hods. 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, o