THE ROLE OF VITAL LAYER (OR

SCHMUTZDECKE) IN SLOW SAND >

BACTERIOLOGICAL PURIFICATION

By DURGA DAS MITRA, m.sc., m.b., d.p.h.

Water Analyst, Pulta Water Works, Calcutta Corporation

The upper layer formed on the surface of the- slow sand filter does not appear to play the most important part in bacteriological purifica- tion. The sand filters at Pulta, with a capacity of about 99 million gallons daily, offer an

excellent opportunity to study such actions, as

it sometimes becomes necessary to rake a run-

ning filter when it tends to choke, which rapidly diminishes the rate of filtration. Doubts were expressed regarding the efficacy

of raking the surface of the filters, in spite of the great amount of investigation done else-

where, although it is obvious that certain types of water may form mud balls very quickly on the surface of the filter bed, and principally along the walls of the filters, resulting in shrink- ing of the sand bed from the walls. The filter rake used consisted of a steel bar

about 5 feet in length and carrying 40 or 50 tines about 5 inches long, pointed and fixed at right angles to the supporting bar. Raking consists of pulling the rake the whole length and breadth of the surface of the sand of the filters leaving shallow cuts about i inch deep. In

many cases raking appeared to prolong the life of the moribund filters, but it has been used only as an emergency measure.

The present discussion is based on 300 ran- dom samples of filtrates from raked filters examined during a period of 2-? years (from June 1940 to December 1942) during the differ- ent seasons of the year. The samples were

examined either the same day or the day following the raking of the filters.

It should be mentioned that raking does not follow scraping but is done to prolong the period between scrapings.

For examination, McConkey's media were

used. Five tubes (double strength) were inocu- lated with 10 c.crn. (^ach) and 5 tubes (single strength) with 1 c.cm. (each) of every sample. Results were noted at the end of 48 hours' incubation at 37?C.

Sept., 1943] VITAL LAYER IN SLOW SAND FILTRATION :MlTRA 441

Table I

Analysis of bacteriological results of 291 samples from raked filters

Examined on the same day

Examined on the next day

Total

Column 1

Number of samples

31

2G0

291

Column 2

+ in 1 c.cm.

Column 3 ! Column 4

-f in 10 c.cm.

4

57

61

in 10 c.cm.

25

203

228

Table I shows the results obtained from the investigations on various filters.

Routine weekly examinations were made of the filters before raking. The 2 positive filters under column 2 were positive in 1 c.cm. on

the weekly examination but became negative the next day.

Of the 61 filters under column '3, 4 were

positive in 1 c.cm. and 20 positive in 10 c.cm. in the weekly examination. An additional 9 filters were examined and

4 were positive in 1 c.cm. and 3 were positive in 10 c.cm. before raking. After raking, these 9 filters became negative

in 1 c.cm. after a 2 to 4 days' filtration run. Of the 291 samples in table I, 228 (78 per

cent) were negative in 10 c.cm.

An effort was made to secure information regarding the efficiency of the filters at different time-intervals after raking (see table II).

Immediately after raking 1 hour after raking 2 hours 3 ?

4 ?

5 ?

6

Table II

Bacteriological results

, Number of Time of taking samples j samples

10 7 8

11 4 1 1

Total ? ? i 42

in 10 c.cm.

10 4 8 9 3 1 1

36

-f in 10 c.cm.

Number of samples in

weekly examination -f- in 10 c.cm.

These results compared with the results of

Weekly analysis showed that, several hours after raking, the filters were as efficient as, or more efficient than, they were before. The raw water after sedimentation in the

settling tanks was usually positive in 1 c.cm.

?r 10 c.cm. It would therefore appear as

stated by Parker (1925) : ' It seems hard to

avoid the deduction that a fair thickness of sand coated with zooglea is in itself a very efficient filter.'

Summary

Three hundred and forty-two samples of fil- trates from raked filters were examined.

Generally speaking, raking does not cause

deterioration in the bacteriological quality of the filtrate in the great majority of cases, even though the rate of filtration is increased.

Acknowledgments

I desire to express thanks and gratitude to

\[r. Sailapati Chatterji, the present Chief Executive Officer, Calcutta Corporation, for

encouragement of scientific work, with a view to which he reorganized the Pulta Laboratory in 1939. I am specially indebted and grateful to Mr. B. R. Dyer, the Professor of Sanitary Engineering, All-India Institute of Hygiene and Public Health, for his kind constructive

criticism and elucidation of several points in the paper. Thanks are also due to Dr. M U Ahmad, the Health Officer, and Dr. S. K Ghose the Analyst, Calcutta Corporation, for en- couragement.

REFERENCE

Parkkh P. A. M. (1925), The Control of Water. George Routledge & Sons, Ltd., London.