Sewerage and Sewage Treatment - Harold E. Babbitt

Fig. 25.—Typical Map Used in the Design of a Storm Sewer System.

45. Drainage Areas.—The quantity of dry weather sewage is determined by the population rather than the topography. Lot lines and street intersections or other artificial lines marking the boundaries between districts are therefore taken as watershed lines for sanitary sewers. The quantity of sewage to be carried and the available slope are the determining factors in fixing the diameter of the sewer. Since there may be no change in diameter or slope between manholes the quantity of sewage delivered by a sewer into any manhole will determine the diameter of the sewer between it and the next manhole above. In order to determine the additional amount contributed between manholes a line is drawn around the drainage area tributary to each manhole. This line generally follows property lines and the center lines of streets or alleys, its position being such that it includes all the area draining into one manhole, and excludes all areas draining elsewhere. An entire lot is usually assumed to lie within the drainage area into which the building on the lot drains. In laying out these areas it is best to commence at the upper end of a lateral and work down to a junction. Then start again at the upper end of another lateral entering this junction, and continue thus until the map has been covered.

The areas are given the same numbers as the manholes into which they drain. The dividing lines for the drainage areas on Fig. 24 are shown as dot and dash lines, and the areas enclosed are appropriately numbered. If more than one sewer drains into the same manhole the area should be subdivided so that each subdivision encloses only the area contributing through one sewer. Such a condition is shown at manhole C2. The areas are designated by subletters or symbols corresponding to the symbol used for the sewer into which they drain. For example, the two areas contributing to manhole C2 are lettered C2_K and C2_D. The sewer from manhole C3 to C2 receives no addition, it being assumed that all the lots adjacent to it drain into the sewer on the alley. There is therefore no area C2. Likewise there is no area A1_C.

46. Quantity of Sewage.—The remaining work in the computation of the quantity of sewage is best kept in order by a tabulation. Table 19 shows the computations for the sewers discharging from the east into manhole No. 142. The computation should begin at the upper end of a lateral, continue to a junction, and then start again at the upper end of another lateral entering this junction. Each line in the table should be filled in completely from left to right before proceeding with the computations on the next line. In the illustrative solution in Table 19, computations for quantity have not been made between manholes where it was apparent that there would be an insufficient additional quantity to necessitate a change in the size of the pipe.

In making these computations the assumptions of quantity and other factors given below indicate the sort of assumptions which must be made, based on such studies as are given in Chapter III. The density of population was taken as 20 persons per acre, the assumption being based on the census and the character of the district. The average sanitary sewage flow was taken as 100 gallons per capita per day. The per cent which the maximum dry weather flow is of the average was taken as M = 500
P^⅕, in which P is the population in thousands. The per cent is not to exceed 500 nor to be less than 150. The rate of infiltration of ground water was assumed as 50,000 gallons per mile of pipe per day.

In the first line of Table 19, the entries in columns (1) to (6) are self-explanatory. There are no entries in columns (7) to (10), as no additional sewage is contributed between manholes 3.5 and 3.4. In column (11), 2250 persons are recorded as the number tributary to manhole No. 3.5 in the district to the north and west. These people contribute an average of 100 gallons per person per day, or a total of 0.346 second foot. This quantity is entered in column (13). The figure in column (14) is obtained from the expression M = 500
P^⅕. Column (15) is .01 of the product of columns (13) and (14). Column (16) is the product of the length of pipe between manholes 3.5 and 3.4, and the ground water unit reduced to cubic feet per second. Column (17) is the sum of column (16), and all of the ground water tributary to manhole 3.5, which is not recorded in the table. Column (18) is the sum of columns (15) and (17).

No new principle is represented in the second and third lines.

In the fourth line the first 10 columns need no further explanation. The (11th) column is the sum of the (10th) column, and the (11th) column in the third line. It represents the total number of persons tributary to manhole 3.4 on lateral No. 8. Column (13) in the fourth line is the sum of column (13) in the third line and the (12th) column in the fourth line, and the (15th) column in the fourth line is the product of the 2 preceding columns in the fourth line. Note that in no case is the figure in column (15) the sum of any previous figures in column (15). With this introduction the student should be able to check the remaining figures in the table, and should compute the quantity of sewage entering manhole No. 142 from the west, making reasonable assumptions for the tributary quantities from beyond the limits of the map.

TABLE 19

Computations for Quantity of Sewage For a Separate Sewerage System

On Street	From Street	To Street	From Manhole	To Manhole	Length Feet	Mark of Added Areas	Area, Acres	Population per Acre	Number of Persons	Total Persons Tributary	Avg. Sanitary Flow, C.F.S.	Cumulative Avg. Sanitary Flow, C.F.S.	Per cent Max. Sanitary is of Average	Total Max. Sanitary, C.F.S.	Increment of Ground Water, C.F.S.	Cumulative Ground Water, C.F.S.	Total Flow, C.F.S.	Line Number
Nebraska St.	Map margin	Alley S. Grant St.	3.5	3.4	338					2250	0.0000	0.346	425	1.47	0.005	0.0187	1.66	1
Alley S. of Grant St.	Railroad	E. of Missouri St.	8.3	8.2	328	8.2	2.7	20	54	54	.0084	.0084	500	0.041	.0048	.0048	0.046	2
Alley S. of Grant St.	E. of Missouri St.	E. of Kansas St.	8.2	8.1	355	8.1	3.41	20	68	122	.0106	.0190	500	0.095	.0052	.010	0.105	3
Alley S. of Grant St.	E. of Kansas St.	Nebraska St.	8.1	3.4	340	3.4₈	2.68	20	54	176	.0084	.0274	500	0.137	.0050	.015	0.152	4
Nebraska St.	Alley S. of Grant St.	Alley S. of Meridian	3.4	3.3	380					2428	.0000	.373	423	1.58	.0058	.208	1.79	5
						7.1
Alley S. of Meridian	Railroad	Nebraska St.	7.2	3.3	800	3.3₇	7.14	20	142	142	.0221	.0221	500	0.111	.0117	.0117	0.123	6
Nebraska St.	Alley S. of Meridian	Alley S. of Smith Av.	3.3	3.2	304					2568	.0000	.395	414	1.63	.0045	.224	1.85	7
						6.1
Alley S. of Smith Ave.	Railroad	Nebraska St.	6.2	3.2	609	3.2₆	3.82	20	76	76	.0119	.0119	500	0.060	.0089	.0089	0.069	8
Nebraska St.	Alley S. of Smith Ave.	S. of Cordovez St.	3.2	3.1	300					2644	.0000	.407	414	1.68	.0044	.237	1.92	9
S. of Cordovez St.	Railroad	Nebraska St.	4.1	3.1	410	3.1₄	3.10	20	62	62	.0096	.0096	500	0.048	.006	.006	0.054	10
S. of Cordovez St.	Map margin	Nebraska St.	5.1	3.1	380	3.1₅	2.69	20	54	54	.0084	.0084	500	0.042	.0056	.0056	0.048	11
Nebraska St.	S. of Cordovez St.	Long St.	3.1	148	172					2760	.0000	.425	409	1.74	.0025	.251	1.99	12
Long St.	Map margin	Nebraska St.	149	148	380	148	1.53	20	31	31	.0048	.0048	500	0.024	.0056	.0056	0.030	13
Long St.	Nebraska St.	N. Carolina St.	148	147	492					2791	.0000	.430	409	1.76	.0072	.264	2.02	14
Long St.	N. Carolina St.	Georgia St.	147	146	430					2791	1.000 [^[33]]	.430	409	1.76	.0064	1.27	3.03	15
Long St.	Georgia St.	Harris St.	146	145	419	146	0.81	20	16	2807	.0025	.433	407	1.76	.0061	1.28	3.04	16
						2.1
Long St.	Harris St.	Tennessee St.	145	143	725	143–145	6.6	20	132	2936	.0205	.454	403	1.83	.024	1.30	3.13	17

Column No. (1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)	(13)	(14)	(15)	(16)	(17)	(18)