le, part of the water-supply is used for an
ornamental fountain in the front yard, or if in the summer time a large
amount of water is used for sprinkling the lawns, that water is not
converted into sewage, and the amount of the latter is thereby
diminished; but, ordinarily, it is safe to say that the quantity of
water supplied to the house and the quantity of sewage taken away from
the house is identical, and since it is much easier to measure the
water-supply than the sewage flow, the former is taken as the quantity
of sewage to be treated.

In the course of its passage through the house, however, the water has
added to it a certain amount of polluting substances, largely derived
from the kitchen sink, where dirt from vegetables and particles of
vegetable material, together with more or less soap, are carried by the
waste water from the sink into the drain. In the bath-room, also, some
small amount of organic matter is added to the water, but the proportion
of such matter to the total volume of water used is very small, probably
not exceeding one tenth of one per cent. This small proportion is
nevertheless sufficient to become very objectionable if allowed to
decompose, and the problem of sewage disposal for a single house is to
drain away the water, leaving behind the solids so disposed that they
shall not subsequently cause offense by their putrefaction.

The process of decay is normal for all organic matter and is due to the
agency of certain bacteria whose duty it is, providentially, to
eliminate from the surface of the earth organic matter which otherwise
would remain useless, if not destructive, to man. It is impossible to
leave any vegetable or animal matter exposed to the air without this
process of decay at once setting in. Apples left in the orchard at the
end of the season inevitably are reduced and disappear in a short time.
Dead animals, whether large or small, in the same way succumb to the
same process of nature, and it has been pointed out that, unless this
provision did exist, the accumulation of such organic wastes since the
settlement of this country would be so great as to make the country
uninhabitable. Fortunately, however, this inevitable process breaks down
the structure of all organic material, partly converting fiber and pulp
into gas, partly liquefying the material and converting the remainder
into inorganic matter which is of vast importance as food for plant
life. A cycle is thus formed w