For any given area, the average amount of domestic sewage to be provided for is estimated by multiplying the estimated maximum population of that area by the average per capita domestic consumption of water. Sewers must, however, have capacity for the maximum flow at any minute of the year, which of course will be at a rate greater than the average for the year. The consumption may run 20 to 30 per cent above the yearly average for several consecutive weeks, 50 per cent higher during several consecutive days, and 100 per cent higher during occasional hours. Sewer gaugings at Des Moines, Ia., are shown graphically in Fig. 3; curves for a number of commercial, an industrial and two residential districts of Cincinnati, in Fig. 4, give the percentage that the flow for each hour of the day was of the average for the entire day. It appears from these that the maximum domestic sewage rate may be assumed as 70 to Ico per cent greater than the daily average; this including seepage in both average and maximum. Allowing for water supplied to residential sections that does not reach the sewer, we may assume that the maximum domestic sewage flow is 75 per cent greater than the average domestic water supply.

This is for the combined flow from several thousand people. The flow from a single household comes in short flushes at considerable intervals. While the total daily flow may average 500 gallons, the discharge of laundry tubs may for two or three minutes equal 10 to 20 gallons per minute, or at the rate of 14,400 to 28,800 gallons per day, or about thirty to sixty times

FIG. 3.-GAUGINGS AT SEWER OUTLETS AT DES MOINES.

the average rate; while during only one hundred to two hundred minutes out of the day would there be any flow at all from a single house into the sewer. With one hundred houses connected to a sewer (say 600 population), the probability of more than two or three of these flushes reaching a given point in the sewer at the same time is remote, and the flow would probably be continuous (except at night) but still not uniform, and the maximum might be expected to be 200 per cent or more of the

average. It will probably be safe practice to allow for fifty times the average flow where only two or three houses are

FIG. 4.-VARIATIONS IN SEWAGE RATES IN CINCINNATI.

Each line represents the record of a single district in Cincinnati, eight being commercial districts, one industrial, and two residential.

tributary; double the average where the tributary population number 1000, and 75 per cent more than the average where they number 3000 or more.

If, for areas containing 3000 or more population, we assume a yearly average of 100 gallons per capita per day, this gives a maximum rate of 175 gallons per day, which equals .1215 gallon per minute, or .00027 cubic foot per second. Therefore, if we use these assumptions in our estimate, the estimated maximum population of a certain area times .00027 will give the maximum number of cubic feet per second of sewage to be provided for in that area.

Water used in flushing separate sewers should be included under domestic sewage, and if much water is to be so employed it is necessary to provide for it in a sewage treatment plant. This is not necessary in designing the sewers, however, since the purpose of such water is to gorge the sewer for as great a distance as possible. Where automatic flush tanks are used, it is not generally advantageous that they should discharge more than 300 or 400 gallons each per twenty-four hours.

ART. 7. COMMERCIAL AND INDUSTRIAL SEWAGE

In the business and manufacturing sections of a city, the amount of domestic sewage reaching the separate sewers is comparatively small, but that of the commercial and industrial sewage may be considerable, especially that coming from manufacturing plants. The amount of sewage coming from stores, office buildings and other non-manufacturing properties will generally be a function of the number of persons occupying the premises during business hours. Figures for the amount per capita or the number of people to be anticipated are very few. In Table No. 1 it is seen that in the cities entered therein the industrial and commercial consumption of water combined amounted to about 45 per cent of the total consumption, but the commercial was less than 15 per cent and apparently was between the 5 and 15 gallons per capita of the total population of the city.

In estimating the amount of commercial sewage per unit area, we may employ the method of estimating population and average amount of sewage per capita. An office building may

have an occupant for each 50 to 200 square feet on each floor. (This including allowances for halls, elevators, etc. New York's labor law requires floor area of 32 square feet for each employee in fire-proof buildings and 36 square feet in non-fire-proof buildings.) The total floor area of a building will be the ground area times the number of floors and the latter will depend upon the size of the city, value of plant, and local laws and customs. Some of the larger cities have buildings of from 15 to 40 stories. One of the latter covering an area of

acre may contain by day more persons than the population of a small city, reaching possible 5000 per acre. In a commercial city it may easily be that 30 per cent of the total population is occupied during the day in the business district.

The water consumption of such day occupants is not nearly so great as in their residences, and the maximum rate of 20 gallons per day for each individual will ordinarily be sufficient allowance for use of water by water-closet flushes, wash basins, etc. Even this low average will give very considerable quantities of sewage to be provided for from office buildings or those containing other industries where no water is employed in the manufacturing, where the occupants average one for each 50 square feet of floor space and the stories run up to 10 or more. In the Cincinnati plans it was estimated that the maximum rate of sewage discharge in a commercial district varied from 12,000 to 77,700 gallons per acre per day, depending upon the extent of development. In New York the consumption in hotels was found to average 526 gallons per day per 1000 sq. ft. of floor area; that in office and manufacturing buildings averaged 250; and that in tenement and apartment houses averaged 230 gallons.

The amount of sewage from manufacturing plants varies so widely with the nature of the manufacturing process that no general rule can be given for even an approximate estimate. Some industries use and discharge through the sewer or some other outlet enormous quantities of water, these including breweries and paper mills, while others use practically no water whatever for manufacturing processes. If it is known