rapid filtration and could be ascribed to the same
causes (viz., insects, air-bubbles and algal growths) which had been recorded previously in
connection with the slow filtration experiments.
        (6) With rates of filtration, more rapid than 4 inches vertical, the filters worked for
comparatively shorter periods.
        (7) The yield of water in gallons during the respective periods the filters worked at
different rates, shows that 12 inches would be about the limit to attain to in practice, as the
tabular statement given below will show:—

        (8) Working at a slow rate, viz., 4 inches vertical, the filter head curve showed a steady
rise to about 15 inches beyond which, there was a sharp rise followed by rapid clogging of
the filter.
        (9) Working at a rapid rate, the filter head curve rises immediately and keeps rising
steadily, until the maximum head is reached (this point is usually reached in a much shorter
time than is the case with slow filtration).
        (10) The bacterial purification attained by rapid filtration, although not so good as that
obtained in slow sand filtration, is still satisfactory and compares not unfavourably with the
results of slow sand filtration obtained in muffassal filters.