with the administration of the distributing systems and the
consumption of water in it. This table has been compiled by Mr.
Chas. W. Sherman of the New England Water-works Association, and
was published in the proceedings of that association for September,
1901. The service-pipes, varying from ½ to 10 inches in diameter,
are of cast-iron, wrought-iron, lead, galvanized iron, tin-lined,
rubber-lined, cement-lined, enamelled and tarred, the practice varying
widely not only from one city to another, but in the same city.
TABLE XIX
LEGEND:
(A) = Kind of Pipe.
(B) = Size of Pipe. Ins
(C) = Total Length in Use, Miles.
(D) = Cost of Repairs per Mile.
(E) = Total Number of Hydrants in Use.
---------------------------+--------+-----+------+------+----------
Name of City or Town. | (A) | (B) | (C) | (D) | (E)
---------------------------+--------+-----+------+------+----------
Albany, N. Y. | | | 129.7| | 808
---------------------------+--------+-----+------+------+----------
Atlantic City, N. J. | C.I. | 4-20| 47.6| | 519
| | | | | 808
---------------------------+--------+-----+------+------+----------
Boston, Mass. | C.I. | 2-48| 713.4| 27.09| 7606
---------------------------+--------+-----+------+------+----------
| C.L. | | | |
Burlington, Vt. | C.I. | 4-30| 38.0| 4.61| 213
| W.I. | | | |
---------------------------+--------+-----+------+------+----------
Cambridge, Mass. | | | | | 968
| | | | |
---------------------------+--------+-----+------+------+----------
Chelsea, Mass. | C.I. | 6-16| 37.8| | 253
| | | | |
---------------------------+--------+-----+------+------+----------
| C.I. | | | |
Concord, N. H. | C.L. | 4-30| 60.2| | 267
---------------------------+--------+-----+------+------+----------
Fall River, Mass. | C.I. | 6-24| 87.3| | 954
| | | | |
---------------------------+--------+-----+------+------+----------
Fitchburg, Mass. | C.I. | 2-20| 66.6| | 499
---------------------------+--------+-----+------+------+----------
| C.I. | | | |
Holyoke, Mass. | W.I. | ½-30| 81.6| 5.14| 860
---------------------------+--------+-----+------+------+----------
Lowell, Mass. | | | 127.8| | 1098
| | | | |
---------------------------+--------+-----+------+------+----------
| W.I. | | | |
Lynn, Mass. | C.L. | 2-20| 129.4| | 952
| C.I. | | | |
---------------------------+--------+-----+------+------+----------
Madison, Wis. | C.I. | 4-16| 34.3| | 169
| | | | |
| | | | |
---------------------------+--------+-----+------+------+----------
| C.L. | | | |
Manchester, N. H. | C.I. | 4-20| 96.9| | 743
---------------------------+--------+-----+------+------+----------
Owned by | C.I. | 6-60| 69.8| |
Metropolitan | C.L. | | | |
Water-works +--------+-----+------+------+----------
| C.I. | | | |
Tot. Sup. by | C.L. | 4-60|1360.3| | 11913
| Kal. | | | |
---------------------------+--------+-----+------+------+----------
| C.I. | | | |
Minneapolis, Minn. | Steel.|1¼-50| 269.2| | 3172
---------------------------+--------+-----+------+------+----------
New Bedford, Mass. | C.I. | 4-36| 92.7| 24.00| 738
---------------------------+--------+-----+------+------+----------
| W.I. | | | |
New London, Conn. | C.L. | 4-24| 50.5| 18.71| 258
| C.I. | | | |
---------------------------+--------+-----+------+------+----------
Newton, Mass. | C.I. | 4-20| 136.6| 6.43| 935
---------------------------+--------+-----+------+------+----------
Providence, R. I. | C.I. | 6-36| 324.6| 0.56| 1886
---------------------------+--------+-----+------+------+----------
” H.P. Fire System | C.I. |12-24| 5.6| | 92
---------------------------+--------+-----+------+------+----------
| W.I. | | | |
Quincy, Mass. | C.I. | 1-36| 144.7| 5.50| 955†
| C.L. | | | |
---------------------------+--------+-----+------+------+----------
| C.I. | | | |
Springfield, Mass. | Kal. | 2-20| 84.1| | 539
---------------------------+--------+-----+------+------+----------
Woonsocket, R. I. | C.I. | 4-20| 45.8| 3.57| 548
---------------------------+--------+-----+------+------+----------
Yonkers, N. Y. | | | 74.1| | 771
---------------------------+--------+-----+------+------+----------
Worcester, Mass. | | 2-40| 173.5| | 1763
| | | | |
---------------------------+--------+-----+------+------+----------
† Public hydrants only.
TABLE XIX. (continued)
LEGEND:
(F) = Total Number of Gates in Use.
(G) = Range of Pressure on Mains at Centre, Pounds.
(H) = Size of Service-pipe in Inches.
(I) = Total Number of Service-taps in Use.
(J) = Total Number of Meters in Use.
(K) = Total Pumpage for the Year in Gallons.
(L) = Average Static Head against which Pumps Work, Feet.
----+--------------+------+-------+--------+-----------------+------
(F)| (G) | (H) | (I) | (J) | (K) | (L)
----+--------------+------+-------+--------+-----------------+------
803| | | | 2030 | |
----+--------------+------+-------+--------+-----------------+------
| | ½-4 | 4,249| 3298 | 955,726,046 | 81.7
| | | | | 148,662,947 | 119.5
----+--------------+------+-------+--------+-----------------+------
8910| 40-90 | ½-8 | 87,525| 4516 | |
----+--------------+------+-------+--------+-----------------+------
| | | | | |
618| 70-85 | ½-6 | 3,350| 2311 | 312,896,525 | 289
| | | | | |
----+--------------+------+-------+--------+-----------------+------
| | | 14,207| 860 | 2,651,277,240 |
399| 48-50 | ⅝-2 | 6,146| 104 | |
----+--------------+------+-------+--------+-----------------+------
757| | | 3,340| 1010 | 142,772,165 |
| | | | | |
----+--------------+------+-------+--------+-----------------+------
940| 80 | ½-2 | 6,943| 6,544 | 1,388,776,336 | 186.2
----+--------------+------+-------+--------+-----------------+------
| 75 L.S. | | | | |
554| 155 H.S. | ¾-8 | 4,432| 2,427 | |
----+--------------+------+-------+--------+-----------------+------
734| 80-100 | ⅝-4 | 3,610| 210 | |
| | | | | |
----+--------------+------+-------+--------+-----------------+------
1188| | | 10,634| 5,586 | 2,042,066,140 | 156.1
| | | | | |
| | | | | |
----+--------------+------+-------+--------+-----------------+------
| | | | | 378,782,675 |
966| 45-60 | ¾-4 | 13,504| 2,571 | 1,330,784,875 |
| | | | | |
----+--------------+------+-------+--------+-----------------+------
234| | | 2,758| 2,586 | 306,637,454 | 223.8
| | | | | |
----+--------------+------+-------+--------+-----------------+-----
| | | | | |
| | | | | |
910| | ½-6 | 5,513| 3,667 | |
| | | | | |
| | | | | |
| | | | | |
----+--------------+------+-------+--------+-----------------+------
268| | |134,496| 10,385 |15,027,410,000(a)|
| | | | | 9,431,140,000(b)|
| | | | | 2,015,130,000(c)|
----+--------------+------+-------+--------+-----------------+------
2195| | ⅝-1 | 20,064| 5,030 | 6,863,135,200 |
----+--------------+------+-------+--------+-----------------+------
| | | | | |
1065| 28-64 | ½-10| 9,280| 1,429 | 2,307,429,372 | 167.2
| | | | | |
----+--------------+------+-------+--------+-----------------+------
318| 40-48 | ½-4 | 3,088| 229 | |
----+--------------+------+-------+--------+-----------------+------
801| 84 | ½-6 | 7,087| 6,001 | 762,876,073 | 234
----+--------------+------+-------+--------+-----------------+------
3399| 64-73 | ½-10| 21,566| 17,813 | 3,833,243,445 | 171.6
| | | | | 34,401,038 | 172.4
----+--------------+------+-------+--------+-----------------+------
31| 114 | | | | 578,940,480 | 111.2
----+--------------+------+-------+--------+-----------------+------
1889| 30-35 H.S. | 1-6 | 9,764| 3,122 | |
| 100-120 L.S.†| | | | |
----+--------------+------+-------+--------+-----------------+------
1001| 78-85 | ⅝-3 | 4,330| 122 | |
| | | | | |
----+--------------+------+-------+--------+-----------------+------
456| 50-120 | ⅝-6 | 2,193| 1,889 | 340,849,628 | 237.6
----+--------------+------+-------+--------+-----------------+------
498| | ¼-8 | 4,968| 4,852 | 1,323,696,099 |
----+--------------+------+-------+--------+-----------------+------
| 70 L.S. | | | | |
2432| 150 H.S.† | | 13,292| 12,529 | |
† Public hydrants only.
C.L. = cement-lined.
(_a_) = Chestnut Hill high service.
(_b_) = Chestnut Hill low service.
(_c_) = Spot Pond Pumping-station.
TABLE XIX. (continued)
LEGEND:
(M) = Average Dynamic Head against which Pumps Work, Feet.
(N) = Duty in Foot-pounds per 100 Pounds of Coal. No Deductions.
----------------------------+--------+-------+-------+------------
| | | |
| | | |
Name of City or Town. |Kind of |Size of| (M) | (N)
| Pipe. | Pipe. | |
| | | |
----------------------------+--------+-------+-------+------------
Albany, N. Y. | .. | .. | .. | ..
----------------------------+--------+-------+-------+------------
Atlantic City, N. J. | C.I. | 4-20 | 123.3| 36,501,217
| | | 119.5| 15,518,455
----------------------------+--------+-------+-------+------------
Boston, Mass. | C.I. | 2-48 | .. | ..
----------------------------+--------+-------+-------+------------
| C.L. | | |
Burlington, Vt. | C.I. | 4-30 | 316 | ..
| W.I. | | |
----------------------------+--------+-------+-------+------------
Cambridge, Mass. | .. | .. | .. | ..
Chelsea, Mass. | C.I. | 6-16 | .. | ..
----------------------------+--------+-------+-------+------------
Concord, N. H. | C.I. | | |
| C.L. | 4-30 | .. | ..
----------------------------+--------+-------+-------+------------
Fall River, Mass. | C.I. | 6-24 | .. | ..
Fitchburg, Mass. | C.I. | 2-20 | .. | ..
----------------------------+--------+-------+-------+------------
Holyoke, Mass. | C.I. | ½-30 | .. | ..
| W.I. | | |
----------------------------+--------+-------+-------+------------
Lowell, Mass. | .. | .. | 163.9| 93,489,048
----------------------------+--------+-------+-------+------------
| W.I. | | |
Lynn, Mass. | C.L. | 2-20 | 167 | 88,780,036
| C.I. | | 167 | 87,265,319
Madison, Wis. | C.I. | 4-16 | 242.4| 47,530,839
----------------------------+--------+-------+-------+------------
Manchester, N. H. | C.L. | 4-20 | .. | ..
| C.I. | | |
----------------------------+--------+-------+-------+------------
| C.I. | 6-60 | 96.5| 121,800,000
Owned by | C.L. | | |
Metropolitan +--------+-------+-------+------------
Water-works | C.I. | 4-60 | 51.8| 109,380,000
Tot. Sup. by | C.L. | | 125.6| 80,400,000
| Kal. | | |
----------------------------+--------+-------+-------+------------
| C.I. | | |
Minneapolis, Minn. | Steel.| 1½-50 | .. | 68,016,609
----------------------------+--------+-------+-------+------------
New Bedford, Mass. | C.I. | 4-36 | 192 | 130,336,508
----------------------------+--------+-------+-------+------------
| W.I. | | |
New London, Conn. | C.L. | 4-24 | .. | ..
| C.I. | | |
----------------------------+--------+-------+-------+------------
Newton, Mass. | C.I. | 4-20 | 254 | 72,500,000
----------------------------+--------+-------+-------+------------
| | | 176.9| 101,301,600
Providence, R. I. | C.I. | 6-36 | 177.7| 60,329,100
| | | 124.7| 68,533,300
” H.P. Fire System | C.I. |12-24 | .. | ..
----------------------------+--------+-------+-------+------------
Quincy, Mass. | C.I. | | |
| Kal. | 2-20 | .. | ..
----------------------------+--------+-------+-------+------------
| W.I. | | |
Springfield, Mass. | C.I. | 1-36 | .. | ..
| C.L. | | |
----------------------------+--------+-------+-------+------------
Woonsocket, R. I. | C.I. | 4-20 | 239.5| 51,024,641
Yonkers, N. Y. | .. | .. | .. | ..
Worcester, Mass. | .. | 2-40 | .. | ..
----------------------------+--------+-------+-------+------------
TABLE XIX. (continued)
LEGEND:
(O) = Cost per Million Gallons raised 1 Foot High.
Pumping-station Expenses.
(P) = Cost per Million Gallons raised 1 Foot High.
Figured on Total Maintenance.
(R) = Rate of Interest Per Cent.
------+-------------+-------------+----------+-------------+--------
| | Net Cost of | Bonded | Value of |
(O) | (P) | Works | Debt | Sinking | (R)
| | to Date. | at Date. |Fund at Date.|
------+-------------+-------------+----------+-------------+--------
.. | $0.264 | $916,723.59| $892,000| $100,407.01| 4½-5
------+-------------+-------------+----------+-------------+--------
.. | .. |23,054,387.81|11,960,272|10,144,647.08| 3½-6
| | | | |
------+-------------+-------------+----------+-------------+--------
0.08 | 0.366 | 468,039.73| 248,000| 64,076.40| 3½-4
------+-------------+-------------+----------+-------------+--------
.. | .. | 5,670,229.52| 3,302,100| 604,326.58|
| | | | |
| | | | |
------+-------------+-------------+----------+-------------+--------
.. | .. | 483,335.52| 300,000| 50,921|
| | | | |
------+-------------+-------------+----------+-------------+--------
.. | .. | 857,440.98| 650,000| |
| | | | |
------+-------------+-------------+----------+-------------+--------
.. | .. | 1,937,862.93| 1,920,000| 581,647.78| 5.1
| | | | |
------+-------------+-------------+----------+-------------+--------
.. | .. | 452,091.09| 648,000| 195,908.91|
------+-------------+-------------+----------+-------------+--------
.. | .. | 1,244,742.23| 300,000| 37,403.46| 4
------+-------------+-------------+----------+-------------+--------
0.0399| .. | .. | 1,274,700| 287,226.20|
| | | | |
------+-------------+-------------+----------+-------------+--------
0.042 | 0.51 | 2,472,821.85| 1,800,300| 524,027.50| 3½-5
------+-------------+-------------+----------+-------------+--------
0.159 | .. | 337,630.13| | |
| | | | |
------+-------------+-------------+----------+-------------+--------
.. | .. | 1,513.012.79| 900,000| 159,466.83| 4-6
| | | | |
------+-------------+-------------+----------+-------------+--------
0.0314| | | | |
| | | | |
------+-------------+-------------+----------+-------------+--------
| | | | |
| | | | |
0.032 | | | | |
| | | | |
| | | | |
------+-------------+-------------+----------+-------------+--------
0.043 | | | | |
------+-------------+-------------+----------+-------------+--------
0.033 | | | | |
------+-------------+-------------+----------+-------------+--------
0.0259| 0.2867| 1,820,107.73| 558,000| 148,793.77|av. 4.44
| | | | |
| | | | |
------+-------------+-------------+----------+-------------+--------
.. | .. | 706,978.44| 410,000| .. | 3.5-4
------+-------------+-------------+----------+-------------+--------
0.05 | 0.59 | 2,034,808.07| 2,075,000| 849,115.40| av. 4.7
| | | | |
| | | | |
------+-------------+-------------+----------+-------------+--------
.. |L.S. = 0.0259| 6,470,093.35| 5,920,000| 713,431.62| av. 3.7
------+-------------+-------------+----------+-------------+--------
.. |H.S. = 0.1134| | | |
| | | | |
------+-------------+-------------+----------+-------------+--------
.. | .. | .. | 720,500| .. | 4
| | | | |
| | | | |
------+-------------+-------------+----------+-------------+--------
.. | .. | 2,128,559.56| 1,500,000| 461,861.90| av. 5.9
------+-------------+-------------+----------+-------------+--------
0.061 | 0.37 | 390,841.78| | |
------+-------------+-------------+----------+-------------+--------
.. | .. | 1,577,105.15| 1,475,000| 310,700 | 3.5-7
------+-------------+-------------+----------+-------------+---------
CHAPTER XX.
=211. Sanitary Improvement of Public Water-supplies.=—In the preceding
consideration of a public water-supply it has been virtually assumed
that the water will reach consumers in the proper sanitary condition;
but this is not always the case. With great increase of population
and corresponding increase of manufacturing and other industries
there arise many sources of contamination, so that pure spring- or
river-water for public supplies becomes less available and at the
present time in this country it is rarely to be had.
The legal responsibility of parties who allow sewage, manufacturing
wastes, or other contaminating matter to flow into streams is already
clearly recognized, and many cities and towns are required to dispose
of their sewage and other wastes in such manner as to avoid polluting
streams of water flowing past sewer outfalls or manufacturing
establishments; but even these restraints are not sufficient. If a
stream has once been polluted it can scarcely be considered safe as
a supply for potable water for public or private purposes. There are
certain diseases whose bacilli are water-borne and which are conveyed
by drinking-water containing them; prominent among such diseases are
typhoid fever and cholera. Experience has many times shown that these
bacilli or disease germs may find their way from isolated country
houses as well as from the sewage of cities into water that would
otherwise be potable. Besides such considerations as these it is
equally well known from engineering experience that many waters of
otherwise fair quality carry the remains of organic matter in one shape
or another which operate prejudicially to the physical condition of
those who drink such water. It is therefore becoming more and more
the conviction of civil engineers and sanitarians that there are
few sources of potable water so free from some degree of pollution
that the supplies drawn from them do not require treatment in order
to put them into good condition for drinking. It is not intended in
this observation to state that there are no streams or springs from
which natural waters may not be immediately used for domestic purposes
without improving them by artificial means, but it may be stated even
at the present time that no water of a public water-supply should
be used without treatment, unless the most thorough bacteriological
examinations show that its sanitary condition is eminently satisfactory.
It is the common experience of many public water-supplies in this
country that during certain seasons of the year, extending through the
summer and autumn months, certain low forms of vegetation flourish,
causing sometimes discoloration and always offensive tastes and odors.
While such waters are usually not dangerous, they certainly are not
desirable and may cause the human system to become receptive in respect
to pathogenic bacilli. The tendency at the present time, therefore,
is to consider the improvement of any water-supply that may be
contemplated for any city or town.
=212. Improvement by Sedimentation.=—The two broad methods of improving
the water of a public supply at the present time are sedimentation and
filtration, the latter generally through clean sand, although sometimes
other fine granular material or porous mass is used. The operation of
sedimentation is carried on when water is allowed to stand absolutely
at rest or to move through a series of basins with such small velocity
that the greater portion of the solid material held in suspension is
given an opportunity to settle to the bottom. All water which is taken
from natural sources, whether surface or underground, carries some
solid matter. Some waters, like spring-water or from an underground
supply, are so clear as to be very nearly free from solid matter in
suspension, but, on the other hand, there are waters, like those from
silt-bearing rivers, which carry large amounts. Observations upon the
Mississippi River at St. Louis have shown that the suspended matter
may reach as much as 1000 parts in one million, although the quantity
held in suspension is usually much less than that. Similar observations
have been made upon other silt-bearing streams. Such large proportions
of suspended solid matter are not usually found in streams used for
potable purposes, but there are few surface sources of water-supply
the water from which will not be sensibly improved by sedimentation in
settling-basins or reservoirs.
The process of sedimentation is usually preliminary to that of
filtration. If raw water, i.e., as it comes from its natural source,
is conducted directly to filtration-beds, the amount of solid matter
is frequently so great that the surface of the filter would be too
quickly clogged; hence it is advisable in almost every case to subject
to sedimentation any water which is designed to be treated subsequently
by filtration.
The degree of turbidity is usually measured by means similar to those
employed in gauging discoloration from vegetable matter. One method
devised by Mr. Allen Hazen, to which allusion will again be made, is
that in which the depth in inches is observed at which a platinum wire
1 mm. in diameter and 1 inch long can be seen. The degree of turbidity
is then represented by the reciprocal of that distance. The permissible
turbidity estimated in this manner is taken by different authorities
at different values running from .025 to .2. Water of this degree of
turbidity appears, when seen through a glass, to be practically clear.
The rapidity with which sedimentation can be performed depends greatly
upon the character and degree of comminution of the solid material.
If it is coarse, comparatively speaking, it will quickly fall to the
bottom; if the solid matter is clay of fine texture, it is dissipated
through the water in an excessively high degree of diffusion and
will remain obstinately suspended. This has been found to be the
case at some points with the Ohio River water. Ordinarily sufficient
sedimentation can be accomplished where the water remains at rest
from twenty-four to forty-eight hours; in general, observations as
to this matter, however, must be applied very cautiously. Water of
the Mississippi River at St. Louis has been found to deposit nearly
all of its sediment within twenty-four hours. At Cincinnati, on the
other hand, the Ohio River water carries so fine a sediment that on
an average not more than 75 per cent of it will be deposited in three
days by unaided subsidence. Again, at Omaha the water of the Missouri
River has been found to be turbid at the end of seventy-two hours.
In some cases, as with the waters of the Delaware and Schuylkill at
Philadelphia, a greater amount of subsidence has been found to exist at
times at the end of twenty-four hours than after forty-eight hours. It
is obvious that some special conditions must have produced such results
that would not ordinarily occur in connection with the operation of
sedimentation.
=213. Sedimentation Aided by Chemicals.=—In cases where simple
unaided subsidence proceeds too slowly it can be accelerated by the
introduction of suitable chemicals. At Cincinnati, for instance, it
was found advantageous to introduce into the water before flowing into
the settling-basins a small amount of alum or sulphate of alumina,
depending upon the degree of turbidity, the average being about 1.6
grains per gallon, rising to perhaps 4 grains in floods. By these
means a few hours of aided sedimentation would produce more subsidence
than could be obtained in several days without the chemicals. A
similar recommendation has been made for the purpose of improving the
water-supply for the city of Washington, D. C., from the Potomac River.
In other cases between 5 and 6 grains of lime per gallon have produced
effective results.
=214. Amount of Solid Matter Removed by Sedimentation.=—Under adverse
conditions, or with sediment which remains obstinately suspended, not
more than 25 to 50 per cent of the solid material will be removed
by sedimentation, but when the process is working satisfactorily,
sometimes by the aid of chemicals acting as coagulants, 90 to 99 per
cent even of the solid material may be removed. The operation of
sedimentation has another beneficial effect in that the solid matter
when being deposited carries down with it large numbers of bacteria,
which, in some cases, have been observed to be 80 or 90 per cent of
the total contents of the water. In other words, the subsidence of the
solid matter clears the water of a large portion of the bacteria.
=215. Two Methods of Operating Sedimentation-basins.=—Sedimentation
is carried on in two ways, one being the “fill-and-draw” method and
the other the “continuous” method. In the former method a basin or
reservoir is first filled with water and then allowed to stand while
the subsidence goes on for perhaps twenty-four hours. The clear water
is then drawn off, after which the reservoir is again filled. In the
continuous method, on the other hand, water is allowed to flow into
a single reservoir or series of reservoirs through which it passes
at an extremely low velocity, so that its contents will not entirely
change within perhaps twenty-four hours or more. In this method the
clear water is continuously discharging at a comparatively low rate,
the velocity in the reservoir being so small that the solid matter may
be deposited as in the fill-and-draw method. Both of these methods
are used, and both are effective. The choice will be dependent upon
local conditions. In the continuous method the solid matter is largely
deposited nearer the point of entrance into the reservoir, but more
generally over the bottom in the fill-and-draw method. The velocity
of flow in the reservoirs of the continuous method generally ranges
between 0.5 inch and 2.5 inches per minute. Occasionally the velocity
may be slightly less than the least of these values, and sometimes one
or two inches more than the maximum value.
=216. Sizes and Construction of Settling-basins.=—The sizes of the
settling-basins will obviously depend to a considerable extent upon the
daily consumption of water. There is no general rule to be followed,
but the capacity of storage volume of those actually in use run
from less than 1 to possibly 14 or 15 days’ supply. Under ordinary
circumstances their volumes may usually be taken from 5 to 6 or 8 days’
supply. Their shape should be such as to allow the greatest economy
in the construction of embankments and bottoms. They may generally
be made rectangular. Their depths is also a matter, to some extent,
of constructive economy. The depth of water will usually be found
between about 10 and 16 feet, it being supposed that possibly 2 or 3
feet of depth will be required for the collection of sediment. These
basins must be water-tight. The bottom surfaces may be covered with
concrete 6 to 9 inches thick, with water-tight firm puddle 12 to 18
inches thick underneath, resting on firm compacted earth. The inner
embankment surfaces or slopes may be paved with 10- or 12-inch riprap
resting on about 18 inches of broken stone over a layer of puddle of
equal thickness with the bottom and continuous with it. Occasionally
the bottom and sides may be simply puddled with clay and lined with
brick or riprap pavement, laid on gravel, or broken stone. It is only
necessary that the sides and bottoms shall be tight and of such degree
of hardness and continuity as to admit of thorough cleaning.
The bottoms of sedimentation-basins may advantageously not be made
level. In order to facilitate cleaning away the solid matter settling
on them, a valley or depression may be formed along the centre line to
which the two portions of the bottom slope. A grade in this channel or
central valley of 1 in 500 with slopes on either side of 1 in 200 or 1
in 300 will be effective in the disposition of the solid matter. At the
lowest end of the central valley there should be suitable gates through
which the accumulated sediment can be moved out of the basin. This
sedimentary matter will in many cases be soft mud, but its movement
will always be facilitated by the use of suitable streams of water. The
frequency of cleaning will depend upon the amount of sediment carried
by the water and upon its accumulation in the basin. Whenever its depth
ranges from 1 to 2 or 3 feet it is removed.
Complete control of the entrance of the water to and its exit from
the basin must evidently be secured by suitable gates or valves and
other appliances required for the satisfactory operation of the basin.
In some cases the cost of sedimentation-reservoirs with concrete
bottoms and sides has risen as high as $9000 per million gallons of
capacity; but where the cheaper lining has been used, as in the case
of reservoirs at Philadelphia, the range has been from about $3300 to
about $4300.
=217. Two Methods of Filtration.=—After the process of sedimentation
is completed there will necessarily always be found the remains of
organic matter and certain other polluting material which should be
removed before the water is allowed to enter the distributing system.
This removal is accomplished usually by filtration through clean sand,
but occasionally through porous material, such as concrete slabs,
porcelain, or other similar material. The latter processes are not much
used at the present time, and they will not be further considered.
The filtration of water through sand is carried on by two distinct
methods, one called slow sand filtration and the other rapid sand
filtration. In the first method the water is simply allowed to filter
slowly through beds of sand from 2 to 3 or 5 feet thick and suitably
arranged for the purpose. In the second method special appliances and
conditions are employed in such manner as to cause the water to flow
through the sand at a much more rapid rate. The method of slow sand
filtration will first receive attention.
=218. Conditions Necessary for Reduction of Organic Matter.=—The most
objectionable class of polluting materials includes organic matter
which from one source or another finds its way into natural waters.
Such material has originally constituted or formed a part of living
organisms and chemically consists of varying proportions of carbon,
oxygen, hydrogen, and nitrogen. As found in public water-supplies it
is usually in some stage of decomposition. The chemical operations
taking place in these decompositions are more or less complicated, but
in a general way it may be said that the first step is the oxidizing
of the carbon which may produce either carbon monoxide or carbon
dioxide and a combination of nitrogen with hydrogen as ammonia. When
the conditions are favorable, i.e., when free oxygen is present, the
ammonia may be oxidized by it, thus producing nitric acid and water.
If, as is generally the case, suitable other substances, as alkalis,
are present, the nitric acid combines with them, forming nitrates more
or less soluble and essentially innocuous. It is therefore seen that
the complete result is a chemical change from the original organic
matter, offensive and possibly dangerously polluting, to gaseous and
solid matter, the former escaping from the water and the latter either
passing off unobjectionably in a soluble state or precipitating to
the bottom as inert mineral matter. In order that these processes may
be completely effective, two or three conditions are necessary, i.e.,
sunlight, free oxygen, and certain species of that minute and low class
of organisms known as bacteria, the nature and conditions of existence
of which have been scientifically known and studied within a period
extending scarcely farther back than ten or fifteen years. The precise
nature of their operations and their relations to the presence of the
necessary oxygen, or just the parts which they play in the process of
decomposition, are not completely known, although much progress has
been made in their determination. It is positively known that their
presence and that of uncombined oxygen are essential. Certain species
of these bacteria will live and work only in the presence of sunlight
and oxygen; these are known as aerobic bacteria. Other species,
forming a class known as anaerobic bacteria, live and effect their
operations in the absence of sunlight and oxygen in that offensive
mode of decomposition which takes place in cesspools and other closed
receptacles for sewage and waste matter. They play an essential part in
what promises to be one of the most valuable methods of sewage-disposal
in which the septic tank is a main feature.
=219. Slow Filtration through Sand—Intermittent Filtration.=—In the
slow sand filtration method of purifying the water of a water-supply
the aerobic bacteria only act. In order that their operations may be
completed, free oxygen and sunlight are essential requisites, and the
first of these is found in every natural water which can be considered
potable. Any water which does not contain sufficient free oxygen for
this purpose is to be regarded with suspicion, and generally cannot be
considered suitable for domestic purposes. The amount of uncombined
oxygen contained in any potable natural water is greatly variable and
changes much with the period of exposure in a quiet state, as well
as with pressure and temperature. In the river Seine it has averaged
nearly 11 parts in a million throughout the year, being lowest in July
and August and highest in December and January. It has been found in
the experimental work of the Massachusetts State Board of Health that
free or dissolved oxygen in potable water may vary from 8.1 parts
at 80° Fahr. to 14.7 parts by weight at 32° Fahr. in 1,000,000 at
atmospheric pressure.
[Illustration: FIG. 4.]
[Illustration: No. 1. CROSS-SECTION AT NORTH END OF BED.]
[Illustration: No. 2. CROSS-SECTION AT BEGINNING OF PIPE UNDERDRAIN.]
[Illustration: No. 3. CROSS-SECTION AT SOUTH END OF BED.]
[Illustration: No. 4. CROSS-SECTION AT END OF LOWEST GRAVEL UNDERDRAIN.]
[Illustration: No. 5. LONGITUDINAL SECTION OF A BED, AT WESTERLY END OF
FILTER.
TYPICAL SECTIONS OF UNIT BEDS IN LAWRENCE CITY FILTER.
APRIL, 1901.
COPIED FROM PLAN FURNISHED BY A.D. MARBLE, CITY ENGINEER.]
In some cases where liability to dangerous contamination exists it may
be advisable to increase the available supply of oxygen in the water by
using a slow sand filter intermittently, as has been done at Lawrence,
Mass. Instead of permitting a continuous flow of water through the
sand, that flow is allowed for a period of 6 to 12 hours only, after
which the filter rests and is drained for perhaps an equal period.
During this intermission another filter-bed is brought into use in the
same manner. Alternating thus between two or more filters, the flow in
any one is intermittent. In this manner the oxygen of the air finds its
way into the sand voids of each drained filter in turn and thus becomes
available in the presence of suitable species of bacteria for reducing
the organic matter in the water next passing through the filter.
Intermittent filters operated in this manner are not much used, but the
most prominent instance is that at Lawrence, Mass. At that place the
water after being filtered is pumped to a higher elevation for use in
the distribution system. The pumps have been run nineteen hours out of
the twenty-four, and the water is shut off from the filters five hours
before the pumps stop. The gate admitting water to the filter is open
one hour before they start. Nine hours of each day the filter does not
receive water, and rests absolutely about four hours.
=220. Removal of Bacteria in the Filter.=—The grains of the sand at
and near the surface of a slow sand filter, within a short time after
its operation is begun, acquire a gelatinous coating, densest at the
surface and decreasing rapidly as the mass of sand is entered. This
gelatinous coating of the grains is organic in character and probably
largely made up of numerous colonies of bacteria whose presence is
necessary for the reduction of the organic matter. It is necessary
to distinguish between these species of bacteria and those which are
pathogenic and characteristic of such diseases as typhoid fever,
cholera, and others that are water-borne. Every potable surface-water
and possibly all rain-water carry bacteria which are not pathogenic and
which apparently accumulate in dense masses at and near the surface of
the slow sand filter. As the water finds its way through the sand it
loses its organic matter and its bacteria, both those of a pathogenic
and non-pathogenic character. Potable water, therefore, is purified and
rendered innocuous by the removal in the filter of all its bacteria,
including both the harmless and dangerous.
=221. Preliminary Treatment—Sizes of Sand Grains.=—In designing
filtration-works consideration must be given to the character of water
involved. There are waters which when standing in open reservoirs
exposed to the sunlight will develop disagreeable tastes and odors, and
it may be necessary to give them preliminary treatment especially for
the removal of such objectionable constituents.
The character and coarseness of the sand employed are both elements
affecting its efficiency as a filtering material. It should not
be calcareous, for then masses of it may be cemented together and
injure or partially destroy the working capacity. Again, if it is
too coarse and approaches the size of gravel, water may run freely
through it without experiencing any purification. Much labor has been
expended, especially by the State Board of Health of Massachusetts,
in investigating the characteristics of sand and the sizes of grains
best adapted to filter purposes. In that work it has become necessary
to classify sands according to degrees of fineness or coarseness.
The diameter of a grain of sand in the system of classification
employed means the cube root of the product of the greatest and least
diameters of a grain multiplied by a third diameter at right angles
to the greatest and least. The “effective” size of any given mass of
sand means the greatest diameter of the finest 10 per cent of the
total mass. There is also a term called the “uniformity coefficient.”
The uniformity coefficient is the quotient arising from dividing
the greatest diameter of the finest 60 per cent of the mass by the
greatest diameter of the finest 10 per cent of the same mass. These are
arbitrary terms which have been reached by experience as convenient for
use in classifying sands. Evidently absolute uniformity in size will be
indicated by a uniformity coefficient of 1, and the greater the variety
in size the greater will be the uniformity coefficient. Sands taken
from different vicinities and sometimes even from the same bed will
exhibit a great range in size of grain.
[Illustration: FIG. 5.—Sizes of Grain or Fineness of Sand.]
Fig. 5 represents the actual variety of size of grain as found in
eight lots of sand among others examined in the laboratory of the
Massachusetts State Board of Health. The vertical scale shows the per
cent by weight of portions having the maximum grains less in diameter
than shown on the horizontal line. The more slope, like No. 5 or 6,
the greater is the variety in size of grain. Those lines more nearly
vertical belong to sands more nearly uniform in size of grain.
=222. Most Effective Sizes of Sand Grains.=—Investigations by the
Massachusetts State Board of Health indicate that a sand whose
effective diameter of grain is .2 mm. (.008 inch) is perhaps the most
efficient in removing organic matter and bacteria from natural potable
waters. At the same time wide experience with the operation of actual
filters seems to indicate that no particular advantage attaches to
any special size of grain, so long as it is not too fine to permit
the desired rate of filtration or so coarse as to allow the water to
flow through it too freely. Experiments have shown that effective
sizes of sand from .14 to .38 mm. in diameter possess practically
the same efficiency in a slow sand filter. The action of the filter
is apparently a partial straining out of both organic material and
bacteria, but chiefly the reduction of organic matter in the manner
already described and probably the destruction to a large extent of
the bacteria, especially those of a pathogenic nature, although at the
present time it is impossible to state the precise extent of either
mode of action.
=223. Air and Water Capacities.=—Another important physical feature of
filter-sands, especially in connection with intermittent filtration, is
the amount of voids between the grains. When the intermittent filter is
allowed to drain, so that the only water remaining in it is that held
between the grains by capillary attraction, generally at the bottom
of the filter unless the sand is very fine, the volume of the water
which remains in the voids is called the water capacity of the sand.
The remaining volume between the grains is called the air capacity of
the same sand. It is evident that the air capacity added to the water
capacity will make the total voids between the sand grains.
[Illustration: FIG. 6.]
Fig. 6 shows the amount of air and water capacities of the same sands
whose sizes of grains are exhibited in Fig. 5. The depth of the sand is
supposed to be 60 inches, as shown on the vertical line at the left of
the diagram, while the percentages of the total volume representing the
amounts of voids is shown on the horizontal line at the bottom of the
diagram. Both air and water capacities for each sand are shown by the
various numbered lines partially vertical and partially inclined. It
will be observed that the fine sands No. 2 and No. 4 have large water
capacities, the water capacity being shown by that part of the diagram
lying below and to the left of each line. It will be noticed that No.
5 sand is made up of approximately equal portions of fine and coarse
grains, the former largely filling the voids between the latter. This
mixture, as shown by the No. 5 line, gives a very high water capacity
and a correspondingly low air capacity. Obviously a sand with a high
water capacity has a correspondingly low air capacity, and in general
would not be a very good sand for an intermittent filter, since it
is the purpose of the latter to secure in the voids between the sand
grains as much oxygen as practicable whenever the filter may be at rest.
=224. Bacterial Efficiency and Purification—Hygienic Efficiency.=—As
the function of a filter is to remove as far as possible the organic
matter and bacteria of the applied water, there must be some criterion
by which its efficiency in the performance of those functions can be
expressed. The bacterial efficiency is represented by the ratio found
by dividing the number of bacteria after filtration in a prescribed
cubic unit, as a cubic centimeter, by the number which the same
volume of raw water held before being applied to the filter. This is
a rather misleading ratio, for the reason that the effluent water may
contain bacteria of certain species which grow in the lower portions
of a filter or in the drains which conduct the effluent from it.
It is possible, therefore, that bacteria may be found in a filter
effluent when all of the bacteria originally held in the water have
been removed. Hence the ratio expressing what is called the bacterial
purification arises from dividing the number of bacteria actually
removed from a cubic centimeter of water by the filter by the number
originally held by a cubic centimeter of raw water. The smaller the
first of these ratios the higher the degree of efficiency. Extended
experience, both in the filters of such laboratories as that of the
Massachusetts State Board of Health and with actual filters of public
water-supplies, show that under attainable conditions of operation 98
to 100 per cent of all the bacteria originally found in the water may
be removed.
There is also used the term hygienic efficiency which is used in
connection with slow sand filters. This means simply the per cent of
pathogenic bacteria removed by the filter, and there is good reason to
believe that it is at least as high as the bacterial purification.
=225. Bacterial Activity near Top of Filter.=—The work of removal of
bacteria and organic matter has been found by extended investigations
to be performed almost entirely within 6 or 8 inches of the top surface
of the sand; indeed the most active part of that operation is probably
concentrated within less than 3 inches of the surface. At any rate the
retained bacteria and nitrogenous matter are found to decrease very
rapidly within a foot from the upper surface, below which stratum the
quantity is relatively very small and its rate of decrease necessarily
slow. A little of this nitrogenous or gelatinous matter is found to
surround to a slight extent the sand grains found at the bottom of the
filter. Some authorities have considered that the more steady uniform
efficiency of the deeper filters is due to this effect.
=226. Rate of Filtration.=—The rate at which water can be made to
flow through a slow sand filter is of economical importance, for the
reason that the higher the rate the less will be the area required
to purify a given quantity per day. Foreign engineers and other
sanitary authorities advocate generally slower rates of filtration
than American engineers are inclined to favor. The usual rate in
Europe is not far from 1.6 to 2.5 million gallons per acre per day.
There is also considerable range in this country, and the rate may
reach 3 million gallons per acre per day. Indeed a considerable number
of tests have shown that for short periods of time, at least, some
waters may be efficiently filtered at rates as high as 7 to 8 million
gallons per acre per day, but probably no American engineer is ready
to introduce such high rates as yet. As a matter of fact the rate will
depend considerably upon the character of water used. Clear water from
mountain lakes and streams uncontaminated and carrying little solid
material may be filtered safely and properly at much higher rates of
filtration than river or other waters carrying more sediment and more
organic matter. This principle is recognized both in Europe and in this
country. It would appear from experience that slow sand filters at
the present time with rates of 2.5 to 3 million gallons per acre per
day may be employed for practically any water that may be considered
suitable for a public supply, and that with these rates high degrees of
both bacterial purification and hygienic efficiency may be reached.
=227. Effective Head on Filter.=—Inasmuch as the depth of sand ranges
from perhaps 3 to 5 feet the water will experience considerable
resistance in flowing through it. The distance in elevation between
the water surface over the filter and that of the water as it leaves
the filter measures the loss of head experienced in passing through
the sand and the drainage-passages under it. It has been maintained
by some foreign authorities that this loss of head should be not
more than 24 to 30 inches; that a greater head would force the water
through the sand at such a rate as to render desired purification
impossible. Experience both in the laboratory and with public filters
in this country does not appear to sustain that view of the matter;
considerably greater heads than 30 inches have been used with entirely
satisfactory results both as to the removal of organic matter and
bacteria. It appears to be best so to arrange the flow of water through
the sand and the underdrains as to avoid in either a pressure below
the atmosphere, as in that case some of the dissolved air in the water
escapes and produces undesirable disturbances in the sand, resulting
in reduced efficiency. No precise rule can be given in respect to this
feature of filtration, but it seems probable that satisfactory results
may be obtained under proper working of filters with a loss of head
not greater than the depth of water on the filter added to the depth
of sand in it, although that maximum limit would ordinarily not be
reached. The depth of water on the filter may be taken from 3 to 5
feet. In this country it is seldom less than the least of these limits,
and perhaps not often equal to the greater limit.
=228. Constant Rate of Filtration Necessary.=—Care should be taken in
the operation of filters to avoid any sudden change in the texture
or degree of compactness of the sand. At the times when workmen
must necessarily walk over the surface they should be provided with
special broad-based footwear, so as to produce as little effect of
this kind as possible where they step. Sudden changes in the degree
of compactness cause correspondingly sudden changes in the rate of
filtration, and such changes produce a deterioration of efficiency.
This may be due to two or three reasons. Possibly such changes may open
small channels through which water finds its way too freely; or the
breaking of the gelatinous bond between the grains of sand may operate
prejudicially. At any rate it is essential to avoid such sudden changes
and maintain as nearly uniform a rate of filtration over the entire
filter as possible. Again, the age of a filter affects to some extent
its efficiency. A month or two of time is required, when a new filter
is started, to attain what may be called its normal efficiency. Even
after that length of time the filter gains in its power to retain and
destroy bacteria. This action is particularly characteristic of filters
formed of comparatively coarse sand.
=229. Scraping of Filters.=—More or less solid inert as well as organic
matter accumulates on the surfaces of the slow sand filters, so that
at the end of proper periods of time, depending upon the character
of the water filtered, this surface accumulation must be scraped off
and removed together with the sand into which it has penetrated. In
scraping the filter it is impossible to remove less than .25 or .5
inch of sand, and at least .5 to .75 inch is removed whenever a filter
is scraped. Sometimes 1 or 2 inches may be removed. This sand may be
washed and again placed upon the filter for use. The operation of
scraping exhibits a fresh sand surface to the applied water. It has
been held, particularly by foreign authorities, that this operation of
scraping militates against the efficiency of the filter for the time
being. The investigations of the Massachusetts State Board of Health
and other experiences in this country do not confirm that view which
is based on the assumption that the top nitrogenous film is essential
to efficiency. These investigations have shown that this film is not
necessary in intermittent filters; that in many instances no diminution
of efficiency has resulted from a removal of the film to a depth of
.3 inch; that even the presence of that film has not given efficiency
to coarse sand when the coating was thick enough to completely clog
the filter; and, further, that the material of this nitrogenous
film is found at a depth of several inches below the surface. It is
practically certain that the scraping to depths not exceeding 1 inch
have no sensible effect upon the efficiency under proper management and
operation of the filters. This is particularly true if the thickness of
sand is from 3 to 5 feet. It is undoubtedly true that with very shallow
sand filters from 1 to 2 feet in depth the scraping of the surface may
have some effect upon bacterial efficiency.
It has been the custom in connection with some European filters to
waste the water which first passes through after cleaning, but the
usual practice in this country is to fill slowly the filter with
filtered water from below and, after the sand is submerged, to permit
it to stand a little while before use. Care taken in this manner will
insure an efficiency to a freshly scraped filter sufficient to avoid
any wastage.
=230. Introduction of Water to Intermittent Filters.=—Where
intermittent filters are used it is of the greatest importance to
conduct the water to them so as not to disturb the sand on their
surfaces. This can readily be done in a number of ways. If the shape
of the filter is not oblong, it will be advisable to form a number of
main drains or passages in the sand from which smaller depressions or
passages near together may lead the water to all parts of the surface.
The flowing of the first water through these depressions will permit
the entire surface to be covered so gradually as not to disturb the
sand grains, and it is essential that such means or their equivalent
be employed. If the filter is long and narrow in shape, the main
ditch along one of the longer sides, with depressions at right angles
to it or across the filter and near together, will be sufficient
to accomplish the desired purpose. Obviously when filters are not
intermittently used such precautions are not needed.
=231. Effect of Low Temperature.=—In the early days of the use of
sand filters in this country it was frequently supposed that the low
temperature of the winter caused decreased bacterial purification and a
decrease in power to reduce organic material. It now appears that such
is not the case. The effects of low temperature, such as is experienced
in winters of this climate, may be overcome by temporarily covering the
filters so that heavy ice cannot form and produce disturbances in one
way or another prejudicial to efficiency of operation. The agencies
which operate to reduce efficiency in cold weather are no longer
believed to be those due to low temperature. They are rather indirect
and mechanical, and may be readily overcome by the prevention of the
formation of ice.
=232. Choice of Intermittent or Continuous Filtration.=—The process of
slow sand filtration when continuous has been shown by experience to
be entirely effective for ordinary potable waters, but in those cases
where the amount of dissolved oxygen may be low and where the amount
of organic matter is relatively high it may be advisable to resort to
intermittent filtration. Neither method, however, can be depended upon
to render potable a water which has been robbed of its free oxygen
by an excessive amount of contaminated organic matter. Nor can these
processes be expected to remove coloring matter produced by peaty
soils or other conditions in which large amounts of vegetable matter
have been absorbed by the water. The methods, therefore, have their
limitations, although their field of application is sufficiently wide
to cover nearly all classes of potable water.
=233. Size and Arrangement of Slow Sand Filters.=—Among the first
questions to arise in the design of slow sand filters are their size
and arrangement. The total area will be determined by the total daily
draft and the rate of filtration. Rates of filtration running from
2.5 to 3 million gallons per acre per day, or even more, have been
found satisfactory and are customary in this country. Having given,
therefore, the total daily quantity required, it is only necessary to
divide that by the rate of filtration per acre and the result will be
the number of acres required for the total filter-bed surface. This net
area, however, is not sufficient. Unless there is requisite storage of
filtered water to meet the variation in the hourly draft for the day,
the capacity of the filters must be sufficient to meet the greatest
hourly rate, which must be taken at least 1½ times the average hourly
demand during the day; indeed this is only prudent in any case.
Again, it is necessary to divide the total filter surface into small
portions called beds, so that one or more of them may be withdrawn from
use for cleaning or repairs, while a sufficient filter-area remains in
operation to supply the greatest hourly draft. This surplus area will
usually run from 5 to 20 per cent of the total area of the filter-beds,
although for small towns and cities it may be much more. The sizes of
the filter-beds will depend upon the local circumstances of each case.
It is evident that as each single bed must have its individual set of
appliances and its separating walls, the purpose of economy will be
best served by making the beds as large as practicable. At the same
time they must not be made too large, for in that case the portion
out of use might form so large a percentage of the total area as to
increase unduly the cost of the entire plant. A size of bed varying
between .5 and 1.5 acres is frequently and perhaps generally found
in foreign filtering-plants. If filter-beds range in area from .5
acre to 2 acres, the latter for large plants, the purposes of economy
and convenience in administration will probably be well served. The
grouping of the beds is an important consideration and will depend
somewhat, at least, upon the shape of the plot of ground taken for the
filters. It is advisable that the inlets to the different beds should,
as far as possible, discharge from a single inlet-pipe or main. This
will generally be most conveniently accomplished by making the beds
rectangular in shape, grouped on each side of the supply-main, with
their longest dimensions at right angles to it. This arrangement is
illustrated by the grouping of the filter-beds in the Albany plant,
shown in Fig. 7. In the case of a single oblong bed, like that at
Lawrence, Mass., shown in Fig. 4, page 284, its relatively great length
and small width makes it possible to run the main supply along one
side, from which branch depressions with concrete bottoms enable the
water to be distributed uniformly over its surface in the manner shown
in the figure. It is further necessary to group the filter-beds, pumps,
sand-cleaning appliances, and other portions of the plant, so that
the ends of economy and efficient administration may be served in the
highest degree. It is always necessary that these features of the whole
filtration system should be carefully kept in view in laying out the
entire plant.
=234. Design of Filter-beds.=—The preparation of the site for a group
of filtration-beds also involves the consideration of a number of
principal questions. In the first place, the depth required for the
sand and underdrains will not be far from 5 feet, and there must be a
suitable bottom prepared below the collecting-drains. Again, the depth
of water above the sand may vary from 3 to 5 feet, making the total
depth, including the bottom, of the filter proper about 10 or 11 feet,
and this may represent the depth of excavation to be made. If the
material on which the filter to be built is soft, it may be necessary
to drive piles to support the superincumbent weight. The bottom must be
made water-tight. This can be done either by the use of a layer of well
rammed or packed clay, 1 to 2 feet in thickness, carrying 6 or 8 inches
of concrete, or by a surface of paved brick or stone. If the sides of
the filter-beds are of embankments with surface slopes, the latter may
be protected in the same manner. If the sides are of walls of masonry,
concrete is an excellent material to be used for the purpose.
[Illustration: FIG. 7.—Sedimentation-basin and Filter-beds at Albany,
N. Y.]
[Illustration: Filtration-plant at Albany, N. Y.]
In designing the sides of filters or of the piers projecting up
through the sand for the support of the roof, in case there is one, it
is imperative that care be taken to prevent water from flowing down
through the joints between the sand and the sides of piers or the
masonry sides of the filter-beds. There should be no vertical joint
of that character, but the faces of masonry in contact with the sand
should both slope and be made in steps, so that any settlement of the
sand will tend to close the joint, while the steps will prevent flow.
Nor should there be angles in which sand is to be packed; filleted
corners are far preferable and should be used.
=235. Covered Filters.=—It has become the custom where the best results
are expected in cold climates, if not in all cases, to cover filters
with masonry roofs of domes and cylindrical or groined arches supported
on masonry columns. Such roofs are usually covered with earth to a
depth of 1 to 2 or 3 feet. They prevent any injurious action on the
sides of the filters produced by thick ice or the effects of such
ice upon the upper portions of the sand. In summer they also protect
against the baking and cracking of the upper surface of the sand when
exposed to the sun and prevent, to a considerable extent, the growth
of algæ in different portions of the beds. They are expensive, filters
with masonry covers costing once and a half to twice as much as open
filters, but they enhance the sanitary value of the water. The height
of the masonry roof must be about 2 to 3 feet above the upper surface
of the water and high enough to offer convenient access to the sand
when it is to be cleaned and renewed. The length of span for the arches
or domes is seldom more than 12 or 15 feet.
=236. Clear-water Drain-pipes of Filters.=—After the water has passed
through the sand it must be withdrawn from the bottom of the filter
with as little resistance as practicable. This necessitates, in the
first place, the bottom of the filter to be so shaped as to induce
the flow of the filtered water toward the lines of drain-pipes which
are laid to receive it. These pipes consist of the main members and
the branches, the main members being laid along the centres of the
beds and the branches running from them. The bottoms of the filters,
therefore, should be formed with depressions in which the main pipes
are laid, and with such grades as to expedite the movement of the water
flowing through the branches. If the bottoms are of concrete, they can
advantageously be made of inverted arches or domes, the drain-pipes
being laid along the lines of greatest depression. In such cases the
loads produced by the weight of the roof are more nearly uniformly
distributed over the bottom. The sizes of the drains will be dependent
upon the areas from which they withdraw water. It is advisable to make
them rather large, in order that the water may flow through them more
freely. They seldom need exceed 6 or 8 inches. They are preferably made
of salt-glazed vitrified pipes laid with open joints, around and in the
vicinity of which are placed gravel or broken stone, the largest pieces
with a maximum diameter of 1 to 2 inches. The largest broken stone or
coarsest gravel is near the pipe and should decrease in size as the
drain-pipe is receded from, so that the final portions of the gravel
farthest removed from the drains will not permit the filter-sand to
pass into it. When properly designed and arranged, the loss of head in
passing from the farthest points of a filter-bed to the point of exit
from the filter will not exceed about .01 to .02 of a foot.
[Illustration: Interior of Covered Filter at Ashland, Wis.]
=237. Arrangement of the Sand at Lawrence and Albany.=—Above this
gravel is placed the filtering-sand, about 4 feet thick in the Albany
filter and 3 to 4 feet thick in the filter at Lawrence, Mass. The sand
in the Albany filter was specified to have not “more than 10 per cent
less than .27 mm.” in diameter and “at least 10 per cent by weight
shall be less than .36 mm.” in diameter. Over the entire floor was
spread not more than 12 inches of gravel or broken stone, the lower
7 inches consisting of broken stone or gravel with greatest diameter
varying from 1 inch to 2 inches; the remaining 5 inches of the lower
1 foot was composed of broken stone or gravel decreasing from 1 inch
in greatest diameter to a grain a little coarser than that of the
sand above it. In all cases, sand for the filter-bed should be free
from everything that can be classed as dirt, including clay, loam,
and vegetable matter. Furthermore, it should be free from any mineral
matter which might change the character of the water and render it less
fit for use.
[Illustration: Partially Filled Covered Sand Filter showing Drain-pipe.]
This filtering-sand is usually placed in position with a horizontal
surface. At Lawrence, however, it was placed with a wavy surface, the
horizontal distance between the crests of two consecutive waves being
30 feet, the concrete gutter for admitting the water being half-way
between, all as shown in the illustrations. The sand of this filter
was of two grades, the coarser sand having an effective size of 0.3
mm. (.118 inch) and the finer an effective size of 0.25 mm. (.098
inch). The two different sizes of sand are seen not to be arranged
in horizontal layers, but so that the finer is over the drains and
the coarser between. The No. 70 sand is capable of passing 70 million
gallons per acre per day with a head on it equal to the depth of sand,
while the No. 50 sand can pass 50 million gallons per acre per day
with a head on it equal to its depth. There appears to be no special
advantage in placing the sand in filters other than in horizontal
layers with an effective size practically uniform.
=238. Velocity of Flow through Sand.=—The velocity with which water
will flow through a given depth of sand with a known depth or head
above the surface of the latter has been carefully investigated by the
Massachusetts State Board of Health with the following results:
_v_ = the velocity at which a solid column of water,
whose section equals in area that of the bed of sand,
moves downward through the sand in meters per day; this
is practically the number of million gallons passing
through the sand per acre per day.
_c_ = a constant, having the value of 1000 for clean sand,
and 800 for filter-sand after having been some time in use.
_d_ = the effective size of the sand-grain in millimeters.
_h_ = the head lost by the water in passing through the sand
at the rate v; this is the effective head of water producing
motion through the sand.
_l_ = the thickness of the sand bed.
_t_ = the temperature of the water in degrees Fahr.
The velocity _v_, as determined by experiment, takes the following form:
_h_ (_t_ + 10)
_v_ = _cd²_ --- (--------).
_l_ ( 60 )
This formula cannot be used for the flow of water through all sands of
all thicknesses and under all circumstances. It is limited to effective
diameters of sand between .1 and 3 mm., having a uniformity coefficient
not greater than 5. _h_ and _l_ may be taken in any unit as long as
both are expressed in the same unit, since the ratio of the two
quantities will then not be affected. If the effective head of water
on the filter or the head lost is equal to the thickness of the bed of
sand, the ratio of _h_ divided by _l_ will be 1. In case the formula is
used to express the quantity of water flowing through the sand per acre
per day, it must be remembered that _v_ will be the number of million
gallons and not the total number of gallons. The formula can only be
used when the sand is well compacted and where the voids of the sand
are entirely filled with water.
=239. Frequency of Scraping and Amount Filtered between Scrapings.=—The
frequency of the scraping of filters will depend upon the amount of
organic matter in the water and upon the rate of filtration. Between
the years 1893 and 1900 the periods between scrapings of the Lawrence
filter ranged generally from 20 to 32 days, although periods as small
as 13 or 19 are found in the records. The quantity of water passed
between scrapings varies generally from 67 million to 90 million
gallons, although it fell as low as 49 millions and rose as high as
109 millions. In the case of the Albany filter-plant, up to the end of
the year 1900 the shortest period between scrapings was about 15 days
and the longest about 42 days, the smallest quantity of water passing
through any filter between scrapings being 26,735,000 gallons and the
largest 76,982,000 gallons. The operation of the Albany filters for
the year 1901 shows that the average run of a bed was 26 days between
scrapings, with a total of 70,000,000 gallons per acre for that period.
These figures represent about the usual workings of slow sand filters
at the present time, the period between scrapings running usually
between 15 and 30 days, and the quantity from 30 million gallons per
acre to 100 million gallons per acre.
[Illustration]
[Illustration: Filters for City of Albany, N. Y.]
=240. Cleaning the Clogged Sand.=—The clogged sand scraped from the top
of the filters at the periods of cleaning is removed to a convenient
point where appliances and machinery are available for washing it. This
is an item of some importance in the administration of filters, as the
sand which is removed and washed is at a later period replaced upon the
filter-bed. Various methods have been tried for the purpose of cleaning
sand efficiently and economically. The continuous ejector sand-washer,
one set of which is used at Albany, is probably as efficient as any
machine yet devised. It is shown in Fig. 8. It will be observed that
the dirty sand is fed to the machine at one end into a hopper-shaped
receptacle. In the bottom of this hopper is a nozzle through which
water is discharged from a pipe running along the entire bottom of the
machine. This jet of water forces the sand upward through a suitable
pipe into a reservoir which discharges the sand and water into another
hopper, and so on through the series of five. Evidently there may be
any number of hoppers in the series, a jet of water being provided at
the bottom of each. In this manner the sand and water are thoroughly
mixed together and compelled to flow upward from each hopper to the
next, the dirty water overflowing also from each hopper into a tank
underneath, whence it runs to waste. The clean sand and water flow out
of the machine at the end opposite to that at which they entered. After
the washed sand is dried it is ready to be replaced in the filter.
=241. Controlling or Regulating Apparatus.=—It is essential to the
proper working of a slow sand filter that the amount of water admitted
to and passing through it shall be as nearly uniform as practicable.
This necessitates controlling or regulating apparatus, of which there
are two general classes, the one automatic and the other worked by
hand. There are a considerable number of appliances of both classes.
The filtered water flows from the end of the drains to one or two
small tanks formed by suitable masonry walls immediately outside of
the filter-beds and rises to a level determined by the loss of head
in passing through the filter. The difference in elevation between
the water surface over the sand and that in the filtered water-tanks
shows the effective head which causes the water to flow through the
sand. The object of the controlling or regulating appliances is to keep
that head as nearly constant as possible. Both the hand and automatic
appliances preserve the value of that head by maintaining constant
discharges through either vertical or horizontal orifices, the orifices
themselves being movable. They may be rectangular or other orifices
with horizontal lips or crests. If the control is automatic it is
accomplished usually by a float which raises and lowers the orifice in
such a way as to maintain a constant difference of level between the
filtered and the unfiltered water. The figures illustrate both types of
regulating appliances, the actions of which will be readily understood.
[Illustration: FIG. 8.—Ejector Sand-washer.]
[Illustration: FIG. 9.—Ball-float Regulator of Rate of Filtration.]
[Illustration: FIG. 10.—Regulator in Use in Zurich, Switzerland. M.
Peter, Engineer.]
=242. Cost of Slow Sand Filters.=—The cost of both the open and
covered slow sand filters will obviously vary according to the cost of
labor and materials at their sites. The original cost of the Lawrence
filter, about 2.44 acres in total area, was nearly $25,000 per acre.
The cost of covered filters, so far as constructed in this country,
varies from about $44,000 to nearly $51,000 per acre excluding the
pipe, pumping plants, and sedimentation-basins. The Albany covered
filters cost about $38,000 per acre including filtering materials, but
excluding excavation, pumps, buildings, sedimentation-basins, piping,
and sand-washing machinery, or nearly $46,000 per acre including those
items except pumps and sedimentation-basins. The roof, included in
the preceding estimate, cost about $14,000 per acre. The smaller the
filters the greater the cost per acre, as a rule, as would be expected.
A single open filter at Poughkeepsie and three open filter-beds at
Berwyn, Pa., cost respectively $42,000 and $36,000 per acre, the
former being little less than .7 acre in area and the latter having an
aggregate area of a little more than one half acre. A covered filter at
Ashland, Wis., consisting of three beds of one sixth acre each, cost at
the rate of about $70,000 per acre.
[Illustration: FIG. 11.—Regulating Apparatus Designed by Allen Hazen
for the Albany Filters.]
[Illustration: FIG. 12.—Regulator of Rate of Filtration.]
=243. Cost of Operation of Albany Filter.=—The cost of operating
the Albany filter, including only the costs of scraping, removing
sand, refilling, incidentals, lost time, and washing the sand during
seventeen months ending December 29, 1900, was $1.66 per million
gallons filtered. The cost of removing the sand (excluding scraping),
washing, and refilling was $1.21 per cubic yard. The total cost of
operating the entire filter-plant, including all items, for the year
1900 was $4.52 per million gallons filtered. This covers all expenses,
including pumping, superintendence, and laboratory, which can be
charged to the operation of the filter-plant. The average removal of
albuminoid ammonia at Albany for the year 1900 was 49 per cent and
of the free ammonia 78 per cent of that in the raw water, while the
average bacterial removal was over 99 per cent, running from 98.3 per
cent to 99.6 per cent. The volume of water used in washing the sand was
about twelve and a half times the volume of the sand. Each cubic yard
of sand washed, therefore, required twelve and a half cubic yards of
water.
[Illustration: FIG. 13.—Regulator Designed by W. H. Lindley for the
Filters at Warsaw, Poland.]
=244. Operation and Cost of Operation of Lawrence Filter.=—It was
originally intended that the Lawrence filter should be worked
intermittently. The Merrimac River water, which is used by the city of
Lawrence, was known to carry at certain periods of the year sufficient
typhoid germs received from the city of Lowell to produce at least
mild epidemics. The intermittent operation was considered necessary to
furnish the filter with the requisite oxygen to destroy beyond a doubt
all pathogenic bacteria. The increasing demands of water consumption
during the years that have elapsed since filtration began in 1894 have
seriously modified these conditions, so that the intermittent feature
of operation of the filter is no longer very prominent. During 1898,
for instance, the filter was drained only four to thirteen times
per month, with an average of eight monthly drainings. In 1899 the
drainings were more frequent, varying from five to fourteen per month
and averaging eleven times. Finally, in 1900, the monthly drainings
ranged from three to thirteen, with an average of eight. It may be
considered, therefore, that the Lawrence filter occupies a kind of
intermediate position between intermittent and continuous operation.
The total cost of operating the filter at Lawrence, including scraping
and washing of sand, refilling, removal of snow and ice, and general
items in the period from 1895 to 1900, both inclusive, varied from a
minimum of $7.70 per million gallons to $9.00 per million gallons. If
the removal of snow and ice be omitted, these amounts will be reduced
to $5.10 and $6.90 respectively. The cost of washing the sand only in
the Lawrence filter during the same period varied from 45 to 67 cents
per cubic yard. The volume of water required for that washing varied
from ten to fourteen times the volume of sand.
=245. Sanitary Results of Operation of Lawrence and Albany
Filters.=—The average number of bacteria in the Merrimac River water
applied to the filter during the period 1894 to 1899, both inclusive,
varied from about 1900 per cubic centimeter to 34,900, and the
percentage of reduction attained by passing the water through the
filter varied in the same period generally from 97 to 99.8 per cent,
with an average of about 99.1 per cent.
In the city of Lawrence the average number of cases of typhoid
fever per 10,000 of population has been about one third, since the
introduction of filtered water, of the number of cases which existed
prior to the installation of the filters, and less than one fourth as
many deaths. A large number of the cases of typhoid occurring after the
installation of the filter have been traced to the use of unfiltered
water, and it is probable that all or nearly all could be similarly
accounted for.
In the city of Albany the experience had been quite similar. The
average number of deaths per year from typhoid fever for ten years
before the introduction of filtered water was 84, while in 1900, with
the filter in operation, the total number of deaths was 39. These
figures are sufficient to show the marked beneficial effect of filtered
water on the public health.
[Illustration: Jewell Filter.]
=246. Rapid Filtration with Coagulants.=—It has been seen that the rate
of filtration through open sand filters does not usually exceed 2 to
4 million gallons per acre per day under ordinary circumstances. Much
greater rates would clog the sand and produce less efficient results.
Experience has also shown that such methods cannot be depended upon to
remove from water coloring matter of a vegetable origin or very finely
divided sediment. In order to accomplish these ends it is necessary to
employ suitable chemicals which, acting as coagulants, may accomplish
results impracticable in the open filter. Resort has therefore been
made first to the adoption of suitable coagulants and then to such
increased heads or pressures as to force the water through the sand
at rates from 25 to 30 or even 50 times as great as practicable in
slow sand filtration. These rapid sand filters are called mechanical
filters. If the water is forced through them under pressure, they
consist of closed tanks in which sand is placed so as to leave
sufficient volume above it for the influent water and, supported upon
a platform carrying perforated pipes, strainers, or equivalent details
through which the filtered water may flow into a suitable system of
effluent pipes in the lower part of the filter. If water is forced
through the sand by the required head, the upper part of the filter may
be open, but of sufficient height to accommodate it. The same filtering
material, clean sand, is used as in the slow filters; the only
differences, aside from the higher rate of filtration, are the greater
head and the introduction of a coagulant to the water. The depth of
sand used may vary from 2 to 4 feet. The thickness of a relatively fine
sand may be less than that of a coarser sand.
=247. Operation of Coagulants.=—The coagulant which has been found to
give the best results is ordinary alum or sulphate of aluminum. If
sulphate of aluminum is dissolved in water containing a little lime or
magnesia, aluminum hydrate and sulphuric acid are formed. The aluminum
hydrate is a sticky gelatinous substance which gathers together in a
flocculent mass the particles of suspended matter in the water, and it
also adheres to the grains of sand when those masses have settled to
the bottom. This flocculent, gelatinous mass covers the sand and passes
into its voids. As the water is forced through it the bacteria and
suspended matter are held, leaving a clear effluent to pass through.
Other coagulants are used, such as the hydrate of iron, but it costs
more than alum and is not so effective in removing color, although
it is an excellent coagulant for removing turbidity. Physicians have
made objection to the use of alum for this purpose, on the ground that
any excess might pass into distribution-pipes and so be consumed by
the water-users to the detriment of health. While it is possible that
further experience may show that there is material ground for this
objection, it has thus far not been found to be so. It is, however,
essential that only the necessary amount of alum should be used and
that there may be a sufficient amount of alkali to combine with the
sulphuric acid. Otherwise the acidulated water may attack the iron and
lead pipes and so injure the water and produce serious trouble. It
can only be stated that the method and operation of these mechanical
filters have thus far been sufficiently successful to avoid any of
these difficulties.
[Illustration: The Jewell Filter-plant at Norristown, Penn.]
=248. Principal Parts of Mechanical Filter-plant—Coagulation and
Subsidence.=—The principal parts of a complete mechanical filter-plant
in the order of their succession are a solution-tank, a measuring-tank,
a sedimentation-basin, and a filter. In case of great turbidity
the sedimentation may be completed in two stages, the first in a
settling-basin prior to receiving the coagulant, and the second in
another basin subsequent to the coagulation. The tanks are usually
of wood, although they may be of steel. The solution-tank is a
comparatively small vessel in which the alum is dissolved. The solution
is then run into the measuring-tank, from which it flows into the water
at a constant rate maintained by suitable regulating apparatus. It is
imperative for the successful working of the mechanical filter-plant
that the coagulant be introduced to the water at a uniform rate.
This rate will obviously depend upon the character of the water. The
coagulating solution runs from the measuring-tank into the pipe through
which the water to be filtered flows and in which it first receives
the alum. The water and the coagulating solution are thus thoroughly
mixed and flow into the sedimentation-basin. The subsidence which is
provided for in this basin may be omitted in very clear waters which
carry little solid matter, but the operation of the filter itself will
be more satisfactorily accomplished if as much work as feasible is done
before reaching it. The mixture must remain in this basin a sufficient
length of time to allow such subsidence as can reasonably be attained.
It appears from experience in this part of the work that it is not well
to introduce the coagulant too long before the water enters the filter,
especially if the water be fairly clear. In the case of the presence
of finely divided solid matter, however, sufficient time must be
permitted for the necessary settlement. A period ranging in length from
½ hour to 6 or 8 hours may be advantageously assigned to this part of
the operation, the shorter period for clear waters and the longer for
very turbid waters. It has been suggested that two applications of the
coagulant might be beneficial, the principal portion being given to the
water before entering the sedimentation-basin and the other just before
the waters enters the filter. The work of the filter, especially with
turbid waters, may be much reduced by simple subsidence for a period
of perhaps 24 hours before receiving the coagulant, the secondary
subsidence taking place in the settling-basin in the manner already
described. Duplicate solution- and measuring-tanks will be required in
order that the process may be continuous while one set is out of use.
In this process it is absolutely essential also that the coagulant
should be of the best quality, inferior grades having been found to be
unsatisfactory in their operation.
=249. Amount of Coagulant—Advantageous Effect of Alum on Organic
Matter.=—The amount of sulphate of alumina will vary largely with the
quality of water. In the investigation made by Mr. Fuller in connection
with the Ohio River supply for the city of Cincinnati, he found that
with very slight turbidity only ¾ grain was required per gallon of
water, but that a high degree of turbidity required as much as 4.4
grains per gallon, with intermediate amounts for intermediate degrees
of turbidity. It was estimated that these quantities would correspond
to an average annual amount of about 1.6 grains per gallon. In case
there should be a period of three days of subsidence preliminary to
filtration, he estimated that for the greater part of the time the
amount of alum would vary from 1 to 3 grains per gallon. Occasionally
more and sometimes less would be required.
Alum has some specially valuable qualities in connection with this
class of purification work. It combines with coloring matter,
particularly that which has been acquired from contact of the water
with vegetation, and precipitates it. It seems to combine also, to some
extent, with the organic matter carried by the water and thus enhances
the efficiency of filtration.
=250. High Heads and Rates for Rapid Filtration.=—The principal work
of investigation of filtration in mechanical or pressure filters has
been made for the cities of Pittsburg, Cincinnati, Louisville, and
Providence, R. I. In the experimental work of those investigations
rates of filtration ranging from 46 million to 170 million gallons per
acre per day have been employed with essentially the same efficiency.
This is a practical result of great importance, particularly if in
the continued use of these filters on a large scale a satisfactorily
high efficiency can be reached and maintained. It was observed that
the number of bacteria in the effluent varied with that in the raw
water. It was also noticed that similarly to the operation of slow sand
filters the rate of filtration should not be changed suddenly, as that
is likely to cause breaks in the sand and militate against continued
efficiency.
In his experimental work at Cincinnati Mr. Fuller found that with
fine sand an available head on the filter of 12 feet gave economical
results. He also states that “high rates are more economical than low
ones, and that the full head which can be economically used should be
provided. Just where the economical limit of the rate of filtration is
can only be determined from practical experience with a wider range of
conditions than exist here, but there seem to be no indications that
the capacity of a plant originally constructed on a medium rate basis
(100 million to 125 million gallons per acre daily) could not readily
and economically be increased, as the consumption demanded, to rates at
least as high as the highest tried here (170 million gallons per acre
daily), provided the full economical increase in loss of head could be
obtained.”
=251. Types and General Arrangement of Mechanical Filters.=—These
mechanical or pressure (by gravity) filters have until lately been
constructed by companies owning patents either on the process or on
the different parts of the filters. The fundamental patent, however,
protecting rapid sand filters with the continuous application of
a coagulant has expired and the city of Louisville, Ky., is now
constructing rapid sand filters different in design from those
heretofore used. The types that have been most common heretofore are
the Jewell subsidence gravity filter, the Continental gravity filter,
the New York sectional-wash gravity filter, and others. They all
possess the main feature of accelerating the rate of filtration by
pressure, either in a closed tank (rarely) with comparatively small
water volume above the sand or by an open filter with sufficient head
of water above the sand to accomplish the high rate desired. This
latter method is that now generally used, as by it the requisite
steadiness of head or pressure can be secured. The closed type is
subject to objectionable sudden changes of pressure which prevent
or break uniform rates of filtration. The sand is supported upon a
platform with a suitable system of pipes fitted with valves or gates
for the withdrawal of the filtered water, the space below the platform
forming a small sedimentation-chamber. They are usually constructed in
comparatively small circular units, so that one or more of a group may
be withdrawn from operation for the purposes of cleaning or repairs
without interfering with the operation of the others. This system of
small units, gives some marked practical advantages, as housing is
readily accomplished, and if necessary the plant may be easily removed
from one point to another.
[Illustration: Continental Filter.]
It is obvious that with the large amount of water forced through
a given area of filter-bed the sand will become clogged within a
comparatively short time, requiring washing and replacing. Mr. Fuller
found at Cincinnati that the periods between washings when fine sand
was used in the filters ranged from 8 to 24 hours, with an average of
15, but with coarse sand the average became 20, with a range of from
6 to 36 hours. The time required for washing the sand at Cincinnati
was 20 minutes for coarse or 30 minutes for fine. At Providence Mr.
Weston found that the average time of washing was about 11 minutes.
The cleaning is accomplished partially by stirring the sand with
revolving arms, as shown in the accompanying figures, but generally
by forcing the water in a reverse direction through the sand and
allowing the wash-water either to run to waste or to be again purified.
The filters are designed for the purpose of cleaning by the reversal
of the direction of the flow of water. Latterly the sand has been
cleaned by forcing compressed air at a low pressure through it and the
superimposed water. The passage of the air or water upward through the
sand produces such a commotion among the grains that they rub against
each other and clean themselves of the adhering material, allowing it
to be carried off by the water above the sand. Both methods are much
used and are satisfactorily effective for the purpose.
It was found at Cincinnati that 4 to 9 per cent, with an average of 5
per cent, of filtered water was required for washing the fine sand, and
only 2 to 6 per cent, with an average of 3 per cent, for the coarse
sand of the mechanical filters used in Mr. Fuller’s experiments. Mr.
Weston has found about the same figures in his experimental work at
Providence. The wash-water need not be wasted at all if it is pumped
back into the subsidence-tanks.
It has been found in some cases that the efficiency of the filters
after washing is not quite normal, and that possibly 2 or 3 per cent
of the water must be wasted unless it is allowed to run back into
the subsidence-tanks and again pass through the filter. Under such
circumstances it has required 20 to 30 minutes of operation of the
filter after washing to regain its normal efficiency.
=252. Cost of Mechanical Filters.=—The cost of these mechanical filters
has been found to range as high as a rate of $500,000 per acre, which
is probably about ten times as much as the rate of cost for the slow
sand filters. On the other hand, the efficiency of the mechanical
filters may be as high as the other class, with a rate of filtration
from thirty to fifty times as great, and with a cost of operation less
than that of the slow sand filters. The cost of the filters per million
gallons of filtered water may, therefore, be reduced to perhaps one
fourth of that of the slow sand type.
=253. Relative Features of Slow and Rapid Filtration.=—It is premature,
even unnecessary, to make a comparison between the slow and rapid sand
filters. The former are well adapted to a large class of potable waters
in which there is not too much or too finely divided solid matter and
in which the coloring from organic origin is not serious. They have
the advantage of requiring no chemicals and are capable of attaining a
high degree of efficiency. The average rate of filtration may be taken
about 3,000,000 gallons per acre per day. The rapid sand filter, on the
contrary, requires the application of a coagulant, but has thirty to
fifty times the capacity of the other class. It is better adapted to
the removal of turbidity and color, and when properly operated it gives
a high efficiency. A sufficiently extended experience has not yet,
however, been attained to enable a complete statement to be made as to
the entire field to which they may be adapted. They have certainly been
shown to possess valuable qualities in a number of respects, and they
are undoubtedly destined to play an important part in the purification
of waters.
PART IV.
_SOME FEATURES OF RAILROAD ENGINEERING._
CHAPTER XXI.
=254. Introductory.=—The first step toward the construction of a
railroad is the location of the line, which requires as an initiative
a careful ocular examination of the general vicinity of the proposed
road, supplemented by simple and approximate instrumental work rapidly
performed. Following this reconnaissance, as it is called, more
complete surveys and examinations are made both in the field and on the
maps plotted from the data of the field-work. The prosecution of this
series of operations produces the final location, together with the
accumulation of such maps, profiles, and other data as may be required
in the construction of the road-bed, bridges, and other structures
constituting the complete railroad line with its ballast and track in
place ready for traffic.
The ultimate purpose of any railroad line is the transportation of
passengers and freight under conditions, including those of a physical
nature connected with the road as well as the rates received, leading
to profitable returns. Competition or other circumstances attending the
traffic of a given road will fix the maximum rates to be charged for
transportation. It is the business, first, of the civil engineer so to
locate and design the road and, second, of the manager so to conduct
the transportation as to make the margin of profits the greatest
possible. It will be the purpose of this lecture to consider in a
general way only some of the features of a railroad and its operation
which are related directly to civil-engineering.
[Illustration: The Royal Gorge.]
=255. Train Resistances.=—It is a fact confirmed by constant daily
experience that, however nicely the machine impelling the railroad
train or the tracks supporting the cars may be built, considerable
frictional and other resistance is offered to the movement of the train
when the latter passes over a perfectly level and straight track.
A considerable portion of the cost of transportation is expended in
overcoming this resistance. When the line fails to be either level
or straight other resistances of magnitude are developed; they are
called the resistances of grades and curves: and it is the business of
the civil engineer so to design the railroad as to reduce these two
classes of resistance to an absolute minimum, in view of certain other
conditions which must be concurrently maintained.
=256. Grades.=—The grade of a railroad is expressed usually in this
country by the number of feet through which 100 feet of length of line
rises or falls, or by some expression equivalent to that. If, for
instance, the line rises 1.5 or 2 feet in 100, it is said to have an
ascending grade of 1.5 or 2 per cent. Or if the line falls the same
amount in the same length, it is said to have a descending grade of
1.5 or 2 per cent. It is evident that a grade which descends in one
direction would be an ascending grade for trains moving in the opposite
direction, so that grades favoring traffic in one direction oppose it
in the other. Hence, other things being equal, that road is the most
advantageous for the movement of trains which has the least grade. The
grades of railroads seldom exceed 2 or 2.5 per cent, although, as will
presently be shown, there are some striking exceptions to that general
observation. The actual angles of inclination of railroad tracks
from a horizontal line are therefore as angles very small, but their
disadvantages for traffic increase rapidly.
A simple principle in mechanics shows that if the railroad train with
a weight _W_ moves up a 2 per cent grade, one component of the train
weight acts directly against the tractive force of the locomotive or
other motive power. If _a_ is the angle of inclination of the track
to a horizontal line, this opposing component will have the value _W_
sin _a_. When angles are small their sines are essentially equal to
their tangents. Hence, in this case, sin _a_ would have the value .02
or ¹/₅₀ of the train weight. If the weight of the train were 500 tons,
which is a rather light train for the present time, this opposing force
would be 10 tons, or 20,000 pounds, which, as we shall see later on, is
more than one half of the total tractive force of any but the heaviest
locomotives built at the present day. This simple instance shows the
advantage of keeping railroad grades down to the lowest practicable
values.
One of the most economical freight-carrying roads in the United States
is the Lake Shore and Michigan Southern of the New York Central system,
running from Buffalo to Chicago. Its maximum grade is 0.4 of 1 per
cent. The maximum grade of the N. Y. C. & H. R. R. R. is 0.75 of 1 per
cent between New York City and Albany and between Albany and Buffalo,
1.74 per cent at Albany, 1.12 per cent at Schenectady, and 1 per cent
at Batavia. Pushers or assistant locomotives are used for heavy trains
at the three latter points. The maximum grade of the Pennsylvania R. R.
on the famous Horseshoe Curve between Altoona and Cresson is 1.8 per
cent. It is advantageous, whereever practicable, to concentrate heavy
grades within a short distance, as in the case of the New York Central
at Albany, and use auxiliary engines, called pushers or assistants.
Some of the heaviest grades used in this country are found on the
trans-continental lines where they pass the summits of the Rocky
Mountains or the Sierras. In one portion of its line over a stretch of
25.4 miles the Southern Pacific R. R. rises 2674 feet with a maximum
grade of 2.2 per cent; also approaching the Tehacipi Pass in California
the maximum grade is about 2.4 per cent. At the Marshall Pass on the
Denver & Rio Grande R. R. there is a rise of 3675 feet in 25 miles with
a maximum grade of 4 per cent. The Central Pacific R. R. (now a part of
the Southern Pacific system) rises 992 feet in 13 miles with a maximum
grade of 2 per cent. The Northern Pacific R. R. rises at one place 1668
feet in an air-line distance of 13 miles with a maximum grade of 2.2
per cent. Probably the heaviest grade in the world on an ordinary steam
railroad is that of the Calumet Mine branch of the Denver & Rio Grande
R. R., which makes an elevation of 2700 feet in 7 miles on an 8 per
cent grade and with 25° curves as maximum curvature. These instances
are sufficient to illustrate maximum railroad grades found in the
United States.
=257. Curves.=—Civil engineers in different parts of the world have
rather peculiar classifications of curves. In this country the railroad
curve is indicated by the number of degrees in it which subtend a chord
100 feet in length. Evidently the smaller the radius or the sharper the
curvature the greater will be the number of degrees between the radii
drawn from the centre of a circle to the extremities of a 100-feet
chord. American civil engineers use this system for the reason that
the usual tape or chain used in railroad surveying is 100 feet long. A
very simple and elementary trigonometric analysis shows that under this
system the radius of any curve will be equal to 50 divided by the sine
of one half of the angle between the two radii drawn to the extremities
of the 100-feet chord. In other words, it is equal to 50 divided by
the sine of one half the degree of curvature. The application of this
simple formula will give the following tabular values of the radii for
the curves indicated:
Curve. Radius in Feet.
1° 5729.65
2° 2864.93
3° 1910.08
4° 1432.69
5° 1146.28
6° 955.36
7° 819.02
8° 716.78
9° 637.27
10° 573.69
12° 478.74
15° 383.06
20° 287.91
=258. Resistance of Curves and Compensation in Grades.=—Inasmuch as
the resistance offered to hauling the train around a curve increases
quite rapidly as the radius of curvature decreases, it is obvious
that in constructing a railroad the degree of each curve should be
kept as low as practicable, and that there should be no more curves
than necessary. While no definite rule can be given as to such
matters, curves as sharp as 10° (573.69 feet radius) should be avoided
wherever practicable. It is not advisable to run trains at the highest
attainable speeds around such curves, nor is it done. Inasmuch as curve
resistance has considerable magnitude, as well as the resistance of
grades, it is natural that wherever curves occur grades should be less
than would be permissible on straight lines or, as they are called,
tangents. If a maximum gradient is prescribed in the construction of
a railroad, that gradient will determine the maximum weight of train
which can be hauled on the straight portions or tangents of the road.
If one of these grades should occur on a curve, a less weight of
train could be handled by the same engine than on a tangent. Hence it
is customary to reduce grades by a small amount for each degree of
curvature of a curve. This operation of modifying the grades on curves
so as to enable a locomotive to haul the same train around them as up
the maximum grade on a tangent is called compensating the curves for
grade. There is no regular rule prescribed for this purpose, because
the combination may necessarily vary between rather wide limits in
view of speed, condition of track, and other influencing elements.
The compensation, however, has perhaps frequently been taken as lying
between .03 and .05 per cent of grade for each degree of curvature. In
other words, for a 5° curve the grade would be .15 to .25 per cent less
than on a tangent. This compensation for grades is carefully considered
in each case by civil engineers in view of experience and such data as
special investigations and general railroad operation have shown to be
expedient.
[Illustration: FIG. 1. GRAVEL BALLAST]
[Illustration: FIG. 2. STONE BALLAST
NEW YORK CENTRAL & HUDSON RIVER RY.]
[Illustration: FIG. 3. PENNSYLVANIA RY.]
=259. Transition Curves.=—High speeds for which modern railroads are
constructed have made it necessary not only to protect road-beds, but
also to make the passage from tangents to curves as easy and smooth as
possible. This is accomplished by introducing between the curve and
the tangent at each end what is called a “transition” curve. This is a
compound curve, i.e., a curve with varying radius. At the point where
the tangent or straight line ceases the radius of the transition curve
is infinitely great, and it is gradually reduced to the radius of the
actual curve at the point where it meets the latter. By means of such
gradual change of curvature the trucks of a rapidly moving train do
not suddenly pass from the tangent to the curve proper, but they pass
gradually from motion in a straight line to the sharpest curvature over
the transition curve. The rate of transition is fixed by the character
of the curves, which have been subjected to careful analysis by civil
engineers, and they can be found fully discussed in standard works on
railroad location.
[Illustration: FIG. 4.—Baltimore Belt-line Tunnel, B. & O. Ry.]
=260. Road-bed, including Ties.=—Not only the high rates of speed of
modern railroad trains but the great weights of locomotives and cars
have demanded a remarkable degree of perfection in the construction of
the road-bed and in the manufacture of rails. The favorite ballast at
the present time for the best types of road-beds is generally broken
stone, although gravel is used. The first requisites are a solid
foundation and perfect drainage whether in cuts or fills. Figs. 1,
2, 3, and 4 show two or three types of road-bed used by the New York
Central and Hudson River R. R., the Pennsylvania R. R., and a special
type adopted by the B. & O. for the belt-line tunnel at Baltimore.
These sections show all main dimensions and the provision made for
drainage. The general depth of ballast is about 18 inches, including
the drainage layer at the bottom. The total width of road-bed for a
double-track line varies frequently between 24 and 25 feet, while the
width of a single-track line may be found between 13 and 14 feet. In
the cross-sections shown the requirements for drainage are found to be
admirably met. Timber ties are almost invariably used at the present
time in this country, although some experimental steel ties have
been laid at various points. Fig. 5 shows the steel tie adopted for
experiment on the N. Y. C. & H. R. R. R. within the city limits of New
York. The time will undoubtedly come when some substitute for timber
must be found, but the additional cost of steel ties at the present
time does not indicate their early adoption.
[Illustration: FIG. 5.]
[Illustration: FIG. 6.]
[Illustration: Cañon of the Rio Las Animas, near Rockwood.]
=261. Mountain Locations of Railroad Lines.=—The skill of the civil
engineer is sometimes severely taxed in making mountain locations of
railroads. Probably no more skilful engineering work of this kind has
ever been done than in the crossings of the Rocky Mountains and the
Sierras in this country by trans-continental railroad lines, although
more striking examples of railroad location for short distances may
perhaps be found in Europe or other countries. The main problem in such
cases is the making of distance in order to attain a desired elevation
without exceeding maximum grades, such as those which have already
been given. Most interesting engineering expedients must sometimes
be resorted to. One of the oldest of these is the switchback plan
shown in Fig. 6. This is probably the simplest procedure in order to
make distance in attaining elevation. The line is run up the side of
a mountain at its maximum grade as far in one direction as it may be
desirable to go. It then runs back on itself a short distance before
being diverted so as to pass up another grade in the reverse direction.
This zigzagging of alignment may obviously be made to attain any
desired elevation and so overcome the summit of a mountain range. The
old switchback coal road near Mauch Chunk, Pa., is one of the oldest
and more famous instances of the method, which has many times been
employed in other locations.
[Illustration: FIG. 7.]
A more striking method, perhaps, is that of loops by which the
direction of a line or motion of a train on it is continuous. Distance
is made by a judicious use of the topography of the locality so as to
run the line as far up the side of the valley as practicable and then
turn as much as a semicircle or more, sometimes over a bridge structure
and sometimes in tunnel, so as to give further elevation by running
either on the opposite side of the valley or on the same. A succession
of loops or other curves suitably located will give the distance
desired in order to reach the summit.
=262. The Georgetown Loop.=—Fig. 7 shows one of these spiral or loop
locations on the Georgetown branch of the Union Pacific Railroad
in Colorado. It is a well-known and prominent instance of railroad
location of this kind. On the higher portion of this loop system
included in the figure there is a viaduct on a curve which crosses the
line 75 feet above the rail below it and 90 feet above the water. This
location is a specimen of excellent railroad engineering. The length
of line shown in the figure, including the spiral, is 8½ miles, and it
cost $265,000 per mile exclusive of the bridges.
=263. Tunnel-loop Location, Rhætian Railways, Switzerland.=—In Figs.
8 and 9 are shown two portions of the Albula branch of the Rhætian
Railways, Canton Graubünden, southeastern Switzerland. The line
connects the valleys of the Albula and the Inn, the former being one of
the branches of the Rhine and the latter of the Danube; it therefore
cuts the divide between the watersheds of those two rivers. It is a
3.28-feet gauge single-track road, and is built largely for tourist
traffic, as the scenic properties of the line are remarkable.
The maximum grade on this line is 3.5 per cent. Over one portion of the
line 7.8 miles long one third of that distance is in tunnel and 15 per
cent of it on viaducts. The radii of the centre lines of the tunnels
are 460 and 394 feet, while the lengths of the tunnels range from 1591
to 2250 feet, with a maximum grade in them of 3 per cent. The weight of
rails used is 50 pounds per yard on grades of 2.5 per cent or less, but
for heavier grades 55-pound rails are employed. The cross-ties are of
mild steel and weigh 80 pounds each except in the long Albula tunnel,
where treated oak ties are used as being better adapted to the special
conditions existing there. It will be observed that in each case the
line rises from the left-hand portion of the figure toward the right.
[Illustration: FIG. 8.]
[Illustration: FIG. 9.]
The tunnels are represented by broken lines, and they are in every
instance on circular curves. Fig. 9 represents the line running from
a point on the east side of the Albula River through a heavy cut and
then across the valley of the Albula into a tunnel 2250 feet long.
The line then runs chiefly in cuts to a point where there are two
tunnels, one over the other; indeed the line over-laps itself in loops
and tunnels a number of times in that vicinity. That portion of the
road shown in Fig. 8 is less remarkable than the other, although it
exhibits extraordinary alignment. This example of railroad location
is one of the most striking among those yet completed. It would
appear to indicate that no topographical difficulties are too great
to be overcome by the civil engineer in railroad location in a most
rugged and precipitous country. Obviously such a line could not be
economically operated for heavy freight traffic.
Railroad lines frequently lead through mountainous regions affording
some of the grandest scenery in the world accessible to the travelling
public. In this country the Canadian Pacific, the Northern Pacific, the
Great Northern, and the Rio Grande Western probably exhibit the most
remarkable instances of this kind.
CHAPTER XXII.
=264. Railroad Signalling.=—The birth of the art of railroad signalling
was probably coexistent with that of the railroad. At the very outset
of the movement of railroad trains it became imperative to insure to
a given train the sole use of the single track at schedule periods.
Both head-to-head and rear-end collisions were liable to occur on main
tracks, as well as false meetings at branches and cross-overs.
=265. The Pilot Guard.=—One of the earliest if not the earliest
of systematic procedures in England to accomplish the safe use of
a railroad track involved the employment of the “pilot guard” on
single-track roads. The pilot was an employé whose duty it was to
accompany every train over a stated section of the line. The authority
to start trains was lodged in him. When it became necessary to start
two or three trains from the same point and in the same direction, it
was also his duty to issue to each train conductor what was called a
pilot ticket, equivalent to a modern train order to run the train over
the section under his control. In that case he was obliged to accompany
the last train to the other end of his section, and no more trains
could move over that section in the same direction until his return
to his first station. As no train could pass over the section without
either him or his pilot ticket, it is clear that the system could
prevent head-to-head collisions, but in itself it is not sufficient to
eliminate rear-end collisions. This system is still employed in Great
Britain on some short branch lines.
=266. The Train-Staff.=—Another method nearly as old as the preceding
is that of the train-staff, used in an improved form at the present
time on some single-track roads. No train under this system can
pass over any given section of the line unless it carries the staff
belonging to that section, the staff being a piece of wood or metal 1
to 1¼ inches in diameter and 18 to 20 inches long. In order to cover
the case of two or more trains starting in the same direction at one
end of a section before running a train in the opposite direction,
tickets were issued, the staff being taken by the last train. The
proper operation of this method, like that of the preceding, would
prevent head-to-head collisions, but is not sufficient in itself to
prevent one train running into the rear of another while both are
proceeding in the same direction in the same section.
=267. First Basis of Railroad Signalling.=—These and other similar
systems answered fairly well the more simple requirements of early
railroad operation. Strictly speaking they are not methods of
signalling, although it may be said that each train is a signal in
itself. With the development of railroad business it was found that
other methods better adapted to a more efficient and rapid movement
of trains were imperative. It was in response to the advancing
requirements of the railroad business that the first approach to
what is now so well known as the block system of signalling was made
in 1842. An English engineer, subsequently, Sir W. F. Cooke, stated
the following sound principles as to the basis of efficient railroad
signalling:
“Every point of a line is a dangerous point which ought to be covered
by signals. The whole distance ought to be divided into sections, and
at the end as well as at the beginning of them there ought to be a
signal, by means of which the entrance to the section is open to each
train when we are sure that it is free. As these sections are too long
to be worked by a traction rod, they ought to be worked by electricity.”