_May 31, 1900._
THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY
_Theodore Clifford Phillips and Edward John Schneider_ ENTITLED
_Comparison of Methods of Sewage Purification_ IS APPROVED BY ME AS
FULFILLING THIS PART OF THE REQUIREMENTS FOR THE DEGREE OF _Bachelor of
Science in Municipal and Sanitary Engineering_.
_Arthur M. Talbot_
HEAD OF DEPARTMENT OF _Municipal and Sanitary Engineering_.
_Contents_
Comparison of Methods of Sewage Purification
Dilution
Irrigation
Intermittent Downward Filtration
Chemical Precipitation
Septic Tank
Contact Bed
Discussion
Conclusion
_Comparison of Methods of Sewage Purification._
The disposal of sewage is one of the most important problems that
confront the municipal and sanitary engineer to-day.
The study of this subject dates back to 1844 when the Healths of Towns
Commission of London made the report which first showed the real course
of the rapid increase in the death rate of the larger cities. In this
country practically nothing was done until 1872 when the Massachusetts
Legislature instructed the State Board of Health to investigate and
report on the subjects of sewerage, sewage disposal, and the pollution
of streams, and to-day Massachusetts leads in the investigation of the
best methods of sewage disposal.
Six different methods for the treatment of sewage are now in use. These
methods are used separately or in combination, the choice depending upon
local conditions and the amount of purification required:—
These methods are:
1— Dilution.
2— Irrigation.
3— Intermittent Downward Filtration.
4— Chemical Precipitation.
5— Septic Tank.
6— Contact Bed.
The purpose of this thesis is to investigate each method and make a
comparison along the following lines:
1— Purification.
2— Capacity.
3— Applicability.
4— Cost.
_Dilution_
Disposal of sewage by dilution is the most common method at the present
time, and no doubt will continue to be for some time to come where the
stream or body of water is not used for domestic purposes. Under certain
conditions this affords a satisfactory solution of the problem. However,
if the water into which the sewage is discharged is a source of water
supply, two very important questions arise. They are: 1—How large must
the dilution be? 2—How far must the polluted stream flow before the
water way may be used with safety to public health?
In 1867 Dr. Letheby and others in England made the statement that if
sewage is diluted at least twenty times its volume, it will not only be
made inoffensive but be thoroughly destroyed after flowing a dozen miles
or more.
The Rivers Pollution Commission of Great Britain in 1878 maintained that
no river in England was long enough to allow of a complete disappearance
of sewage matter discharged into it.
Mr. F. P. Stearns in an article on The Pollution and Self-Purification
of Streams in the Massachusetts State Board of Health Report 1890 gives
a table showing, “The calculated composition of sewage for different
degrees of dilution in a running stream”. A comparison was made of the
Blackstone and Merrimack rivers, and extended over three years. It was
from this investigation that the Commissioner of the City of Chicago
reported that the flow of the Chicago Drainage Canal should be four
cubic feet per second per 1000 inhabitants.
The purification by dilution may be due to three things:
1— Oxidation of organic matter.
2— Subsidence.
3— Destruction of the organic matter by small animals and plants.
The oxidation is a very slow process and depends mainly upon bacterial
life and conditions favorable to it. Subsidence depends upon the
difference in the specific gravity of the suspended matter, and the
clarification is increased as the velocity is diminished.
In regard to the destruction of the organic matter by small animals and
plants Dr. Sorby says, (_Journal Royal Microscopic Society, 1884 p.
988_.) “It appears to me that the removal of impurities from rivers is
more of a biolytic than a chemical question, and it is most important to
consider the action of minute animals and plants, which may be looked
upon as being indirectly most powerful agents.”
_Irrigation_
Although the method of broad irrigation has been carried on successfully
in several of the largest cities of the old world, it has not been used
to any great extent in the United States, except at a few state
institutions in the east, and in the arid districts of the west.
Several years ago this method of disposal was given much attention as it
was thought the sewage if used would yield large profits, but more
recent information shows conclusively that such is not the case.
Perhaps the most noticeable examples of this system that are now in
operation in the United States are the plants at the State Hospital of
the Insane, Worcester, Mass., at the Rhode Island State Institute, and
at the Reformatory at Concord, Mass. In all cases the labor was done by
the inmates so that it is not possible to get a fair statement of the
cost. General information regarding the use of sewage for irrigation in
the west may be obtained from Baker and Rafters’ “Sewage Disposal in the
United States”.
Undoubtedly the best example of sewage farming is at Berlin. The
following summary is given by James H. Fuentes, M. Am. Soc. C. E., in
No. 2. Vol. 40 of the Engineering Record. The average quantity of sewage
pumped to the farms and distributed amounts to from 6,000,000 to
7,000,000 gallons per acre per year, or in other words each acre serves
for 750 people. The area farmed is divided into small beds about 150 by
200 feet separated from each other by slight embankments and ditches for
distributing the sewage over the surface. The sewage is admitted into
the carriers from the force mains through checking chambers made of
woven willow and posts driven into the sand.
The farms are said to yield small profits over the expenses of working
them, consequently contribute scarcely anything toward the cost of
pumping the sewage.
The great reason why this method of disposal has been possible and
profitable in a degree at Berlin is that this large and beautiful city
lies in the midst of an extensive sandy area, the greater part of which
in the immediate neighborhood is quite sterile and therefore
comparatively thinly populated. Better conditions for such sewage farms
could not exist.
Statistics compiled by Chas. S. Swan, M. Am. Soc. C. E. in his paper,
Notes on European Practice in Sewage Disposal, in Jour. Assn. of Eng.
Soc. Vol. 7 No. 7 shows the volume of sewage per acre per day (yearly
average) to vary from 2155 to 29450 gallons, and the average depth per
annum varies from 2.4 to 32.8 feet.
The nineteenth annual report of the Massachusetts State Board of Health
1888 states that on an ordinary farm in Massachusetts 2500 gallons per
acre per day are as much as could be applied to any valuable grass crop.
This would require such a large area for large cities that irrigation
could not be depended upon for preventing the pollution of streams.
It is evident from the above report that the irrigation system can be
used to advantage only where large tracts of low land can be obtained at
a low cost, and where help and plenty of cheap labor can be obtained.
_Intermittent Downward Filtration_
Filtration means the concentration of sewage on an area of especially
chosen porous ground, or on an especially prepared bed as small as will
absorb and purify it. The intermittency of application is a necessary
requirement even in suitably constituted soils wherever complete success
is arrived at. The reason for this is that free oxygen is indispensable
to nitrification and the nitrifying organism.
This method has been in operation only during the last thirty years, and
the correct theory of its action has been found out within the last ten
years.
Dr. Frankland, in the first report of Rivers Pollution Commission of
England, shows that not only a chemical action but also a biolytic
action takes place with the assistance of minerals and the oxygen of the
air. This biolytic action is known as nitrification, and consists in the
oxidation of the nitrogen of ammonia and its ultimate conversion into
nitric acid. This change takes place in two stages, each characterized
by a distinct organism. The office of one of these organisms is to
convert ammonia into nitrite, while the other converts nitrites into
nitrate.
The bacteria are present or quickly develop in the sewage which may be
considered a nutrient medium for them by reason of containing a large
amount of their natural nitrogenous food.
These organisms, according to Warrington, may be separated by successive
cultivations in favorable media; from a potassium nitrate solution with
no ammonia we may obtain nitric organism alone; from an ammonium
carbonate solution we may obtain nitrous organisms in a pure state.
It is a noticeable fact that frost checks nitrification, but several
satisfactory methods have been used so that no serious objection can be
raised to intermittent filtration on this ground.
“The purification of sewage by this method depends upon the oxygen and
the time of action” (Hazen). To obtain the highest efficiency the sewage
must be applied intermittently and there must be a large percentage of
voids in the sand.
Experiments made on especially prepared beds at Lawrence, Mass. show
that by treating 85000 gallons per acre per day, 94 percent of the
organic matter and 98 percent of the bacteria are removed, and by
treating 60,000 gallons per acre per day, 97 to 99 percent of the
organic matter may be removed. The above data shows that 100,000 may be
successfully treated with a five foot bed of sand.
The writers inspected the intermittent downward filtration plant at
Mendota Illinois and the following data were obtained. The city has a
population of about 6000 with 8 miles of sewers and 210 house
connections. The average daily flow is about 350,000 gallons although
during the rainy season there is a considerable amount of ground water
present.
The disposal plant was constructed under the supervision of the Iowa
Engineering Company of Clinton, Iowa, on a 15 acre tract of land, one
and a half miles south of the city. Although original plans called for
18 beds with five feet of gravel, only four beds each 275 feet by 75
feet were finished, and instead of 5 feet of ground only 24 inches was
used. The plant was complete in July 1899 and was operated until
December, when it was discontinued on account of cold weather, it not
being considered necessary to operate it during the winter. From
December until April the sewage was discharged directly into a creek
which runs along one side of the land.
The beds have no automatic device for distributing the sewage at
definite intervals of time, but are connected with the main sewer by a
number of gate valves which were regulated by the attendant. The sewage
is distributed by means of a wooden sluice way which extended the entire
length of the bed and has six inch openings on the side every 16 feet
which gave a fairly even distribution.
Although no analysis of the sand and gravel was made, it was apparent
that the uniformity coefficient was very large.
The material was shipped a distance of 90 miles and resembles ordinary
ballast used for rail roads. The largest pieces were thrown along the
sluice way to prevent excessive washing. The finest of the sand was of a
size which would be considered too fine for good mortar sand. No attempt
was made to screen the material. The gravel would not be considered
first class for filtering beds.
Samples of the sewage and effluent were taken at two different times.
The first was between 9:00 and 10:30 A. M., April 20, 1900. Until the
day before no sewage had been applied to the beds since last fall. The
second collection was made two weeks later, when samples of the sewage
and effluent were collected at intervals of one hour during the day, and
mixed into composite samples. The sewage was taken from a manhole just
above the beds, and the effluent from the outlet of the underdrain about
900 feet below the beds.
The analysis is given in Table I. and the results show that the beds are
not working as efficiently as might be expected from a first class
plant. This may be accounted for from the fact that the filtration is
too rapid and that the material which is less than two feet in depth is
not suitable for such a purpose.