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 “ |