Health > European conference on rural hygiene (June 29th-July 7th, 1931)

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The system of disposing of sewer effluents described above is the simplest and may
be used under the prescribed conditions. Obviously, unpurified sewage should not be
permitted to flow into a stream above or in the neighbourhood of the point at which water
is taken for drinking-purposes.
4. Purification of Sewer Effluents.
When it becomes necessary to purify sewer effluents as in:
(a) Closely populated rural districts;
(b) Districts without an abundance of surface water;
(c) Districts where it is desirable to limit stream pollution to a certain maximum,
a number of methods may be adopted in rural districts. These should be simple, adapted to
local conditions and require a minimum of care by unskilled staff. These methods are
mechanical and biological.
A. Mechanical Methods.— The simplest mechanical method is the use of fixed racks
intended to retain the solids.
Settling-tanks also effect a certain amount of purification depending upon the condition
of the sewage and the velocity of flow in the tank.
With these methods, only matter in suspension is removed. If the water-course which
receives the effluent is not too small, this treatment is generally adequate, particularly
in rural districts where the volume of sewage is slight. It should be remembered that the
most harmful substances passing into the water-course consist of matter in suspension.
The simplest mechanical method is to retain matter in suspension by means of fixed
racks
A higher degree of purification may be obtained by using settling-tanks. The time
required for settling depends on the kind of sewage, but two hours is generally sufficient.
To avoid evil-smelling fermentations, the period should not exceed four hours. In practice,
a tank with a capacity of 0.62 of the daily volume of the dry-weather flow, the width
equalling one-third of the length, gives good results. Care must be taken to avoid
sludge being carried from the settling-tank into the water-course which receives the
effluent (production of hydrogen sulphide).
The disposal of sludge offers a difficult problem in towns, but is a simple matter m
small rural installations.
B. Artificial Biological Methods. — These are always preceded by mechanical methods.
The principal artificial biological methods are:
(1) Sprinkling filters: this is one of the methods best adapted to rural conditions,
being inexpensive to instal and maintain, requiring little attention and no trained
personnel and being capable of easy repair;
(2) The remaining methods — such as activated sludge (as at present consti¬
tuted), contact beds or sand filters ■— are not adapted to rural conditions.
The biological methods (both artificial and natural) remove matter in suspension
from the effluent, partly dissolve such matter and prevent the putrefaction of the sewage.