We will next deal with the conditions which should be fulfilled where
it is sought to utilize a river or the sea into which to cast the
sewage of a town. If it can be ascertained beyond question that at the
proposed point of discharge the currents at all times will carry the
sewage right away, and will not at the same time produce mischief at
a distance (which is often omitted from the consideration), then that
arrangement may be accepted as a good one. This, however, seldom occurs.
A river has been looked upon by manufacturers and local authorities
as the natural carrier of their refuse from their district. This view
has been persevered in, in spite of the River Pollution Prevention Act
of 1876, which is practically a dead letter. The public, however, who
use a river either for pleasure purposes or for obtaining their water
supply, have of late years grown more and more united in their efforts
to stop this abuse; and there is no doubt that these efforts will
eventually succeed. In a paper which we read last year at the Congress
at Glasgow, we pointed out the steps that were necessary to be taken
to render this act operative, and we refer our hearers to that paper
if they wish to follow the matter further. The effect of discharging
sewage matter into a river has been the subject of much controversy
among chemists. Some allege most positively that the injurious
properties in the sewage are indestructible. This has led to alarmists
demanding that under no circumstances ought sewage to pass untreated
into a river.
We have given considerable attention to this vexed question, as it
requires to be grasped by any engineer who has to advise on the
selection of sewer outfalls, and it appears to us that the balance of
evidence is against the alarmists. Every river has a certain power of
oxidizing impurities in proportion to the extent of oxidation of the
river itself. Besides this, there are the powerful purifying influences
exercised by the plants and animalcules which exist in rivers.
It has been ascertained that entomostraca consume dead animal matter;
and where this is wanting they do not live, but where it is in
abundance they thrive. It follows, then, these minute animals exercise
an important function in absorbing sewage impurities. They multiply
prodigiously in these impurities, and are both created by them and fed
upon them, converting foul and dangerous matters into harmless ones, in
a similar way to that which we have referred to as nitrification when
speaking of the action of bacteria in the soil. Considering that these
organisms arise from and are fed on concentrated filth, it is obvious
that they cannot live when the conditions favorable to their existence
disappear. This would be the case when the sewage is discharged into a
large volume of water with a different temperature to that which suits
them, and with powerful oxidizing influences at work. These conditions,
added to the difficulty they must experience to find their natural
food--namely, concentrated sewage--where the sewage matter becomes so
greatly diluted, accounts for the fact that in a short run of a good
river sewage impurities largely disappear. The action of weeds and
plants also aids purification to a very large extent. Minute plants,
such as confervoid algæ and the like, also assist in oxygenating the
river, as when exposed to light they decompose carbonic acid, and
liberate oxygen.
The practical question which has to be answered in every case where
sewage is proposed to be discharged into a river requires to be
approached from two points. The first is whether a nuisance will be
caused at the spot to which objection would be taken. If this is likely
to be the case, then the fact that the sewage will get purified in a
short run of the river does not meet the objection. The second point
requires a careful consideration of the condition of the river, both
from an engineering as well as from a chemical and biological point
of view. Decisions on these matters have too often been arrived at
in a rough and ready way. They require skillful treatment, as the
interests--both commercial and hygienic--which are affected are too
great to permit of them being dealt with by any who are not well
informed and careful. The general conclusions which we deduce from our
observations are as follows:
1. That chemical precipitation is not so necessary now as it was
considered to be a few years ago, in cases where land for irrigation is
not procurable.
2. That the efforts to profitably remove the manurial elements from
sewage by chemicals not having been successful, the system should be
adopted _per se_ only where a filtration area cannot be obtained.
3. That the success which has attended the construction of filtration
areas where the land is clayey, and the successful results which have
been obtained from a combined straining of sewage and of subsequent
filtration through small areas of artificial filters, point to the
adoption of one or other of these systems in many cases where chemical
treatment would previously have been advised.
4. That the injurious effects of passing untreated sewage into a river
depend upon not merely the relative volumes of the sewage and the
river, but chiefly upon the power of the river to oxidize the sewage,
which power is in proportion to the extent of oxidation of the river
itself.