USA > Massachusetts > Worcester County > Fitchburg > Address of the mayor, together with the annual reports for the city of Fitchburg, Massachusetts for the year .. > Part 30
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time to warrant an estimate as high as either of those above mentioned, which would require an increase of approximately 20.2 and 18.3% respectively.
PRESENT AND ESTIMATED FUTURE SEWERED AREA. In estimating the quantity of sewage likely to be received at the disposal works in the future a careful study has been made of the probable increase in the sewered area. The estimates made are liberal and provide for all districts which it seems at all likely will be built up during the next 30 years. The area drained by sewers now built is approximately 2,000 acres. This, however, includes only that portion of the city actually sewered, the area included being only that back to a line about 100 feet from each sewer. As a result of the study given to this problem it is estimated that the area of the districts which will be sewered in 1940 will be 8,134 acres, equivalant to about 12.7 square miles. This area, however, is taken on a much more liberal basis than the area given as that served by existing sewers, and it is not probable that it will be cempletely sewered. It represents rather the area of the districts which will then be tributary to the sewer system. The limits of the area served by existing sewers in 1911, and the assumed sewered area in 1940, are shown by the solid and dotted red lines on Plate XI.
SEWER DISTRICTS. As the design of the sewer systemn depends largely upon topography, each valley being drained independently, it is necessary to define the limits of the main drainage areas. That portion of the city which, it has been assumed, will be sewered in 1940, has been divided into twenty- eight sewer districts, which vary greatly in size. The area of each sewer district is given in Column 4 of Table 5.
Much larger quantities of water are used, and consequently larger quantities of sewage are produced, in mercantile and manufacturing districts than in purely residential sections. Such districts are relatively small in area and contain business blocks, mercantile houses and manufacturing establishments. They also generally contain some residences as it is not prac- ticable to prescribe the boundaries in such manner as to entirely exclude them. After careful study of the natural and artificial conditions which favor manufacturing, certain areas have been
584
REPORT OF SEWAGE DISPOSAL, COMMISSION
defined as comprising the present and probable future manufac- turing districts. Obviously in this as in other estimates it is not possible to define these limits with great accuracy, but it is believed that the areas designated are ample for the period in question and that they will permit of great industrial expansion.
Nearly all of the sewer districts contain some area which will probably in the future be devoted to industrial works. The area in each sewer district, which it is estimated will be devoted to manufacturing or commercial works and to residential pur poses, is given in columns 2 and 3 of Table 5.
The total residential area, as of 1940, has been assumed to be 6,878 acres, and the manufacturing area 1,256 acres. On this basis about 85% of the area will be devoted to residential purposes and about 15% to manufacturing or industrial pur- poses.
DENSITY OF POPULATION IN SEWER DISTRICTS. The quantity of sewage from the several sewer districts depends not only upon the area of these districts, but also upon the density of the population. It is therefore important to estimate as accurately as possible the population which will be served by . the sewer system in each district.
The number of polls in each sewer district has been ob- tained from the printed lists, and by applying a factor determined by dividing the total population in 1910 by the total number of polls, it has been possible to estimate with approxi- mate accuracy the distribution of the present population among the various sewer districts. The total population in each dis- trict and the corresponding density of population, or number of persons per acre, are given in columns 2 and 3 respectively of Table 6. The density of population varies greatly as from .3 in district No. 1, lying south of Waite's Corner, to 77.7 persons per acre, in district No. 21, a small area in the center of the city, which is the most densely populated of any district in the city.
It is probable that the density of population in district 21, as well as in district 18, will not increase as much in the future as that of the other districts. For this reason the population as of 1940 in these two districts has been estimated at 1,200 and 900 respectively, assuming a rate of growth somewhat smaller
TABLE 6 ESTIMATED POPULATION AND DENSITY OF POPULATION IN SEWER DISTRICTS IN 1910 AND 1940 .
DISTRICT NUMBER
ESTIMATED POPULATION IN 1910
ESTIMATED ESTIMATED POPULATION POPULATION PER ACRE IN 1940 IN 1910
ESTIMATED POPULATION PER ACRE IN 1940
(1)
(2)
(3)
(4)
(5)
1
280
0.3
659
0.7
2
190
0.6
436
1.3
3
350
1.2
821
2.8
4
960
2.1
2 271
4.9
5
| 210
5.7
2 858
13.5
6
130
0.7
304
1.6
7
640
1.3
1 521
3.2
8
80
0.8
182
1.7
9
5 200
7.3
12 338
17.2
10
320
9.9
750
23.1
11
380
1.7
902
4.1
12
430
6.8
1 014
16.1
13
1 360
5.6
3 224
13.4
14
1 280
18.9
3 031
44.7
15
..
5 500
13.6
13 078
32.3
16
3 330
11.4
7 898
27.1
17
60
2.9
142
6.8
18
770
37.6
1 200
58.6
19
20
5 410
18.6
12 855
44.1
21
660
77.7
900
104.6
22
70
1.3
162
3.0
23
2 720
7.9
6 448
18.7
24
2 680
14.0
6 357
33.2
25
200
4.1
2 850
9.7
26
540
1.7
1 277
4.0
27
190
0.5
446
1.2
28
1 380
1.3
3 276
3.0
TOTAL
37 320
4.6
87 200
10,7
.
DAVID A. HARTWELL
Chief Engineer HARRISON P. EDDY
Consulting Engineer .
J.A.L.
C.W.A. Jan 17 1912
585
REPORT OF SEWAGE DISPOSAL COMMISSION
than that assumed for the remainder of the city. The density of population in these two districts in 1940, based on these estimates, will be 104.7 and 58.6 persons per acre respectively. In estimating the future population in all of the other sewer districts it has been assumed that the rate of growth will be the same as that of the entire city after deducting the increase in population in districts 18 and 21. The estimated density of population in each district in 1940 is given in column 5, Table 6.
WATER SUPPLY. The quantity of sewage produced by a city depends in part upon the water supply. Recent measure- ments indicate that the average quantity of water supplied to the city in the fall of 1910 was about 119 gallons per capita per day, while the maximum rate was equivalent to about 172 gallons per capita.
A large proportion of the water used by the city does not find its way into the sewers, some being used for locomotives and for steaming purposes in industrial plants and some for watering streets and sprinkling lawns and gardens. A portion leaks into the ground from the distribution system, and much of that which is used for domestic purposes is lost, in one way or another, so that it fails to reach the sewers. Furthermore, a substantial proportion of the water consumers, probably not far from 15%, are not served by the sewer system, consequently none of the water used by them reaches the sewers. In view of all the circumstances it seems fair to estimate that the average quantity of sewage which will be produced by the residential population will be about 75 gallons per capita per day. Obviously the consumption is much greater in the day time than at night and on certain days of the week than on others. After giving consideration to local conditions and to the maxi- mum rates of water consumption in Fitchburg and other similar cities, the maximum rate of sewage flow from residential districts in Fitchburg has been estimated as 150 gallons per capita per day.
STORM WATER. Most of the sewers thus far built in Fitchburg, provide for both sewage and storm water. The act of 1910 requires that "the separation of the sewage from the storm water in said city (Fitchburg) shall be completed in
586
REPORT OF SEWAGE DISPOSAL COMMISSION
accordance with the provisions of Section 5 of the said Chapter 354." There seems to be reasonable grounds for questioning the desirability of making a complete separation of sewage from storm water. Such a scheme will require the construction of duplicate sewers in practically every street, and will also require many changes in existing plumbing, as well as more or less duplication of plumbing in buildings to be built in the future, wherever the roof water is to be conveyed to the storm drains. It will undoubtedly be desirable to build storm drains parallel to the sewers in many of the outlying districts where neither system is yet constructed.
In order to secure the benefits resulting from the purifica- tion of the sewage at as early a date as possible, it has been de- cided to build the intercepting sewer and the sewage disposal works before attempting to separate the sewage from the storm water. When this part of the work is completed the sewage from the existing combined sewers will be discharged into the intercepting sewer, through regulating gates, which will auto- matically close when the intercepter is full. When these gates are closed in this manner the sewage and storm water will flow past the intercepter directly to the river.
In this way it will be possible to treat all of the sewage in dry weather, and a portion of the combined flow of sewage and storm water in times of storm. The quantity of sewage which will be discharged into the river during storms will be relatively small, and will not cause its serious pollution.
While no specific allowance has been made for storm water, in the design of the intercepting sewers, the allowances for ground water and leakage have been made very liberal, so that if a small quantity of roof water is unintentionally admitted to the sewers it will not cause the intercepter to become over- charged. From the time when the intercepting sewer is com- pleted, until the city reaches such a size as to produce enough sewage to tax its capacity, it will have a surplus capacity which may be used for a relatively small quantity of storm water. At the outset, the quantity of storm water which can be provided for in this manner will exceed that of the sewage, although it is not intended to convey the entire flow in the intercepter to the purification works, an overflow being provided at the upper end of the siphon, near the Hollow Mill, which will permit about
587
REPORT OF SEWAGE DISPOSAL COMMISSION
one-half of the flow collected by the intercepter above this point, to flow directly into the Nashua river.
GROUND WATER. It is very difficult to build water-tight sewers, and many of the present sewers are known to leak and admit substantial quantities of ground water. The amount of such leakage varies greatly from season to season, with the height of ground water. During rains and at times when snow is melting, material quantities of storm water may leak into the sewers through perforated manhole covers or around solid covers. In a city as large at Fitchburg there are usually some connections with admit storm water to the sewers even though it is the intention to prohibit such connections. Taking into account all the sources of leakage and the experience of other cities, it has been decided that provision should be made for an ultimate maximum rate of leakage of 1,960 gallons per acre per day, which is equivalent to about 74,000 gallons per day per mile of sewers, assuming the tributary area to be com- pletely sewered.
INDUSTRIAL WASTES. The quantity of industrial wastes, which are likely to find their way into the sewers is most diffi- cult to estimate. Fitchburg is unusually favorably situated to permit of discharging clean wastes into the river, and thus avoiding the necessity of providing for the disposal of such wastes through the sewer system. On the other hand the manu- facturing sites along the river are now nearly all taken up and new plants will be likely to seek locations such that it may be necessary for the city to provide facilities for disposing of a reasonable quantity of industrial wastes. It has therefore been deemed wise to provide for a maximum rate of 8,000 gallons of industrial wastes per day per acre of area included in the manu- facturing and mercantile districts. This rate is based upon the assumption that three quarters of the industrial wastes will be discharged during the working day of ten hours, which appears to be a fair assumption in most cities. This would not be a correct assumption for Fitchburg if all of the paper mill wastes were to be admitted to the sewers, which, however, is not probable. Assuming that three-quarters of the wastes will be discharged during ten hours, and that the maximum rate is
588
REPORT OF SEWAGE DISPOSAL COMMISSION
uniform during the ten hours and is equivalent to 8,000 gallons per acre per day of 24 hours, the quantity of industrial wastes to be expected is 4,444 gallons per acre of industrial area per day of 24 hours, equivalent to a total for the entire city of approximately 5,500,000 gallons daily, which would appear to be a liberal allowance for the period under consideration. This provision is in addition to that made for residential sewage and leakage.
QUANTITY OF SEWAGE FOR WHICH INTERCEPTING SEWERS SHOULD PROVIDE. As a result of the studies de- scribed, it appears that the maximum rate of flow for which the intercepting sewer* should provide will be about 39,000,000 gallons per day, equivalent to nearly 450 gallons per capita per day, for a population of 87,200 persons, assumed to be a reason- able estimate of the probable population of Fitchburg in 1940. The quantities of domestic sewage, ground water and industrial wastes for each district are given in Tables 7 and 8.
The basic data used in the design of the intercepting sewer may be summarized as follows :
Total residential area
6,878.3 acres
Total industrial area 1,256.1 acres
Total sewered area 8,134.4 acres
Population 87,200 persons
Average density of population over whole area 10.7 persons per acre.
Maximum rate of domestic sewage flow 150 gallons per day per cap.
Maximum rate of leakage into sewer 1,960 gallons per day per acre.
Maximum rate of flow of industrial wastes for industrial area 8,000 gallons per day per acre.
*A portion of this quantity may be conveyed to low level purification works, or, by means of pumping, direct to the main purification works without first entering any portion of the intercept- ing sewer.
TABLE 7 ESTIMATED MAXIMUM RATE OF SEWAGE FLOW INTO INTERCEPTER FROM SEWER DISTRICTS IN 1940
GALLONS
PER 24.
HOURS
DISTRICT NUMBER
DOMESTIC SEWAGE
INDUSTRIAL WASTES
GROUND WATER
TOTAL
& OTHER LEAKAGE | MAXIMUM RATE
AT 150 GALLONS AT 8000 GALLONS AT 1960 GALLONS PER CAP. PER DAY PER ACRE PER DAY PER ACRE PER DAY
OF SEWAGE FLOW
(1)
(2)
(3)
(4)
(5)
1
98 800
776 000
1 911 000
3 785 800
2
65 400
76 800
653 300
795 500
3
123 200
252 800
574 500
950 500
4
340 600
304 000
915 300
1 559 900
5
428 700
120 000
416 300
965 000
6
45 600
140 800
375 100
561 500
7
228 200
1 096 000
946 700
2 270 900
8
27 300
160 000
208 700
396 000
9
1 850 700
860 000
1 407 500
4 118 200
10
112 500
14 400
63 700
190 600
11
135 300
27 200
435 100
597 600
12
152 100
504 000
123 500
779 600
13
483 600
92 000
472 600
1 048 200
14
454 600
72 000
132 900
659 500
15
1 961 700
323 200
794 200
3 079 100
16
1 184 700
40 000
571 500
1 796 200
17
21 300
36 800
41 200
99 300
18
180 000
128 800
31 500
160 300
20
1 928 300
200 000
571 300
2 699 600
21
135 000
16 900
151 900
22
24 300
304 000
104 900
433 200
23
967 200
60 000
674 400
1 701 600
24
953 500
375 300
328 800
25
427 500
329 600
577 000
1 334 100
26
191 600
346 400
620 700
1 158 700
27
66 900
741 900
808 800
28
491 400
2 784 000
2 146 200
5 421 600
TOTAL
13 080 000
10 048 800
15 943 400
39 072 200
40 200
220 200
19
DAVID A. HARTWELL .... Chief Engineer HARRISON P. EDDY. Consulting Engineer
J.A.L. Jan. 17, 1912
CW . R .
TABLE 8 ESTIMATED MAXIMUM RATE OF SEWAGE FLOW INTO INTERCEPTER FROM SEWER DISTRICTS IN 1940
CUBIC
FEET
PER
SECOND
DISTRICT NUMBER
DOMESTIC SEWAGE AT 150 GALS PER CAP. PER DAY
INDUSTRIAL WASTES AT 8000 GALS. PER ACRE PER DAY
GROUND WATER TOTAL & OTHER LEAKAGE MAX. RATE OF AT 1960 GALS.
PER ACRE PER DAY SEWAGE FLOW
(1)
(2)
(3)
(4)
(5)
1
0.15
2.75
2.96
5.86
2
0.10
0.12
1.01
1.23
3
0.19
0.39
0.89
1.47
4
0.53
0.47
1.42
2.42
5
0.66
0.19
0.64
1. 43
6
0.07
0.22
0.58
0.87
7
0.35
1.70
1.46
3.51
8
0.04
0.25
0,32
0.61
9
2.86
1.33
2.18
6.37
10
0.17
0.02
0.10
0.29
11
0.21
0.04
0.67
0.92
12
0.24
0.78
0.19
1.21
13
0.75
0.14
0.73
1.62
14
0,70
0.11
0.21
1.02
15
3.04
0.50
1.23
4.77
16
1.83
0.06
0.88
2.77
17
0.03
0.06
0.06
0.15
18
0.28
0.06
0.34
19
0,20
0.05
0.25
20
2.99
0.31
0.88
4.18
21
0.21
0.03
0.24
22
0.04
0.47
0.16
0.67
23
1,50
0.09
1.04
2.63
24
1.48
0.58
2.06
25
0.66
0.51
0.89
2.06
26
0.30
0.54
0.96
1.80
27
0.10
1.15
1.25
28
0.76
4.31
3.32
8.39
TOTAL
20.24
15.56
24.65
60.45
DAVID A HARTWELL
Chief Engineer HARRISON P. EDDY
Consulting Engineer
UNA: Jan 17.1912
589
REPORT OF SEWAGE DISPOSAL COMMISSION
Gallons per day
Per cent
Total maximum rate of flow of domestic sewage
13,080,000
33.5
Total maximum rate of flow of leakage
15,943,000
40.8
Total maximum rate of flow of
industrial wastes
10,048,800
25.7
Total maximum rate of flow of sewage from all sources
39,071,800
100.0
Total maximum rate of sewage flow per capita
450
In designing the intercepting sewer provision has been made for the maximum rate of flow during the day in that sea- son of the year when the quantity of ground water and the leak- age into the sewer system is the greatest.
QUANTITY OF SEWAGE FOR WHICH PURIFICATION WORKS SHOULD PROVIDE. Obviously the average rate of flow for which the purification works should provide will be very much less than the maximum rate upon which the size of the inter- cepting sewer is based.
The quantity of water supplied to the city in 1910 was found to be approximately 100 gallons per capita per day, and the measurements of flow which were made several years ago indicate that the normal flow of sewage is about equivalent to the water supply. It has therefore been decided to base the design of the purification plant upon an assumed quantity of sewage equivalent to 100 gallons per day per capita.
While certain portions of the plant, such as the influent and effluent conduits, will at first be built of sufficient size to accomodate the ultimate flow to be passed through the purifica- tion works, the tanks and filters will be built of capacity to pro- vide only for a population of 55,000 persons, which will probably be reached about the year 1925. The normal quantity of sewage, upon which the designs are based, is 100 gallons per capita or 5,500,000 gallons per 24 hours. If the assumed rate of flow, of 100 gallons per day per capita, is correct for the present time, the quantity of sewage to be expected when the disposal
:
590
REPORT OF SEWAGE DISPOSAL COMMISSION
works are first completed will be approximately 4,000,000 gal- lons per day.
The maximum capacity of the siphon to be built at the outset will be approximately 14,000,000 gallons per day. This capacity will be reaclied only in time of storm. Provision will be made for passing this quantity of sewage and storm water through at least a portion of the purification plant. In time of storm the total flow delivered to the purification works will be three and one-half times the average rate of flow in 1912, and probably two and one-half times the average rate of flow in 1930, dependent upon the population of the city at that time.
The essential quantities of sewage which will be delivered to the purification works are estimated to be as follows :
GALLONS PER DAY
1925
Normal rate of flow
1912 4,000,000
5,500,000
Maximum rate of flow 140% of normal
5.6
7.7
Minimum rate of flow 70% of normal
2.8
3.85
Maximum rate of flow, time of storm
14.2
14.2
HIGH AND LOW LEVEL DISTRICTS. Nearly all of that portion of the city which is now provided with sewers is at such an elevation that the sewage can be delivered to the disposal works by gravity, but a substantial portion of the assumed sewered area as of 1940, lies at an elevation so low that it will be necessary to pump the sewage to the disposal works or to construct auxiliary low level works to care for it. The low level districts lie in the easterly and southeastly por- tions of the city, the largest being No. 28 which is the section naturally drained by Baker Brook and comprises nearly 1,100 acres, while the other districts, No. 25 and No. 26, in South Fitchburg, are relatively small. The total area of these three districts is 1706.1 acres, equivalent to about 21% of the assumed ul imate sewered area.
It is possible that the sewage from Districts No. 24 and No. 27 also may be pumped to the disposal area, although they lie at an elevation which will permit of their drainage to the main works by gravity. At the present time no sewers are built in district No. 27, but district No. 24, which may be defined as the district lying just west of Boutelle street, is almost com-
591
REPORT OF SEWAGE DISPOSAL COMMISSION
pletely sewered. If this district is served by a gravity outfall it will be necessary to conduct the sewage from a point near the intersection of Summer and Boutelle streets, through an inverted cast iron siphon laid in Summer street and Bemis Road, to the main intercepter at a point near the intersection of Bemis and Falulah roads. The sewage from district No. 27, which may be approximately defined as the area between Mechanic Street and Rindge road can be discharged into a sewer to be built through district No. 24, and thus conveyed to the branch in- verted siphon connecting with the outfall sewer. The studies upon this local problem have not as yet been completed and it is possible that it may prove more economical in the end to pump this sewage than to convey it to the intercepter by means of a branch siphon. Should it finally be decided to treat the sewage from districts No. 24 and No. 27 with that from the low level area 378.5 acres will be added to the low level district, making its total area 2276.1 acres, equivalent to nearly 28% of the assumed ultimate sewered area.
The sewage from the Baker Brook district and districts No. 25 and No. 26, and possibly also districts No. 24 and No. 27 can be treated upon low level sand filter beds at the disposal area, or it can be pumped to the main disposal works as may, after detailed studies, appear to be the more economical and practical.
INTERCEPTING SEWER.
SECTION 1. Section 1 of the intercepting sewer will extend from the purification works across land owned by the city to Falulah road, thence in Falulah road to Bemis road, thence along the Nashua river to a point opposite the foot of Newton street, thence in land of the American Woolen Company between the mill and the location of the New York, New Haven & Hartford Railroad Company. From the purification works to a point opposite the foot of Newton street, the intercepter will be laid through land which lies at an elevation much lower than the tanks into which the intercepter will discharge, and it is therefore necessary to lay this portion of Section 1 as an inverted siphon, which will be about 5,000 feet in length. The siphon will be of cast iron pipe and will pass under the Nashua river at 38
'592
REPORT OF SEWAGE DISPOSAL, COMMISSION
a point near the northerly boundary line of the disposal area purchased by the city, and it will pass under the tail race in Falulah road at Sheridan street and under the head-race near the intersection of Falulah and Bemis roads.
The upper portion of Section 1, about 900 feet in length, will be 48 inches in diameter, having a slope of one in one thousand and will be built of concrete. At the lower end of this concrete section a siphon chamber will be constructed, through which the sewage will flow into the cast iron pipe inverted siphon. From this chamber there will be provided an overflow sewer, 24 inches in diameter, extending to the Nashua river.
The capacity of the 48-inch intercepter above the siphon chamber will be approximately 46 cubic feet per second, sufficient for the ultimate needs of that portion of the city lying west of Arden Mill. For a number of years, however, its capacity will be reached only in times of storm.
In order to provide a sufficient velocity in the siphon to assure its freedom from deposits, it will be necessary to provide two or more pipes so that low flows may be confined to a single pipe and thus a relatively high velocity maintained. A con- sideration of the average and maximum quantities of sewage which will be delivered by the 48-inch out-fall sewer during the next fifteen years, indicates that it will be wise to construct the siphon of two pipes, one 30 inches in diameter and the other 36 inches in diameter. It will not be necessary, however, to provide more than one of these pipes until the city has far exceeded its present size, -- probably not until after 1925 or 1930, at which time the population should have reached about 60,000 persons, and it is possible that it may be deferred for a still longer period.
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