Engineering Wonders of the World
Volume III

THE DRAINAGE SYSTEM OF LONDON. 215 DIVERSION CHAMBERS ON NORTHERN OUTFALL SEWERS NEAR ABBEY MILLS. The Chemical Treatment of Sewage introduced. mined to construct chemical precipitation works at both Barking and Crossness. Those at the northern outfall were begun in 1887 and completed about the end of 1890, by the London County Council, which succeeded the Metropolitan Board of Works in 1889 ; while the Crossness works were commenced in 1888, and were ready for operation by 1891. The treatment of sewage in such a way as to render it practically innocuous is carried out on so colossal a scale at Barking and Cross- ness that no apology is needed for describing a process which, though unsavoury, is by no means devoid of interest. The diagram of the Barking Outfall Works, reproduced by the kind permission of the London County Council, will assist the reader to follow the course of operations. On its way from Abbey Mills the sewage passes by a liming station, where there is an elaborate installation of, machinery for churn- ing lime and water together to form a milk-coloured liquid, Outfall Works. . . . L containing about 110 grains of lime to the gallon of water. This liquid is run into the crude sewage in such proportions that there shall be about four grains of lime to the gallon of sewage, which means the con- sumption of 14,800 tons of lime yearly at the Barking works. A further addition is made to the sewage of a solution of sulphate of iron, in the propor- tion of one grain of sulphate to the gallon of sewage—3,300 tons of the chemical being used in a year. The lime and iron together pre- cipitate the solid matter. At the Barking outfall are thirteen precipi- tation channels, varying in length from 1,200 to 860 feet, and 30 feet wide. Their united capacity is 20,000,000 gallons. The channels are separated Palpitation Channels. from one another by walls, and are roofed over. At the sewer end of each are two penstocks or inlet valves for admitting sew- age ; at the other end is a weir wall over which the effluent—that is, the clarified sewage after precipitation of the heavier matter—passes through old reservoirs into the river. The channels are closed in rotation once in about sixty hours—the period varies according to the nature of the sewage and of the weather — for the treatment of the precipitated sludge. When the penstocks of a channel have been shut, the top water is drained off through VIEW FROM THE INTERIOR OF A NORTHERN OUTFALL SEWER DURING CONSTRUCTION.