History of the Water Supply of the World / arranged in a comprehensive form from eminent authorities, containing a description of the various methods of water supply, pollution and purification of waters, and sanitary effects, with analyses of potable waters, also geology and water strata of Hamilton county, Ohio, statistics of the Ohio river, proposed water supply of Cincinnati.

by Thomas J. Bell

FILTRATION

is the artificial method of clarification by mechanical and chemical means. By the mechanical system heavier impurities are held in suspense by percolation of water through carefully prepared beds of sand, gravel, coke, shells, and like substances.

The total area of the London filter beds in 1874 was 68 acres, and rate of filtration per hour in inches and depth of water, or head on beds, were:

	RATE.		DEPTH.
Lambeth Works	10	inches.	7	feet.
Southwark & Vauxhall	4	“	4	“
Grand Junction	3	“	4	“
West Middlesex	4	“	3	“
Chelsea	6	“	5	“
New River	4½	“	5	“
East London	3	“	5	“

The efficiency of filtration is inversely to rate of flow. Humber says:

“It is now generally admitted that filtration through sand, to be effective, should not proceed at a higher rate than 6 inches of descent per hour; or, in other words, there should be at least 1½ square yards of filtering area for each 1,000 gallons per day. This is, of course, exclusive of reserve area, which will be necessary to permit of at least one bed being cleansed while sufficient area remains in operation in the other beds.”

The maintenance of these beds enhance the cost of supplying water, because they must be cleansed frequently—in some cases once a week. The regulation and control of the water consumption is an important consideration, that the rate of increase will be proportioned to the growth of the city; and not, as in this country, an unaccountable rapid increase due to the profligate use of water that makes filtration impossible. Over eleven-twelfths of the water supplied to London is filtered with the following efficiency:

	BEFORE FILTRATION.		AFTER FILTRATION.
	ORGANIC	ORGANIC	ORGANIC	ORGANIC
	CARBON.	NITROGEN.	CARBON.	NITROGEN.
	In parts of	In parts of	In parts of	In parts of
	100,000.	100,000.	100,000.	100,000.
West Middlesex Works	.209	.071	.198	.043
Grand Junction Works	.262	.042	.231	.032
Southwark & Vauxhall,
Hampton Works	.321	.063	.273	.042
Battersea “	.239	.047	.226	.035
Lambeth Works	.273	.067	.258	.038
Chelsea Works	.325	.076	.258	.032
New River, Lea River	.287	.067	—	—
“ New River	.375	.059	.227	.043
“ “	.350	.084	.246	.042
East London Co.,
Lea Water	.363	.082	—	—
Waltham St. Res	.481	.092	.305	.041
Thames Water	—	—	.159	.030

Dimensions of filter beds for given volumes (from Fanning):

For	1	million gallons per diem	3 beds	60 feet	×	100 feet.
For	2	“ “ “	3 “	80 “	×	150 “
For	3	“ “ “	3 “	100 “	×	180 “
For	4½	“ “ “	4 “	100 “	×	180 “
For	6	“ “ “	4 “	100 “	×	240 “
For	8	“ “ “	4 “	120 “	×	270 “
For	10	“ “ “	5 “	120 “	×	270 “

Analysis of sand from filter beds (in 100,000 parts)

	ORGANIC	ORGANIC	ORGANIC
	MATTER.	CARBON.	NITROGEN.
As removed from filter bed, unwashed	1523.40	314.160	38.674
After washing	804.41	94.921	16.973

It can not be doubted that a small amount of organic matter undergoes oxidation and destruction during the passage of the water through the sand; but, independent of this, it appears, from the above analytical numbers, that one ton of dry sand, washed after previous use, is capable of removing from water and retaining 16.1 lbs. of peaty matter.

Chemical filtration may be arranged under the following heads:

1. By use of alum and borax to reduce turbidity.

2. Dr. Gunning’s experiment of the waters of the River Maas, by reducing the turbidity with .032 gramme of perchloride of iron into one litre of water.

3. Dr. Bischoff’s (Jr.) process of removing organic matter by spongy iron, prepared by heating hydrated oxide of iron with carbon.

4. Spencer’s process of sand filtration with crushed grains of a carbide of iron. The carbide, it is claimed, does not require frequent removal.

5. Sheet-iron strips placed in water decomposes organic matter rapidly. It is recommended by eminent authority.

6. Charcoal is, possibly, the best substance for removing organisms chemically; but its efficiency is destroyed by an insoluble precipitate of either lime or iron. Messrs. Adkins & Co., of London, have patented a method to overcome this objection, by use of charcoal plates that may be easily scraped.

Filtration through spongy iron (by Rivers Pollution Commission—Parts in 100,000 parts):

	ORGANIC	ORGANIC	PREVIOUS
	CARBON.	NITROGEN.	SEWAGE.
Thames water, before	.120	.013	1340
Thames water, after	.025	.004	10

Filtration through animal charcoal:

	ORGANIC	ORGANIC	PREVIOUS
	CARBON.	NITROGEN.	SEWAGE.
Grand Junction Co.’s water, before	.164	.030	320
Grand Junction Co.’s water, after	.010	.002	950