_RATE OF SEWAGE TREATMENT._
System. Location. Sewage treated in
gallons per acre per
day.
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IRRIGATION.
Berlin, Germany. 18,000.
Manchester, Eng.—Estimated but not
constructed. 18,500.
INTERMITTENT DOWNWARD FILTRATION.
Framingham, Mass. 19,000.
Leicester, Mass. 38,750.
Brockton, Mass. 44,000.
Marlborough, Mass. 54,000.
Gardner, Mass. 140,000.
Mendota, Ills. 184,000.
Worcester,[1] Mass. 322,000.
Worcester,[1] Mass. (Fletcher’s estimate). 500,000.
Madison,[1] Wis. 8,000,000.
CONTACT BEDS.
Manchester, Eng. (crude sewage) 500,000.
Manchester,[1] Eng. 700,000.
Manchester, Eng. (storm water sewage) 2,500,000.
Footnote 1:
Secondary filtration.
Table III shows the capacity of representative plants. Best authorities
consider 100,000 gallons per acre per day to be the maximum rate
permissible under the best conditions for the treatment of crude sewage
by the intermittent downward filtration system. With unfavorable
conditions the quantity of sewage should be limited to as low as 20,000
gallons. Wherever too large a dose is applied to the bed the sewage is
not properly purified and the beds soon become clogged up and unfit for
further use.
When the beds are used for secondary purification, 750,000 gallons may
be treated per acre per day.
The results of extensive experiments made at Manchester, England in 1898
and 1899 show that by means of contact beds crude sewage may be treated
at the rate of 500,000 gallons per acre per day. When the beds are used
for secondary purification, 750,000 gallons may be successfully treated,
and storm water sewage treated at the rate of 2,500,000 gallons per acre
per day.
The capacity of the chemical precipitation plant at Madison, Wisconsin
is 68 percent of the daily flow, which in 1899 was estimated at 300,000.
The company that built the plant agreed that the plant should have a
daily capacity of 1,200,000 for each and every day in the year. This
shows the estimated capacity of the tank to be 28 percent.
At Worcester, one of the best examples of this process is in operation.
The size of the tanks is equal to 28 percent of the total flow of 17.1
million gallons per day of which 10.1 million gallons is the out flow
from a comparatively clear pond. If the actual amount of sewage is
considered the tanks have a capacity of 65 percent of the total flow.
Rafter and Baker in their discussion of the size of tanks recommends
that the total capacity should be nearly 50 percent of the average daily
flow.
The two typical types of septic tanks are those at Exeter, England and
at Champaign Illinois. The first has a cubic capacity of 93 percent of
the total daily flow, while the second has a capacity of 7.5 percent of
the daily flow.
When the capacity of the tank is as small as at Champaign the bacteria
do not have time to act upon the sewage as they would if the flow was
not so rapid.
It is conceded now by the highest authorities on this subject that the
tank should have a capacity equal to from one fourth to one half of the
average daily flow.
Concerning the conditions for which the various processes are
applicable, it may be said that the dilution process is used when
advantage may be taken of natural resources. This method can be utilized
by most cities situated along rivers or streams large enough to
sufficiently dilute and carry away the sewage, and fulfill sanitary
requirements.
As an example of the use of dilution the Chicago River may be cited.
There the uncertain flow of the river was made to pass inward toward the
Illinois River with a speed of 2½ miles per hour and a discharge of
20,000 gallons per minute per 100,000 inhabitants. The Blackstone River
was used by the cities of Massachusetts until the pollution of the river
became unbearable and the state was compelled to pass a law forcing the
cities to purify their sewage.
Broad irrigation is a method which cannot generally be used on account
of the large amount of land and labor required. This method is
especially applicable to asylums, alms houses, and reformatories where
cost of labor is small. It is used in the West where the value of all
available water leads to the application of sewage to crops, and also on
account of the low stage of western streams during the summer which
renders sewage discharged into them an unbearable nuisance.
Intermittent downward filtration may be used where there is a
considerable area of sandy soil, and also where a high degree of
purification is necessary.
Chemical precipitation does not require a large area for its operation
and used alone does not give a high degree of purification. Where land
and material for beds is expensive, and partial purification is
sufficient, this system may be used.
The septic tank requires a small area. The odors are not offensive and
cannot be noticed 100 feet away. The effluent may be discharged into
small streams and soon loses its identity.
Contact beds are used where a high degree of purity is required; if the
effluent is emptied into rivers or sources of water supply. The beds do
not require a large area and the process may be recommended if suitable
material is at hand.
The cost of construction and maintenance of the different systems vary
so largely according to local conditions that a fair estimate is not
possible. As the most striking example of maximum and minimum cost the
disposal system at Chicago and at Lowell, Massachusetts may be cited.
The purification in both cases is by dilution, the cost being
practically nothing at Lowell while the Chicago Drainage Canal cost
$33,000,000. It may not be fair to charge this entire cost to the
sewerage system; nevertheless up to date it has been used for no other
purpose.
At Berlin, Germany where broad irrigation is used, the sewage has to be
pumped to the farms and involves a considerable cost. Aside from this
the receipts derived from the sale of farm products are enough to pay
the running expenses of the farm.
The first cost of the intermittent downward filtration plant is
estimated by the writers at $90,000 per million gallons per day of
sewage treated, where the beds are artificial. Where natural beds can be
used the first cost may be reduced to $7,500 per million gallons per
day. The cost of maintenance is about $2.00 per 1,000,000 gallons.
The average cost for operating a chemical disposal plant is about 58
cents per inhabitant per year, or $16.00 per 1,000,000 gallons treated.
At Manchester England where broad irrigation and chemical precipitation
are used in combination, the average cost was $63.00 per 1,000,000
gallons treated.
A septic tank for treating 1,000,000 gallons of sewage per day will cost
less than $10,000 and the cost of maintenance will be about $1.00 per
1,000,000 gallons treated.
_Conclusion_
In selecting a method of sewage disposal, the conditions, surroundings,
and requirements of the city should be carefully studied and analyzed,
and judgment and discretion must be used. A matter of so much importance
to the community should be placed in the hands of men qualified to make
a proper solution of the problem. While in general several methods of
purification may be applied to the requirements of the city, usually
local conditions and considerations will narrow the choice to two plans
or possibly to a single method.
The governing features of the dilution process are so distinct that it
is not usually difficult to determine where this method is applicable,
and likewise, the broad irrigation process has peculiar conditions. In
both, the plans involve only matters of construction.
The other methods have more in common and the determination of their
relative value is not so easy. As before stated, the recently developed
biolytic processes promise to displace chemical precipitation. Where a
suitable deposit of sand or gravel is conveniently located on cheap land
which may be made without great expense into filtration areas and the
sewage discharged upon them by gravity, intermittent downward filtration
may be the most satisfactory, especially if a highly purified effluent
is desired. The item of expense of attendance, and labor of maintenance
must be considered in connection with the cost of this method. In the
absence of such favorable conditions and especially where complete
purification is not required the septic tank may be the most suitable.
For higher purification the combination of the septic tank with filter
beds, or contact beds run at comparatively high rates, makes a
satisfactory purification and is applicable to a wide range of
conditions. A bacteria bed may be substituted for the septic tank for
the roughing process but its applicability in not so general.
In conclusion it may be said that if the next ten years gives as much
development in sewage purification as has the last decade, some of the
processes herein outlined will have been discarded and sanitary
engineering will have achieved still greater triumphs.
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