----------------------+--------+---------+-----------+-------------- |Average |Effective| Maximum | Per cent |Depth of| Size of | Rate of | Bacteria | Sand, | Sand- |Filtration.| passing | Inches.| grain. | | 1 r^2d | | | |= - ---------- | | | | 2 [sqrt]sand ----------------------+--------+---------+-----------+-------------- Hamburg | 32 | 0.31 | 1.60 | 0.07 Altona | 28 | 0.34 | 2.57 | 0.21 Berlin, Stralau | 20 | 0.34 | 2.57 | 0.25 Berlin, Müggel | 20 | 0.34 | 2.57 | 0.25 Berlin, Tegel | 20 | 0.37 | 2.57 | 0.27 London, Southwark & | | | | Vauxhall | 36 | 0.34 | 2.81 | 0.22 London, West Middlesex| 39 | 0.37 | 2.81 | 0.23 London, Chelsea | 54 | 0.36 | 3.27 | 0.26 London, Grand Junction| 30 | 0.40 | 3.27 | 0.39 London, Lambeth | 36 | 0.36 | 3.75 | 0.42 Middlesborough | 20 | 0.42 | 5.85 | 1.58 Zürich | 26 | 0.35 | 7.50 | 1.90 ----------------------+--------+---------+-----------+-------------- The numbers actually observed are in every case higher than the calculated per cents passing, as indeed they should be on account of those coming from the underdrains, accidental contamination of the samples, etc. It may be said that filtration now practised in European works under ordinary conditions never allows over 1 or 2 per cent bacteria of the raw water to pass, and ordinarily not over one fourth to one half of one per cent, although exact data cannot be obtained owing to masking effect of the bacteria which come from below and which bear no relation to those of the raw water. By increasing the size of the filters, fineness and depth of sand (as at Hamburg), the efficiency can be materially increased above these figures. At the same time it must be borne in mind that the effectiveness of a filter may be greatly impaired by inadequate underdraining, by fluctuating rates of filtration where these are allowed, by freezing in winter in the case of open filters in cold climates, and by other irregularities, all of which can be prevented by careful attention to the respective points.