Fig. 148.—Diagram Showing Rate of Depreciation of Pipe Sewers.
Eng. News, Vol. 71, p. 86.
The depreciation of brick sewers as studied in New York, is shown graphically in Fig. 147. At zero the sewer is in good condition and at 100 it is in such a state of dilapidation as to require instant rebuilding. Repairs are not considered economical in this condition. In the preparation of this diagram each condition on the list above was given a certain number of points, which when added together represented the state of depreciation of the sewer. These sums were plotted as ordinates and the corresponding ages of the sewer were plotted as abscissas. The various points were taken cumulatively, and where the bond of the brickwork was broken (given a value of 72) plus other defects gave a total of 164 the sewer was considered as valueless and not worth repair. The scale of 164 was later reduced to a percentage basis as shown on the right of the figure. Fig. 148 shows a similar diagram for the depreciation of pipe sewers.
It was concluded that the life of a brick sewer in New York is 64 years. Some of the sewers examined were over 200 years old. The total original cost of 483 miles of brick, pipe and wood sewers was figured as $23,880,000 with a present worth of $18,665,000 and an average annual depreciation of 2.2 per cent. In figuring these amounts no account was taken of obsolescence. The deterioration of catch-basins proceeded at about the same rate as for brick sewers.
CHAPTER XIII
COMPOSITION AND PROPERTIES OF SEWAGE
208. Physical Characteristics.—Sewage is the spent water supply of a community containing the wastes from domestic, industrial, or commercial use, and such surface and ground water as may enter the sewer.[[118]] Sewages are classed as: domestic sewage, industrial waste, storm water, surface water, street wash, and ground water. Domestic sewage is the liquid discharged from residences or institutions and contains water closet, laundry, and kitchen wastes. It is sometimes called sanitary sewage. Industrial sewage is the liquid waste resulting from processes employed in industrial establishments. Storm water is that part of the rainfall which runs over the surface of the ground during a storm and for such a short period following a storm as the flow exceeds the normal and ordinary run-off. Surface water is that part of the rainfall which runs over the surface of the ground some time after a storm. Street wash is the liquid flowing on or from the street surface. Ground water is water standing in or flowing through the ground below its surface.
Ordinary fresh sewage is gray in color, somewhat of the appearance of soapy dish water. It contains particles of suspended matter which are visible to the naked eye. If the sewage is fresh the character of some of the suspended matter can be distinguished as: matches, bits of paper, fecal matter, rags, etc. The amount of suspended matter in sewage is small, so small as to have no practical effect on the specific gravity of the liquid nor to necessitate the modification of hydraulic formulas developed for application to the flow of water. The total suspended matter in a normal strong domestic sewage is about 500 parts per 1,000,000. It is represented graphically in Fig. 149. The quantity of organic or volatile suspended matter is about 200 parts per 1,000,000. It is shown graphically in the smaller cube in Fig. 149.
Fig. 149.—Graphical Representation of Relative Volumes of Liquids and Solids in Sewage.
The odor of fresh sewage is faint and not necessarily unpleasant. It has a slightly pungent odor, somewhat like a damp unventilated cellar. Occasionally the odor of gasoline, or some other predominating waste matter may hide all other odors. Stale sewage is black and gives off nauseating odors of hydrogen sulphide and other gases. If the sewage is so stale as to become septic, bubbles of gas will be seen breaking the surface and a black or gray scum may be present. Before the South Branch of the Chicago River was cleaned up and flushed this scum became so thick in places, particularly in that portion of the Stock Yards where the river became known as Bubbly Creek, that it is said that weeds and small bushes sprouted in it, and chickens and small animals ran across its surface.