SANDS USED IN EUROPEAN FILTERS.

To secure definite information in regard to the qualities of the sands actually used in filtration, a large number of European works were visited in 1894, and samples of sand were collected for analysis. These samples were examined at the Lawrence Experiment Station by Mr. H. W. Clark, the author’s method of analysis described in Appendix III being used. In the following table, for the sake of compactness, only the leading points of the analyses, namely, effective size, uniformity coefficient, and albuminoid ammonia, are given. On page 28 full analyses of some samples from a few of the leading works are given.

ANALYSES OF SANDS USED IN WATER FILTRATION.
Source.	Effective Size; 10% Finer than (Milli- meters).	Uni- formity Coeffi-cient.	Albu- minoid Ammo- ia. Parts in 100,000.	Remarks.
London, E. London Co.	0.44	1.8	0.45	New sand, never used or washed.
London, E. London Co.	0.39	2.1	26.20	Dirty sand, very old.
London, E. London Co.	0.37	2.0	8.60	Same, washed by hand.
London, Grand Junc.	0.26	1.9	1.90	Sand from rough filter.
London, Grand Junc.	0.40	3.5	10.00	Old sand in final filter.
London, Grand Junc.	0.41	3.7	2.70	Freshly washed old sand.
London, Southw’k & V.	0.38	3.5	5.00	Freshly washed old sand.
London, Southw’k & V.	0.30	1.8	2.80	Freshly washed new sand.
London, Lambeth	0.36	2.3	2.60	Freshly washed old sand.
London, Lambeth	0.36	2.4	0.35	New unused sand, washed.
London, Lambeth	0.25	1.7	0.70	New extremely fine sand.
London, Chelsea	0.36	2.4	2.10	Freshly washed old sand.
Middlesborough	0.42	1.6	17.60	Dirty sand, ordinary scraping.
Middlesborough	0.43	1.6	7.30	Same, after washing.
Birmingham	0.29	1.9	33.20	Dirty sand.
Birmingham	0.29	1.9	7.20	Sand below surface of filter.
Reading	0.30	2.5	4.00	Dirty sand.
Reading	0.22	2.0	1.50	Same, after washing.
Antwerp	0.38	1.6	7.80	Dirty sand.
Antwerp	0.39	1.6	3.40	Same, after washing.
Hamburg	0.28	2.5	8.50	Dirty sand.
Hamburg	0.31	2.3	0.80	Same, after washing.
Hamburg	0.34	2.2	7.90	Dirty sand, another sample.
Hamburg	0.30	2.0	0.90	Same, after washing drums.
Hamburg	0.34	2.3	1.50	Same, after washing ejectors.
Altona	0.32	2.0	9.00	Dirty sand, old filters.
Altona	0.37	2.0	1.50	Same, after washing.
Altona	0.33	2.8	0.50	Washed sand for new filters.
Berlin, Stralau	0.33	1.9	12.20	Dirty sand-pile.
Berlin, Stralau	0.35	1.7	4.50	Filter No. 6, 3″ below surface.
Berlin, Stralau	0.34	1.7	6.30	Filter No. 7 3″ below surface.
Berlin, Stralau	0.35	1.7	4.00	Filter No. 10 3″ below surface.
Berlin, Tegel	0.38	1.6	11.00	Dirty sand, old filters.
Berlin, Tegel	0.38	1.5	2.80	Same, after washing, old filters.
Berlin, Tegel	0.35	1.6	3.20	Same, after washing, new filters.
Berlin, Müggel	0.35	1.8	0.80	Sand from filters below surface.
Berlin, Müggel	0.33	2.0	6.30	Dirty sand, ordinary scraping.
Berlin, Müggel	0.34	2.0	15.30	Dirty sand, another sample.
Charlottenburg	0.40	2.3	7.20	Dirty sand.
Chemnitz	0.35	2.6	0.20	New sand not yet used.
Magdeburg	0.39	2.0	9.50	Dirty sand.
Magdeburg	0.40	2.0	2.80	Same, after washing.
Breslau	0.39	1.8	1.40	Normal new sand.
Budapest	0.20	2.0	0.80	New washed Danube sand.
Zürich	0.28	3.2	6.20	Dirty sand.
Zürich	0.30	3.1	1.50	Same, after washing.
Hague	0.19	1.6	0.70	Dune-sand used for filtration.
Schiedam	0.18	1.6	5.60	Dune-sand used for filtration; dirty.
Schiedam	0.31	1.5	13.50	River-sand; dirty.
Amsterdam	0.17	1.6	2.40	Dune-sand.
Rotterdam	0.34	1.5	2.30	River-sand; new.
Liverpool, Rivington	0.43	2.0	0.76	Sand from bottom of filter.
Liverpool, Rivington	0.32	2.5	1.00	New sand unwashed and unscreened.
Liverpool, Rivington	0.43	2.7	4.10	Washed sand which has been in use 30 to 40 years.
Liverpool, Oswestry	0.30	2.6	9.40	Dirty sand.
Liverpool, Oswestry	0.31	4.7	2.20	Same, after washing.

Note.—It is obvious that in case the sands used at any place are not always of the same character, as is shown to be the case by different samples from some of the works, the examination of such a limited number of samples as the above from each place is entirely inadequate to establish accurately the sizes of sand used at that particular place, or to allow close comparisons between the different works, and for this reason no such comparisons will be made. The object of these investigations was to determine the sizes of the sands commonly used in Europe, and, considering the number and character of the different works represented, it is believed that the results are ample for this purpose.

The English and most of the German sands are washed, even when entirely new, before being used, to remove fine particles. At Breslau, however, sand dredged from the river Oder is used in its natural state, and new sand is used for replacing that removed by scraping. At Budapest, Danube sand is used in the same way, but with a very crude washing, and it is said that only new unwashed sand is used at Warsaw.

In Holland, so far as I learned, no sand is washed, but new sand is always used for refilling. At most of the works visited dune-sand with an effective size of only 0.17 to 0.19 mm. is used, and this is the finest sand which I have ever found used for water filtration on a large scale. It should be said, however, that the waters filtered through these fine sands are fairly clear before filtration, and are not comparable to the turbid river-waters often filtered elsewhere, and their tendency to choke the filters is consequently much less. At Rotterdam and Schiedam, where the raw water is drawn from the Maas, as the principal stream of the Rhine is called in Holland, river-sand of much larger grain size is employed. It is obtained by dredging in the river and is never washed, new sand always being employed for refilling.

The average results of the complete analyses of sands from ten leading works are shown in the table on page 28. These figures are the average of all the analyses for the respective places, except that one sample from the Lambeth Co., which was not a representative one, was omitted.

The London companies were selected for this comparison both on account of their long and favorable records in filtering the polluted waters of the Thames and Lea, and because they are subject to close inspection; and there is ample evidence that the filtration obtained is good—evidence which is often lacking in the smaller and less closely watched works. For the German works Altona was selected because of its escape from cholera in 1892, due to the efficient action of its filters, and Stralau because of its long and favorable record when filtering the much-polluted Spree water. These two works also have perhaps contributed more to the modern theories of filtration than all the other works in existence. The remaining works are included because they are comparatively new, and have been constructed with the greatest care and attention to details throughout, and the results obtained are most carefully recorded.

Some of the most interesting of these results are shown graphically on page 29. The method of plotting is that described in Appendix III.

TABLE SHOWING THE AVERAGE PER CENT OF THE GRAINS FINER THAN VARIOUS SIZES IN SANDS FROM LEADING WORKS.
	Per Cent by Weight Finer than
	0.106 mm.	0.186 mm.	0.316 mm.	0.46 mm.	0.93 mm.	2.04 mm.	3.89 mm.	5.89 mm.
East London	0.2	0.5	3.6	22.2	69.7	89.8	95.0	99.0
Grand Junction	0	0.2	3.1	17.4	47.1	68.2	84.7	93.6
Southwark and Vauxhall		0.7	8.0	34.1	69.7	83.5	90.0	94.0
Lambeth	0	0.5	5.5	26.6	63.0	79.2	88.0	94.3
Chelsea	0	0.1	5.0	28.6	63.0	76.7	86.0	93.6
Hamburg	0.2	1.5	10.9	33.2	74.4	95.7	99.5
Altona	0.1	1.1	7.8	28.7	72.1	92.1	95.8
Stralau		0.3	7.0	37.3	86.9	95.4	97.6
Tegel		0.2	4.5	35.4	94.3	98.5	99.1
Müggel	0.1	0.5	7.9	33.6	79.7	94.3	98.5
Average of all	0.06	0.56	6.33	29.71	71.99	87.34	93.42	(97.45)

AVERAGE EFFECTIVE SIZE, UNIFORMITY COEFFICIENT,AND ALBUMINOID AMMONIA IN SANDS FROM TEN LEADING WORKS.
I. LONDON FILTERS.
	Effective Size; 10% Finer than (Millimeters).	Uniformity Coefficient.	Albuminoid Ammonia.
	Effective Size; 10% Finer than (Millimeters).	Uniformity Coefficient.	Dirty Sand.	Washed Sand.
East London	0.40	2.0	26.00	8.60
Grand Junction	0.40	3.6	10.00	2.70
Southwark and Vauxhall	0.34	2.5	.....	3.90
Lambeth	0.36	2.4	.....	2.60
Chelsea	0.36	2.4	.....	2.10
Average	0.37	2.6	18.00	3.98
II. GERMAN WORKS.
Stralau	0.34	1.7	12.20	4.00
Tegel	0.37	1.6	11.00	3.00
Müggel	0.34	2.0	10.80	0.80
Altona	0.34	2.3	9.00	1.50
Hamburg	0.31	2.3	8.20	1.07
Average	0.34	2.0	10.25	2.07

Placing Sand in a Filter, Hamburg.

[To face page 28.

The averages show the effective size of the English sands to be slightly greater than that of the German sands—0.37 instead of 0.34 mm.—but the difference is very small. The entire range for the ten works is only from 0.31 to 0.40 mm., and these may be taken as the ordinary limits of effective size of the sands employed in the best European works. The average for the other sixteen works given above, including dune-sands, is 0.31 mm., or, omitting the dune-sands, 0.34 mm.

Fig. 3a.—Sand Analysis Sheet, with Analyses of Several European Filter Sands.

It is important that filter sands should be free from lime. When water is filtered through such sands, no increase in hardness results. When, however, water is filtered through sand containing lime, some of it is usually dissolved and the water is made harder. The amount of lime taken up in this way depends both upon the character of the sand, and upon the solvent power of the water; and it does not necessarily follow that a sand containing lime cannot be used for filtration, but a sand nearly free from lime is to be preferred.

The presence of lime in sand can usually be detected by moistening it with hydrochloric acid. The evolution of gas shows the presence of lime. Some idea of the amount of lime can be obtained from the amount of gas given off, and the appearance of the sample after the treatment, but chemical analysis is necessary to determine correctly the amount.

Experiments with filters at Pittsburg were made with sand containing 1.3 per cent of lime, the result being that the hardness of the water was increased about one part in 100,000; but the amount of lime in the sand was so small that it would be washed out after a time, and then the hardening effect would cease. Larger amounts of lime would continue their action for a number of years and would be more objectionable.

Turning to the circumstances which influence the selection of the sand size, we find that both the quality of the effluent obtained by filtration and the cost of filtration depend upon the size of the sand-grains.

With a fine sand the sediment layer forms more quickly and the removal of bacteria is more complete, but, on the other hand, the filter clogs quicker and the dirty sand is more difficult to wash, so that the expense is increased.