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REPORT ON THE WORKING OF THE KING INSTITUTE, GUINDY

APPENDIX No. II-A.

     A note on the experiments on filtering with alum in slow sand filters carried out at the
Guindy Experimental Filters between November 1923 and March 1925.

     1. The following methods of forming the film were carried out:—

          (i) By letting in the solution of alum along with the raw water while filling the bed and

          (ii) by first filling the bed to maximum depth and then sprinkling the solution over the
surface of water followed by surface agitation.

     The first method was found to be efficient.

     2. Refilming at intermediate stages of each period of the filter-run resulted in rapid clogging
of the bed.

     3. Inferences to be drawn from the experiments on filming:—

          (i) Initial filming produced a filtrate of superior quality immediately on starting the
filter.

          (ii) This treatment did not clog the bed in any way quicker than the control; on the other
hand filmed filter ran consistently longer than the control by at least 30 per cent of the number
of days in two cases and 15 per cent in one case.

          (iii) Initial filming was found to arrest silt and other impurities on the top of the sand bed
effectively, whereas in the control bed the penetration of silt was deep into the sand bed.

          (iv) The continuous use of a film on a slow sand filter followed by careful working, is
bound eventually to result in economy of working slow sand filters, as there will be fewer chances
of overhauling such a bed frequently than the one worked without the initial film.