EFFECT OF SIZE OF GRAIN UPON EFFICIENCY OF FILTRATION.
It is frequently stated that it is only the sediment layer which performs the work of filtration, and that the sand which supports it plays hardly a larger part than does the gravel which carries the sand, and under some circumstances this is undoubtedly the case. Nevertheless sand in itself, without any sediment layer, especially when not too coarse and not in too thin layers, has very great purifying powers, and, in addition, acts as a safeguard by positively preventing excessive rates of filtration on account of its frictional resistance. As an illustration take the case of a filter of sand with an effective size of 0.35 mm. and the minimum thickness of sand allowed by the German Board of Health, namely, one foot, and let us suppose that with clogging the loss of head has reached two feet to produce the desired velocity of 2.57 million gallons per acre daily. Suppose now that by some accident the sediment layer is suddenly broken or removed from a small area, the water will rush through this area, until a new sediment layer is formed, at a rate corresponding to the size, pressure, and depth of the sand, or 260 million gallons per acre daily—a hundred times the standard rate. Under these conditions the passing water will not be purified, but will pollute the entire effluent from the filter. Under corresponding conditions, with a deep filter of fine sand, say with an effective size of 0.20 mm. and 5 feet deep, the resulting rate would be only 17 million gallons per acre daily, or less than seven times the normal, and with the water passing through the full depth of fine sand, the resulting deterioration in the effluent before the sand again became so clogged as to reduce the rate to nearly the normal, would be hardly appreciable.
The results at Lawrence have shown that with very fine sands 0.09 and 0.14 mm., and 4 to 5 feet deep, with the quantity of water which can practically be made to pass through them, it is almost impossible to drive more than an insignificant fraction of the bacteria into the effluent. Even when the sands are entirely new, or have been scraped or disturbed in the most violent way, the first effluent passing, before the sediment layer could have been formed, is of good quality. Still finer materials, 0.04 to 0.06 mm., as far as could be determined, secured the absolute removal of all bacteria, but the rates of filtration which were possible were so low as to preclude their practical application.
With coarser sands, as long as the filter is kept at a steady rate of filtration, without interruptions of any kind, entirely satisfactory results are often obtained, although never quite so good as with the finer sands. Thus at Lawrence the percentages of bacteria (B. prodigiosus) appearing in the effluents under comparable conditions were as follows:
| 1892 | 1893 | |
|---|---|---|
| With effective grain size 0.38 mm | 0.16 | .... |
| With effective grain size 0.29 mm | 0.16 | .... |
| With effective grain size 0.26 mm | 0.10 | .... |
| With effective grain size 0.20 mm | 0.13 | 0.01 |
| With effective grain size 0.14 mm | 0.04 | 0.03 |
| With effective grain size 0.09 mm | 0.02 | 0.02 |
We may thus conclude that fine sands give normally somewhat better effluents than coarser ones, and that they are much more likely to give at least a tolerably good purification under unusual or improper conditions.
EFFECT OF GRAIN SIZE UPON FREQUENCY OF SCRAPING.
The practical objection to the use of fine sand is that it becomes rapidly clogged, so that filters require to be scraped at shorter intervals, and the sand washing is much more difficult and expensive. The quantities of water filtered between successive scrapings at Lawrence in millions of gallons per acre under comparable conditions have been as follows:
| 1892 | 1893 | |
|---|---|---|
| Effective size of sand grain 0.38 mm | .... | 79 |
| Effective size of sand grain 0.29 mm | .... | 70 |
| Effective size of sand grain 0.26 mm | .... | 57 |
| Effective size of sand grain 0.20 mm | 58 | .... |
| Effective size of sand grain 0.14 mm | 45 | 49 |
| Effective size of sand grain 0.09 mm | 24 | 14 |
The increase in the quantities passed between scrapings with increasing grain size is very marked.