=Disposal of Sewage=
=Waste Products.=--There is a large amount of waste products in human
and social economy. The products of combustion, such as ashes,
cinders, etc.; the products of street sweepings and waste from houses,
as dust, rubbish, paper, etc.; the waste from various trades; the
waste from kitchens, e. g., scraps of food, etc.; the waste water from
the cleansing processes of individuals, domestic animals, clothing,
etc.; and, finally, the excreta--urine and faeces--of man and animals;
all these are waste products that cannot be left undisposed of, more
especially in cities, and wherever a large number of people
congregate. All waste products are classified into three distinct
groups: (1) refuse, (2) garbage, and (3) sewage.
The amount of _refuse_ and _garbage_ in cities is quite considerable;
in Manhattan, alone, the dry refuse amounts to 1,000,000 tons a year,
and that of garbage to 175,000 tons per year. A large percentage of
the dry refuse and garbage is valuable from a commercial standpoint,
and could be utilized, with proper facilities for collection and
separation. The disposal of refuse and garbage has not as yet been
satisfactorily dealt with. The modes of waste disposal in the United
States are: (1) dumping into the sea; (2) filling in made land, or
plowing into lands; (3) cremation and (4) reduction by various
processes, and the products utilized.
=Sewage.=--By sewage we mean the waste and effete human matter and
excreta--the urine and faeces of human beings and the urine of domestic
animals (the faeces of horses, etc., has great commercial value, and is
usually collected separately and disposed of for fertilizing
purposes).
The amount of excreta per person has been estimated (Frankland) as 3
ounces of solid and 40 ounces of fluid per day, or about 30 tons of
solid and 100,000 gallons of fluid for each 1,000 persons per year.
In sparsely populated districts the removal and ultimate disposal of
sewage presents no difficulties; it is returned to the soil, which, as
we know, is capable of purifying, disintegrating, and assimilating
quite a large amount of organic matter. But when the number of
inhabitants to the square mile increases, and the population becomes
as dense as it is in some towns and cities, the disposal of the human
waste products becomes a question of vast importance, and the proper,
as well as the immediate and final, disposal of sewage becomes a
serious sanitary problem.
It is evident that sewage must be removed in a thorough manner,
otherwise it would endanger the lives and health of the people.
The dangers of sewage to health are:
(1) From its offensive odors, which, while not always directly
dangerous to health, often produce headaches, nausea, etc.
(2) The organic matter contained in sewage decomposes and eliminates
gases and other products of decomposition.
(3) Sewage may contain a large number of pathogenic bacteria (typhoid,
dysentery, cholera, etc.).
(4) Contamination of the soil, ground water, and air by percolation of
sewage.
The problem of sewage disposal is twofold: (1) immediate, viz., the
need of not allowing sewage to remain too long on the premises, and
its immediate removal beyond the limits of the city; and (2) the final
disposition of the sewage, after its removal from the cities, etc.
=Modes of Ultimate Disposal of Sewage.=--The chief constituents of
sewage are organic matter, mineral salts, nitrogenous substances,
potash, and phosphoric acid. Fresh-mixed excrementitious matter has an
acid reaction, but within twelve to twenty hours it becomes alkaline,
because of the free ammonia formed in it. Sewage rapidly decomposes,
evolving organic and fetid matters, ammonium sulphide, sulphureted and
carbureted hydrogen, etc., besides teeming with animal and bacterial
life. A great many of the substances contained in sewage are valuable
as fertilizers of soil.
The systems of final disposal of sewage are as follows:
(1) Discharge into seas, lakes, and rivers.
(2) Cremation.
(3) Physical and chemical precipitation.
(4) Intermittent filtration.
(5) Land irrigation.
(6) "Bacterial" methods.
_Discharge into Waters._--The easiest way to dispose of sewage is to
let it flow into the sea or other running water course. The objections
to sewage discharging into the rivers and lakes near cities, and
especially such lakes and rivers as supply water to the
municipalities, are obvious. But as water can purify a great amount of
sewage, this method is still in vogue in certain places, although it
is to be hoped that it will in the near future be superseded by more
proper methods. The objection against discharging into seas is the
operation of the tides, which cause a backflow and overflow of sewage
from the pipes. This backflow is remedied by the following methods:
(1) providing tidal flap valves, permitting the outflow of sewage, but
preventing the inflow of sea water; (2) discharging the sewage
intermittently, only during low tide; and (3) providing a constant
outflow by means of steam-power pressure.
_Cremation._--Another method of getting rid of the sewage without
attempting to utilize it is by cremation. The liquid portion of the
sewage is allowed to drain and discharge into water courses, and the
more or less solid residues are collected and cremated in suitable
crematories.
_Precipitation._--This method consists in separating the solid matters
from the sewage by precipitation by physical or chemical processes,
the liquid being allowed to drain into rivers and other waters, and
the precipitated solids utilized for certain purposes. The
precipitation is done either by straining the sewage, collecting it
into tanks, and letting it subside, when the liquid is drawn off and
the solids remain at the bottom of the tanks, a rather unsatisfactory
method; or, by chemical processes, precipitating the sewage by
chemical means, and utilizing the products of such precipitation. The
chemical agents by which precipitation is accomplished are many and
various; among them are lime, alum, iron perchloride, phosphates, etc.
_Intermittent Filtration._--Sewage may be purified mechanically and
chemically by method of intermittent filtration by passing it through
filter beds of gravel, sand, coke, cinders, or any such materials.
Intermittent filtration has passed beyond the experimental stage and
has been adopted already by a number of cities where such a method of
sewage disposal seems to answer all purposes.
_Land Irrigation._--In this method the organic and other useful
portions of sewage are utilized for irrigating land, to improve garden
and other vegetable growths by feeding the plants with the organic
products of animal excretion. Flat land, with a gentle slope, is best
suited for irrigation. The quantity of sewage disposed of will depend
on the character of the soil, its porosity, the time of the year,
temperature, intermittency of irrigation, etc. As a rule, one acre of
land is sufficient to dispose of the sewage of 100 to 150 people.
_Bacterial Methods._--The other biological methods, or the so-called
"bacterial" sewage treatment, are but modifications of the filtration
and irrigation methods of sewage disposal. Properly speaking the
bacterial purification of sewage is the scientific application of the
knowledge gained by the study of bacterial life and its action upon
sewage.
In intermittent filtration the sewage is passed through filter beds of
sands, etc., upon which filter beds the whole burden of the
purification of the sewage rests. In the bacterial methods the work of
purification is divided between the septic tanks where the sewage is
first let into and where it undergoes the action of the anaerobic
bacteria, and from these septic tanks the sewage is run to the contact
beds of coke and cinders to further undergo the action of the aerobic
bacteria, after the action of which the nitrified sewage is in a
proper form to be utilized for fertilization of land, etc. The septic
tanks are but a modification of the common cesspool, and are
constructed of masonry, brick, and concrete.
There are a number of special applications of the bacterial methods of
sewage treatment, into which we cannot go here.
=Sewage Disposal in the United States.=--According to its location,
position, etc., each city in the United States has its own method of
final disposition of sewage. Either one or the other, or a combination
of two of the above methods, is used.
The following cities discharge their sewage into the sea: Portland,
Salem, Lynn, Gloucester, Boston, Providence, New York, Baltimore,
Charleston, and Savannah.
The following cities discharge their sewage into rivers and lakes:
Philadelphia, Cincinnati, St. Louis, Albany, Minneapolis, St. Paul,
Washington, Buffalo, Detroit, Richmond, Chicago, Milwaukee, and
Cleveland.
"Worcester uses chemical precipitation. In Atlanta a part of the soil
is cremated, but the rest is deposited in pits 8 x 10 feet, and 5 feet
deep. It is then thoroughly mixed with dry ashes from the crematory,
and afterwards covered with either grain or grass. In Salt Lake City
and in Woonsocket it is disposed of in the same way. In Indianapolis
it is composted with marl and sawdust, and after some months used as a
fertilizer. A portion of the sewage is cremated in Atlanta, Camden,
Dayton, Evansville, Findlay, Ohio; Jacksonville, McKeesport, Pa.;
Muncie, and New Brighton. In Atlanta, in 1898, there were cremated
2,362 loads of sewage. In Dayton, during 30 days, there were cremated
1,900 barrels of 300 pounds each." (_Chapin, Mun. San. in U. S._)
=The Immediate Disposal of Sewage.=--The final disposition of sewage
is only one part of the problem of sewage disposal; the other part is
how to remove it from the house into the street, and from the street
into the places from which it is finally disposed.
The immediate disposal of sewage is accomplished by two methods--the
so-called _dry_, and the _water-carriage_ methods. By the _dry method_
we mean the removal of sewage without the aid of water, simply
collecting the dry and liquid portions of excreta, storing it for some
time, and then removing it for final disposal. By the _water-carriage
method_ is understood the system by which sewage, solid and liquid, is
flushed out by means of water, through pipes or conduits called
sewers, from the houses through the streets to the final destination.
=The Dry Methods.=--The dry or conservacy method of sewage disposal is
a primitive method used by all ancient peoples, in China at the
present time, and in all villages and sparsely populated districts; it
has for its basic principle the return to mother earth of all excreta,
to be used and worked over in its natural laboratory. The excreta are
simply left in the ground to undergo in the soil the various organic
changes, the difference in methods being only as regards the vessels
of collection and storage.
The methods are:
(1) Cesspool and privy vault.
(2) Pail system.
(3) Pneumatic system.
_The Privy Vault_ is the general mode of sewage disposal in villages,
some towns, and even in some large cities, wherever sewers are not
provided. In its primitive and unfortunately common form, the privy
vault is nothing but a hole dug in the ground near or at some distance
from the house; the hole is but a few feet deep, with a plank or rough
seat over it, and an improvised shed over all. The privy is filled
with the excreta; the liquids drain into the adjacent ground, which
becomes saturated, and contaminates the nearest wells and water
courses. The solid portion is left to accumulate until the hole is
filled or the stench becomes unbearable, when the hole is either
covered up and forgotten, or the excreta are removed and the hole used
over again. This is the common privy as we so often find it near the
cottages and mansions of our rural populace, and even in towns. A
better and improved form of privy is that built in the ground, and
made water-tight by being constructed of bricks set in cement, the
privy being placed at a distance from the house, the shed over it
ventilated, and the contents of the privy removed regularly and at
stated intervals, before they become a nuisance. At its best, however,
the privy vault is an abomination, as it can scarcely be so well
constructed as not to contaminate the surrounding soil, or so often
cleaned as to prevent decomposition and the escape of poisonous gases.
_The Pail System_ is an economic, simple, and, on the whole, very
efficient method of removing fresh excreta. The excreta are passed
directly into stone or metal water- and gas-tight pails, which, after
filling, are hermetically covered and removed to the places for final
disposal. This system is in use in Rochedale, Manchester, Glasgow, and
other places in England.
The pails may also be filled with dried earth, ashes, etc., which are
mixed with the excreta and convert it into a substance fit for
fertilization.
_The Pneumatic System_ is a rather complicated mechanical method
invented by Captain Lieurneur, and is used extensively in some places.
In this system the excreta are passed to certain pipes and
receptacles, and from there aspirated by means of air exhausts.
=The Water-carriage System.=--We now come to the modern mode of using
water to carry and flush all sewage material. This method is being
adopted throughout the civilized world. For it is claimed a reduction
of the mortality rate issues wherever it is introduced. The
water-carriage system presupposes the construction and existence of
pipes from the house to and through the street to the place of final
disposition. The pipes running from the house to the streets are
called house sewers; and when in the streets, are called street
sewers.
=The Separate and Combined Systems.=--Whenever the water-carriage
system is used, it is either intended to carry only sewage proper,
viz., solid and liquid excreta flushed by water, or fain water and
other waste water from the household in addition. The water-carriage
system is accordingly divided into two systems: _the combined_, by
which all sewage and all waste and rain water are carried through the
sewers, and the _separate_ system, in which two groups of pipes are
used--the sewers proper to carry sewage only, and the other pipes to
dispose of rain water and other uncontaminated waste water. Each
system has its advocates, its advantages and disadvantages. The
advantages claimed for the separate system are as follows:
(1) Sewers may be of small diameter, not more than six inches.
(2) Constant, efficient flow and flushing of sewage.
(3) The sewage gained is richer in fertilizing matter.
(4) The sewers never overflow, as is frequently the case in the
combined system.
(5) The sewers being small, no decomposition takes place therein.
(6) Sewers of small diameter need no special means of ventilation, or
main traps on house drains, and can be ventilated through the house
pipes.
On the other hand, the disadvantages of the separate system are:
(1) The need of two systems of sewers, for sewage and for rain water,
and the expense attached thereto.
(2) The sewers used for sewage proper require some system for
periodically flushing them, which, in the combined system, is done by
the occasional rains.
(3) Small sewers cannot be as well cleaned or gotten at as larger
ones.
The separate system has been used in Memphis and in Keene, N. H., for
a number of years with complete satisfaction. Most cities, however,
use the combined system.